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Offline SMPS needs metal enclosure to pass Radiated Emissions.


Advanced Member level 5
Jun 13, 2021
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We are spec’ing a 400W offline SMPS, 100-265VAC for a consumer product.

It will be PFC then LLC.

It will be in totally sealed plastic enclosure, no vents and no fans. We want it to pass Radiated Emissions, so we must ensure they put a metal foil shield around the inside of the plastic enclosure. But preferably they will use an aluminium heat spreading box. There will be gaps in it in places, but do you believe if we make sure no aperture is wider than 2cm then we should be OK with radiated emissions?

Lambda = v/f = 3e8/1e9 gives 30cm……… some gaps will be OK?

The following works well it appears...

No idea why you'd assume a shield is necessary to pass radiated emissions. There are countless open-frame supplies available off the shelf with certifications for various EMC standards. Putting it in a plastic enclosure won't make EMC any more difficult (though it may be an issue for thermals).
There are countless open-frame supplies available off the shelf with certifications for various EMC standards.
Thanks, as you know, to pass radiated emissions, they will get placed in a metal enclosure.
Look at every fire panel in the world, they use metal enclosure, if they use plastic enclisure, then the internal offline SMPS is in its own metal enclosure.

Please see point number 8 of this.....

As you know, a radiated emissions test for an offtheshelf offline SMPS is meaningless, because the radiated emissions test happens to the "whole product"...not just the SMPS.
It is a well known fact that offline SMPS in a non-metallic enclosure will fail Radiated Emissions.
Usually, plastic enclosed offline SMPS have a metal foil lining for shielding to allow it to pass Radiated Emissions.
No one thinks its strange that we shield things like comms cables...the di/dt, and the dv/dt can be far worse in an offline SMPS than in low voltage it obviously needs shielding aswell.

I went to one co who did not shield a 70W offline flyback....they failed radiated EMC.

Best way to pass radiated EMC for offline to shield the SMPS in earthed metal or metal foil (can have holes and gaps).....the earth should come from the mains cable.

Many products of course, cheat......and dont bother certifying for Radiated EMC.

Once we drilled a hole in a metal box which contained an open frame offline SMPS....we brought a cable out from the offline SMPS, and took it to a charger box outside of the metal box......and we damned well needed to shield that cable......Radiated EMC was fail fail fail when we didnt shield that cable!

Why is it that people do not appreciate that an offline SMPS will fail Radiated EMC if it is not shielded in metal? offline SMPS, with its high voltages, and hard switching PFC stage, is the most violent thing known in the world of EMC.........and people dont think it needs shielding, when other far less "EMC violent" things do need shielding.

Is anyone going to pretend that a low voltage comms cable, or a 3V3 micro running at a few MHz, is going to be more "EMC offensive" than a hard switching offline switcher?

Next time one passes a plastic fire panel.........ring up the manufacturer...they will tell you, when those fire panels got converted to plastic case, instead of metal case...the open frame offline SMPS's inside tham had to get put in metal enclosures.........why does the industry try so hard to cloak this in secrecy? it because mitigating hardware for passing radiated EMC is more expensive? it because getting through Radiated compliance testing is extremely expensive?
Or is it because nobody fines you if you do produce something that fails radiated EMC.....after all, imagine how long it would take the authorities to do a radiated scan test on every product on the market.
Your competitor may threaten to report you if they see that you fail radiated on the competing prodct....but then they dont,, becauase everyone just declares "amnesty", and promises not to report competitors, as long as they dont report them for their own Radiated EMC fails.
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