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Overview of Power Semiconductor Switches

Overview of Power Semiconductor Switches. Presently available power semiconductor switches can be divided into three groups according to their degree of controllability: Diodes : ON and OFF states controlled by power circuits

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Overview of Power Semiconductor Switches

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  1. Overview of Power Semiconductor Switches Presently available power semiconductor switches can be divided into three groups according to their degree of controllability: • Diodes:ON and OFF states controlled by power circuits • Thyristors:latched on by a control signal but turned OFF by the power circuit • Controllable switches:turned ON and OFF by control signals

  2. Diodes • On and off states controlled by the power circuit • Forward biased – conduction • Reverse biased – small leakage current flow until break down voltage reached

  3. Thyristors • Semi-controlled device • Latches ON by a gate-current pulse if forward biased • Turns-off if current tries to reverse

  4. Thyristor in a Simple Circuit • For successful turn-off, reverse voltage required

  5. Generic Switch Symbol • Idealized switch symbol • When on, current can flow only in the direction of the arrow • Instantaneous switching from one state to the other • Conduct large current with zero voltage drop in on-state • Block large forward and reverse voltages with zero current flow when off • Infinite voltage and current handling capabilities

  6. Bipolar Junction Transistors (BJT) • BJT is a current-controlled device • A sufficiently large base current will turn the device ON • Base current must be supplied continuously to keep it in the ON state • Used commonly in the past • Now used in specific applications, replaced by MOSFETs and IGBTs

  7. Various Configurations of BJTs • dc gain is in the order of 5-10 of one BJT • To achieve larger current gain, these devices are sometimes connected in the above configurations.

  8. MOSFETs • MOSFET is a voltage-controlled device • Easy to control by the gate – continuous application of vGS required to keep the device in the ON state • Faster switching speed (in the nanosecond range) than BJTs • Switching loss is lower compared to BJTs

  9. Gate-Turn-Off Thyristors (GTO) • GTO as an ON/OFF switch • Once forward biased GTO can be turned ON by a gate pulse • GTO will stay ON • However, can be turned off by applying a negative gate-cathode voltage • Used at very high power levels • Require elaborate gate control circuitry

  10. IGBT • High impedance gate – requires small amount of energy to switch the device • Current rating: ~1700 A • Voltage rating: 2~3 kV

  11. Comparison of Controllable Switches

  12. Review of Basic Electrical and Magnetic Circuit Concepts

  13. Sinusoidal Steady State

  14. Three-Phase Circuit

  15. Steady State in Power Electronics • Voltage produced by an inverter in an ac motor drive • Often line currents drawn from the utility by the power electronic circuits are highly distorted as shown in b

  16. Fourier Analysis

  17. Phasor Representation

  18. Response of L and C

  19. Inductor Voltage and Current in Steady State In steady-state, the average inductor voltage (over one time period) must be zero.

  20. Capacitor Voltage and Current in Steady State In steady-state, the average capacitor current (over one time period) must be zero.

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