Electronics > Projects, Designs, and Technical Stuff
Unexpected amount of heat
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mikerj:
You seem to be fixating on the junction to case thermal impedance, but it's almost irrelevant in this situation.  Junction to case gives you a idea of how hot the junction will run given a case temperature and a power dissipation value, but you primary issue here is case temperature since you don't have sufficient heat sinking. 
schratterulrich:
I have written a tool to simulate the thermal behaviour of pcbs.
It can at least give you a feeling of how copper planes and vias affect thermal spreading.
Unfortunately it is only useful for steady state scenarios.



You can find it at leiterplatte.jimdo.com/thermal-pcb-sim/
OM222O:
Thanks for the simulation tool but I can't seem to be able to do anything and constantly get this error message:


it couldn't even create an error log which is interesting ... can you please provide a guide on how to use this app?
schratterulrich:
Hmm...
It seems the program tries to save the project in the c:\Program Files\... directory, where it doesn't have write permission.

Possible solutions:
a) Please save the project first in a directory where you have read/write permissions

b) If a) doesn't work: copy the file XVIIx64.exe in your project directory and set the LTspice Path in the tool accordingly

If a) and b) don't work look for "Show Console" in the settings tab and activate it. Then post me a screenshot of the log.
jmelson:

--- Quote from: Siwastaja on May 12, 2019, 02:18:08 pm ---. If your PCB is acting as a heatsink, I recommend you google some similar D2PAK datasheets to find example PCB layouts with their evaluated thermal resistances junction to ambient. Some device datasheets or appnotes list full junction-to-ambient resistances for two or even three different PCB layouts.

--- End quote ---
11 W into a PC board pad?  No, NOT going to work (at least, not without a fan blowing directly on it.)
You need a "real" heat sink, like a chunk of aluminum with fins.  A TO-220, or maybe even TO-247 package might be better, once you get into 10's of Watts being dissipated.

Just because the data sheet says it can dissipate this power doesn't mean it can handle it without external means of cooling.
And, the data sheet SOA figures are often with an infinite heat sink at 25 C.

Jon
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