PRINCIPAL OF OPERATION

PRINCIPAL OF OPERATION : 

                                 The principle of operation of thyristors can be explained by the two transistor analogy. Here a thyristor is replaced by a PNP and NPN transistor connected in regenerative feedback. If the gate current Ig is injected into the transistor T2, its collector current Ic2 amplifies the collector current Ic1 of of transistor T1. This in turn reinforces the gate current IG. Eventually, T1 and T2 go into complete saturation and all the junctions become forward biased. 

      

               A silicon transistor has the property thatὰ1 and ὰ2 are the common- base current gains and ICBO1 and ICBO2 are the common base leakage currents of  T1 and T2 respectively.

       

     A silicon  transistor has the property that ὰ is very low at low emitter current and rise rapidly as the emitter current builds up. When the device is off, Ig =0, and Ia will be the leakage current. If it is possible to raise the emitter currents of T1 and T2, such that (ὰ1 +ὰ2) apporaches unity, then the device triggers into saturation. There are several means of achieving this :

1. Injection of gate curent(normal turn -on)
2. By increasing the forward voltage above a limit, Vbo called break-over voltage. In this case, the minority-carrier leakage current at middle junction increases due to avalanche effect.
3. By increasing the anode voltage at a rate such that the depletion layer capacitance at the middle junction will create a displacement current(dv/dt turn-on).
4. At a high enough junction temperature, the leakage current increases and casues a turn-on.
5. Direct irradiation of light on silicon creates electron-hole pairs, which under the influence of electric field result in a current to trigger the thyristor.

           Triggering the device into saturation is called turn-on.Controlled turn-on without damaging the device is only feasible through gated turn-on. The device remains in a conducting state untill the current is maintained by the circuit action, above the holding current. During this period, the gate has no control on the conduction. The turn-off process which results in the device regaining its blocking state achieved either by:(i)line communication or (ii)forced communication.

             In both cases, the circuit voltage source is reversed which in turn will drive the current to zero. After a time lapse of tq, the turn-off time, the voltage can be reversed again, when the device regains its blocking state.