This Circuit is used to switch to a secondary power supply without interruption, if the voltage of the primary supply drops below 22V.
The OP-Amp works as comparator, driving the N-Channel FET as driver for the P-Channel FET.
The 3.3V supply was selected to connect an analog input of some microcontroller to sense the state of the circuit, but it could be changed to any other source, like a Z-diode.
