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Unrealistic SMD power dissipation ratings

Discussion in 'Electronic Design' started by Adam Seychell, Feb 12, 2004.

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  1. Surface mount power devices such as D-pack often have power
    dissipation ratings over 80W power at Tj=25²C, but almost always these
    devicea will be mounted on a PCB where the junction to ambient thermal
    resistance shoots up to around 50²C/W, meaning practical power
    dissipation is 1 or 2 watts. So My question is if the design requires
    more than say 2 watts dissipation does that normally mean using a
    through hole component with heat sink ?
    When the number of paralleled SMD power MOSFETs devices become
    impractical is that when through hole and heat sinks are considered ?

    The application is for synchronous current doubler rectifier having an
    output of 50 amps at 2.7 volts. The Ron required to limit power
    dissipation at 1 watt per MOSFET would mean many paralleled devices. A
    9 mOhm MOSFET can pass only 10.5 amps RMS for 1 W dissipation.
  2. Robert Baer

    Robert Baer Guest

    For SMD, one could use 2 Oz copper for better heatsinking, a 1 mil
    laminate thickness from top layer to bottom layer which is
    sweat-soldered to a copper heatsink or heatpipe.
    The laminate material in this case is Kapton and, due to the thickness
    is called "flex".
    Otherwise, use TO-220 (same package size as D2-pak) and clamp the tabs
    onto similar heatsinks.
  3. Can I suggest that you pepper the area under the tab with plated vias. It
    makes a big difference. You might have to experiment to find out how much.

    Graham Holloway
    WPS Electronics
  4. Boris Mohar

    Boris Mohar Guest

    There are surface mount heatsinks for these. Use 2oz copper it the rest of
    the board permits it.


    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs
  5. N. Thornton

    N. Thornton Guest

    To diss 80w in a smd at Tj = 25C I would imagine you'd need liquid nitrogen cooling.

    Regards, NT
  6. Mac

    Mac Guest

    The OP misspoke. The spec. is usually with Tcase at 25C.

  7. N. Thornton

    N. Thornton Guest

    Even then, I'm thinking LN2 would be needed.

    Regards, NT
  8. Graham Holloway wrote...
    There's an app note that quantitatively details the efficacy of this
    technique, showing it works surprisingly well. Sorry, I've forgotten
    which app note. :>(

    - Win

  9. onestone

    onestone Guest

    This works extremely well, IIRC an old Linear tech app note had some
    calcs on this, but can't rememeber, I last used it around 1995 when I
    was working on a 36W PSU for a transceiver using the LT1370, hence why I
    think it might have been an LT app note. The PSU was in a wholly
    enclosed armoured box (for mining purposes), with no possibility of a
    fan or heat sink on the PCB. the PCB itself was used as the heat sink,
    It worked like a charm, in the Australian desert, with cab temperatures
    often hitting 70C with no engine running. The PCB was 4 layers. The area
    immediately around the switcher was almost a sea of vias, there were
    radiating areas on the top or bottom layer, but never both in the same area.

  10. Nico Coesel

    Nico Coesel Guest

    In that case, the manufacturor is merely specifying the thermal
    resistance between the junction and the case. Perhaps they printed a
    nice derating graph somewhere further on for the engineers who are
    unfamiliar with Mr. Ohms thermal law.
  11. Mac

    Mac Guest

    Yes, indeed. Please note that I didn't go and read the datasheet for this
    part. I'm just going on the datasheets I've seen.

  12. Mac

    Mac Guest

    You mean to keep the case at 25C while dissipating 80 Watts? Maybe. But no
    one is saying that this is a requirement. They are just saying that if you
    can keep the case at 25, then you can dissipate 80 Watts. Since no one is
    going to be able to keep the case at 25 with that kind of dissipation,
    they'll have to keep the dissipation under 80 Watts. I have to agree that
    it borders on dishonest to spec these devices this way, but it seems to be
    the industry standard.
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