Hello,(adsbygoogle = window.adsbygoogle || []).push({});

This is my first post on this forum and I better warn you that there maybe some poor grammer and spelling mistakes. Given what I had to aggree to in order to register I'd like appologies in advance to all those people who are offended by such things.

I'm looking at a problem with a PSU that may end up operating with a high ambient temperature in the enclosure and I'm looking for some advise on calculating expected junction temperatures for various components.

The high ambient temperature is cause by a high current, (200 A), circuit that shares the enclosure. I've done some testing at [tex]23^{\circ}[/tex] and get around a [tex]30^{\circ}[/tex] rise with 100 A in the high current circuit. The unit needs to operate with an ambient temperature of [tex]70^{\circ},[/tex] so I believe this gives me an enclosure ambient of [tex]100^{\circ}.[/tex] I can't supply 200 A so can't know the temperature rise at this level but I assume that the temperature rise will be two to four times greater than that at 100 A. Which means that before I even energise the PSU the junction temperature rating of the components could be exceeded.

I know about themal resistance and calculating junction temperatures as normal; e.g. [tex]T_{j}=T_{a}+(P \times R_{ja}).[/tex]

It was put to me that the junction temperature can be lower than ambient and the heat flow will then be from ambient to the junction. Hence the junction will not rise above ambient.

Does anybody know if this is correct?

This does seem to make some sense in that generally things flow from hi to low. However, my basic understanding is that the energy dissipated by the circuit can be view as an ideal current source, (where Watts is equivelnt to Amps), such that no matter how high the ambient temperature gets the junction temperature will always be higher.

**Physics Forums | Science Articles, Homework Help, Discussion**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Power dissipation at high ambient temperatures

Loading...

**Physics Forums | Science Articles, Homework Help, Discussion**