SWITCHING CHARACTERISTICS

SWITCHING CHARACTERISTICS  :

              Turn-on :When a gate drive is applied with the forward voltage above latching voltage (the minimum anode to cathode voltage that will successfully turn-on a thyristor with a given gate drive), turn-on occurs. Becauseof the finite sheet resistance of the p-base region, only those regions of the cathode nearest to the gate are influnced by the gate current. Regenerative switching action is initially restricted to these regions. The establishment of the equilibrium current flow over the cathode area follows by outward spreading froms this conducting plasma, by diffusion. The plasma spreading is relatively slow and occurs with a typical velocity of 0.1mm/. When the area of conduction is small, the voltages across the device is considerable has an upper limit on the di/dt, which in modern amplifying gate thyristor is up to 500 A/. Saturable reactors in series with the thyristor are used to limit di/dt, particularly arising from the discharge of current due to stray capacitances and snubber circuits.

               There are three phases of turn-on .The delay time is associated with the establishment of regenerative action in response to the gate current. Its duration depends upon the level of the gate drive. Regeneration is well established during the rise time. The current continues to increase of 10-20% of that predicated for normal conduction using on-state voltage of the thyristor. The spreading phase may last over hundred microseconds.

               Turn-off :  All the three junctions are forward biased during on-state and the base regions contain excess minority and majority charge. This charge must either be swept out by an electric field or decay through regenerative processes within the silicon.

           When the circuit voltage is reversed, the current falls to zero at a certain rate. Once the current reaches zero, the flow reverses,since the minority carrier concentration at the junctions can support this current by diffusion without build-up of depletion layer. The peak value of this reverse  current is reached when the excess hole concentration at the anode junction has fallen to zero. At this time, the voltage across the thyristor reverses with the development of the depletion layer and the voltage across the thyristor reverses with the development of the depletion layer and the current decays in a near exponential manner as a result of charge recombination within the n-base region. The decay of current is dependent on the mean life-time of carriers in the n-base region. Immediately after current zero, a thyristor is unable to support forward voltage. Gradually, the thyristor acquires some forward blocking capability is attained only after a millisecond or so has elapsed from current zero. This characteristic is circuit and temperature dependent