JaBa:
Tinman, thanks for explaining thus far. If I am correct that R2/R3 creates a voltage divider which reduces the gate potential on the MOSFET to approximately half what the switched voltage is, then I have to make sure that the Zener is rated for at least that voltage. So, assuming 30 Volts max, I need at least a 15V D2, right?
No, that's not quite it.
R2 limits the voltage across the opto isolator so it doesn't burn up, but must be small enough to permit the
gate to rise to 12V for any valid supply voltage - its value depends on the rest of the circuit.
R3 turns off the MOSFET and needs to be low enough in value to do that fast enough (MOSFET
gates are very capacitive). However the opto-isolator current is going to be a limiting factor - it
must be able to pull the gate voltage upto 12V despite R3, so R3 can't be too low either.
D2 sets the on voltage for the gate which should be close to 12V. Use a 12V zener.
This circuit is not suitable for PWM at high rates (like 10's of kHz), its not able to switch the MOSFET
particularly fast.