Switching a higher voltage using MOSFETs and circuit design help.

I am helping to design a circuit at work which switches a battery supply off once a DC supply from a wall adapter is applied.
I have found many circuits around that do this but only when the wall supply DC voltage is greater than the battery.

For my case, the wall supply is at 13.8 V and the battery is at ~16 V. The circuit needs to be able to supply up to 17A to the load.

MOSFETS are quite new to me but I was hoping to use them to make the circuit as small as possible as the size available for the circuit is quite limited.

The circuit I’ve designed is attached but I’m open to suggestions :slight_smile:
It is based on this circuit http://ruggedcircuits.com/html/circuit_-11.html - But with “Digital Out” being the DC wall supply at 13 V and the wall supply “switching off” the battery supply.

Could anyone please let me know:
a) Will this circuit work?
b)If so, great, but is there a better way to do it?
c) If not, how should I go about doing this?

Thanks, in advance, for any help.


My circuit design is weak, but my first inclination is Battery into MOSFET, p-channel like a PNP BJT, out to load to ground. Wall source and pull down resistor into gate. Wall source on into load.

If Wall is 0v, gate is low, MOSFET conducts, load is on Battery.
If Wall is High, gate is high, MOSFET does not conduct, load is on Wall.

I dunno.

Could anyone please let me know:
a) Will this circuit work?
b)If so, great, but is there a better way to do it?
c) If not, how should I go about doing this?

a) no, the 13.8 supply switches the 16V supply on, not off...
c) You need a 3V3 zener between 13.8 supply and the p-channel gate,
lose the n-channel but keep the pull-down on the 13.8V rail. When 13.8 is
present the p-chan gate will be pulled right up to the source and will switch it off.
When the 13.8V rail drops the p-channel will be driven on by the pull-down and

The schottky diode is going to get hot at 17A, big heatsink needed, better
to actively switch at that point (2 p-channel FETs)

Thanks to both of you for the replies.

MarkT, thank you for your suggestions, but I am struggling to decipher which circuit you are referring to when you talk about changes that I need to make. It is probably my fault for not labelling the diagrams properly. I really don't want you to think I am being rude.

This is what I thought was happening in the rugged circuits diagram, the link that i provided:
When the digital output goes high, the gate of Q1, the n-channel enhancement MOSFET, goes higher than the source, which then shorts T1's gate pin to ground. This then turns the 16V supply on, like you said.

The changes you suggest in my question "c)", are they changes you are suggesting I make to the rugged circuits one? By suggesting that I put a zener between the 13.8 and p-channel gate, I think you are. But I could have just drawn my circuit with the wrong symbols, I just wanted to make sure. When the 13.8 V supply is present, the Vgs will be -2.2 V? Should I be able to find a MOSFET which has a higher turn on threshold than that, so that -2.2 V will turn the MOSFET off?

Do you mean like this?

What do you mean by actively switching at that point? Do you mean one enhancement mode MOSFET and one depletion mode MOSFET so that when one is on, the other is off?

I advise you to get hold of a version of SPICE or PSPICE.
It is a program to simulate circuits for it's DC and AC behavior.

If you feed SPICE with the circuit you can run it once with the 16V DC applied and simulate a DC sweep at the other input from 0 to 14V to see how your circuit behaves. And vice versa run it with 13.8V applied to one input and do a dc-sweep at the other input fom 0 to 16 volt and see what happens.
How the circuit exactly behaves will be dependent of the types of MOSFETS and diode and the load. A variable load can also be simulated with SPICE.

You need to have the right spice-models for your components (library) to do a reliable simulation, but these are not so hard to find.

A free student version of PSPICE for windows can be found here :


limited to 63 nodes i think.

Great, thanks.

I'll see what I can do on SPICE.

I'm curious about the eventual solution, but have nothing to add in terms of circuit design except: one thing to pay attention to is that as time progresses the battery's voltage will drop. While its great to assume its 16V most of the time, it could end up in the triode mode, which would eat power, get hot and generally not be helpful. So whatever you do, you need to make sure you get a distinct digital signal and not something that will fluctuate around with battery voltage.

I got hold of a copy of LTSpice and have had a play with some MOSFETs.

MarkT, I tried to make the circuit that you suggested and this is what I came up with, including a graph with the wall supply on and off.
The lowest breakdown of any zener I could find in the library was 4V7, is there any particular reason you suggested a 3V3 one?

I can't find depletion MOSFETs anywhere in the library, unless I'm missing something. Could anyone point me in the direction of them?