Download Power Electronics - Dr. Imtiaz Hussain

Power Electronics
Lecture-9
Power Transistors & GTO
Dr. Imtiaz Hussain
Associate Professor
email: [email protected]
URL :http://imtiazhussainkalwar.weebly.com/
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Outline
• Power Transistors
– Power BJT
– Power MOSFET
– IGBT
• GTO
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Power Transistors
• Power transistors are fully controlled semiconductor
switches.
• These are turned ON when the signal (voltage or current)
is given to control terminal.
• Types
– Power BJT
– Power MOSFET
– Insulated Gate Bipolar Junction Transistor (IGBT)
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Power BJT
• The symbol of the Power BJT is same as signal
level transistor.
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Power BJT
• The construction of the Power Transistor is different from the
signal transistor as shown in the following figure.
• The n- layer is added in the power BJT which is known as drift
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region.
Power BJT
• A Power BJT has a four layer structure of alternating P and
N type doping.
• In most of Power Electronic applications, the Power
Transistor works in Common Emitter configuration.
• In power switches npn transistors are most widely used
than pnp transistors.
• The thickness of the dirft region determines the
breakdown voltage of the Power transistor.
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VI Characteristics
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VI Characteristics
• The VI characteristics of the Power BJT is different from signal
level transistor.
• The major differences are Quasi saturation region & secondary
breakdown region.
• The Quasi saturation region is available only in Power transistor
characteristic not in signal transistors. It is because of the lightly
doped collector drift region present in Power BJT.
• The primary breakdown is similar to the signal transistor’s
avalanche breakdown.
• Operation of device at primary and secondary breakdown regions
should be avoided as it will lead to the catastrophic failure of the
device.
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Power MOSFET
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Power MOSFET
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Power MOSFET
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Power MOSFET
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IGBT
• IGBT is a new development in area of Power
MOSFET technology.
• This device combines into it the advantages of
both MOSFET and BJT.
• So an IGBT has high input impedance like a
MOSFET and low on state power loss like a BJT.
• Further, IGBT is free from second breakdown
problem presented in BJT.
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Cross-Sectional View of an IGBT
Metal
Silicon Dioxide
Metal
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Cross-Sectional View of an IGBT
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IGBT
• It is an FET integrated with a bipolar transistor in a form
of Darlington configuration.
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IGBT I-V Characteristics
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IGBT
• “IGBT Transistor” has the output switching and conduction
characteristics of a bipolar transistor but is voltagecontrolled like a MOSFET.
• IGBTs are mainly used in power electronics applications,
such as inverters, converters and power supplies, were the
demands of the solid state switching device are not fully
met by power BJTs and power MOSFETs.
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Comparison Table
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Antiparallel diode
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GTO (Gate Turn-off Thyristor)
• GTO can be turned-on by a gate signal, and it can
also be turned-off by a gate signal of negative
polarity.
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GTO (Gate Turn-off Thyristor)
• Applications of GTO: They are used in
– Motor drives
– Static VAR compensators (SVCs)
– AC/DC power supplies with high power ratings
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GTO (Gate Turn-off Thyristor)
• Compared to a conventional SCR, the device has the
following disadvantages
– Magnitude of latching, holding currents is more. The latching
current of the GTO is several times more as compared to
conventional thyristors of the same rating.
– On state voltage drop and the associated loss is more.
– Due to multicathode structure of GTO, triggering gate current is
higher than that required for normal SCR.
– Gate drive circuit losses are more.
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END OF LECTURE-9
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