overview of power semiconductor switches

Copyright © 2003 by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor Switches: An Overview

2-12-1

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



2-22-2

DiodesDiodes

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

break down voltage reached



2-32-3

ThyristorsThyristors

• Semi-controlled device

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

• Turns-off if current tries to reverse



2-42-4

Thyristor in a Simple CircuitThyristor in a Simple Circuit

• For successful turn-off, reverse voltage required



2-52-5

• 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



2-62-6

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



2-72-7

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.



2-82-8

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



2-92-9

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



2-102-10

IGBTIGBT

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

• Current rating: ~1700 A

• Voltage rating: 2~3 kV



2-112-11

Comparison of Controllable SwitchesComparison of Controllable Switches



2-122-12

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



2-132-13

Sinusoidal Steady StateSinusoidal Steady State



2-142-14

Three-Phase Circuit



2-152-15

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



2-162-16

Fourier Analysis



2-172-17

Phasor Representation



2-182-18

Response of L and C



2-192-19

Inductor Voltage and Current in Steady State

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

period) must be zero.



2-202-20

Capacitor Voltage and Current in Steady State

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

overview of power semiconductor switches

Documents