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?


3 Answers 3


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.

This datasheeet


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.

  • \$\begingroup\$ Thanks for your reply. What I meant about the junction to case thermal resistance varying with the heatsink is that in addition to the factors you mention, some heatsinks have powder coatings while others don't, some like the Wakefield sinks I have here have extra mounting holes under the contact point between the case and the sink, etc. I would think these factors would also influence the case to sink thermal resistance to some small degree. \$\endgroup\$
    – MattyZ
    Jul 17, 2011 at 2:58
  • \$\begingroup\$ The KSC1173 is a Fairchild part - I don't see the case to sink resistance anywhere on the datasheet here: datasheetcatalog.org/datasheet/fairchild/KSC1173.pdf unless I've somehow I've overlooked it. \$\endgroup\$
    – MattyZ
    Jul 17, 2011 at 3:00
  • \$\begingroup\$ @Russell - I don't think you should change manufacturer because he does publish \$R_{JC}\$. Bitrex mentions differences in finishing (coating, holes), and in my answer I mention the mounting method (screw, clip). There's no unique figure for it. \$\endgroup\$
    – stevenvh
    Jul 17, 2011 at 5:28
  • \$\begingroup\$ #1. A FET manufacturer can never (well, a least should not) specify a Theta-cs because that is out of their control. Similarly for Theta-ja, but they all do it anyway. (where j=junction, c=case, s=sink, a=ambient) \$\endgroup\$
    – Frederick
    May 19, 2017 at 15:43
  • \$\begingroup\$ @Frederick Agree. Edited. That seems to have been a 'typo' on my part. I said "(Usually Rjc or similar)" BUT also wrote "case to sink" - which is clearly not withing the manufacturer's ability or responsibility. \$\endgroup\$
    – Russell McMahon
    May 21, 2017 at 21:03

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


TO-220 Case-to-heatsink thermal resistance is discussed in these two application notes:Fairchild: AN-4166 Heat Sink Mounting Guide, and Philips Semiconductors: Mounting instructions

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.


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