I'm comparing the thermal characteristics of two ethernet switch ICs. They both come in the same package and their thermal resistances are:
θja 51.42 C/W
θjc 20.66 C/W
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?