ECE 4501

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# ECE 4501 - PowerPoint PPT Presentation

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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Presentation Transcript

### 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
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
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
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