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SOT23 BJT dissipating 720mW

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treez

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Hi
Our software engineer just phoned us to say that a SOT23 NPN transistor (FMMT491A) was accidentally turned ON all the time for 10 minutes. It would have been dissipating 720mW. Its pads did not have extra cooling copper joined to them. The PCB was in open air in the lab at 25degC.
Do you think that this SOT23 will be destroyed? The software engineer is now not contactable.
Should we order some new in for Monday, in anticipation of dead transistors?
 

Hi treez,

First, call Europol and report a missing software engineer. Next, it says 500mW, 10 minutes isn't that long but it might have degraded the device somewhat which would make it questionable to continue using it/them, at an amateur guess. Do you think going through the calculations to guess/estimate junction temperature will answer the question with more certainty? I ask as I don't know. If ordering more is not an issue, I would just to be on the safe side.
 
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Hi,

An SOT23 dissipating 720mW....I expect it to burn the skin away in less than 2 minutes.

Klaus
 
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260C/W would make it about a 220C temp rise, toasty and
perhaps enough to compromise the mold compound over time.
Not enough to damage the silicon.

I'd say replace it if the case has changed color or sheen,
compared to a virgin unit.
 

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