Chapter 2 Overview of Power
Semiconductor Devices
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-1
Diodes
• On and off states controlled by the power circuit
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-2
Diode Turn-Off
• Fast-recovery diodes have a small reverse-recovery time
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-3
Diode Turn-Off
• Fast-recovery diodes have a small reverse-recovery time
• reverse-recovery current sweeps out the excess carriers
in the diode and allow it to block a negative polarity voltage.
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-4
Diode Turn-Off
•The reverse-recovery current can lead to overvoltages in
inductive circuits.
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-5
Diode Types
• Schottky diodes.
Low forward voltage drop  0.3 V
limlited blocking voltage capabilities 50100V
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-6
Diode Types
• Fast-recovery diodes.
– used in high frequency circuits where small
reverse-recovery time is needed.
– At power level of several hundred volts and
serveral amperes trr  few microseconds.
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-7
Diode Types
• Line-frequency diodes.
– The on-state voltage is low.
– Have larger trr.
– Reverse blocking voltages several kilovolts.
– Current ratings several kiloamperes.
– Can be connected in series and in parallel to
satisfy any voltage and current requirements.
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-8
Diode Types
• Silicon Carbide (SiC).
– Lower swithing losses.
– Forward voltage drop ~ 1.7 V.
– Reverse blocking voltages around 1200V.
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-9
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
2-10
Thyristors
• Usually voltage ratings for forward- and
reverse-blocking voltages are the same .
• Current ratings are specified in terms of maximum
rms and average currents.
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-11
Thyristor in a Simple Circuit
•
For successful turn-off, reverse voltage required
for an interval greater than the turn-off interval
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-12
Types of Thyristors
• Phase-control thyristors.
– used for phase-controlled converters
– Large voltage current and voltage capabilities
– Low on-state voltage drop.
– Available in 4000 A and 5-7 KV capability
with on-state voltage 1.5-3 V.
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-13
Types of Thyristors
• Inverter-grade thyristors.
– Have small turn-off times tq and low on-state
voltage.
– 2550 V and 1500 A.
– tq few microseconds to 100 ms
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-14
Types of Thyristors
• Light-activated thyristors.
– Can be triggered on by a pulse of light guided
by optical fibers to special sensitive region of
the thyristor.
– Used in high voltage applications.
– 4 kV, 3 kA, on-state voltage of 2 V and light
trigger power requirements of 5 mW.
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-15
Transistor analogy
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-16
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
• Zero voltage drop in on-state
• Infinite voltage and current handling capabilities
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-17
Switching Characteristics (linearized)
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-18
Switching Characteristics (linearized)
Switching Power Loss is proportional
to:
•
•
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-19
Switching Characteristics (linearized)
Switching Power Loss is proportional
to:
• switching frequency
•
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-20
Switching Characteristics (linearized)
Switching Power Loss is proportional
to:
• switching frequency
• turn-on and turn-off times
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-21
Concept Quiz #1
In the switching circuit, while the switch is
turning-on, the voltage across it remains
the same as the input voltage:
A. Because of the switch characteristic.
B. Because the diode current is still flowing.
C. Neither.
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-22
Concept Quiz #1
B. Because the diode current is still flowing.
(correct)
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-23
23
Bipolar Junction Transistors (BJT)
• Used commonly in the past
• Now used in specific applications
• Replaced by MOSFETs and IGBTs
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Chapter 2 Power Semiconductor
Switches: An Overview
2-24
Various Configurations of BJTs
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-25
MOSFETs
• Easy to control by the gate
• Optimal for low-voltage operation at high switching frequencies
• On-state resistance a concern at higher voltage ratings
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-26
MOSFETs
D
iD
iD
iD
RDS (on )  1/slope
VGS  11V

9V
7V
VDS
G

VGS
S

(a)
Io
VGS
0
5V
 VGS (th)
VDS
0
VGS (th) VGS ( I o )
(c)
(b)
VGS
RDS ( on )  V
2.5 to 2.7
DSS
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-27
Gate-Turn-Off Thyristors (GTO)
• Slow switching speeds
• Used at very high power levels
• Require elaborate gate control circuitry
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-28
GTO Turn-Off
• Need a turn-off snubber
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-29
IGBT
C
iC

G
iC
VGE

VGE

E
VCE
VCE

(a)
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(b)
Chapter 2 Power Semiconductor
Switches: An Overview
2-30
Power (VA)
108
Thyristor
IGCT
IGBT
MOSFET
106
Thyristor
Choice of Power Transistors
IGCT
IGBT
104
102
MOSFET
(a)
101 102 103 104
Switching Frequency (Hz)
(b)
• MOSFET Figure 15-1 Power semiconductor devices.
• IGBT
• IGCT
• GTO
• Others
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by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-31
31
Comparison of Controllable Switches
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-32
Summary of Device Capabilities
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-33
SELECTION OF POWER TRANSISTORS
AND POWER DIODES
• Voltage Ratings
• Current Ratings
• Switching Speeds
• On-State Voltage Drop
34
Switching in a Power-Pole
iD

VGG
RGG
Vin

iD

vDS

Io
on
Io
off
0
(a)
(b)
Vin
vDS
35
Turn-on Characteristics
D
G


VGG
0
Vin

0
G
vGG
S
vGS
iDiD
D
vDS 
 vDS
S
iD
vGS ( Io )
vGS (th )
iD

Io
on
B
vGG
0
t
Vin
Vin
iD
A

off
I oI 0 0
(a)
idiode  I o  iD
Vin
(b)
vDS
vDS
Io
0
td ( on )
tri
t fv
t
(c)
iD  I o
36
Concept Quiz #1
In a switching power-pole, while the transistor is
turning-on, the voltage across it remains the
same as the input voltage:
A. Because of the transistor characteristic.
B. Because the diode current is still flowing.
C. Neither.
37
Turn-off Characteristic
vGG
iD
G
vGG

Vin

vGS ( Io )
vGS (th )
D

vDS
S
iD
Io
0
on
C
0
D
Io
(a)
off
Vin vDS
0
(b)
vGS
t
Vin
Io
vDS
iD
0
td ( off )
trv
t fi
t
(c)
38
• Concept Quiz #2
In the switching power-pole, while the transistor
is turning-off, the current through it remains
the same as the output current:
A. Because of the transistor characteristic.
B. Because the diode is reverse-biased. (correct)
C. Neither.
• Concept Quiz #2
In the switching power-pole, while the transistor
is turning-off, the current through it remains
the same as the output current:
A.
B. Because the diode is reverse-biased. (correct)
C.
Calculating Power Losses Within the MOSFET
(assuming an ideal diode)
Vin
vDS
Vin
vDS
Io
iD
0
iD
t fv
tri
tc , on
0
t
tc , off
Vin I o
psw
t fi
trv
tc , on
Switching Losses:
Vin I o
psw
tc , off
1
Psw  Vin I o (tc , on  tc ,off ) f s
2
t
tc , on  tri  t fv
tc , off  trv  t fi
41
Conduction Loss:
Pcond  d  R
2
DS ( on ) o
I

42
Summary
• Design of a Switching Power-Pole
– Power Semiconductor Devices
• Diodes
• Transistors
– Losses in Switching Power-Poles
• Switching Losses
• Conduction Losses
43
Clicker Question #1
D
G
iD

vDS

VGG

0
S
iD
Vin

In the circuit shown, Vin  100V and I 0  10 A . During turn-on of
the MOSFET, the drain current has risen to iD  8 A . If the diode
forward voltage drop is VFM  1.0V , what is the value of vDS
across the MOSFET at this time?
A.
B.
C.
100 Volts
101 Volts (correct)
0 Volts
I0
Clicker Question #2
In the circuit shown, the MOSFET has the threshold value of the
gate voltage VGS (th )  3V . The slope of the transfer characteristic,
assumed to be linear, is 2 Amperes / Volt . This circuit has
Vin  100V and I 0  10 A .
During the turn-on of the MOSFET in this circuit, a gate-drive
voltage of VGG  15V is applied. What is the value of the drain
current iD when the gate-source voltage has risen to VGS  11V ?
A.
B.
C.
22 Amperes
16 Amperes
10 Amperes (correct)