Makeshift Passive USB Charger 5V OVP
If you just try to protect against some &quot;should never happen&quot; case, a simpler solution may be to put a 5.6 - 6 V varistor like this across the power terminals.
The varistor can handle 250 A. I suggest to add also a fuse that would quickly disconnect the rogue power source. There are self-resetting fuses available on the market, say hold 2 A, trip at 3.5.
The only problem I see with this solution is the quite wide range of the trigger voltages of the varistor. If the lower boundary is 6 V or about, the upper boundary may go as high as 11 V. It will trigger at 20 V anyway, but under some 8 V it may not.