supply
Yet another N-channel MOSFET, in series with the other one, joined source to source, drain sinking the negative return current. It is permanently biased on from the positive supply. It's gate drive is reduced if the input is detected to be overvoltage. It can be set to turn off completely if there is an excessive input voltage. Generally, it is designed to operate linearly to simply reduce the input voltage to the PSU for the MIL/DEF standard surges of known duration.
Regards
Graham Holloway