Difference between placing unidirectional TVS diode before fuse or after P mosfet
A transient voltage suppressor (TVS) diode will shunt overvoltages to ground and help protect the circuit. If you place it &quot;after&quot; the fuse, then overvoltage events will cause excess current to flow through both the fuse and the TVS. If you place it &quot;before&quot; the fuse, then current will only flow through the TVS. Generally you would want to have it after the fuse so that the fuse can open (&quot;blow&quot;) during a prolonged event.
If the TVS is before the fuse, and an overvoltage event occurs which damages the TVS, the fuse may not blow before the circuit is exposed to the now-high voltage in the absence of the working TVS. I think of a TVS as something that's extremely fast acting, but cannot handle sustain high current for very long. The fuse is slower (even a &quot;fast blow&quot;), and I think of it as relying on the TVS to react first so that it then has time to heat up and blow the fuse element as necessary.
Edit:
In case you're wondering whether to handle reverse polarity or overvoltage &quot;first&quot;, consider a bidirectional TVS diode which would still be between the fuse and FET, to protect the FET from reverse over-voltage, should you feel that to be necessary.