ECE 4501

1. ECE 4501 Lecture 11: Rectifiers, Switches and Power Supplies

2. Transformers and Isolation • Primary and Secondary connected only through magnetic circuit (Electrically Isolated) • Implies that Grounding Point of Primary Need Not Be Coordinated with that of Secondary

3. Isolation

4. Diode - AC Performance • Vd = 0.6 V for Forward Current • Open Circuit for Reverse Current • Reverse Recovery Characteristic – a measure of the time it takes to ‘turn off’ the current during trnasition from forward bias to reverse bias

5. Half-Wave Rectifier • One Diode • Only Forward Current - Positive Average (Vpk/PI) Vavg approximately (Vo – Vdiode)/PI

6. Full-Wave Rectifier • 2 Diodes -Reverse Current Commutated • Center-Tapped Transformer - Isolation allows change of grounding point Vavg approx. 2(Vo-Vdiode)/PI

7. Bridge Rectifier • 4 Diodes - No Need for Center-Tapped Transformer Vavg approx. 2(Vo - 2Vdiode)/PI

8. Ripple Current Filter • Use Capacitor to Minimize “AC Ripple” • Ic = C dV/dt

9. Conventional Power Supplies • Basic Features of Power Supply (AC to DC): • Rectifier Circuit -Transformer & Diode Bridge & Filter • Overcurrent Protection - Fuse or Breaker • Voltage Regulator - Constant Output Volts Across Current Range • Anti-Reverse - Diode Blocks Reverse Current from Entering Supply • Crowbar - Overvoltage Applied to Terminals Initiates Short-Circuit to Blow Fuse

10. Conventional Power Supply

11. Switch-Mode Power Supplies • Use Power Electronics to “Chop” AC waveform • Used in Modern Computers • Many Other Applications • Compact and Efficient

12. Power Electronics • High Voltage (100’s of Volts) • High Current (10’s of Amps) • High Power Transistors, SCR’s • Power BJT, IGBT • Power MOSFET • Power Diode • Thyristor (Power SCR), GTO

13. High Power DC Switch • Use Power Transistor as a Switch (On/Off) on a Power Circuit • Small Signal (Low power) Controls Large Signal (Like a Relay) • Combine with Inductors and Capacitors for Wave-Shaping

14. Power MOSFETs • Hundreds of Volts • Tens of Amps • Low Gate Voltages • Vgs < +/- 20 Volts (DO NOT EXCEED) • Fairly Fast Switching times (200 nS)

15. DC-DC Chopper • Power Transistor “Chops” High Voltage DC into Low Voltage DC (DC to DC Transformation)

16. Chopper Output Waveforms • Transistor Chops Voltage into Square Wave • Inductor Smoothes Current

17. Biasing Circuit for P-MOSFET Switch • Design Goals: • 5V Logic to turn on/off switch • Want MOSFET in saturation when on (Vgs=10-15V) [Avoid approaching Vgs=+/-20V] • Want to control a 24V circuit • Want to protect Logic Source from Transients

18. Design of Biasing Circuit for MOSFET Switch IMPORTANT: |Vgs| < 20 Volts!

19. Circuit Isolation • IMPORTANT to electrically isolate delicate electronics from power circuits (Pulse Width Modulation motor drives, etc)

20. Relays • Provide Electric Isolation (magnetic circuit) • Provide “electro-mechanical Amplification” • Low Power Signal Controls Large Power Circuit • AC or DC • Not for Repetitive Operations

21. Opto-Couplers • Provide Electric Isolation (Energy Transfer via Photons) • Many Types of Output: BJT, Darlington Pair, SCR, etc

22. Tri-State Drivers (Buffers) • Enable Pin = 0 puts driver in High Impedance State (Open Circuit A to B) • High Input Z, Low Output Z (10 GE output) • Non-Inverting or Inverting

23. References • Heathkit, Electronic Circuits, EB-6104A, 2002 • Alexander, Fundamentals of Circuit Analysis – 2nd Edition, McGraw-Hill, 2004