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Getting heat out of a box

Discussion in 'Electronic Design' started by markp, Aug 5, 2010.

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  1. markp

    markp Guest

    Hi All,

    I'm looking for a company that could help solve some thermal management
    issues. Basically I've got a design that dissipates about 30W and need to
    encase it in a sealed box (not hermetically, but to all intents and purposes
    there can be no airflow through the box).

    Current thoughts are an aluminium extruded box with large heatsinks on the
    outside, and a method of thermally bonding or piping any higher powered
    components directly to to the box inside. I also need an AC-DC supply which
    I think needs to be a baseplate cooled type and bond that directly to the
    side of the box that has the heatsink. I'm unsure of size of box needed or
    even whether this is a viable solution.

    I know it's a bit vague, but anyone know a company who could help
    (preferably one with a vested interest in solving it, like an aluminium
    extrusion box manufacturer)?

  2. Joerg

    Joerg Guest

    Sounds like the only viable approach, short of oil-filling the whole
    thing. Try to do your best to reduce dissipation in the electronics. I
    suggest to avoid electrolytic capacitors or similar components where the
    MTBF significantly drops with rising temperature.

    The AC-DC power supply presents a challenge here. Not because you can't
    get enough efficiency but because it requires at least one electrolytic
    capacitor. Any chances to at least send in DC, at whatever voltage?

    I know a company (client of mine) that can probably do the heat transfer
    simulations but it's not going to be cheap. Would be strictly as an
    engineering service, they don't make boxes. I don't think a box makers
    would do that or even have the SW tools and engineering know-how to do that.

    If interested send me an email. Clicking on the reply-to address works.
  3. markp

    markp Guest

    Thanks Joerg. I'm going to do a bit of hunting first, if I can get into the
    right ballpark with an aluminium extrusion manufacturer and get some sample
    boxes I may even protoype something using dummy resistors and do some
    temperature profiling myself. This might not catch hotspots in the real
    thing of course, but it'll give me a good idea of how far off a solution it
    might be overall. Thanks for the offer, I may take you up on it later :)

  4. Joerg

    Joerg Guest

    I use the Dale RH-50 series for that a lot. You can even get resistors
    in TO-220, should nicely mimic power transistors and large Schottkys:

    Another method that easily lets you "dial in" the dissipation without
    needing a large variety of power resistors is to take a bunch of LM317
    and then use them as constant current sources into ground. It takes only
    one (smaller) resistor to set the desired current and thus dissipation.
    The LM317 comes in TO-220 and all sorts of other packages, including
    some that can sink heat into a chunk of copperclad so you can mimic a
    circuit board that heats up.
  5. markp

    markp Guest

    Do you mean and area of the sidewalls of the box externally > 30 square
    inches (that's equavalent to 5.48" x 5.48" for ease of visualisation), with
    an internal fan, will get rid of 30W? What's the max temp rise above ambient
    internally do you think, and would I need to put heatsink fins inside the
    box as well as outside?

    The box is going to need to be big for the components, probably 12" x 4" x
    3" external dimensions or so. That's a good 120 square inches available if
    you ignore the base and the two end plates. Those other free-air surfaces
    can also have quite large surface area heatsinks on them too.

    Having said that, I would really like it to be rugged and not have any
    moving parts in it at all. What do you think the chances I could keep all
    internal temps below, say, 20 degrees above ambient if I were to bolt the
    high powered stuff directly to the box walls inside? The 30W is spread out a
    bit component wise, but 20W of that comes from the PSU.

  6. markp

    markp Guest

    No, this design is actually a custom power supply. The total heat
    dissipation inside my box is 30W, there's an AC-DC converter in there which
    is dissipating 20W of this, and is powering another circuit internally which
    is dissipating the remaining 10W. This additional circuit provides an
    output from the box and delivers power to another box. The overall
    efficiency of my box is about 70% (series combination multiple of the
    efficiencies of the AC-DC supply and my additional circuit), so basically it
    is consuming 100W and delivering 70W to another box.

  7. markp

    markp Guest

    Well everyone, I've found this site which if I can learn to drive it might
    be really useful. It's got some seriously interesting stuff in there,
    including online calculators:

  8. markp

    markp Guest

    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.

  9. Ian Iveson

    Ian Iveson Guest

    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.

  10. Grant

    Grant Guest

    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.

    The box was lightly finned extruded channel with sheet metal ends.

    No fan, sealed box but not airtight. Made in Taiwan.

  11. legg

    legg Guest

    ~160, actually.

  12. legg

    legg Guest

    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.

  13. markp

    markp Guest

    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).

  14. Tim Williams

    Tim Williams Guest

    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.

  15. legg

    legg Guest

    Yes. 30 in^2 is a small package to dissipate 30W.

    If this were a conventional isolated converter, you'd not find much
    free air left inside this size of a package anyways.

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