1. Rectifiers, Switches and Power Supplies
EE 4501
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