Home

Agilent Technologies 90B User's Manual

1. OPERATING REGION Es SHORT CIRCUIT LOAD 0 lt 15 Eg FRONT PANEL VOLTAGE CONTROL SETTING Ig FRONT PANEL CURRENT CONTROL SETTING JES CRITICAL OR CROSSOVER VALUE Tg LOAD RESISTOR Figure 18 Operating Locus of a CV CC Power Supply Full protection against any overload condition is inherent in the Constant Voltage Constant Current design principle because all load conditions cause an output that lies somewhere on the operating locus of Figure 18 For either constant voltage or constant current operation the proper choice of ES and Is insures optimum pro tection for the load device as well as full protection for the power supply The slope of the line connecting the origin with any operating point on the locus of Figure 18 is proportional to the value of load resistance connected to the output terminals of the supply The critical or crossover value of load resistance is defined as RC ES IS and adjustment of the front panel voltage and current controls permits the crossover resistance to be set to any desired value from 0 to co If is greater than Rc the supply is in constant voltage operation while if R is less than the supply is in constant current operation CONSTANT VOLTAGE CURRENT LIMITING SUPPLIES Current limiting supplies provide overcurrent protection for applications where a constant voltage is the only requirement There are two basic types of current limiting
2. Figure 61 Speed of Response Programming Up Figure 62 shows that when the power supply is programmed down the regulator senses that the output voltage is higher than desired and turns off the series transistors entirely Since the control circuit can in no way cause the series regulator transistors to conduct backwards the output capacitor can only be discharged through the load resistor The output voltage decays exponentially with a time constant and stops falling when it reaches the new output voltage which has been demanded If no load resistor is attached to the power supply output terminals the output voltage will fall slowly the output capacitor being discharged only by internal bleed paths within the power supply Whether the supply is required to increase or decrease its output voltage the output capacitor tends to slow the change Many Agilent power supplies therefore make it possible to remove a major portion of the output capacitance simply by removing a strap on the rear barrier strip After this has been accomplished the output voltage can in general by programmed ten to one hundred times more rapidly but the regulator loop may need readjustment of the transient recovery control so that the supply does not oscillate under certain load conditions Beyond a certain point further reduction in the size of the output capacitor Co will not result in greater speed of programming since other power supply circuit elements
3. ce eee a eet EEUU 72 oro imo loop definition MENS C Nc p in a Raed oh asm en ikea 65 isolation from dc distribution 5 14 Ground 71 79 Ground power cord safety e pet REI cada Ra oen to so Ene ead 68 Guard amplifier in high performance constant current supply 47 H High performance constant current supply 45 High performance constant voltage current limited supply 45 High performance power supply definition 45 High voltage power supply eerte e SUR EAR CA ES FN eas YN RR Ee E ER AE EVE 45 output voltage conteolmethogs qui dee dpt ad 46 High voltage regulator see Piggy Back regulator Impedance output calculation eats 64 OL CONSTANE current 33 Ol constant voltage SUDDD o 17 12 122 of high performance constant current supply esee eene enne 49 measurement 0 112 119 Inrush current definition iie tete ei dee eee Aaa oad 11 Protection satan tel peerage 22 28 L Linear regulators see seri
4. Since any variation in the resistance value of R1 and R2 will result in a change in the voltage divider ratio and hence the output of the slave supply it is important that both these resistors have a low temperature coefficient 20 ppm C or better and have a power rating at least 10 times their actual dissipation Resistors and R2 should be selected so that at the nominal operating levels the current through them will be of the order of 1 to 5mA 97 ET Es RI4R2 ul RI Figure 67 Auto Series Operation of Two Supplies Comparing Figure 67 with previous block diagrams for the constant voltage power supply there is no difference in the circuit location of Resistor R2 and the front panel voltage control normally found in Agilent laboratory type power supplies Thus Auto Series operation can be achieved using only one external resistor R1 and employing the front panel voltage control on the slave supply as the element which determines the ratio of its voltage to that of the master Mixed model numbers may be employed in Auto Series combination without restriction provided that each slave is specified as being capable of Auto Series operation The master supply need not be an Auto Series supply since the internal circuit aspects of the master supply in no way affect the AutoSeries principle of operation If the master supply is set up for constant current operation then the master slave combination will
5. so cesta tees a 98 124 S Safety ground ni pOWEE COE end su ducite 68 Sampling resistor see current monitoring resistor SCRs defrmitiQpz n locos cetero ec eiiis esee ats teet leote 24 IM ocerudrioquiii m M 24 Deu Selen 32 Senes Opera IC ost ide we 97 Series regulator advantages disadvantages 18 T J S 19 Source ekfectsdefinttion z o oed acie eid dde i o a Rea ndoe ese 16 104 116 108 117 Stability see Drift LET TONE c Cc 85 Switching regulator advantages disadvantages essere 25 COIT pote M sat eus o t E M feta 26 Circuit pEI netples uie ad e qp n 27 T Temperature coefficient Definition 16 Aes mE p et 119 of remote programming resistors a cod rai der tad 82 Tracking operation see Auto Tracking Transient recovery time see load effect transient recovery M P 25 29 V Voltage divider for dual output ee a t ERO TR C a ba e ONES da 94 Voltage drop in ac power input cable fais tae cod eet pese 94 consta
6. CC Temperature Coefficient Definition The change in output current per degree Celsius change in the ambient temperature following a 30 minute warm up During the measurement interval the ac line voltage output current setting and load resistance are held constant The constant current power supply must be placed in an oven and operated over any temperature span within the power supply rating The current monitoring resistor RM should not be placed in the oven but must be held at a constant temperature while this measurement is made Other Constant Current Specifications The measurement of output impedance programming speed and other performance specifications is less often required in the case of constant current power supplies Complete information on proper methods of measuring any other constant current specifications beyond those listed here can be obtained by contacting your nearest Agilent field sales office 119 INDEX A AC power input connections inp t dicc rm 61 interchanging ac and ace leads seinen erre tpi i dito eir Sale 60 interchanging amd ground leads oec ete Ih ei ope ten 60 Ambient temperature definition c iiec ev eati euis 6 Amplifier comparison see Comparison amplifier Amplifier power SUp ply te ena deoa baeo 14 Auto Parallel operation connections 96 defo ks s oto se cia SEEE TE R 6 Auto Series operati
7. Current Source described earlier in this section As mentioned previously these guarding techniques prevent the flow of leakage current and allow a more precise regulation of the output current 58 AC AND LOAD CONNECTIONS Modern power supplies are flexible high performance instruments designed to deliver a constant or controlled output with a maximum of reliability and control versatility In many cases however the user inadvertently degrades this performance capability by making improper wiring connections to the input or output At best this can result in excessive output ripple a tendency toward oscillation poor load and line regulation and unnecessary degradation of stability temperature coefficient and transient recovery specifications At worst the result can be power supply failure and potential shock hazards Careful attention to the guidelines presented in this section will improve the safety and usefulness of power supplies As a general rule the guidelines should be followed in the sequence given e g dc distribution terminals must be considered before common or ground connections The following checklist is included for quick reference to the most important rules in connecting dc power supplies these rules are repeated with greater detail on the pages indicated CHECKLIST FOR AC AND LOAD CONNECTIONS Each rule should be followed in the sequence indicated AC Power Input Connections Page 1 The ac acc and th
8. Unless otherwise specified Agilent measures efficiency at maximum rated output power and at worst case conditions of the ac line voltage ELECTROMAGNETIC INTERFERENCE EMI Any type of electromagnetic energy that could degrade the performance of electrical or electronic equipment The EMI generated by a power supply can be propagated either by conduction via the input and output leads or by radiation from the units case The terms noise and radio frequency interference RFI are sometimes used in the same context INRUSH CURRENT The maximum instantaneous value of the input current to a power supply when ac power is first applied 11 LOAD EFFECT LOAD REGULATION Formerly known as load regulation load effect is the change in the steady state value of the dc output voltage or current resulting from a specified change in the load current of a constant voltage supply or the load voltage of a constant current supply with all other influence quantities maintained constant LOAD EFFECT TRANSIENT RECOVERY TIME Sometimes referred to as transient recovery time or transient response time it is loosely speaking the time required for the output voltage of a power supply to return to within a level approximating the normal dc output following a sudden change in load current More exactly Load Transient Recovery Time for a CV supply is the time X required for the output voltage to recover to and stay within Y millivolts of the
9. of current monitoring resistors in the master and slave supplies the output current contribution will always be equal regardless of the output voltage or current requirement of the load Normally only supplies having the same model number should be connected for Auto Parallel operation since the two supplies must have the same voltage drop across the current monitoring resistor at full current rating As is also true of Auto Series and Auto Tracking operation no internal wiring changes are necessary All that is required is a screwdriver to change the strapping pattern on the terminals of the rear barrier strip and one extra lead running from the barrier strip of each slave supply to another supply in the same master slave system SERIES OPERATION Series operation of two or more Agilent power supplies can be accomplished up to the output isolation rating usually 300 volts off ground of any one supply Series connected supplies can be operated with one load across both supplies or with a separate load for each supply All Agilent semiconductor power supplies have reverse polarity diodes connected across the output terminals so that if operated in series with other power supplies reverse polarity will not occur across the output terminal of any supply if the load is short circuited or if one power supply is turned on separately from its series partners AUTO SERIES OPERATION Auto Series or automatic series operation of power supplies permit
10. 101 AUTOMATIC CONSTANT VOLTAGE LOAD SWITCH POWER SUPPLY UNDER TEST VARIABLE AUTO TRANSFORMER DIFFERENTIAL OR DIGITAL DC VOLTMETER TRUE RMS AC VOLTMETER Figure 70 Constant Voltage Measurement Setup Failure to connect the monitoring instrument to the proper points shown in Figure 71 will result in the measurement not of the power supply characteristics but of the power supply plus the resistance of the leads between its output terminals and the point of connection Even using clip leads to connect the load to the power supply terminals and the monitoring instrument to the load leads can result in a serious measurement error Remember that the power supply being measured probably has an output impedance of less than 1 milliohm and the contact resistance between clip leads and power supply terminals will in most cases be considerably greater than the specified output impedance of the power supply Use Separate Leads to All Measuring Instruments All measurement instruments oscilloscope ac voltmeter differential or digital voltmeter must be connected directly by separate pairs of leads to the monitoring points indicated in Figure 71 This is necessary to avoid the subtle mutual coupling effects that may occur between measuring instruments unless all are returned to the low impedance terminals of the power supply Twisted pairs or shielded cable should be used to avoid pickup on the measuring le
11. CV Output Impedance The output impedance of a power supply is normally not measured since the measurement of load effect transient recovery time reveals both the static and dynamic output characteristics with just one measurement The output impedance is commonly measured only in those cases where the exact value at a particular frequency is of engineering importance consult the Agilent data sheet or the Operating and Service Manual for further details CONSTANT CURRENT POWER SUPPLY MEASUREMENTS For the most part the instruments methods and precautions necessary for the proper measurement of constant current power supply characteristics are identical to those already described for the measurement of constant voltage power supplies As Figure 79 shows there are only two major differences which distinguish the constant current measurement setup from the constant voltage measurement setup 1 The load switch is connected in parallel rather than in series with the power supply load since the power supply performance will be checked between short circuit and full load rather than open circuit and full load 2 Acurrent monitoring resistor is inserted between the output of the power supply and the load To simplify grounding problems one end of this monitoring resistor should be connected to the same output terminal of 113 the power supply which will be shorted to ground All constant current measurements are made in terms of the change in
12. act as a composite constant current source In some applications remote programming of the master supply is employed thereby achieving simultaneous control of the output of two sources from a single remote resistance or voltage input When the center tap of such an Auto Series combination is grounded coordinated positive and negative voltages result This technique is commonly referred to as rubber banding and an external reference source may be employed if desired Any change of the internal or external reference source e g drift ripple will cause an equal percentage change in the outputs of both the master and slave supplies This feature is of considerable use in any application where the load requires a positive and a negative power supply and is less susceptible to an output voltage change occurring simultaneously in both supplies than to a change in either supply alone 98 AUTO TRACKING OPERATION Auto Tracking or automatic tracking operation of power supplies is similar to Auto Series operation except that the master and slave supplies have the same output polarity with respect to a common bus or ground Figure 68 shows two supplies connected in Auto Tracking with their negative output terminals connected together as a common or ground point A fraction R2 R1 R2 of the output of the master supply is provided as one of the inputs to the comparison amplifier of the slave supply thus controlling the slave s output The master suppl
13. 10 of power rating will be accompanied by approx imately a 50 C temperature rise above ambient at the surface of the resistor the bobble or slow variation in this surface temperature will amount to about 20 of the rise above ambient in this case a bobble of about 10 C peak to peak Using a 20ppm resistor this 10 C variation will cause roughly a 0 02 variation in the measured current even though the monitoring resistor is being operated at only 1 10 of its power rating 114 LOAD CURRENT TERMINALS RM VOLTAGE MONITORING TERMINALS Figure 80 Four Terminal Current Monitoring Resistor Keep Temperature of Ry Constant Resistor Ry should be protected against stray air currents open doors or windows air conditioning vents since these will change the resistance value degrading the stability and temperature coefficient measurements Check Voltage Control Setting When measuring constant current performance specifications the power supply s voltage control must be set above the maximum output voltage that the supply will deliver since voltage limiting action will cause a drop in output current increased ripple current and other performance changes not properly ascribed to the constant current operation of the supply Do not Connect DC Voltmeter Directly Across Power Supply Output Terminals Note that in Figure 79 the DC voltmeter used to monitor the output of the power supply is connect
14. 27 Included but not shown in the modulator chip are additional circuits that establish a minimum dead time off time for the switching transistors This ensures that both switching transistors cannot conduct simultaneously during maximum duty cycle conditions INPUT REGULATOR Que SZ RA mae ion NA 1 i I 1 h E t DRIVE ei PASE NOT MODULATOR CHIP M E E OUTPUT RECUFILTER Se ae ee A SWITCH OUTPUTS XENA PRIMARY Figure 10 Switching Regulated Constant Voltage Supply Ac Inrush Current Protection Because the input filter capacitors are connected directly across the rectified line some form of surge protection must be provided to limit line inrush currents at turn on If not controlled large inrush surges could trip circuit breakers weld switch contacts or affect the operation of other equipment connected to the same ac line Protection is provided by a pair of thermistors in the input rectifier circuit With their high negative temperature coefficient of resistance the thermistors present a relatively high resistance when cold during the turn on period and a very low resistance after they heat up A shorting strap J1 permits the configuration of the input rectifier filter to be altered for different ac inputs For a 174 250Vac input the strap is removed and the circuit functions as a conventional full wave brid
15. CL POWER SUPPLY A supply similar to a CV CC supply except for less precise regulation at low values of load resistance 1 in the current limiting region of operation One form of current limiting is shown above CONSTANT VOLTAGE CURRENT LIMITING CV CL OUTPUT CHARACTERISTIC CONSTANT VOLTAGE OUTPUT Es CURRENT LIMITING P4 R gt EA R Ren 7 quM d LS Rory 10 CROWBAR CIRCUIT An overvoltage protection circuit that monitors the output voltage of the supply and rapidly places a short circuit or crowbar across the output terminals if a preset voltage level is exceeded CURRENT FOLDBACK Another form of current limiting often used in fixed output voltage supplies For load resistance smaller than the crossover value the current as well as the voltage decreases along a foldback locus PEUT CONSTANT VOLTAGE eS OPERAT ING REGION tRATED CROSSOVER Eour DRIFT The maximum change in power supply output during a stated period of time usually 8 hours following a warm up period with all influence and control quantities such as load ac line and ambient temperature maintained constant Drift includes periodic and random deviations PARD over a bandwidth from dc to 20Hz At frequencies above 20Hz PARD is specified separately EFFICIENCY Expressed in percent efficiency is the total output power of the supply divided by the active input power
16. DC Amplifier Regulator Output Stage Series Regulating Transistor Bias Power Supply Rectifier Filter Gain Control Output Voltage Control As a result of the specific method used in transforming an operational amplifier into a power supply some restrictions are placed on the general behavior of the power supply The most important of these are 1 The large output capacitor Co limits the bandwidth 22 2 The use of a fixed dc input voltage means that the output voltage can only be one polarity the opposite of the reference polarity 3 The series regulator can conduct current in only one direction This together with the fact that the rectifier has a given polarity means that the power supply can only deliver current to the load and cannot absorb current from the load Special design steps have been added to the design of most Agilent low voltage supplies to permit a significant reduction in the size of the output capacitor merely by changing straps on the rear terminal strip see page 89 n Agilent s line of Bipolar Power Supply Amplifiers this output capacitor is virtually eliminated by using a special feedback design In addition BPS A instruments are capable of ac output conduct current in either direction and their outputs are continuously variable through zero see page 54 AC IN Figure 6 Operational Amplifier Representation of Adjustable CV Power Supply Series Regulator with P
17. Measure Performance at Front or Rear Terminals Before attaching the load and monitoring devices shown in Figure 70 determine whether the supply is connected for front or rear terminal sensing because the load and monitoring devices must be connected to the same pair of output terminals to which the feedback amplifier within the power supply is connected In the case of small laboratory supplies that feature Automatic Error Sensing performance measurements can be made at either the front or rear output terminals but are normally accomplished at the rear terminals Connect Leads to Power Supply Terminals Properly Casual clip lead connections will inevitably result in serious measurement errors in most cases exceeding the power supply s specifications even though the power supply is operating perfectly The load and monitoring leads must be connected to the power supply terminals exactly as shown in Figures 71A and B If performance measurements are made at the front terminals Figure 71A the load should be plugged into the front of the terminal at B while the monitoring device is connected to a small lead or bus wire inserted through the hole in the neck of the binding post at A If performance is being measured at the rear barrier strip Figure 71 B the measuring instrument should be connected to the plus and minus sensing terminals in this way the monitoring device sees the same performance as the feedback amplifier within the power supply
18. SEE DRIFT TEMPERATURE COEFFICIENT For a power supply operated at constant load and constant ac input the maximum steady state change in output voltage for a constant voltage supply or output current for a constant current supply for each degree change in the ambient temperature with all other influence quantities maintained constant WARM UP TIME The time interval required by a power supply to meet all performance specifications after it is first turned on 16 PRINCIPLES OF OPERATION Electronic power supplies are defined as circuits which transform electrical input power either ac or dc into output power either ac or dc This definition thus excludes power supplies based on rotating machine principles and distinguishes power supplies from the more general category of electrical power sources which derive electrical power from other energy forms e g batteries solar cells fuel cells Electronic power supplies can be divided into four broad classifications 1 ac in ac out line regulators and frequency changers 2 dc in dc out converters and dc regulators 3 dc in ac out inverters 4 ac in dc out This last category is by far the most common of the four and is generally the one referred to when speaking of a power supply Most of this Handbook is devoted to ac in dc out power supplies although a brief description of a dc to dc converter is presented later in this section Four basic outputs or modes or opera
19. SUPPLY ACTIVE LOAD DEVICE NORMAL LOAD CURRENT SSS Ig REVERSE LOAD CURRENT IN 5A 1 Figure 64A Reverse Current Loading Problem T Thus a change in the current requirement of either load results not only in a change in its own dc voltage but also in a change of the dc voltage feeding the other load and extreme conditions of imbalance can develop In many cases a simultaneous need for positive and negative dc voltages necessitates the use of two separate regulated power supplies 94 POWER SUPPLY ACTIVE LOAD DEVICE CURRENT FLOW DURING tw Eo CURRENT FLOW DURING tg Rp mom Figure 64B Reverse Current Loading Solution REGULATED POWER SUPPLY RLI ZI RI 82 R 2 Z2 R2 R RL Rie Figure 65 Center tapped Power Supply Output 95 PARALLEL OPERATION The operation of two constant voltage power supplies in parallel is normally not feasible because of the large circulating current which results from even the smallest voltage difference which inevitably exists between the two low impedance sources However if the two power supplies feature CV CC or CV CL automatic crossover operation then parallel operation is feasible since the supply with the higher output voltage setting will deliver its constant current or current limiter output and drop its output voltage until it equals the output of the other supply which
20. Terminals Remote Sensing 62 If remote sensing is employed the DT s should be located as close as possible to the load terminals sensing leads should then be connected from the power supply sensing terminals to the DT s see Figure 36 See Figure 47 for further details on remote sensing One pair of wires should be connected directly from the power supply output terminals to the DT s and a separate pair of leads from the DT s to each load There should be no direct connection from one load to another except by way of the DC Distribution Terminals Although for clarity the diagrams show the load and sensing leads as straight lines some immunity against pick up from stray magnetic fields is obtained by twisting each pair of and load leads In addition all sensing leads should be shielded POWER SUPPLY CDT S ARE SHOWN SOLID A WITH ONE LOAD POWER SUPPLY KEEP THESE FOUR LOAD WIRES AS SHORT AS POSSIBLE USE LARGE WIRE SIZE B WITH MULTIPLE LOAD USING DT s SEPARATE FROM POWER SUPPLY AND LOAD TERMINALS Figure 36 Location of DC Distribution Terminals with Remote Sensing Load Wire Rating As an absolute minimum each load wire must be of sufficient size to carry the power supply output current which would flow if the associated load terminals were short circuited However impedance and coupling considerations usually dictate the use of load current wires larger than required simply to
21. The current limit or constant current setting Co The output capacitor in farads and AT Duration of overload condition in seconds This approximation is pessimistic since it assumes that the discharge of the output capacitor is linear at the rate of I C instead of decaying exponentially REVERSE CURRENT LOADING In some applications it is necessary for a power supply to retain its normal regulated output voltage in the presence of reverse current flow during part of the operating cycle of an active load device connected to the power supply Such situations can arise for example in pulse and digital circuitry and in bias supplies for class C amplifiers Figure 64A illustrates the nature of this problem It is assumed that the active load device normally draws a current of 5 amperes but that during part of its operating cycle it delivers a current of 3 amperes Since the series transistor cannot conduct current in the reverse direction the reverse current furnished from the load device would charge the output capacitor of the power supply causing an increase in the output voltage with loss of regulation and possible damage to the output capacitor and other components within the power supply To correct these deficiencies and permit the normal operation of a regulated power supply with loads of this type it is only necessary to add a shunt or dummy load resistor such as RD Figure 64B thus shifting the zero bias level with respect to
22. This value is obtained by multiplying the programming coefficient X ohms volts by the maximum rated output voltage of the supply The programming coefficient is included on both the data sheet and the Operating and Service Manual for each model 4 Forsupplies with programming speeds of less than 8 milliseconds a mercury wetted relay of the type used for checking transient recovery time can be employed to switch the programming resistance between zero and maximum at a 60Hz rate The relay is connected as shown on Figure 77 For supplies with slower programming speeds above 8 milliseconds a hand operated switch must be substituted in place of the mercury wetted relay across the programming resistance A dc coupled oscilloscope is connected across the output terminals to allow observation of the one shot displays Figure 78 illustrates the programming speed relationship between the remote programming control input RP 112 and the output voltage EOUT in both the up and down programming directions REMOTE CONTROL INPUT SIGNAL i TOLERANCE BAND EouT TOLERANCE UP PROGRAMMING DOWN PROGRAMMING TIME TIME Figure 78 Typical Programming Speed Waveforms The constant voltage programming speed of a power supply using a remote programming voltage is identical to the speed obtained when using a remote resistance provided that the remote voltage changes rapidly enough
23. Voltage Constant Current CV CC Power Supply Figure 18 illustrates the output characteristic of an ideal CV CC power supply With no load attached RL 0 and gour Es the front panel voltage control setting When a load resistance is applied to the output terminals of the power supply the output current increases while the output voltage remains constant point D thus represents a typical constant voltage operating point Further decreases in load resistance are accompanied by further increases in with no change in the output voltage until the output reaches Is a value equal to the front panel current control setting At this point the supply automatically changes its mode of operation and becomes a constant current source still further decreases in the value of load resistance are accompanied by a drop in output voltage with no accompanying change in the output current value Thus point B represents a typical constant current operating point Still further decreases in the load resistance result in output voltage decreases with no change in output current until finally with a short circuit across the output load terminals Iour Is and EOUT 0 By gradually changing the load resistance from a short circuit to an open circuit the operating locus of Figure 18 will be traversed in the opposite direction 35 OPEN CIRCUIT CONSTANT VOLTAGE LOAD 5 OPERATING REGION C CONSTANT NS CURRENT RL gt Rc 2
24. a linear manner When the switches are turned off the collapsing magnetic field reverses the voltage across the primary and the previously stored energy is transferred to the output filter and load The two diodes in the primary protect the transistors from inductive surges that occur at turn off Flyback techniques have long been used as a means of generating high voltages e g the high voltage power supply in television receivers and as you might expect this configuration is capable of providing higher output voltages than the other two methods Also the flyback regulator provides a greater variation of output voltage with respect to changes in duty cycle Hence the flyback configuration is the most obvious choice for high and variable output voltages while the push pull and forward configurations are more suitable for providing low and fixed output voltages SCR Regulation SCR regulation techniques permit the design of low cost compact power supplies with efficiencies of approximately 70 Their main disadvantages are a higher ripple and noise a less precise regulation and a slower transient recovery time relative to the other three regulation methods However these supplies are widely used in high power applications where a lower degree of performance can be tolerated 31 Figure 14 illustrates a typical SCR regulated supply whose output is continuously variable down to near zero volts Circuit operation is very similar to the S
25. can be predicted by dividing the constant voltage specification by the value of Ry and then adding on a percentage basis any change in the value of Ry due to temperature effects The lowest constant current output level is limited to the programming current Ip typically 5 milliamps 100 PERFORMANCE MEASUREMENTS CONSTANT VOLTAGE POWER SUPPLY MEASUREMENTS Figure 70 illustrates a setup suitable for the measurement of the six most important operating specifications of a constant voltage power supply source effect load effect PARD load effect transient recovery time drift and temperature coefficient The automatic load switch shown in Figure 70 is used to periodically interrupt the load when measuring transient recovery time Full details of a suitable load switch and the method of employing it are given later under CV Load Effect Transient Recovery Time Measurement Instrument Necessary Characteristics Suitable Model Number Oscilloscope Sensitivity and bandwidth 100u V cm Agilent 180C with 1821A time base and 400KHz for all measurements except and 1806A vertical plug in 1803A noise spike 5mV sensitivity and 20MHz plug in for spike measurement bandwidth for noise spike measurement Differential or Digital DC Resolution 1 millivolt or better at Agilent 3420B Voltmeter voltages up to 1000 volts Agilent 3455A True RMS Voltmeter Sensitivity 1mV full scale crest factor Agilent 3400A 10 1 Precautions
26. capacitor Figure 45 POWER SUPPLY Figure 45 Floating Load In some special applications however e g bridge load circuits neither conductive nor capacitive grounding of the dc load distribution system is appropriate since such grounding would also short out the desired output signal being generated by the bridge 71 DC Ground Point The CP should be connected to the GP as shown in Figures 40 through 43 unless one load is already grounded making certain there is only one conductive path between these two points This connection should be such that the total impedance from the DC Common for example be the separate ground terminal located on one of the power supplies or loads in a system or it may be a special system ground terminal bus or plane established expressly for ground connection purposes CP should be connected to the GP as shown in Figure 40 through 43 unless one load is already grounded making certain there is only one conductive path between these two points This connection should be such that the total impedance from the DC Common Point to the DC Ground Point is not large compared with the impedance from the GP to earth ground Braided leads are sometimes used to further reduce the high frequency component of this ground load impedance Sometimes the impedance between the CP and the GP is minimized by using a single terminal or bar for both In these cases care should be tak
27. circuits used today conventional current limit and current foldback Current Limit Current limiting is similar to constant current except that the current feedback loop uses fewer stages of gain Because of this regulation in the region of current limiting operation is less precise than in constant current 36 operation Thus the current limiting locus of Figure 19 slopes more than that of Figure 18 and the crossover knee is more rounded A sharp knee indicates continuous regulation through the crossover region while a rounded knee denotes loss of regulation before the crossover value is reached To avoid any possibility of performance degradation the current limit crossover point must be set somewhat higher than the maximum expected operating current when using a Constant Voltage Current Limiting CV CL Supply CIRCUIT CONSTANT VOLTAGE OPERATING REGION CURRENT LIMITING OPERATING REGION SHORT CIRCUIT LOAD Figure 19 Current Limiting Characteristic CV CL supplies employ either a fixed current limit or a continuously variable limit In either case the change in the output current of the supply from the point where current limiting action is first incurred to the current value at short circuit is customarily 3 to 5 of the current rating of the power supply By their current limiting action CV CL supplies prevent damage to components within the power supply and will also protect the load provided that
28. feedback 1 eeesceeeeneeessececeeeeeceeeeceaeeeesueecseeeeneeeeseeeenaees 19 VOUT OM sie ete queo ote ipic a i pee 9 output characteristic 9 14 Output impedance tenia ee elle eee A 15 s ries gies aaoseevasnbonas 16 15 Mecum 32 Dui Rl 22 Crossover operation see Constant voltage constant current supply Crow protection circuit de ee eee RAN CERA MEE ERBEN 11 indicator light rr e De REV ROGER EAE RIS EN ARR REIR C YT RR Ee C ee RAE Pot 41 overvoltage control 41 OVELVOMASES 12121540 41 Current Monitoring TESIStO tact 34 115 D Decouplthe eco ie ee RO aatia Eds 78 Differential oscilloscope in performance measurements 105 Digitally controlled power supplies voltage 56 CULT ONG SOULC Cis 26 ofa cad Soya ae 57 Distribution terminals dc definition nnne nnne retener nnn 58 ett eter seeds Ud eut eee uds 57 79 Drift 11 oce oats stent A 111 DUIS Cy CLS
29. is induced in the leads or picked up from the grounds the scope lead should be shorted to the scope lead at the power supply terminals If the ripple magnitude of the shorted test approaches the actual ripple measurement then the measurement results are unreliable In most cases the single ended scope method of Figure 72B will be adequate to eliminate non real components of ripple and noise so that a satisfactory measurement may be obtained However in more critical cases or in 106 measurements where both the power supply and the oscilloscope case are connected to ground e g if both are rack mounted it may be necessary to use a differential scope with floating input as shown in Figure 72C If desired two single conductor shielded cables may be substituted in place of the shielded two wire cable Because of its common mode rejection a differential oscilloscope displays only the difference in signal between its two vertical input terminals thus ignoring the effects of any common mode signal introduced because of the difference in the ac potential between the power supply and scope case Before using a differential input scope in this manner however it is imperative that its common mode rejection be verified by shorting together the two input leads at the power supply and observing the trace on the CRT If this trace is a straight line the scope is properly ignoring any common mode signal present If it is not a straight line t
30. key sections which determine its unique regulating pro perties the Programming Guard Amplifier the Main Current Regulator and the Voltage Limit Circuit The Programming Guard Amplifier is an independent variable constant voltage source whose output voltage Eg is linearly dependent upon the setting of Ro being equal to ESRo Rs The guard aspects of this circuit are discussed in detail later it is sufficient to note here that this circuit permits linear output current control while facilitating the common point connection at the inboard side of the current monitoring resistor The Programming Guard Amplifier provides the programming voltage Eg for the Main Current Regulator this dc voltage which is negative with respect to circuit common is applied to one of the inputs of the differential Current Comparison Amplifier The other input of this differential amplifier is connected to the current monitoring resistor Ry The Current Comparison Amplifier continuously compares the voltage drop across the current monitoring resistor IoRy with the programming voltage If these voltages are momentarily unequal due to a load disturbance or a change in the output current control setting this error voltage is amplified and applied to the series regulator transistors altering the current conducted through them and forcing the voltage drop IoRy to once again equal The output current is related to the programming voltage and reference voltage by
31. malfunctioning of power supplies particularly if they employ SCR or switching type regulators or pre regulators Moreover since the control action of the most common line voltage regulators is accompanied by a change in the output waveshape their advantage in providing a constant rms input to a power supply is small Often these waveshape changes are just as effective in causing output voltage changes of the power supply as the original uncorrected line voltage amplitude changes Input AC Wire Rating When connecting ac to a power supply it is necessary to use a wire size which is rated to carry at least the maximum power supply input current In addition a check should be made to determine whether a still larger wire size will be required to retain a sufficiently low impedance from the service outlet to the power supply input terminals particularly if a long cable is involved As an extreme example many power supplies would fail to function properly if the IZ drop in the input cable approached 10 of the line voltage even though the wire size had an adequate current rating and even though the voltage at the power supply terminals was not below the rated input range specified by the manufacturer As a general guideline input cables should employ wire size sufficient to insure that the IZ drop at maximum rated power supply input current will not exceed 1 of the nominal line voltage LOAD CONNECTIONS FOR ONE POWER SUPPLY The simp
32. place the rectifier voltage more than 300 volts across the series transistors Utilizing a sufficient number of high voltage series transistors to achieve output of several thousand volts would be too costly and unreliable Even the preregulator circuit of Figure 7 is not suitable for a higher voltage supply because a shorted output causes the rectifier capacitor to discharge through the series regulator and the energy stored in this capacitor is enough to destroy the power transistors in the regulator High voltage Agilent supplies utilize a circuit technique that extends the usefulness of series regulating transistors rated for 30 volts to short circuit proof power supplies rated for outputs of well over 3000 volts As shown in Figure 24 the basic technique consists of placing a well regulated low voltage power supply in series with a less well regulated high voltage supply Notice however that the amplified error signal from the voltage comparison amplifier is dependent upon the total output voltage not just the output of the low voltage power supply alone Thus the well regulated piggyback supply continuously compensates for any ripple load regulation or line regulation deficiencies of the main power source by adjusting the voltage across its series regulator to maintain the total output voltage at a constant level 43 PIGGY BACK SUPPLY FROM REFERENCE SERIES CIRCUIT REGULATOR VOLTAGE LOW
33. power supply and the current monitoring resistor Ry as constant as possible Variations of the voltage across this current monitoring resistor over the specified 8 hour interval are measured on the digital or differential voltmeter and may be recorded on a strip chart recorder Since such voltage measurements are generally being made at a rather low level it is important to check that the stability of the measurement instruments is adequate 117 POWER SUPPLY CASE OSCILLOSCOPE CASE A INCORRECT METHOD GROUND CURRENT IgPRODUCES 60Hz DROP IN NEGATIVE LEAD WHICH ADDS TO THE POWER SUPPLY RIPPLE DISPLAYED ON SCOPE POWER SUPPLY CASE OSCILLOSCOPE CASE BREAK LENGTH OF LEAD BETWEEN RM AND OUTPUT TERMINAL OF POWER SUPPLY MUST BE HELD TO ABSOLUTE MINIMUM B A CORRECT METHOD USING A SINGLE ENDED SCOPE OUTPUT FLOATED TO BREAK GROUND CURRENT LOOP TWISTED PAIR REDUCES STRAY PICKUP ON SCOPE LEADS TWISTED PAIR POWER SUPPLY CASE OSCILLOSCOPE CASE TWO WIRE LENGTH OF LEAD BETWEEN Ry AND GROUNDED OUTPUT TERMINAL OF POWER SUPPLY MUST BE HELD TO ABSOLUTE MINIMUM C A CORRECT METHOD USING A DIFFERENTIAL SCOPE WITH FLOATING INPUT GROUND CURRENT PATH IS BROKEN COMMON MODE REJECTION OF DIFFERENTIAL INPUT SCOPE IGNORES DIFFERENCE IN GROUND POTENTIAL OF POWER SUPPLY amp SCOPE SHIELDED WO WIRE FURTHER REDUCES STRAY PICKUP ON SCOPE LEAD Figure 82 Measurement of PARD for a CC Power Supply 118
34. satisfy current rating requirements Power supplies and load wires are usually thought of in terms of their schematic equivalents the battery symbol and line connections The simplistic circuit models which these symbols imply are adequate for many purposes but more exact models must be used when evaluating the regulation properties of a power supply connected to its load s 63 The battery symbol represents an ideal constant voltage source with perfect regulation and zero output impedance at all frequencies but every regulated power supply has some small output impedance at high frequencies Thus a more exact circuit model for a power supply includes an equivalent source resistance and inductance as shown in Figure 37 is the power supply output impedance at dc and is found by dividing the load regulation by the current rating for example a power supply which has a load regulation of 10mV for a full load change of 10 amps has an equivalent Rs of 1 milliohm a typical value Similarly a power supply with an output impedance of 0 2 ohms at 100KHz and 2 ohms at 1MHz has an equivalent high frequency output inductance Ls of 0 3uH again a value typical of high performance power supplies The connecting lines on a schematic represent ideal connection between two points but the physical wires used to connect any two terminals such as power supply and load are characterized by distributed resistance inductance and capacitance For de
35. supply Besides high efficiency and reduced size and weight switching supplies have still another benefit that suits the needs of the modern environment That is their ability to operate under low ac input voltage brownout con ditions and a relatively long carryover or holdup of their output if input power is lost momentarily The switching supply is superior to the linear supply in this regard because more energy can be stored in its input filter capacitance To provide the low voltage high current output required in many of today s applications the series regulated supply first steps down the input ac and energy storage must be in a filter capacitor with a low 26 voltage across it In a switching supply however the input ac is rectified directly Figure 9 and the filter capacitor is allowed to charge to a much higher voltage the peaks of the ac line Since the energy stored in a capacitor 0 5CV2 while its volume size tends to be proportional to CV storage capability is better in a switching supply Although its advantages are impressive a switching supply does have some inherent operating characteristics that could limit its effectiveness in certain applications One of these is that its transient recovery time dynamic load regulation is slower than that of a series regulated supply In a linear supply recovery time is limited only by the speeds of the semiconductors used in the series regulator and control circuitry In a swit
36. the input current to the amplifier can be considered negligibly small and all of the input current Ig flows through both resistors and Rp As a result 20 Eg Es Es Eo 1 Then multiplying both sides by RRRP we obtain ERR EsRp EsRg EoRa 2 Figure 4 yields a second equation relating the amplifier output to its gain and voltage input Eo Es A 3 which when substituted in equation 2 and solved for Es yields Eg Rp Es Rp Ra A 4 Normally the operational amplifier gain is very high commonly 10 000 or more In equation 4 If we let A gt 5 Then 5 This important result enables us to say that the two input voltages of the comparison amplifier of Figure 4 and Figure 3 are held equal by feedback action In modern well regulated power supplies the summing point voltage Es is at most a few millivolts Substituting Es 0 into equation 1 yields the standard gain expression for the operational amplifier Rp 7 6 Notice that from equation 6 and Figure 4 doubling the value of Rp doubles the output voltage To convert the operational amplifier of Figure 4 into a power supply we must first apply as its input a fixed dc input reference voltage Eg see Figure 5 21 SERIES COMPARISON REGULATOR AMPL Figure 5 Operational Amplifier with DC Input Signal A large electrolytic capacitor is then added across the output terminals of the oper
37. the 25k control permits adjustment of the duty cycle of the load current switching and reduction in jitter of the oscilloscope display The maximum load ratings listed in Figure 76 must be observed in order to preserve the mercury wetted relay contacts Switching of larger load currents can be accomplished with mercury pool relays with this technique fast rise times can still be obtained but the large inertia of mercury pool relays limits the maximum repetition rate of load switching and makes the clear display of the transient recovery characteristic on an oscilloscope more difficult LOAD SWITCH CHASSIS W E TYPE 276B OR CLARE TYPE HGPIOO2 MAXIMUM LOAD RATINGS OF SWITCH 5AMPS 500 VOLTS 250 WATTS NOT 2500 WATTS TO POWER SUPPLY CONTACT OUTPUT PROTECTION TO EOAD TERMINALS NETWORK X USE WIRE WOUND RESISTOR Figure 76 Automatic Load Switch for Measuring Transient Recovery Time 110 CV Drift Stability Definition The change in output voltage dc to 20Hz for the first eight hours following a 30 minute warm up period During the warm up and measurement interval all parameters such as load resistance ambient temperature and input line voltage are held constant Drift includes periodic and random deviations over a bandwidth from zero frequency dc to an upper limit of 20Hz The 20Hz upper limit for drift must coincide with the lower frequency limit for PARD so that all deviations un
38. the current limiting crossover point is set at a current value that the load can handle without damage All of Agilent s current limiting supplies are self restoring that is when the overload is removed or corrected the output voltage is automatically restored to the previously set value Current Foldback Current foldback Figure 20 provides better load protection than conventional current limiting because as the load resistance decreases below the crossover value both the voltage and current decrease simultaneously along the foldback locus The short circuit foldback point ranges typically between 20 and 50 of rated output current with the exact point for a specific supply depending on design considerations and circuit tolerances For series regulated supplies this reduction in short circuit output current decreases dissipation in the 37 regulating elements Thus current foldback is especially useful if the supply is operating in a remote location and a long term short circuit occurs For switching regulated supplies current foldback does not significantly reduce dissipation within the supply It does however provide superior load protection as mentioned previously All Agilent supplies that employ current foldback are self restoring and are designed to withstand worst case dissipation which occurs under partial overload conditions when the supply is operating along the upper half of the foldback locus Foldback current limiting i
39. the load current waveform so that the power supply is only required to deliver current In terms of the numerical example shown in Figure 64B it is necessary to add a resistor Rp which will draw 3 or more amperes at the operating voltage of the power supply With this resistor added the power supply output current varies between 0 and 8 amperes rather than between 3 and 5 amperes During the interval when the load device is absorbing current current flow follows the paths indicated by the solid lines of Figure 64B whereas when the load device delivers current current flow follows the path indicated by the broken line Since the power supply is operating normally under both conditions the voltage across the active load device is maintained continuously at the regulated level 93 DUAL OUTPUT USING RESISTIVE DIVIDER Often it is required to use both a positive and negative dc power source having approximately the same voltage and current capability It might seem reasonable to meet such requirements using a single regulated dc supply with a resistive voltage divider center tapped to ground Figure 65 shows however that such an arrangement results in a drastic increase in the effective dc source impedance feeding each load assuming that the power supply has a zero output impedance each load looks back into a source impedance consisting of the two arms of the voltage divider in parallel with each other and the other load resistance POWER
40. the precision power amplifier The reference output signal is either positive or negative in accordance with the polarity of the input data Bipolar Power Amplifier The accurate reference signal from the D A converter goes directly to the power amplifier To preserve the accuracy of the input signal large amounts of negative feedback are used in the amplifier circuits The amplifier can be programmed either side of or through zero without notch effects or the use of polarity switches The power amplifier has a self contained voltage limit circuit which prevents the output voltage from exceeding 110 of rating despite possible programming errors It also contains a gross current limit circuit which prevents the output current from exceeding 110 maximum of the rated output current This circuit provides backup protection for the programmable overcurrent circuits Overcurrent Protection Both the load and the DVS are protected against overcurrent conditions by a current comparator and latch circuit When activated this circuit sends a latch signal to the power amplifier which shuts off the output stages and reduces the output current to under 10 of the current rating The current latch trip point can be programmed by three external current latch program bits to one of eight values ranging from 2 to 100 of the output current rating The current latch bits from storage are first converted to a corresponding analog reference value within the c
41. the programming current is the inverse of the programming coefficient Kp Using the same example a 200 ohms volt programming coefficient corresponds to 5mA programming current and for 30 volts output and thus 30 volts across the programming resistor 150 milliwatts will be dissipated in Rp A stable programming resistor must be used since a percentage change in its resistance value will result in the same percentage change in the output voltage of the power supply being controlled To avoid short term temperature dependent shifts in the resistance value and hence the power supply output voltage the programming resistor used should have a temperature coefficient of 20 ppm C or less and a wattage rating in excess of ten times the actual dissipation Thus in the previous example the programming resistor should have a minimum power rating of 1 5 watts 81 OT POWER SUPPLY Figure 55 Remote Programming Connections The wire size of the programming leads must be adequate to withstand any programming surges consider effects of any large storage capacitors which have to be charged or discharged through the programming leads The temperature coefficient of a very long programming leads may degrade power supply temperature coefficient and drift specifications This is particularly true if the power supply is exceptionally well regulated or the programming leads are subjected to considerable ambient temperature c
42. through the dc and load wiring cannot be avoided as long as separate loads connected to the same power supply or dc system have separate ground returns Figure 44 One solution is to break the circuit connection to ground in all of the loads and then select the DC Common Point following alternative b on page 68 or break the circuit connection to ground in all but one of the loads and treat as in c In other cases the only satisfactory solution is to increase the number of power supplies operating each grounded load from its own separate supply and treating each combination of power supply and load as in c However in this case any conductive path remaining between the loads may degrade load performance and any conductive path between power supplies except via their respective load grounds will probably degrade both power supply and load performance 70 POWER SUPPLY GROUND VOLTAGE SOURCE Figure 44 Ground Connections for Multiple Loads Two or More Grounded e Load System Floated as a DC Potential Above Ground In some applications it is necessary to operate the power supply output at a fixed voltage above or below ground potential In these cases it is usually advantageous to designate DC Common Point using whichever of the preceding four alternatives is ap propriate just as though conductive grounding would be employed Then this DC Common Point should be shorted to the DC Ground Point through a
43. to the negative output terminal The front panel control or remote programming control is used to determine the voltage E across the current monitoring resistor Ry Since this voltage E will be held equal to the voltage Ep across the control resistance by feedback action a constant current E RM will be caused to flow through the current monitoring resistor Ry The load current I consists of the current flowing through the monitoring resistor plus the programming current Ip normally negligibly small compared to Both the current through the monitoring resistor and the programming current are held constant by regulator action thus the net load current is also constant CONSTANT VOLTAGE POWER SUPPLY SERIES REGULATOR COMPARISON AMPL REMOVE THIS RESISTOR Figure 69 Converting a CV Supply to CC Output Since any change in the value of the resistance Ry will result in a change in the load current the current monitoring resistor should have a low temperature coefficient and should be operated at less than 1 10 or even 1 100 of its power rating This plus the restriction that the total IR drop across Ry and R in series cannot exceed the voltage rating of the power supply means that Ry will be selected so that its IR drop will be of the order of 1 volt depending upon the constant current value required Generally speaking the constant current performance of a supply connected in the method shown in Figure 69
44. utilizing the oscilloscope to insure that the input waveform to the rms voltmeter has 2f fj ac input line frequency fundamental component and is free of extraneous signals not coming from the power supply output 116 Most of the comments pertaining to the ground loop and pickup problems associated with constant voltage ripple and noise measurement also apply to the measurement of constant current ripple and noise Figure 82 illustrates the most important precautions to be observed when measuring the ripple and noise of a constant current supply The presence of a 2fj waveform on the oscilloscope is normally indicative of a correct measurement method A waveshape having a fundamental component at fj is typically associated with an incorrect measurement setup As before the basic measuring instrument is an oscilloscope The measurement of CC noise spikes is similar to CV noise spikes as discussed previously except that an appropriate load resistor RL and current monitoring resistor RM must be included as illustrated in Figure 82C The peak to peak rms conversion factors suggested by Figure 73 and comments in the previous sections of this Handbook dealing with constant voltage pickup and ground loop effects as well as the section dealing with the measurement of constant voltage ripple and noise apply in full to constant current ripple and noise measurements CC Load Effect Transient Recovery Time Definition The time X for output curr
45. voltage across this resistor the current performance is calculated by dividing these voltage changes by the ohmic value of RM CONSTANT CURRENT POWER SUPPLY UNDER TEST AUTOMATIC LOAD SWITCH CURRENT AC MONITORING VARIABLE AUTO TRANSFORMER TRUE RMS AC VOLTMETER DIFFERENTIAL OR DIGITAL DC VOLTMETER Figure 79 Constant Current Measurement Setup Many of the precautions listed for the previous constant voltage measurement setup are equally applicable to a constant current setup In addition other precautions peculiar to a constant current measurement setup are listed on the following pages Precautions Ry Must be Treated as a Four Terminal Device In the manner of a meter shunt the load current must be fed from the extremes of the wire leading to this resistor while the voltage monitoring terminals connected to the three measuring instruments should be located as close as possible to the resistance portion itself as shown in Figure 80 Use Precision Low T C Monitoring Resistor Resistor Ry should be a precision ammeter shunt or a wire wound resistor 20ppm C or better and should be operated at a power less than 1 10 preferably 1 100 of its rating so that its surface temperature will not be high compared with ambient and therefore not subject to slow thermal fluctuations that cause similar changes in the resistance value With typical wire wound power resistors operation at
46. 2 DC POWER SUPPLY ee HANDBOOK 1 e g Application Note 90B Agilent Technologies TABLE OF CONTENTS INTRODUCTION Regulated power supplies employ engineering techniques drawn from the latest advances in many disciplines such as low level high power and wideband amplification techniques operational amplifier and feedback principles pulse circuit techniques and the constantly expanding frontiers of solid state component development The full benefits of the engineering that has gone into the modern regulated power supply cannot be realized unless the user first recognizes the inherent versatility and high performance capabilities and second understands how to apply these features This handbook is designed to aid that understanding by providing complete information on the operation performance and connection of regulated power supplies The handbook is divided into six main sections Definitions Principles of Operation AC and Load Connections Remote Programming Output Voltage and Current Ratings and Performance Measurements Each section contains answers to many of the questions commonly asked by users like What is meant by auto parallel operation What are the advantages and disadvantages of switching regulated supplies When should remote sensing at the load be used How can ground loops in multiple loads be avoided What factors affec
47. 58 Rpcan be selected so that the resulting voltage gain is either less or greater than unity It is possible to use the front panel control on the supply as the voltage gain control RP EXTERNAL VOLTAGE SOURCE CHOOSE lt P gt Ip NORMAL R SERIES REGULATOR EouT COMPARISON AMPL PROG CONTROL Z EXTERNAL OR INTERNAL Rp EQUI Figure 58 Voltage Programming with Variable Voltage Gain When programming the output using a remote voltage source the use of a zener diode across the programming terminals will prevent the power supply output from exceeding a predetermined limit even though the programming source may provide an excessively high input command The relationship between the zener diode and the output limit value depends upon the power supply design and the programming connection but in any case can be determined by considering the power supply as equivalent to an operational amplifier The zener diode must have a current rating equal to or greater than the largest current which the remote programming source can provide in some cases the power rating of the zener diode can be reduced by employing a fixed resistance in series with the programming path 85 In situations where only low programming voltages are being used forward conducting silicon diodes 0 7V per junction can be used in place of zener diodes CONSTANT CURRENT REMOTE PROGRAMMING Most of the general princ
48. A IN Saar gc vett este vase ccs nai Eig 91 Dual output with resistive divider see eati 94 121 E Efficiency definitionis siet i RR EUIS AR SN GR RU 11 or preregulated Supplies N 23 of SCR regulated supplies ae vi gue 27 regulated 22 of switching regulated supplies cei ne a rese ra te eR Eae eR Pee Sa ORTU 26 Electronic power supply definition de 17 sce tini tion rette estes cut oeste ceteri coercere air med 11 protection NEAL Supplies 53 protection switching supplies meiner iai es 26 43 Equivalent circuit power supply with load 64 89 Error SENSING normal 72 cesis ton DU mesi tUe cad Dude rus ec i tad Sage LOL am cme 74 Extended range power SUPPLY sacetncdisudeaeiansaecadisgevadeaspavcaealbesecteaveraceensoaceatenduendeaspaveaveseooentens 48 F Feedback in constant current sUpplys ioeo puse Uto Ier p cedri eee 33 in constant voltage Supplies ein titer e Pan ek eA Arv YE eds 18 26 in high performance constant current supply seeeeseeeeeeeeneeeenee enne 47 AT operational amplifier s cesses ires Rida quis 22 Floating operato ce eee at ta e ea stes 71 G ral dte BUT ALOT eeclesiae mde 64 Ground leads
49. ARE SHOWN SOLID peed 1 EQUI VALENT A ONE LOAD CIRCUIT POWER SUPPLY B MULTIPLE LOADS Figure 38 Local Decoupling Capacitors The ideal concept of a single quiet ground potential is a snare and a delusion No two ground points have exactly the same potential The potential differences in many cases are small but even a difference of a fraction of a volt in two ground potentials will cause amperes of current to flow through a complete ground loop any circuit with more than one ground point To avoid ground loop problems there must be only one ground return point in a power supply system the power supply and all its loads and all other power supplies connected to the same loads However the selection of the best DC Ground Point is dependent upon the nature and complexity of the load and the dc wiring and there are practical problems in large systems which tend to force compromises with the ideal grounding concept For example a rack mounted system consisting of separately mounted power supplies and loads generally has multiple ground connections each instrument usually has its own chassis tied to the third Safety Ground lead of its power cord and the rack is often connected by a separate wire to Safety Ground the cold water pipe With the instrument panels fastened to the rack frame circulating ground currents are inevitable However as long as these ground currents are confined to the Gr
50. AUTO SERIES POWER SUPPLY SYSTEM EMT EI Eo 3Ey AUTOMATIC AUTO TRACKING OPERATION A master slave connection of two or more power supplies each of which has one of its output terminals in common with one of the output terminals of all of the other power supplies Auto Tracking operation is characterized by one knob control proportional output voltage from all supplies and no internal wiring changes Useful where simultaneous turn up turn down or proportional control of all power supplies in a system is required AUTO TRACKING POWER SUPPLY SYSTEM O O SLAVE NO I SLAVE NO 2 2 5 0 CARRYOVER TIME The period of time that a power supply s output will remain within specifications after loss of ac input power It is sometimes called holding time COMPLEMENTARY TRACKING A master slave interconnection similar to Auto Tracking except that only two supplies are used and the output voltage of the slave is always of opposite polarity with respect to the master The amplitude of the slaves output voltage is equal to or proportional to that of the master A pair of complementary tracking supplies is often housed in a single unit COMPLIANCE VOLTAGE The output voltage rating of a power supply operating in the constant current mode analogous to the output current rating of a supply operating in the constant voltage mode CONSTANT CURRENT POWER SUPPLY A regulated power supply that acts to maintain its out
51. COMPARISON 56 VOLTAGE R INPUT XFM RECTIFIER DC OUTPUT MAIN VOLTAGE SOURCE HIGH VOLTAGE CONTROL CIRCUIT HIGH VOLTAGE XFMR amp RECTIFIER HIGH VOLTAGE CONTROL SIGNAL Figure 24 Piggy back Power Supply As an illustrative example assume that the low voltage rectifier supplying the series transistor of the piggy back supply develops approximately 40 volts and that the main voltage source is capable of providing a maximum of 300 volts With 20 volts normally dropped across the series regulator the maximum output of this supply would be 320 volts 20 volts from the piggy back supply and 300 volts from the main source Thus the series regulator of the piggy back supply would have a 20 volt range available for accomplishing the dynamic changes necessary to compensate for the output voltage variations of the main source Short circuit protection for the series regulator in the piggy back supply Figure 24 is provided by diode CR which if the output terminals are shorted provides a discharge path for rectifier capacitor Cy Since CRp prevents the output of the piggy back supply from ever reversing polarity the series regulator will never be called upon to withstand a voltage greater than the 40 volts from its own rectifier Fuse F1 is included so that the path between the output terminals and the rectifying elements of the main voltage source will be opened under overload conditio
52. CR preregulators described previously except that the SCR control circuit receives its input from the voltage comparison amplifier The control circuit computes the firing time for the SCRs varying this in a manner which will result in a constant output despite changes in line voltage or load resistance The control circuit is capable of making a nearly complete correction within the first half cycle 8 3msec following a disturbance AUX XFMR REFERENCE REGULATOR MAIN POWER p RECTIFIERIFILTER 4 XFMR ee RR VOLTAGE COMPARISON AMPL SCR REGULATOR Fag OUT SCR CONTROL CIRCUIT Figure 14 SCR Regulated Power Supply CONSTANT CURRENT POWER SUPPLY The ideal constant current power supply exhibits an infinite output impedance zero output admittance at all frequencies Thus as Figure 15 indicates the ideal constant current power supply would accommodate a load resistance change by altering its output voltage by just the amount necessary to maintain its output current at a constant value Constant current power supplies find many applications in semiconductor testing and circuit design and are also well suited for supplying fixed currents to focus coils or other magnetic circuits where the current must remain constant despite temperature induced changes in the load resistance Just as loads for constant voltage power supplies are always connected in parallel never in s
53. DO CO X EIER SS SER USER CY YN RR Eee ESO dS 39 42 Complementary tracking depo bed lees 8 Compliance volidg6 4 oie a epo Bees 8 Conduction time SCR see Preregulator Connections checklist for power 1 59 Constant current supply applications ete 32 Dasic feedback ta on bestie a etui id 33 120 constructed from constant voltage supply nnne 99 dre oed efr iita eod uq 8 ced 33 33 OUtput Hapedange 32 perfotniance measurements ties 113 Constant voltage constant current supply crossover load 1 400 35 GIOSSOVOL aes 34 Soo ps pes ieee aera stele o a s bae 9 output CH ARACLETISUC seo ate te lacte toL E adr 10 36 overload protectio PE 36 Constant voltage current limiting supply current limiting action 17 DU 10 o tput characteristics Cio cus talo 10 36 relation to constant voltage constant current supply eene 32 c rrent TOLD DAC qe T t 34 Constant voltage supply basic
54. ENCE VOLTAGE CONSTANT CURRENT COMPARISON SERIES REGULATOR CURRENT MONITORING RESISTOR Figure 16 Constant Current Power Supply CONSTANT VOLTAGE CONSTANT CURRENT CV CC POWER SUPPLY Because of its convenience versatility and inherent protection features many Agilent supplies employ the CV CC circuit technique shown in Figure 17 Notice that only low power level circuitry has been added to a constant voltage supply to make it serve as a dual purpose source Two comparison amplifiers are included in a CV CC supply for controlling output voltage and current The constant voltage amplifier approaches zero output impedance by varying the output current whenever the load resistance changes while the constant current amplifier approaches infinite output impedance by varying the output voltage in response to any load resistance change It is obvious that the two comparison amplifiers cannot operate simultaneously For any given value of load resistance the power supply must act either as a constant voltage or a constant current supply it cannot be both Transfer between these two modes is accomplished automatically by suitable decoupling circuitry at a value of load resistance equal to the ratio of the output voltage control setting to the output current control setting 34 CURRENT COMPARISON SERIES REGULATOR VOLTAGE COMPARISON AMPL Figure 17 Constant
55. NECTION VIA 3RD WIRE SAFETY GROUND LEAD AND RACK FRAME S G SAFETY GROUND LEAD IN POWER CORD GND POWER SUPPLY GROUND TERMINAL POWER SUPPLY OUTPUT CAPACITORS REMOVED AND PLACED ACROSS DT S 1 2 5 LOAD DECOUPLING CAPACITORS RI R2 R3 R4 REMOTE SENSING PROTECTION RESISTORS Figure 53 Load Connections for Multiple Power Supplies DC Distribution Terminals There must be only one point of connection between the dc outputs of any two power supplies in the multiple 78 power supply system this point must be designated as one of the two DT s for both power supplies Thus there are exactly N 1 DT s in any system where N is the number of power supplies excluding the possibility of parallel supplies sharing the same distribution terminals or series power supplies with unused intermediate terminals This rule eliminates the possibility of circulating dc currents while insuring the optimum connection of load and sensing leads and lays the groundwork for avoiding ground loops DC Common One of the 1 DT s determined in accordance with the preceding paragraph is designated as the CP for the system There can be only one CP per system it is the DT which is to be held at Ground potential For other constraints affecting the choice of the CP see DC Common starting on page 68 DC Ground Point There must be only one GP per multiple power supply system This rules out the possibili
56. R TERMINATION Figure 74 Measurement of Noise Spikes The circuit of Figure 74 can also be used for the normal measurement of low frequency ripple and noise by simply removing the four terminating resistors and the blocking capacitors and substituting a higher gain vertical plug in in place of the wide band plug in required for spike measurements Notice that with these changes Figure 74 becomes a two cable version of Figure 72C 108 CV Load Effect Transient Recovery Time Load Transient Recovery Definition The time X for the output voltage to recover and to stay within Y millivolts of the nominal output voltage following a 7 amp step change in load current where Y is specified separately for each model but is generally of the same order as the load regulation specification The nominal output voltage is defined as the dc level half way between the steady state output voltage before and after the imposed load change Z is the specified load current change typically equal to the full load current rating of the supply Load effect transient recovery time may be measured at any input line voltage combined with any output voltage and load current within rating If a step change in load current is imposed on the output of a power supply the output voltage will exhibit a transient of the type shown in Figure 75 The output impedance of any power supply rises at high frequencies giving rise to an equ
57. The triac control circuit also monitors the unregulated dc to provide ac line compensation Variations in the amplitude or frequency of the ac line modify the amplitude of the unregulated dc voltage which in turn alter the position of the IOD1 and IOD2 decision lines For example both IOD lines decrease move to the left if the ac line voltage increases The reverse occurs if the ac line voltage decreases This action prevents excessive regulator dissipation during periods of high line yet assures an adequate voltage drop across the series regulator during periods of low line Noise Reduction Triacs as well as any other switching devices can generate a considerable amount of EMI Because this interference is proportional to the turn on dv dt the triacs in this supply are fired only when the voltage across them is low thus minimizing EMI The current through the triacs however must be higher than the minimum latching current that is required to sustain conduction Thus triac firing must occur late enough in the ac input cycle for the current to be above the latching level yet early enough in the cycle so that the dv dt will not generate excessive EMI Faster Down Programming Speed As mentioned previously the programming speed of this unit is considerably better than that of the preregulated power supplies that exist at this power level Up programming is not a particular problem in either type of unit because it is aided by the conduction of t
58. Y B WITH REMOTE SENSING Figure 40 Preferred Ground Connections for Single Isolated Load b Multiple Ungrounded Loads Select the positive or negative DC Distribution Terminal as the DC Common Point This alternative is applicable when there are two or more separate loads with separate pairs of load leads and none of the load circuits has internal connections to chassis or ground Figure 41 68 POWER SUPPLY A WITHOUT REMOTE SENSING B WITH REMOTE SENSING Figure 41 Preferred Ground Connections for Multiple Loads All Isolated POWER SUPPLY B WITH REMOTE SENSING Figure 42 Preferred Ground Connections for Single Grounded Loads c Single Grounded Load The load terminals of the grounded load must be designated as the DT s and the grounded terminal of the load is necessarily the CP Figure 42 This method of CP selection is followed when there is only one load and it has an essential internal 69 connection to ground or chassis or when there are multiple loads and only one has an internal connection to ground or chassis Figure 43 POWER SUPPLY A WITHOUT REMOTE SENSING POWER SUPPLY B WITH REMOTE SENSING Figure 43 Ground Connections for Multiple Loads One Grounded d Multiple Loads Two or More of Which are Individually Grounded This is an undesirable situation and must be eliminated if at all possible Ground loop currents circulating
59. ads Use an Adequate Load Resistor In general the load resistance and wattage selected should permit operation of the supply at its maximum rated output voltage and current When measuring the transient recovery time of power supplies requiring low resistance loads it may be necessary to use non inductive loads so that the L R time constant of the load will not be greater than the inherent recovery time of the power supply thus impeding the measured transient recovery performance 102 OUTPUT TERMINAL MONITOR HERE A FRONT PANEL MEASURING INSTRUMENT SCOPE TRUE RMS VTVM DIFFERENTIAL OR DIGITAL VOLTMETER BARRIER STRIP MONITORING LEADS B REAR PANEL Figure 71 Proper Connections for Monitoring and Load Leads Check Current Limit Control Setting When measuring the constant voltage performance specifications the constant current or current limit control must be set well above the maximum output current that the supply will draw The onset of constant current or current limiting action can cause a drop in output voltage increased ripple and other performance changes not properly ascribed to the constant voltage operation of the supply Check Setup for Pickup and Ground Loop Effects Avoid degradation of the measured performance caused by pickup on the measuring leads or by power line frequency components introduced by ground loop paths Two quick checks will determine if the measure
60. al This problem is eliminated by enclosing the primary winding in an electrostatic shield which is connected to earth ground A second source of ripple current is capacitive coupling between the secondary winding and ground To keep this current from affecting the output the secondary winding is enclosed in an electrostatic shield which is connected to the negative output terminal This causes the ripple current generated by the secondary winding to be confined to a closed loop inside the instrument Extended Range Power Supplies This type of supply automatically extends the number of output voltage and current combinations that are available within the maximum power rating of a single unit Before discussing circuit techniques the following paragraphs first outline the advantages of the extended range concept itself Advantages of Extended Range Figure 28 compares the output characteristic of a conventional CV CC supply with that of an extended range supply of the same output power 200W in the example As indicated in 28 A maximum output power from a conventional supply can be obtained at only one point 20V 10A on its rectangularly shaped locus Maximum output power for an extended range supply however is available not only at 20V 10A but at 50V 4A and anywhere along the curved portion of the characteristic ranging between 20 and 50V To duplicate this capability would require either one large 500W power supply operating inefficiently at l
61. ational amplifier The impedance of this capacitor in the middle range of frequencies where the overall gain of the amplifier falls off and becomes less than unity is much lower than the impedance of any load that might normally be connected to the amplifier output Thus the phase shift through the output terminals is independent of the phase angle of the load applied and depends only on the impedance of the output capacitor at medium and high frequencies Hence amplifier feedback stability is assured and no oscillation will occur regardless of the type of load imposed In addition the output stage inside the amplifier block in Figure 4 is removed and shown separately After these changes have been carried out the modified operational amplifier of Figure 5 results Replacing the batteries of Figure 5 with rectifiers and a reference zener diode results in the circuit of Figure 6 A point by point comparison of Figures 3 and 6 reveals that they have identical topology all connections are the same only the position of the components on the diagram differs Thus a series regulated power supply is an operational amplifier The input signal to this operational amplifier is the reference voltage The output signal is regulated dc The following chart summarizes the corresponding terms used for an operational amplifier and a power supply Operational Amplifier Constant Voltage Power Supply Input Signal Reference Voltage Output Signal Regulated
62. ative advantage applies under all possible combinations of load and line Protecting Against Open Sensing Leads The possibility of an open remote sensing path which might occur on a long term or transient basis should be avoided Such open circuit conditions are likely if the remote sensing path includes any relay switch or connector contacts any interruption of the connections between the power supply sensing terminals and the DC Distribution Terminals should be avoided wherever possible When a Sensing open occurs the regulator circuit within the supply reacts as though the load voltage were zero usually the output voltage corrects this deficiency by climbing rapidly toward the maximum rectifier voltage a value which is significantly larger than the power supply s maximum rated output voltage Even if the power supply output circuitry is designed to withstand this extreme the chances are that the load is not 75 To reduce the degree of output overshoot which can result from accidentally opened remote sensing connections many regulated power supplies include internally wired resistors or small silicon diodes as shown in Figures 50 and 51 If they are not part of the power supply and if the power supply application involves long sensing leads sensing paths which include relay switch or connector contacts or any other likely cause of even momentary open circuits in the remote sensing paths then the user should add either resist
63. basic feedback circuit principle used in Agilent series regulated power supplies The ac input after passing through a power transformer is rectified and filtered By feedback action the series regulator alters its voltage drop to keep the regulated dc output voltage constant despite variations in the ac line the load or the ambient temperature The comparison amplifier continuously monitors the difference between the voltage across the voltage control resistor RP and the output voltage If these voltages are not equal the comparison amplifier produces an amplified difference error signal This signal is of the magnitude and polarity necessary to change the conduction of the series regulator which in turn changes the current through the load resistor until the output voltage equals the voltage EP across the voltage control SERIES REGULATOR COMPARISON AMPL Figure 3 Series Regulated Constant Voltage Power Supply Since the net difference between the two voltage inputs to the comparison amplifier is kept at zero by feedback action the voltage across resistor is also held equal to the reference voltage Thus the programming current Ip flowing through Re is constant and equal to The input impedance of the comparison amplifier is very high so essentially all of the current Ip flowing through also flows through Rp Because this programming current Ip is constant Ep and hence the outpu
64. by N1 N3 Finally if both triacs are fired simultaneously Cl charges to its highest voltage level N1 N2 N3 oc AC LINE Figure 29 Extended Range Power Supply Using Tap Switching The triac control circuits determine which triac is to be fired by monitoring the output voltage and current IR drop across current monitoring resistor RM and comparing these values against a set of three internally derived reference levels These reference levels are translated into boundary lines to allow the output characteristic to be mapped into four operating regions Figure 30 The boundary lines which are invisible to the user are named VOD for output voltage decision line and IOD1 and IOD2 for output current decision lines one and two Whenever the output voltage is below the sloping VOD line the control circuit inhibits both triacs and the input capacitor charges to a voltage determined by N1 Whenever the output voltage is greater than the VOD line the control circuit looks at the output current level to determine which triac should be fired Figure 30 indicates the windings that are connected as a result of the current decisions 52 NOM LINE NOM LINE SOV 40V 1 2 N3 30V CR1 CR2 ON NI N2 CR ON CR2 OFF 20V 1 2 OFF CR2 ON 10V N1 CR1 CR2 OFF 0 2 4 6A 8A 10A Figure 30 Output Power Plot
65. ching supply however recovery is limited mainly by the inductance in the output filter This may or may not be of significance to the user depending upon the specific application Also electro magntic interference is a natural byproduct of the on off switching within these supplies This interference can be conducted to the load resulting in higher output ripple and noise it can be conducted back into the ac line and it can be radiated into the surrounding atmosphere For this reason all Agilent Technologies switching supplies have built in shields and filter networks that substantially reduce EMI and control output ripple and noise Reliability has been another area of concern with switching supplies Higher circuit complexity and the relative newness of switching regulator technology have in the past contributed to a diminished confidence in switching supply reliability Since their entry into this market Agilent has placed a strong emphasis on the reliability of their switching supplies Field failures have been minimized by such factors as careful component evaluation MTBF life tests factory burn in procedures and sound design practices Typical Switching Regulated Power Supplies Currently switching supplies are widely used by Original Equipment Manufacturers OEMs This class of switching supply provides a high degree of efficiency and compactness moderate to good regulation and ripple characteristics and a semi fixed
66. constant voltage output Figure 10 shows one of Agilent s higher power yet less complex OEM switching supplies Regulation is accomplished by a pair of push pull switching transistors operating under control of a feedback network consisting of a pulse width modulator and a voltage comparison amplifier The feedback elements control the ON periods of the switching transistors to adjust the duty cycle of the bipolar waveform E delivered to the output rectifier filter Here the waveform is rectified and averaged to provide a dc output level that is proportional to the duty cycle of the waveform Hence increasing the ON times of the switches increases the output voltage and vice versa The waveforms of Figure 10 provide a more detailed picture of circuit operation The voltage comparison amplifier continuously compares a fraction of the output voltage with a stable reference EREF to produce the VCONTROL level for the turn on comparator This device compares the VCONTROL input with a triangular ramp waveform A occurring at a fixed 40KHz rate When the ramp voltage is more positive than the control level a turn on signal B is generated Notice that an increase or decrease in the VCONTROL voltage varies the width of the output pulses at B and thus the ON time of the switches Steering logic within the modulator chip causes switching transistors Q1 and Q2 to turn on alternately so that each switch operates at one half the ramp frequency 20KHz
67. ction Angle Control of Preregulator Output The reaction time of an Agilent preregulator control circuit is much faster than earlier SCR or magamp circuits Sudden changes in line voltage or load current result in a correction in the timing of the next SCR trigger pulse which can be no farther away than one half cycle approximately 8 milliseconds for a 60Hz input The large filter capacitance across the rectifier output allows only a small voltage change to occur during any 8 millisecond interval avoiding the risk of transient drop out and loss of regulation due to sudden changes in load or line The final burden of providing precise and rapid output voltage regulation rests with the series regulator while the preregulator handles the coarser and slower regulation demands The preregulator SCRs together with the leakage inductance of the power transformer limit high inrush currents during turn on A slow start circuit allows gradual turn on of the SCRs while the leakage inductance acts as a small filter choke in series with the SCRs Thus both the supply s input components and other ac connected instruments are protected from surge currents Switching Regulation The rising cost of electricity and continuous reductions in the size of many electronic devices have stimulated recent developments in switching regulated power supplies These supplies are smaller lighter and dissipate less power than equivalent series regulated or series preregulated
68. d its twin the guard voltage EG Since there is zero volts across the series combination of isolation diode CR2 and resistor R1 5 kilohms or less no current flows through them and potential EG is also present at their junction thus back biasing isolation diode Any small leakage through back biased diode flows through R1 and the output of the Programming Guard Amplifier but does not flow into CR2 or the positive output terminal The Shunt Voltage Regulator conducts a standby current through shunt regulator bias resistor RB this current insures that the Shunt Voltage Regulator is operating in its linear region ready to react quickly when voltage limiting action is required thus preventing crossover transients If the output voltage exceeds the preset voltage limit value CR1 and CR2 conduct and the Shunt Voltage Regulator conducts a portion of the current which otherwise would flow to the load thus clamping the output voltage to the preset limit value Even during voltage limiting action Eg continues to be maintained at a value equal to the potential at the positive output terminal both guarding action and the normal control action of the Main Current Regulator continue minimizing any output transients which might tend to occur when the output transfers from voltage limiting to its normal output current mode High Output Impedance The high output impedance of these current sources is a result of several factors both el
69. d replaces the former term ripple and noise PARD is the residual ac component that is superimposed on the dc output voltage or current of a power supply It is measured over a specified bandwidth with all influence and control quantities maintained constant PARD is specified in rms and or peak to peak values over a bandwidth of 20Hz to 20MHz Fluctuations below 20Hz are treated as drift Attempting to measure PARD with an instrument that has insufficient bandwidth may conceal high frequency spikes that could be detrimental to a load DC OUTPUT OF POWER SUPPLY AND SUPERIMPOSED PARD COMPONENT 13 PROGRAMMING SPEED The maximum time required for the output voltage or current to change from an initial value to within a tolerance band of the newly programmed value following the onset of a step change in the programming input signal Because the programming speed depends on the loading of the supply and on whether the output is being programmed to a higher or lower value programming speed is usually specified at no load and full load and in both the up and down directions PROGRAMMING SPEED WAVEFORMS REMOTE CONTROL INPUT SIGNAL OPERATION OR i louT GONSTANT CURRENT _ TOLERANCE OPERATION BAND 1 H UP PROGRAMMING DOWN PROGRAMMING TIME TIME REMOTE PROGRAMMING REMOTE CONTROL Control of the regulated output voltage or current of a power supply by means of a remotely varied resistance or vo
70. deliver its full value of regulated output voltage during the peak load interval Examples of the first category are dc motors and filaments for large vacuum tubes While the starting resistance of these loads is very low compared to the normal operating value it is not necessary that the power supply be able to deliver this peak current it is necessary that the supply withstand without damage this initial peak load condition and that it continue to operate through the peak load interval until normal load conditions are established For such loads Constant Voltage Constant Current or Constant Voltage Current Limiting supplies rated for the normal not the peak load condition are adequate and in some cases preferable since the limited output current can provide protection for the load device during the peak load interval Peak load demands in excess of the current rating of the power supply will not result in damage to the power supply the output voltage will merely drop to a slightly lower value Normal output voltage will be restored automatically by the power supply after the peak or transient load condition has passed As for the second category if it is desired to meet a duty cycle requirement similar to that illustrated in Figure 63 while retaining the full value of regulated output voltage during peak load conditions then a power supply must be selected which has a current rating equal to or greater than the peak load requirement However if t
71. der constant operating conditions are covered by specifying one or the other This measurement is made by monitoring the output of the power supply on a differential voltmeter or digital voltmeter over the stated measurement interval a strip chart recorder can be used to provide a permanent record A thermometer should be placed near the supply to verify that the ambient temperature remains constant during the period of measurement The supply should be put in a location immune from stray air currents open doors or windows air conditioning vents if possible the supply should be placed in an oven which is held at a constant temperature Care must be taken that the measuring instrument has a stability over the eight hour interval which is at least an order of magnitude better than the stability specification of the power supply being measured Typically a supply may drift less over the eight hour measurement interval than during the 112 hour warm up period Drift measurements are frequently made while the supply is remotely programmed with a fixed wire wound resistor thus avoiding accidental changes in the front panel setting due to mechanical vibration or knob twiddling CV Temperature Coefficient Definition The change in output voltage per degree Celsius change in the ambient temperature following a 30 minute warm up During the measurement interval the ac line voltage load resistance and output voltage setting are held constant The te
72. e Minimizes spikes at output of supply by slowing down turn on of triac C Rectifier Damping Network RC network protects other elements in supply against short duration input line transients D Series Regulator Diode Protects the series regulator against reverse voltages which could be delivered by an active load or parallel power supply E Slow Start Circuit A long time constant network that reduces turn on overshoot and helps limit inrush current When supply is first turned on this circuit holds off both the series regulator to reduce output overshoot and the preregulator triac to limit inrush current F Amplifier Input Clamp Diodes Limit the maximum input to the amplifier to protect it against excessive voltage excursions G Output Diode Protects components in the power supply against reverse voltages that might be generated by an active load or series connected power supply H Sensing Protection Resistors Protect the load from receiving full rectifier voltage if remote sensing leads are accidentally open circuited VOLTAGE COMP AMPL Figure 21 Protection Circuits Linear Type Supply Overcurrent and Overvoltage Protection All Agilent supplies are short circuit proof and can operate under any current overload condition indefinitely without risk of internal damage Overvoltage protection is also available if required during constant current operation The CV CC and CV CL automatic crossover circuitr
73. e protection circuits perform functions that are similar to those of the linear supply of Figure 21 However their circuit placement or the manner in which they affect the operation of the supply is often different Several protection circuits such as the ac undervoltage detector are required only in switching supplies The following is a brief description of the protection circuits shown in Figure 23 A 42 RFI Filter Helps prevent RFI spikes from being conducted to the load or back into the ac line Agilent switching supplies also contain built in shields for additional control of conducted and radiated interference Thermistor Limits ac inrush current by its negative temperature coefficient of resistance Has a high resistance when cold during turn on and low resistance after it heats up Regulator Overcurrent Limit This circuit is much faster than the current limit comparator and protects the regulator switches from overcurrent conditions of a transient nature It monitors current flow through the switches and prevents it from exceeding a harmful level Output Rectifier Diodes Besides final rectification these diodes also protect internal components against reverse currents that could be injected into supply by an active load or series connected supply AC Undervoltage Performs a dual function Protects supply from damage that could result from a prolonged condition of low ac input voltage and limits output overshoot during tu
74. ect is the change AEour in the steady state value of dc output voltage due to a change in load resistance from open circuit to a value that yields maximum rated output current or vice versa Load effect is measured by closing or opening the switch in Figure 70 and noting the resulting static change AEour in the output voltage on the digital voltmeter or differential voltmeter connected to the output terminals 104 The power supply will perform within its load effect specification at any rated output voltage combined with any rated input line voltage CV PARD Ripple and Noise Definition The term PARD replaces the former term ripple and noise PARD is the Periodic and Random Deviation of the dc output voltage from its average value over a specified bandwidth and with all other parameters maintained constant The PARD measurement of an Agilent constant voltage power supply can be made at any input ac line voltage combined with any dc output voltage and load current within rating PARD is measured in rms and or peak to peak values over a 20Hz to 20MHz bandwidth Fluctuations below the lower frequency limit are treated as drift The peak to peak measurement is particularly important for applications where noise spikes could be detrimental to a sensitive load such as logic circuitry The rms measurement is not an ideal representation of the noise since fairly high output noise spikes of short duration could be present in the ripple and not a
75. ectrical and mechanical The series regulator transistors are in a cascade configuration which inherently has a high output impedance Since the open loop gain of the error amplifier is high the closed loop output impedance is greatly increased by feedback Output capacitors have been eliminated and although the output impedance falls off with frequency because of the necessary gain and phase compensation in the amplifier circuits it is much higher than it would be if a capacitor were connected across the output terminals The importance of low output capacitance should not be underestimated Excessive output capacitance would cause the output impedance of the current source to fall off with increasing frequency producing undesirable transients in rapidly changing loads Large capacitors store large amounts of energy which if discharged sud denly through the load may cause damage negative resistance devices are particularly susceptible to this kind of damage Finally an output capacitor would slow down the response of the current source to changes in the external programming signal In the interest of keeping the output impedance high the impedances of internal leakage paths have been made as high as possible by careful mechanical design and hygienic construction techniques Leakage both internal and external is further reduced by guarding the positive output terminal Guarding In addition to eliminating leakage currents the guard can a
76. ed by the source I is load current Z is load impedance and 7 is the total impedance shunting then Io Zs ZA Zs L When the output impedance of the current source is high then even very small leakage currents can become significant see Figure 26 Such things as the input impedance of a voltmeter measuring the load voltage the insulation resistances of wiring and terminal blocks and the surface leakage currents between conductors on printed circuit boards will all take current away from the load unless special design precautions are used Agilent Current Sources In Agilent Technologies Current Sources leakage at the output terminals is negligible owing to a combination of techniques including guarding shielding and physical isolation Feedback regulation makes the output impedance high 3 3 to 10 000 megohms and there is no output capacitor to lower the output impedance or store energy Low leakage and high output impedance result in precise current regulation 46 IL loZs 21 Zg _ CURRENT SOURCE Zj OUTPUT INTERNAL IMPEDANCE OF CURRENT SOURCE 2 o FOR IDEAL CURRENT SOURCE Z INSULATION IMPEDANCE ANYWHERE INSIDE OUTPUT TERMINAL OF CURRENT SOURCE Zp CIRCUIT BOARD IMPEDANCE Z LOAD IMPEDANCE 2 METER IMPEDANCE Zy TEST LEAD INSULATION IMPEDANCE Figure 26 Impedances Shunting the Load Degrade Current Regulation As shown in Figure 27 the CCB design includes three
77. ed for the application As suggested previously capacitor Co is commonly included in order to suppress load transients and reduce the 77 power supply impedance at the load at high frequencies However the capacitor must be chosen with care if power supply oscillation is to be avoided since any capacitor resonances or other tendency toward high impedance within or near the bandpass of the power supply regulator will reduce loop stability It is therefore common in extreme remote sensing applications to remove Co from the power supply and use it as Co Proper Current Limit Operation Check for proper current limiting operation while the power supply is connected in the system for remote sensing With some power supply designs the resistance of one of the current carrying leads adds to the resistance used for current limit monitoring thereby reducing the threshold value at which current limiting begins The current limit value should not change significantly while shorting S to OUT and S to OUT at the power supply If it does refer to the instruction manual for corrective adjustments or contact the manufacturer LOAD CONNECTIONS FOR TWO OR MORE POWER SUPPLIES The extension of the preceding single power supply concepts to multi power supply systems Figure 53 is simple and direct requiring only the application of the following additional rules POWER SUPPLY A d 4 O 5 6 O POWER SUPPLY CHASSIS GROUND CON
78. ed outside the current monitoring resistor Thus the true output voltage of the supply is obtained by adding this voltmeter reading to the voltage across the current monitoring resistor If the voltmeter were placed on the left side of the current monitoring resistor a change in output voltage of the constant current supply would result in a change in current through the voltmeter input resistance As can be seen from Figure 81 this change in current through the incorrectly connected voltmeter will be accompanied by an equal magnitude change in current through the load and the current monitoring resistor thus degrading the measured constant current performance Of course if a sufficiently high resistance dc voltmeter is used this precaution need not be observed provided the voltmeter input current is small compared to the current change being measured Other precautions associated with the proper measurement of constant current power supply specifications are given in the following paragraphs as required 115 DIGITAL VOLTMETER R DIFFERENTIAL VOLTMETER Figure 81 External Voltmeter Measurement Error on CC Power Supply CC Source Effect Line Regulation Definition The change Alour in the steady state value of dc output current due to a change ac input voltage over the specified range from low line e g 104 volts to high line e q 127 volts or from high line to low line Measurement is accompli
79. ed with multiple loads drawing pulse currents with short rise times Without local decoupling these current changes can cause spikes which 64 travel down the load distribution wires and falsely trigger one of the other loads POWER SUPPLY DT S ARE SHOWN SOLID A EQUIVALENT CIRCUIT WITH ONE LOAD POWER SUPPLY B EQUIVALENT CIRCUIT WITH MULTIPLE LOAD Figure 37 Power Supply and Load Wiring Equivalent Circuits To be effective the high frequency impedance of local decoupling capacitors Co C1 C2 and C3 Figure 38 must be lower than the impedance of wires connected to the same load Thus a decoupling capacitor must be chosen with care with full knowledge of its inductance and effective series resistance as well as its capacitance Moreover it is imperative that the shortest possible leads be used to connect local decoupling capacitors directly to the load and DT terminals not to other points along the dc wiring paths so that the wiring impedance between the capacitor and its connection point is minimized Ground Loops This is the most persistent subtle difficult to analyze and generally troublesome problem connected with power supply wiring The origins of ground loop problems are so diverse that empirical solutions are frequently resorted to Nevertheless a little extra thought and care will reduce and in most cases eliminate the need for an empirical approach 65 POWER SUPPLY DT S
80. edance presents an opportunity for a variation of one load current to cause a dc voltage variation at another load If the loads are pulse or digital circuits false triggering may result Similarly if one load is the output stage of a high gain amplifier and another load contains low level stages feeding the same signal path unintentional feedback may occur via this mutual impedance with resulting amplifier oscillation Connecting remote sensing to the load terminals of Figure 37A or the DT s of Figure 37B has the effect of reducing by a factor equal to the loop gain of the power supply regulator usually of the order of 10 10 or 10 However remote sensing does not in general alter the effective value of L seen by the load since Lo predominates at frequencies above the bandwidth of the power supply regulator Since remote sensing affords little or no reduction in the effective load wiring impedance at high frequencies some amount of capacitive load decoupling is sometimes desirable particularly when multiple loads are connected to a power supply Load Decoupling A local decoupling capacitor if required should be connected across each pair of load and distribution terminals This reduces the high frequency impedance seen by any individual load looking back toward the power supply and reduces high frequency mutual coupling effects between loads fed from the same supply The use of load decoupling capacitors is most often employ
81. edance should be as high as possible of course and should remain high with increasing frequency to limit current transients in rapidly changing load A capacitor across the output terminals should be avoided since it will lower the output impedance store energy which can result in undesirable current transients and decrease the programming speed One approach to the design of a current source is to add a high series resistance to an ordinary voltage source However it is difficult to achieve good current regulation with this design Typical applications for current sources call for output impedances of a few megohms to a few hundred megohms and currents of tens or hundreds of milliamperes This means the source voltage would have to be 45 tens of kilovolts or more Such a high voltage supply would cause noise problems would be difficult to modulate or to program rapidly would be dangerous very large and would waste considerable power IDEAL CURRENT SOURCE 2 0 VOLTAGE PRACTICAL CURRENT SOURCE LIMIT Zj lt AV INTERNAL IMPEDANCE i aL OUTPUT CURRENT Figure 25 An Ideal Current Source Electronic current regulation is a much more tractable way to obtain high output impedance although there are still design problems such as leakage Leakage Versus Regulation The current regulation of a current source as seen at the load is degraded by any impedance in parallel with the load If Io is the current generat
82. emote programming terminals and trimming the internal programming resistance or offset control adjustment to obtain exactly zero volts Once a power supply has its programming characteristic aligned perfectly in accordance with the characteristic shown in Figure 59 this alignment will retain an absolute accuracy within a tolerance found by adding the power supply specifications for load regulation line regulation temperature coefficient X ambient temperature change drift ao gp Any change in the load resistance input line voltage ambient temperature or warmup time can be expected to cause slight variations in the output voltage of the supply even though the value of the programming resistance has not been altered The capability for remote programming accuracy therefore increases with improvements in the four specifications mentioned and high stability power supplies are capable of greater long term programming accuracy than standard supplies REMOTE PROGRAMMING SPEED A constant voltage regulated power supply is normally called upon to change its output current rapidly in response to load resistance changes In some cases however notably in high speed remote programming applications and constant current applications involving rapidly changing load resistance the power supply must change its output voltage rapidly If the power supply does not employ a preregulator the most important factor limiting the speed of output volta
83. en that all DC System connections are made at one end of the terminal or bar and any Ground System connection at the other so that the DC and Ground System currents are not intertwined When checking for unintentional paths from dc to ground be sure that any straps or wires between power supply output and ground terminals have been removed unless of course this is the single desired connection between the CP and the GP Remote Error Sensing Constant Voltage Operation Only Normally a power supply achieves its optimum load and line regulation its lowest output impedance drift ripple and noise and its fastest transient recovery performance at the power supply output terminals Figure 46 POWER SUPPLY AEoXO 0 0 lt O Figure 46 Regulated Power Supply with Local Normal Error Sensing If the load is separated from the output terminals by any lead length some of these performance characteristics will be degraded at the load terminals usually by an amount proportional to the impedance of the load leads compared with the output impedance of the power supply 72 Some idea of how easily even the shortest leads can degrade the performance of a power supply at the load terminals can be obtained by comparing the output impedance of a well regulated power supply typically of the order of 1 milliohm or less at dc and low frequencies with the resistance of the various wire sizes listed in the foll
84. en you use Agilent equipment we can verify that it works properly help with product operation and provide basic measurement assistance for the use of specified capabilities at no extra cost upon request Many self help tools are available Your Advantage Your Advantage means that Agilent offers a wide range of additional expert test and measurement services which you can purchase according to your unique technical and business needs Solve problems efficiently and gain a competitive edge by contracting with us for calibration extra cost upgrades out of warranty repairs and on site education and training as well as design system integration project management and other professional engineering services Experienced Agilent engineers and technicians worldwide can help you maximize your productivity optimize the return on investment of your Agilent instruments and systems and obtain dependable measurement accuracy for the life of those products For more assistance with your test amp measurement needs or to find your local Agilent office go to www agilent com find assist Or contact the test and measurement experts at Agilent Technologies 1 800 452 4844 8am 8pm EST Product specifications and descriptions in this document subject to change without notice Copyright 1978 2000 Agilent Technologies Reprinted in USA October 1 2000 5952 4020 Agilent Technologies 126
85. ent recovery to within Y milliamps of the nominal output current following a Z amp step change in load voltage where Y is generally of the same order as the load regulation specification The nominal output current is defined as the dc level half way between the static output current before and after the imposed load change Z is the specified load voltage change normally equal to the full load voltage rating of the supply The test set up used for measuring constant voltage transient recovery time should be used for measuring constant current transient recovery time except that the contacts of the mercury relay are connected in parallel rather than in series with the load resistance refer to Figure 79 The waveforms obtained are similar to those indicated on Figure 73 but keep in mind that Y in millivolts must be converted to milliamps by dividing the value of Y by the ohmic value of the current monitoring resistor Ry All other comments and conditions mentioned previously under CV Transient Recovery Time apply equally to the constant current measurement CC Drift Stability Definition The change in output current for the first 8 hours following a 30 minute warm up period During the interval of measurement all parameters such as load resistance ambient temperature and input line voltage are held constant The stability of a power supply in constant current operation must be measured while holding the temperature of the
86. er CV CC supplies The CV or CC comparison amplifiers compare the output voltage or current with the bipolar reference and generate amplified error signals that control the conduction of the applicable regulating transistor Q1 or Q2 Amplifier Figure 32 shows the BPS A redrawn as an amplifier Transistors Q1 and Q2 are arranged in a single ended push pull configuration and the operational amplifier aspects are more readily suggested by this configuration For simplicity the constant current control circuits are not included in Figure 32 In the amplifier mode the BPS A controls the gain of an externally applied dc or ac signal In Figure 32 an external ac input signal has been substituted for the internal bipolar reference supply shown in Figure 31 54 CONTROL Figure 31 Bipolar Power Supply Amplifier Drawn as a CV CC Power Supply The rear terminal strip on BPS A instruments includes numerous control terminals to facilitate remote resistance programming of the CV or CC output in the power supply mode or remote dc or ac programming in the amplifier mode Digitally Controlled Power Sources Digitally controlled power sources DCPS s are designed specifically for use in modern automated systems which require power supplies capable of being programmed by a digital controller Although tailor made for computer based automatic test systems DCPS interface circuitry can be readily modified to permit control by a programmab
87. eries loads for constant current power supplies must always be connected in series never in parallel 32 E out Figure 15 Ideal Constant Current Power Supply Output Characteristic Any one of the four basic constant voltage regulators can also furnish a constant current output provided that its output voltage can be varied down to zero or at least over the output voltage range required by the load Besides the regulator the reference and control circuits required for constant current operation are nearly identical to those used for constant voltage operation As a result of these many common elements most constant current configurations are combined with a constant voltage circuit in one Constant Voltage Constant Current CV CC power supply The following paragraphs describe the current feedback loop generally employed in Agilent Technologies CV CC supplies This particular approach to constant current while sufficiently effective for most applications does have limitations that are caused by its simplified nature For example although output capacitor Co minimizes output ripple and improves feedback stability it also increases the programming response time and decreases the output impedance of the supply a decrease in output impedance inherently results in degradation of regulation If precise regulation rapid programming and high output impedance are required improvements on the basic feedback loop are necessary as de
88. es regulating components and other components within the regulator circuitry Therefore any constant current programming mechanism involving switches must use make before break switches A good safety precaution is to place directly across the constant current programming terminals of the power supply a control resistance corresponding to the maximum output current The remote switching mechanism can then be used to shunt this safety resistor to the degree necessary to achieve any lower values of output current The resistor can only be used if non linear programming of the output current can be tolerated The speed of response associated with constant current programming is determined by the output voltage change required as a result of change in output current being programmed The equations given in Figure 61 are applicable in determining the time required for the newly programmed value of constant current to be achieved REMOTE PROGRAMMING ACCURACY Figure 59 shows the relationship between programming resistance and output voltage for a power supply with perfect remote programming Zero ohms across the programming terminals results in exactly zero volts output and all other values of programming resistance result in the output voltage predicted by the programming coefficient Kp 86 0 max Emax Kp Figure 59 Ideal Remote Programming Characteristics As Figure 60 indicates all power supplies deviate somewhat fro
89. es regulator Line regulation see source effect Line regulators used with regulated power 61 e edendo ei eii HR te HODIE 90 Load connections multiple supply System 78 sinale supply cas PH 29 L ad deco plins capacitor seen ME Pm 78 Poad 12 ineasure meter nce eee ete o 104 116 Load effect transient recovery time definition nennen 12 E e doce 109 116 Load Switch automatic ete d cobs esee teri ete edt deeds etri tide el ee dede eee otras id 110 WATE PAVING Coco eA SORS 63 T7 TSOP SAI RT p 64 Loop around see Ground loop O Off line power Supply ea Prts 12 Operational amplifier as a dc power 20 Output capacitance in high performance constant current supply eene 49 removal of to increase programming 89 Output impedance see Impedance output Overlodd protectlOI us sous osea er eds 35 36 39 Overvoltage protection circuit see Crowbar protection circuit P Parallel peratloni 96 PARD Pde finition Dt e 13 measufter
90. ess than rated output at many operating points or else several equivalent supplies a 50V 4A supply a 20V 10A supply and others for in between voltage and current ratings Both of these solutions are expensive and require a significant amount of bench or rack space 50 OPERATING REGION Pour MAX 20 200 WATTS OPERATING REGION CONVENTIONAL CV CC SUPPLY 200W 50V PouT MAX 200 WATTS EOUT 20V 4A 10A lout B EXTENDED RANGE SUPPLY 200W Figure 28 Output Characteristics of CV CC Supplies Conventional vs Extended Range Example of Extended Range Power Supply Agilent Technologies uses two different design techniques in their extended range power supplies In one type shown on Figure 29 extended range is achieved by adding a special tap switching network ahead of a standard CV CC series regulated feedback loop A more recent design utilizes a regulator type whose output characteristic naturally assumes the shape of Figure 28 B without the need for an electronic tap switching network However for convenience only the extended range supply with the tap switching network is described in the following paragraphs This supply achieves the high level of performance normally associated with a preregulated series regulated supply plus two additional advantages The first advantage is the extended range concept described previously and the second is that the programmi
91. g the exact limitation applicable to a particular supply In addition it must be remembered that voltage lost in the load leads reduces the voltage available for use at the load This is usually not significant at high voltages but a typical 10 volt power supply will only have 6 volts left for load use if 2 volts are dropped in each load lead remote sensing does not increase the total voltage available from the power supply rectifier and regulator Either of these two factors will in some cases lead to a wire size selection which is larger than dictated by a consideration of wire current rating or impedance Output Oscillation Check for the possibility of power supply oscillation when connected in the system for remote sensing Figure 52 illustrates that the impedance of the load leads is included inside the power supply feedback loop In remote sensing applications involving small or long load wires there is a tendency for power supply oscillation to occur due to the phase shift and added time delay associated with the load and sensing leads POWER SUPPLY RECTIFIER AND REGULATOR Figure 52 Effect of Load Lead Impedance on Remote Sensing Removal of such tendency toward oscillation is usually done empirically In some cases readjusting a transient recovery or loop stability control inside the supply will be adequate in more severe cases the power supply loop equalization may have to be redesigned and tailor
92. ge For 87 127Vac inputs the strap is installed and the input circuit becomes a voltage doubler Switching Frequencies Present and Future Presently 20KHz is a popular repetition rate for switching regulators because it is an effective compromise with respect to size cost dissipation and other factors Decreasing the switching frequency would bring about the return of the acoustical noise problems that plagued earlier switching supplies and would increase the size and cost of the output inductors and filter capacitors Increasing the switching frequency however would result in certain benefits including further size reductions in the output magnetics and capacitors Furthermore transient recovery time could be decreased because a higher operating frequency would allow a proportional decrease in the output inductance which is the main constraint in recovery performance Unfortunately higher frequency operation has certain drawbacks One is that filter capacitors have an Equivalent Series Resistance and Inductance ESR and ESL that limits their effectiveness at high frequencies Another disadvantage is that power losses in the switching transistors inductors and rectifier diodes increase with frequency To counteract these effects critical components such as filter capacitors with low ESRs fast recovery diodes and high speed switching transistors are required Some of these components are already available others are not Switching transi
93. ge change is the output capacitor and load resistor The equivalent circuit and the nature of the output voltage waveform when the supply is being programmed upward are shown in Figure 61 When the new output is programmed the power supply regulator circuit senses that the output is less than desired and turns on the series regulator to its maximum value the current limit or constant current setting This constant current I charges the output capacitor Co and load resistor Ry in parallel The output therefore rises exponentially with a time constant toward a voltage level I a value higher 88 than the new output voltage being programmed When this exponential rise reaches the newly programmed voltage level the constant voltage amplifier resumes its normal regulating action and holds the output constant Thus the rise time can be determined using a universal time constant chart or the formula shown in Figure 61 If no load resistor is attached to the power supply output terminals then the output voltage will rise linearly at a rate of Co I when programmed upward and TR Co E2 E1 Iz the shortest possible up programming time LEVEL IR 2 Eout TC R F Eg INITIAL E TIME L ETT LTR 150 ILRL E2 L t2 NEW IS REACHED L t NEW IS PROGRAMMED EQUIVALENT CIRCUIT FOR ty lt t lt to
94. gulator switches does not absorb a large amount of power Hence the addition of the preregulator does not significantly reduce the overall efficiency of this supply Figure 11 Switching Supply with Preregulator Single Transistor Switching Regulator At lower output power levels a one transistor switch becomes practical The single transistor regulator of Figure 12 can receive a dc input from either one of two sources without a change in its basic configuration For ac to dc requirements the regulator is connected to a line rectifier and SCR preregulator and for dc to dc converter applications it is connected directly to an external dc source Like the previous switching supplies the output voltage is controlled by varying the ON times of the regulator switch The switch is turned on by the leading edge of each 20KHz clock pulse and turned off by the pulse width modulator at a time determined by output load conditions While the regulating transistor is conducting the half wave rectifier diode is forward biased and power is transferred to the output filter and the load When the regulator is turned off the flywheel diode conducts sustaining current flow to the load during the off period A flywheel diode sometimes called a freewheeling or 29 catch diode was not required in the two transistor regulators of Figures 10 and 11 because of their full wave rectifier configuration Another item not found in the previous re
95. gulators is flyback diode This diode is connected to a third transformer winding which is bifilar wound with the primary During the off periods of the switch is forward biased allowing the return of surplus magnetizing current to the input filter and thus preventing saturation of the transformer core This is an important function because core saturation often leads to the destruction of switching transistors In the previously described two transistor push pull circuits core saturation is easier to avoid because magnetizing current is applied to the core in both directions Nevertheless matched switching transistors and balancing capacitors must still be used in these configurations to ensure that core saturation does not occur OUTPUT RECTIFIERIFILTER REGULATOR SWITCH FROM PREREGULATOR OR EXTERNAL OUT DC SOURCE VOLTAGE COMPARISON AMPL E OUT SAMPLE REF TURN OFF COMPARATOR Figure 12 Single Transistor Switching Regulator Summary of Basic Switching Regulator Configurations Figure 13 shows three basic switching regulator configurations that are often used in today s power supplies Configuration A is of the push pull class and this version was used in the switching supplies shown in Figures 10 and 11 Other variations of this circuit are used also including two transistor balanced push pull and four transistor bridge circuits As a group pus
96. h pull configurations are the most effective for low voltage high power and high performance applications Push pull circuits have the advantage of a ripple frequency that is double that of the other two basic configurations and of course output ripple is inherently lower 30 PUSH PULL SINGLE ENDED FIGURES 10811 FORWARD FEED THROUGH FIGURE 12 C FLYBACK RINGING CHOKE Figure 13 Basic Switching Regulator Configurations Configuration B is a useful alternative to push pull operation for lower power requirements It is called a forward or feed through converter because energy is transferred to the power transformer secondary immediately following turn on of the switch Although the ripple frequency is inherently lower output ripple amplitude can be effectively controlled by the choke in the output filter Two transistor forward converters also exist wherein both transistors are switched simultaneously They provide the same output power as the single transistor versions but the transistors need handle only half the peak voltage Configuration C is known as a flyback or ringing choke converter because energy is transferred from primary to secondary when the switches are off during flyback In the example two transistors are used and both are switched simultaneously While the switches are on the output rectifier is reverse biased and current in the primary inductance rises in
97. hanges or when programming is done with low resistance values Protecting Against Momentary Programming Errors Using remote programming several different values of fixed output voltage are obtainable with resistors and a switch so that the output voltage of the supply can be switched to any pre established value with a high degree of reproducibility Figure 56 illustrates several switching techniques that can be used in conjunction with resistance programming Suppose it is desired to program a supply having a programming coefficient Kp of 200 ohms volt to any of three values 5 volts 10 volts and 15 volts the circuit of Figure 56A is a typical configuration However if a break before make switch is used in the configuration of Figure 56A there will occur for a short interval during the switching action a very high resistance between the two programming terminals and the power supply during that interval will raise its output voltage in response to this high resistance input To eliminate this output overshoot corresponding to an infinite programming resistance a make before break switch should be employed However this solution has the disadvantage that during the short interval when the swinger of the switch is contacting two switch terminals two programming resistors will momentarily be paralleled across the power supply programming terminals and the supply will for this short interval seek an output voltage which is lower than either
98. he peak load condition is of relatively short duration then the stored energy in the power supply output capacitor may prevent an excessive output voltage sag Thus for peak loads of either category 1 or 2 it is of interest to know how much the output voltage will drop for a peak load condition in excess of the power supply current rating and how long it will take for the supply to recover to its normal output voltage following the removal of the overload Figure 63 illustrates the equivalent circuit and output voltage waveform which are characteristic of a power supply experiencing a short term overload When the overload condition is first imposed the power supply goes into the current limit mode and is therefore equivalent to a constant current generator I feeding the output capacitor Co already charged to Enorm in parallel with the lowered value of load resistance peax Thus the capacitor begins discharging exponentially toward the final output voltage value which would result if the overload condition were retained namely peax The amount of voltage sag AV depends upon the output time constant and the duration of the overload peak load condition the equation for this voltage sag is given in Figure 63 When the peak load condition is removed is restored to its normal value and the supply continues in the current limiting mode charging the output capacitor on another exponential curve This time the asymptotic level approac
99. he series regulator Down programming however is much slower in a preregulated unit because 1 an active element is not normally available to aid in the discharge of the output capacitor and 2 any decrease in the charge across the output capacitor must be accompanied by a decrease in the voltage across the input filter capacitor to avoid damaging the series regulator This problem exists because the series regulator which normally operates with a Vce of only a few volts does not have the heat sink capability to absorb the extra energy dissipated when discharging the filter capacitor during down programming 53 The extended range power supply overcomes the latter problem through the use of series regulating transistors with higher voltage ratings and with thermally improved heat sinks The heat sinks allow the series transistors to be properly cooled during the worst case conditions that are encountered during rapid down programming In addition a special transistor circuit not shown on Figure 29 provides for a more rapid discharge of the output capacitor during down programming Bipolar Power Supply Amplifier In some applications a power supply is required that has a faster programming speed than standard designs see Remote Programming Chapter for programming speed limitations Still other applications require a power supply that can be controlled continuously through zero over a wide span in either the positive or negative direction Bip
100. hed by the exponential curve is I norm However this charging action stops when the voltage level has risen to the normal level and the regulator changes from the current limit mode to the normal constant voltage mode Figure 63 also gives the equation for the time required for this voltage recovery following the removal of the 91 peak load condition EQUIVALENT CIRCUIT DURING SAG PEAK LOAD AVERAGE LOAD NORMAL LOAD Ij RL NORM 2 77 ASYMPTOTIC LEVEL ENORM an rues EMIN ILR PEAK ASYMPTOTIC LEVEL t Lts NORMAL OUTPUT VOLTAGE RESTORED HNORMAL LOAD CONDITION RESTORED L tg PEAK LOAD CONDITION IMPOSED VOLTAGE SAG E NORM EMIN 4719 RC LRL peak tee E70 SAG RECOVERY TIME TLRL NORM EMIN 170 76 IL RL NORM ENORM Figure 63 Short term Overload Equivalent Circuit and Output Voltage Thus the equations can be used to evaluate whether the voltage sag and recovery time resulting from a overload condition lie within acceptable limits permitting the use of a power supply having a current rating less than the 92 peak load demand For short term overloads a quick approximation can be made to determine the amount of voltage sag Ip hb AT AV Co where AV The voltage sag Enorm Peak load current demand I
101. hen the scope is not rejecting the ground signal and must be realigned in accordance with the manufacturer s instructions To be absolutely certain that the measurement setup is free from extraneous signals turn off the power supply and with the scope connected across S and S terminals ascertain that no signals are present on the CRT The presence of noise signals under these conditions is indicative of pickup on the leads between the power supply and the scope Figure 73 shows the relationship between the peak to peak and rms values of three common waveforms The output ripple of a dc power supply usually approximates the sawtooth of Figure 73B which is 1 3 464 of the peak to peak value displayed on the oscilloscope The square wave is included in Figure 73 because it has the highest possible peak to rms ratio Thus the rms ripple and noise present on the output terminals of a power supply cannot be greater than 1 2 the peak to peak value measured on the oscilloscope In most cases the rms ripple on Agilent power supplies is between 1 3 and 1 4 of the peak to peak value A SINE 2 2 Epps 2 828 ERMS B SAWTOOTH WAVE 2 3 3 464 ERMS C SQUARE WAVE 2ERMS Figure 73 Three Ideal Ripple Waveshapes 107 Noise Spike Measurements When a high frequency spike measurement is being made the oscilloscope must have a bandwidth of 20MHz or more Measuring noise with an in
102. identally thus 83 causing the output voltage to rise to some value higher than the maximum voltage rating of the supply With some loads this could result in serious damage To protect loads from accidental opening of the remote programming leads a zener diode should be placed directly across the power supply programming terminals This zener diode is selected to have a breakdown voltage equal to the maximum power supply voltage that can be tolerated by the load Thus if the programming terminals open the programming current will cause the zener diode to break down and the output voltage will be limited to the zener diode voltage Such a zener diode must be capable of dissipating a power equal to the product of its breakdown voltage times the programming current Ip CONSTANT VOLTAGE REMOTE PROGRAMMING WITH VOLTAGE CONTROL Instead of controlling a power supply by means of a programming resistance it is possible to control the output of any remotely programmable supply with an input voltage Thus the power supply becomes a low frequency dc amplifier Remote Programming Speed later in this section stresses the bandwidth and speed of response of this configuration whereas these paragraphs deal only with the method of control Two distinct methods can be employed to voltage program unity and variable voltage gain Programming with Unity Voltage Gain This method shown in Figure 57 requires that the external voltage be exactly equal
103. ing the zener reference diode for the voltage comparison amplifier and the low level portions of the feedback amplifier are enclosed in a temperature controlled oven Moreover the less critical components that are not oven enclosed are high quality components with low temperature coefficients These techniques together with the utilization of a high gain feedback amplifier result in an exceptionally stable and well regulated supply with a 0 1 accuracy Precision Constant Current Source The concepts and circuits used in basic constant current power supplies were shown in Figure 16 This section is devoted to the refinements necessary to upgrade a basic constant current supply to a precision class with characteristics that more closely approach an ideal current source An ideal current source is a current generator that has infinite internal impedance It provides any voltage necessary to deliver a constant current to a load regardless of the size of the load impedance It will supply this same current to a short circuit and in the case of an open circuit it will attempt to supply an infinite voltage see Figure 25 In practical current sources neither infinite internal impedance nor infinite output voltages are possible In fact if the current source is to be used as a test instrument it should have a control for limiting its maximum output voltage so its load will be protected against the application of excessive potentials Its output imp
104. iples discussed under Constant Voltage Programming are also applicable when considering remote programming for constant current supplies Remote programming of the constant current output of any programmable supply can be accomplished either by 1 Applying a resistance of voltage to the programming terminals of a CV CC supply or 2 Modifying a constant voltage programming supply for constant current operation and then controlling the output current by means of a resistance or voltage applied to the terminals normally used for constant voltage control A Constant Voltage Supply is modified for Constant Current operation by adding an external current monitoring resistor as described later in this chapter Method 1 is used with any Constant Current or Constant Voltage Constant Current Agilent power supply while Method 2 is used for any remotely programmable Constant Voltage Current Limiting supply Method 1 has one important disadvantage the normal current limiting protection which is dependent upon the constant current setting of a CV CC power supply is negated if the constant current programming terminals are accidentally opened Particular care must be taken in the design of the constant current programming network to insure that no open circuit condition can exist even for a short interval of time because such an open circuit will program the power supply to an output current in excess of its rating with almost certain destruction of the seri
105. ird wire safety ground continuity should be retained without accidental 60 interchange from ac power outlet to the power supply input terminals 2 If an autotransformer or isolation transformer is connected between the ac power source and 61 the power supply input terminals it should be rated for at least 20096 of the maximum rms current required by the power supply 3 The autotransformer common terminal should be connected to the acc not ac terminals of 61 both the power supply and the input power line 4 Most ac input line regulators should not be used with regulated power supplies without first 61 checking with the power supply manufacturer 5 When connecting ac to a power supply it is necessary to use a wire size which is rated to carry 61 at least the maximum power supply input current Load Connections for One Power Supply 6 A single pair of terminals are designated as the positive and negative DC Distribution 62 Terminals DT s 7 One pair of wires should be connected directly from the power supply output terminals to the 63 DT s and a separate pair of leads from the DT s to each load 8 As an absolute minimum each load wire must be of sufficient size to carry the power supply 63 output current which would flow if the associated load terminals were short circuited 9 A local decoupling capacitor if required should be connected across each pair of load and 64 distribution terminals 10 Oneofthe DC Distribution Termina
106. ivalent output inductance if the load current is switched rapidly enough so that the high frequencies associated with the leading edge of the step change can react with this effective output inductance a spike will occur on the output terminals of any power supply It is not possible to specify the amplitude of an output voltage spike caused by a load current change unless the rise time of the load change is first established A power supply with an effective output inductance of 0 16 microhenries will exhibit a load transient spike of about 0 16 volts if the load is switched with a rise time of 1 but the spike amplitude will be only 160uV if the load is switched at 1 amp millisecond In this latter case the output spike would not be evident since it would be small compared to the static change in output voltage associated with the full load change FULL LOAD Tour NO LOAD LOAD CURRENT T NOMINAL OUTPUT VOLTAGE VOLTAGE RECOVERY 41 Figure 75 Load Effect Transient Recovery of a Constant Voltage Power Supply While an oscilloscope with a bandwidth of the order of 100KHz is adequate to observe and measure the 109 transient recovery time of a power supply the spike amplitude for load switching times of less than 1 microsecond cannot be accurately determined unless a very wideband scope is used Of all power supply specifications transient recovery time is subject to the widest variation i
107. l have been made available on rear terminals so that an external control can be substituted The current flowing through Rp and Rg is constant and independent of the output voltage and the voltage across the programming resistor and therefore the output voltage is a linear function of the resistance Rp 80 REFERENCE SUPPLY SERIES RR REGULATOR VOLTAGE COMPARISON AMPL Rp Z EXTERNAL PROGRAMMING Eout CONTROL CONNECTED Kp TO SUPPLY S REAR TERMINALS Figure 54 Constant Voltage Supply with Resistance Programming Programming a power supply with a 200 ohms volt programming coefficient to an output level of 30 volts would require and Rp of 6K The power supply will force through this programming resistor a 5mA constant current thus resulting in 30 volts across the power supply output terminals Remote Programming Connections Shielded two wire cable should be used to connect the power supply programming terminals to the remote programming source following the manufacturer s instructions for connections The shield should not be used as one of the programming conductors One end of the shield should be connected to the DC Common Point and the other end of the shield should be left unconnected Figure 55 Output Drift Check that programming leads and source will not contribute to output drift noise etc The power consumed in the programming resistor can be readily determined by remembering that
108. le calculator coupler or other digital data source The basic function of the DCPS is to convert the digital signals from the controller into analog form with speed and accuracy Agilent Technologies family of DCPS s includes several digital voltage sources and one current source Digital Voltage Source DVS The DVS is a Constant Voltage Current Limiting power source that can be continuously programmed throughout its bi polar output voltage range Figure 33 is a simplified block diagram showing a typical DVS manufactured by Agilent The unit consists basically of a digital to analog D A converter followed by a bi polar power amplifier 55 56 EXTERNAL COMPARISON Figure 32 Bipolar Power Supply Amplifier Drawn as an Amplifier BI POLAR POWER AMPL ANALOG VOLTAGE FROM CURR CONTROLLER LATCH DATA STATUS SIGNALS T CONTROLLER Figure 33 Digital Voltage Source Block Diagram Additional circuits are also included to facilitate operation within the systems environment The additional circuitry performs interface isolation storage overcurrent protection and status feedback functions as explained in subsequent paragraphs Interface and Isolation Each input and output signal to and from a DCPS passes through interface and isolation circuits Interface circuits are designed to match the unit to a variety of controllers Isolation circuits isolate the digital input from the ana
109. lest and most common example of improper load wiring is illustrated in Figure 34 Each load sees a power supply voltage which is dependent upon the current drawn by the other loads and the IZ drops they cause in some portion of the load leads Since most power supply loads draw a current which varies with time a time varying interaction among the loads results In some cases this interaction can be ignored but in most applications the resulting noise pulse coupling or tendency toward inter load oscillation is undesirable and often unacceptable 61 POWER SUPPLY Figure 34 Improper Load Connections DC Distribution Terminals A single pair of terminals are designated as the positive and negative DC Distribution Terminals DT s These two terminals may be the power supply output the B at the load or a separate pair of terminals established expressly for distribution Proper location of the DT s results in improved over all performance and reduced mutual coupling effects between separate loads using the same power supply If remote sensing is not used locate the DT s as close as possible to the power supply output terminals optimum performance will result when the power supply output terminals themselves are used as the DT s See Figure 35 POWER SUPPLY DT S ARE SHOWN SOLID A WITH ONE LOAD DT S ARE SHOWN SOLID B WITH MULTIPLE LOADS PREFERRED Figure 35 Location of DC Distribution
110. log output voltage allowing the output to be floated if desired Isolation also prevents troublesome loops between the output ground and controller ground and prohibits potentially destructive current surges which could occur if some point in the load were inadvertently grounded Storage The digital voltage and current programming input data are transferred into integrated circuit storage buffers upon receipt of the storage pulse from the controller Once the data is stored the controller can perform other tasks without the need for maintaining the input data This increases controller operating efficiency and even allows party line operation where one set of data lines can be used to program several DCPS The storage capability also minimizes voltage programming overshoots or undershoots by ensuring that all voltage program inputs reach the D A converter simultaneously The gate pulse is delayed 50 from the arrival of the input data to allow time for all input lines to settle If the programming source does not normally generate gate signals the storage circuits can be bypassed by means of a switch on the DVS The voltage program data now passes directly into the D A converter as soon as received from the isolation circuits but without the benefit of storage D A Converter The heart of a DCPS is the D A converter This bi polar high speed circuit converts the digital voltage programming inputs into an analog reference signal which drives
111. ls should be designated as the DC Common Point CP 68 11 Oneofthe terminals which is connected to ground should be designated as the DC Ground 72 59 Point GP 12 The CP should be connected to the GP as shown in Figures 40 through 43 unless one load is 72 already grounded making certain there is only one conductive path between these two points 13 Connections between the power supply sensing and output terminals should be removed and 74 using shielded two wire cable the power supply sensing terminals should be connected to the DC Distribution Terminals as shown in Figure 49 14 One end of the shield should be connected to the CP and the other end should be left 75 unconnected 15 The possibility of an open remote sensing path which might occur on a long term or transient 75 basis should be avoided 16 The minimum wire size for the load current leads from the power supply output terminals to TI the DT s should be determined for remote sensing 17 Check for the possibility of power supply oscillation when connected in the system for remote 77 sensing 18 Check for proper current limiting operation while the power supply is connected in the system 78 for remote sensing Load Connections for Two or More Power Supplies 19 There must be only one point of connection between the dc outputs of any two power supplies 79 in the multiple power supply system this point must be designated as one of the two DT s for both po
112. lso be used to measure the output voltage without drawing current away from the load Connecting a voltmeter between the negative output terminal and the positive output terminal will lower the output impedance but a voltmeter connected between the negative 49 output terminal and the guard has no effect on the output impedance The meter still measures the output voltage because the guard is at the same potential as the positive output terminal The front panel voltmeter is internally connected to guard and if greater accuracy is needed a voltmeter can be connected externally Unlike other guards such as those used on digital voltmeters the guard in the Current Source is active and internally referenced to the positive terminal For this reason the guard is labeled meter on the front panel and must not be connected to either output terminal since this interferes with the closed loop performance Transformer Shielding Eliminates Ripple Agilent Current Sources meet their low ripple specifications regardless of which output terminal if either is connected to earth ground High gain current regulation is one reason for the low ripple Another is special shielding to keep ac voltages in the power transformer from being coupled into the output via the capacitance between the transformer windings and the output or ground One potential source of ripple current is capacitive coupling between the primary winding and the negative output termin
113. ltage The illustrations below show examples of constant voltage remote programming CC applications are similar REMOTE PROGRAMMING USING RESISTANCE CONTROL POWER SUPPLY REMOTE PROGRAMMING USING VOLTAGE CONTROL POWER SUPPLY Eout 14 REMOTE SENSING REMOTE ERROR SENSING A means whereby a constant voltage power supply monitors and regulates its output voltage directly at the load terminals instead of the power supply output terminals Two low current sensing leads are connected between the load terminals and special sensing terminals located on the power supply permitting the power supply output voltage to compensate for IR drops in the load leads up to a specified limit POWER SUPPLY AND LOAD CONNECTED NORMALLY POWER SUPPLY POWER SUPPLY AND LOAD CONNECTED FOR REMOTE SENSING POWER SUPPLY SENSING LEAD SENSING LEAD RESOLUTION The smallest change in output voltage or current that can be obtained using the front panel controls SOURCE EFFECT LINE REGULATION Formerly known as line regulation source effect is the change in the steady state value of the dc output voltage of a CV supply or current of a CC supply due to a specified change in the source ac line voltage with all other influence quantities maintained constant Source effect is usually measured after a complete change in the ac line voltage from low line to high line or vice versa 15 STABILITY
114. m the DC System to ground can provide a return path enabling additional ground loop current to link both the DC System and Ground System 67 DC Common One of the DC Distribution Terminals should be designated as the DC Common Point CP There should be only one DC Common Point per DC System If the supply is to be used as a positive source then the minus DC Distribution Terminal is the DC Common Point if it is to be a negative source then the plus DT is the CP Here are some additional suggestions for selecting the best DC Common Point for five different classes of loads a Single Isolated Load Select either the positive or negative DC Distribution Terminal as the DC Common Point A single isolated load exists when a power supply is connected to only one load and that load circuit has no internal connections to the chassis or ground If the power supply output terminals are to be used as the DC Distribution Terminals then the DC Common point will be either the positive or negative power supply output terminal Figure 40A On the other hand if remote sensing is to be employed and the load terminals will serve as the DT s then either the positive or negative load terminal is designated as the CP Figure 40B POWER SUPPLY S G SAFETY GROUND LEAD IN POWER CORD CONNECTED TO CHASSIS AND GROUND TERMINALS OF POWER SUPPLY AND TO EARTH GROUND GND POWER SUPPLY GROUND TERMINAL A WITHOUT REMOTE SENS ING POWER SUPPL
115. m the ideal The application of a short circuit across the programming terminals results in an output voltage which is slightly different from zero typically between 20 millivolts and 50 millivolts While the linearity of the programming characteristic is nearly perfect the overall slope may differ from the value predicted by the programming coefficient by from 1 to 5 The fact that this slope is extremely linear can be utilized in improving the absolute accuracy in programming a supply since by pinpointing two points on this straight line segment all other points are thereby determined The two points which are the easiest and best to fix are zero and maximum output voltage If these two points are successfully relocated the graph of Figure 60 can be changed into one closely approximating that shown in Figure 59 which is the characteristic of an ideal supply having perfect programming accuracy Figure 60 Practical Remote Programming Characteristic Regardless of the programming coefficient an ideal programmable supply having absolute programming 87 accuracy will deliver zero volts with zero programming resistance Thus the first step in improving the programming accuracy of Figure 60 is to short the programming terminals and note the output voltage Normally this voltage will be slightly negative If this is not the case the comparison amplifier packages can sometimes be interchanged the output voltage with zero program
116. mal operation of the supply thus changing the current that flows through the current monitoring resistor Diode keeps this extra current at a fixed level for which compensation can then be made in the constant current comparator circuit 3 In preregulated supplies the crowbar turns off the preregulator circuit when the SCR fires reducing the voltage drop across the series regulator and the current flow through the SCR 4 Anauxiliary winding is included on the blocking oscillator transformer for connection to an additional crowbar Tandem crowbar operation is then available for coincident firing of all crowbars in a system Crowbar Response Time The crowbar s speed of response to an overload is a critical parameter If the response time is too slow the output could rise to a level high enough to damage the load If the response is too fast then spurious noise can cause false tripping and create a nuisance condition There are three time delays that place a practical limit on how fast crowbars can react In order of decreasing magnitude they are 1 The typical SCR turn on time from to around 5 2 The reaction time of the trigger circuit and 3 The time delay associated with the crowbar voltage sensing circuit Since only a fraction of the output voltage is compared with an internal crowbar reference voltage the voltage sensing circuit incorporates a large voltage divider which combines with discrete and st
117. ment tei deeem 105 116 Peak load power supply equivalent circuit during eene 93 Performance measurements introduction 404 060000 0 nnn eren eee 101 113 Mee Mc PEE 111 117 scrinio 104 117 OBIDUE dITIDE AN de PE ea 114 118 PARD cc an acne ds 105 116 source elfeet oc mE LU tr rc pU tt 104 116 qe c PET 108 116 temperature coefficient ea 111 119 123 transient 107 117 Piggy Back regulator del milione tree a 44 Power s pply amplifiet 54 Prereeulator de emn 23 Preresulator circuit 1 23 29 POAC MOM CINE oio 24 SUPPression OF REL m 24 39 Bro Sal MIN coeficients n P E Shane aay 81 87 Programming elimination of switching 45 81 protection against open 82 Ine rS sib ME 80 5 iut todo daten Gases eee 81 Programmi remote yolldge 84 andi and tinens taa utile asta pice oleo cens 84 85 Programming speed definition EAS RUN RR ee YEN Una Eva eara ag eO TVs 14 e
118. ment setup is free of extraneous signals a Turn off the power supply and observe the CRT for evidence of unwanted signals with the scope connected between S and S b Instead of connecting the oscilloscope leads separately to the positive and negative sensing terminals of the 103 supply connect both leads to either the positive or the negative sensing terminals whichever is grounded to chassis Signals on the face of the CRT as a result of either of these tests are indicative of shortcomings in the measurement setup The most likely causes of these defects and proper corrective measures are discussed further under CV PARD Ripple and Noise Connect AC Voltmeter Properly It is important that the ac voltmeter be connected as close as possible to the input ac terminals of the power supply so that its indication will be a valid measurement of the power supply input without any error introduced by the IR drop present in the leads connecting the power supply input to the ac line voltage source Use an Auto Transformer of Adequate Current Rating If this precaution is not followed the input ac voltage presented at the power supply may be severely distorted and the rectifying and regulating circuits within the power supply may operate improperly Do Not Use an AC Input Line Regulator Such regulators tend to increase the impedance of the ac input as explained previously Further precautions necessary for the proper measureme
119. ming resistance will then in most cases become slightly negative In some supplies an internal control is provided for adjusting this zero offset voltage It is also possible to insert permanently a small resistor in series with the programming leads this value of resistance being just sufficient to bring the output voltage up to exactly zero volts One point of the ideal programming characteristic has now been established Next the slope of Eour versus Rp characteristic must be adjusted so that this straight line will pass through the maximum output voltage with the proper value of programming resistance Assume for example that we are adjusting a power supply which has a programming coefficient of 200 ohms per volt and a maximum output voltage of 20 volts Having inserted internally a series programming resistance of sufficient value to bring the output voltage to zero volts with zero ohms external programming resistance the next step would be to attach a precision 4000 ohm resistor across the programming terminals and adjust the programming current so that the output voltage would equal 20 volts In some supplies this programming current can be adjusted by means of an internal pot In most cases however it will be necessary to trim up a precision resistor by means of shunt resistors which determines the programming current Having adjusted this constant current it may be necessary to readjust the zero output crossing point by shorting the r
120. mperature coefficient of a power supply is measured by placing the power supply in an oven and varying it over any temperature span within its rating Most Agilent laboratory type power supplies are rated for operation from 0 C to 55 C The power supply must be allowed to thermally stabilize for a sufficient period of time at each temperature of measurement The temperature coefficient specified is the maximum temperature dependent output voltage change which will result over any 5 C interval The differential voltmeter or digital voltmeter used to measure the output voltage change of the supply should be placed outside the oven and should have a long term stability adequate to insure that its drift will not affect the overall measurement accuracy CV Programming Speed Definition The time required following onset of a step change in the programming input for the output to change from an initial value to within a certain band of the newly programmed value This band is typically specified in millivolts for a well regulated CV supply and in milliamps for a CC supply The measurement is made by monitoring the output voltage while rapidly changing a remote programming resistance or voltage Up programming requires that the remote programming source RP in Figure 77 be varied from zero ohms to a value that will produce maximum rated output voltage while down programming involves changing the resistance from the value that produces maximum rated outpu
121. n definition and is not defined at all by some power supply manufacturers Specifying that a power supply has a transient recovery time of 50 microseconds is incomplete and conveys no information Such a specification leaves to the imagination whether the power supply will recover during the 50usecond interval to within 37 1 e of its initial value to within 10 or all the way Since the falling portion of the transient remains reasonable constant in spite of wide variations in the spike amplitude and the speed of the load change causing it Agilent Technologies has chosen to define transient recovery time in terms of recovery to a certain voltage level For ease in oscilloscope measurement this voltage level is referenced to a nominal output voltage half way between no load and full load Reasonable care must be taken in switching the load resistance on and off A hand operated switch in series with the load is not adequate since the resulting one shot displays are difficult to observe on most oscilloscopes and the arc energy occurring during switching action completely masks the display with a noise burst Transistor load switching devices are expensive if reasonably rapid load current changes are to be achieved Agilent Technologies employs a mercury wetted relay using the load switching circuit of Figure 76 When this load switch is connected to a 60Hz input the mercury wetted relay will open and close 60 times per second Adjustment of
122. ng speed is considerably faster than a comparable preregulated unit The simplified schematic of Figure 29 shows that this supply consists basically of a CV CC series regulator feedback loop preceded by an electronic tap switching network Operation of the main series regulator loop is virtually identical to many other supplies developed by Agilent Technologies The tap switching circuits however relate only to extended range supplies and it is these circuits that govern the overall output characteristic of Figure 28 B Electronic Tap Switching As shown on Figure 29 tap switching is accomplished by a pair of triac switches and CR2 and an associated control circuit By selecting different triac firing combinations these circuits allow the input capacitor Cl to charge to one of four discrete voltage levels depending on the output voltage and current required 51 The main secondary winding of the power transformer has three sections each of which has a different turns ratio with respect to the primary winding At the beginning of each half cycle of the input ac the control circuit determines whether one both or none of the triacs will be fired If neither triac is fired the rectifier receives an ac input voltage that is determined by N1 turns and the input capacitor charges to a corresponding level If triac CR2 is fired capacitor Cl charges to voltage determined by N1 N2 turns Similarly if CR1 is fired the capacitor is charged
123. nominal output voltage following Z amp step change in load current where TRANSIENT RECOVERY TIME FULL LOAD lout NO LOAD LOAD CURRENT NOMINAL OUTPUT VOLTAGE VOLTAGE RECOVERY tim 1 Y is specified separately for each model but is generally of the same order as the load regulation specification 2 The nominal output voltage is defined as the dc level halfway between the steady state output voltage before and after the imposed load change 3 Z is the specified load current change typically equal to the full load current rating of the OFF LINE POWER SUPPLY A power supply whose input rectifier circuits operate directly from the ac power line without transformer isolation OUTPUT IMPEDANCE OF A POWER SUPPLY At any frequency of load change AEOUT AIOUT Strictly speaking the definition applies only for a sinusoidal load disturbance unless the measurement is made at zero frequency dc The output impedance of an ideal constant voltage power supply would be zero at all frequencies while the output impedance for an ideal constant current power supply would be infinite at all frequencies 12 TYPICAL OUTPUT IMPEDANCE OF A CONSTANT VOLTAGE POWER SUPPLY 100 o z 1 u lt lt ul a 0 001 10 10 102 103 104 105 106 107 FREQUENCY Hz M9 PARD RIPPLE AND NOISE The term PARD is an acronym for Periodic and Random deviation an
124. ns to protect the rectifiers and transformer The high voltage control circuit does not derive its input control signal from the total voltage across the load resistor or the voltage across the terminals of the high voltage supply itself Instead the control circuit monitors the voltage across the combination series regulator and current monitoring resistor and maintains this voltage 44 drop at approximately 20 volts leaving approximately 20 volts across the output terminals of the piggy back supply Agilent Technologies supplies may use any of three basic methods of controlling the high voltage output of the Main Voltage Source 1 the control signal from the High Voltage Control Circuit fires SCRs in the rectifier circuit to vary the dc output 2 the control signal varies the coupling of the high voltage input transformer to adjust the ac input to the rectifiers or 3 the control signal pulse modulates the input to the rectifier to vary the dc output High Performance Power Supplies Agilent Technologies manufactures several types of high performance dc power supplies with specifications at least an order of magnitude superior to the normal well regulated laboratory supply Foremost among these are the precision voltage and current sources Precision Voltage Sources This line includes both CV CC and CV CL supplies similar to those described previously with a few important exceptions The critical components of the supply includ
125. nt across senes Tegulator oie ete e teste 24 LOADS ac 73 IM SENSING P 75 bENDEIEIDT 16 Wireratings ae power Lo Fe RR CEP ot 61 S 63 77 WATE SIZ6 resistance 0 oss uli o MEER e nA 73 125 Agilent Technologies Test and Measurement Support Services and Assistance Agilent Technologies aims to maximize the value you receive while minimizing your risk and problems We strive to ensure that you get the test and measurement capabilities you paid for and obtain the support you need Our extensive support resources and services can help you choose the right Agilent products for your applications and apply them successfully Every instrument and system we sell has a global warranty Support is available for at least five years beyond the production life of the product Two concepts underlie Agilent s overall support policy Our Promise and Your Advantage Our Promise Our Promise means your Agilent test and measurement equipment will meet its advertised performance and functionality When you are choosing new equipment we will help you with product information including realistic performance specifications and practical recommendations from experienced test engineers Wh
126. nt of specific power supply specifications are given as required in the following paragraphs CV Source Effect Line Regulation Definition Formerly known as line regulation source effect is the change A Eovr in the steady state value of dc output voltage due to a change in ac input voltage over the specified range from low line e g 104 volts to high line e g 127 volts or from high line to low line Actual measurement is accomplished by turning the variable autotransformer Figure 70 through the specified range from low line to high line and noting the change in the reading of the digital voltmeter or differential voltmeter connected to the output terminals of the supply The measurement is performed with all other parameters maintained constant The power supply will perform within its source effect specifications at any rated output voltage combined with any rated output current the most severe test normally involves measuring source effect at maximum output voltage combined with maximum output current Notice that the line regulation specification for Agilent power supplies is not prefixed by Thus the line regulation specification sets a limit on the total excursion of the output voltage resulting from the total input ac change from low line to high line thereby allowing only one half the output deviation of a specification CV Load Effect Load Regulation Definition Formerly known as load regulation load eff
127. olar Power Supply Amplifiers BPS As which utilize the operational amplifier concept of power supplies have been developed to meet these needs A BPS A is not only a high speed programmable power supply but can also be used as direct coupled amplifier with low output distortion and a bandwidth from dc to as high as 40KHz in certain operating modes Bipolar Power Supply Figure 31 shows a simplified representation of the instrument drawn as a CV CC power supply Note that this circuit differs from the typical CV CC power supply of Figure 17 in that 1 The regulating elements consist of two series transistors connected in a push pull pseudocomplementary configuration Q1 and Q2 are actually npn s Q1 is connected to a positive rectifier and controls positive outputs while Q2 connected to a negative rectifier controls negative outputs 2 bipolar CV reference source is used instead of a unipolar one to allow a bipolar output in the constant voltage operating mode 3 Two current comparison amplifiers and an associated bipolar reference source are used to provide a bipolar output in the constant current mode 4 The output capacitor has been eliminated to increase programming speed Additional modifications not shown in Figure 31 insure that the power supply will remain stable for capacitive resistive or inductive type loads Aside from the above differences basic operation of this bipolar power supply is very similar to most oth
128. oltage The best trigger circuit is the one that turns the SCR on the fastest Fastest SCR turn on is accomplished by a fast rise time pulse circuit such as a blocking oscillator or Schmitt trigger The Agilent Technology crowbar illustrated in Figure 22 compares the output voltage with a reference voltage V The overvoltage potentiometer adjusts the reference voltage on the comparison amplifier and sets the voltage level at which the crowbar will activate Normally the overvoltage control is located on the front panel and can be adjusted from approximately 20 to 120 of the maximum rated output voltage of the power supply SERIES REGULATOR PREREGULATOR TURN OFF OVERVOLTAGE COMPARISON AMPLIFIER UNREG DC BLOCKING OSCILLATOR AUXILIARY FIRING PULSES REFERENCE OVERVOLTAGE CROWBAR PROTECTION CIRCUIT Figure 22 Typical Crowbar Overvoltage Protection Circuit When the output voltage exceeds the reference the comparison amplifier triggers the blocking oscillator which then sends firing pulses to the SCR When the SCR fires it places a very low impedance across the output reducing the voltage to near zero Several beneficial features are included in most Agilent crowbar circuits 1 Anovervoltage indicator lights when the SCR fires the lamp conducts a holding current to prevent the SCR from oscillating on and off 40 2 The crowbar circuit creates an extra current path during nor
129. on Connection saei enses osa n Sn enean e UST e HS e ene e sank T le Pn e va rare 97 IC ety ek 7 Auto Tracking operation CONNECTIONS ccesececsseecesececesceececeeceseceesceecsaceesaecessaeceeeeeeseeeeaaees 99 LC TALEO WY Be ee ee Rae eod diede tae eats 8 Autotransformer rating for use with regulated 61 B Bipolar power 54 Braided around leads 22 eie n ie ttu Medii doe e etd sas esa 72 C Common point constant current supply 1 enne enne enne enhn enne tenentes 54 Common point do deftmfiOr uuu e e tene ve a 68 for individually grounded multiple 1 66 70 for loads floated at dc potential above ground eee 71 for multiple ungrounded loads eye tti teer aea eh 68 for smgle erounded loads ei en looi qeu soie 69 POL single isolat d load PE 68 Comparison amplifier in constant current 33 in constant voltage supply eei TAIN Un Eo SON NUS ka PRSETER ee Ro eU E NE eS 19 27 gere ic Am 41 in high performance constant current supply esses enne nenne 48 pigey back Supply i55 2 oae eit 41 input voltage limiter ee
130. ors or silicon diodes POWER SUPPLY X 100 OHMS TYPICAL Figure 50 Remote Sensing Protection with Resistors If the diode configuration of Figure 51 is employed operation will be satisfactory up to about 0 5 volts drop in either load lead between a power supply output terminal and the corresponding DC Distribution Terminal for greater drops use two or more diodes in series POWER SUPPLY Figure 51 Remote Sensing Protection with Diodes 76 If the resistor configuration of Figure 50 is included by the manufacturer or added by the user it may be necessary to check that the power rating of this resistor is adequate particularly for sizable sensing drops Remember that the actual dissipation in the remote sensing protection resistors is ED2 R where ED is the IR drop from either power supply output terminal to the corresponding DT and R is the ohmic value of the protective resistor Load Wire Ratings The minimum wire size for the load current leads from the power supply output terminals to the DT s should be determined Most well regulated power supplies have an upper limit to the load current IR drop around which remote sensing may be accomplished without losing proper regulation control This maximum limitation is typically 0 5 1 or 2 volts and may apply to the positive negative or both the positive and negative output leads consult the instruction manual or the manufacturer if in doubt concernin
131. ound System and do not flow through any portion of the power supply dc distribution wiring the effect on system performance is probably negligible To 66 repeat separating the dc distribution circuits from any conductive paths in common with ground currents will in general reduce or eliminate ground loop problems DC SYSTEM CONSISTING OF ALL INTERCON NECTED POWER SUPPLY OUTPUTS THEIR DC DISTRIBUTION WIRING AND ASSOCIATED LOAD CIRCUITS DC COMMON POINT CP ONL Y ONE WIRE OR CONNECTION IS PERMITTED IF GROUND LOOP CURRENTS ARE TO BE KEPT OUT OF DC SYSTEM DC GROUND CONNECTION DC GROUND POINT GP GROUND SYSTEM CONSISTING OFALL CHASSIS FOR POWER SUPPLIES AND THEIR LOAD DEV ICES THEIR GROUND TERMINALS SAFETY GROUND AC GROUND WIRING RACK FRAMES X f ALL EVENTUALLY LEAD TO EARTH GROUND Figure 39 Isolating Ground Loop Paths from DC System The only way to avoid such common paths is to connect the dc distribution system to ground with only one wire Figure 39 illustrates this concept DC and signal currents circulate within the DC System while ground loop currents circulate within the Ground System Providing there is only one connection between the two systems the ground loop currents do not affect the power supply dc output and load circuits Notice that any magnetic coupling between the DC System and Ground System or any capacitive leakage fro
132. output ripple and malfunction of associated instrument 60 Autotransformers An autotransformer or isolation transformer connected between the ac power source and the power supply input terminals should be rated for at least 200 of the maximum rms current required by the power supply Because a power supply input circuit does not draw current continuously the input current wave is not sinusoidal and the peak to rms ratio is generally greater than V2 and can be as high as two or more at full output To avoid autotransformer saturation and consequent limiting of peak input current the autotransformer must have a rating higher than suggested by the power supply s rms input current Failure to follow this precaution may result in the power supply not meeting its specifications at full output voltage and current The autotransformer common terminal should be connected to the acc not ac terminals of both the power supply and the input power line If acc is not connected to the common terminal of the autotransformer the input acc terminal of the power supply will have a higher than normal ac voltage connected to it contributing to a shock hazard and in some cases greater output ripple Line Regulators Most ac input line regulators should not be used with regulated power supplies without first checking with the power supply manufacturer Such regulators tend to increase the impedance of the line in a resonant fashion and can cause
133. oved and using shielded two wire cable the power supply sensing terminals should be connected to the DC Distribution Terminals as shown in Figure 49 Do not use the shield as one of the sensing conductors Although for clarity the diagram shows the load leads as straight lines some immunity against pick up from stray magnetic fields is obtained by twisting each pair of and load leads 74 POWER SUPPLY DT AND CP GP Figure 49 Remote Sensing Connections Typically the sensing current is 10mA or less To insure that the temperature coefficient of the sensing leads will not significantly affect the power supply temperature coefficient and stability specifications it is necessary to keep the IR drop in the sensing conductors less than 20 times the power supply temperature coefficient stated in millivolts C This requirement is easily met using readily available small size shielded two wire cable except in applications involving very long sensing leads of unusually well regulated power supplies with very low TC and stability specifications One end of the shield should be connected to the CP and the other end should be left unconnected In nearly all cases this method of connecting the sensing shield will minimize ripple at the Load Distribution Terminals However in rare cases a different ground return point for this shield is preferable it is important in such cases to experimentally verify that this rel
134. owing chart AWG B amp S Annealed Copper Resistance at Nominal current rating amps WIRE SIZE 20 C milliohms ft 22 16 1 5 20 10 2 7 18 6 39 10 16 4 02 13 14 2 53 20 12 1 59 25 10 0 999 40 8 0 628 55 6 0 395 80 4 0 249 105 2 0 156 140 0 0 0993 195 00 0 0779 260 Single conductor in Free Air at 30 C with rubber or thermoplastic insulation With remote error sensing Figure 47 a feature included on nearly all Agilent power supplies it is possible to connect the feedback amplifier directly to the load terminals so that the regulator performs its function with respect to the output terminals of the power supply Thus the voltage at the power supply output terminals shifts by whatever amount necessary to compensate for the IR drop in the load leads thereby retaining the voltage at the load terminals constant POWER SUPPLY SENSING LEAD SENSING LEAD Figure 47 Regulated Power Supply with Remote Error Sensing Figure 48 shows remote sensing connections to the regulator circuit Remote error sensing simply involves operating the input comparison amplifier Q1 with reference to the load terminals instead of the output terminals of the power supply 73 REFERENCE SUPPLY SENSING LEAD Figure 48 Constant Voltage Regulator with Remote Error Sensing Remote Sensing Connections Connections between the power supply sensing and output terminals should be rem
135. percentage of a supply s input power that it can deliver as useful output Pour Pin ranges typically between 30 and 45 for series regulated supplies With the present need for energy conservation this inefficiency is being scrutinized very closely For many of today s applications the size and weight of linear supplies constitute another disadvantage The power transformer inductors and filter capacitors necessary for operation at the 60Hz line frequency tend to be large and heavy and the heat sink required for the inherently dissipative series regulator increases the overall size The Series Regulated Supply An Operational Amplifier The following analogy views the series regulated power supply as an operational amplifier Considering the power supply as an operational amplifier can often give the user a quick insight into power supply behavior and help in evaluating a power supply for a specific application An operational amplifier Figure 4 is a high gain dc amplifier that employs negative feedback The power supply like an operational amplifier is also a high gain dc amplifier in which degenerative feedback is arranged so the operational gain is the ratio of two resistors Figure 4 Operational Amplifier As shown in Figure 4 the input voltage Eg is connected to the summing point via resistor Rp and the output voltage is fed back to this same summing point through resistor Rp Since the input impedance is very high
136. ppreciably increase the rms value The technique used to measure high frequency noise or spikes on the output of a power supply is more critical than the low frequency ripple and noise measurement technique therefore the former is discussed separately Figure 72A shows an incorrect method of measuring PARD because a continuous ground loop exists as illustrated by the dashed line Any ground current circulating in this loop as a result of the difference in potential EG between the two ground points causes an IR drop which is in series with the scope input This IR drop has a 60Hz line frequency fundamental and is magnified by pickup on the unshielded leads interconnecting the power supply and scope The magnitude of this resulting noise signal can easily be much greater than the true power supply ripple and can completely invalidate the measurement The same ground current and pickup problems can exist if an rms voltmeter is substituted in place of the oscilloscope in Figure 72 However the oscilloscope display unlike the true rms meter reading tells the observer immediately whether the fundamental period of the signal displayed is one half cycle or one full cycle of the ac input Since the fundamental ripple frequency present on the output of an Agilent supply is 2f where is the line frequency due to fullwave rectification an oscilloscope display showing 2f fundamental component is indicative of a clean measurement setup while the
137. presence of a fundamental frequency usually means that an improved setup will result in a more accurate and lower value of measured ripple Figure 72B shows a correct method of measuring the output ripple of a constant voltage power supply using a single ended scope The ground loop path is broken by floating the power supply output Note that to ensure that no difference of potential exists between the supply and the scope it is recommended that they both be plugged into the same ac power buss whenever possible If the same bus cannot be used both ac grounds must be at earth ground potential 105 POWER SUPPLY CASE OSCILLOSCOPE CASE A INCORRECT METHOD POWER SUPPLY CASE OSCILLOSCOPE CASE B CORRECT METHOD USING A SINGLE ENDED SCOPE POWER SUPPLY CASE OSCILLOSCOPE CASE AC AC SHIELDED AGE GND TWO WIRE B GND VERTICAL INPUT C CORRECT METHOD USING A DIFFERENTIAL SCOPE Figure 72 Measurement of PARD Ripple and Noise for a CV Supply Either a twisted pair or preferably a shielded two wire cable should be used to connect the output terminals of the power supply to the vertical input terminals of the scope When using shielded two wire it is essential for the shield to be connected to ground at one end only so that no ground current will flow through this shield preventing induced noise signals in the shielded leads To verify that the oscilloscope is not displaying ripple that
138. primary while SCR s are included in two arms of the bridge rectifier as shown in Figure 7 No matter which type of element is used the basic operating principle of the preregulator circuit is the same During each half cycle of the input the firing duration of the SCR s is controlled so that the bridge rectifier output varies in accordance with the demands imposed by the dc output voltage and current of the supply REFERENCE RECTIFIER amp REGULATOR PREREG SCR s SERIES REGULATOR AC IN VOLTAGE COMPARISON AMPL PREREG CONTROL CIRCUIT Figure 7 Constant Voltage Power Supply with SCR Preregulator The function of the preregulator control circuit is to compute the firing time of the SCR trigger pulse for each 24 half cycle of input ac and hold the voltage drop across the series regulator constant in spite of changes in load current output voltage or input line voltage Figure 8 shows how varying the conduction angle of the SCR s affects the amplitude of the output voltage and current delivered by the SCR bridge rectifier of Figure 7 An earlier firing point results in a greater fraction of halfcycle power from the bridge and a higher dc level across the input filter capacitance For later firing times the dc average is decreased AC INPUT EARLIER FIRING POINT _ LARGE DC LEVEL SCR LATER BRIDGE FIRING POINT OUTPUT 22 SMALL DC LEVEL Figure 8 SCR Condu
139. put current constant in spite of changes in load line temperature etc Thus for a change in load resistance the output current remains constant while the output voltage changes by whatever amount necessary to accomplish this CONSTANT CURRENT POWER SUPPLY OUTPUT CHARACTERISTICS lour CONSTANT VOLTAGE POWER SUPPLY A regulated power supply that acts to maintain its output voltage constant in spite of changes in load line temperature etc Thus for a change in load resistance the output voltage of this type of supply remains constant while the output current changes by whatever amount necessary to accomplish this CONSTANT VOLTAGE POWER SUPPLY OUTPUT CHARACTERISTIC Es CONSTANT VOLTAGE CONSTANT CURRENT CV CC POWER SUPPLY A power supply that acts as a constant voltage source for comparatively large values of load resistance and as a constant current source for comparatively small values of load resistance The automatic crossover or transition between these two modes of operation occurs at a critical or crossover value of load resistance Rc Es Is where Es is the front panel voltage control setting and Is is the front panel current control setting CONSTANT VOLTAGE CONSTANT CURRENT CV CC OUTPUT CHARACTERISTIC CONSTANT VOLTAGE OUTPUT Es 5 CONSTANT RRENT Roy gt OUTPUT Eout CONSTANT VOLTAGE CURRENT LIMITING CV
140. quivalent circuit during DEOPEATDIBIHTS 88 Meas r ment method tcc Le Mo Cose 111 method OEMS Goes asad edet 53 88 of power supply amplifiets 54 Protection circuits switching supplies 43 linear 36 37 60 Py 44 SCR 22 32 E E M ate Re E eae ae ead 18 ks usn P 25 Remote programming BCCI TAC 86 adjustment of eret titt deoa e 87 cables and 8 eee Se dba du ee 8l Saeco uos nd reet na 14 TESIS ANCE COMMON 80 85 Remote seusmbp CONMECH ONS 72 73 ates cae acacia tye cok 15 effect on current limit 71 with external output capacitor ree eid aq odio 77 oscillation WHET Using 77 protection against opem oa o tortus ia 39 Reverse current Loading sis I 93 RFI see EMI Ripple and noise see PARD Rubber
141. r control knob In digital control the value of the remote resistance or voltage is selected by an Agilent Desktop Computer or Minicomputer No matter what method of control is used the basic principles of remote programming are the same and all of the basic drawing and rules throughout this section apply equally to both analog and digital control Additional Information on Digital Interfaces For Power Supplies Although this section does not describe the equipment nor the programming techniques necessary for digital control all of the existing digital interfaces for power supplies are summarized in Agilent s DC Power Supply Catalog In addition specific details on digital control of power supplies on the Agilent Technologies Interface Bus GPIB are given in Application Note 250 1 GPIB Power Supply Interface Guide GPIB is Agilent Technologies implementation of IEEE Standard 488 ANSI Standard MC1 1 CONSTANT VOLTAGE REMOTE PROGRAMMING WITH RESISTANCE CONTROL Using an external resistor and or rheostat the output voltage can be set to some fixed value or made continuously variable over the entire output range or made variable over some narrow span above and below a nominal value Figure 54 illustrates the essential details of resistance programming of a constant voltage power supply Note that this differs from the normal constant voltage circuit in only one respect the circuit points normally connected to the front panel contro
142. ray capacitance in the sensing circuit to form an RC delay network If the output voltage is rising relatively slowly there will be essentially no time delay of types 2 and 3 The SCR will be triggered within a fraction of a microsecond after the relatively slow rising output voltage crosses the trip level and the only noticeable time delay will be the turn on time of the SCR If on the other hand the output voltage waveform approaches a step the sense circuit will not follow the fast rising wavefront and an added time delay type 3 will result For a step that is only a few millivolts above the trip level this time delay can be as much as a few microseconds but as the magnitude of the step increases the sense circuit charges faster and the delay decreases In practice it is unrealistic to specify either the time delay or the maximum overvoltage as shown in Figure 22 because they vary according to operating levels load and line impedances and the exact failure mode Instead Agilent Technologies specifies the overvoltage trip margin which is defined as the minimum crowbar trip setting above the desired operating output voltage to prevent false crowbar tripping 41 TRIP VOLTAGE LEVEL MARGIN OPERATING VOLTAGE DURATION OF OVERVOL TAGE gt Figure 22 Crowbar Response Figure 23 shows typical protection circuits that are used in Agilent switching regulated power supplies Most of thes
143. reregulator Adding a preregulator ahead of the series regulator allows the circuit techniques already developed for low power supplies to be extended readily to medium and higher power designs The preregulator minimizes the power dissipated in the series regulating elements by maintaining the voltage drop across the regulator at a low and constant level This improves efficiency by 10 to 20 while still retaining the excellent regulation and low ripple and noise of a series regulated supply In addition fewer series regulating transistors are required thus 23 minimizing size increases Figure 7 shows an earlier Agilent power supply using SCR s as the preregulating elements Silicon Controlled Rectifiers the semiconductor equivalent of thyratons are rectifiers which remain in a non conductive state even when forward voltage is provided from anode to cathode until a positive trigger pulse is applied to a third terminal the gate Then the SCR fires conducting current with a very low effective resistance it remains conducting after the trigger pulse has been removed until the forward anode voltage is removed or reversed On more recent preregulator designs the SCR s are replaced by a single triac which is a bidirectional device Whenever a gating pulse is received the triac conducts current in a direction that is dependent on the polarity of the voltage across it Triacs are usually connected in series with one side of the input transformer
144. riginates from circuit common via this amplifier bypassing RM only the output current flows through RM In this way leakage current flowing directly between the supply s two output terminals is eliminated and precise load regulation is obtained The Programming Guard Amplifier output may also be used as a convenient point to connect indicating meters since the current to drive these meters will not affect the regulated output current Io The Voltage Limit Circuit is designed to eliminate dangerous high voltage or high current transients that might occur under certain load conditions For example when the load is suddenly removed from an ordinary constant current power supply the output voltage attempts to rise to the raw supply voltage of the instrument which can be hundreds of volts Or when the load is suddenly reconnected to a supply operating in the voltage 48 limit mode a high current transient can occur if the current regulator saturates while the instrument is still in voltage limit The Voltage Limit Circuit in Constant Current Sources virtually eliminates voltage or current overshoots and undershoots when going in and out of voltage limit without adding any significant leakage path across the output terminals Normally when voltage limiting action is not occurring the setting of the Voltage Limit Control establishes across the Shunt Voltage Regulator a preset voltage limit EL which is higher than the positive output voltage an
145. rn on During undervoltage or turn on conditions the low ac input level reduces the voltage and activates the undervoltage detector When activated the modulator pulses are inhibited and the regulator switches turned off Overvoltage Detector Monitors output voltage and turns off regulator switches if output attempts to rise above a preset value Similar to crowbar circuit described previously except that output voltage is removed by turning off regulator rather than by shorting the output Temperature Switch Opens in case of high ambient temperature that could be caused for example by a misapplication or cooling fan failure Opening switch removes and activates ac undervoltage detector Switch closes again after temperature cools to a safe level REGULATOR 0 LIMIT Figure 23 Protection Circuits Switching Type Supply Additional Protection Although not shown on Figure 23 all Agilent switching supplies contain some form of overcurrent protection usually a current foldback circuit Also included are remote sensing protection resistors and input protection components for the comparison amplifier SPECIAL PURPOSE POWER SUPPLIES High Voltage Power Supply Normal series switching or SCR regulation techniques may not be suitable for all semiconductor short circuit proof power supplies with outputs greater than 300 volts For example in a series regulated supply shorting the output terminals would
146. s equal or proportional voltage sharing under all load conditions with complete control of the AutoSeries ensemble being obtained from the master supply alone Figure 67 illustrates the circuit principle involved The slave supply is connected in series with the negative output terminal of the master supply and a voltage divider R1 and R2 is placed across the series voltage span One input of the comparison amplifier of the slave supply is connected to the junction of these two resistors while the other input is connected to the positive output terminal of the slave Since normal feedback action of the slave supply is such as to maintain a zero error between the two comparison amplifier inputs the slave supply will contribute a fraction of the total output voltage determined by the voltage divider R1 and R2 For example if these two resistors are equal the slave supply will contribute half the total output voltage with the master supply contributing the other half Notice that the percent of the total output voltage contributed by each supply is independent of the magnitude of the total voltage When using fixed resistors R1 and R2 the front panel voltage control of the slave supply will be inoperative Turning the voltage control of the master supply will result in a continuous variation of the output of the series combination with the contribution of the master s output voltage to that of the slave s voltage always remaining in the ratio of R1 to R2
147. s incorporated more often into fixed output voltage supplies because full rated output current can be obtained only at the maximum rated output voltage As indicated in Figure 20 reductions in output voltage are accompanied by linear decreases in the available output current Conventional current limiting techniques on the other hand are used mostly in supplies that furnish continuously variable output voltages Note in Figure 19 that maximum rated current can be obtained at any rated output voltage OPEN CONSTANT VOLTAGE OPERATING REGION ERATED TYPICAL CURRENT LOAD Tout FOLDBACK HORT I RATED I CROSSOVER Figure 20 Current Foldback Characteristic PROTECTION CIRCUITS Today many power supply manufacturers are placing increasing emphasis on protection for both the power supply and its load A supply with extensive protection networks is of obvious benefit to the user particularly if used in a crucial application such as a system power source or in the testing of critical components Protection Circuits for Linear Type Supplies The following paragraphs describe protection circuits that are commonly used in Agilent series regulated and series preregulated type supplies Figure 21 A Varistor A voltage dependent resistor that protects preregulator triac against ac line spikes Its resistance decreases abruptly if line voltage exceeds a harmful level 38 B RFI Chok
148. scribed under Precision Constant Current Sources later in this section Figure 16 illustrates the elements constituting a basic constant current power supply using a linear type regulator As mentioned previously many of these elements are identical to those of a constant voltage supply with the only basic difference being in their output sensing techniques While the constant voltage supply monitors the output voltage across its output terminals the constant current supply monitors the output current by sensing the voltage drop across a current monitoring resistor RM connected in series with the load The current feedback loop acts continuously to keep the two inputs to the comparison amplifier equal These inputs are the voltage drop across the front panel current control and the IR drop developed by load current IL flowing through current monitoring resistor RM If the two voltages are momentarily unequal the comparison amplifier output changes the conduction of the series regulator which in turn corrects the load current voltage drop across RM until the error voltage at the comparison amplifier input is reduced to zero Momentary unbalances at the comparison amplifier are caused either by adjustment of current control Ra or by instantaneous output current changes due to external disturbances Whatever the cause the regulator action of the feedback loop will increase or decrease the load current until the change is corrected 33 REFER
149. shed by turning the variable autotransformer of Figure 79 through the specified input voltage range and noting the change in the reading on a digital voltmeter or differential voltmeter connected across the current monitoring resistor this change when divided by the value of the current monitoring resistor yields the change in output current The power supply will perform within its source effect specification at any rated output current combined with any rated output voltage CC Load Effect Load Regulation Definition The change Alour in the steady state value of dc output current due to a change in load resistance from short circuit to a value which yields maximum rated output voltage Load effect is measured by closing or opening the switch in Figure 79 and noting the resulting static change on the digital voltmeter or differential voltmeter connected across the current monitoring resistor The power supply will perform within its load effect specifications at any rated output current combined with any rated line voltage CC PARD Ripple and Noise Definition The residual ac current which is superimposed on the dc output current of a regulated supply PARD may be specified and measured in terms of its rms or preferably peak to peak value The peak to peak voltage measured on the oscilloscope across RM is divided by RM to obtain the peak to peak ripple current For the rms value a true rms voltmeter reading is taken across RM after first
150. speed at least an order of magnitude faster than can be achieved with any standard power supply with reduced output capacitance More details on these versatile supplies are given in the Principles of Operation Section Supplies using SCR preregulator circuits cannot in general be expected to respond as rapidly as shown in Figures 61 and 62 since a change in output voltage must be accompanied by a change in rectifier voltage the large value of the rectifier filter plus protection circuits within the SCR preregulator prevent the rectifier voltage from changing rapidly 90 OUTPUT VOLTAGE AND CURRENT RATINGS DUTY CYCLE LOADING In some applications the load current varies periodically from a minimum to a maximum value At first it might seem that a regulated power supply having a current rating in excess of the average load requirement but less than the peak load value would be adequate for such applications However the current limit or constant current circuit within a semiconductor power supply limits the output current on an instantaneous not an average basis because extremely rapid protection is necessary to provide adequate safeguard against burnout of the regulating elements The first question which must be answered when powering a dc load that draws a large current during some portion of its operating cycle is whether 1 the power supply need only withstand the peak load condition or whether 2 the power supply must continue to
151. stors are improving but remain one of the major problems at high frequencies However further improvements in high speed switching devices such as the new power Field Effect Transistors FETs would make high frequency operation and its associated benefits a certainty for 28 future switching supplies Preregulated Switching Supply Figure 11 shows another higher power switching supply similar to the circuit of Figure 10 except for the addition of a triac preregulator Operation of this preregulator is similar to the previously described circuit of Figure 7 Briefly the dc input voltage to the switches is held relatively constant by a control circuit which issues a phase adjusted firing pulse to the triac once during each half cycle of the input ac The control circuit compares a ramp function to a rectified ac sinewave to compute the proper firing time for the triac Although the addition of preregulator circuitry increases complexity it provides three important benefits First by keeping the input to the switches constant it permits the use of lower voltage more readily available switching transistors The coarse preregulation it provides also allows the main regulator to achieve a finer regulation Finally through the use of slow start circuits the initial conduction of the triac is controlled providing an effective means of limiting inrush current Note that the preregulator triac is essentially a switching device and like the main re
152. strument that has insufficient bandwidth may conceal high frequency spikes detrimental to the load The test setups illustrated in Figures 72A and 72B are generally not acceptable for measuring spikes a differential oscilloscope is necessary Furthermore the measurement concept of Figure 72C must be modified if accurate spike measurement is to be achieved 1 As shown in Figure 74 two coax cables must be substituted for the shielded two wire cable Impedance matching resistors must be included to eliminate standing waves and cable ringing and the capacitors must be connected to block the dc current path 3 The length of the test leads outside the coax is critical and must be kept as short as possible the blocking capacitor and the impedance matching resistor should be connected directly from the inner conductor of the cable to the power supply terminals 4 Notice that the shields of the power supply end of the two coax cables are not connected to the power supply ground since such a connection would give rise to a ground current path through the coax shield resulting in an erroneous measurement 5 Using the setup in Figure 74 the measured noise spike values must be doubled because the impedance matching resistors constitute a 2 to 1 attenuator 50 f TERMINAT ION POWER SUPPLY T TOR OSCILLOSCOPE CASE CONNECTS CASE 0 Oluf OT VERTICAL SoN INPUT AC VERTICAL ACC GND 0 Oluf 500 we T CONNEC TO
153. supplies Although basic switching regulator technology and its advantages have been known for years a lack of the necessary switching transistors rectifier diodes and filter capacitors caused certain performance problems that were costly to minimize As a result these supplies were used only in airborne space or other applications where cost was a secondary consideration to weight and size However the advent of high voltage fast 25 switching power transistors fast recovery diodes and new filter capacitors with lower series resistance and inductance have propelled switching supplies to a position of great prominence in the power supply industry Presently switching supplies still have a strong growth potential and are constantly changing as better components become available and new design techniques emerge Concurrently performance is improving costs are dropping and the power level at which switching supplies are competitive with linear supplies continues to decrease Before continuing with this discussion a look at a basic switching supply circuit will help to explain some of the reasons for its popularity Basic Switching Supply In a switching supply the regulating elements consist of series connected transistors that act as rapidly opened and closed switches Figure 9 The input ac is first converted to unregulated dc which in turn is chopped by the switching element operating at a rapid rate typically 20KHz The resultan
154. t 20KHz pulse train is transformer coupled to an output network which provides final rectification and smoothing of the dc output Regulation is accomplished by control circuits that vary the on off periods duty cycle of the switching elements if the output voltage attempts to chance INPUT SWITCHING OUTPUT RECTIFIER ELEMENTS RECTIFIER FILTER REGULATED DC OUT DUTY CYCLE CONTROL Figure 9 Basic Switching Supply Operating Advantages and Disadvantages Because switching regulators are basically on off devices they avoid the higher power dissipation associated with the rheostat like action of a series regulator The switching transistors dissipate very little power when either saturated on or nonconducting off and most of the power losses occur elsewhere in the supply Efficiencies ranging from 65 to 85 are typical for switching supplies as compared to 30 to 45 efficiencies for linear types With less wasted power switching supplies run at cooler temperatures cost less to operate and have smaller regulator heat sinks Significant size and weight reductions for switching supplies are achieved because of their high switching rate The power transformer inductors and filter capacitors for 20KHz operation are much smaller and lighter than those required for operation at power line frequencies Typically a switching supply is less than one third the size and weight of a comparable series regulated
155. t programming speed What are the techniques for measuring power supply performance In summary this is a book written not for the theorist but for the user attempting to solve both traditional and unusual application problems with regulated power supplies DEFINITIONS AMBIENT TEMPERATURE The temperature of the air immediately surrounding the power supply AUTOMATIC AUTO PARALLEL OPERATION A master slave parallel connection of the outputs of two or more supplies used for obtaining a current output greater than that obtainable from one supply Auto Parallel operation is characterized by one knob control equal current sharing and no internal wiring changes Normally only supplies having the same model number may be connected in Auto Parallel since the units must have the same IR drop across the current monitoring resistors at full current rating AUTO PARALLEL POWER SUPPLY SYSTEM MASTER 1 I2 3Iy AUTOMATIC AUTO SERIES OPERATION A master slave series connection of the outputs of two or more power supplies used for obtaining a voltage greater than that obtainable from one supply Auto Series operation which is permissible up to 300 volts off ground is characterized by one knob control equal or proportional voltage sharing and no internal wiring changes Different model numbers may be connected in Auto Series without restriction provided that each slave is capable of Auto Series operation
156. t voltage is variable and directly proportional to Rp Thus the output voltage becomes zero if Rpis reduced to zero ohms Of course not all series regulated supplies are continuously adjustable down to zero volts Those of the OEM modular type are used in system applications and thus their output voltage adjustment is restricted to a narrow range or slot However operation of these supplies is virtually identical to that just described Pros and Cons of Series Regulated Supplies Series regulated supplies have many advantages and usually provide the simplest most effective means of satisfying high performance and low power requirements In terms of performance series regulated supplies have very precise regulation properties and respond quickly 19 to variations of the line and load Hence their line and load regulation and transient recovery time are superior to supplies using any of the other regulation techniques These supplies also exhibit the lowest ripple and noise are tolerant of ambient temperature changes and with their circuit simplicity have a high reliability Power supply performance specifications are described in the Definitions section of this handbook The major drawback of the series regulated supply is its relatively low efficiency This is caused mainly by the series transistor which operating in the linear mode continuously dissipates power in carrying out its regulation function Efficiency defined as the
157. t voltage to zero ohms As shown on Figure 77 the load resistance RL is included when checking up programming and is removed for 111 downprogramming This is done to present the worst possible conditions for programming in each direction A method for measuring the programming speed of an Agilent power supply is as follows POWER SUPPLY REMOTE OUTPUT PROGRAMMING Q 5 OSCILLOSCOPE R L SHOULD BE REMOVED WHEN CHECKING DOWN PROGRAMMING SPEED TO SIMULATE WORST CASE CONDITIONS MERCURY WETTED RELAY Figure 77 CV Programming Speed Test Setup 1 Restrap the power supply rear barrier strip for remote resistance programming constant voltage The strapping pattern for remote resistance programming of laboratory type power supplies is illustrated in each Agilent Operating and Service Manual 2 Disconnect the output capacitor On most Agilent supplies the output capacitor can be disconnected by simply removing the appropriate straps on the rear barrier strip as illustrated in the Operating and Service Manual A minimum amount of output capacitance is permanently wired to the output and should not be removed to increase the programming speed because the supply could oscillate under certain load conditions The programming speed increases by a factor of from 10 to 100 when the output capacitor is removed 3 Select the value of the programming resistor that will produce maximum output voltage of the supply
158. termining necessary load wire sizes it is usually sufficient to consider only the equivalent lumped constant series resistance and inductance Lo L1 L2 and Ro R1 R2 Given the wire size and length these lumped equivalents can be determined from wire tables and charts In general the power supply performance degradation seen at the load terminals becomes significant whenever the wire size and length result in a load wire impedance comparable to or greater than the equivalent power supply output impedance With one load this degradation can be evaluated by comparing 2Ro with Rs and 2Lo with Ls The total impedance seen by the load is Zr Rs 2Ro Ls 2Lo and the variation of the dc load voltage caused by a sinusoidal variation of load current is Eac Iac ZT If load current variations are more pulse or step shaped than sinusoidal then the resulting load voltage spike will have a magnitude of e Ly di dt where Ly Ls 2Lo and di dt is the maximum rate of change of load current If these calculations indicate that the resulting variations in dc voltage provided to the load are greater than desired then shorter and or larger load leads are required With multiple loads Figure 37B it is necessary to consider separately the common or mutual impedance seen by the loads Rs 2Ro Ls Lo and the added impedance seen by each load individually 2 R1 jo L1 2 R2 j L2 etc Remember that the mutual imp
159. the initial or the final value being programmed This output undershoot increases the time required for the supply to settle to its new value The switching circuit of Figure 56B using a make before break switch eliminates both the overshoot and the undershoot problems associated with Figure 56B When the switch is rotated clockwise the resistance value between the two programming terminals will go directly from 1000 to 2000 ohms and then from 2000 to 3000 82 ohms It appears at first glance that the circuit of Figure 56B also has one drawback namely the output voltage must always be switched in ascending or descending sequence As Figure 56C shows however the same voltage divider can have its tap points returned to the switch contacts in any sequence permitting output voltage values to be programmed in any desired order without overshoot or undershoot 5 lov 2K TO POWER SUPPLY 15 3K PROGRAMMING TERMINALS A UNRECOMMENDED CONFIGURATION IK TO V POWER SUPPLY 2 PROGRAMMING TERMINALS IK 10V 15 RECOMMENDED IN SEQUENCE PROGRAMMING CIRCUIT USE MAKE BEFORE BREAK SWITCH TO POWER SUPPLY PROGRAMMING TERMINALS C RECOMMENDED OUT OF SEQUENCE PROGRAMMING CIRCUIT USE MAKE BEFORE BREAK SWITCH Figure 56 Remote Programming Switching Circuits Backup Protection for Open Programming Source In some applications it is possible for the programming switching circuit to be opened acc
160. the relationship b Ru Rs As this equation suggests Ry is a critical component and is selected to have low noise low temperature coef ficient and low inductance 47 Its ohmic value is large enough to give an adequate current monitoring voltage yet small enough to minimize its temperature rise and the resulting resistance change caused by its own power dissipation VA CIRCUIT COMMON PROGRAMMING OUTPUT GUARD CURRENT AMPLIFIER CONTROL um Eg Rs CASCODE SERIES REGULATOR CURRENT COMPARISON AMPL a VOLTAGE REGULATOR LIMIT CIRCUIT Figure 27 Precision Current Source Block Diagram Returning to the guard duties of the Programming Guard Amplifier the output of this amplifier Ec is connected to a guard conductor which surrounds the positive output terminal the current monitoring resistor and the input to the Current Comparison Amplifier Since Eg is held at the same potential as the positive output terminal by the Main Current Regulator no leakage current flows from the positive output terminals or any of the internal circuit elements connected to it The leakage currents that would normally flow from the positive output circuitry flow instead from the guard conductor whose current is supplied by the Programming Guard Amplifier Notice that since the Programming Guard Amplifier is a low impedance source referenced to C any leakage current fed by the guard o
161. tion can be provided by dc output power supplies e Constant Voltage The output voltage is maintained constant in spite of changes in load line or temperature e Constant Current The output current is maintained constant in spite of changes in load line or temperature e Voltage Limit Same as Constant Voltage except for less precise regulation characteristics e Current Limit Similar to Constant Current except for less precise regulation As explained in this section power supplies are designed to offer these outputs in various combinations for different applications CONSTANT VOLTAGE POWER SUPPLY An ideal constant voltage power supply would have zero output impedance at all frequencies Thus as shown in Figure 1 the voltage would remain perfectly constant in spite of any changes in output current demanded by the load Es now Eout Figure 1 Ideal Constant Voltage Power Supply Output Characteristic 17 A simple unregulated power supply consisting of only a rectifier and filter is not capable of providing a ripple free dc output voltage whose value remains reasonably constant To obtain even a coarse approximation of the ideal output characteristic of Figure 1 some type of control element regulator must be included in the supply Regulating Techniques Most of today s constant voltage power supplies employ one of these four regulating techniques a Series Linear b Preregulator Series Reg
162. to the desired output voltage SERIES REGULATOR COMPARISON AMPL EouT 7 EXTERNAL VOLTAGE SOURCE REPLACES INTERNAL REFERENCE SUPPLY Figure 57 Voltage Programming with Unity Voltage Gain The current required from the voltage source Ep is at most several milliamps Of course this voltage source must be free of ripple and noise and any other undesired imperfections since within the regulator bandwidth the power supply will attempt to reproduce on its output terminals the programming voltage input on a one for one 84 basis Programming with Variable Voltage Gain Figure 58 illustrates the method by which the power supply can be programmed using an external voltage with a voltage gain dependent upon the ratio of Rp to Rp Note that this method is no different from the circuit normally used for constant voltage control of the output except that an external reference the programming voltage source has been substituted for the internal reference On most supplies external terminals are available so that the connections shown in Figure 58 can be accomplished without any internal wiring changes In all Agilent remotely programmable power supplies the summing point S is made available and the configuration of Figure 58 can always be accomplished using the external programming voltage source and external precision wirewound resistors Rp and Rr should not exceed 10K As indicated by the equation in Figure
163. ty of connecting two grounded loads in the same system For other notes on designating the GP refer to DC Ground Point on page 72 Connect the System to the System GP unless one load is already grounded making certain there is only one conductive path between these two points for the entire system This rule also appears on page 72 and is repeated here as a reminder because of the far greater number of possible paths from dc to ground associated with multiple power supply systems The notes on page 72 are fully applicable to multiple power supply systems 79 REMOTE PROGRAMMING Remote programming a feature found on many Agilent power supplies permits control of the regulated output voltage or current by means of a remotely varied resistance or voltage It is generally accomplished by restrapping the supply s rear terminals so that the front panel control is disabled and a remote control device is connected to the supply There are four basic types of remote programming 1 controlling the constant voltage output using a remote resistance 2 controlling the CV output using a remote voltage 3 controlling the constant current output using a remote resistance and 4 controlling the CC output using a remote voltage Notice that the remote resistance or voltage can be controlled by either analog or digital means As used here analog control means that the value of resistance or voltage is selected by means of a switch o
164. ulator c Switching d SCR Series Regulation Series regulated power supplies were introduced many years ago and are still used extensively today They have survived the transition from vacuum tubes to transistors and modern supplies often utilize IC s the latest in power semiconductors and some sophisticated control and protection circuitry The basic design technique which has not changed over the years consists of placing a control element in series with the rectifier and load device Figure 2 shows a simplified schematic of a series regulated supply with the series element depicted as a variable resistor Feedback control circuits continuously monitor the output and adjust the series resistance to maintain a constant output voltage Because the variable resistance of Figure 2 is actually one or more power transistors operating in the linear class A mode supplies with this type of regulator are often called linear power supplies RECTIFIER SERIES AC INPUT FEEDBACK RL POWER CONT ROL OUTPUT TRANSFORMER Figure 2 Basic Series Regulated Supply Notice that the variable resistance element can also be connected in parallel with the load to form a shunt regulator However this type of regulator is seldom used because it must withstand full output voltage under normal operating conditions making it less efficient for most applications 18 Typical Series Regulated Power Supply Figure 3 shows the
165. urrent comparator and latch circuit Next this reference value is compared with a sample of the output current IOUT If the output current equals or exceeds this reference value a current overload condition exists Approximately 5 after a current overload is detected a latch signal is generated to reduce the output current Should the load require a heavy initial current the delay period between overload and latch can be extended up to 2msec by adding an external capacitor 57 Status Feedback Three feedback lines are available to furnish continuous status information to the controller A flag line informs the computer when new voltage programming data is being processed by the DVS Current overload and latch lines are activated if the DVS experiences a current overload or latch condition Digital Current Source DCS The DCS is a bipolar Constant Current Voltage Limiting power source A DCS unit contains most of the same circuit elements that were shown for a DVS unit including interface and isolation circuits storage D A converter and a bi polar output amplifier One difference is of course that a DCS unit has a programmable voltage limit circuit rather than a programmable overcurrent circuit The voltage limit circuit returns two overload status signals to the controller in a manner similar to the DVS overcurrent circuit The DCS also employs guarding techniques that are conceptually identical to those of the Precision Constant
166. wer supplies 20 One of the N 1 DT s determined in accordance with the preceding rule is designated as the 79 CP for the system 21 There must be only one GP per multiple power supply system 79 22 Connect the System CP to the System GP unless one load is already grounded making 79 certain there is only one conductive path between these two points for the entire system AC POWER INPUT CONNECTIONS The ac acc and third wire safety ground continuity should be retained without accidental interchange from ac power outlet to the power supply input terminals Accidental interchanging of ac and safety ground leads may result in the power supply chassis being elevated to an ac potential equal to the line input voltage This could result in a potentially lethal shock hazard if the chassis is not grounded or blown fuses if the chassis is grounded If ac and acc are accidentally interchanged the power supply switches and fuses are thereby placed in series with the ground side of the power line instead of the hot side if the power supply ac line switch is turned off or the fuse opens the hot side of the power line will be connected to exposed components within the power supply Accidental interchanging of acc and ground leads places the chassis at the acc potential giving rise to circulating ground currents flowing through the power supply chassis and other associated ground return paths the result is often excessive power supply
167. will eventually limit the maximum rate of change of the output voltage For example of Figure 54 eventually limits the speed of programming but reduction or elimination of this capacitor would degrade the ripple performance Thus high speed programming applications can involve special circuit considerations which ultimately lead to a distinctly different power supply design 89 E4 INITIAL Egut R Four lt TC Co I RE2 NEW Ei TF FALL TIME R Co LOGel gt 1 ASYMPTOTIC LEVEL 0 EQUIVALENT CIRCUIT l to NEW EguT 5 ACHIEVED FOR Sere L t 4 NEW 1 PROGRAMMED Eout Figure 62 Speed of Response Programming Down Since up programming speed is aided by the conduction of the series regulating transistor while downprogramming normally has no active element aiding in the discharge of the output capacitor laboratory power supplies normally program upward more rapidly than downward In many Agilent laboratory power supplies however a special transistor circuit provides for the more rapid discharge of the output capacitor for downprogramming With this circuit and the unstrapping of the major portion of the output capacitance these laboratory power supplies have up and down programming speeds in the order of 1ms High performance bipolar power supply amplifiers provide ultimate performance in high speed programming operation with programming
168. will remain in constant voltage operation and only deliver that fraction of its rated output current which is necessary to fulfill the total load demand For example if two CV CC power supplies each rated for 10 amperes were connected in parallel across a 15 amp load with one of the supplies set for 30 0 volts and the other supply set for 30 1 volts the 30 1 volt supply would deliver 10 amperes as a constant current source thus dropping its output voltage to 30 0 volts The second supply would continue to act as a constant voltage source delivering 5 amps at the 30 0 volt level AUTO PARALLEL OPERATION Auto Parallel or automatic parallel operation of power supplies permits equal current sharing under all load conditions and allows complete control of the Auto Parallel ensemble utilizing only the controls of the master supply Figure 66 Auto Parallel Operation of Two Supplies Figure 66 illustrates the circuit principle involved The master supply operates in a completely normal fashion and may be set up for either constant voltage or constant current operation as required The slave supply employs its regulator circuit to compare the voltage drop across the current monitoring resistor of the master supply with the voltage drop across the current monitoring resistor of the slave supply and adjusts the condition of the series regulator in the slave supply so that these two IR drops are held equal Therefore with equal values 96
169. y discussed previously is ideal for these purposes since it allows the user to select the maximum safe current or voltage for the particular load device Overvoltage Crowbar The crowbar circuit is connected across the output terminals and provides protection against any output overvoltage condition which might occur because of operator error or failure within the power supply or load Most Agilent power supplies of this type have either a built in or optional crowbar circuit Because of its importance in many applications the crowbar circuit is discussed in more depth on the following pages Overvoltage Crowbar Circuit Details An operator error or a component failure within the regulating feedback loop can drive a power supply s output voltage to many times its preset value The function of the crowbar circuit is to protect the load against this 39 possibility The circuit insures that the power supply voltage across the load will never exceed a preset limit This protection is valuable because of the extreme voltage sensitivity of present day semiconductor devices The basic elements used in most crowbars are some method of sensing the output voltage an SCR that will short the output and a circuit that will reliably trigger the SCR within a time period that is short enough to avoid damage to the load The sense circuit can be a simple bridge or voltage divider network that compares the output voltage to some internal crowbar reference v
170. y in an Auto Tracking system must be the positive supply having the largest output voltage Auto Series addition of still more slaves permits the expansion of an Auto Tracking system to both positive and negative power supplies Figure 68 Auto Tracking of Two Supplies Like Auto Series operation Auto Tracking permits simultaneous turn on and turn off of power supplies in the same system thereby preventing accidental application or removal of main power sources without proper bias potentials being present CONVERTING A CONSTANT VOLTAGE POWER SUPPLY TO CONSTANT CURRENT OUTPUT Many but not all Agilent power supplies are capable of constant current operation Those which are not designed for normal operation as a constant current source can readily be converted provided the supply has remote programming capability 99 As Figure 69 indicates it is only necessary to add a single external current monitoring resistor to a remote programming constant voltage power supply in order to convert it to constant current operation Also any remote sensing protection resistor or diode connected inside the supply from S to OUT must be removed Because the proper operation of the regulator circuitry requires that the positive output and positive sensing terminals be at nearly the same potential the external current monitoring resistor RM must be connected to the positive output terminal while the constant current load must be connected

Agilent Technologies 90B User's Manual

Contents

Download Pdf Manuals

Related Search

Related Contents