Passing radiated EMC with an SMPS is just luck....how much do you agree?

[Flyback]

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.

 

[AnalogKid]

Disagree. If adding parts to your best-design-practices device changes it from non-passing to passing, then maybe your "best" design practices aren't.

ak

 

[Flyback]

Thanks, but sorry,, what i meant is that if cable ferrites arent used in the supply cable, then passing radiated EMC to EN55032 class B with eg an offline flyback SMPS, is virtually impossible...unless it is in a metal shielded enclosure.......or there is a metal screen over eg the switching node.

 

[AnalogKid]

what i meant is that if cable ferrites arent used in the supply cable, then passing radiated EMC to EN55032 class B with eg an offline flyback SMPS, is virtually impossible...unless it is in a metal shielded enclosure.......or there is a metal screen over eg the switching node.

Probably, but there are circuit techniques such as slowing the edge rates that have a large effect on EMI.

Also, I still disagree with the thread title. Relying a filter component such as a ferrite bead to filter stuff is not "luck"; it is using the right part for the job.

If your concern is that a ferrite core clamped onto the output cable, preventing it from acting as a radio antenna, is fat and ugly, consider that it is a common practice because it is cost-effective. That does not mean it is the only viable technique.

ak

 

[Flyback]

Probably, but there are circuit techniques such as slowing the edge rates that have a large effect on EMI.

Thanks , yes very true, but as you know, there is a limit to how much edge rates can be slowed in smps, before efficiency gets too low.

 

[ronsimpson]

Thanks , yes very true, but as you know, there is a limit to how much edge rates can be slowed in smps, before efficiency gets too low.

That is why I love resonant supplies. There is no sharp edge.