JohnRob, thanks for the explanations. That really helps.
As I previously asked TinMan, is there any reason not to use the 18V version of the Zener?
You said "at any significant frequency this circuit will not be capable of switching the FET on fast enough". Is that because of the values of R2 and D2 only or are there other problems as well?
I really would like to make the circuit a) generic enough for various switching needs as far as watts goes b) generic enough for 12 or 24V as needed c) isolated d) PWM capable if possible.
Thanks again!