Overview of Power Semiconductor Switches

Copyright © 2003 by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor Switches: An Overview

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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– Thyristors: latched on by a control signal but turned OFF

by the power circuit– Controllable switches: turned ON and OFF by control

signals



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DiodesDiodes

• On and off states controlled by the power circuit• Forward biased – conduction• Reverse biased – small leakage current flow until

break down voltage reached



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ThyristorsThyristors

• Semi-controlled device

• Latches ON by a gate-current pulse if forward biased

• Turns-off if current tries to reverse



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Thyristor in a Simple CircuitThyristor in a Simple Circuit

• For successful turn-off, reverse voltage required



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• 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

Generic Switch SymbolGeneric Switch Symbol



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Bipolar Junction Transistors (BJT)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



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Various Configurations of BJTsVarious 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.



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MOSFETsMOSFETs

• 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



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Gate-Turn-Off Thyristors (GTO)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



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IGBTIGBT

• High impedance gate – requires small amount of energy to switch the device

• Current rating: ~1700 A

• Voltage rating: 2~3 kV



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Comparison of Controllable SwitchesComparison of Controllable Switches



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Review of Basic Electrical and Magnetic Review of Basic Electrical and Magnetic Circuit ConceptsCircuit Concepts



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Sinusoidal Steady StateSinusoidal Steady State



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Three-Phase Circuit



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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



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Fourier Analysis



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Phasor Representation



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Response of L and C



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Inductor Voltage and Current in Steady State

In steady-state, the average inductor voltage (over one time

period) must be zero.



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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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Overview of Power Semiconductor Switches