Understanding Heat Sinks

Discussion in 'General Electronics Chat' started by RodneyB, Jun 2, 2014.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I am working on a project currently being helped under the project forum.

    So as not to confuse topics I have posted a separate thread.

    I am using a 3 watt LED and under thermal management it says for a 3 Watt LED I need 50cm square of Heat sink So for 3 LED's I would need 150 cm square of Heat Sink.

    Now what I am not sure of is how to work the area out of a heat Sink. Is Each side of a fine added and the Surface?

    The datasheet talks about thermal conductivity which I am assuming is the material which I can find off the datasheet.

    I just need the area clarified and if there are any additional tips I may need.
     
  2. #12

    Expert

    Nov 30, 2010
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    150 square centimeters is a square that is 12.25 centimeters on each edge.
    Or it might be a circle that is 13.8 centimeters in diameter.
    Or a lot of other shapes.

    Use transistor heat sink compound to get better thermal conductivity from the LED to the metal.
    Be sure the metal is thick enough as the instructions say.
     
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  3. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    If I was to use 25mm x 25mm x25mm x 100mm Aluminium channel would this be suitable.

    =

    If I am correct the surface are is a total of 150 cm Squared. The aluminium channel has a wall thickness of 3mm.

    ((2.5*10)*3)*2

    I then will screw it onto the aluminium housing which will add to the cooling. This will be through two screws tapped into the housing.
     
    Last edited: Jun 2, 2014
  4. #12

    Expert

    Nov 30, 2010
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    2.5 x 3 = 7.5 square centimeters per centimeter of length.
    10 cm x 7.5 = 75 square centimeters.

    Was the recommended heat sink a piece of metal? If so, it would have 2 surfaces.
    If you flatten a channel out, it is a flat piece of metal with 2 surfaces.
    You can't count both sides and multiply by 2 to get the equivalent of the recommended heat sink.

    In addition, the aspect ratio can ruin the heat sink. 20 meters of 3 mm wire will not make a good heat sink. Your heat sink and the spacing between the LEDs should have vaguely similar distances between the LEDs and the ends of the channel.
     
  5. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Centimeters or millimeters? There seems to be some confusion as to the units of measurement.
     
  6. #12

    Expert

    Nov 30, 2010
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    I didn't have any difficulty tracking the numbers.:confused:
     
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Oops.

    54321...
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Yes, each side that is exposed to the cool ambient air.
    Some heatsinks state their circumference, so that you can calculate the total surface area for the length of the heatsink. However, it would be better if you could post the whole datasheet of the the LEDs you are going to use, as there might be better figures that would get you more appropriate answers. Maximum junction temperature and Thermal resistance junction to case would be much better than the recommended area of some heatsink with unknown parameters (surface finish, shape, material, air flow.. all these are quite important).
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Here is an example when thermal resistances are known
    Example:
    Natural Convection (no fan)
    Cree XML LED
    Thermal resistance (Junction to solder point) = 2.5 deg C/W
    Star metal clad PCB = 5 deg C/W
    Thermal Compound = 1 deg C/W
    CUI VHS-45 half brick heatsink (Sink to air) = 10 deg C/W

    So 2.5 + 5 + 1 +10 = 18.5 deg C/W
    If you run the LED at 3W then 3 x 18.5 = 55.5 deg C rise over ambient
    If ambient is 25 deg C then the LED junction is at approx 80.5 deg C (= getting toasty but should be fine)

    With forced air (a fan)
    you can get that heatsinks thermal resistance down to 2 deg C/W
    Which would lead to a
    2.5 + 5 + 1 + 2 = 10.5 deg/W so at 3W you would be running 31.5 deg C over ambient and with the 25 deg C ambient the LED junction is at approx 56.5 deg C (= good long life running cool)
     
  10. #12

    Expert

    Nov 30, 2010
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    Apparently kubeek has a different configuration in mind. That's why we need you to keep providing information.
     
  11. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    Thank you for the information so Far, I will try and post as much information as possible.

    Firstly the datasheet of the LED.

    I want to fit the LEDs Onto a heat sink and into a 150 Watt halogen fitting. The heat sink can be secured to a bracket which is also secured to the housing for additional heat sinking.

    The available heat sinks to me are shown at

    http://www.mantech.co.za/stock.aspx?Query=HEATSINK+FINNEDand

    The LED's are 23mm apart, 3 connected in series. to the LDD-600H LED driver. on advice I am not driving the LED's at the full 700mA But thought for extra precaution to try and design a heat sink that can handle the 9 watts.
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The PCB will make a thermal isolation barrier between the leds and heatsink.
    It would be better to mount the leds directly to the heatsink.

    Bertus
     
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  13. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Such LEDs are meant to be attached to an aluminium or ceramic based PCB, which will allow you to mount the backside directly to a heatsink or to a plate that will be thermally connected to the case. 3W is quite a lot of power for such small LEDs, and as you can see there is no other way to cool them than through the back side of the package.
     
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