Fail Safe Mode for Trisils

[Entropy2]

We are considering using ST TRISIL (TSS)  devices for crowbar protection . We have been using an alternative type of protection device from LITTELFUSE (SIDACTOR) in an application where we require the TSS to fail as a short circuit. In our application we require the TSS to be a single use device that fails to a very low impedance in a current limited circuit (~3A) . The LITTELFUSE devices have been tested extensively in our circuit and do fail safe to a very low impedance of ~0.1 ohms however some other brands (not ST) have a percentage that fail with impedance of say 5-7 ohms which is too high a resistance  with 3A flowing and this creates excessive power and fire risk. The ST data for the SMP100LC series of TSS device says that it will fail safe and our testing on 10pcs so far shows this to be correct. Can anyone confirm that the design of the TRISIL device will never fail in a higher impedance manner therefore minimizing fire risk?

[Peter BENSCH]

Welcome @Entropy2, to the community!

The SMP100LC TRISILs are indeed designed as crowbar protectors and, when overstressed, they are intended to go to a low‑impedance / short‑circuit state rather than open‑circuit. Your test results are consistent with this design goal.

However, from a reliability and safety point of view no semiconductor vendor can guarantee that a particular failure mode will never occur. The datasheet therefore does not exclude the possibility of rare abnormal failures (for example a residual resistance of a few ohms if the device is used well outside its ratings or suffers mechanical damage).

For an application where fire safety depends on a guaranteed end‑of‑life behaviour, you should:

• Qualify the chosen SMP100LC device in your own circuit with a statistically meaningful sample size, and
• Contact your local ST representative for official reliability data and any safety‑related statements needed for your risk analysis.

TRISILs are designed to fail short, but your system design must not rely on 'never' seeing any other failure mode.

Hope that helps?

Regards
/Peter