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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.

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ECE 4501

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ece 4501

ECE 4501

Lecture 11:

Rectifiers, Switches and Power Supplies

transformers and isolation
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
diode ac performance
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
Half-Wave Rectifier
  • One Diode
  • Only Forward Current - Positive Average (Vpk/PI)

Vavg approximately (Vo – Vdiode)/PI

full wave rectifier
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
Bridge Rectifier
  • 4 Diodes - No Need for Center-Tapped Transformer

Vavg approx. 2(Vo - 2Vdiode)/PI

ripple current filter
Ripple Current Filter
  • Use Capacitor to Minimize “AC Ripple”
  • Ic = C dV/dt
conventional power supplies
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
switch mode power supplies
Switch-Mode Power Supplies
  • Use Power Electronics to “Chop” AC waveform
  • Used in Modern Computers
  • Many Other Applications
  • Compact and Efficient
power electronics
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
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
DC-DC Chopper
  • Power Transistor “Chops” High Voltage DC into Low Voltage DC (DC to DC Transformation)
chopper output waveforms
Chopper Output Waveforms
  • Transistor Chops Voltage into Square Wave
  • Inductor Smoothes Current
biasing circuit for p mosfet switch
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
circuit isolation
Circuit Isolation
  • IMPORTANT to electrically isolate delicate electronics from power circuits (Pulse Width Modulation motor drives, etc)
  • Provide Electric Isolation (magnetic circuit)
  • Provide “electro-mechanical Amplification”
    • Low Power Signal Controls Large Power Circuit
    • AC or DC
    • Not for



opto couplers
  • Provide Electric Isolation (Energy Transfer via Photons)
  • Many Types of Output: BJT, Darlington Pair, SCR, etc
tri state drivers buffers
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
  • Heathkit, Electronic Circuits, EB-6104A, 2002
  • Alexander, Fundamentals of Circuit Analysis – 2nd Edition, McGraw-Hill, 2004