Dwayne Reid wrote:
> Nice job on the power supplies.  I particularly like the use of the
> current sink level shifter (Q9) in the 12-31 Vdc SMPS section - it
> looks as if will do a nice job of ensuring that the output switch
> (Q5) does not receive excess gate drive.  Quick glance at the circuit
> suggests that the highest Vgs the FET will see is about 14V.  Nice!

Thanks.  The wide range of input power options is one thing I think sets
this board apart.

The nominal gate drive on Q5 is about -11V when on.  The base of Q9 is
driven with about 5V, so R14 sees about 4.3V accross it.  That makes a
current sink of 4.3V / 750ohms = 5.7mA.  That accross R7 is about 11.5V,
which minus the B-E drop of Q6 leaves about -11V base drive on Q5.  The
resistors are only 5% and there is some variation in the original 5V drive
voltage, but there is also a lot of room with this FET.  It has a Rdson of
20mOhm at -10V and still does 35mOhm at -4.5V gate drive.  The FET's
absolute maximum allowed drive is -20V, which there is no danger of getting
anywhere near.


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