I am using two N-channel enhancement mode power MOSFETs to switch 12V and 5V power rails via a single low-voltage control signal. I have connected the control signal to a third MOSFET to toggle 12V as the gate input to the power MOSFETs. Will this work or are there problems with this circuit?

 

Will this work or are there problems with this circuit?

Don't see how that will work as all 3 mosfets have the source connected to the + side of the supply voltages.
Even connected correctly the outputs will not be 12 volts but equal to the control voltage minus the gate threshold voltage.
Use P channel mosfets to switch high side.

 

I am using two N-channel enhancement mode power MOSFETs to switch 12V and 5V power rails via a single low-voltage control signal. I have connected the control signal to a third MOSFET to toggle 12V as the gate input to the power MOSFETs. Will this work or are there problems with this circuit?
View attachment 301763

Hello,

The first problem is you have all the MOSFETs drawn upside down therefore all the MOSFETs will look like they are turned on already because the parallel diodes will conduct immediately regardless what the control voltage is.
You have to flip Q2 and Q3, and use a bipolar for Q4 or as already suggested a P channel MOSFET.

To turn on an N channel MOSFET, the gate voltage has to be higher than the source voltage, and once you flip the MOSFETs the sources will all be on the bottom with the drains on the top. This in turn means you can never get 12v out to the load either. You would need a voltage higher than 12v like at least 17v using the right choice for the MOSFETs.

Here's a new drawing but you can see now the control input has to go up to 17v or close to that in order to turn the P MOSFET off. You can however use a bipolar to drive Q4.
17v is needed to turn on Q3.

 

It would work with P-channel MOSFETs.
With no input, Q4 would be on. Q3's gate would it at its source voltage and therefore off. Q2's gate would be 7V above its source voltage and therefore off.
With the input at Vin. Q4 would be off. Q3's gate would be -12V with respect to the source, and Q2's gate would be at -5V, so both would switch on.

 

MrAl already mentioned the power issue and you also need to change the input to output from left to right.

 

.... but welcome to AAC!