Transcript EUT 1040
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