I'm still working on the Mosfet power switching design I was fussing about a fortnight ago. I'm a bit hamstrung by the fact that I've used Mosfets before in other peoples designs but have never had to actually think about them. I can make a stab at bipolar transistor design but FETs are a closed book to me.
I've selected my Mosfet, a BUZ72A which has the current and voltage rating I want. The question is, how to turn it on. Flowers, chocolate, liquor and soft music don't work.
The data sheet Max Ratings says the max Gate/source voltage is +/- 20V.
The electrical characteristics say that the gate threshold voltage is Min 2V Typical 2.9V and Max 4V The same section gives the static Drain/Source resistance (low, as I want it) for Gate/Source at 10V and Drain current of 5A.
I'm intending to send the gate low with an NPN transistor (BC548) turned on hard when I want the FET turned off.
Question, what is the optimum voltage to apply to the FET gate when I want the FET turned on hard? 10V?
Furthermore. Phil Allison in his reply to my previous post suggested a 63V Zener, drain to source, to protect the FET from inductive spikes. I've since seen a suggestion (on a 'Net hobby page) that a 15V Zener connected gate to source will protect the gate from spikes. It occurs to me that when I have decided on the optimum gate voltage, I should connect a zener of that voltage gate to source. Feed it with a resistor from the 12V supply which is also the load resistor of the NPN transistor. When NPN transistor is not conducting, the zener-controlled voltage is applied to the FET gate. When NPN transistor is turned on hard, the gate/collector/resistor point goes low. Certainly lower than the the 2V Min threshold voltage of the FET. Obviously, I'll choose a load resistor for the NPN transistor which limits the collector current to a safe value. The attraction of this idea is that the zener will alsodetermine the voltage to be applied to the gate as well as protect against spikes.
Question, is it a stupid idea? Has any real designer ever done it this way?