Verification of approach to calculating power dissipation, temperature and use of heatsink

Thermal resistances should be additive. Your thermal resistance should be the Rjc + Rpaste + Rha. I suppose you might think of it being negative in the sense it's supposed to improve conduction into the heatsink, but that goes against what I have been taught in the past. The assumption for Rha is that the heat is already "inside" the heatsink and Rha would capture the resistance of the flow from within the heatsink to the ambient. The Rpaste probably omits the resistance from case to paste, and from paste to heatsink surface. I might be wrong, but that's just how I've been taught to interpret it.

If you have the thermal paste areaC/W (or areaK/W) then by balancing units, you have to multiply by the application area. Lets say we take your figure of 0.061cm^2C/W. If you have a TO-220 package (a fairly common one), the area of the contact is about the rectangular portion behind the chip area (excluding the tab hole to simplify calculations) - lets say 12.95mm x 8.5mm based on this drawing. This means the area is 1.107225cm^2, multiplied by the figure before gives us 0.067540725C/W as the result. In other words, given proper thermal coupling (i.e. paste application), the paste contribution should be negligible. Using the other figure - say 2.4W/mK, would probably need to know the thickness of the film.

Despite this, I've rarely ever bothered working out this because most of the time, the rule of thumb I often use is to add another 0.5 to 1C/W for the case-to-heatsink resistance contribution just to be pessimistic and lump in all of the unknown case-to-paste reistance, paste bulk resistance, paste-to-heatsink resistance. If you can't handle this amount of additional thermal resistance, you can't handle a 1-degree rise in ambient and that would be extremely borderline in terms of design.

- Gough

Thanks Gough, that helps with the thermal paste.  Was I right in my other calcs? That is, without a heatsink, I use Rja and with a heatsink, I use Rjc?

In general, yes,

Rja is Junction to Ambient, so that's the figure you use assuming no case. Note that some datasheets will have some specific stipulations about Rja - e.g. assuming a certain leg length or soldered with a certain leg to a minimum amount of area of copper for example.

Rjc is Junction to Case, so that's the figure you're interested in if you are trying to work out the total thermal resistance of a system with a heatsink which is mounted to the case of the device. It's also useful to work out how far above Tcase the Tjunction is for for a given power dissipation so you can compensate and work out what Tjunction is, given a Tcase measurement and knowing the Pdissipated.

- Gough

In general, yes,

Rja is Junction to Ambient, so that's the figure you use assuming no case. Note that some datasheets will have some specific stipulations about Rja - e.g. assuming a certain leg length or soldered with a certain leg to a minimum amount of area of copper for example.

Rjc is Junction to Case, so that's the figure you're interested in if you are trying to work out the total thermal resistance of a system with a heatsink which is mounted to the case of the device. It's also useful to work out how far above Tcase the Tjunction is for for a given power dissipation so you can compensate and work out what Tjunction is, given a Tcase measurement and knowing the Pdissipated.

- Gough

Thanks Gough. 

Just for completeness of the thread, W/mK is Watts per metre Kelvin.  The higher the number, the more efficient the heat transfer.