GTO

1. The GTO is a power switching device that can be turned on by a short pulse of gate current and turned off by a reverse gate pulse. This reverse gate current amplitude is dependant on the anode current to be turned off. Hence there is no need for an external communication circuit to turn it off.
2. Because turn-off is provided by bypassing carriers directly to the gate circuit, its turn-off time is short, thus giving it more capability for high-frequency operation than thyristors. The GTO symbol and static characteristics (similar to that of SCR) are shown in Fig. below.

GTO Structure

A high degree of interdigitation is required in GTOs in order to achieve efficient turn-off. The most common design employs the cathode area separated into multiple segments (cathode fingers) and arranged in concentric rings around the device center. The internal structure is shown in Fig. A common contact disk pressed against the cathode fingers connects the fingers together. It is important that all the fingers turn off simultaneously; otherwise the current may be concentrated into fewer fingers, with damage due to overheating more likely.

The high level of gate interdigitation also results in a fast turn-on speed and high di/dt performance of GTOs. The most remote part of a cathode region is no more than 0.16mm from a gate edge and hence the entire GTO can conduct within 5 ms with sufficient gate drive and the turn-on losses can be reduced. However, interdigitation reduces the available emitter area end therefore the low-frequency average current rating is less than for a standard thyristor with an equivalent diameter.

 Switching Characteristics of GTO

Switching phases of GTO

Turn-on: A GTO has a highly interdigited gate structure with no regenerative gate. Thus it requires a large initial gate trigger pulse. Minimum (I_GM) and maximum values of gate pulse can be derived from the device data sheet. The rate of rise of gate current di/dt will affect the device turn-on losses. The duration of the I_GM pulse should not be less than half the minimum for time given in data sheet rating. A longer period will be required if the anode current di/dt is low such that I_GM is maintained until a sufficient level of anode current is established.

On-state: The GTO operates in a similar manner to the thyristor. If the anode current remains above the holding current level then positive gate drive may be reduced to zero and the GTO will remain in condition. However, as a result of the turn-off ability of the GTO, it does posses a higher holding current level than the standard thyristor and, in addition, the cathode of the GTO thyristor is subdivided into small finger elements to assist turn-off. Thus, if the GTO thyristor anode current transiently  dips below the holding current level, localized regions of the device may turn-off, thus forcing a high anode current back into the GTO at a high rate of rise of anode current after this partial turn-off. This situation could be removed during conduction but held at a value at least 1% of the turn-on pulse to ensure that the gate does not unlatch.

Turn-off: The turn-off performance of a GTO is greatly influenced by the characteristics of the gate turn-off circuit. Thus the characteristics of the turn-off circuit must match with the de-ice requirements. The gate turn-off process involves the extraction of the gate charge, the gate avalanche period, and the anode current decay. The amount of charge extraction is a device parameter and its value is not affected significantly by the external circuit conditions. The device data sheet gives typical values for I_GQ.

·         For reliable operation of GTOs, the critical aspects are proper design of the gate turn-off circuit and the snubber circuit. A GTO has a poor turn-off current gain of the order of 4 to 5. For example, a 2000-A peak current GTO may require as high as 500 a of reverse gate current. Also, a GTO has the tendency to latch at temperature above 125˚C.

·         GTOs have the I²t withstand capability and hence can be protected by semiconductor fuses. GTOs are available up to about 4500 V, 2500A.

GTO Gate Drive Requirement

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