Electronic – Why are input MOSFETs in Laptop power supply always in pairs

laptopmosfetpower supply

For example, on https://youtu.be/CW87dyYIimE?t=340 (and related work on motherboard), it says the managment chip uses two MOSFETs as a switch (as overheat protection, emergency poweroff on 10-second powerbutton press etc).

While the story makes sense and I can understand how it would work, I'm wondering why such MOSFETs need to come in pair?
To my (begginers) view, it looks like one MOSFET should be enough to act as switch for mentioned purposes. So why are there two MOSFET connected in series? What else am I missing?

Best Answer

You're missing the fact that discrete MOSFETs generally have their substrates tied directly to their source terminals. This creates a "body diode" in parallel with the drain-source path, and this means that such a MOSFET can only block current in one direction.

Therefore, many applications use two identical MOSFETs back-to-back (sources and gates tied together) so that when they're off, they can block current in either direction.