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Cooling fans inside totally enclosed SMPS enclosure?

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treez

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Hello,
Is it worth putting fans into a totally enclosed (no vent holes in the enclosure) 3kw offline EV charger? It will be in an aluminium case.
 

KlausST

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Hi,

This may spread the heat and avoid hot spots, while avoiding to bring dust and dirt into the case.

Klaus
 

treez

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thanks, yes, I would have though it (having fans) was essential to allow the internal thermistors to accurately sense the true temperature inside the case?
 

chuckey

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I think if the case is matt black (inside and out) it will be thermally transparent. So perhaps you won't need fans?
Frank

- - - Updated - - -

On reflection, I don't know how big your case is or your losses. 3 KW rating, losses 200W? Dummy something up and see what temperature the case gets to. Say Some filament bulbs inside your case. No point in having fans inside when they should go on the outside :) .
Frank
 

treez

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thanks, there wont be room outside the enclosure for fans.
If it was thermally transparent (the enclosure), then that would definitely make putting fans inside worthwhile.
 

andre_teprom

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Enclosed cases are always critical thermal designs. Apart from having to use a large fan to ensure equal distribution of air flow through the greatest possible internal surface of the cabinet, it may also be necessary an additional small fan over the hottest components in order to prevent the junction temperature exceeds the specified limit.
 

treez

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thanks, I agree with you, but feel its uncommon to have fans inside totally enclosed enclosures with no air vents in them.
 

andre_teprom

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I was rather referring to a specific air spread system aimed for reflow ( not necessarily target so directly on the hot components ), and another cooling system for the hottest components themselves. in general the geometry of the big power components inside the cabinet do not help to efficiently spread the heat and also cool hot components by a single fan.
 

mtwieg

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How do you suppose heat is going to exit the enclosure? Is there an external heatsink anywhere on the outside?

First you should worry about the average internal temperature before considering the temperature variation inside the enclosure.
 

treez

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the unit will be screwed to a coldplate. I think fans with no vent holes is bad because the bearing grease will overheat and deteriorate.
 

andre_teprom

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There are fans that provide a third wire in addition to both those for power supply, whereby it is possible to monitor the fan's rotation, and consequently alarm the need for immediate maintenance.

Anyway, even considering the additional heat generated by the fan, would be negligible if compared to the dissipation of whole power devices, and the heat accumulation concentrated on a silicon device can lead it to operate in a critical thermal condition.
 

mtwieg

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the unit will be screwed to a coldplate. I think fans with no vent holes is bad because the bearing grease will overheat and deteriorate.
Then you should thermally couple the power components directly to that side of the enclosure, so that convection within the enclosure won't even be necessary. This is where chassis mount semiconductor modules come in handy. Surface mount FETs with top-side cooling can work very well also.
 

BradtheRad

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Apple's first generation of their Time Capsule became plagued by units dying. Components (particularly capacitors) had cooked in the confined space. It was caused by ineffective ventilation. The fan was placed where it did practically nothing. Air flow was almost non-existent.

It became notorious as a careless design fail on Apple's part.

A fix involved (I had to do this on my Time Capsule):

* detaching and re-orienting the fan
* drilling larger holes, to channel air flow directly through the fan
* replacing the fried bulging capacitors.

The unit's power requirements are much less than 3kW.
 

SunnySkyguy

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Thermal resistance of 300W of heat with a fan inside a box is like putting a current source into 100 Ohms when you need 1 Ohm.

Seal it with purified vegetable oil with inhibitors would be an improvement with perfect sealed box.

Look at how many PC's have a 30'C rise internal ambient and another 30'C rise on the CPU even with half a dozen case fans, when it should be 10 & 10 with optimal heat transfer such as water cooling loop to outside.

Thermal transfer by the path of least resistance is best with suitable heat conductors such as copper shells with heat pipes.
 

treez

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Look at how many PC's have a 30'C rise internal ambient
Thanks, Are you speaking of PC's that have vent holes? My boss always used to say that our UV print heads , that had vent holes , would never have a rise of more than 10 degc of internal ambient, because of the vent holes. (even if they had no fans in them)
 

SunnySkyguy

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Yes I'm talking about poorly designed PC boxes with lots of vent holes and fans blow air not l very hot because they are drawing cool air thru the mean path of least resistance rather than directly over the hot spots , CPU fans that just circulate the heat and poor heat transfer outside . THe fans with vents are like the jet stream which is far different temperature from the ground surface. with 1kW/m^2 solar power. YOu end up heating all the components according the the box's thermal resistance in deg/watt.

And this is far better than what you are describing. A flat wall has very little surface conduction area and although a fan may reduce some nearby hotspots, without a large heatsink attached on the inside of the outerwall , you may as well have a toaster inside.

A serious proper thermal design needs to be done. on heat flux and heat velocity.
 

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