There is a question each electronics guy ask himself one time on his life, “Why people use N mosfet instead of P mosfet?”.
P mosfet is really more easy to use because the ground is the same everywhere, you just have to deal with different tension, but everyone tells us N-mos is better!
There is a lot of subject and reply about that on the web but most of them just speak about performance and price (which is a big point here) but for me they miss the point.
That’s right N-mos have better performances and are cheaper than P-mos. This is due to 2 things N-mos is easier to build and easier to sell because a lot of people use it. Most of the case that’s the reason why people choose N-mos.
In our case we have to choose N-mos because we can’t use P-mos. We want to use mosfet to isolate a power source from a circuit as described here : Robus power management
In this case we can’t use P-mos because we have a big range of different voltage to deal with.
All mosfets have a VGS threshold representing the tension needed between Gate (G) and Source (S) pin of the transistor. The Gate pin is the one you use to command your mosfet to switch around the VGS threshold and the source pin is your power reference (V+ for P-mos and GND for N-mos).
P channel typical circuit :
For P mos the Vgs is generally a negative value for example -1V. For example if we have a source at 12V with this one we need a commande under 12-1=11V to be passing and above 11V to be not. To be able to control it using a 3.3v microcontroler we will need a transistor with (3.3/2)-12 = -10.5Vgs treshold. But after that you can’t change the value of V+.
N channel typical circuit :
If we use the same example with a N mos with a Vgs of 1v (N mos Vgs threshold are generaly positiv) the source is connected to GND so we need a command above 1V to be passing and under to be not. You can control it using a 3.3v microcontroler no matter what tension your power circuit have
In our case, with robus the input tension value can vary between less than 5v and 24V, we have to use N mos.