RE: An extremely good MOSFET driver

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Original poster: "Mccauley, Daniel H by way of Terry Fritz <twftesla-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>




Never had any problems.  This driver isn't really my design, but its a
proven design we use extensively in DC-DC Power supplies where I work.  The
only possible problem is that this circuit never entirely turns off.  There
is always that diode drop voltage.  But when dealing with FETs that don't
turn on until a much higher voltage, we don't really care.

Dan




I'd love to turn them both off & on blazingly fast...except that if I do
that with any given pair of them, then both transistors of the pair find
themselves on at the same time while one is turning off & the other, on. 
That shorts out the power supply during that instant of time.  I notice,
in your circuit, the other secondary winding, and suppose that it drives
an identical MOSFET circuit, which perhaps is in a 1/2-H configuration
with Q1.  Do you not have that problem?

Other than that, the scheme is surely OK.  Have you considered using a
paralleled-gate CMOS logic IC in place of the two bipolars?  Or are the
older 15V-rated ICs becoming obsolete, nowadays?

Re David's comment:

"I've always been told to NEVER forward bias a zener diode,
you will hose its knee characteristics.  Comments from the
list???
 
Regards
Dave Sharpe",

I've never heard that although I have heard it about bipolar base:emitter
junctions, as to degrading the transistor's gain--but I don't know if I
believe it.  I have an old Motorola Zener Diode Handbook that goes into a
lot of detail and there's no mention in it of that kind of problem. 
Anyone else have info?

As to Malcolm's comment re gate resistors--yes, I'd want 10 ohms or so in
series with the gate(s) for parasitic suppression, and also a pull-down
resistor, gate:source, for Q2.  Also, the transformer might need a load
resistor.

Ken Herrick