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?


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.

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    \$\begingroup\$ OK, that makes sense in general case, but why would it be needed on laptop input power jack? It has well defined + and - terminals, so direction of current flow is known in advance. Only way that makes sense if manufacturer also wants to protect against users cutting their powercords and connecting them with reverse polarity (which should not happen all that often, and it would void the warranty anyways). I could undertand if it was some odd laptop with such extra protection, but not (looks like) all of them? \$\endgroup\$ – Matija Nalis Nov 6 '18 at 3:06
  • \$\begingroup\$ Because the laptop has power internally, it might send current back into the charger if you don't block it. So when the switch is off, it blocks both current from the charger into the laptop and from the laptop into the charger. \$\endgroup\$ – C. K. Dec 3 '19 at 3:01

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