EE 4501 Rectifiers, Switches and Power 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 (Vpk/PI) Vavg approximately (Vo Vdiode)/PI Full-Wave Rectifier
2 Diodes -Reverse Current Commutated Center-Tapped Transformer - Isolation allows change of grounding point Vavg approx. 2(Vo-Vdiode)/PI Bridge Rectifier 4 Diodes - No Need for Center-Tapped
Transformer Vavg approx. 2(Vo - 2Vdiode)/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 (100s of Volts) High Current (10s of Amps) High Power Transistors, SCRs 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=1015V) [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 2nd Edition, McGraw-Hill, 2004
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