Meanwell power supply screams blue murder at light load on EMC test - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: November 23, 2022, 08:58:35 pm
content: I have been testing a machine that runs on this power supply:https://docs.rs-online.com/6da3/0900766b816e7457.pdfthe 48V version.The machine and power supply are separate, with the power supply in a dedicated box. Now having screened the power cable and used shielded glands we seem to have fairly good emissions. but when the machine is idle we got the worse emissions. The machine uses about 12A max running motors so with no motors running very little power is drawn and the converter will be in discontinuous mode. I get this on a dummy box with resistors in it. As soon as I switch in 1A of load the noise goes.Now I could deal with this by using a dummy load but I'd hope the required load is less than 1A and will experiment. Is this actually the best way? I have a filter of my own design already, I think having taken away lots of noise with a filter and the shielding we have regained it by upping our 48V cable to 5m.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: November 23, 2022, 09:17:45 pm
content: Do the emissions correlate with the switching frequencies (55kHz/70kHz)?73

author: ogden
author_rank: Super Contributor
timestamp: November 23, 2022, 09:31:13 pm
content: It is important- EMC failed at high or (relatively) low frequencies. As "worse emissions" is not defined, advice regarding possible solution is: "it depends".

author: Simon
author_rank: Unknown
timestamp: November 23, 2022, 09:33:00 pm
content: Quote from: wilhe_jo on November 23, 2022, 09:17:45 pmNo it does not work like that in EMC, emission frequencies are due to edge rates not just and less due to the actual frequency

author: Simon
author_rank: Unknown
timestamp: November 23, 2022, 09:35:28 pm
content: Quote from: ogden on November 23, 2022, 09:31:13 pmFrequencies are around 70-80MHz, which is where I have always seen problems with most stuff, it's always sub 100MHz and due to some edge rate, in this case the spikes you get with discontinuous conduction, put 50W (5%) load on and 20dB just falls off.

author: ogden
author_rank: Super Contributor
timestamp: November 23, 2022, 09:52:11 pm
content: If somebody experience problems like mentioned by Simon - filtering is solution, not dummy load. Use choke coils + ceramic caps.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: November 23, 2022, 09:55:57 pm
content: That's usually the diodes stopping conducting...But you're lucky. At these frequencies even budget ferrites should do quite well.Howevery at most instances I was hunting down the actual path for the emissions longer than it took me to get the right countermeasure...Do you have a current probe to measure the RF cm-currents?73

author: Simon
author_rank: Unknown
timestamp: November 23, 2022, 10:00:34 pm
content: We made a filter, we used a common mode choke that also has quite some differential inductance too, maybe I should put a straight differential choke in.

author: Simon
author_rank: Unknown
timestamp: November 23, 2022, 10:03:20 pm
content: Quote from: wilhe_jo on November 23, 2022, 09:55:57 pmI don't have a current probe. I still don't know if this is conducted or radiated, it feels like as fast as I filter it just gets picked up again. I'm going to look at adding to the shielding of the existing frame as it belches out of there, so it feels like filters won't help unless I firewall them.

author: ogden
author_rank: Super Contributor
timestamp: November 23, 2022, 10:13:46 pm
content: Quote from: Simon on November 23, 2022, 10:03:20 pmRight. Any length of wire is antenna converting radiated emissions into conducted. You either have perfect design which does not radiate or you shall have perfect enclosure with perfect filtering right on the exit.

author: nctnico
author_rank: Super Contributor
timestamp: November 24, 2022, 01:00:28 am
content: Quote from: Simon on November 23, 2022, 10:03:20 pmJust make a DIY current probe from a split bobbin cable clamp and you'll see what is going out of the wires of your device.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2022, 02:26:42 am
content: What frequencies? By how many dB? Do you know if it's more primary or secondary?Quote from: wilhe_jo on November 23, 2022, 09:55:57 pmRecovery doubtful has enough energy (thus slew rate) to do those kinds of frequencies... more likely turn-on, and what's missing is valley mode switching or something like that, the higher initial drain voltage being cause for harmonics exciting the transformer winding or something like that. Hm, I wonder if that would be flyback, or 2-switch forward? Or anything else; even LLC can have pretty nasty waveforms if the controller uses burst mode. Not that modifying the PSU is an option, sadly...Anyway, upside is it's high frequency so it doesn't take much reactance to filter. Small inductors and capacitors I mean. Downside is the layout needs to be good, and it probably doesn't matter primary or secondary, both probably needed.Ferrite beads are tempting but remember they do nothing under load (normal mode), or work CM only; and this may be DM at point of filtering, so a bead around the pair may not help.Because of, say, velocity differences, lumpy cabling, whatever, the measurement at the LISN need not be pure DM or CM, it may be a mixture. So, even if you try measuring from the LISN at those frequencies, the phase need not be representative. Measuring right at the cable gland out to be representative, but you'd have to mangle a cable to probe it.But you just need a little reactance, so something like a rod core or powder toroid is fine.As for the capacitors, something approximating a bulkhead / passthru capacitor is needed. The "bulkhead" can be transformed into a thicker, flatter shape, and the hole in it opened up to one side, placing the components on top or beside the opening; that is, a microstrip arrangement, a broken trace spanning a rectangular PCB, the connections (ports) being made on opposite edges, and making multiple GND points (to chassis) around its edge. The trace is broken where the inductor is placed, and capacitors are placed to ground, flanking the traces as needed. Such a transformation of course exposes the outside-facing traces and wires, and capacitor ESL, to internal (noise) fields, so some compromise is made in this process.Tim

author: srb1954
author_rank: Super Contributor
timestamp: November 24, 2022, 03:39:36 am
content: Quote from: Simon on November 23, 2022, 09:33:00 pmNo, for any periodic waveform the emissionfrequencieswill always be at harmonics of the fundamental frequency. This is basic Fourier transform theory.However, theamplitudeof any particular emission will be dependent on both the edge rates and the frequency and amplitude of the fundamental along with frequency response of the system from source to measuring equipment.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: November 24, 2022, 10:33:58 am
content: well that all just one part of the whole truth...In the MHz range, you can/will have some resonance which gets excited periodically.My last 10yrs of EMC consulting told me that currents through diodes can stop pretty abruptly (in german you call this "dioden abriss"... I can't find an approrite translation... it's just the rapid change from the conducting region to the non-conducting state of the diode... especially Si diodes tend to be problematic since they can still conduct when the voltage reverses due to some remaining charge).Having this said, you don't see the emission at multiples of the switching frequency - you see emissions happening that are multiples of the switching frequency appart from each other.Again, that's true for emissions >1...10MHz. Below this, my bet is on the direct multiples of the switching frequencyLooking on the datasheet, it's very, very unlikely that this is a flyback... I mean, 1kW is a "little" high, even though it seems to me that there are 2 transformers visible on the product photo.Most meanwell supplies in this power-region I've seen so far are some kind of push-pull style (so some sort of forward converter).So you'd have some bridge rectifier on the output of the transformers. Thus, I'd say they would have 110/220kHz (my guess is that they use only 1 transformer in some kind of burst mode in a light-load condition...so most likely you'll see 110kHz) spacing on the emissions if they come from the DC/DC and 70kHz if they relate to some bad behavior on the booster of the PFC.Now the fun begins: Depending on the root cause, the problem could be fixed as easy as getting a big y-cap on the output-side (the y-caps on the input are usually fine, already).Just adding CM-chokes can actually even increase the problem. Especially if the CM-choke is not directly at the input/output of the supply.If the emissions come from energy in some stray inductance, the currents will flow anyway.Hence, increasing impedance can/will increase the voltage spikes.Depending on the other possible paths, you now may find the emissions leaking from some other cable...So it's _usually_ (remember, EMC is kind of black-magic) safer (in terms of getting some solution quickly) to have y-caps close to the input/outputs AND adding CM-Chokes towards the rest to present a defined and smaller (and thus less radiating) path to the energy.Quote from: nctnico on November 24, 2022, 01:00:28 amI prefer a common closed core with big inner diameter... the less expensive split-core ones have usually a pretty small ID.7-8 windings of coax on the core gives you a pretty decent current probe good up to maybe 200MHz.Just cut the outer shield of the coax at about the mid.Then connect both halfs + one of the ends of the coax to GND of a coax connector and the other on to the center pin.That way you get a nice E-field shield, and you don't hunt some ghost-signals.73

author: Simon
author_rank: Unknown
timestamp: November 24, 2022, 01:10:03 pm
content: Quote from: srb1954 on November 24, 2022, 03:39:36 amThe switching frequency is no more than 200kHz, at 70MHz that is the 350th harmonic, but the frequencies due to the edge rates will start out much higher than the switching frequency.

author: Simon
author_rank: Unknown
timestamp: November 24, 2022, 01:19:16 pm
content: The main issue is that the thing radiates like hell. I found the problem, my (junior) colleague had an earth connection on the input and output of the filter, the filter stands to attention right in front of the opening in the PSU shielding and and earth path was created across the the filter PCB so that was that. Having spent the morning doing some cable management we have now got back to where we expected to be.I am yet to mount the filter PCB on a bracket that hopefully will act as a good firewall to showing the output cabling the full force of the radiated emisions from the PSU. There is a reason they say that it passes EMC testing for the purposes of being used inside equipment only. The AC side is well filtered, the DC side is drier that a desert for filtering circuitry.There is a folded metal can over a transformer in the middle of the PSU with a thick non conductive pad stopping it from coming into contact with the cover plate, my guess is that they had some fun getting this through EMC testing. Also there are various capacitors mounted on vertical daughter boards, I don't know if this was to add more capacitors in place of fewer ones or if it was to make better use of space like laying down tall electrolytic capacitors.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: November 24, 2022, 01:40:50 pm
content: Forget (mostly) about rise/fall times!They "just" define where you see a drop in the spectrum.The first article explaining this I found is this:https://incompliancemag.com/article/spectra-of-digital-clock-signals/Oscillations >1...10MHz come mostly from parasitic capacitances and inductances of your non-perfect components, which get triggered by some switching event.Yes, smoother edges help, but that's not where you should start.Anyway, changing circuits is nothing you would do, right?The problem you see is pretty common.They got through EMC testing, because there they put everything on a table and used some power resistor on some heatsink.There, the path from output to input via ground had basically an infinite resistance.Now you are presenting the RF currents a viable path... Hence, it starts radiating...Y-Caps close to the converter can minimize the current loop, and hence the radiating structure...However, that's not always possible and does not solve the problem in all cases.Without knowledge about the internals, there's always a little trail-and-error involved.BTW: a commercial clamp-on current-probe by Tekbox is reasonably priced (I have no affiliation with them! It's just that their webshop shows way lower prices as comparable stuff on eBay).A DIY thing costs around 10-15.- EUR...Both would help you a lot with this particular problem because you can easily see/measure the differences.Even a simple oscilloscope would be fine for this kind of debugging.73

author: Simon
author_rank: Unknown
timestamp: November 24, 2022, 03:58:26 pm
content: Well we seem to have it solved, the filter has capacitors to earth as well. Just waiting on the proper metal work mods and we can reassemble it and do a final test. I chose a common mode choke that has it's self resonance frequency well beyond the noise frequency so that providing the noise does not turn into something else of a much higher frequency we should not see any weird stuff happen. We already noticed that when we filtered just the machine at the other end we just got noise at a higher frequency instead but more spread out, but still beyond the fail point.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2022, 05:21:52 pm
content: Harmonics aren't really meaningful anyway, as the generation mechanism may be different, and the harmonics need not be coherent up there.By that I mean, sure, ultimately it might be driven by the edge rate ofsomething, but that something can also exhibit stray oscillation, or it pings a local loop that therefore emits a tone burst at some high frequency. It doesn't matter how often it gets pinged, just that it does at all, and then you get a forest of peaks which outline the frequency response of that loop (along with everything else between it and the detector). Which really are outlining the continuous spectrum of that system (the source plus signal path).As for coherency, I don't think any SMPS controllers -- maybe outside of some handmade demo that uses a crystal controlled MCU -- exhibit low enough phase noise to be able to pick out harmonics that far up there (100s) in the first place. That is, it might be a forest of peaks, within any given  time window, but those peaks vary from window to window, and over an extended period of time (wider windows, multiple acquisitions, etc.) you'll see them all fluctuate. A statistical model is more effective, in which case you can average over that ever-swaying forest and approximate the underlying continuum.Let alone SMPS that intentionally modulate frequency, whether as a cheat (spread spectrum), or for control (fixed on time, BCM, LLC, etc.).This is a point that I've long been concerned about; people hold up the Fourier transform as some godly miracle, but it's not, it's just another tool in our toolbox. It's a rather simple transform, as things go. Its limitations are rapidly evident with more complex signals; if the above isn't enough of an example, then consider the spectrum of a musical instrument for example. Real, rich instruments are rarely harmonic. Flutes contain turbulent sounds, as well as vibrato or tremelo; Fourier analysis is fundamentally inapplicable to attack and decay phases -- how exactly would you express the spectrum of blowing or puffing into a flute, the sound of air hissing past before it's started oscillating, or the shift in pitch as the tube fills with humid breath? And that's not even to complain about the simpler (periodic, but anharmonic) effects, like the overtones of an acoustic resonator.Or for something like a plucked string (guitar, harpsichord, etc.), we can model the system as a "plucked" transmission line for which we might expect a series of harmonics which decay over time (higher frequencies dropping faster), but how do you express "decay" in a transform that is fundamentally time-independent? We can interpolate between spectra and inverse FT as we go, sure, but that's a sloppy hack. We might be better off with something more mixed and expressive -- like wavelets.So, when it comes to spectra like this, it may not be all that useful to think in terms of edges and such. The peaks you see, might not show up in obvious form anywhere in the circuit; you could have a case like, there's a 400V edge here, but only the 5V squiggles at the top and bottom (that you'd almost never realize from looking at the scope) are actually your problem. Or they only show up under limited circumstances, and the EMC test averages over that because it's a long duration, peak/average/QP detector, and the antenna sweeps position picking up more response from the wiring than you might otherwise realize based on a static test.Tim

author: Simon
author_rank: Unknown
timestamp: November 25, 2022, 10:54:36 am
content: They do appear to be doing spread spectrum as the frequencies shift all the time, but at light load it is not enough if you want a 5m antenna on the PSU.