An Example of Intelligent Power-Supply Design
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1. An Example of Intelligent Power Supply Design By Keith Curtis Principal Applications Engineer Microchip Technology Chandler Ariz A low pin count microcontroller enhances the features and functionality of a switching regulator allows designers to implement new microcontroller to add a number of novel programmable programmable features previously features unavailable from stand alone control One such feature is an adaptive current limit control ler chips M any of these features can be demonstrated with that lets the designer specify separate current limit values the design of a 24 V to 5 V dc dc buck regulator This for different phases of the power supply s operation It s design may seem generic because it has many of the stan possible to program an elevated current limit at startup sing a microcontroller to manage the dard features common to a switching power supply operation of a switching power supply design However this design exploits the power of the 25 V C6 0 1 uF yo 1 a lt p DaN 0 SA oe Slk 1001 Uk 24 V supply 18 U3 5 ply a ic 0 Ah e w e1 C227 G23 1 8050040 L1 1 nF 1 nF 0 1 uF 04 uF ut C16 C17 C18 C19 E PCV 2 974 10 5 V out ie hoo hioo hoo a uF 8080040 T495x10 M016AS CT 1 gbAT54s CT321 100 2 P8 Trigger supply wok i o Power good Temperature 2 Overcurrent a zE GND p1 aa Power good CEEDED 33k ve ay nam emperature tet tetirah T e2. Ss ae 3 k D1 EA E A GS lt x Overcurrent 3 3 k 8 8 ICSPCLK Rn Ean YY Ba i PICkit 1 Flash programming connector HDR1x14 HDR1x14 HDR1 x14 Fig 1 Hardware schematic of the power supply Power Electronics Technology September 2004 www powerelectronics com for capacitive loads a reduced current limit for high tem perature operation and thenormal operating current limit Another featureis programmable output voltage which lets the designer set the output voltage through program ming rather than a manual adjustment A calibration op tion also allows the designer to do an automated setting of the output voltage by simply connecting a reference volt ageto the output and powering up thesupply with ajumper in place Also an output voltage ramp up lets the designer specify the speed at which the output voltage rises On power up Using the microcontroller allows designersto implement supervisory features such as a programmable undervoltage lockout that makes the minimum supply voltage require ments a programmed value rather than a manual adjust ment Another possibility is a trigger voltage input which lets designers coordinate the startup of the power supply with other suppliesin thesystem Other options arean out put overcurrent detect that doesn t use a high side current sense an automatic power up restart in the event of a seri ous fault and aretry counter to limit restarts M ost of the features li
3. microcontrollers for handling fault conditions H ardware reliability involves two concerns the behavior of the microcontroller at power up power down and its responseto EM I RFI Software concerns are limited to detecting and correcting fault condi tions that arise from corrupted data and or programming memory To manage the behavior of the microcontroller during power up and power down conditions two systems are built into the microcontroller hardware the power on reset POR circuit and the brownout reset BOR circuit The POR circuit holds the microcontroller in reset during power up until the supply voltage for the microcontroller reaches the mini mum safe operating voltage and the main oscillator has stabilized The BOR operates during power down generating and holding a reset when the microcontroller power sup ply voltage drops below the user de fined minimum supply voltage To gether these two systems combine to hold the microcontroller in reset whenever the power supply for the microcontroller is insufficient for stable operation of the device M anaging the microcontroller s www powerelectronics com POWER SUPPLY DESIGN response to EM and RFI is a matter of combining proper static protection for the device proper selection of by pass capacitors and good layout Good software design is important because EM I and RFI events can corrupt data and program memory Although safe guards are impl
4. e combination of the UC3847 s easy access to internal signals and the general controllability of the device made it a good candidate for the power chain controller The choice of a microcontroller also was influenced by the features required for the design Two PWM outputs were needed One output would require a minimum of www powerelectronics com POWER SUPPLY DESIGN Fig 2 Power supply test hardware 10 bits of resolution to generate the reference voltage in put to the power chain and a second 6 bit to 8 bit resolu tion PWM was needed for controlling the current limit input to the UC 3847 Other requirements included a multiple input 10 bit analog to digital converter for monitoring the various lev els in the power chain and an EEPROM memory for stor ing configuration constants and the output voltage set point Low current operation also would besought so the microcontroller could be powered from the reference volt age in the U C3847 Because of the microcontroller s scal ing of the reference voltage it would be necessary for the PWM output driver to be powered by the reference Also the ability to reprogram and configure the microcontroller required that the device have Flash program memory Themicrocontroller selected to fit the requirements was aM icrochip PIC 16F 684 In addition to meeting the design requirements its small size 14 pins low cost and the avail ability of low cost development tools recommend
5. e inactive until the microcontroller exits reset following power up These design precautions arereasonable and Wanna Discover a Secret Yokogawa s SignalExplorer prudent for any design that uses a microcontroller with in circuit programming M any features associated with the power supply are implemented in software This is advantageous In that changes and up grades arejust a matter of reprogram ming The disadvantage is that the software design becomes more com plex requiring the ability to handle multiple simultaneous operations Handling simultaneous operation or multitasking is accomplished by building the software using state ma chine constructs M uch like hardware state machines software state ma chines usea counter to sequencetheir Operation Based on the value or state of the counter the state machine ex ecutes different software functions The current state and external inputs Output voltage and ScopeCorder families of aes digital oscilloscopes may be a 7 C le the best kept secret in test T B amp measurement However it ee becomes less of a secret every year as more engineers discover the unique capabilities that Yokogawa s scopes offer Request a two week no commitment evaluation of one of our scopes and discover for yourself the secret to making your most complicated measurements easier KEY FEATURES of the SignalExplorer and ScopeCorder Families E Lots of C
6. ed it for the design Once the main control devices were selected the hard ware design was simply a matter of interfacing the PIC16F684 to the UC3847 and filling in the required peripheral components The U C3847 and its power chain are based on a design example from its datasheet The PIC16F684 s programming interface is taken from the PICkit 1 Flash Starter Kit user s manual Fig 1 shows the final schematic for the power supply and Fig 2 shows the final development board A couple of points in the design should be highlighted The power supply connection to the microcontroller iS connected through jumper J6 This is done because the power supply for the microcontroller must be isolated during programming so the programmer can properly reset the part The bypass capacitance on the microcon Power Electronics Technology September 2004 POWER SUPPLY DESIGN Cal jumper Vsupply Vtrigger Temperature Verror amp Vout Fig 3 Software control and dataflow diagram troller supply must also be below 0 5 uF for proper programming Also noteworthy is the combina tion of an isolation diode and a pull up resistor on pin RA3 of the microcontroller These components prevent the leakage of the 12 V pro gramming voltage into the 5 V sup ply during programming Further more all control pins on the UC3847 that are driven by the microcontroller are biased to their inactive states with a large value resistor to keep the de vic
7. emented in software they shouldn t be taken as a license to neglect proper hardware protection Two important software safeguards have been incorporated into this de sign to prevent unpredictable behav ior by the software The watchdog timer WDT is enabled and set for a time out that is slightly longer than the worst case timing for the main loop in the program In addition a clear WDT instruction exists in only one place and that isin the main soft ware loop This protects the system from lockup conditions in the soft ware by creating a means for the soft ware to recover from a fault serious enough to have corrupted the pro gram counter in the microcontroller Another safeguard is the inclusion of a default state in all of the state de coding in the state machines The de fault state is to be called whenever the state variable doesn t decodeto a valid state Within the default state is a rou tineto reset all of thecritical state ma chine data variables including the state counter variable Together the hardware and soft ware systems help the microcontroller recover from unpredictable behavior Careful consideration of their use should be part of any design incor porating microcontroller manage ment of a power supply system Those interested in building the power supply design presented within the text of this article may visit http techtrain microchip com downloads pet_0904 zip for a zipped file with source
8. hannels Both Analog and Logic E 1 C CAN and SPI Triggering Analysis E Full size and Portable Scopes E Long Memory E Power Analysis E Easy PC Interface To discover more about Yokogawa s scopes contact your local Yokogawa representative or go to promo us yokogawa com and enter key code AD9409 Power Analyzers Digital Oscilloscopes Data Acquisition 800 447 9656 yokogawa com im YOKOGAWA lt gt QRCLE 225 on Reader Service Card or freeproductinfo net pet Power Electronics Technology September 2004 are then combined to determine the next state to be executed Multitasking is done by building separate state machines for the vari ous functions that operate simulta neously and calling them one after the other inside an infinite loop This system allows each state machine to execute one state each time through the loop The overall effect is that all state machines appear to be operat ing simultaneously although slower than if executed individually For this power supply controller design four state machines were writ ten Monitor _sm Control _sm Crnt_Imt_sm and Out_volt_sm Control_sm coordinates the activ ity of the other state machines and sets the overall operational mood of the system M onitor_sm performs all of the analog to digital conversion for the system monitoring the supply output error amplifier and trigger voltages as well as the temperature of the powe
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10. listings a hex file gerber files and a bill of materials PETech Author s Note Thank you to Datatronics and Wirebenders for supplying the mag netic components in the design For more information on this article CIRCLE 334 on Reader Service Card Intelligent MotionControl Enlarged View 4x MC73110 Brushless MotorControllenC Ultra high performance e Low cost Sinusoidal or 6 step commutation Internal profile generation High speed index capture 6 signal PWM with shoot through protection e Digital current loop Velocity loop The Best Engineered Products in Motion a gt F M PERFORMANCE MOTIONDEVICES www pmdcorp com CIRCLE 228 on Reader Service Card or freeproductinfo net pet Power Electronics Technology September 2004
11. olt_sm operates in much the same way as Crnt_Imt_sm with Vsupply good present Vtrigger good Current OK Cool Hi temperature Fig 4 Control _sm state transition diagram Shutdown Fig 5 Crnt_Imt_sm state transition diagram afew exceptions It doesn t contain the timer monitoring or pulse generation sections and it uses three states not four Shutdown Ramp and Normal Shutdown and Normal behave like the Shutdown and Normal states in Crnt_Imt_sm driving the output voltage PWM to zero for Shutdown and to the preprogrammed setpoint during Normal operation The new state is Ramp in which Out_volt_sm ramps up the output voltage over time using increment and delay values set by the designer Once the ramp up of the output is complete Control sm is notified and the mood of the system and its state machines changeto Normal Fig 6 diagrams Out_volt_sm When combined the four state machines create a single management system that monitors and controlsthe operation of the power supply re www powerelectronics com Shutdown Fig 6 Out_volt_sm state transition diagram sponding to both internal and exter nal events based on configuration val ues stored in the microcontroller s EEPROM memory Another issue in power supply de sign is reliability With the inclusion of a managing microcontroller addi tional mechanisms must exist within the hardware and software design of the
12. r MOSFET Crnt_Imt_sm controls the loop timing for the system and generates the current limit control voltage Finally Out_volt_sm controls the output voltage of the power supply and gen erates the ramp up function on power up Fig 3 shows how the control and data flows through the state machines Monitor sm is a simple state machine that continuously cycles through the various analog signals converting each into a 10 bit number for storage in the measured array The sequence is supply voltage trig ger voltage transistor temperature error amplifier and output voltage beforelooping back to the supply volt age The exception to the sequence is for the calibration mode When cali brating Monitor _sm repeatedly converts only the output voltage sig nal as the power chain is inactive Control_sm directs the activity of all the other state machines through its state variable mood The state machine s states correspond to the six moods of the system Based on the value of the mood state variable the system s other state machines alter their states accordingly to coordinate www powerelectronics com No T better tantalt Capacitors Inductors NEC TOKIN e Tantalum Capacitors NEC TOKIN SMD Chip Inductors s innovation performance product quality and reliability NEC TOKIN is the first and the best We introduced the world s first tantalum capacitors in 1955 We led the way in innovat
13. r an overtemperature condition Cal is active in response to a re quest for calibration But notethat Cal iSa dead end state requiring a power down to exit Fig 4 shows the states of Control_sm and the possible state transitions with their causes The circles denote the specific state with the arrow denoting a transition The tag on each arrow notes the specific condition causing the transition Crnt_Imt_sm performs three functions It synchronizes the start of each pass through the loop by delay ing until the hardware timer Timer0 rolls over from FF to 00 Then Crnt_Imt_sm generates the current limit PWM output pulse before mov ing on to execute the current state The Crnt_Imt_sm has four pos siblestates Shutdown Boost Normal and Reduce The Shutdown state holds the current limit PWM output at a duty cycle of zero forcing the U C3847 to shut down the output of the power supply The Boost state in creases the current limit duty cycle during the ramp up of the output voltage The Normal state sets the current limit duty cycleto the steady state current limit Reducereducesthe current limit duty cycle in response to a high temperature condition in the power chain transistors The transitions from state to state in the Cnrt_Imt_sm state machine occur in response to changes in the systems as indicated by the mood state variable Fig 5 shows the transi tion diagrams for the Crnt_Imt_sm state machine Out_v
14. sted aren t new to power supply design H owever the designer s ability to program and con figure the features as well as specify complex sequences for the startup and fault recovery is new Design Overview Once specifications for the buck regulator were defined the next step was to design hardware to support them For clarity the design was broken into two sections the power chain and the microcontroller The power chain was based on Texas Instruments U C3847 chip Not a new device the U C3847 provided ac cess to several internal signals required for the features de scribed above Specifically it allowed access to the Vref in put of the error amplifier The microcontroller had to be able to control the out put voltage of the power chain Access to the Vref input of theerror amplifier was needed so the microcontroller could inject its own variable reference voltage in place of the U C3847 s reference Access to the UC3847 s voltage refer ence also provided a convenient supply voltage for the microcontroller and gavethe microcontroller s variable ref erence voltage the same stability as the original reference Another feature of the UC3847 an external current limit input allowed the microcontroller to set the limit for the power chain The U C3847 also offered access to the output of the error amplifier Measuring this voltage al lowed the microcontroller to determine the average out put current of the power chain Th
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