Thyristor technology is inherently superior to transistor for blocking voltage values above 2.5kV, plasma distributions equal to those of diodes offering the best trade-off between the on-state and blocking voltages. Until the introduction of newer power switches, the only serious contenders for high-power transportation systems and other applications were the GTO (thyristor), with its cumbersome snubbers, and the IGBT (transistor), with its inherently high losses. Until now, adding the gate turn-off feature has resulted in GTO being constrained by a variety of unsatisfactory compromises. The widely used standard GTO drive technology results in inhomogenous turn-on and turn-off that call for costly dv/dt and di/dt snubber circuits combined with bulky gate drive units.
Rooting from the GTO is one of the newest power switches, the Gate-Commutated Thyristor (GCT). It successfully combines the best of the thyristor and transistor characteristics, while fulfilling the additional requirements of manufacturability and high reliability. The GCT is a semiconductor based on the GTO structure, whose cathode emitter can be shut off "instantaneously", thereby converting the device from a low conduction-drop thyristor to a low switching loss, high dv/dt bipolar transistor at turn- off.
The IGCT (Integrated GCT) is the combination of the GCT device and a low inductance gate unit. This technology extends transistor switching performance to well above the MW range, with 4.5kV devices capable of turning off 4kA, and 6kV devices capable of turning off 3kA without snubbers. The IGCT represents the optimum combination of low loss thyristor technology and snubberles gate effective turn off for demanding medium and high voltage power electronics applications.
The thick line shows the variation of the anode voltage during turn-off. The lighter shows the variation of the anode current during turn-off process of IGCT.
GTO and thyristor are four layer (npnp) devices. As such, they have only two stable points their characteristics-'on' and 'off'. Every state in between is unstable and results in current filamentation. The inherent instability is worsened by processing imperfections. This has led to the widely accepted myth that a GTO cannot be operated without a snubber. Essentially, the GTO has to be reduced to a stable pnp device i.e. a transistor, for the few critical microseconds during turn-off.
To stop the cathode (n) from taking part in the process, the bias of the cathode n-p junction has to be reversed before voltage starts to build up at the main junction. This calls for commutation of the full load current from the cathode (n) to the gate (p) within one microsecond. Thanks to a new housing design, 4000A/us can be achieved with a low cost 20V gate unit. Current filamentation is totally suppressed and the turn-off waveforms and safe operating area are identical to those of a transistor.
IGCT technology brings together the power handling device (GCT) and the device control circuitry (freewheeling diode and gate drive) in an integrated package. By offering four levels of component packaging and integration, it permits simultaneous improvement in four interrelated areas; low switching and conduction losses at medium voltage, simplified circuitry for operating the power semiconductor, reduced power system cost, and enhanced reliability and availability. Also, by providing pre- engineered switch modules, IGCT enables medium-voltage equipment designers to develop their products faster.