Passing radiated EMC with an SMPS involves an element of luck...how much agree?

I once worked with one of the UK’s finest SMPS design engineers. They were working for the same company as myself at the time. They had already designed many of the SMPS PSU’s as used by the British military.
They once were talking about trying to get SMPS’s through radiated EMC without the SMPS being in a metal case, and without any metal shielding being used around the PSU or its components, whatsoever…..
….In relation to this, this excellent designer said to me…..if you have designed an (non metal enclosed) SMPS and it has passed conducted EMC, and it has been layed out as good as it possibly can be using EMC layout rules (eg keep power switch current loops as minimal in area as possible etc etc)……and supposing this SMPS fails on radiated emissions…..then there are no exact calculations that you can do to get it to pass radiated EMC…..but rather, one simply has to “sprinkle” in ferrite beads and common mode chokes, and Y capacitors and cable ferrites like “currents in a cake”, until you get a pass.
Please discuss if you agree with this?

Passing radiated EMC for an SMPS when one has a metal casing is far , far easier, of course, but thats not what they were talking about.

Many of the "sprinkles" you mention look more like
conducted emissions stuff. Of course power lead
can do both.

Why you'd expect a calculation to do anything for
radiated emissions after the fact eludes me.

Practices aimed at radiated emissions overlap but
are not all covered by conducted emissions practices.
Large current loops and long wires carrying switching
waveforms don't do much of anything as regards
conducted emissions. Likewise open magnetic path
inductors.

What I think you'd want, is on-the-bench ability to
do a good enough "poor boy radiated emissions" test.
Like maybe a cast-off microwave oven for a quiet
chamber, and a spectrum analyzer with a sniffer
probe. You'd have some work to do in figuring out
what kind of reading there, equates to a passing
reading at a real EMI test house, but that might be
as simple as putting one of your already-tested
versions that you have a detailed test report for,
and see what the spectrum analyzer says. Draw
your "frequency mask" on the screen in crayon
and there's your cut-and-try goal / feedback.

For plastic case you need a topology that is inherently very quiet in the first place, and heatsinks all tied to +Vcc or 0v, - this is why we design resonant converters for many of our products ...

Sprinkling ferrite beads, etc, is best done by some one who has a fair idea of their exact effect in the partic location ...

Why you'd expect a calculation to do anything for
radiated emissions after the fact eludes me.

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Thanks, sorry i mis-worded, i meant that whilst one is doing the design and PCB, there's no fixed way of caluclating what radiated emissions combating components are needed

For plastic case you need a topology that is inherently very quiet in the first place, and heatsinks all tied to +Vcc or 0v

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Thanks, I have worked in places where none of the plastic cased , hard-switched converters pass radiated EMC, but they still sell the product, on the basis that they are "working on it".
I very much agree that passing radiated emissions to domestic level, with a hard-switched SMPS in a purely plastic enclosure, is not particularly possible at all.

I believe that with eg an offline flyback, it is not possible to pass radiated EMC to domestic level (EN55032 class B) unless one uses some kind of metal foil screen round the product or metal case......or at least a metal box over the switching node, say.

Sprinkling ferrite beads, etc, is best done by some one who has a fair idea of their exact effect in the partic location ...

Click to expand...

Yes i agree, sprinkling the beads is best eg as close as possible to the output rail of an on-board smps converter.....eg, right by the output caps of it....also, ferrite beads on the pcb traces just before they go to the outside world via the connectors.

I've done SMPS's in non metal cases that have passed EMC requirements, care and understanding of the beast is what you need, break it down into little bits and its easier, like all circuits, split it into loops and minimise the loops, avoid interaction between loops both capacitive and inductive as much as possible and keep switching currents contained, that includes the 0V especially.