Discussion
The turn-off time of thyristor is 30 m sec at 50°C. Its turn-off time at 100° is :
Answer: C
The key gives 60 m sec; the paper writes 'm sec' where microseconds are meant, since thyristor turn-off times are measured in microseconds.
Turn-off time is the time the anode current must stay at zero, with reverse bias, before forward voltage can be reapplied without the thyristor turning on again. It depends on how quickly the excess carriers stored in the inner junction recombine. Carrier lifetime increases with temperature, so the stored charge takes longer to disappear, and turn-off time rises roughly in proportion to junction temperature in the working range. The standard textbook statement used for this question is that doubling the temperature from 50°C to 100°C doubles the turn-off time, from 30 to 60.
The turn-off time therefore does not stay the same or fall to 15, and 100 is not supported by that proportionality.
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