Electronics > Power & Renewable Energy

Gap in metal enclosure for SMPS

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Faringdon:
Hi
We need to pass EN55032 with an offline isolated 20w flyback SMPS.
We have an earthed metal screen all round it, apart from a 33mm by 8mm bit which is open. Will this be OK from a radiated EMC viewpoint?
(After all, the 1GHz wavelength is 30cm.)

langwadt:
http://cdn.lairdtech.com/home/brandworld/files/EMI%20Rule-of-Thumb%20for%20Calculating%20Aperture%20Size%20Technical%20Note%20Download.pdf

TimFox:
Also, for a more general case, Google “waveguide beyond cutoff” for the penetration of an evanescent wave into a hole.

Faringdon:
Thanks, looks like a 33mm  by 8mm gap ("slot") is going to let through to much radiation.

I am wondering about doing some trick with inner layer copper to do the shielding across the isolation barrier?
I wonder if an inner layer of secondary  ground copper can be sneaked across the  Offline SMPS isolation barrier to do the under-side shielding?
(sec ground is actually earthed anyway)

I doubt  it would be violating regulations as its covered by FR4?

...otherwise it'll be very awkward to get a metal enclosure to cover the entire Mornsun LS15-13B12 SS power module as it stands  mounted on the PCB.

Mornsun LS15-13B12 power module
https://www.mornsun-power.com/index/sitesearch/partlink/keyword/LS15-13B12SS.html

dmills:
Treez,
How much energy are you actually making up in the UHF region?

For a mains power supply application I would be somewhat surprised if there was much above low VHF, and you don't need to shield that which you do not produce.

Usually my issues up where that slot radiator will screw you are more in the way of harmonics from video clocks and the like rather then harmonics from power converters.

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