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What is this MOSFET for? schematic

First I thought it's some sort of over-voltage protection but right after this, there's Zener diode doing the same?

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2 Answers 2

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It's for reverse polarity protection. Note that, even if it were turned off, the body diode will conduct under normal conditions. It will block when the input voltage polarity is reversed.

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  • \$\begingroup\$ That makes sense now, thanks a lot! \$\endgroup\$
    – Konidem
    Mar 19, 2020 at 15:05
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That PMOS is being used for reverse polarity protection. I see the net DC_5V_IN is connected before the PMOS so it is not meant to protect reverse polarity from the DC barrel jack J2. That makes me think it is there to prevent current from being pushed to the 5V DC bus from the "wall" side of the circuit.

It looks like R1 and R2 are simply jumpers (0 ohm) that if you added to the circuit would bypass the PMOS altogether.

DNP may mean "Do Not Populate".

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  • \$\begingroup\$ DC_5V_IN goes only to OVP_LED and there's jumper for selecting either this or USB 5V so I think reverse polarity protection makes sense \$\endgroup\$
    – Konidem
    Mar 19, 2020 at 15:19
  • \$\begingroup\$ Could you tell as well why the FDMA530PZ has a back-to-back-diodes on the transistor's Gate? For a novice like me, it looks like these diodes will never conduct anything except for ultra-low reverse leakage current. So, what they are for there? \$\endgroup\$ Mar 19, 2020 at 20:37
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    \$\begingroup\$ @quetzcoatl Sure, the image shown in the schematic isn't accurate. The data sheet shows that each diode is a zener diode. The data sheet doesn't note anything about them but it is safe to assume they are meant to prevent Vgs from getting too high and damaging the PMOS. \$\endgroup\$ Mar 19, 2020 at 21:45

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