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Comment by rarecoil | Hacker News Reader

> It is very likely that the reason the transistor failed was because of poor cooling, as it is a low cost, high-resistance transistor that is running near its design limits. To prevent the problem from re-occuring, a soft / deformable silicone heat transfer pad can be applied to sink excess heat to the aluminum shell of the computer. In the 1990s, this was a very popular way to keep the motor controllers in CD-ROM drives cool without any additional hardware.

So it seems like the problem is a cooling issue, which doesn't surprise me. "Ultrabooks" often seem to sacrifice decent cooling for the sake of thickness, and I think it is because most people really do not tax their hardware all that much. If you do - and you do not have a proper workstation-class laptop - you're likely to run into similar issues.

My Thinkpad X280 suffers from poor cooling that quickly leads to throttling, even undervolted. My old work A485 wasn't that great, either.

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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.

It's not cooling. A switching transistor in that application shouldn't make too much heat.

We don't know the voltage on the rail, so we can't say exactly what it is, but the likely candidates are an SOA violation because they're switching it absurdly slowly, or the gate drive is below a reasonable threshold.

> So it seems like the problem is a cooling issue

Not just a cooling issue. It's a MOSFET thermal runaway, as it is being driven to its limit, higher temperature => higher resistance => higher temperature. It can be stopped by cooling it, but the bigger picture is that it is either being driven improperly, or cheap, low power components are used to cut costs without adequate a safety margin.

Laptop audio is ~1-2W of power from 5V rail. Thats at most 500mA going thru this 225 milliohm rdson mosfet.

From the article, it seemed the issue was more just bad design. As the author demonstrated, adequate cooling could be achieved within the existing space constraints.

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