MOS Controlled Thyristor

Basically a thyristor with two MOSFETs built into the gate structure. In which, one MOSFET is used for turning on the MOS Controlled Thyristor (MCT) and the other for turning off the device.
  • An MCT is a high frequency, high power, low conduction drop switching device.
  • In MCT, anode is the reference with respect to which all gate signals are applied.
  • A practical MCT consists of thousands of these basic cells connected in parallel, just like a P-MOSFET.
  • An MCT is turned-ON by a negative voltage pulse at the gate with respect to the anode and is turned-OFF by a positive voltage pulse.

Turn-ON Process of MCT

MCT must be initially foward biased, then gate is made negative with respect to anode by the voltage pulse between gate and anode. With the application of negative voltage pulse, ON-FET (p-channel) gets turned ON. (whereas OFF-FET is already OFF).
  1. Current begins to flow from anode A, through ON-FET and then as the base current and emitter current of npn transistor and then to cathode K. This turns ON npn transistor.
  2. Collector current begins to flow in npn transistor, and acts as the base current of pnp transistor. This turns ON pnp transistor.
  3. Once both transistors are ON, MCT is turned ON.
Note: pnp transistor has better conduction property then ON-FET.

Turn-OFF Process of MCT

With the application of positive voltage pulse, OFF-FET is turned ON and ON-FET is tuned OFF.
  1. Emitter-base terminal of pnp transistor are short circuited by OFF-FET. Anode current begins to flow through OFF-FET and base current of pnp transistor begins to decrease.
  2. Collector current of pnp transistor that forms the base current of npn transistor also begins to decrease.
This regenerative action turn OFF the MCT.

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