ECE 4501
Lecture 11:
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 (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=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