Relation of case temperature to junction temperature

Question

I'm comparing the thermal characteristics of two ethernet switch ICs. They both come in the same package and their thermal resistances are:

IC1
θja 51.42 C/W
θjc 20.66 C/W

IC2
θja 41.54 C/W
θjc 19.78 C/W

Test data for IC1 shows that the case temperature was 58.8 C and it dissipates about 1.2W. I ran a test for IC2 and the case temperature was 65 C and it disspates 0.7W. The ambient temperature for both tests was 23C.

I was expecting for the case temp of IC2 to be lower since it dissipates lower power. How does the case temperature relate to the junction temperature. The Tjmax for both ICs is 125C. Is it safe to assume that the Tj in IC2 is higher than in IC1?

Answer 1

One or more of your assumptions is not correct.

You should have \$\theta_{ja} = \theta_{jc} + \theta_{ca}\$, giving \$\theta_{ca}\$ of 30.76 C/W for IC1 and 21.76 C/W for IC2.

With those values, IC1 should have a case temperature of

23 C + (1.2 W) (30.76 C/W) = 59.9 C,

which is pretty close to what you measured; but IC2 should have

23 C + (0.7 W) (38.23 C/W) = 38.23 C,

which is way off the measurement.

A likely cause are if the datasheet's \$\theta_{ja}\$ value makes some assumption about board layout that is not met in your application. If your board doesn't provide sufficient copper area attached to the chip to spread heat away, \$\theta_{ja}\$ will increase substantially. It's also possible that the datasheet value assumes some forced air cooling over the chip.