P-Channel MOSFET as Switch vs. Rectifier

[harvie]

Hello,
i've understood that it's possible to use P-channel MOSFET as rectifier. Eg:

https://www.youtube.com/watch?v=IrB-FPcv1Dc

But they wire Drain to power supply + and Source to load (-). However P-channel MOSFET is used as switch it's wired by Source to supply + and by Drain to load (-).

What's the difference? I know it's "unipolar" so it should not really matter, but there must be some difference, otherwise the pins wouldn't had different names.

Can i connect P-channel mosfet by Drain to + power, when using as switch? I'd like to use it for reverse voltage protection also, but if i wire it other way round it would not work because of parasitic diode... Will it loose performance if i'll do so?

I am designing low voltage cutoff circuit using P-MOS as switch and i think it would be cool if it could also prevent the reverse voltage...

 

[Analog.World]

@ Harvie, Mosfet is a four terminal device. Mosfet would be truely "Unipolar", if the fourth terminal is available in the package. But as shown in the video, usually standalone (n/p)Mosfet chip has only three terminals. Where it is understood that, the substrate and the source terminals are internally shorted.

Now, if you connect the three terminal device in reverse way, i.e. source connected to power supply and drain to the load. Then, during the reverse power supply condition(VIN = -12V), the substrate will be biased at -12V and the drain at 0V . Hence, PN diode between drain-substrate will be forward biased and current will start flowing, i.e. no rectification.