Switching between two DC supplies with polarity protection using common return

[x4ce]

Dear,

I have a setup where I need to connect an external power source of 12V & a 4S battery with common return (negative). I got a PCB with following schematics of each source which has polarity protection along with switching of power i.e. 12V supersede 4S. But, the intended circuit is not working. When I connect both 12V and 4S, a short-circuit is observed. There could be something that is missing.

Please help/ guide in identifying issue.

Appreciated!

Regards

[m k]

What have you measured?

[jwet]

Unfortunately, there is a big hidden body diode between source and drain.  These kind of circuits that want to allow bidirectional current flow need two back to back MOSFET's.  Just google for back to back n channel low side for battery isolation.  Hopefully you can salvage what you have and add a FET.  Good luck.

edit: original has "source and gate"

[Konkedout]

Unfortunately, there is a big hidden body diode between source and gate.  These kind of circuits that want to allow bidirectional current flow need two back to back MOSFET's.  Just google for back to back n channel low side for battery isolation.  Hopefully you can salvage what you have and add a FET.  Good luck.

I think you are referring to the drain body diode between source and drain, not source and gate.  When you use a normal MOSFET for ORing control (or as a synchronous rectifier) your normal conduction is generally reversed from using the MOSFET as a switch.  So a Pch drain will be positive relative to source, and an Nch drain will be negative relative to source.  The drain body diode will then conduct in the direction that you want to conduct anyway.

https://www.vishay.com/docs/68934/si7149dp.pdf

I strongly recommend that you replace your MOSFET box symbols with correct, complete MOSFET symbols which include those diodes.  That will help you to see the error of your ways...

[jwet]

Correction made- thanks