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Mosfets usually fail with a short circuit

It happens because gate is actually isolated from semi-conductor npn doped sandwich

As soon as it has small leak, it exponentially grows up at some point

And you have full short between gate drain and source - insulator simply is not there anymore. ESD damage is a real thing since higher voltages might make a small "hole" in the insulator and mosfet will smoke as soon as running current reaches enough rage to "find" this little hole

On p mosfets you have basically the same situation except you are supossed to have continuity between drain and source if gate is low but not between these two and gate, since it is always isolated on FET(edited)

P mosfets sometimes fail with no short to gate, it could be diagnoses off-circuit

You can positively "charge" mosfet gate with a multimeter probe, you just use MM metering voltage

Putting power with a red probe on gate should close continuity between source and drain


Basically MOSFETs have a diode allowing current to flow from drain to source even when gate isn't triggered.

But as it is a diode ,it has a threshold voltage/forward voltage drop.

So when MOSFET isn't turned on and current is flowing from drain to source, you'll measure a voltage on the source lower (by like 0.3-0.7V) than on the drain.

(in the case of the P-channel, since the context was about DC-in MOSFETs on Macbook boards that are P-channel)

But basically current flowing through body diode is an unwanted effect. The forward voltage drop means you waste power proportional to the current that needs to be dissipated (P = Vf * I). So you want to turn on the MOSFET anyway if you want current flowing through it.

Meaning that in general it's not normal to measure a voltage drop similar to a diode forward voltage drop across a MOSFET.

In the case of DC-in MOSFETs, what can happen is that the MOSFET that normally blocks the current shorts across drain/source/gate. This short causes the gate to be at the same voltage as drain/source, and it usually also causes the gate for the other MOSFET to have the same voltage, so it's turned off, either because they're tied together or the charger IC tries to turn them off. So current reaches the reverse polarity protection MOSFET, which would normally block the current from flowing in the other direction, so it flows through it but it's not turned on, so you measures a voltage drop after the DC-in MOSFETs compared to before them which is not normal.

The other failure mode of the DC-in MOSFETs is the reverse polarity protection MOSFET shorts across drain/source/gate, gate has the same voltage as source/drain, it's tied to the other MOSFET, the one that blocks current flowing in the correct direction, so it's turned off and never lets the current flow.

What's confusing for beginners in this situation is that the MOSFET that's bad is the one that lets the current flow, not the one that is blocking the current. Every beginner will jump on replacing the one that blocks the current and obviously it'll not solve the problem. Then they Sorin it and they don't understand why they have 19V on the main power rail but it's still not turning on, or it's turning on but the battery is not charging, or the laptop is throttling or whatever weird stuff happens.

This second situation is much more common on non-Apple laptops.