Gate Turn-off Thyristor

The thyristor, also called a silicon-controlled rectifier (SCR), is basically a four-layer three-junction pnpn device. It has three terminals: anode, cathode, and gate. The device is turned on by applying a short pulse across the gate and cathode. After the device turns on, the gate loses its ability to turn off the device. The turn-off is achieved, instead, by applying a reverse voltage across the anode and cathode.

The GTO type of thyristor is a power switching device that can be turned on by a short pulse of gate current – but unlike a standard thyristor, it can be turned off by a reverse gate pulse. The required reverse gate current amplitude is dependent on the anode current to be turned off. Hence, there is no need for an external commutation circuit to turn off the device.

Because turn-off is provided by bypassing carriers directly to the gate circuit, the turn-off aspects are proper design of the gate turn-off circuit and the snubber circuit. The GTO has poor turn-off current gain, on the order of four to five. For example, a 2000 A peak current GTO may require as much as 500 A of reverse gate current. Also, the GTO has a tendency to latch at high temperatures (above approximately 125ᴼC). GTO devices are commonly available for operation up to 4500 V at 2500 A.