Calculating Junction Temperature
We use the excellent thermal resistance number of the JEDEC “look-alike board” mentioned previously. We know that its thermal resistance was around
RthBA = 10.5°C/W
The thermal resistance of the chip from junction to lead (or board) is published as 0.5°C/W. We must add typically 15°C to the room ambient to get the “local (chip) ambient” TA (see Fig. 12.3). Further, if AC disconnect is being used we need to add another 10°C to the local ambient, to account for the heating up of the PCB by the AC disconnect diodes.
For example, suppose the chip’s dissipation is 1.6 W and its datasheet declares: RthJB = 0.5°C/W and RthJA = 25.5°C/W (we ignore any published RthJC for reasons discussed previously). We also keep ...