PIC Controlled DC/DC Power Converter

1. PIC Controlled DC/DC Power Converter Project By: John Prehn Randall Matrin TA: Tom Houlihan Project #42

2. Project Objective • Create a Switched Mode Power Supply • Accepts and Outputs DC • Suitable for Lab Power Supply • Medium Power Level • Wide Input/Output Range • Excellent Regulation • Local and Remote Control • Onboard Display and Control • Serial interface for Data/Instruction

3. Power Circuit Design Overview • Switched Mode Buck Converter • 500 Watt Max Output • 60-120 Volt Input • ~0-50 Volt, ~0-10 Amp Output • >1% Output Voltage and Current Ripple • >85% Power Efficiency

4. Buck Converter Circuit • Major Components to Design • -Input/Output Capacitors • -Switches • -Inductor

5. PIC Control Circuit Design • Hardware: • dsPIC30F2010 Controller Board – PWM • 30MHz operation, 2xUART @115.2k baud • 75kHz PWM w/ 8-bit resolution -> 0.2% duty cycle increments • PIC18F8722 Controller Board – Peripherals • 10MHz operation, 2xUART @115.2k baud • 500ksps ADC, 5mV accuracy • Software • LabWindows/CVI – RS232 Interface between hardware and software • Installer available for any Windows PC

6. Diagram

7. Passive Component Design - Inductor • Initial Design Based on: • Vin: 60-120 V • Vout: 5-50 V, 1% Ripple • Current: ~0 – 10 Amps, 1% Ripple • Switching Freq: 200 kHz • Inductor: L ≥ (Vin – Vout) · (D / Fsw ) / Iripple L ≥ (120-50) · (.417 / 50k ) / .1 L ≥ 5.83 mH

8. Passive Component Design – Output Capacitor • C ≥ L(Iomax + ΔI/2)^2 / ((ΔV + Vo)^2 – Vo^2) • Cout≥ 5.83mH(10 + .1/2 )^2 / ((.5 + 50)^2 – 50) • Cout≥ 11.725 mF

9. Switch Design • MOSFET • Main Consideration is Heat • Pdis = Vout/(Vinmin)*Iout^2*Rdson • Pdis = (50/60)*10^2*Rdson • Passive Diode • Id = (1-D) · Iload • Id = (1-.04167)*10 = 9.583 A • Pdis = Vf · Id

10. Hardware Choices

11. Gate Driver • MOSFET controlled by Vgs voltage • High side gate driver with bootstrap cap

12. Design Challenge: Gate Capacitance • Early driver – not enough current • Final Choice: FAN7371 (4 Amps)

13. Gate Drive Waveform

14. Design Schematic

15. Verification • Low and Slow Approach • Ensure circuit works at low outputs • Proper Operation • Correct Output Given • Input parameters • Duty Cycle • Ripple within Limits

16. Output Example

17. PIC Reading

18. PIC Design • Controller boards used for easy interfacing • 4-pin connecter on PCB • PIC PWM • 75kHz wave, 8-bit resolution • 8 bits = 256 divisions => 0.19% duty cycle increments • Background operation • RS232 • 115.2k baud, ~10 bytes/instruction • ADC • 500ksps divided between 3 channels

19. General Firmware Routine Run Initializations; while(1) { Check Serial Port; // Checks for new outputs If(New Output Selected) Disable Output; Calculate New Duty Cycle; Enable Output; Wait 50ms; Check Current and Voltage Levels; if(Current Too High) // Overcurrent Protection Disable Output; if(Output Voltage Differs) Closed-Loop Duty Cycle Calculation; // Reiterate until output voltage agrees Send New Data to PC; // Send ADC values to GUI }

20. Graphical User Interface

21. GUI Results • Pros: • Real Time Data results • User Programmable Output Voltage • Restricts levels outside of converter’s range • Printable Results • Cons: • Noisy ADC Ports • Slow throughput on RS232 • Could not show switching function

22. Analysis • More precise duty cycles achievable with faster PIC models • Faster PIC also allows for faster RS232 • More data available on GUI • Can allow for addition of switching function graph • Segregate power circuit and data circuit to reduce noise on PIC’s ADC ports • Cascaded resistor circuit prone to noise

23. Power Circuit Analysis • Work still to be done • Full output • Over-current Protection Finalized • Less noise to PIC • Efficiency • Average power efficiency of 92% • Smaller Components • Less heat, safer

24. Cost Analysis

25. Questions?

26. Appendix50V – 5V, 3x50 load

27. Appendix50V – 40V, 3x50 load, Print Screen

28. Appendix50V – 25V, 8x500 load