pic microcontroller discussion list wrote:
> Since the FET is a P-channel, it turns on if it's gate is pulled down
> so pulling it up turns it off.  Equilibrium will be reached when Iout
> = Vbe(PNP)/.5
>

To add my two cents:

Note that the Vbe drop is not all that accurate, either. A
small-signal bipolar will start conducting if the Vbe voltage is
~0.5V, more or less. Its also greatly dependent on temperature. The
0.7V they teach in school is an approximation only. You have to
understand when it is valid to use the approximation and when not.

As Bob Blick pointed out in another post, the circuit you mentioned is
not designed to protect MOSFETs in an H-Bridge application.

A relatively simple method is to sense the voltage drop across the
switch. A fully-on MOSFET has a maximum Rds(on). Be aware that this also
varies! So the idea is that if you know the on-state resistance, you can
calculate the voltage dropped across the low-side FET while it is on.

Note this can cause problems while the other leg is on. Obvious when
you think about it, but when the other leg is on, then the voltage
across the FET is equal to the supply voltage.

Or insert a small resistor in the return path, and measure the voltage
drop across that resistor.

The other option is to use a current-limited supply feeding the bridge,
which is probably what you were thinking to use the original circuit
for? I personally like the per-cycle limiting, but that may just be
a personal preference.



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