# What factors affect a thyristor's “circuit commutated turn-off time”?

In datasheets, they give this value at a certain operation point (voltage, current, and di/dt). My question is... What are the effects of changing these parameters.

For example, if di/dt is increased, will this make the circuit commutated turn-off time less? Or is the circuit commutated turn-off time an inherent property of the thyristor?

Thanks!

Some parameters do effect others. For example, see the following from the General Electric SCR Manual 5th ed. 1972

• very good. I forgot that the GE Manual had that list. Not many manuals around anymore. – Marla May 24 '19 at 20:21
• Could you please share the picture with these $t_1$ to $t_{10}$ as well? – Huisman May 24 '19 at 21:03
• @Huisman: Image added – Charles Cowie May 24 '19 at 23:47
• Thanks a lot Charles, for the information. In the circuit I have designed, I can control the reverse voltage across the thyristor during turn-off. According to (5), an increase in reverse voltage can result in a shorter tq. Now, do you know how it could be calculated how short tq can realistically be made? In a datasheet, I am seeing 400us as a common tq (this is at a given reverse voltage). If the reverse voltage is doubled (compared to the datasheet) is it reasonable to say that the time tq will be halved? Would other parameters scale linearly? – CaptainFantastic May 29 '19 at 21:20
• I doubt that doubling the reverse voltage will halve the tq. If you are looking at thyristors with tq of 400us, I think you are looking at devices that are not intended for force commutation. Depending on voltage and current rating, you should be able to find devices with tq in the 10 to 60 us range. What voltage and current rating do you need? @Maria, what do you think? – Charles Cowie May 29 '19 at 23:26

A thyristor (SCR) turn off time is affected by the reverse voltage applied to the thyristor during turn off time. There is even a patent (lapsed) for increasing reverse voltage across the thyristor to reduce turn off time.

Patent # US4009430 (A) ― 1977-02-22 : PARTRIDGE DONALD F

ABSTRACT : A control circuit for regulating power to a resonant load from a dc source including switching devices, magnetic means and antiparallel means for increasing the reverse voltage on the switching devices, reducing the dv/dt stresses on the switching devices, reducing the likelihood of a crowbar or shoot through of the switching devices and obtaining the ability of recovering from a shoot through without device failure or circuit interruption.