appnotes/bias current
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1. RING z 5V N u n SLEEVE TIP Figure 5 Page 8 of 8 WDDAKN SOLENOID ENABLE Cc MICRO SWITCH SHOWN WITH DOOR OPEN SOLENOID RELAY OR f SOn ATE SUPPLY2. Application Note 9901 Transformer Characterisation Ln UUR Using External Low Bias Current wayne Kerr Electronics Limited Introduction There is an increasing demand for telecommunications devices such as mobile phones and modems With this comes an increasing demand on component manufacturers to produce telecomm components The area of interest in this application note is telecomms transformers Transformers used in modems will quite often be used in circuits where a small DC current will pass through the primary winding For transformer manufacturers it is often imperative that their transformers are tested in a way that simulates the intended operating conditions Using the technique described in this application note it is possible to provide an extremely accurate DC bias current which can be used to simulate in circuit operation This application note is written around a worked example and it explains how to make an external DC bias current device The aim is to give an understanding of the external bias design so that it can be modified to suit any low DC bias current application The technique described uses a 3260A Precision Magnetics Analyser The same technique can be used in conjunction with the 3255 Inductance Analyser Figure 1 below shows an example test set up using and external bias current Figure 1 Low bias currents It is common for telecomms transformers to have a small DC current flowing through the pri
3. r Fa Replaced with short for Ful measurements Figure 3 Resistor R1 R1 in the circuit diagram above acts as a V I converter When selecting the value for R1 you should consider the bias current required and the output range of the PSU In this worked example 4k7Q was chosen primarily because it was a readily available value but also because it required a relatively high base drive voltage of 4 7V This meant that to deliver the full range of required current 1 to 2mA the voltage range required was 4 7 9 4V This gave a large margin for fine adjustment of the voltage therefore following the current to be set very accurately Calculations Current required was 1 2mA Voltage range of PSU 0 30V V IRR V D R 47 1 10 R1 4 7 10 O 4k7Q The power rating of R1 needed to be selected to allow for the maximum voltage available from the PSU This is because there is not a limiter on the PSU so the possibility of accidental over voltage needed to be considered The maximum voltage available was 30V Before the power rating could be calculated the current through R1 at 30V needed to be calculated Page 3 of 8 I V R I 30 4 7 10 I 0 0064A 6 4mA With this information the power rating required for R1 could be calculated P VI P 30 6 4 10 Pri 0 192W 192mW A quarter watt resistor was chosen for this application so that the maximum power was always well within the specification of the resistor Ca
4. 1 Vac Internal DC Bias OA off External DC Bias 1 2mA Set by adjusting voltage source Measurement Parameters L Q Series equivalent circuit and Slow measurement speed Measurement Mode Repetitive Range See notes below Range settings This method of generating DC bias forces some restrictions on the ranges and drive levels available These restrictions are indicated in the table below Range Impedance Drive Type Idc max Range 1 lt 1Q I drive 50mA N A 2 lt 10Q I drive 50mA N A 3 lt 50Q I drive 30mA N A 4 gt 500 V drive 30mA lt 0 5V 5 gt 250Q V drive 3mA lt 0 5V Page 7 of 8 Measurements Once the instrument is set up with the DUT in place all of the required inductance measurements can be taken in the usual way Refer to the instrument user manual for more specific measurement instructions If the readings are blanked by the application of the bias current a range error has occurred To overcome this try reducing the drive level until the reading updates Appendix 1 For reasons of safety an interlock should be used in conjunction with the external bias The diagram below shows the diagram as recommended for internal bias protection This can easily be adapted for external bias Ideally the DUT should be housed within a box that has an interlock door For further details please refer to you user manual BOOR MICRO SWITCH N O o SOCKET ON REAR PANNEL
5. 0uF The voltage rating of the capacitors needed to be enough to take the maximum voltage that the PSU could deliver In this application example the upper voltage of the PSU was 30V so the capacitors chosen were rated at 50V this gave ample operational margin Ammeter It is very important to design and build the circuit to allow the ammeter to be connected in series with the DUT This will give the most accurate reading of the current at the DUT It is also important to use a suitably accurate preferably digital ammeter Power Supply The power supply needs to meet certain requirements It must have a stable finely adjustable output If the voltage output fluctuates or does not have fine enough adjustment then the required current at the DUT may be unobtainable This situation will be made worse with an inaccurate ammeter If possible a linear power supply should be used If the only power supply available is switch mode then you need to ensure that the switching frequency will not interfere with the measurement frequency Switches S1 S2 and S3 None of the switches shown in figure 3 are required for the basic operation of the external bias source However S2 and S3 will allow certain operations to be performed that would otherwise require the external bias source to be removed In addition though not required switch S1 is recommended S1 Bias Current On Off Fitting a switch single pole double throw in the position marked S1 i
6. al bias current to be introduced in to the circuit Figure 2 below shows how the measurement leads connect with the external bias DW2 27148 Four way BNC link cable 1EVA40100 Kelvin leads PMA3260A OR DUT Connecting 1A3255 Leads Voltage Source Ammeter Figure 2 Construction Details Parts List 1 off General purpose metal box Choose size to suit requirement 1 off 5kQ Resistor 0 25 Watt Current resistor t 2 off 10uF Capacitor 50V Blocking capacitors for Drive High and Sense High 1 off Single sided strip board Choose size to suit requirement 4 off 4mm sockets 8 off BNC bulkheads Non insulated 4 off Insulating PCB pillars nylon or similar Values are specific to each design Please see the Design Notes section for further information on how to select these components Tolerances not quoted as the absolute value is not important Page 2 of 8 Additional Equipment 1 set of Kelvin leads fine jaw 1EVA40100 Used to connect the external bias unit to the DUT 1 set of Four way BNC link cable DW2 27148 Used to connect the test instrument to the external bias unit 2 sets of leads terminated in 4mm banana plugs Stabilised power supply Farnell LT30 1 used in worked example See notes Ammeter Keithley 2000 Multimeter used in worked example See notes Design Notes Brown Red Orange Yellow Sense Low Drive Low Drive High sense High DLT T oa F
7. in at this stage of assembly Finally fit the lid to the case and connect the bias source as described in the section called Connections Page 6 of 8 Test Procedure Trimming Before making any measurements with the external bias source connected an open and short circuit trim should be performed in the normal way Both trims should be performed as spot frequency trims The reason for performing a spot trim being that an all frequency trim includes a trim at DC The blocking capacitors prevent DC measurements and will cause the trim to fail Also since each bias source will be designed for use over a very narrow or even fixed frequency range it is not necessary to perform an all frequency trim Instrument set up Setting DC bias Connect the ammeter into the circuit as indicated in figure 3 in series with the DUT Adjust the voltage to give the desired current and replace the ammeter with a shorting link Important note If a different type of transformer is to be tested then the voltage should be readjusted to obtain the correct current for the particular DUT Setting measurement conditions Note the following instrument set up only applies for this particular example Each bias box will generally be designed to meet a specific specification Therefore each box will be designed to work under the desired test conditions for the DUT Instrument should be set to Frequency 1kHz AC Drive Level 0
8. mary winding It is important that when transformers are tested the effects of any current that will be present when the device is in circuit are taken into account In some telecomm transformers the effect of the DC current can be quite significant and a very small variation in the DC current a few micro amps up or down can have dramatic effects on the transformers characteristics For this reason transformers that will be used with a DC current flowing though the primary need to be tested under the same conditions The PMA3260A can be used to provide a DC bias current between 1mA and 1A Although the lower current limit is 1mA it is not always possible to deliver low level DC bias current with enough accuracy for all applications However using the technique detailed in this application note it is relatively easy to obtain a level of current accuracy that is adequate for most applications Page 1 of 8 Safety Warning WARNING Back EMF Lethal back EMF potentials can be generated if an inductor under test is disconnected whilst current is still flowing in it This warning is applicable even at the relatively low currents covered in this application note NEVER touch the test connections while the direct current is flowing ALWAYS USE a Safety Interlock see Appendix 1 Safety Interlock Connections The instrument connections to the DUT remain the same The measurement leads however need to be intercepted to allow the extern
9. n figure 3 will allow the PSU output to be disconnected from the circuit and connected directly across R1 This removes the need to switch off the power supply for measurements with no bias current Just switching off the power supply may leave a small residual voltage which could cause measurement errors S2 Internal Bias Switch S2 when closed will short out the blocking capacitor in the drive high line This will allow the internal DC current bias to be used without removing the external bias source IT IS IMPORTANT TO NOTE THAT THE INTERNAL AND EXTERNAL DC CURRENT BIAS SHOULD NEVER BE USED SIMULTANEOUSLY S3 DC Resistance The final optional switch S3 will allow DC resistance measurements It will also allow an all frequency trim to be performed without having to remove the external bias source This switch needs to be used in conjunction with switch S2 Page 5 of 8 Build Instructions Figure 4 below shows a basic sketch of the assembled bias current box 1 2 3 4 5 Inpot Sense Dure Dre Sense Low Low High High b eanmermnt Ternnale Quiput Sense Dorre Drive Jense Low Low High High Figure 4 The general purpose case needs to be modified to accommodate the eight BNC connectors required four inputs and four outputs The BNC connectors should then be fitted in such a way that the outer ground connection makes a permanent contact with the case The case will therefore form the earth termi
10. nal and act as a shielding box The general purpose case also needs to be modified to accommodate four 4mm connectors or similar Two of the connectors will be used for the voltage input and two for the current measurement Mount the connectors either on the side of the case or on the lid The voltage input low terminal should be directly connected to the case as this is a ground connection However all of the other connections for the voltage source and the current measurement should be suitably insulated from the case Also at this stage any of the optional switches that are required should be mounted in the case The external bias circuit should be put together following the circuit sketched in figure 3 The circuit can be built on strip board or similar prototype material If a large number of bias boxes are required then another method of producing the circuit boards may be feasible Once constructed the circuit board needs to be securely fitted in to the case The board should be mounted in the box ideally on pillars in such a way that the circuit cannot short out on the metal case The ground rail s should be connected to the case An ideal way to do this would be to connect the ground rail s to a tag on one of the BNC connectors or the Earth terminal 4mm socket At this point the circuit board is ready to be wired in all connections to the BNC s and 4mm connectors should be made Any of the optional switches required should be wired
11. pacitors C1 and C2 C1 and C2 act as blocking capacitors to prevent the external bias flowing into the instrument The specification of the blocking capacitor C2 is critical C2 will have an effect on the measurement accuracy of the instrument C1 however is less critical and its value and type matches C2 purely for simplicity of design Both capacitors must be non polarised 10uF capacitors were used in this application The details of why these were chosen are shown in the calculations below Calculations The impedance of C2 needed to be relatively low when compared with sense line input impedance which is approximately 4kQ Z of 10uF Capacitor at 1kHz Z 1 2nfC Z 1 2 m 1000 10 10 Z 162 This introduced a negligible magnitude error and a phase error of approximately 0 4 Design Note If the required frequency is higher than IkHz then a 10uF capacitor will still be suitable This is because as the frequency goes up the impedance of the capacitor will reduce therefore reducing the phase error However should the requirement be for a frequency lower than IkHz then another capacitor value should be chosen The impedance should be kept to 16Q or less e g If a frequency of 500Hz is required The target impedance is 16Q So by rearranging the impedance formula in terms of capacitance we can calculate the required capacitor value C 1 2nfZ C 1 2 1 500 16 Page 4 of 8 C 1 50265 48 C 1 989 10 2
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