Gate triggering

This is the simplest, reliable and efficient method of firing the forward biased SCRs. First
SCR is forward biased. Then a positive gate voltage is applied between gate and cathode. In
practice the transition from OFF state to ON state by exceeding 𝑉𝐵𝑂 is never employed as it
may destroy the device. The magnitude of 𝑉𝐵𝑂, so forward breakover voltage is taken as final
voltage rating of the device during the design of SCR application.
First step is to choose a thyristor with forward breakover voltage (say 800V) higher than the
normal working voltage. The benefit is that the thyristor will be in blocking state with normal
working voltage applied across the anode and cathode with gate open. When we require the
turning ON of a SCR a positive gate voltage between gate and cathode is applied. The point
to be noted that cathode n- layer is heavily doped as compared to gate p-layer. So when gate
supply is given between gate and cathode gate p-layer is flooded with electron from cathode
n-layer. Now the thyristor is forward biased, so some of these electron reach junction 𝐽2 .As a
result width of 𝐽2 breaks down or conduction at 𝐽2 occur at a voltage less than 𝑉𝐵𝑂.As 𝐼𝑔
increases 𝑉𝐵𝑂 reduces which decreases then turn ON time. Another important point is
duration for which the gate current is applied should be more then turn ON time. This means
that if the gate current is reduced to zero before the anode current reaches a minimum value
known as holding current, SCR can’t turn ON.
In this process power loss is less and also low applied voltage is required for triggering.