Spread Time (ts)
During spread time anode voltage falls from 10% Ia to the on state voltage drop and anode current rises from 90% Ia to final value of anode current i ,e Ia in this period whole charges injected from gate spreads over the entire section of cathode and anode current reaches at steady state condition.
There the total turn on time ( ton ) can be written as
Ton = td + Ir + ts
In these intervals td tr and ts the rise time is high a s compared to other. If rise time it is reduced beyond a limit thyrisotr may get destroyed because rate of rise of anode current may be large as compared to the spread velocity of charges and this may forms a local hot spot on the cathode surface of SCR. Turn on time depends on gate current. It can be reduced by using higher value of gate currents.