This is a charging regulator, and with over voltage regulation across Cload, which is used to provide power for up to 3ms in case of a power micro cut.
What I do understand in this schematic :
How the current limiting works with the PNP and PMOS, using the 10V zener to bias the gate
How it is necessary to have current flowing in the 33V zener to keep it in its breakdown region
Simulating, the current across D33V is 16uA when input voltage is at 72V, this allows to regulate the voltage across Cload at about 33.65V, which is D33V + the NPN's Vbe.
What I don't understand, and what I'd like you to help me with is why we don't just use a zener, and a series resistor to keep the current in an acceptable range? I've tried to simulate such a circuit, using a resistor that would put the current through D33V at about the same as the current design (16 uA), and the zener can't regulate. It will only regulate across Cload when approx. 100mA is flowing through it, which is unacceptable in terms of power dissipation. I don't understand the reason why it doesn't work.