pfet backward

[certs]

i want to use a BSS84 power pfet to be used as a reverse battery protection circuit. i want to connect the drain to the the positive side of the battery and the gate to the negative side of the battery. usually pfets have to have the source connected to the higher potential.

i have seen this setup before and i wanted to know if pfets can be used backward and what implications it would cause.

thanks.

 

[Heydave]

FETs use the charge on the gate to effectively remove the opposing PN junctions between the gate and drain. This only works to turn on the transistor in one polarity. Your idea should work. Is this application meant to reduce the voltage drop you would encounter with a diode?

 

[certs]

ya this meant to replace the diode, since the diode has a higher voltage drop.

so your saying a pfet is symmetrical no mater what direction you put it, it will work?

 

[melone]

Can you get away with a Schottky diode? Using a FET sounds like a good idea, except you will no longer have voltage blocking capabilities. There is a substrate diode inherent to the FET device, and it is biased in the opposite direction to normal current flow. Therefore, you will not be able to block current from flowing in the opposite direction (which is the entire purpose of the diode). Really cool idea, but a schottky might be the easiest solution.

I tried the same idea using a PNP (no sustrate diode and ~100mV drop), but the PNP BJT doesn't like it much when current flows from Emitter->Collector.

Hope this helps!