We do not apply a continuous signal rather than a pulse to the thyristor gate.
We trigger scr using the pulse output of the trigger circuit for the following reasons:
- Using a continuous signal would mean that scr would be a default diode, and we do not want it. we need to have control over the switch even at high frequencies, so we could sometimes use pulses that are of the order of microseconds, depending on the type of thyristor used.
- The duration of the pulse must be long enough to generate the current level equal to or greater than the latching current, for successful activation of the scr. a pulse longer than that is not necessary in the circuit, and it would simply be a loss of energy.
- In addition, a greater amplitude of the gate voltage can be used for the same power dissipation in the gate circuit if pulse triggering is used.
In case of thyristor like scr (silicon controlled rectifier), the gate signal can not turn off the device but it only helps in switching the device (because scrs is semi controlled device). so if we have scr to turn on, we just need to provide a signal that has the voltage and duration sufficient to activate it. once the switch is on, the remaining signal can be continuous or pulsed or even zero.
In a converter application, the scr should be turned on for an alternate half cycle. so the signal should not be zero and the remaining option is a continuous or pulsed signal.