MOS Controlled Thyristor (MCT) Its Working and Applications

A MOS-controlled thyristor is a type of power semiconductor device. It has the ability to transfer current and thyristor voltage through a MOS gate used for switching on and off.

MCT was developed by V.A.K. Temple in 1984. It is known as a voltage-controlled fully controlled thyristor with two MOSFETs built into the gate structure. MOSFETs control this device.

Characteristics of MCT

  • Low forward voltage drop during conduction
  • Fast turn-on and turn-off times
  • Low switching losses
  • Low reverse voltage blocking capability
  • High gate input impedance
  • High-Speed switching capabilities

Symbol of MCT


Working of MCT

The following diagram shows the working principle of a MOS-controlled thyristor. It is a combination of current and voltage functions using MOS gating. A MOS gate is used for the ON/OFF switch of the MCT.


When the MOSFET is turned ON MCT

The device switches ON w.r.t the anode using a negative voltage pulse. The Gate terminal is made negative with respect to the anode by a voltage pulse between the Anode and Gate terminals. Therefore the MOS-controlled thyristor is turned on. During the start-up phase, the MOS-controlled thyristor is forward-biased.

When a negative voltage is applied to the negative voltage pulse, the ON-mode FET is turned ON and the FET-mode off already exists as the off state. When the FET is in the ON state, current flows from the anode through the ON FET, through the base current and the n-p-n transistor at the emitter terminal, and finally current flows to the cathode. So this process turns on the n-p-n transistor. The NPN transistor acts as the base current for the P-N-P transistor when the OFF FET is in OFF mode. Similarly, when both transistors are in their on state and the appropriate action occurs, the P-N-P transistor turns on, turning on the MCT.

When the MOSFET is turned OFF MCT

The device is turned off with the help of a positive voltage pulse. It is applied to the gate terminal with respect to the anode. Then the OFF-FET switched to ON mode and the ON-FET switched to OFF state. When the OFF FET is turned on, the p-n-p transistor is shorted through the emitter and base terminals. Therefore, anode current flows through the OFF FET. Therefore, the base current of the N-P-N transistor is reduced. Reverse voltage blocking capability is the negative point of this device.

Equivalent Circuit Diagram of MCT

The figure below is an equivalent circuit of a MOS-controlled thyristor. This circuit consists of two MOSFET transistors, one is N-channel, and the other is P-channel. The P-channel is used to turn on the ON-FET and the N-channel is used to turn off the OFF-FET. This circuit consists of two transistors, an n-p-n transistor, and a p-n-p transistor. Combining these two transistors forms the n-p-n-p structure of a MOS-controlled thyristor. A p-channel MOSFET is indicated by an arrow connected from the gate terminal.


Applications of MCT

  • Discharge Applications
  • Circuit breakers
  • Electric Vehicles
  • Smart Vars
  • Induction heating
  • High power conversions

Leave a Comment