Thermal Calculations

[ElectricGuy]

Hi all;

I'm trying to calculate a thermal caracterisic of a regulator.
Since i will not use dissipator, the formula is;

Tj = Ta + (Rja x PD )

or

Tj = Ta + (Rja x PD ) + (Rjc x PD )

I look at  google, but i allready see the 2 formulas, so i don't know each on should i use.

[bobcat]

The first formula is for a free standing device like a TO-220 without a heatsink.

The second formula is for a device that in addition to dissipation into free air, it also uses another heatsink, such as a pc board. 

[kxenos]

I think the second formula is wrong.
You either leave the device free standing in which case you need the R(junction-ambient) or you're fixing the device on a heatsink in which case you need the R(junction-case), the R(case-heatsink) and the R(heatsink-ambient).

[KerryW]

You are right, kxenos, the 2nd formula IS wrong.
Rja = Rjc + Rcs + Rsa

The spec sheet should give values for Rjc and Rja.  The Rja value assumes no added heat sink, so the case IS the heat sink, and Rja = Rjc + Rca

jc - junction to case
ja - junction to ambient
ca - case to ambient
cs - case to sink
sa - sink to ambient

So for a TO220 regulator that specifies 3 d/w Rjc and 75 d/w Rja, without a heat sink, 1 Watt would generate a 75 degree rise above ambient.  With a 5.5 d/w heat sink, you would have
3 d/w Rjc
.5 d/w Rcs (approximately)
5.5 d/w Rca
9 d/w Rja, for a 9 degree rise at 1 Watt.

[ElectricGuy]

Thank you all;

Like i said before, i will not use heatsink, so Tj = Ta + (Rja x PD ).

Thank you once again!