“Simple” thermal analysis of component on PCB over time

Question

I'm trying to be pragmatic about designing a PCB and circuit that will have to dissipate heat. It's pretty simple: a surface-mount power transistor charges up a 4F 5v supercapacitor with a roughly constant current of 2A. It will not do this repeatedly - so it only needs to dissipate about 20 joules over 10 seconds, once, from room temperature.

I like the idea of killing two birds with one stone: keep calculations simple by building in safety margins and taking worst-case simplifications. But calculations or thermal dissipation over time, especially combined with electrical dynamics is not simple, mainly since the PCB's copper tracks and their layout have such a significant effect, but also since the transistor is not ideal.

So I imagine the only two practical ways of designing this are one or both of: with specialist CAD software, or testing of a physical prototype, simply measuring the temperature parts of the PCB and its components reach.

Realistically, what would be the simplest, effective way of doing the electrical and thermal calculations/assessments necessary?

Answer 1

20 Watt-seconds/10seconds * (Rjc+Rcs+Rsa) = 2[W] * Rth(total)['C/W] = temp rise

I^2*RdsOn is one of your choices in addition to choice of mechanical thermal resistances to apply the thermal version Ohm's Law for convection based on datasheets and after you define the max Tj rise for reliability concerns.