BJT power dissipation - Which value to use? (Ta vs Tc)

The ON datasheet is rather confusing (or rather doesn't explain its notations). The 65W refers to the [max] power dissipation if you manage to keep the case at 25C. The 2W refers to an ambient temp of 25C, but no restriction on the case temp. This is a bit more clear from the Bourns datasheet of their similar product.

What this means in practice is that 65W is the max you can hope for with an ideal [possibly very large] heatsink.

Both of these data are actually a rather convoluted way of saying the same thing, namely that the max junction temperature allowed is 150C. This can be verified using the following data:

• 1.92*65 + 25 = 124.8 + 25 = ~ 150C
• 62.5*2 + 25 = 125 + 25 = 150C.

Which is actually given as such in the datasheet:

Now for practical purposes, I would suggest using a small heatsink rather than betting you won't fry it at exactly the dissipation limit for use without one.

If you want to calculate the temp rise with a heatsink, say one which gives 13C/W, then you add the heatsink's thermal resistance to that of the case (1.92C/W) and the interface material, let's say 1C/W, which would give you about 16C/W total resistance. For 2W that translates into 32C temp rise over ambient, so at 25C you'd have 57C. That's pretty decent for not frying yourself when accidentally touching it.