MCT & SCS Thyristors

There are several low power devices belong to Thyristor family that are mainly used in trigger circuits to turn the thyristor ON. The principal ones are the Silicon Control Switch (SCS) and the MOS-Controlled thyristor (MCT). The MCT is a new but very promising switching device.

The Silicon Controlled Switch (SCS)

The silicon controlled switch SCS is a four-layer PNPN device. Figure 1 shows the structure and the symbol for the device. The SCS has two gates, an anode gate (AG) and a cathode gate (KG).

SCS Structure & Symbol
Figure 1: The SCS (a) Structure (b) Symbol​​​​​​

Figure 2 show the electrical equivalent circuit. As shown, both transistor bases are accessible for applying gate pulses. Like an SCR, an SCS can be turned on by applying a positive pulse at the cathode gate. The device can also be turned on by applying a negative gate pulse at the anode gate. If the SCS is ON, a positive pulse at the anode gate or a negative pulse at the cathode gate is required to turn it OFF. Usually, the turn ON anode gate is larger magnitude than the required cathode gate current.

Equivalent Circuit for an SCS
Figure 2: Equivalent circuit for an SCS​​​​​​

The MOS-Controlled Thyristor (MCT)

The MOS-controlled thyristor is a new device that combines the characteristics of a MOSFET and an SCR. It has a low forward voltage drop in the on-state and a low turnoff time. It has high di/dt and dv/dt capabilities. It is similar in functionality to the GTO but has a lower turnoff gate current requirement. Its main disadvantage is that its reverse voltage blocking ability is very low. Figure 3 show the symbol and equivalent circuit for the MCT. In an MCT, an SCR and two MOSFET’s are combined into a single device. The two have the same source terminal, which is the anode of the MCT. The N-channel MOSFET QOFF, which is connected between the anode and one of its internal layers, turns the SCR off while the P-channel MOSFET QON, connected between the gate and anode, turns it on.

MCT Symbol & Structure
Figure 3: MCT (a) Symbol (b) Equivalent Circuit​​​​​​

Unlike a GTO, which is turned ON and OFF by establishing a gate current, the MCT is switched by establishing the proper voltage from gate to anode. When the gate to anode voltage is approximately -5V, ON turns ON and supplies gate current to the SCR. This turns on the SCR. The MCT is turned OFF by applying a gate to anode voltage of approximately +10V, which turns OFF ON. This shunts current away from the SCR and turns it OFF.

MCT V-I Characteristic

Figure 4 shows the V-I Characteristics of an MCT. If the anode (A) is made positive with respect to the cathode (K). When no voltage is applied to the gate, the MCT remains in the blocking state, allowing only a small leakage curent (ILEAK). The MCT remains in the off state until a breakover voltage VBO is reached, at which point the MCT breaksdown. However, the MCT is not turned ON this way.

MCT VI Characteristics
Figure 4: MCT V-I Characteristic

If the cathode (K) is made positive with respect to the anode (A) with either positive or negative voltage applied to the gate, the MCT breaks down at a low voltage. This situation is to be avoided.

The normal way to turn the device ON is to forward bias the MCT by making the anode positive with respect to the cathode and applying a negative voltage to the gate and anode. When ON, the voltage drop across the MCT (VON) is very small (about 1V) and the anode current is limited only by the load resistance. Once the MCT turns ON, removal of the gate voltage will not turn it off. If the MCT is ON, the application of a positive voltage to the gate turns the device off until a negative voltage is applied to the gate again.