Similar to that John, but with much larger heatsinks on both side of the box along the full length of the box (unless I can get away with a large heatsink on the top, but then there's going to be very little natural convection so probably inefficient as it'll be mostly by radiation).
As a matter of interest, do you know how much power those hestsinks dissipate, or roughly the temperature rise internally?
BTW the environment will just be still air externally, I can't rely on any air movement other than natural convection.
Why not make less heat? You said your 100W AC-DC converter is dissipating 20W. That's 80% efficient. 90% under full-load (when dissipation is highest) isn't unreasonable.
Or thermally bond the culprit components to the top, attach the potential victims to the bottom, stop air from circulating inside, and blow on the outside if necessary. That'd also relieve the inside from the heat generated by the fan. Depends on how effectively the hot bits can be heatsunk to the top extrusion, and how much the circuit is open to layout changes, but generally it would be better to isolate the heat generators from the bits that don't like heat, no?
Just to be contrary.
Different strategies, horribly complicated situation for analysis. Experienced thermal engineers are probably making good money.
On a smaller scale, this photo is of the inside of box bits from a ~150W power converter.
The mains side power switch heat spreader to the left, the other heat spreader is for the secondary TO247 dual schottky rectifier.
Insulation is that non-woven 0.4mm thick paper like stuff. The aluminium plates are 2.5mm thick. One on left measures 86 x 56mm.
A screw each side of the power device applies pressure through an elastic pad (blue, dunno what it's made of) and PCB to the TO220 or TO247 case onto the heat spreader.
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The box was lightly finned extruded channel with sheet metal ends.
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No fan, sealed box but not airtight. Made in Taiwan.
You have never heard of hermetically sealed, mil EL caps?
There are other ways of capacitive storage as well.
Ahhh... basic... internal radiation coupled to non-moving or slowly moving internal air that is already hot is going to "work"?
Hahahaha... You cite!
You missed the point. Try *more* elevated than convection coupled methods.
To remove heat, there MUST be external coupling. Either to the air or to an attachment (conduction) plate or both (ideal).
You obviously do not know much about conduction cooling. One takes all of the heat generating internal sources and couples them to a LARGE thermal MASS INSIDE the case, and couples that to that same integrated thermal mass that is the outside of the case. This should result in a fairly homogenous temperature throughout with a few slight variations near the actual source elements, which increases as the designer falls below the minimum mass spec'd for conduction cooling efficiently. A well done design may or may not benefit from an external matte black finish, and may or may not benefit from the inclusion of some certain sized "fin array" incorporated into the case exterior.
Metal conducts better than air, so an internal fin array and trying to use air internally to perform cooling as you stated earlier, was a bad method. Using METAL MASS to CONDUCT the internally generated heat into the mass, and then conductively pass it to the outside of the mass, and then to the conduction plate that you mount it onto. THAT MAXIMIZES the amountof internally generated heat that you can bring to the outside to dispense with it in whatever way you can. So conduction cooling IS the best INTERNAL management method, and the external design is all that one would need to fancy with based on space considerations, etc.
Then, one presumes that the outside temp is lower than the heat generating elements of your product
If it isn't the discussion is moot for conduction or air operated internals. He DID specifiy a sealed enclosure in both cases, so AIR inside would have to STAY inside. Something I fear you forgot to notice as well.
That depends on how you bottle it up, and what parts might change operation under cold. Failure under cold is easily managed. Don't let it happen.
I did suggest conformal coating, but full encapsulation would work as well, just be far less servicable. There are a few encapsulants that are 'approved' for 'space use' in the US. For HV circuitry, there is only one. CONAP.
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That stuff will stop a sharp point, thick blade knife with a forceful blow, and I think it would even stop bullets. The knife tip only penetrates about a quarter inch. Anyway, it does NOT gas,and it does NOT detach from a surface very easily once it has made adhesion and cure.
Sounds scary. A square inch of surface in free air can have theta of over 100 k/w. And that's just the outside of the box... ther would be a similar theta on the inside.
If the box has, say, 5 k/w to the world (30 sq inches, 150 K/w per) and you dump 30 watts from inside, the wall temperature averages 150K above ambient. That's bad enough, without the parts inside having another bunch of theta between themselves and the hot wall. That's why it's best to bolt hot stuff directly to the inside of the walls, and not use convenction, even with a circulating fan, inside.
So I guess we agree.
This one uses oversize transistors and a heat spreader plate to avoid needing a heat sink. The chassis is a bit too thin to dump the heat into directly.
Actually, in a situation where the major dissipators are more closely coupled to the box wall, the box wall temperature can become a fairly accurate indicator of internal air temperatures. Dissipation from other isolated internal sources into this air has to be restricted simply due to the increased internal anbient. The result is that the sealed box wall becomes the dominant regulator, however backwards this may sound.
Not true, of course. The inside has forced airflow, not 'free air', and the only way you'd get 100K/W would be on a horizontal surface with no convection. No box is entirely horizontal surfaces, and no plausible scenario raises the case temperature by 100K without initiating convection.
A wall-wart in a plastic case with circa 10 square inches of area has no difficulty in shedding 4W. Metal should dump heat faster.
Well, back to the question at hand, internally the OP should mount the hot parts to the box walls, and externally he needs a heatsink that gets him to ~1C/W.
Even better than getting the heat out of the box is... not putting so much in in the first place.
I think that is what I'm intending to do. I'll mount the higher powered stuff directly to the side walls, and probably use an aluminium backed PCB thermally coupled to the side walls too so that lower powered devices have a thermal path to the heatsinks.
The AC-DC converter will be a baseplate cooled type mounted probably directly on one of the walls (or find some way of thermally connecting it to both walls with a thick bracket maybe and mount it horizonaally).
"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...
Hmm, I've got a large power supply that occupies a 5 x 10 x 13" aluminum box (lots of free space). After a few hours, it gets maybe 20C above ambient. That's 360 in^2 (not counting the bottom, which is against the table). Measured power 100W, so the thermal conductivity is about 0.2 C/W, or 72 in^2*C/W. I may be grossly off with my power estimate, and the top panel may not contribute much by convection, being in stall.
Eating mashed red potatoes. Not just redskin, these are red *all the way through*. It looks like strawberry ice cream, and is exactly as delicious, but savory instead of sweet.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
It's because of turbulent flow; the local cooling of a bit of plastic in airflow doesn't cause heat flow laterally from nearby plastic surfaces that are in 'dead air', but lateral heat flow in a metal case will be significant. Admittedly, turbulent flow outside the box would have to exist for this to be important, but laminar flow is not common in uncontrolled environments.
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