Heat sink design

Discussion in 'General Electronics Chat' started by tagada47, Feb 5, 2018.

  1. tagada47

    Thread Starter New Member

    Nov 13, 2017
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    Hi all,

    I'd like to implement some kind of heat sink to cool down the following step-down power converter: <https://www.pololu.com/product/2851>, but I have zero experience with heat sink techniques. The converter board has components on both sides of the PCB, so it's not obvious to me if I can use some kind of sil-pad.

    What are my options? Adding a fan is not one of them, and the whole package needs to remain reasonably small.

    Thanks in advance for your suggestions.

    - Mathieu
     
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Kindly explain why you think you need a heat sink.
     
  3. tagada47

    Thread Starter New Member

    Nov 13, 2017
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    I plan to use this to continuously to power 144 NeoPixel leds with an average overall DC current of 3 A. The instructions state that "the maximum achievable output current [...] depends on many factors, including the ambient temperature, air flow, heat sinking, and the input and output voltage". The converter will be placed within a closed 40 mm diameter PCB cylinder.

    - Mathieu
     
  4. dl324

    AAC Fanatic!

    Mar 30, 2015
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    What is the input voltage?

    What are the manufacturer recommendations for heatsinking? Only the switch will dissipate any significant power. Can you identify which component that is?
     
  5. bertus

    Administrator

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  6. tagada47

    Thread Starter New Member

    Nov 13, 2017
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    The input voltage will be around 7.4 V (2S L-ion pack). So far I could find no specific manufacturer recommendation. I'll look into locating the switch. Should I think about sticking a largish heat sink to the switch itself?

    Thanks for helping me out. I realize I'm clueless.

    - Mathieu
     
  7. tagada47

    Thread Starter New Member

    Nov 13, 2017
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    Please see my reply to @dl324 above.

    Thanks, I'll look into it. I have doubts about using these on anything other that a flat surface, but both sides of my PCB have components on them. Should I only stick a sink to the switching component?

    - Mathieu
     
  8. tagada47

    Thread Starter New Member

    Nov 13, 2017
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    Alternatively, I could use a similar converter from the same manufacturer, but rated at 9 A (https://www.pololu.com/product/2866) instead of 5 A.

    Am I right to suspect that given the same input and the same load, the 9A-rated converter should run significantly cooler than the 5A-rated one?

    Thanks again for helping out a noob,

    - Mathieu
     
  9. dl324

    AAC Fanatic!

    Mar 30, 2015
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    I think you're worrying too much about nothing. The input to output differential is 2.4V. Look up the thermal characteristics of the switching transistor and calculate what the junction temperature will be without a heatsink.
    Only the switching transistor will dissipate any significant power.
     
  10. Audioguru

    Expert

    Dec 20, 2007
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    The manufacturer of the step down converter gives no detailed spec's about how much heat. They say the max allowed voltage but probably with no current. They say the max allowed current but probably with the minimum input voltage. They say efficiencies but without saying the currents.

    Guessing: Your lithium battery will average 7.4V during a discharge but will be 8.4V when fully charged. 8.4V - 5V= 3.4V x 3A= 10.2W, 15% of 10.2W= 1.53W which is probably too much heat for that small circuit board.
     
    RichardO likes this.
  11. tagada47

    Thread Starter New Member

    Nov 13, 2017
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    Wait, from the plots near the bottom of <https://www.pololu.com/product/2851> it seems that I'd be functioning at ~95% efficiency, so I would expect to be dissipating about 5V x 3A x 0.05 / 0.95 = 0.8W, no? Might be still too much, I have no idea...

    - Mathieu
     
  12. Audioguru

    Expert

    Dec 20, 2007
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    I didn't see the graph.
    5V is across the load, not across the step down circuit. The input is 8.4V when the battery is fully charged and the output is 5V so it has 3.4V across it x 3A= 10.2W x 5%= 0.51W. The manufacturer does not say how much heating it can survive. That is not a good way to sell something.
     
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