Spread Time (t_s)

During  spread time anode  voltage  falls from 10% I_a  to the  on state  voltage  drop  and anode  current  rises  from 90% I_a  to final  value  of anode  current i ,e I_a 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 = t_d + I_r + t_s

In these  intervals td t_r and t_s 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.