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Why did a copper foil screen over an SMPS have no effect on radiated emissions?

treez

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Hi,
At the Radiated EMC lab (30MHz to 1GHz) , putting a copper foil over a 10W (12V TO 5V) Buck SMPS had no effect on radiated emissions.
Why was this?
The copper foil cover only covered top and sides of the PCB...the back of the PCB was facing a metal enclosure....raised above it by some 10mm.
The copper foil was soldered to a short wire which was ring terminalled to the metal semi-open-enclosure in which the smps PCB sat.
The EMC antenna was "facing" the copper foil shield. (in the emc chamber).

The metal enclosure is open at the front...thats why we added the copper foil shield over the smps.
 

betwixt

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That's a bit like saying
"if I place a blindfold on my left knee, why can I still see through my right ear?"

Without knowing how the radiation leaves the SMPS it is impossible to know why copper tape didn't stop it.

Brian.
 

crutschow

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Generally radiation will get through any opening, unless it's very small.
The 10mm gap at the bottom is one possibility.
It may also be coming from the attached wires.

Is the case and shield connected to earth ground?
 

danadakk

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The copper foil was soldered to a short wire which was ring terminalled to the metal semi-open-enclosure in which the smps PCB sat.

Short wires, depending on length and gauge, not exactly a good RF ground for a shield.
Normally one thinks of RF gasket material, copper braid, or spring loaded multiple ground
contact clips as minimum. There are also housing coating techniques to provide EMI
reduction.....

1614623954627.png

or 1614623989165.png


Regards, Dana.
 

treez

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Is the case and shield connected to earth ground?
The case and shield are connected, via a 2 metre cable, to the chassis of another panel, whose metal work is connected to earth ground via the earth wire in a long mains lead.
Generally radiation will get through any opening, unless it's very small.
The 10mm gap at the bottom is one possibility.
Thanks, i thought unless the gap was on the order of a quarter of a wavelength or more, then there woudl not be that much radiation from a gap? For 100MHz, the wavelength is 3 metres. Why wasnt 100MHz radiation massively attenuated by the shield.....the 2cm by 1cm gap is nowehere near 3/4 metre.
 

treez

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Thanks, the attached is the setup
 

Attachments

  • Radiated emissions.jpg
    Radiated emissions.jpg
    66.9 KB · Views: 13

betwixt

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One if the most efficient ways to radiate a signal is by placing the source in the center of a dipole antenna. If you forget the AC and DC power pathways for now and think of the SMPs as a signal transmitter, you can see both have cables leaving them at the input and output sides. Now consider that virtually every commercial and most amateur/home made transmitters are built inside screened boxes but still work efficiently, you may start to see things from the signals perspective.

Another analogy: imagine a long power cable feeding a completely screened RF source. The cable rises from the ground up a few metres to the screened box. First thoughts are the box is screened so AC goes in but nothing can get out. Now turn it upside down, the screened box is ground and the cable rises up from it. Doesn't that look like a conventional transmitter arrangement with a vertical antenna?

We really need to see a photograph of the test set up to be sure but I would be pretty sure the box is almost irrelevant in this test and the difference in RF between the input and output cables is causing the problem.

Brian.
 

treez

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I would be pretty sure the box is almost irrelevant in this test and the difference in RF between the input and output cables is causing the problem.
Thanks, you mean the difference in RF between input and output cables of the Buck converter? The buck's input cable is the "12V cable" shown in post #7 above, and its output cable is within the semi-open enclosure that contains the buck also.

I have also just thought, maybe i should have connected the metal screen (the one covering the buck) to the Buck's 0V rather than to the chassis of the semi-open enclosure? (chassis and buck 0V are not connected)
 

dick_freebird

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Either the screen is not adequately tied to earth
and thus becomes just another antenna, that
gets pushed around by what's inside, or the
radiating element(s) is / are elsewhere.
 

    treez

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