gtothyristors - MITSUBISHI HIGH POWER SEMICONDUCTORS...

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Aug.1998 Gate turn-off (GTO) thyristors are able to not only turn on the main current but also turn it off, provided with a gate drive cir- cuit. Unlike conventional thyristors, they have no commuta- tion circuit, downsizing application systems while improving efficiency. They are the most suitable for high-current, high- speed switching applications, such as inverters and chopper circuits. 1. GTO thyristor operation principles A GTO thyristor consists of four layers, pnpn, as like conven- tional thyristors. Functions except for turn-off are the same as those of conventional thyristors, therefore, we mainly de- scribe the turn-off operation here. When a GTO thyristor is in the on-stats, the central base re- gions are filled with holes supplied from the anode and elec- trons supplied from the cathode. If reverse bias is applied to make the gate negative in respect to the cathode, part of holes in the p-base layer are extracted through the gate, sup- pressing the injection of electrons from the cathode. In re- sponse to this suppression, more hole current is extracted through the gate, further suppressing the electron injection. In the course of this process, the cathode emitter junction (J 3 ) is put into a reverse-bias state entirely, GTO thyristor is turned off. Fig. 1 illustrates the turn-off operation, using a two-transistor model. Suppose that a GTO thyristor is divided into npn transistor T r1 on the cathode side and pnp transistor T r2 on the anode side, and that they are connected as shown in Fig. 1(b). In this figure, the current amplification factor of transistor T r1 is called α 1 , and that of transistor T r2 , α 2 . If reverse current I GQ flows through the gate, base current I B at transistor T r1 is re- duced when I GQ is increased. This relationship can be ex- pressed by the following equation: I B = α 2 · I A – I GQ On the other hand, electron current I RB , which disappears due to the recombination in the T r1 base layer, can be ex- pressed as follows: I RB = (1 – α 1 ) · I K The relationship between GTO thyristor anode current (I A ) and cathode current (I K ) is expressed by the following equa- tion: I A = I K + I GQ To turn off the GTO thyristor, I B must be smaller than I RB . The magnitude of the reverse-bias current I GQ that satisfies this condition can be calculated by the following equation: I GQ = ( α 1 + α 2 – 1) · I A / α 1 As can be seen from what has been discussed, it is possible in theory that a GTO thyristor can carry out the turn-off if an adequate magnitude of reverse bias current is supplied to the gate. Actually, however sheet resistance exists in the T r1 base region, making it difficult to turn off the on state current flowing at the emitter junction that is far from the gate. To minimize the resistance, GTO thyristors for high power appli- cations are finely patterned: unit constructions as illustrated in Fig. 1 are placed in parallel with one another in whole sili- con area. (See Fig. 2.) Fig. 2 GTO Thyristor Fine Pattern Gate Gate Cathode Cathode (K) Anode (A) Anode n n p p I GQ I GQ I k I k I B I A α 1
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