Russell McMahon wrote:
>> http://www.embedinc.com/products/qprot01/qprot2.pdf sheet 2

> As Vasile intimates, you *MAY* be able to save a few cents by removing
> R5,

Yeah, R4 thru Q2 should accomplish mostly the same thing.  Still, for less
than $.01 in quantity...

> adding a base drive resistor for Q5 (parts neutral),

I think you mean Q6?  Q5 is the power supply regulator for the PIC, and does
get its base current thru resistor R6.

> removing C11,
> adding a cap across the Q6 base drive resistor as a speed up cap
> (still parts neutral so far),

But then Q6 no longer acts as a controlled current sink.

> remove Q4, place diode across Q2 emitter
> to base cathode to emitter. Saves one transistor, costs a diode. Diode
> can often be a 1N4148.

Note that a diode is about the same price as a transistor.

> 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.

There is no chance my original circuit can overdrive the gate of Q3.  Even
if Q2 and Q4 have infinite gain and therefore they present the R4 voltage to
Q3 with zero impedence, the rise and fall time on R4 just isn't and can't be
fast enough to cause problems.

> 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.

This is a problem only with your altered version.  I think you are missing
the big point that Q6 acts as a controlled current sink when the PIC output
is high.  This causes a fixed voltage drop accross R4 regardless of the
input supply rail voltage.  See my other response to Vasile.  The Q3 gate
drive remains about 11V regardless of input supply voltage level.


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