>>Take a critical look to the Q6, Q4 and Q2 and see the redudancy.

    http://www.embedinc.com/products/qprot01/qprot2.pdf sheet 2

> I guess you have little experience of driving power MOSFETs in high 
> frequency switching applications?
>
> The driver needs to have a high current capability in order to 
> charge/discharge the (significant) gate capacitance of the MOSFET to 
> minimise the time spent in the linear region.  Failure to do this 
> results in poor efficiency and high power dissipation.  A single NPN 
> with resistive collector load does not (usualy) satisfy these 
> requirements unless the resistor has a very low value, which again 
> increases power dissipation and reduces efficiency.

/>

_____________________

The driver is fairly standard and a good safe bet.

As Vasile intimates, you *MAY* be able to save a few cents by removing 
R5, adding a base drive resistor for Q5 (parts neutral), removing C11, 
adding a cap across the Q6 base drive resistor as a speed up cap 
(still parts neutral so far), remove Q4, place diode across Q2 emitter 
to base cathode to emitter. Saves one transistor, costs a diode. Diode 
can often be a 1N4148. Diode now provides the low side drive for Q3 
via Q6.

R5 is strictly not needed BUT I'd consider adding a small series gate 
drive resistor her to limit gate currrent to stop really really hard 
switching which can also increase losses.

The reduction by one transistor is unlikely to be worthwhile.
R4 can be increased until speed becomes an issue - which may be at 2K 
as that's what Olin's used.

******************   BUT   *******************

                 as shown this circuit has a major problem.

Vgs_max for the IRF7416 is -20 volts and when supply voltage is much 
above 20 volts the FET will be driven into a potentially (pun almost 
intended) fatal state.
Adding a series resistor     Q6_C to Q2/4_B will allow this to be 
fixed, but will introduce drive speed issues.

Returning Q4_C to a positive potential less than 20v below Vin with a 
suitable reservoir capacitor will fix the overdrive problem. This can 
be a resistor/zener/cap supply and needs not supply much current as, 
while the FET gate drive should be 100's of mA to 1A ish (as Michael 
noted) the mean current is low as the peaks occur only during 
switching.



        RM


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