Make a simple soft-shutdown circuit

[ratatax]

Hi,
I have a polarity protection circuit using a P channel mosfet that is working (Q1, R9, D28). R9 was initially connected to ground.



I'd like to add a switch to:
- Power on the circuit when it's pressed (no need for latching circuit, the switch keeps its position)
- When the switch is turned OFF, the MCU keeps the circuit active until it decides to shut it down by itself.



Since the switch I use is DPST, I guess one contact can be used to tell the MCU when the user wants to power down the circuit, then with the additional MOSFET force Q1 to switch off by setting MCU_POWEROFF_CONTROL to input (high impedance). I'd like to use a p-ch mosfet too for BOM simplicity.

I'm a beginner, would that work ?

[Ian.M]

That wont work.  The body diode of the reverse polarity protection MOSFET will conduct so all the power on/off circuit will do is lower the power rail by the body diode Vf drop when off.

To make it work, you need a separate P-MOSFET for power control, with its source fed by your reverse polarity protection MOSFET.

[ledtester]

I would look at Dave's video on a soft latching power switch for ideas:

https://youtu.be/Foc9R0dC2iI

His goal is to have a momentary switch both turn power on and off, but there's enough details of how the circuit works to see how to shut the power off via a digital signal.

[Peabody]

Here's a circuit that uses a push button to power everything up, and it's an input to the processor as well.  Then the processor maintains the power and turns it off when ready.  But the polarity protection would need to be added ahead of all this.  The orientation of the protection mosfet is opposite that of the switch mosfet.

Also,  you might try to find a mosfet that will handle a GS differential of something like 20V so you wouldn't need to use zeners.