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

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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Diodes

•
On and off states controlled by the power circuit

•
Forward biased

–

conduction

•
Reverse biased

–

small leakage current flow until
break down voltage reached

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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Thyristors

•
Semi
-
controlled device

•
Latches ON

by a gate
-
current pulse
if forward biased

•
Turns
-
off if

current tries to reverse

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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Thyristor in a Simple Circuit

•
For successful turn
-
off, reverse voltage required

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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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 Symbol

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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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

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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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.

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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MOSFETs

•
MOSFET is a voltage
-
controlled device

•
Easy to control by the gate
–

continuous application of
v
GS

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

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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2
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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

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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IGBT

•
High impedance gate
–

requires small
amount of energy to switch the device

•
Current rating: ~1700 A

•
Voltage rating: 2~3 kV

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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Comparison of Controllable Switches

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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2
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Review of Basic Electrical and Magnetic
Circuit Concepts

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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2
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Sinusoidal Steady State

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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2
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Three
-
Phase Circuit

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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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


Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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Fourier Analysis

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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Phasor Representation

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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Response of
L

and
C

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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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
.

Copyright
©
2003

by John Wiley & Sons, Inc.

Chapter 2 Power Semiconductor
Switches: An Overview

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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
.