Power Transistors

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Power Transistors

• Lakshmi S
• Department
• of
• Electronics Communication Engineering
• M.S. Ramaiah Institute of Technology

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References for further information

• Power Electronics by Mohan / Undreland / Robbins.
• Power Electronics by Dr. P.S. Bimbhra,
• Power Electronics by Muhammad H Rashid

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Overview of the Session

• Structural features of power transistors.
• Power transistor characteristics.
• Switching characteristics.
• Switching limits.
• Base drive control.

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Introduction to Power Transistors

• Have controlled turn-on turn-off
  characteristics.
• Used as switches in dc-ac and dc-dc converters.
• Switching speeds are higher than Thyristors.
• Voltage current ratings are lower than
  thyristors.

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Types of Power Transistors

• Bi-polar junction transistors (BJTs).
• Metal oxide semiconductor field effect
  transistors (MOSFETs).
• Static induction transistors (SITs).
• Insulated gate bi-polar transistors (IGBTs).

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BJTs

• Signal level transistor structure

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Power Transistor Structure
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Vertical Cross-section
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Steady State Characteristics of Signal level
BJT
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Steady State Characteristics of Power Transistor
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Transfer Characteristics
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Transistor as a Switch
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Problems

• Problem 1 (July / August 2004)
• Calculate
• Forced ?.
• ODF.
• Power Loss PT

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• Problem 2 (Jan / Feb 2005)
• Sketch device current waveform
• w.r.t. to input pulse.
• Calculate
• ICS.
• RB for an ODF of 2.
• Power Loss PT

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Transient Model of BJT
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Switching Characteristics of BJT
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Switching Limits

• Breakdown voltages
• BVSUS
• BVCEO
• BVCBO

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• Second
• Breakdown

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• Forward
• biased safe
• operating
• area
• (FBSOA)

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• Reverse
• biased safe
• operating
• area
• (RBSOA)

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• Power Derating
• 
  
• Thermal Equivalent
• 
  Circuit of Transistor

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Base Drive Control

• Used for increasing switching speeds.
• Types
• Turn-on control.
• Turn-off control.
• Proportional base control.
• Antisaturation control.

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Turn-on Control
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Turn-off Control
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Turn-on Turn-off Control
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Proportional Base Control
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Antisaturation Control
Without Clamping
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• The load resistance RC should satisfy the
  condition
• IC gt IL
• ?IB gt IL
• ?IBRC gt ILRC
• But ILRC VCC VBE VD1 VD2

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Advantages of BJTs

• Have high switching frequencies.
• Turn-on losses are small.
• Controlled turn-on turn-off characteristics.
• No commutation circuit required.

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Demerits of BJT

• Drive circuit is complex.
• Has the problem of charge storage.
• Has the problem of second breakdown.
• Cannot be used in parallel due to problems of
  negative temperature coefficients.

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