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Nat Semi 60watt LM4780 Insulator

Discussion in 'Electronic Design' started by Joe G \(Home\), Jun 27, 2007.

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  1. Hi All,

    I want to use a mica washer or impregnated rubber insulator - to isolate a
    Nat Semi LM4780 power block from the heat sink.

    See the package info here http://www.national.com/pf/LM/LM4780.html


    The problem is - I can't find a suitable insulator.


    I am hoping you have suggestions.


    Thanks in advance?


    Joe
     
  2. Meat Plow

    Meat Plow Guest

    Did you buy it without the insulator?
    I can't recall buying something like that packaged for OEM that didn't
    come with the proper insulator.
     
  3. Eeyore

    Eeyore Guest

    You can certainly buy the thermally conductive rubber pads in sheet form and cut
    it to size. Ditto sheet melinex.

    Really ?

    I've almost never seen semiconductors come with insulators.

    Graham
     
  4. Meat Plow

    Meat Plow Guest

    All the parts we bought from NTE, RCA, Sylvania (mostly TO220, TO3 come to
    think of it) came with new insulators. But you're right a lot of the
    bigger stuff STK, LM stuff doesn't. The convergence outputs for my
    Panasonic big screen came without.
     
  5. Rich Grise

    Rich Grise Guest

  6. Mook Johnson

    Mook Johnson Guest

    I read the datasheet and they do not mention a heatsink isolatior in the
    mounting section. If the part is black plastic all the way around, it
    doesn't need it. As small dab of the white thermal grease stuff everyone
    hates is all your need. Putting anything else will degrade the thermal
    performance.
     
  7. Eeyore

    Eeyore Guest

    You're talking drivel. There are any number of reason one might want to insulate
    the part. For example the heatsink cannot be grounded unless the device is
    insulated.

    From the datasheet......

    Note 14: The TA27A is a non-isolated package. The package’s metal back, and any
    heat sink to which it is mounted are connected to the V- potential when using
    only thermal compound.

    Would you want V- on an expsoed heatsink ?

    Graham
     
  8. Mook Johnson

    Mook Johnson Guest


    You are correct, I missed that note.

    We use two layers of .002" kapton tape in this case.
    Mica is also fine.
     
  9. Ross Herbert

    Ross Herbert Guest

    It seems that this is a common problem for this device. The best
    suggestion is to get a sheet of thermally conductive insulator and cut
    your own. RS Components have Berquist Sil-pad 400 in 12"x12" sheets
    for $40.90 + gst.

    Go to http://www.rsaustralia.com/ and type in 507-5605 into the search
    window and select RS Stock No and then <Find>.

    A higher quality sheet of Sil-Pad K6 (RS 468-2224) will cost $99.40 +
    gst.
     

  10. Thanks for the great suggestion.

    All I need to know is it thermal properties.... to take this in to
    consideration for the heat disipation calculations.


    Joe
     
  11. Eeyore

    Eeyore Guest

    You'll find that data on the manufacturer's sites e.g.
    http://www.google.com/search?q=sil-pad

    Graham
     
  12. John Larkin

    John Larkin Guest

    Sil-pads and kapton ("thermafilm") are rotten heat conductors. The
    best way to cool a semi is to just use silicone grease and isolate the
    heat sink. Or hard anodize the sink and use grease. Next best is an
    AlN or BeO insulator. Thin mica is pretty good.

    One nice trick is to mount the semi to a heat spreader block, ideally
    copper, and use an insulator between the block and the main sink. That
    increases the area over which the insulator acts, reductig theta
    proportionally.

    John
     
  13. Eeyore

    Eeyore Guest

    Some 'sil pads' are much better heat conductors than others. Beware of the
    differences.

    Very purist approach.

    Graham
     
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