Best practices to avoid a MOSFET switch failing closed

Question

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?

Answer 1

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.

Answer 2

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.