Thyristors

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Thyristors

• Most important type of power semiconductor
  device.
• Have the highest power handling capability.they
  have a rating of 1200V / 1500A with switching
  frequencies ranging from 1KHz to 20KHz.

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• Is inherently a slow switching device compared to
  BJT or MOSFET.
• Used as a latching switch that can be turned on
  by the control terminal but cannot be turned off
  by the gate.

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Different types of Thyristors

• Silicon Controlled Rectifier (SCR).
• TRIAC.
• DIAC.
• Gate Turn-Off Thyristor (GTO).

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SCR

• Symbol of
• Silicon Controlled Rectifier

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Structure
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Device Operation

• Simplified model of a thyristor

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V-I Characteristics
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Effects of gate current
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Two Transistor Model of SCR

• ?

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Turn-on Characteristics
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Turn-off Characteristics
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Methods of Thyristor Turn-on

• Thermal Turn-on.
• Light.
• High Voltage.
• Gate Current.
• dv/dt.

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Thyristor Ratings
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Voltage Ratings
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dv/dt Triggering
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Current Ratings
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Gate Specification
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Gate Triggering Methods

• - Efficient reliable method for turning on SCR.
• Types
• R - Triggering.
• RC - Triggering.
• UJT - Triggering.

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R-Triggering

• Resistance firing circuit

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Waveforms of R-triggering with R2 at large value
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Waveforms of R-triggering with R2 at optimum value
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Waveforms of R-triggering with R2 at small value
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RC Triggering

• RC half-wave trigger circuit

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Waveforms of RC half-wave triggering with R at
high value
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Waveforms of RC half-wave triggering with R at
low value
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• RC full-wave trigger circuit

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Waveforms of RC full-wave triggering with R at
high value
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Waveforms of RC full-wave triggering with R at
low value
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Uni-Junction Transistor
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UJTCharacteristics
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UJT Relaxation Oscillator

• Circuit Diagram Waveforms

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Expression for period of oscillation T

• Voltage of capacitor

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VD ltlt VBB
VP ?VBB
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Design

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SYNCHRONIZED UJT OSCILLATOR
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