I understand why some transistors have a built-in (normal) diode from drain to source, so that when the transistor turns off, back-emf can flow back up, specially important in inductive charges, but why do some MOSFET like IRF540 have a zener? From what I understand, a zener diode conducts backwards with a specified drop voltage. I can only suspect that this zener has something to do with the absolute maximum voltage sustained between source and drain, specified in the datasheet, is that correct?
Also, I am trying to build this circuit: MOSFET gate drive using a transformer http://oi43.tinypic.com/bfkyza.jpg
But the transistor I am using as Q1 is currently IRF540 that has a zener diode instead of a normal one. As expected, it doesn't work very well. As proof of concept, with a normal diode (no Q1), Q2 I can successfully charge the gate and it lets current pass through.
I tried to put a normal diode in parallel with the intrinsic zener and it still doesn't work...