Gate Turnoff Thyristor Symbol:
A Gate Turnoff Thyristor Symbol (GTO), unlike the SCR, has the additional capability that it can be turned off by injecting negative gate current pulse. This is because of its highly interdigitated gate emitter which permits the diversion of PNP collector current by the gate and thereby break up the current regenerative effect in the local PNP-NPN loop. However, Gate Turnoff Thyristor Symbol has poor turn-off current gain (typically 4 or 5) and so a 2000A peak current device would require 500A negative current pulse. But the corresponding average power is small and can be easily absorbed by a power MOSFET.The turn-off phenomenon is somewhat complex. It sometimes leads to a voltage spike at of the anode caused by snubber circuit reactance and di/dt of anode current decay. This creates a possibility of a second break-down. Further anode circuit can show a long current tail with accompanying losses. This necessitates a well designed snubber circuit with large capacitor. Larger switching losses in the turn-off process limit the PWM (pulse width modulation) frequency of Gate Turnoff Thyristor Symbol to within 1 to 2 kHz.
Gate Turnoff Thyristor Symbol Applications:
In spite of this disadvantage, Gate Turnoff Thyristor Symbol has almost replaced SCRs in force-commutated voltage-fed converter application because of its reduced size, circuit complexity and improved efficiency. These are popular in ac machine drives UPS systems, etc. The state-of-the-art devices are now available for about 4500 V, 3000 A ratings.