This is a simple Over-Voltage Protection circuit that locks out at around 20.4V. The working is simple. There is a voltage divider at the input which is used to set the lockout voltage. When the input voltage is high enough, the NPN transistor turns on, which inturn turns the PNP transistor on. The PNP transistor pulls the gate of the PMOS high, thus turning it and the load (the lamp) off. There is a small "transition" region between the on and off states (which is bad. The fet gets hot there). I shall try to make it sharper.
Edit: I breadboard-ed the circuit and the lockout is quite sharp.
