Stop thinking of it as a switch between the positive supply and the load. An N-channel MOSFET is used as a switch between the load and ground. If you need a switch between the positive supply and the load you need a P- channel MOSFET.

The same is true for NPN and PNP bipolar transistors.

Bob

Okay that makes a lot more sense.

Stop thinking of it as a switch between the positive supply and the load. An N-channel MOSFET is used as a switch between the load and ground. If you need a switch between the positive supply and the load you need a P- channel MOSFET.

The same is true for NPN and PNP bipolar transistors.

Bob

I actually didnt know that. Thank you

 

Here is how the IRF540 Mosfet works as a follower:

Oh okay thank you, I think I am understanding it now. It seems like I need to use something else as the switch since the control voltage cant be above around 10V. Ideally less than that though.

 

I copied the Mosfet symbols wrong.
Here are the corrections:

 

I copied the Mosfet symbols wrong.
Here are the corrections:

Thank you, I can see why a PMOS is needed, but why is a bjt needed?

 

The P-channel Mosfet is turned off only when its gate voltage is the same as its source voltage, +20V.
The NPN transistor turns on and turns on the Mosfet when its base voltage is a little more than 0.7V. They both turn off when the base voltage is less than about 0.4V.

 

When you say 'in front of the PMOS' do you mean attached only to the PMOS gate? Like the image? What would the emitter be attached to if anything?
View attachment 220863

The circuit like this:

 

I thought the load should be on the source terminal for NMOS?
View attachment 220866

You COULD use your IRF540N as you have it in the above diagram, but you'd have to drive the gate to at least 10 volts higher than your power source V+, ie. 30Volts. Not practical in your case!

 

The emitter of the transistor is shown properly connected to ground, then a positive voltage on its base turns it on and a low voltage or ground on its base turns it off.

When the transistor turns on then its collector goes to ground which applies the 20V across the 10k and 12k resistors and the resulting 11V across the 12k resistor turns on the P-channel Mosfet which turns on the load.

 

If you can put the load between the supply voltage and the MOSFET drain, then you can use an N-MOSFET with the source grounded as a switch.
If the load must be grounded, then you need to use a P-MOSFET with the source to the supply and the drain to the load.
You will then also need an NPN transitor to drive the gate of the P-MOSFET.

 

The P-channel Mosfet is turned off only when its gate voltage is the same as its source voltage, +20V.
The NPN transistor turns on and turns on the Mosfet when its base voltage is a little more than 0.7V. They both turn off when the base voltage is less than about 0.4V.

Thats something I was trying to work through actually. Wow thank you, I was wondering how I could turn it off given the requirements.