Electronics > Power/Renewable Energy/EV's

Passing radiated EMC with an SMPS involves an element of luck...agreed?

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ocset:
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.

T3sl4co1l:
We should be eternally grateful to work in a field where every single last thing can be modeled, emulated or solved, without having to touch a single tool.

There are no guesses here.  There can only be a lack of knowledge.

Anyone who insists that it is a required design step, to perform physical testing, and that luck is involved: they are admitting their ignorance.

Now, it often is the case that, for a variety of reasons, an engineer is unable to attain full knowledge of a system, and therefore must make assumptions, guesses, and perform experiments.  One must understand that this is not a strictly necessary route, and these steps are more as exceptions that we make for various reasons (insufficient time or budget to study or model the system; insufficient working knowledge; insufficient time/budget to pursue the education to obtain that working knowledge; etc.).

It's a practical problem, not a theoretical one.

A very good engineer might go her entire career without having undertaken such a project -- that is, one with knowledge so comprehensive as to be practically complete -- but this is only statistical evidence, and not proof!

Conversely, a less skilled engineer might go his whole career assuming that such processes are necessary, and never taking up the education (or making the realization) that there are other ways to do it.

Mind, it's not that one or the other approach is necessarily superior.  Perfectly merchantable products can be made either way.  It's perhaps interesting to consider the ways in which the two approaches might fail.  For example, the low-level method might be fragile under variation in parts value (something of a liability for long-term products, as parts age in use, or as substitutes parts are put into production over the years), whereas the high level method might get all the basics right, but have a structural quirk that makes the design unexpectedly difficult to evolve over time, as the customer's requirements or production needs change (e.g., using just the perfect part, and then the requirement changes just outside its capability; or it's from Maxim and they discontinue it, amirite?).  Either way, if the absolute best results are demanded, one should allow for multiple cycles of refinement.

So, in regards to EMC, we can -- at least in principle -- model everything perfectly, make just a few component changes, and have high confidence in passing tests the first time.  We don't need to rely on luck in the physical test, but we might have no reasonable alternative due to the amount of effort required to create such a simulation (namely, ca. $6-figures for the EM simulator, plus some months setting up the model).

(Note that the argument applies recursively, so that a more talented engineer might change just a few values in the simulation and be done, whereas the other may make hundreds of relatively undirected changes.  Simulations are just virtual testing procedures, after all.  Or in the meta, so that ones' level of knowledge might be approached in a comprehensive way, or through scattered experience.)

(Note also that this is not an endorsement of "CAD jockey" design.  If one can reach a level of practical experience from their armchair, by all means do, but do not leave it unchecked against reality.  Many will need lots of hours in the lab to reach a functional level; most prople, I suppose, will never reach it at all.  (That is, a lot of people don't care for engineering at all, or tried and didn't turn out to be any good at it.))

Tl;dr: disagree.

Tim

ocset:
Thanks, on a related topic, all the places where I have worked where they have designed offline flybacks in plastic cases, did not pass radiated EMC to EN55032 class B unless cable ferrites were used in the supply cable. As you know, unfortunately,  having a supply cable with a cable ferrite on it is a lot more expensive than just a supply cable.

These companies  still sold the product because it passed conducted EMC, and they declared that they were “working on” a pass for the radiated EMC.

In fact, I think many would agree, that it is  not possible to pass radiated emissions to “EN55032 class B “ with a hard-switched SMPS unless one encloses the SMPS in a metal enclosure, or encloses at least the switching node in a “metal can”, or  uses a cable ferrite in the supply cable.

T3sl4co1l:
It's possible to pass any arbitrary level with suitable construction, not necessarily needing explicit shields as such.  Those are just easier.

Tim

MagicSmoker:
Paraphrasing - if not echoing - what Tim just wrote, it is often most cost-effective to build a prototype and do pre-compliance testing on it in-house. It may be theoretically possible to fully model an SMPS but the expense of the software and the time it takes to both become proficient in it as well as characterize every last aspect of the design, typically render this option impractical for all but the highest volume products, where saving a few cents (to dollars, even) per unit will be worth the effort.

I typically design very high power converters for niche markets and rarely have to contend with consumer product EMC testing, but some of the things I have designed over the years ended up needing to go through some form of EMC testing as regulations (or the enforcement thereof) changed over time and they usually made it through on the first try, with only one thing in recent memory needing any kind of remediation (adding common mode chokes to internal cables and shielding a terminal block - rather kludgy stuff, but cheaper than a full redesign, especially since product volume was in the 10s per year).

Passing consumer EMC testing on the first spin of a product design is usually pretty easy once you have experienced the process yourself. It tends to present one with an enormously steep learning curve on the first go, however.

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