MCT | MOS- controlled thyristor | Power electronics

MOS Controlled thyristor (MCT)

• It is basically a thyristor with two MOSFETs built into the gate structure.
• One MOSFET is used for turning on the MCT and the other for turning off the device.
• MCT is a high frequency, high power, low-conducting drop switching device.
• A practical MCT consists of thousands of these basic cells connected in parallel just like a power MOSFET.
• This is done to achieve a high current carrying capacity of the device.

Basic structure of MCT

• The equivalent circuit of MCT consists of one on-FET, one off-FET and two transistors.
• The on-FET is a p-channel MOSFET and off-FET is an n-channel MOSFET.
• An arrow towards the gate terminal indicates n-channel MOSFET and the arrow away form the gate terminal is the p-channel MOSFET.
• The two transistor in the equivalent circuit represents that there is regenerative feedback in the MCT just same as in an ordinary thyristor.

(a) Equivalent circuit of MCT and (b) Circuit symbol of MCT

Turn-on process

• An MCT can be turned on by applying a negative voltage pulse at the gate with respect to anode.
• By getting this negative voltage pulse, on-FET gets turned-on and the off-FET is off.
• During this time current begins to flow from anode A, through on-FET and then as the base current (i_B) and emitter current (i_E) of npn transistor and then to cathode C.
• It turns on transistor and therefore collector current (i_C) begins to flow in npn transistor.
• Since off-FET is off, so this i_C of npn transistor acts as the i_B of pnp transistor, and hence pnp transistor is also turned on.
• When both the transistors are on, the regenerative action of the connection scheme takes place, and the MCT is turned on.

Turn-off process

• For turning-off the MCT, off-FET is energized by positive voltage pulse at the gate with respect to anode.
• So this FET is turned on, and on-FET is tuned off.
• After off-FET is tuned on, either base terminals of pnp transistor are short circuited by it.
• So the anode current begins to flow through off-FET and therefore i_B of pnp begins to decrease.
• So i_C of pnp transistor that forms the i_B also begins to decrease.
• In this way i_B of pnp and npn transistors divide storage charge in their n and p bases respectively, begins to decay.
• This regenerative action turns off the MCT.

Advantages of MCT

• MCT has low forward conduction drop.
• The turn-on and turn-off time of MCT is very fast.
• The switching loss of MCT is very low.
• The gate input impedance of MCT is very high.

To know about this lecture in more detail please visit on https://youtu.be/KtNQJ3Sueo0