turn-off time

When a thyristor or other power semiconductor switches from the conducting state to the blocking state, it does not stop instantly. Turn-off time is the interval between the moment when anode current falls to zero and when the reverse voltage across the device settles at its steady-state value. During this window, charge carriers are still being swept from the junction, and parasitic circuit elements continue to influence the voltage rise.

Turn-off time matters because it determines how quickly you can stop current flow in a circuit. In a half-controlled bridge rectifier or chopper circuit, this delay creates a dead-band where neither the upper nor lower switch should conduct; if you do not account for turn-off time, both devices may conduct briefly and cause a short circuit across the supply. For devices rated at 600 V or higher, turn-off time typically ranges from tens of nanoseconds for modern fast-recovery diodes to several microseconds for older thyristors.

The phenomenon has two components. During the first part, reverse current actually flows as stored minority carriers exit the junction, and the reverse voltage applied by the circuit accelerates this process. Once carriers are depleted, the junction capacitance dominates and voltage rise becomes steeper. A designer must distinguish between tail current (the slow trailing edge of reverse current) and the transition time for voltage to reach 90 percent of its final value, as data sheets may specify each separately.

Turn-off time is not fixed. It increases with forward current just before turn-off, with junction temperature, and with how much reverse voltage the circuit applies. Soft-switching topologies deliberately shape the turn-off transient to reduce electromagnetic interference and switching losses, accepting higher conduction loss to trade for cleaner turn-off behavior. Hard-switched circuits like boost converters or induction heating inverters live with the full turn-off time and plan protection circuits accordingly.

The term itself describes a measurement interval, not a property of the device alone. The same thyristor will exhibit different turn-off times depending on whether you measure in a snubber circuit, a fully inductive load, or a resistive bench test. Circuit inductance, snubber networks, and control gate drive strength all compress or extend the measured interval. This is why device data sheets specify turn-off time under defined conditions: typically a specified forward current, reverse voltage rate, and temperature.