Design Heat Sink!!!!!!!!!!!!!!!!!

Discussion in 'The Projects Forum' started by sneh_km, Apr 7, 2008.

  1. sneh_km

    Thread Starter New Member

    Apr 7, 2008
    We are facing a problem related to heat sink design for ICs LM317 and LM7805 for power supply.....we are confused wether the design depends on the area of the heatsink or something else....please tell us how to design it.....we have to submit our project urgently....!!!!!!!!!!:( :confused: :)
  2. DC_Kid

    Distinguished Member

    Feb 25, 2008
    i'm rather certain the complete surface area of the heatsink is a design metric when engineering the power management of a "power" device. however, common sinks have thermal ratings which need to meet or exceed the operational specs of your design, so choosing a sink becomes a fairly simple task.

    but to answer your question, a short list of metrics when designing the sink:
    1. area
    2. geometric shape
    3. sink material, thickness, etc
    4. forced or free air flow
    5. sink paste

    LM7805 (5v 1amp to220) - a 2" piece of 1x 1/8" thick aluminum flat stock bolted to 7805 should suffice.
  3. thingmaker3

    Retired Moderator

    May 16, 2005
  4. Audioguru

    New Member

    Dec 20, 2007
    The amount of heat that the regulators dissipate is their input voltage minus their output voltage times their load current. With a huge aluminum finned heatsink their max allowsd dissipation is about 18W each.
    How much dissipation is each?
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
    One thing unmentioned, the heatsink 'coating' is important. Yes, a bare aluminum heatsink dissipates less than a black annodized heatsink.. Various coatings have different radiation rates..

  6. John Luciani

    Active Member

    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 *)