Radiated EMC testing of SMPS equipment with lots of pluggable filter boards? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: March 09, 2021, 08:32:32 pm
content: Hi,Supposing you are going to put an equipment containing SMPS’s to Radiated EMC testing for 30MHz to 1GHz.You have many different common mode and differential mode chokes which you want to trial in an attempt to pass Radiated EMC. Time in the EMC chamber is extremely expensive, so you want to swap out the different CM and DM chokes very quickly. So you have various strip boards housing various CM AND DM chokes in doubles and singles etc, with Diff mode caps between them etc. You have wires coming off input/output of each filter board going to pluggable connectors so you can quickly whack the various filter boards in and out and see which one works best. Do you think that such a setup could in itself compromise the Radiated EMC performance? (all the wiring off to all the pluggable connectors)The equipment comprises Offline Flyback SMPS’s and low power Bucks. Offline Flyback is in an metal earthed enclosure, which sends low voltage DC cable to another enclosure which houses a 10W Buck DCDC. The 10W Buck DCDC sends a cable to low power loads in its enclosure via a wiring loom. The buck enclosure is not totally enclosed, but wide open one side.

author: ocset
author_rank: Super Contributor
timestamp: March 14, 2021, 03:25:34 pm
content: Hi,We have got to pass radiated EMC (30MHz to 1GHz), but in a “real” situation where we cannot afford “proper” RF EMC techniques…and in truth…”real” EMC techniques just wouldn’t be mandated, or realistic.Our setup is as attached.The following doc calls for a “RF Reference plane” for use with EMC filtering and shielding….https://interferencetechnology.com/wp-content/uploads/2016/10/2016_IT_EMC_Filters_Guide.pdf…That sinks us from the start. We have two panels, whose metal enclosure is earthed, but that earth comes from the earth wire in the long 3-core power cord…..so that connection is so high Z to RF that its NOT a connection at all. So we have no RF reference plane!!!…and our metal chassis is basically floating at RF !!!So straight away no official EMC literature is of any help to us!!!Does anyone have an “EMC guide for real-life, cost constrained situations” document, instead of these “perfect EMC” guides which tell how to do EMC for super-expensive medical devices or military devices with RF gasketted connectors etc? I’ve read tons of them and they are no use.I mean, Feedthrough capacitors, etc etc aren’t much use without an RF reference plane.Take the blue cable between the two enclosures in the attached.What are the options?(a)…Two core cable unscreened?(b)…Three core cable with third connector connected to chassis at each end?(c)…Two core cable with screen…screen left floating…it may as well float…both our chassis’s are already floating at RF?(d)…Two core cable with screen connected to chassis at “A”?(e)…Two core cable with screen connected to chassis at “B”?…and what if we just don’t bother connecting our chassis’s (A and B) to earth at all?....after all, the power cord earth connection is NOT a connection at RF…its too high Z. And then what should we connect our Y capacitors to?....there is no Earth. How are the common mode chokes working at all?No EMC literature discusses these real options. Do you know the real answers?

author: twospoons
author_rank: Frequent Contributor
timestamp: March 15, 2021, 06:49:39 am
content: your metal box is your RF reference plane, as it is conductive and completely surrounds your RF generator (the switchmode).First rule of passing EMC is to not generate any.Given that this is usually extremely difficult, next best is to generate as little as possible. With very careful choices in topology and layout, this is where you will get the most bang for your buck. Often its better to invest more money here, than to go cheap and then need expensive EMC bandaids in an effort to mask over the real problem. Those ferrites are not cheap!For the blue cable : two core plus shield and connect both ends. This is because you have RF generators at both ends of the cable. If thats insufficient I would add bypass caps to both wires (to chassis) at one end at least. Next step up would be a cable core to add some CM impedance.Of course, if you design the first supply properly then the cable need only be two wires.Does that help?

author: ocset
author_rank: Super Contributor
timestamp: March 15, 2021, 10:52:26 pm
content: Yes, thankyou, do you think the length of the blue cable will change the plot radiated spectrum much?I mean, this cable can be from 1 metre to 3 metres long or anywhere in between...i reckon if we fail EMC at one peak frequency, we can probably change the cable length a little and end up passing?

author: twospoons
author_rank: Frequent Contributor
timestamp: March 16, 2021, 01:43:49 am
content: The cable length will definitely change the EMC profile, but if you are that close to failing then that is not the place to be looking for fixes. Since bends, loops, twists, proximity to other metal will all affect the EMI radiated from a wire, you're in a realm where you really have very little control. You need to stop the EMIbeforeit hits the cable.Passing EMI certification not just about squeaking through an artificial test. Its about not generating interference out in the real world. You want to be minimum 6dB clear of the limits ( I prefer 10dB), so that variations out in the field don't push you over. Remember that if some third party tests your gear and finds it fails there could be some expensive consequences.

author: penfold
author_rank: Frequent Contributor
timestamp: March 16, 2021, 03:59:28 am
content: Quote from: treez on March 09, 2021, 08:32:32 pmIf done properly (absolutely minimal loop area on connectors and lots and lots of ground links between host board and filter board) then probably the influence of the "carrier board" should be negligible at the lower end of the spectrum, approaching the 500MHz to 1GHz mark there'll be a much greater influence, but you're much less likely to get emissions from a switch mode in that area unless someone has really tried to design badly.You could do some pre-compliance tests with the daughter-board filter and repeat after selecting the correct filter and integrating it onto main PCB, that would normally be sufficient defence to say "the PCB+filter than went into EMC testing is equivalent to PCB-including-filter based on these tests repeated before and after official testing"

author: ocset
author_rank: Super Contributor
timestamp: March 16, 2021, 07:05:20 am
content: QuoteThanks, i understand that current loops should be made minimal in area, but dont understand the term "loop area on connectors"?Also, please advise on the "ground links between host board and filter board"...by host board you mean the SMPS board?Sorry i dont understand the concept of the multiple ground links between the SMPS board and the filter board.....there will be a twisted pair wire between them...thats it?

author: penfold
author_rank: Frequent Contributor
timestamp: March 16, 2021, 05:59:47 pm
content: Quote from: treez on March 16, 2021, 07:05:20 amWith regard to the loop area of connectors, what I mean is depending on the connector technology the pins will have some separation (pitch) and some length (how far it separates the filter board off the SMPS board) which will all account for some inductance in the signals.With the ground etc, you would need to make the filter PCB to the same stackup as the SMPS board, so for example you may have a ground plane (could be local 0V or chassis earth etc) which gets quite close to the filter components, this ground plane would need a very low inductance path made by lots of connector terminals to give the illusion that it is part of the SMPS board.Just spitballing an idea... would it be possible to make the SMPS board with a physical cutout into which the filter board sits and be creative with the connections... I'm imagining copper foil would be good to join ground planes.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 16, 2021, 06:06:31 pm
content: Quote from: treez on March 09, 2021, 08:32:32 pmYes, any addiitonal wiring will affect performance. Consider the chamber time cost with the cost of making up several units with different filter options properly installed. If you have at least two units, you can be changing things on one while testing the other ( just make sure they started off teh same)Note that when selecting things like this, you can generally do it relatively quickly as you should fairly quickly find where the problem frequencies are and limit the scan to looking at peaks here, rather than a quasi-peak scan over the entire frequency range.

author: ocset
author_rank: Super Contributor
timestamp: March 17, 2021, 06:51:32 am
content: Thanks,As discussed, the attached is our setup going for radiated emissions testing. (It’s a kiddies play panel for hospitals and doctors gaming.)What do you think to the following edicts on the radiated emissions testing ?.....(30MHz to 1GHz)A…..Adding an RC snubber from switching node to ground at the Buck SMPS may well have a good effect. However, the sudden discharge of the snubber capacitor simply means there will be another radiated emissions problem, but above 1GHz, where formal testing does not take place?B…….If we use common mode chokes with Y capacitors to the chassis, then any failure peaks that we may get are just as likely to be coming from resonance in the common mode LC filter, as from genuine noise emissions? As such, tweaking the Y capacitor value to try and bring peaks down is likely to be futile, as it will just mean getting a resonant peak at a different frequency?C……The cable connecting units A and B could be made of 2 core cable (not twisted) with a metal foil screen…or twisted pair with a foil screen. Can you confirm that 2 parallel conductors covered in metal foil screen would radiate more than twisted pair inside a screen?D…….If I do a radiated emissions test in a proper radiated EMC chamber…but of a battery supplying a Buck converter in a metal chassis via a cable (ie, a cable from the battery to the chassis containing the buck)…then will the emissions be worse if the chassis is connected to earth ground?……or will emissions be worse if the chassis is not connected to earth ground at all?E……The buck converter in the attached supplys some radio pagers via a wire loom. (5V, 0.5A to each pager). Suppose we put a common mode choke at the output of the buck, just before the connector that connects to the loom…..is there any circumstance where adding such a common mode choke could make radiated emissions worse?F….For the blue cable in the attached…..which is best for low emissions?...a)….2 core cable (unscreened)b)….twisted pair (unscreened)c)….3 core cable with 3rd connector connected between the two chassis’s (unscreened)d)… 3 core cable with 3rd connector connected between the two chassis’s (screened)G.......Why on the entire web are their No forums, or sub-forums, for people to ask questions about EMC testing of products?....is it because the actions taken to get kit through EMC testing is the deepest secret of any company?H....For good EMC performance, why is it said that a capacitor with high SRF is needed to be connected right at the input/output connectors? (diff mode)

author: ocset
author_rank: Super Contributor
timestamp: March 18, 2021, 05:32:53 pm
content: The blue cable in the attached picture comes out of enclosure A. The blue cable is the output of a common mode choke PCB, which filters the output of a flyback SMPS, which is also in A...For the purposes of reducing radiated emissions from 30MHz to 1GHz…..1…Do you think its worth using screened cable for the wired interconnection between the Flyback SMPS and the filter PCB in A? (instead of just twisted pair).2….Supposing the assembly person strips back the screened cable too far, and before the blue cable goes into A, it is stripped into its separate go, return, and screen wires, which are then out in the open…..would it then be necessary to cover these in copper tape, so as to reduce emissions? (the copper tape being connected to the cable screen).How far into enclosure A should the blue cable go before it is allowed to be stripped into its three internal wires?

author: penfold
author_rank: Frequent Contributor
timestamp: March 19, 2021, 01:14:00 pm
content: If A is mains powered and B is a metal box, I'm about 90% sure that B needs to have a strong earth connection for safety reasons (it'll be detailed in whatever safety standard you're working to).If you have common-mode noise on the A-B cable conductors then screening may help you, but it will need to be a very hefty copper braid plus foil screen to prevent CM leakage from the inner conductor to screen via mutual capacitance.Terminating the screen to chassis earth at each box may result in problems since the CM return currents can flow through the shield (that is a huge problem with single foil screening since it has a relatively high resistance, braid has lower resistance).RC snubber network does indeed create an additional radiative path, but it is your job as the designer to select R and C and find the correct layout for minimal "loop area" and minimal radiative efficiency from that snubber loop, thus containing the HF in that small loop vs the bigger LC circuit which has more parasitic and higher radiative efficiency.Y-caps do indeed create resonant peaks and other annoyances, but its the responsibility of the designer to select components appropriately. Blindly selecting the values without calculation or knowledge of components' parasitics will almost definitely make things worse.Twisted pair will help diff mode noise, but not common mode.If the assemblyperson strips back the screened cable too far... tell them to do it again, there's no point designing anything if it's not going to be made correctly.I'm surprised there aren't any forums for EMC discussions, but discussing EMC issues without being able to see the full design files is often counterproductive, so that excludes most commercial organisations from discussing EMC on public forums and it is rarely anything a hobbyist needs to worry about.If you can't/(don't want to/don't have the time to/don't have the information to/whatever the reason is) design the flyback and buck correctly around all the appropriate EMC considerations then your best bet is to book some pre-compliance time at the test-house (most places will charge a lot less for quick investigations in a non-certified approach when you subtract the costs of report writing and test spec writing etc) or discuss with a consultant (with whom you can share the design files under an NDA) and at least get a good idea of where the problems are coming from and whether they can be stopped at the source... if indeed you are actually going to have issues. It will be significantly more proactive than any answers you can get on a forum where you can't share complete design details.

author: ocset
author_rank: Super Contributor
timestamp: March 19, 2021, 02:11:25 pm
content: Thanks,If a battery in A (of the diag in #10 above) alone supplies B......then what if A's metal enclosure is not earthed?....will A's enclosure then act as an antenna and make the radiated EMC scan worse?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 19, 2021, 03:57:46 pm
content: I'd also look at ferrites on cables, may be more effective than shielding

author: penfold
author_rank: Frequent Contributor
timestamp: March 19, 2021, 04:23:01 pm
content: Quote from: treez on March 19, 2021, 02:11:25 pmI can't say for sure, it'll probably depend on the anechoic chamber (or open area test site) being used, it shouldn't affect the distribution of currents and voltages from which the radiation propagates but I'm pretty sure it will affect the propagation characteristics, you would need to speak to your test house to determine how to make it into a representative test given the equipment and capabilities they have.Is any of this based on actual measurements? because you really shouldn't be having that much trouble with such a setup, just book the test session, inform them you're concerned about radiated emissions... a good test house will look through any design file items you wish to send them anyway to anticipate problems.