I am designing a high-side solid-state relay to replace a mechanical solenoid. The solid-state relay uses an array of high-power MOSFETs connected in parallel. It has been tested and successfully switches a 100A DC load at 12V.

I am concerned about the possibility of the switch failing closed. The load is a high-speed DC motor intended for intermittent duty and leaving it on could damage the motor and create a safety issue. Apart from large flyback diodes near the source, what other precautions can be taken to avoid damage to the MOSFETs that could lead to a failed-closed condition?

  • \$\begingroup\$ Q1 why are you replacing the solenoid Q2 what steps have you taken to current share between the FETs. ,Q3 Circuit diagram and component types extremely useful. \$\endgroup\$
    Commented Dec 31, 2021 at 2:53
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    \$\begingroup\$ I have never seen a FET fail any way other than closed. I think it is your responsibility to make sure that if a FET fails closed, for any reason, the consequences are acceptable (no major injury/death, and no major property losses). \$\endgroup\$
    – user57037
    Commented Dec 31, 2021 at 6:53
  • \$\begingroup\$ The load is a high-speed DC motor intended for intermittent duty and leaving it on could damage the motor and create a safety issue. - that's become a showstopper in my book. Do you normally do risk assessments this way? \$\endgroup\$
    – Andy aka
    Commented Dec 31, 2021 at 9:32
  • \$\begingroup\$ Do you have control over the power supply? \$\endgroup\$
    – bobflux
    Commented Dec 31, 2021 at 10:17
  • \$\begingroup\$ @Andyaka, I'm not sure what you mean by that? I'm trying to determine if this failure condition is preventable. \$\endgroup\$ Commented Dec 31, 2021 at 22:37

2 Answers 2


If you are using the SSR for speed or wear out reasons, then you could put another mechanical relay in series. You would normally keep that one activated, but of you detect the SSR has failed, you would deactivate the mechanical relay.

You could consider the same concept with another SSR, but it is possible that whatever supply V transients that might damage the 1st SSR would also damage that one.

Be careful that your real-world environment is the same as your test setup -- specifically the inductance of the wires carrying the 100 A -- switching this quickly (as a SSR can) can induce very large high energy V spikes. If these wires are significantly longer in the real application than your test setup, you could have difficulties.


I assume you mean " array of high-power MOSFETs connected in parallel", since 12V does not require multiple MOSFETs.

Well, solid-state devices inherently tend to fail 'on' (unless you literally blow the leads or wirebonds off the device) and there is really nothing you can do about that except operate them conservatively, and even then they will probably eventually fail. With multiples in parallel, the problem is multiplied.

You can add a mechanical enable relay or perhaps do something with a fuse that would blow if the motor was powered continuously. In consumer devices where cost is paramount, a thermal fuse which directly interrupts the current may suffice (detecting overheating), but they usually are good for 10 or 15A not 100A.

  • \$\begingroup\$ Yes, you are correct, I meant parallel. I have updated the question. \$\endgroup\$ Commented Dec 31, 2021 at 1:43
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    \$\begingroup\$ Also it is difficult to find DC rated thermal fuses. At least I have not been very successful at it. Using an SCR to blow a fuse might work if you can detect that the FET's have failed. \$\endgroup\$
    – user57037
    Commented Dec 31, 2021 at 2:00
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    \$\begingroup\$ I was thinking a slow-blow fuse or circuit-breaker might work if the duty cycle of the motor is sufficiently short. A thyristor blowing a fuse is certainly a workable approach. \$\endgroup\$ Commented Dec 31, 2021 at 3:37

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