Circuit to enable / disable VCC and GND

Question

I need to design a circuit able to enabling and disabling a voltage source and its respective reference (GND). The power supply has a adjustable voltage output and can supply 12V, 5V, 3.3V and up to 5A at 12V. It will be a source for powering external circuits that can be any, as long as they are within the capabilities of the source.

To accomplish this task, I thought about using the following circuit:

Does anyone have any experience or observation about this type of circuit?

Corrections: The mosfets were mirrored

Answer 1

Your circuit needs a small modification: connect the low-side n-FET gate directly to the control input, as follows (simulate it here):

Choose appropriate pass FETs, or even parallel multiples of them to get the desired current handling you seek. (Yes, it's ok to connect MOSFETs in parallel.)

Here's an improved version that counteracts the effect of the FET body diodes (simulate it here):

The back-to-back FETs prevent backdrive from the load.

Answer 2

I believe that the circuit below can meet your need:

I didn’t carry out a sizing with so much attention. The components are just a suggestion, so you should check the electrical characteristics, and see if they meet your needs.

The OpAmp used is of the rail-to-rail type, so I can get the voltage levels to polarize the MOSFET Gate.

The selected MOSFETs can conduct a current of about 7A, and have a voltage VGS(th) of at most 1 V, therefore, these values end up being suitable since you want to use for voltages of 3.3V to 12V.

Below, I leave images of some simulations, working with 3.3V and 12V as supply voltages:

- 3.3V:

Output switched off

Output switched on

- 12V:

Output switched off

Output switched on

It is observed that the voltage drops are very small in the transistors. When the circuit is turned off, the impedance of the MOSFETs is so high that they end up being like voltage dividers, so that when turned off, the voltage over them ends up being half of VCC.

You just have to be careful only with the type of load you will connect to the output, because if not, there will be voltage in it anyway (MOSFETs do not isolate the circuit, for this, consider using a relay):

Observe the multimeter of the output, the VCC voltage is still there even with the output turned off, due to the impedance of the multimeter in relation to the impedance of the MOSFETs.

I believe that the circuit meets your needs.

Answer 3

There's a potentially flawed assumption here. Your enable/disable signal will more than likely come from a micro-controller (hence the pull-down resistor). But unfortunately you'll need the gate of device U15 to be pulled all the way to supply or ground to be conduction or in cutoff.

Sometimes the simple solution is the most elegant one. And this comes up on hardware interviews and it's something like this:

I have an MCU that outputs voltages from 0 to 5V but need to control an ADC that only takes logic levels from 0 to 10V, what can I do?

You can be as fancy as you'd like an make circuits up that behave like level-shifters (level translators). But why reinvent the wheel? the same circuit you have but instead of U15 and the resistors associated with them, you can just use a level shifter that takes inputs from your MCU and level shifts them to your PSU output.