**** WARNING ****
Your FETs life may depend on reading this ;-)

>> > The attached circuit can be used for FET gate drive. As shown 
>> > it's for
>> > the low side.  You could adapt it for high side but it's not 
>> > really
>> > needed if you are slow switching the high side and PWMing only 
>> > the low
>> > side. R4 sets the peak gate current which ideally will be 100's 
>> > of mA
>> > to amps for fast switching.

>>Wouldn't it be better to eliminate R4 altogether and put the zener
>>at the base of Q8?

NO!
The zener is not principally to limit the drive voltage level - its 
job is to clamp transisents on the gate coupled in by eg the Milelr 
capacitance from the Drain. Placing it elsewhere removes this 
protection and FET death has been know to follow.

>> Not only does this reduce the drive impedance to
>>the FET, it also eliminates the steady-state power dissipation in 
>>Q8,
>>and the zener can be a low-power unit as well.

You can add preceding zeners for otehr purposes if you wish.

> You beat me to it - the board I am laying out right now places the 
> zener across Q6.

As above.

> But I disagree with you about the need for the series gate 
> resistor - I've had *far* too many problems with parasitic 
> oscillations when I omit that resistor.

The resistor limits maximum gate drive / suck out current. If you 
provide very high currents the FET can switch very fast and 
dissipation can suffer and can be hard on FET.

Too large and you limit the time taken to charge/discharge the FET 
gate capacitance and make switching times too long.
Something around an amp seems to work OK. This is only required at 
tturn on/ turn off so average current is small - typcially 10 mA or 
less at 20 kHz.


> It doesn't need to be huge (I'm using 22R)

12 volt drive and 22R give about 12/22 =~ 500 mA which is fine.

> and  needs to be placed physically as close to the FET as possible.

Yes

> A ferrite bead on the FET gate lead works well also.

Yes.

> I've been using that driver circuit ever since I saw it in a TI data 
> sheet about 15 years ago.  The project I'm working on right now uses 
> 3- IRF1407 FETs in parallel for the low side switches - each FET has 
> its own gate resistor.  But I'm cheating - my PWM rate is fairly low 
> and I'm using sections of a uln2003 in place of Q6 in Russell's 
> drawing.

I use an ULN2803 in one application.

> BTW - the reason for putting the zener before the emitter follower 
> is so that the whole thing can run from 24V and not need a separate 
> 12 or 15V regulated rail.

Having two zeners may be appriopriate.



        Russell McMahon

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