Power dissipation of SOT23

Thread Starter

Wolframore

Joined Jan 21, 2019
2,283
Can you guys help me finish a project... this one has been a lot of fun (too much of it) and I'm on the last leg.

I have a MMBT3904 which can dissipate up to 200 mW and just want to be sure it's going to be ok. I can always change the package and go bigger if it's a problem.

MMBT3904 Datasheet

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The circuit has great air flow but is hand held, The dissipation is constant but will depend on user setting... it's the worst case.

So taking the worst case scenario of 403 C/W for RθJA we area looking at about 80C dissipating 200 mW. And since we are talking about 25C ambient and this is under their max of 150, it should work?

Ok part 2: 100C will boil water and it seems too hot for my liking, Is there are safe point as a rule of thumb where you aim for?
 
Last edited:

crutschow

Joined Mar 14, 2008
27,241
Is there are safe point as a rule of thumb where you aim for?
I would keep the junction temperature below 100°C for the maximum power dissipation at the maximum expected device ambient temperature to give good reliability and minimize the change in transistor characteristics with temperature.

What is the transistor function/use in the circuit?
 

Thread Starter

Wolframore

Joined Jan 21, 2019
2,283
It’s the pass transistor for an opamp controlled variable current source. 20 mA max. Problem is sometimes it’s dropping almost 8v
 
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