# Estimating case to ambient thermal resistance when it is not given in the datasheet

I'm trying to estimate the case to ambient thermal resistance of a GaN switching device (specifically the GS66508T device from GaN Systems). The datasheet provides the junction to case thermal resistance, but not the case to ambient. Is there any way I can estimate the case to ambient value? The reason why I need that value is because I know the case temperature of the device and I want to calculate the power dissipation and junction temperature. The device package is GaNpx which is fairly obscure - do you know if I can just find a package with a similar structure/shape with a known thermal resistance and use that? The device is top-cooled.

• I was going to say something without looking up the package. Then I saw how it's built and what it is. I think you are in boutique-land. Have you contacted the supplier about this question, yet? (I wonder also about the hidden implications behind your wording that suggests only one "junction temperature" to find.) And don't forget that radiation/surface emissivity/environment may also be worth a look-see. (Must be some serious reasons for all the trouble that goes into that package. Makes me curious.)
– jonk
Commented Jun 1, 2021 at 4:00
• I contacted the manufacturer and hopefully they have some typical values to go off of. That's an interesting point you bring up about the junction temperature. Is it typical to have numerous values for a single "controlled (to the best of my abilities)" condition? Commented Jun 1, 2021 at 5:17
• Can you not obtain the power dissipation some other way, for example by electrical measurements? Or you need to work backward from case temp? The junction and case are gong to be about the same temp in a package like that, unless you put a big heatsink on the topside pad. Commented Jun 1, 2021 at 7:32
• @ggg123 I'm assuming there's more than one junction. Consider the idea of a plate, round, square, or rectangular I don't care, and think about heating; the various ways heat leaves such an "odd" shape. There are edge effects (matter near the "edges") which have quite different surface radiation, conduction, and convection issues from the more central regions, for example. Personally? I don't worry about the average over some entire chunk of matter. I worry about the "hottest spot" -- which is usually not near an edge. If you imagine it is "all of a piece" then that lumped view may be trouble.
– jonk
Commented Jun 1, 2021 at 9:35