EMC testing of an LED light  - Page 1
Forum section: electronics / projects

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 11, 2017, 01:14:11 pm
content: HiI have helped a little on an LED light project. The client basically bought an off the shelf 12V - 19V 500mA mains driver and connected it via 30cm, 2 core unshielded cable to an LED board which his housed in a metal case. The LED board is circular with 5 strings of 5 LEDs on it. The measured output to the led board is 14V@500mA. There is a 100Hz ripple on the led driver output but gives no noticeable difference in visual light output.The client took the unit for EMC pre-approvals. The whole fixture was tested for emitted radiation and from 50MHz - 200MHz it failed by about 10dBuV at various peaks. They added a ferrite on the 2 core cable and this made a maybe 3dBuV difference. Test house seems to think its the LED board and not the driver that is the problem. For me the driver is the active component in this product. The LEDs are being driven from a long cable from a driver.I know this does not give people a lot to go on but I was wondering if this rings any EMC bells with anyone? Anyone have any thoughts why the LED light would be the thing that is failing? Is it much more likely to be the driver generating the noise with the cable? I wondered if maybe there was some RC filter I could put on the LED inputs.I actually tried to use my near field probes and SA on the LED board and I could not find any noise peaks. The only bit of noise I did find was conducted on the cable.Looking for some ideas to try and ways that I could go about detecting this noise in the workshop before going back to the pre-approvals.Trev

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 11, 2017, 06:08:26 pm
content: Pictures please

author: ocset
author_rank: Super Contributor
timestamp: October 11, 2017, 07:31:42 pm
content: I think you need a common mode choke on the PCB, just before the cable leads off to the led board.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 11, 2017, 08:14:43 pm
content: I think a common mode choke might do it but I would prefer to be able to put a shunt on the input of the led board or on the output of the driver. I dont think there is much money for a common mode choke or the room.Have attached the main sweep of interest.I have the light in the workshop now and am looking at it with my own SA. I can look on the actual wires and I have near field probes. I think I need to find a setup that shows me the noise and then I can try and work out how to reduce it. I am new to using an SA so this is a learning curve.Trev

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2017, 08:31:51 pm
content: What is the driver?Does it have approvals, and did you ask them how they were measured?It's quite common for power supply modules to be tested with negligible lead length and no grounding, so there is no path for common mode conduction or radiation. Always reserve space for a CMC.Where was the ferrite bead placed?It is very unusual that even a stack of ferrite beads does much over 10dB. This is easily understood by their properties, and the equivalent circuit of the common mode noise path.Tim

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 11, 2017, 08:36:59 pm
content: Hi. Driver is fully approved but the conditions under which it passed I do not know. I do not have the driver at hand.What's the likely chance an rc would make a difference?Sent from my SM-G950F using Tapatalk

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 13, 2017, 02:48:57 pm
content: Hi, had a further update on the LED driver with regard to emission testing. They manufacturer has stated the following regarding the tests they did on the driver."Measurement of Radiated electromagnetic disturbances (Radiated Emissions) in band 30-300MHz - normally you have to have chamber with special antenna in 10m distance from DUT, but there is alternative method called "CDN method" and this measurement we provide to them. "Anyone got any links to how CDN method is implemented and what is required. It sounds interesting if that can be used instead of a test chamber.Trev

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2017, 05:09:07 pm
content: What is it, EN 55022 or something? You'd have to look it up there...CDN is Coupling Decoupling Network. In other words, a LISN, but, it would seem, intended for a high frequency range.Tim

author: chris_leyson
author_rank: Super Contributor
timestamp: October 13, 2017, 05:31:13 pm
content: A bit off topic, but I would love to find out some of the theory behind CDN measurement and how radiated emissions correlate to conducted emissions, EN55015 was for lighting and similar eqipment but it looks like it's been withdrawn and replaced by another standard.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 13, 2017, 05:41:19 pm
content: Quote from: chris_leyson on October 13, 2017, 05:31:13 pmYeah I would like to understand too. What has EN55015 been replaced with?Sent from my SM-G950F using Tapatalk

author: chris_leyson
author_rank: Super Contributor
timestamp: October 13, 2017, 05:58:22 pm
content: Sorry EN55015:2006 was replaced by EN55015:2013 which is due to be replaced by EN55015:2017 so the standard is still in force.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 13, 2017, 06:00:01 pm
content: Quote from: chris_leyson on October 13, 2017, 05:58:22 pmOkay thanks. It seems this company passed their product using CDN method of tests to EN55015. I wonder if the approvals company are performing the right tests?Sent from my SM-G950F using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2017, 06:19:02 pm
content: Off the cuff --I'm not privy to any background, the origins of these standards; or the details of the standards themselves, for that matter. But the intent seems clear, given an understanding of impedances and fields, and the specifications for the tests and equipment.The general idea is this:The AC line, or connected cabling or whatever, will have a wildly varying impedance with frequency. It will be resonant at some frequencies, antiresonant at others, and a good dissipator at still others. In circuit terms, that means the impedance is very high, very low, or near nominal (Zo).While the line may be radiating at all of those frequencies, the frequencies where it radiates most strongly will be when it is matched to the source: when the impedance is near Zo.For a simplified example, consider a dipole antenna in a box.When the box is conductive (shielding), it's not an antenna at all, but a resonant stub transmission line (stripline, coax, whatever you like). The impedance at resonance will be extremely low (series resonant). If connected to a, say, 50 ohm source, it will reflect a lot of power (very high VSWR). Which is, of course, what we should expect: inside this conductive shield, there is little place for power to go, so it gets reflected back to the source, instead of dissipated.Suppose we remove the box, but drive the antenna at twice its natural resonant frequency. It is antiresonant here. Radiation is prohibited because the waves interfere in all directions; the energy is reflected back to the source. The impedance will be extremely high (parallel resonant). Again, power is reflected back to the source, if it is again only a nominal impedance.If we drive the antenna at its natural resonant frequency, however (again, without a shield), it is resonant, and couples efficiently to free space. Rather than having considerable standing waves on the antenna, the radiation resistance dampens it, and waves flow back and forth, between free space and the feedpoint. The impedance is nominal (say 73 ohms), and VSWR to a 50 ohm source is small.If we apply the same thought process to a random mains wiring network, or communications cables or what have you, then it will be apparent that some accidental antenna structures will exist, and peaks and valleys will appear throughout the spectrum, where reflection is strong, and where absorption is strong between them.Now, a word about mains wiring: it is usually PVC insulated, which is pretty lossy at radio frequencies. It may well happen that differential mode conducted emissions are absorbed efficiently within the mains network, without being radiated.Note that conducted emissions can become radiated emissions, when sufficient cable length is provided -- like a houseful of mains wiring. It would be impractical for a testing lab to setup and measure the radiation, so conducted is used instead. (However: industrial equipment, subject to open field testing, is done on a large enough scale -- enough distance between EUT and receiver -- that this can be relevant.)So, whatever power is left, that is not absorbed by the lossy wiring, or other loads, is radiated into space. And most of that radiation will fall on the sweet spots where the mains impedance is nominal.But what about mismatched loads? A switching supply, with 1uF diff mode filter cap, sure doesn't have a 50 ohm output impedance! This is a very valid concern, and can contradict the LISN measurement method. The effect is this: in an impedance valley, where the mains wiring happens to be series resonant, one can force power into that, given a low enough source impedance. The radiation resistance is small, but when driven hard, it radiates just as well as anything. (The location that's actually doing the radiation, might not be the circuit elements that are resonant. For example, it might be a wiring stub attached to a voltage peak. Compare the dipole-in-a-box condition, to a dipole-in-a-box where one element just barely pokes through a hole in the box: the voltage on the dipole's tip is very high, so the electric field around that tip radiates some power, even if it's fairly inefficient, as the tip sticking out of the box looks like an electrically short monopole.)So, to my mind, that is where the LISN / CDN method comes from, and where it can fall over.Regarding an ISN or CDN for telecom pairs (typically applied to Ethernet and the like), if the wiring is routed through walls rather than metallic raceways, then there is ample chance for it to radiate, and it will have an impedance that's reasonably representative. Such networks are usually point-to-point, so you don't have to worry about the complexity of a mains wiring network, and the source and load can be reasonably controlled (Ethernet has a common mode choke facing the cable on each end, helping to decouple any radiation resistance from any unbalanced signals).Regarding CDN vs. radiated testing: the same applies at any frequency where the EUT is electrically small. And ideally, shielded as well. Then it can be tested as a two-port against ground, and all potentially radiating signals can be captured by the CDN.Downside: is the test done per wire (in which case, a single-output power supply would be a 4-port), or per pair (in which case, is it diff or CM or both)? (Of course, if both diff and CM are done, you've collected data equivalent to the 4-port test, give or take phase information.) Was the test done with a typical use-case, or as naked as possible? Examples: additional bypass capacitors, excessive lead lengths*, load or not -- and was it an LED, or resistor, or SMU programmed for something or other?*Note that power cords are looped upon themselves to take up extra length. This could have an LPF effect. But, it's in most standards, so... whatever?One hopes that the manufacturer specifies the setup in a reasonably representative way, but who knows until you find their actual report.Tim

author: chris_leyson
author_rank: Super Contributor
timestamp: October 13, 2017, 06:50:11 pm
content: Thanks Tim I think you summed things up quite nicely, I gave it a bit thought and yes it's the cable that radiates and not the light. Would be interesting to see the internal construction of a 30-300MHz CDN. This made interesting readinghttp://www.emcia.org/documents/DM4.pdfthe LLA method, never seen that used before.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 13, 2017, 06:50:12 pm
content: Wow, fantastic reply. Thanks for sharing the knowledge. This driver is not shielded. It's in a plastic case and I sense noise through the case using my near field h probe.How do we test for radiated noise output from the driver to the led pcb? The line is 30cm long and at the end is an led pcb with just leds. I would need to test if the led board generates noise or the driver.Sent from my SM-G950F using Tapatalk

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 13, 2017, 08:07:24 pm
content: Quote from: chris_leyson on October 13, 2017, 06:50:11 pmChris, thanks for this. It seems the lighting industry has an exception in the way of being able to use a CDN. That seems to be what the pdf you attached indicates. I think this might bring everything together and explain how they managed to pass the driver. I also wonder what the CDN results would be for the LED assembly as a whole. I will have to contact the pre approvals company on Monday.Thanks, this has really helped.Trev

author: chris_leyson
author_rank: Super Contributor
timestamp: October 13, 2017, 10:52:18 pm
content: Thanks Trev and Tim, CDN testing makes a lot more sense now and it would be interesting to see the test results from the pre approvals company.