MOS Controlled Thyristor (MCT)
Power Semiconductor devices are the heart of the modern Power Electronics Field. Every year the market receives some new developments/enhanced power devices. An MCT-MOS Controlled thyristor is one of the newly developed device. In this post we will discuss about this power semiconductor device in detail.
- The MOS Controlled Thyristor (MCT) is a new emerging Power semiconductor Device.
- It is first introduced by General Electric's Power semiconductor operations(Harris Semiconductor, USA).
- Later ABB has introduced a new device called "Insulated Gate Commutator Thyristor(IGCT)" which is in the same family of devices.
- Basically MCT is a thyristor with two MOSFETs built into the gate structure (see the equivalent circuit). These internal MOSFETs are known as ON-FET and OFF-FET. It is a high frequency, high power, low conduction drop switching device.
- The ON-FET is responsible for turning on the MCT.
- The OFF-FET is responsible for turning off the MCT.
- Various sub-classes of MCTs can be made: P-type or N-type, symmetric or asymmetric blocking, one or two-sided Off-FET gate control, and various turn-on alternatives including direct turn-on with light.
- All sub-classes have one thing in common: turn-off is accomplished by turning on a highly interdigitated Off-FET to short out one or both of the thyristor's emitter-base junctions.
- The i - v characteristics of various sub-classes of MCTs are one and same.
- The MCTs have the following advantages:
- Low on-state losses
- large current carrying capability ( Comparing with the same voltage rating of the IGBT, the MCT typically has 10 to 15 times the current carrying capability at the same voltage drop)
- Controlled turn on and turn off
- Fast switching speeds
- Most of the MCT characteristics can be understood easily by referring the equivalent circuit of P-Channel MCT shown above.
- MCT closely approximates a bipolar thyristor with two opposite polarity MOSFET transistors connected between its anode and the proper layers to turn it on and off.
- As MCT is NPNP device ( not PNPN device) output terminal or cathode must be negatively biased.
- Driving the gate terminal negative with respect to the common terminal or anode turns the P channel FET on, firing the bipolar SCR.
- Driving the gate terminal positive with respect to the anode turns on the N channel FET on shunting the base drive to PNP bipolar transistor making up part of the SCR, causing the SCR to turn off.
- When no gate to anode voltage is applied to the gate terminal of the device, the input terminals of the bipolar SCR are unterminated. Operation without gate bias is not recommended.
MCTs can be operated in parallel for proper current sharing at high current applications. Several MCTs can be paralleled to form larger modules with slight derating of individual devices.
The MCT is a power switch with a MOS gate for turn-on and turn-off. It is derived from a SCR by adding the features of a MOSFET. It has several advantages compared to various modern devices like Power MOSFET, IGBT. It has low forward voltage drop and high current density. Thus it is used in high power applications.