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I'm a bit new in electronic repairs. I am trying to learn by myself.

This circuit is from a printer SMPS:

enter image description here

That's an MDD3752 P-channel MOSFET. I took it out of the board and tested it with a multimeter and it opens/closes normally.

When the board is plugged in, I have 24 volts at its source and gate but at the drain I have 15 and it slowly goes down.

The only thing between the MOSFET drain and the board output is a 330μF capacitor which I replaced with brand new.

Is the MOSFET the cause of my problem or am I missing something?

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    \$\begingroup\$ The symptoms you have described exactly tallies with my mental image of the circuit you also described. So, either my interpretation of your circuit is wrong or, it's working as it should. You should post a schematic to resolve this. \$\endgroup\$
    – Andy aka
    Dec 29, 2023 at 9:50

1 Answer 1

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A P channel MOSFET works by have a negative voltage difference between the GATE and the SOURCE pins. You wrote that the voltage on those two pins were the same, which means that the MOSFET is working correctly, it should be turned OFF.

As for the reason of the voltage at the drain, it could be from a residual voltage when turning on(maybe the MOSFET is ON during startup and it charges capacitors) or maybe a voltage generated by another source. A possibility could be to add a 1K or 10K resistor between the DRAIN and GND to check if it discharges the capacitors and understand what else is connected to the DRAIN that could be generating that voltage.

PS: Normally those residual voltages are normal when you have turned off MOSFETs

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  • \$\begingroup\$ This makes perfect sense! Thank you!!! \$\endgroup\$
    – gkoukoul
    Jan 4 at 20:46

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