O'Reilly logo

Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

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 ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required