Thyristors conduction

These are unidirectional and uncontrollablet static electronic devices and used as static switches and shown in Figure 6.14. A diode turns ON at the instant it becomes forward biased and OFF when it becomes reverse biased. By connecting them in series parallel combinations, they can be made suitable for any desired voltage and current ratings. Whether it is a transistor scheme or a thyristor scheme, they are used extensively where a forward conduction alone is necessary and the scheme calls for only a simple switching, without any control over the switching operation. They are used extensively in a rectifier circuit to convert a fixed a.c. supply to a fixed d.c. supply. 

Unlike an SCR, which is unidirectional, a triac is a bidirectional thyristor switch and conducts in both directions. It can be considered as composed of two SCRs, connected back to back with a single gate, as shown in Figure 6.22(a). Since the thyristor now conducts in both directions there is no positive (anode) or negative (cathode) terminals. 

The triac may, however, have some limitations in handling frequencies higher than normal. In such cases, they can be simulated by using two SCRs in inverse parallel combinations as illustrated in Figure 6.22(b). Now it is known as a reverse conducting thyristor. An SCR has no frequency limitations at least up to ten times the normal. The required voltage and current ratings are obtained by series-parallel connections of more than one thyristor unit. 

Figure 6.22(b) Use of two SCRs in inverse parallel combination to simulate a triac (reverse-conducting thyristors)... 

Once fired, a thyristor cannot be controlled. Ft requires a forced commutation to switch it off and (he gate control is quite cumbersome. To swdtch OFT-, (he conducting current Is reduced to less than its holding current. The commutation circuitry is ihcrctbre highly complex an[Pg.118]

Voliage tran.siems (dv/dl) When a thyristor switches from a closed to an open condition, i.e. from a conducting to a non-conducting mode, a transient recovery voltage (TRV) appears. This is a transient condition and the rate of change of voltage can be expressed by... 

For reasons of safety, the short-circuit device of UR 1/400 as given in Fig. 12.6 (V1-V3) may be replaced by a bridge-connected rectifier (Fig. 12.9) V1-V3 and V4-V6, followed by two thyristors V7 and V8 fired in parallel. The lack of any current zero passage at the output of V1-V3/V4-V6 ensures a short-circuit time not affected by the availability of firing pulses for the thyristors. They return to their non-conducting state with neglectable current. 

In recent times there has been a rapid development in the design of high-power transistors, to such an extent that they are feasible alternatives to thyristors for many applications. The main advantage of transistors is that they can be switched on and off at any point in the conducting half-cycle that can appear across their emitter and collector terminals. They must be protected against the reversal of voltage when the second half-cycle appears across the terminals. It is therefore possible to synthesise the waveforms in such a manner as to reduce the harmonic distortion at the supply terminals to a low level. 

A thyristor rectifier uses four silicon semiconductors of alternating types in a pack. Figure 8.7 shows that without some outside influence the three junctions can not simultaneously be conductive. When the cathode side is negative, two junctions are conductive, but the center, or control, junction is not. Applying a current pulse to the interior P-layer neutralizes the bias in that layer and allows current to flow through the rectifier pack. 

Molybdenum has a coefficient of expansion very similar to that of siHcon and borosilicate glass. This allows molybdenum plates to be used to conduct heat away in glass components integrated circuits and thyristors. 

When SWB only is closed the lamp will not light, but when SWA is also closed, the lamp lights to full brilliance. The lamp will remain illuminated even when SWA is opened. This shows that the thyristor is operating correctly. Once a voltage has been applied to the gate the thyristor becomes forward conducting, like a diode, and the gate loses control. 

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