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.
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.
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
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.
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