Turn on time
It is the time during which it changes from forward blocking state to ON state. Total turn
on time is divided into 3 intervals:
1. Delay time
2. Rise time
3. Spread time
Delay time
If ð¼ð‘”and ð¼ð‘Ž represent the final value of gate current and anode current. Then the delay time
can be explained as time during which the gate current attains 0.9 ð¼ð‘” to the instant anode
current reaches 0.1 ð¼ð‘” or the anode current rises from forward leakage current to 0.1 ð¼ð‘Ž.
1. Gate current 0.9 ð¼ð‘” to 0.1 ð¼ð‘Ž.
2. Anode voltage falls from ð‘‰ð‘Ž to 0.9ð‘‰ð‘Ž.
3. Anode current rises from forward leakage current to 0.1 ð¼ð‘Ž.
Rise time (ð’•ð’“
)
Time during which
1. Anode current rises from 0.1 ð¼ð‘Ž to 0.9 ð¼ð‘Ž
2. Forward blocking voltage falls from 0.9ð‘‰ð‘Ž to 0.1ð‘‰ð‘Ž. ð‘‰ð‘Ž is the initial forward blocking
voltage.
Spread time (ð’•ð’‘)
1. Time taken by the anode current to rise from 0.9ð¼ð‘Ž toð¼ð‘Ž.
2. Time for the forward voltage to fall from 0.1ð‘‰ð‘œ to on state voltage drop of 1 to 1.5V.
During turn on, SCR is considered to be a charge controlled device. A certain amount
of charge is injected in the gate region to begin conduction. So higher the magnitude
of gate current it requires less time to inject the charges. Thus turn on time is reduced
by using large magnitude of gate current.