Copyright
©
2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
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2
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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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2
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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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2
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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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2
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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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2
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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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2
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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
2
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8
2
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8
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
2
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9
2
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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
Copyright
©
2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2
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2
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10
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
2
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2
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Comparison of Controllable Switches
Copyright
©
2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2
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12
2
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12
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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2
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Fourier Analysis
Copyright
©
2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
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2
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Phasor Representation
Copyright
©
2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
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2
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18
Response of
L
and
C
Copyright
©
2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
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2
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19
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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2
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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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