JohnRob:
Hi,The zener protects the gate from excessive voltage. The MosFet Gate to Source max is 20V, So an 18V zener will be fine.
No, that's far from being an adequate margin of safety. Use a 12V zener. It is so super-easy to pop a
MOSFET if you fail to take these precautions. Over-voltage on the gate is immediate death within
nanoseconds to the gate-oxide layer. Its not unknown for upto 5V to appear across the lead inductance
of a MOSFET source when it switches, and that will add to the Vgs that the oxide sees.
If the layout of the gate-drive circuit isn't super-tight, you should place the zener directly across the
MOSFET gate and source pins.