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Re: An extremely good MOSFET driver



Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

HI Ken,
        The bipolar drive scheme is very often used because:

On 7 Aug 2002, at 11:30, Tesla list wrote:

> Original poster: "K. C. Herrick by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
> 
> Daniel, Dave Sharpe, Malcolm Watts (& all)-
> 
> Comments interspersed...
> 
> On Tue, 06 Aug 2002 22:03:43 -0600 "Tesla list" <tesla-at-pupman-dot-com>
> writes:
> > Original poster: "Daniel McCauley by way of Terry Fritz 
> > <twftesla-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>
> > 
> > 
> > This circuit shown below:
> > 
> > http://www.spacecatlighting-dot-com/datasheets/mosfetdriver.pdf
> > 
> > is an extremely fast mosfet driver circuit ...
> 
> [snipped]
> > 
> > BTW, why would you want to turn on your MOSFET slowly???  Doing this 
> > will
> > most likely put the FET in its linear operating range and start 
> > really
> > heating it up.  You want something really fast.
> > 
> > Dan
> 
> [snipped]
> 
> 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?

the bases are tied together as are the emitters. It is not possible 
for both transistors to conduct simultaneously. While it works 
extremely well, it does mean that there is something of a high 
impedance connection to the power device drive element while the 
driver is transitioning between its power rails. Not a perfect 
situation.

Regards,
malcolm

> 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
> 
> 
> 
>