Ruggeduino Output Rails Protection Doesn't Make Sense To Me

[yrrapt]

Hi,

I've been looking at the Ruggeduino board (http://ruggedcircuits.com/html/ruggeduino.html) for some inspiration for circuit protection in one of my projects.  What interests me in particular is the output protection to stop applied voltages raising the rails and blowing other components connected to those rails.

They show the circuit they use but as far as I can understand it the P-type MOSFET is connected backwards and supplies power to IO through the diode across the drain and source regardless of gate voltage.  When the IO voltage is higher than the regulator voltage the diode blocks any current and the comparator turns the MOSFET off.  But then surely just using a Schottky would have sufficed without the extra circuitry.



Can someone tell me what I'm missing here?

Cheers,
Tom

[Short Circuit]

Guess they wanted to elimate the voltage drop that comes with just a diode.

btw Too bad their mosfet will be destroyed with any 5VIO transient/spike above 20-25V due to the limited Vgs(max) of the mosfet...  So much for rugged

[dannyf]

the P-type MOSFET is connected backwards

They are trying to protect against a high transient on the +5VIO line (assuming that it is within Vgs(th) ratings of the mosfet).

So much for rugged

Being rugged doesn't mean it is unfallable.

[sleemanj]

They show the circuit they use but as far as I can understand it the P-type MOSFET is connected backwards and supplies power to IO through the diode

Watch:

[dannyf]

Watch

That came from an old IRF application note. N-ch mosfets can be utilized as well.

[yrrapt]

I was under the impression that the P-channel wouldn't conduct current in this direction.  Obviously a bit of a fundamental understanding failure on my part, back to the books!

Thanks for the help guys.
Tom

[Harvs]

I was under the impression that the P-channel wouldn't conduct current in this direction.

Common misconception.  A mosfet will have a low impedance in both directions with a Vgs into saturation.  That's what makes them good as synchronous rectifiers.