When the 12VDC is cut, I want to the load to stay powered for a few seconds but, I want no voltage back at point A.

But no. The capacitor voltage shows up at point A. Why don't both fets shut off and the voltage remain only at the load?

 

You need to disconnect the N-FET gate also via a second switch.

 

Gate-source capacitance it keeping both devices on. Why not just add a series diode instead of that complex switching network?

 

When the switch is open, both FETs are off as far as I can tell.

Bob

 

Why not just add a series diode instead of that complex switching network?

I was trying to use the PFET as reverse polarity protection. I know a diode will do that as well but, the actual circuit incorporates a 358 op amp controlling power Pfets and needs to be as close as possible to the supplied high rail.

I've got better op amps to try and the series diode works even with the 358. I am just learning and was puzzled by this. I guessed it had to do with capacitance but wanted an answer from experts.

I knew it wouldn't take long to get the answer.

 

I'm not a MOSFET expert by any means, but I think this is what is happening.

The gate-source capacitance prevents the devices from turning off *immediately*.

Start with the source voltage on. Both devices are turned on and the capacitor on the output charges. Now MOSFETs are pretty much bidirectional devices, and the channel will conduct in both directions when the device is turned on.

Remove the input voltage. The upper MOSFET continues to conduct and passes the capacitor voltage back to the input. That voltage on the input turns on the lower MOSFET which turns on the upper MOSFET which passes the capacitor voltage to the input which turns on the lower MOSFET which turns on the upper MOSFET...........

 

Try this circuit
https://www.electro-tech-online.com/articles/simple-inexpensive-ideal-diode-mosfet-circuit.817/

 

I'm sure you are right, they must both be staying on. Maybe I'll try a harder pull down.

 

Thanks Jony, I will check that out.