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Aluminium foil placed round inside of plastic SMPS enclosure

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treez

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On page 29 of the below app note, there is a “heat spreader”. This is simply a “wrapping” of aluminium foil to go round the inner wall of a totally enclosed SMPS.
It does not actually connect physically to any of the heatsinking or components on the SMPS PCB.
The wall of the SMPS enclosure is totally enclosing and made of plastic.
Do you think such a “heatspreader” will really have a significant effect in cooling the components of the SMPS PCB? Surely not?

https://ac-dc.power.com/sites/default/files/PDFFiles/der243.pdf?download=1
 

betwixt

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The clue is in its name. It isn't for cooling, it is there to stop 'hot spots' damaging the plastic enclosure by dispersing internal heat over a bigger area.

Brian.
 
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FvM

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Do you think such a “heatspreader” will really have a significant effect in cooling the components of the SMPS PCB? Surely not?
I don't agree.

Similar aluminium sheets are used in many note book power supplies, suggesting that they serve a purpose. The aluminium "foil" is rather a thin sheet of 0.25 mm thickness with certain thermal conductivity. It's in extensive contact with the heat sinks and helps to dissipate the heat more uniformly over the enclosure surface. In addition, the aluminium sheet may help in reducing EMI.
 
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c_mitra

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Do you think such a “heatspreader” will really have a significant effect in cooling the components of the SMPS PCB? Surely not?
Total heat is spread over all the surface; this surely will result in a overall cooling.

Most of the heat is lost by radiation; conduction and convection is effective at lower temp gradients.

Fan helps in convection and heat sinking is effective for conduction.

The radiation is useful to dissipate the total heat produced (say 15% of the total; assuming a worst case 85% efficiency) from the external surface of the enclosure.

I guess the SMPS does not have even a fan. This can be quite effective if the surface temp of the enclosure is 70-90C (just a guess).

If you allow hot spots to develop, locally the temp may rise and locally the radiation will be more efficient but the total effect will be perhaps lower.
 

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The article describes a totally enclosed SMPS similar to the one used on laptop computers. It has no fan and no (or minimal) ventilation. The heat is essentially trapped inside and can only escape through the plastic case which is a relatively poor thermal conductor. The metal foil is to distribute the heat more evenly to make best use of the whole plastic surface and more importantly, to prevent the plastic deforming close to hot components. It isn't a heat sink as such because it isn't attached to anything, it just does as the name suggests, spreading the heat around a bigger area. The bonus is, as FvM states, it also helps to shield EM emissions.

Brian.
 

treez

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Thanks i see your point ref hot spots. I must admit though, i wonder why they dont just mount the PCB in such a way that the hot heatsinks and components do not abutt the plastic enclosure?
 

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