U of M ECE 4501 - Rectifiers, Switches and Power Supplies

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ECE 4501Transformers and IsolationIsolationDiode - AC PerformanceHalf-Wave RectifierFull-Wave RectifierBridge RectifierRipple Current FilterConventional Power SuppliesConventional Power SupplySwitch-Mode Power SuppliesPower ElectronicsHigh Power DC SwitchPower MOSFETsDC-DC ChopperChopper Output WaveformsBiasing Circuit for P-MOSFET SwitchDesign of Biasing Circuit for MOSFET SwitchCircuit IsolationRelaysOpto-CouplersTri-State Drivers (Buffers)ReferencesECE 4501Lecture 11:Rectifiers, Switches and Power SuppliesTransformers 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 SecondaryIsolationDiode - 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 biasHalf-Wave Rectifier•One Diode•Only Forward Current - Positive Average (Vpk/PI)Vavg approximately (Vo – Vdiode)/PIFull-Wave Rectifier•2 Diodes -Reverse Current Commutated•Center-Tapped Transformer - Isolation allows change of grounding point Vavg approx. 2(Vo-Vdiode)/PIBridge Rectifier•4 Diodes - No Need for Center-Tapped TransformerVavg approx. 2(Vo - 2Vdiode)/PIRipple Current Filter•Use Capacitor to Minimize “AC Ripple”•Ic = C dV/dtConventional 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 FuseConventional Power SupplySwitch-Mode Power Supplies•Use Power Electronics to “Chop” AC waveform•Used in Modern Computers•Many Other Applications•Compact and EfficientPower 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), GTOHigh 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-ShapingPower 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 CurrentBiasing 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 TransientsDesign of Biasing Circuit for MOSFET SwitchIMPORTANT: |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 OperationsOpto-Couplers•Provide Electric Isolation (Energy Transfer via Photons)•Many Types of Output: BJT, Darlington Pair, SCR, etcTri-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 InvertingReferences•Heathkit, Electronic Circuits, EB-6104A, 2002•Alexander, Fundamentals of Circuit Analysis – 2nd Edition, McGraw-Hill,


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U of M ECE 4501 - Rectifiers, Switches and Power Supplies

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