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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
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
diodes
Diodes
  • On and off states controlled by the power circuit
  • Forward biased – conduction
  • Reverse biased – small leakage current flow until break down voltage reached
thyristors
Thyristors
  • Semi-controlled device
  • Latches ON by a gate-current pulse if forward biased
  • Turns-off if current tries to reverse
slide4

Thyristor in a Simple Circuit

  • For successful turn-off, reverse voltage required
generic switch symbol
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
slide6

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
slide7

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.
slide8

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
slide9

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
slide10

IGBT

  • High impedance gate – requires small amount of energy to switch the device
  • Current rating: ~1700 A
  • Voltage rating: 2~3 kV
slide15

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
slide19

Inductor Voltage and Current in Steady State

In steady-state, the average inductor voltage (over one time period) must be zero.

slide20

Capacitor Voltage and Current in Steady State

In steady-state, the average capacitor current (over one time period) must be zero.