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 ID: 612033
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Slide1
EE 4501
Rectifiers, Switches and P
ower
SuppliesSlide2
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 SecondarySlide3
IsolationSlide4
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 biasSlide5
Half-Wave Rectifier
One Diode
Only Forward Current - Positive Average
(
V
pk
/PI
)
Vavg approximately (Vo – Vdiode)/PISlide6
Full-Wave Rectifier
2 Diodes -Reverse Current Commutated
Center-Tapped Transformer - Isolation allows change of grounding point
Vavg approx. 2(V
o
-V
diode
)/PISlide7
Bridge Rectifier
4 Diodes - No Need for Center-Tapped Transformer
Vavg approx. 2(V
o
- 2V
diode
)/
PISlide8
Ripple Current Filter
Use Capacitor to Minimize “AC Ripple”
Ic = C dV/dtSlide9
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 SupplyCrowbar - Overvoltage Applied to Terminals Initiates Short-Circuit to Blow FuseSlide10
Conventional Power SupplySlide11
Switch-Mode Power Supplies
Use Power Electronics to “Chop” AC waveform
Used in Modern Computers
Many Other Applications
Compact and EfficientSlide12
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), GTOSlide13
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-ShapingSlide14
Power MOSFETs
Hundreds of Volts
Tens of Amps
Low Gate Voltages
Vgs < +/- 20 Volts
(DO NOT EXCEED)
Fairly Fast Switching times (200 nS)Slide15
DC-DC Chopper
Power Transistor “Chops” High Voltage DC into Low Voltage DC (DC to DC Transformation)Slide16
Chopper Output Waveforms
Transistor Chops Voltage into Square Wave
Inductor Smoothes CurrentSlide17
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 TransientsSlide18
Design of Biasing Circuit for MOSFET Switch
IMPORTANT: |Vgs| < 20 Volts!Slide19
Circuit Isolation
IMPORTANT
to electrically isolate delicate electronics from power circuits (Pulse Width Modulation motor drives, etc)Slide20
Relays
Provide Electric Isolation (magnetic circuit)
Provide “electro-mechanical Amplification”
Low Power Signal Controls Large Power Circuit
AC or DC
Not for
Repetitive
OperationsSlide21
Opto-Couplers
Provide Electric Isolation (Energy Transfer via Photons)
Many Types of Output: BJT, Darlington Pair, SCR, etcSlide22
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 InvertingSlide23
References
Heathkit,
Electronic Circuits
, EB-6104A, 2002
Alexander,
Fundamentals of Circuit Analysis – 2
nd
Edition
, McGraw-Hill, 2004