Thyristor Types

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Thyristor Types

• Phase-control Thyristors (SCRs).
• Fast-switching Thyristors (SCRs).
• Gate-turn-off Thyristors (GTOs).
• Bidirectional triode Thyristors (TRIACs).
• Reverse-conducting Thyristors (RCTs).

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• Static induction Thyristors (SITHs).
• Light-activated silicon-controlled rectifiers
  (LASCRs).
• FET controlled Thyristors (FET-CTHs).
• MOS controlled Thyristors (MCTs).

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Phase Control Thyristor

• These are converter thyristors.
• The turn-off time tq is in the order of 50 to
  100?sec.
• Used for low switching frequency.
• Commutation is natural commutation
• On state voltage drop is 1.15V for a 600V device.

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• They use amplifying gate thyristor.

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Fast Switching Thyristors

• Also called inverter thyristors.
• Used for high speed switching applications.
• Turn-off time tq in the range of 5 to 50?sec.
• On-state voltage drop of typically 1.7V for
  2200A, 1800V thyristor.
• High dv/dt and high di/dt rating.

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Bidirectional Triode Thyristors (TRIAC)
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Mode-I Operation

• MT2 Positive,
• Gate Positive

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Mode-II Operation

• MT2 Positive,
• Gate Negative

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Mode-III Operation

• MT2 Negative,
• Gate Positive

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Mode-IV Operation

• MT2 Negative,
• Gate Negative

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Triac Characteristics
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Gate Turn-off Thyristors

• Turned on by applying positive gate signal.
• Turned off by applying negative gate signal.
• On state voltage is 3.4V for 550A, 1200V GTO.
• Controllable peak on-state current ITGQ is the
  peak value of on-state current which can be
  turned-off by gate control.

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Advantages over SCRs

• Elimination of commutating components.
• Reduction in acoustic electromagnetic noise due
  to elimination of chokes.
• Faster turn-off, therefore can be used for higher
  switching frequencies.
• Improved efficiency of converters.

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Advantages over BJTs

• Higher voltage blocking capabilities.
• High on-state gain.
• High ratio of peak surge current to average
  current.
• A pulsed gate signal of short duration only is
  required.

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Disadvantages of GTOs

• On-state voltage drop is more.
• Due to multi cathode structure higher gate
  current is required.
• Gate drive circuit losses are more.
• Reverse blocking capability is less than its
  forward blocking capability.

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Reverse Conducting Thyristors
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• Anti-parallel diode connected across SCR on the
  same silicon chip.
• This diode clamps the reverse blocking voltage to
  1 or 2V.
• RCT also called Asymmetrical Thyristor (ASCR).
• Limited applications.

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Static Induction Thyristors

• Turned-on by applying positive gate voltage.
• Turned-off by applying negative gate voltage.
• Minority carrier device.
• Low on-state resistance low voltage drop.
• Fast switching speeds high dv/dt high di/dt
  capabilities.

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• Switching time in order of 1 to 6 ?sec.
• The rating can go upto 2500V / 500A.
• Process sensitive.

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Light-Activated Silicon Controlled Rectifiers

• Turned-on by direct light radiation on silicon
  wafer.
• Gate structure is sensitive for triggering from
  practical light sources.
• Used in high voltage and high current
  applications. Example HVDC transmission, Static
  reactive power compensation.

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• Offers complete electrical isolation between
  light triggering source power circuit.
• Rating could be has high as 4KV / 1500A.
• di/dt rating is 250A / ?sec.
• dv/dt rating is 2000V / ?sec.

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FET Controlled Thyristors

• Combines a MOSFET a thyristor in parallel as
  shown.
• High switching speeds high di/dt dv/dt.

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• Turned on like conventional thyristors.
• Cannot be turned off by gate control.
• Application of these are where optical firing is
  to be used.

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MOS-Controlled Thyristor

• New device that has become commercially
  available.
• Basically a thyristor with two MOSFETs built in
  the gate structure.
• One MOSFET for turning ON the MCT and the other
  to turn OFF the MCT.

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Structure
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Equivalent Circuit
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Features

• Low on-state losses large current capabilities.
• Low switching losses.
• High switching speeds achieved due to fast
  turn-on turn-off.
• Low reverse blocking capability.

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• Gate controlled possible if current is less than
  peak controllable current.
• Gate pulse width not critical for smaller device
  currents.
• Gate pulse width critical for turn-off for larger
  currents.

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Thank you .
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