Elevated Temperature Question

[questionseeker]

Hello all!

So let's say I have a power supply with a case temperature of max 90degC and placed in a 3/16" thick 16"x9"x5" die cast aluminum chamber, fully enclosed.
During normal operations, the case temperature was measured at 74degC at a 25degC ambient temperature (room temperature).

Suppose I can elevate the ambient temperature up to 50degC. What is the relationship between the ambient, aluminum chamber, and the power supply.
Would it reach its 90degC limit? Can you walk me through the steps of how to relate all factors together? Is there a possible solution to maintain a 90degC max case temperature by only increasing the thickness of the chamber? let me know your thoughts and thank you.

 

[IRstuff]

I suggest that you read up on heat transfer:

http://en.wikipedia.org/wiki/Convective_heat_transfer

To the first order, deltaT is solely dependent on heat load and thermal conductivity. You have a deltaT of 49ºC, so the ambient need only go 16ºC for the case temperature to hit 90ºC. Aluminum is very thermally conductive; it's used for heat sink material, after all, so it's not a good choice for a thermal chamber.

In general, thermal test chambers are insulated, rather than conductive, and the ambient temperature is controlled with heaters, etc., to maintain a specific ambient temperature to the unit under test.

TTFN
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[questionseeker]

Even if the power supply has a metallic body and mounted straight to the aluminum chamber? Shouldn't the chamber itself be acting as a heat sink? Is case temperature vs. ambient temperature in a linear form? The chamber is made of aluminum die cast 6082 alloy.

 

[IRstuff]

?? Why would you thermally short your case to the chamber? Is that the intent? What exactly are you trying to achieve? What are your design requirements?

TTFN
faq731-376

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[marty007]

questionseeker:

Here's my answer based on how I read your question.

Assuming the heat output from the power supply is constant, first determine the case temperature as follows:
1) Current case temperature is limited by external convection by the formula Q=h*A*Delta(T).
2) You know A and Delta(T) for your current operating conditions. You know your case temperature, so find a convective heat transfer correlation, and calculate Q.
3) With Q now in hand, go back to Q=h*A*Delta(T). Guess a case temperature, calculate a new convective heat transfer coefficient, calculate Q.
4) Iterate for a new case temperature until Q matches that calculated in step 2.

First guess, I think you're going to have an issue. Assuming convective heat transfer coefficient is the same, the effective delta(T) would remain constant. So, the case temperature would likely rise to: 50C + (74C-25C)= 99C.

Cheers,
Marty

 

[mcgyvr]

If its 74 C @ 25 amb it will be 99 C @ 50 amb
Its that simple and thats how testing agencies correct for elevated ambient specifications/requirements without having to be in that ambient for testing.

If you need to be 90 C @ 50 amb you will have to make changes to conduct more heat through the enclosure (or chamber as you called it).. or make changes to this power supply to increase its efficiency

Funny how the choice of the word "chamber" leads some to think you are talking about a "test chamber" where had you used "enclosure" or "chassis" or similar the confusion would not exist.

 

[chicopee]

My way to solve such problem is first assume steady state under the conditions you stipulated; then draw a free body diagram of the entire assembly showing known and unknown parameters such as internal heat source, temperatures, thermal conductivities, convective heat transfer coefficients, radiation emissivities and then develop your equations for each side of the boxes.

 

[prex]

Even if the power supply has a metallic body and mounted straight to the aluminum chamber?

Yes,but you should insure a good thermal contact between the two.

Shouldn't the chamber itself be acting as a heat sink?

Yes

Is case temperature vs. ambient temperature in a linear form?

In that temperature range, substantially yes, assuming nothing else changes.

Is there a possible solution to maintain a 90degC max case temperature by only increasing the thickness of the chamber?

It is possible, but, as aluminum is a good thermal conductor, you won't necessarily get a benefit.

prex
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