MOSFET protection with TVS: at the source pin or at the drain pin
If you put the Fat load-dump TVS downstream of the reverse-polarity protection MOSFET , then the load dump current would have to flow through the MOSFET. The MOSFET would have to be rated for that current.
If you put the Fat load-dump TVS upstream of the reverse-polarity protection MOSFET, the load dump current would not have to flow through the MOSFET. But the MOSFET wouldn't protect the TVS from the reverse polarity. You'll need a fuse further upstream, and it would open when the TVS starts to conduct due to reverse polarity.
Reverse polarity protection element in general. In this case it's a MOSFET. But it could be a Schottky in some cases. ↩︎
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Using a P FET for polarity protection like this means that in case you flip + and -, the gate will be inactive, meaning that GND which is now connected to drain will get disconnected - no current will flow.
If you put a TVS before the MOSFET, you cancel this out. Instead the TVS will short + and - and take the blow instead, so the polarity protection no longer works as intended.
No matter the TVS picked, something will get fried in case the TVS is placed before the MOSFET. In fact, in case you have some gigantic 6.6kW peak pulse TVS, it's so damn rugged that it will probably survive that short. Meaning something else will give instead, like a wire or a trace - and there you have a big problem.
Therefore the TVS should be placed after the MOSFET and zener. The MOSFET is close to an ideal diode in this circuit so it will happily forward a spike and then the TVS will take the spike before it does any damage.
You could however place a non-polarized cap before drain to even out the supply, without affecting anything else.
There is no need for a fuse unless you have EX/intrinsic safety requirements, in which case you'll also need multiple TVS.
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