Gate-off thyristor

Gate-off thyristor

Gate Tur-OThy (GTO for short) is a device that is turned on and off by applying a pulse current to the gate, so it is a current-driven fully-controlled device. GTO is a high-power switching device currently used in high-voltage and large-capacity applications.

The structure and working principle of GTO

GTO is similar to ordinary thyristors and is a PNPN four-layer three-terminal semiconductor device. The structure, graphic symbols and appearance of GTO are shown in Figure 1. The three poles derived from the outside of the GTO are the cathode (K), the anode (A) and the gate (G). GTO is a multi-element power integrated device, which contains dozens or even hundreds of small GTO elements with a common anode. The cathodes and gates of these GTO elements are connected in parallel inside the device. The multiple integrated structure makes the cathode area of each GTO element small, and the distance between the gate and the cathode is greatly shortened, so that the lateral resistance of the P2 base is very small, so that it is possible for the gate to draw a larger current to achieve the purpose of gate control shut-off.

Gate-off thyristor
Figure 1 – The structure, graphic symbols and appearance of GTO

The working principle of GTO can still be analyzed with the two-transistor model shown in Figure 2. Like ordinary thyristors, the two transistors V1 and V2 composed of P1N1P2 and N1P2N2 have common base current gains α1 and α2, respectively. Unlike ordinary thyristors, α12 when GTO is turned on is closer to 1. Ordinary thyristor is designed to be α12≥1.15, and GTO is designed to be α12≈1.05, so that when GTO is turned on, it is in a critical saturation state, which is a favorable condition for gate control. Since GTO is in a critical saturation state when it is turned on, the pressure drop of the tube increases when GTO is turned on.

Gate-off thyristor
Figure 2 – Internal structure, dual transistor model and equivalent circuit of thyristor

The equivalent circuit of GTO shutdown is shown in Figure 2. A negative voltage EG is applied to the gate, and the collector current IC1 of the transistor V1 is drawn, that is, draw current from the gate, so that the base current IB2 of the transistor V2 is reduced, so that IK and IC2 are reduced, and the reduction of IC2 causes a further reduction of IA and IC1. In this cycle, when the two transistor emitter currents IK and IC1 decrease so that α12<1, the device quickly exits saturation and turns off.

Gate-off thyristor
Figure 3 – GTO shutdown equivalent circuit

GTO’s multi-integrated structure is not only beneficial to turn-off, but also makes it faster than ordinary thyristor turn-on process, and has a stronger ability to withstand di/dt.

Figure 4 shows the waveforms of the gate current iG and anode current iA during the turn-on and turn-off of the GTO. The delay time td and the rise time tr need to elapse during the opening process. The turn-off process requires time to extract a large number of carriers stored during saturation and turn-on—storage time ts, so that the transistor exits the saturation state, and then the equivalent transistor retreats from the saturation region to the amplification region, the anode current gradually decreases for the time—falling time tf, and finally there is the residual carrier recombination time—the tail time tt.

Gate-off thyristor
Figure 4 – Current waveforms during GTO turn-on and turn-off

Usually tf is much smaller than ts, and tt is longer than ts. The greater the magnitude of the gate negative pulse current, the steeper the leading edge, the faster the speed of pumping away stored carriers, and the shorter the ts. In the tail time tt, there are still remaining carriers being drawn out, but the anode voltage has been established. At this time, the excessive du/dt will cause the GTO to turn on again, resulting in a turn-off failure. In order to ensure the reliable shutdown of the GTO, an appropriate negative voltage is maintained during the tt phase.

The main parameters of GTO

Many parameters of GTO have the same meaning as the corresponding parameters of ordinary thyristors. Here only briefly introduce some parameters with different meanings.

(1) The maximum shutdown anode current IATO

This is also the parameter used to nominal GTO rated current. This is different from the thyristor using the average on-state current as the rated current.

The anode current of GTO is limited by two conditions: the first is the heating limit, that is, the average on-state current value determined by the device’s rated operating junction temperature; the second is the turn-off failure. Although the thermal limit is not exceeded, a larger current will deepen the full conduction of the device, resulting in Failed to turn off the gate. Therefore, the maximum shutdown anode current IATO is used as its rated current.

(2) Current turn-off gain βoff

The ratio of the maximum turn-off anode current IATO to the maximum negative gate pulse current IGM is called the current turn-off gain, namely

(1-1)

βoff is generally small, only about 5, which is a major disadvantage of GTO. A 500A GTO, the peak value of the negative pulse current of the gate electrode can reach 100A when it is turned off, which is a considerable value. The gate of GTO is turned off and the negative pulse voltage is not high, but the current is large, which requires higher design requirements for the drive circuit.

(3) Opening time ton

The turn-on time is composed of the delay time td and the rise time tr. The delay time of GTO is generally 1~2μs, and the rise time increases with the increase of anode current value.

(4) Turn-off time toff

The turn-off time generally refers to the sum of the storage time ts and the fall time of tf, and does not include the tail time. The storage time of GTO increases with the increase of anode current, and the fall time is generally less than 2μs.

In addition, it should be pointed out that many GTOs are manufactured into reverse-conducting type, similar to reverse-conducting thyristors. When it is necessary to withstand the reverse voltage, it should be used in series with the power diode.