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No, cooling is not the issue,. This is not a power transistor, its not meant to pump a lot of amps. Whats more once its turned ON its internal resistance is insignificant (225mΩ).

What can be happening here:

- badly designed output Audio section shorting power in some rare circumstances, might be as weird as mechanically stressed audio jack touching traces underneath it.

- software glitch around audio power enable routine enabling/driving that transistor hundred/thousand of times per second (pwm) in some circumstances, keeping it in the linear region

- badly designed under powered mosfet driver, either voltage too low or not enough current keeping it in the linear region

You do not cool power rail switches like this one, they arent meant to dissipate any meaningful power when designet properly.

So the author is wrong in his conclusion? I am happy to know the correct answer.

The author's solution may still help somewhat, but yes, they are almost certainly wrong. This transistor really just isn't being driven properly, because it really shouldn't generate heat in this application.

It could a whole slew of problems and it's impossible to tell without measuring. But, I would guess that gate voltage is not far enough above the threshold to drop the Rds(on) to it's low loss on-state.

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