Power Semiconductor Devices

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A brief survey of power semiconductor devices

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● Power diodes● Power MOSFETs● Insulated Gate Bipolar Transistors (IGBTs)● Thyristors (SCR, GTO)

● On resistance vs. breakdown voltage vs. switching times

● Minority carrier and majority carrier devices

The power diode

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Appearance

Construction Symbol

Reverse-biased power diode

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Forward-biased power diode

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Typical power diode characteristics

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Typical diode switching waveforms

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Types of power diodes

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Standard recoveryReverse recovery time not specified, intended for 50/60Hz

Fast recovery and ultra-fast recoveryReverse recovery time and recovered charge specifiedIntended for converter applications

Schottky diodeA majority carrier deviceEssentially no recovered chargeModel with equilibrium i-v characteristic, in parallel withdepletion region capacitanceRestricted to low voltage (few devices can block 100V or more)

Characteristics of several commercial power rectifier diodes

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The power MOSFET

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Appearance Symbol

n-channel p-channel

The construction of Power MOSFET

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MOSFET: Off state

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MOSFET: On state

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MOSFET body diode

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Typical MOSFET characteristics (1)

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Typical MOSFET characteristics (2)

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Power MOSFET switching waveform

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A simple MOSFET equivalent circuit

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Characteristics of several commercial power MOSFETs

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MOSFET: conclusion

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● A majority-carrier device: fast switching speed● Typical switching frequencies: tens and

hundreds of kHz● On-resistance increases rapidly with rated

blocking voltage● Easy to drive● The device of choice for blocking voltages less than

500V● 1000V devices are available, but are useful only at low power

levels(100W)● Part number is selected on the basis of on-resistance rather than

current rating

The Insulated Gate Bipolar Transistor (IGBT)

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Appearance Symbol

The construction of IGBT

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The equivalent circuit of IGBT

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Typical IGBT characteristics

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IGBT switching waveform

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Current tailing in IGBTs

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Characteristics of several commercial devices

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Conclusions: IGBT

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● Becoming the device of choice in 500-1700V applications, atpower levels of 1-1000kW

● Positive temperature coefficient at high current —easy to parallel and construct modules

● Forward voltage drop: diode in series with on-resistance. 2- 4V typical

● Easy to drive —similar to MOSFET● Slower than MOSFET, but faster than Darlington, GTO, SCR● Typical switching frequencies: 3-30kHz● IGBT technology is rapidly advancing —next generation:

2500V

The Thyristor (silicon controlled rectifier, SCR)

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Appearance Symbol

The equivalent circuit and construction of thyristor

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Typical thyristor characteristics

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Thyristor switching waveform

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Why the conventional SCR cannot be turned off via gate control

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The Gate Turn-Off Thyristor (GTO)

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Summary: Thyristors

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● The thyristor family: double injection yields lowest forwardvoltage drop in high voltage devices. More difficult toparallel than MOSFETs and IGBTs

● The SCR: highest voltage and current ratings, low cost, passive turn-off transition

● The GTO: intermediate ratings (less than SCR, somewhatmore than IGBT). Slower than IGBT. Slower than MCT. Difficult to drive.

● The MCT: So far, ratings lower than IGBT. Slower than IGBT.Easy to drive. Still emerging devices?

Summary of power semiconductor devices

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1. Majority carrier devices, including the MOSFET and Schottky diode, exhibitvery fast switching times, controlled essentially by the charging of thedevice capacitances. However, the forward voltage drops of these devicesincreases quickly with increasing breakdown voltage.

2. Minority carrier devices, including the BJT, IGBT, and thyristor family, canexhibit high breakdown voltages with relatively low forward voltage drop.However, the switching times of these devices are longer, and arecontrolled by the times needed to insert or remove stored minority charge.

3. Energy is lost during switching transitions, due to a variety of mechanisms.The resulting average power loss, or switching loss, is equal to this energyloss multiplied by the switching frequency. Switching loss imposes anupper limit on the switching frequencies of practical converters.

Two classifications based on carriers

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Classification I Classification II