Thermal Analysis( When to use heat sink)

Discussion in 'General Electronics Chat' started by sharath_412, Nov 28, 2007.

  1. sharath_412

    Thread Starter Active Member

    May 7, 2007
    Can anyone explain me about Thermal analysis of IC's. When to use Heat sink and all related stuff. How to calculate the temperatue rise etc. Any link .........

    thanks & regards
  2. eeboy

    Active Member

    Sep 27, 2007
    You would use a heat sink in the case that power dissipated by the component in question causes you to exceed the maximum power dissipation listed on the data sheet. At this point you need something to conduct the heat away such that the junction temp remains below the absolute maximum. This can be achieved via heat sink, thermal pads, forced air.... many methods.

    I've read several good application notes on the subject but I can't seem to locate one at this time. Try International Rectifier to see if you can find something from them.

    It actually turns out to be fairly simple. Although I've read other documents which downplay the industry standard method at calculating the junction temperature. My take on it after reading quite a bit of info is that it's not exact and so one should build in a good safety factor.
  3. nanovate

    Distinguished Member

    May 7, 2007
  4. John Luciani

    AAC Fanatic!

    Apr 3, 2007
    To determine the temperature rise you need to calculate the junction
    to ambient thermal resistance (Tja) ---

    Tja = Tjc + Tcs + Tsa all the dimensions are DegC/W

    Tjc thermal resistance between the IC junction and IC case. This is
    from the IC datasheet.

    Tcs thermal resistance between the IC case and your heatsink. This
    should also be in the IC datasheet. Usually you place a thermal
    compound between the IC case and the heat sink to decrease the
    thermal resistance. The decrease occurs because the thermal
    compound fills the air gaps between the two surfaces and conducts
    heat better than the air.

    Tsa thermal resistance between the heatsink and ambient air. This is
    in the heatsink datasheet. Most heatsink datasheets specify Tsa
    with and without airflow. You can determine if you need a fan by
    reviewing this number.

    For a single heatsource the junction temperature would be

    Tj = Tja * P + Tamb (DegC)

    where P is the power dissipated in the IC and Ta is the ambient

    (* jcl *)