I'm using a KSC1173 TO-220 power transistor in a design, and I need to calculate the case temperature for the maximum average power dissipation, to see if the power dissipation falls under the temperature derating curve in the datasheet for a particular heatsink. Unfortunately I'm not sure what the case-to-sink thermal resistance of a TO-220 package is; it's not listed in the datasheet. I suppose it will vary somewhat with the type of heatsink and mounting used, but does anyone have a ballpark figure?
Total thermal resistance from junction to air is
Rja = Rjc + Rcs + Rsa.
Where: a = air, j = junction, c = case, s = (heat)sink Rjc & Rsa are reasonably fixed by component choices. Rcs is somewhat more variable - see below.
If your data sheet does not give junction to case thermal resistance (usually Rjc or similar)* then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.
Your comment about varying with heatsink suggests you don't really understand the subject. Finding one of the many many good tutorials on the net would be advised.
Thermal resistances are like electrical resistance - they can be added in series.Heat flows from junction to case outer, from case to sink (cia washer and thermal paste, and from heatsink to air. So you get
Rthermal = Rjc + Rcs + Rsa Trise jc = Rjc x Watts etc Trise = Tjc + Tcs + Tsa
see prior paragraph for meanings.
Rjc is specified by the device manufacturer.
Rcs is set by thermal washer material, heatsink compound, pressure of mounting etc. It should be a minor contributor.
Rsa is set by heatink design and size, air flow etc.
Temperature rise = Rja x Watts.
Search eg www.digikey.com for TO220 and look at a few Tjc ratings.
gives an unusually large number of Rjc and Rja ratings for various package versions of the same part and is a good starting point. For the TO220 package Rjc is 2 C/W.
Rja here is from about 30 to 60 C/W but the heatsinks are "wimpy". Large heatsinks with fans can be around 1 C/W - much less with much care.
A well heatsunk TO220 can manage Tja = 10C/W without too much effort. But it can be far worse without due effort.
The initial paragraph originally read as follows. It is evident that I wrote "case to sink" but intended "junction to case".
If your data sheet dies not give case to sink thermal resistance (usually Rjc or similar) then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.
You won't always find a number for the case-to-sink thermal resistance, because it depends on how it's mounted (screw, clip) and the amount of heat-conducting paste you're using.
In my answer to this question (dated yesterday!) I explain that the case-to-heatsink thermal resistance is much lower than the heatsink-to-ambient thermal resistance, because the first is conduction and the second convection.
To give you some idea, for this MOSFET, which the above-mentioned question was about:
junction-to-case (conducted): 0.4 K/W
case-to-ambient (convected): 62.5 K/W
case-to-heatsink (conducted): 0.5 K/W
The first gives a series of junction-case values for different mounting options and different mounting torque with different packages: it also quotes the thermal resistance of a few "conventional interface materials" in the range 0.10-0.45 K/W per inch-squared
The second (from 1996) gives these "typical" values:
- with thermal grease 0.3-0.5 K/W
- without thermal grease 1.4 K/W
- with mica insulator 2.2 K/W
- with alumina insulator 0.8 K/W
- with High Voltage mica 3.0-4.5 K/W
- with HV mica and grease 1.4-1.6 K/W
For modern phase/change materials, probably first stop would be to look to the supplier.