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I Understanding a practical cooling situation

  1. Jun 15, 2016 #1
    hey guys and gall's

    i'm builing an amplifier for audio, basically a 6 channel amplifier, each amplifier is a pcb which is mounted on a solid aluminium base and will need to dissipate about 30W.

    I will mount these upside down under a circular sheat of metal in a " circular" 6-way pattern around the centre of theis metal plateand in a way the alumium base can dissipated it's warmth to this metal plate.

    on this metal plate there will be several heatsinks mounted in a way thatare will pass through gettting it to rise and make the air flow. in total i need to get about max 180W of power to dissipate in a fairly s,all space. but that is only at max performance which will likely never be reached and only in (very) short periods in time and the entire system is protected against overheating.

    my main question is this: should the choice of metal be copper or aluminium.I basically initially choose copper but the price of this copper plate vs alumium makes a large difference so i'm not sure if it will be wurth it in this particular situation. which is what i want to discuss.

    the coolin will be done through passive air flow.
    1) air will first flow over the amplifier itself. so the amplifier can dissipate directly to the air (minorly) and to the metal plate via the intended base.
    2) the air will flow through a heatsink mounted on the metal plate.
    3) go THROUGH the metal plate via holes and pass through another heatsink.
    these first two heatsinks are monted on each side of the metal plate and are "common" heatsinks.
    4) After which. the air will need to flow through six separete compartments (i think, but i'm not so sure about that) each of them containing 2 stacked heat sinks on top of each other and mounted on the metal plate exactly on the opposite side of the metal plate where the alumium base is mounted from amplifiers while the top one sticks through to the outside.

    so for this entire story i'm going to neglect the part where there is direct air flowing past the amp and purely focus on the heat going from the amp to the metal plate to the heat sinks to the air.

    as said the amp bases are aluminium.
    the metal plate is the point of discussion but is intended to be 2 mm thick.
    the heatsinks are copper cored with aluminium "wings"

    i understand the benefits of copper vs aluminium.
    thermal conductivity: copper: 401 W/(m·K), aluminium: 237 W/(m·K)
    molar heat capacity:copper: 24.440 J/(mol·K), aluminium: 24.20 J/(mol·K)

    what i can't get my head around is understanding is if there will be such difference between the heat going from.
    1) alu base, 2) 2mm copper plate, 3) copper core, 4) aluminium heatsink flares, vs:
    1) alu base, 2) 2mm alumiunium plate, 3) copper core, 4) aluminium heatsink flares.

    taken in considering that I have 3 options.
    1) fairly reasonable price of aluminim being 25 euro's
    2) less reasonable option of copper being 160 euro's
    3) lightly less expensive option of mounting 2 pieces of 1mm copper together mechanically or via lead soldering. dropping the price down to a mere 90 euro's but taking in account the less desirable heat physics vs price compared to aluminium.

    the wallet is tight. but if it will make a really hugh difference i will have to go for copper, but personlly i believe that the mere 2 mm will not make the difference so much.

    i know this is just mere go by gut feeling zituation but i would love to make a more educated decision.

    Kind regards.

    ps: sorry for the long story but i really wanted to get the complex setup as clear as possible.
     
  2. jcsd
  3. Jun 15, 2016 #2

    CWatters

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    Ok so you have a heat path from some hot power transistors at one end to cool ambient air at the other. Between those two points there a number of parts in series through which the heat must flow. Each part has it's own thermal resistance. The temperature rise between ambient air and the transistors will be equal to the overall thermal resistance of the path multiplied by the power you are sending through it.

    You want to know if reducing the thermal resistance of one part in the chain will make much difference to the temperature rise. To answer that you need to know how much of the overall thermal resistance that part accounts for. I suspect 2mm of aluminium in the path you describe won't account for much so reducing it's thermal resistance by using copper wouldn't have much effect. I would be inclined to build it in aluminium and test it. Measure the temperature drop through the plate. If it's large then perhaps switch it to copper.

    I note you are mounting several amplifiers onto one aluminium plate. Have you checked if they need to be electrically isolated? eg the "solid aluminium base" on one amplifier might have to be isolated from the "solid aluminium base" of the other five.
     
  4. Jun 15, 2016 #3
    thanks for your reply CWatters.

    also really nice abreviation of my storry. i'm never good in using few words :/
    indeed i though so to. these 2 mm won't matter to much in the entire chain. I'll build it and test it.

    about mounting alle these on one plate. this is actually the way it's generally build. instead a plate it's the entire cabinet... which is earthed. meaning that even in different cabinets it's still electrically connected. Also hypex actually makes these in pcb's with three on one pcb so concidering both i dobt it' but i'll definately double check it as it would mean a big change and disadvantage in the design but also a rather financial benefit.

    kind greatz,

    Matthieu
     
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