At 08:34 AM 2/27/2005, Dave Tweed wrote: > > > 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? 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 beat me to it - the board I am laying out right now places the zener across Q6. 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. It doesn't need to be huge (I'm using 22R) and needs to be placed physically as close to the FET as possible. A ferrite bead on the FET gate lead works well also. 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. 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. dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 21 years of Engineering Innovation (1984 - 2005) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist