ECE 4501 Lecture 11: Rectifiers, Switches and P ower Supplies

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

Isolation

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

Half-Wave Rectifier One Diode Only Forward Current - Positive Average (V pk /PI) Vavg approximately (V o – V diode )/PI

Full-Wave Rectifier 2 Diodes -Reverse Current Commutated Center-Tapped Transformer - Isolation allows change of grounding point Vavg approx. 2(V o -V diode )/PI

Bridge Rectifier 4 Diodes - No Need for Center-Tapped Transformer Vavg approx. 2(V o - 2V diode )/ PI

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

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

Conventional Power Supply

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

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

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

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

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

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

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

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

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

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

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

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

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