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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
2-1
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
2-2
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
2-3
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
2-4
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
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
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-5
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
• Copyright
Now used
in specific applications,
replaced by MOSFETs and IGBTs
© 2003
Chapter 2 Power Semiconductor
by John Wiley & Sons, Inc.
Switches: An Overview
2-6
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
2-7
MOSFETs
• 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
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-8
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
2-9
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
2-10
Comparison of Controllable Switches
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-11
Review of Basic Electrical and Magnetic
Circuit Concepts
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-12
Sinusoidal Steady State
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-13
Three-Phase Circuit
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-14
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
2-15
Fourier Analysis
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-16
Phasor Representation
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-17
Response of L and C
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-18
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
2-19
Capacitor Voltage and Current in Steady State
In steady-state, the average
capacitor current (over one
time period) must be zero.
Copyright © 2003
by John Wiley & Sons, Inc.
Chapter 2 Power Semiconductor
Switches: An Overview
2-20