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Heatsink calculation

Discussion in 'Electronic Basics' started by Rok Sitar, Jul 19, 2004.

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  1. Rok Sitar

    Rok Sitar Guest

    Hi.
    I'm wondering if anybody can help me. I made a voltage regulator with LM338T
    and everything works OK. The problem is in heatsink. In have to calc it and
    I don't know how. Can somebody help me. Information from datasheet of LM338
    are:
    Rjc=4°C/W
    Rja=50°C/W
    Rcs=0,5°C/W
    Ta=40°C
    Pd=25W
    Now can somebody give me the whole calc of heatsink.
    THX to anybody.
     
  2. Rja is thermal resistance junction to ambient, when the device is used
    without a heat sink, so you can forget that value.

    Rjc is thermal resistance junction to the mounting surface of the
    case. Rcs is thermal resistance between case and heat sink (under
    some specified mounting setup, probably just a bit of zinc oxide paste
    between case and sink and a specified clamping pressure). You still
    need Rsa, the thermal resistance of your sink to ambient to add up the
    total thermal resistance between junction and ambient. That would be
    the sum of Rjc, Rcs, and Rsa. The total thermal rise above ambient is
    the power being dissipated by the junction (difference of input to
    output voltage times load current, for a regulator) times the total
    thermal resistance.
     
  3. Rich Grise

    Rich Grise Guest

    Zinc oxide? Isn't that the stuff the lifeguards put on their nose?
     
  4. Yes, except that the heat transfer version is silicone grease based.
     
  5. Rok Sitar

    Rok Sitar Guest

    John THX.
    But I was wondering if Anybody can calculate the heatsink from these given
    information.
    Heatsink info is in format xxx °C/W.
    So The question is CAN U CALC THE HEATSINK or NOT?

    Rja is thermal resistance junction to ambient, when the device is used
    without a heat sink, so you can forget that value.

    Rjc is thermal resistance junction to the mounting surface of the
    case. Rcs is thermal resistance between case and heat sink (under
    some specified mounting setup, probably just a bit of zinc oxide paste
    between case and sink and a specified clamping pressure). You still
    need Rsa, the thermal resistance of your sink to ambient to add up the
    total thermal resistance between junction and ambient. That would be
    the sum of Rjc, Rcs, and Rsa. The total thermal rise above ambient is
    the power being dissipated by the junction (difference of input to
    output voltage times load current, for a regulator) times the total
    thermal resistance.
     
  6. Not without knowing the maximum allowable junction temperature. This
    is often 125 to 150 C, but I usually derate to something like 100 C.

    Using 100 C as the maximum allowable junction temperature, That leaves
    a 60 degree rise junction to 40 degree ambient.

    So, (100 - 40) degrees = 25 watts *(Rjc + Rcs + Rsa)

    Rsa = 60/25 - (Rjc + Rcs) = 2.4 - 4.0 - .5 = -2.1 degrees per watt.

    Obviously it is impossible to keep this device below 100 degrees C in
    a 40 degree ambient while it dissipates 25 watts.

    So, hang the engineering safety factor and go for the full 150 degree
    junction temperature.

    150-40=25*(4 + .5 + Rsa)
    Rsa=-.1 degree per watt.

    Nope, still not possible. You will either have to use a bigger device
    with a lower junction to case resistance, lower the ambient
    temperature (water cooled sink), parallel two devices, so that each
    only has to get rid or 12.5 watts, or do something else.
     
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