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Heatsink question

Discussion in 'Electronic Design' started by pimpom, Feb 26, 2009.

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  1. pimpom

    pimpom Guest

    I want to couple two TO-220 transistors to their common heatsink
    by mounting them on a slab of aluminium and then bolting the slab
    to the heatsink.

    The Al piece is 4.5mm thick and 30mm wide. From the center of the
    transistor mounting screws to the point where the slab starts
    mating with the heatsink is about 25mm. The section of the slab
    covering the heatsink is bent 90 deg and is 25mm long, i.e., the
    slab-heatsink contact area is 30x25mm. The contact area is
    reasonably smooth and will have heatsink compound (thermal paste)
    applied between the two surfaces.

    | |
    _| |
    || |
    || |
    25mm|| |
    Tr || |
    _____ || |HS
    ___|____|===________|| |
    4.5mm |____________________| |
    |<--25mm-->| |
    | |

    Can anyone please calculate or give an educated estimate of the
    thermal resistance added by the indirect mounting?
  2. PeterD

    PeterD Guest

    How thick is that 25mm flange that attaches to the main heatsink? How
    thick is the main heatsink?
  3. pimpom

    pimpom Guest

    The heat conduit is of the same thickness throughout - 4.5mm,
    including the flange that attaches to the main HS.
    The section of the heatsink covered by the flange is 4.7mm
    thick.The main heatsink is a readymade one of extruded (not cast)
    finned blackened aluminium, made by a company that's been in the
    business for at least 30 years. The heatsink I'm going to use is
    rated at 2 deg C/watt.

    The two transistors together are expected to dissipate 10-20W
    total in normal use, with occasional 30W peaks. A bit marginal,
    which is why I'm concerned about the additional thermal
    resistance introduced by the indirect mounting. At least the HS
    will be mounted outside the cabinet in free air.
  4. PeterD

    PeterD Guest

    Personally I would not do this... 4.5 is a bit on the thin side, but
    without doing the computations I can't tell you what the actual result
    would be.

    Personally I think you should reconsider your design...
  5. pimpom

    pimpom Guest

    Thanks. That's the kind of guesstimate I was looking for and it
    about agrees with mine.
    Please also read my reply to PeteD.
  6. pimpom

    pimpom Guest

    It may come to that. If possible, I'd rather change the HS and
    flange dimensions than the mounting method.

    My problem is that I live in a remote corner town of India from
    where getting the stuff I need is not as simple as, say, ordering
    from RS in the US. The 4.5mm Al piece is from a building
    materials shop and the heatsink is to be cut from two 2-ft bars I
    bought during a visit to one of the larger cities some 15 years
    ago and which I've been using sparingly since.

    What I can do is 1) look for a thicker flange *and* 2) use a
    longer length heatsink from the bar, though using a longer HS
    would require some modification of the enclosure. Or, I could add
    a small fan, which will also need some modification.
  7. PeterD

    PeterD Guest

    Your problem is that the 'flange' (the piece you bolt on to the heat
    sink and mount the transistors to) won't conduct sufficient heat to
    teh main heatsink.
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