Switching Characteristics of a Thyristor
Switching characteristics of a Thyristor:
- A thyristor can be switched off if the external circuit causes the anode to become negatively biased, a method known as natural, or line, commutation. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.
Basically we can classify the switching characteristics of a Thyristor as
- Turn ON switching characteristics
- Turn OFF switching characteristics
- Static and switching characteristics of thyristors are always taken into consideration for economical and reliable design of converter equipment. During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents through it.
- The time variations of the voltage across a thyristor and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics of a thyristor
Turn ON switching characteristics:
A forward biased thyristor is turned on by applying a positive gate voltage between the gate and cathode. A forward-biased thyristor is usually turned on by applying a positive gate voltage between gate and cathode. There is, however, a transition time from forward off-state to forward on state. This transition time called thyristor turn-on time, is defined as the time during which it changes from forward blocking state to final on-state. Total turn-on time can be divided into three intervals ; (i) delay time td , (ii) rise time tr and (iii) spread time tp .
Turn OFF switching characteristics:
Thyristor turn-off means that it has changed from on to off state and is capable of blocking the forward voltage. This dynamic process of the SCR from conduction state to forward blocking state is called commutation process or turn-off process.The turn-off time tq of a thyristor is defined as the time between the instant anode current becomes zero and the instant SCR regains forward blocking capability. During time tq ,all the excess carriers from the four layers of SCR must be removed. This removal of excess carriers consists of sweeping out of holes from outer p-layer and electrons from outer n-layer. The carriers around junction J2 can be removed only by recombination. The turn-off time is divided into two intervals ; reverse recovery time trr and the gate recovery time tgr ;
i.e. tq = trr tgr.