Chapter 2 Overview of Power
Semiconductor Devices
• Introduction
• Diodes
• Thyristors
• Ideal switches
• Bipolar transistors
• MOSFETs
• MOS-Controlled Thyristors
• Comparison of controllable switches
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Switches: An Overview
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Switches: An Overview
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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
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Switches: An Overview
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Thyristor in a Simple Circuit
•
For successful turn-off, reverse voltage required
for an interval greater than the turn-off interval
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Switches: An Overview
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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
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Switches: An Overview
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Switching Characteristics (linearized)
dW (t )
 p(t )  v i
dt
W   v i dt
1
W  Vd I otc
2
Switching Power Loss is
proportional to:
• switching frequency
• turn-on and turn-off times
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Switch: ideal characteristics
• Small leakage current in off state
• Small on state voltage
• Short turn-on and turn-off
• Large forward and reverse voltage
• High on state current rating
• Positive temperature coefficient
• Small control power required to switch
• Capability to survive rated voltage and rated current simultaneously
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Bipolar Junction Transistors (BJT) #1
• Used commonly in the past
• Now used in specific applications
• Replaced by MOSFETs and IGBTs
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Bipolar Junction Transistors (BJT) #2
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Bipolar Junction Transistors (BJT) #3
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Various Configurations of BJTs
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Conclusions BJTs
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MOSFETs #1
• Easy to control by the gate
• Optimal for low-voltage operation at high switching frequencies
• On-state resistance a concern at higher voltage ratings
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MOSFETs #2
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IGBT
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Conclusions IGBT
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Gate-Turn-Off Thyristors (GTO)
• Slow switching speeds
• Used at very high power levels
• Require elaborate gate control circuitry
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MOS CONTROLLED THYRISTORS (MCT)
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Comparison of Controllable Switches
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Summary of Device Capabilities
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