LED driver conducted EMI affecting things I didn't expect. - Page 1
Forum section: electronics / general

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2022, 01:47:52 am
content: I recently built a new workbench and I put some LED spotlights under the shelf. I used some 3-watt (supposedly) 2" spotlights that each have their own 120V to ~3V constant current driver. Everything worked nice, except I found that my scope traces were hairy and the lights were obviously putting out some noise. I should have thought of that, but they are inside a grounded steel box and I didn't think that they would be that bad. I'll have to tear it apart again and wire them all in series and install a noise-free constant current power supply. So I'm not looking to mediate the noise or fix/redesign the drivers, at least not for the purpose of making my workbench work. But in the meantime, I noticed something strange, at least to me, and that was that the EMI was severely affecting two instruments I wouldn't have expected--an HP 403B AC voltmeter and a Fluke 731B DC voltage reference.Both of these instruments have NiCad batteries and will run for a while without line power. I determined that the issue would go away if the power cord was disconnected, so presumably this is a conducted EMI issue--the steel box part isn't going to help. The 403B would read 55 volts in the 300 volt range from the interference, even with the inputs shorted. The 731B would drop 70ppm (a huge error) when I turned the lights on, and again, the issue went away with the power cord disconnected. So whatever is happening is going in the line cord, in through a transformer and a linear power supply that uses batteries and apparently affecting a DC circuit somehow. Both instruments have the characteristic in common that they don't function properly without good batteries. Neither of them is sensitive to the battery voltage and will remain accurate over a 30% or more change in battery voltage.I don't have an LISN and EMI isn't something I'm any sort of expert at, but I decided to try and take some readings. I simply used an isolation transformer to power a single LED driver and then connected the scope across the power leads using two .0068uF capacitors and a 600 ohm resistor in series, with a 10X scope probe across the resistor. Have a look and let me know what you think--I'm not looking for any specific answer, but I'd like to know if this level of EMI is particularly bad, what might be the reason it interferes with these instruments and how bad the LED driver circuit is. It uses a chip marked 'HA3021' which is a specific constant-current LED driver designed for this type of step-down setup, but I cannot find a datasheet on it. It obviously uses an input bridge, a 7uF capacitor and then this chip plus a transformer and diode, but no output filter. Putting a .01uF capacitor across the output made no difference.The last screenshot is the scope directly across the LED when powered up. The current measures 0.5 amps on a DMM.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 16, 2022, 02:06:22 pm
content: Huh, DCM. Makes sense I guess, might have to be an exorbitantly large inductor otherwise.This is evident from the line ripple zoom, which shows a tall thin spike, a short duration pulse (ramp up then fast valley down), relative quiet, then a wiggle before restarting. That wiggle will be free ringdown of the inductor. So the relatively flat section before the wiggle is the inductor, diode and LED conducting in a loop (flyback period), and the little pulse bit is the on-time, which needs to be brief of course for the large stepdown ratio this thing has, and that means the tall spike is hard-switching turn-on, which makes sense as the switch node voltage is basically nil once it's settled and it has to chunk in a hard 160V or so at turn-on. Which is evidently fairly rapid, and not well damped (the switch's risetime is faster than the switching loop inductance and node capacitance ringing frequency, thus exciting it as a resonant circuit).The ringing on the spike can be dampened by putting some R+C across the diode. The spike peak itself likely cannot, and must be filtered elsewhere. Possibly grounding can be improved to reduce impedances (inductances), making the circuit more suitable for the fast risetime, but this is hard to do without designing the board itself. Perhaps a bypass cap (>= 10nF ceramic or film?) can be placed between +V/GND near the chip, to this end.Oh, uh, hm... SS26 is a low voltage schottky. So unless there's something hidden under the inductor or on the bottom, it's not a buck type supply as I'd have assumed. Aux power? Oh, or maybe it's actually a transformer, so the reg is more like TOPSwitch (flyback) than a direct chopper thingy. Oh... fuck, well if it's isolated then there's your noise problem. Even if it's common ground, the coupling through the transformer (incompletely sunk by thin ground-return traces on the board?) can leave quite a bit. Which, I mean you've got several volts of splat on the output there, that could well be it.And that splatting is quite high frequency so you'll see it in most points of the circuit, reaction mass (so to speak) being the length of the LED wire, or mains on the other side (most isolation transformers are nothing but dead weight capacitance at these frequencies).Hmm, that's not actually all that fast, is it? 1us/div zoom on #25? Well, the divs are, really hard to see on this display, but looks wide, so that ringing is still some 140ns or so? Some MHz in any case. There's some even faster crunch close in, which will radiate notably from a meter of wire; the 7MHz not so much.Are there no capacitors on the bottom side? Under the transformer?So, if it's isolated, toss in a 1nF Y1 cap between primary and secondary ground/negative, that should help. Capacitance on the output I think would not hurt, you just didn't use enough: more like 10uF for rectifier output filtering. That'll make the LEDs run more efficient too (ripple current is just wasted heat into LED ESR).Note that your measurement method is ambiguous between CM and DM noise; the faster spikes are likely more CM, but you'll read some apparent DM due to probe connection.I'm not seeing how this is regulated, so I'm not sure that adding a lot of capacitance won't make it unstable or something. But that's easy enough to check later. And I mean, LEDs have low impedance, it should be fine, if it's indeed constant current like it says.So if the 7MHz is the SS26, and it's around 50-100pF at LED voltages, that suggests LL = 5uH (quite high??), and then the RC snubber should be 220R + 220pF or thereabouts.Ah, so to wrap up the #23 waveform, the wiggle is relatively strong because it's the free ringdown of a flyback, so, on the order of 100V peak and decaying (at the primary side). This can be damped with some R+C as well (across the primary), probably like 1k + 47p or whatever; I need a bit more information to calculate it exactly (namely primary inductance and/or node capacitance).And, lastly about the waveforms, the envelope changing throughout the mains cycle, I'd have to see what's happening between phases. Looks like it's a bit better behaved around peaks, and worse on the flanks? Not sure. Is it set for fixed output current, or is it slowed so the average output is constant, but cycle-to-cycle it's proportional to input (giving somewhat better power factor I suppose)? Nah, it can't be power factor, with that electrolytic there... Oh, is it just because, when the FWB is conducting, that provides some shunt impedance (radiating up the line cord!), and when not, it's free? Wait, but you're probing on L/N right, not DC +/-? Dunno. Maybe something about phasing, where the peaks are effectively sharpened by the rectifier's capacitance, Idunno.(Note that the rectifier acts as a PIN diode, so -- generally speaking, at least -- EMI is only conducted up the line cord while it's forward-biased, during line peaks. Which is also to say, the noise is amplitude-modulated at 120Hz, hence why anywhere on the dial you find this noise, it's always a terrible BUZZ. Every harmonic has close-in sidebands at 120Hz and harmonics.)And then, not much else to do beyond the usual -- a couple 100 uH in series with the mains input, and some X1/X2 caps shunting before/after it (pi filter), will help reduce DM noise. CM noise is between line and LED cables; consider some kind of pi filter there as well (in the common mode, that is -- most likely a CMC on mains, with 'Y' caps to GND or output, either side of the choke). It's wide open throttle, basically as much noise as you can get, short of intentionally hooking the switch node to the line itself.As for a less intensive solution, simply buying something that doesn't lie about its approvals, and has effective isolation and all that, is the way to go. I'd recommend refunding those, if you can.Tim

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2022, 10:58:51 pm
content: Quote from: T3sl4co1l on February 16, 2022, 02:06:22 pmThanks for the detailed reply! I'm not going to send them back for a refund because the lights themselves work for what I want and I've already drilled a whole lot of large holes in the bottom of my new riser shelf to accommodate them. My solution will be a low-noise constant current source for all of them based on loose junk I have laying around. However, this has piqued my interest for a few reasons--this would be an opportunity to learn something detailed about EMI output issues and also I'm a bit alarmed by the effect it had on devices that I didn't expect would react that way. In theory I understand how HF EMI can affect a DC circuit--rectification, just like an AM detector--but not knowing how, where and why this happens on a microneolithic device like the 731B makes me worry that some less outrageous EMI issue could cause a smaller error, one that wouldn't immediately yell 'broken!'.QuoteSorry, I omitted a photo of the other side. There is a small capacitor on the hidden side of the transformer, right near the LED output connections. I assume this circuit is similar to a flyback design?QuoteOK, so if I look at #24, the primary turns on at the trigger point, then off about 3us later, the secondary conducts for ~10us and then when that cuts off the transformer rings until the next primary turn on.QuoteI took most of the captures with the 20MHz BW limiter on, but at the end I realized that was a mistake--there's plenty of energy above that. And it seems to travel right through almost everything in its path, including the in-guard isolation of at least one old DMM.QuoteYes, the measurement was quick and primitive, now I'll actually have to think about a homemade LISN and the LISN-mate device from another thread to separate out CM and DM. Or, if you have a suggestion how to better do it quick and cheap... When I get a bit of time--after I've scraped out the 9 LED drivers that I glued in place and replaced them with a single CC driver--I'll disassemble one and draw out it's circuit. I can then try your various suggestions to mitigate the noise. I'm assuming the spikes are the EMI component that is actually causing the issues I see. I'd also be interested to see what can be done to effectively filter those types of EMI out other than modification of the source.

author: TimFox
author_rank: Super Contributor
timestamp: February 16, 2022, 11:05:22 pm
content: I had a similar problem with a shop-bench LED light and magnifier (articulated arm) that I clamped to the right front corner of my work/test bench.Unfortunately, that corner is adjacent to the left edge of a shelf that holds my -hp- 339A audio analyzer and Marconi 2382 spectrum analyzer.Both of them see much higher noise when that LED light is switched on.Luckily, I have enough (halogen) incandescent bulbs to light that part of the basement when I need to make measurements.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2022, 02:22:08 am
content: Quote from: bdunham7 on February 16, 2022, 10:58:51 pmIt's certainly a textbook example!QuoteAh, then that explains that. Yes, it sounds like a flyback, so, primary winding, some ratio, secondary, and the duty cycle is more modest than a direct buck would. Should be able to identify similar circuits from western manufacturers; although, at a glance, I don't see much quite like that from Power Integrations, they've advanced a bit I think (e.g. regulators with integrated isolated feedback, or secondary side synchronous rectification -- sweet!).QuoteYes, exactly.QuoteAha; yes, it'll tend not to "stay in wires" at those frequencies. Much to the chagrin of your other equipment.As to how they're susceptible, dunno; at worst, it could be mere gaps in the housing, but more likely it's picked up on wires/cables hanging off it. I'm not sure exactly how "wireless" you had those things; battery power at least suggests no conducted mains EMI, but an attached cord could still be an antenna, and probes/cables can be as well.You'd hope that they're more immune to such things regardless, but eh, it depends. A lot of classic BJT-based hardware is susceptible by RF rectification at junctions; the inputs could be better filtered (or indeed, the mains or any other means of RF ingress), but maybe it wasn't important at the time, or may not even be feasible (when the input bandwidth and impedance preclude meaningful RFI filtering). That includes BJT-based ICs, of course. Which nowadays, often come with RFI filtering on chip (e.g. OPA192), but also, you can't do that so well when you need 100s of MHz of GBW.QuoteDon't worry too much about a precise well-specified LISN -- any kind of impedance stabilization will help.First, get everything on a ground plane so you have something to work against.Second, get your isolation transformer down there: maybe some bypass caps (a couple uF from each line to GND, maybe with some resistance to keep it from ringing with the transformer's capacitance -- 4.7 ohms is typical), maybe a line filter (to improve immunity from mains noise), etc.Third, the impedance stabilization itself: you want an R || L characteristic, so the load sees fixed R at HF, and L at LF (in particular, the impedance at mains frequency will still be quite low). The R could be straight in parallel with the L, but that wouldn't be very helpful; instead, we wire it to GND, and avoid burning mains voltage by using a series coupling cap. Furthermore, we can use a BNC instead, and hook up whatever radio receiver we like (scope, spec an..) as the resistance. Or a terminator if unused.So, if nothing else, you could assume the iso trans is some (well enough behaved) source impedance. In general it's going to have some resonances in the 100s kHz to MHz, some weird combinations of transmission line effects due to the layers of windings -- and that's probably not going to make a good LISN, so you'd want to test it to be sure. But as a hand-wave, I mean, that's basically what you've done already, and as you can see, it's hardly a game changer, you can still see the thing is blasting out the noise!And so we get typical values like the 50uH for a FCC Part 15 or CISPR 11 sort of network, maybe some bypass cap on the mains side, and a ~0.2uF coupling cap to the 50 ohm receiver.Then the EUT can be elevated modestly above the ground plane. In a test lab, it might be on an insulated table, and you're just measuring any and all noise in the chamber itself; for a more compact environment, you might prefer keeping that noise near the ground plane (so, a few inches elevation say), where it's less prone to picking up ambient radiation / losing energy to the ambient. Note this gives impedances in the 150 ohm range for cables elevated over the ground plane, handy for calculating impedances/transformations -- if you're, y'know, into that sort of thing.And then the whole thing is floating, so you won't need any other connections, no additional LISN or CDN or whatever. You can use one on the LED side if you like, to see what's going between LED and mains cables, (somewhat) independent of frequency (since the capacitance of the EUT over ground will be the only CM "reaction mass", and thus give an LF cutoff). Should also be okay to ground one side of the LED, to the same end but with slightly different gain, of course inviting some possible reflections at much higher frequency.Anyway, dealing with the noise at the source is of course ideal, but if you can't do that for , then consider putting the module in a metal box and using pass-thru filters on all connections. Maybe one can be hard grounded (LED neg?) and the rest get whatever appropriate filtering. So, other LED terminal, probably a small inductor and relatively large cap (0.1uF or more?). Mains, a CMC, with X1 caps between lines, on both sides, and Y1 cap(s) from one or both lines to the enclosure, will do.I tend to leave off pairs of Y1 caps, but eh, it depends. I mean you often see a Y cap from each line to GND. It's not very important, most of the time, because the X cap has a low impedance: the two lines already act together at most frequencies, so the second Y cap is basically acting in parallel. Maybe it helps to improve balance (so, CMRR), maybe it's not needed.For better sake of discussion, consider:"AC line" is the LISN equivalent at pass frequencies (so, 100s kHz to 10s MHz).By paired Y caps, I mean you might have one from neutral to GND like C4, and one from hot to GND as well. C1 is much larger than C4 or the other one (absent here), so L/N tend to act in parallel. But you can see they act as a balanced divider with respect to DM noise, so that can be a reason to use two.C3 represents isolation (inter-winding) capacitance, in a typical DC supply. That might not apply here (i.e. if secondary is grounded to primary), or maybe it does. We most often place C7, to shunt this at the source, i.e. between pri/sec common nodes, greatly reducing the amount of filtering needed around it.The matter of C5 vs. C7 vs. C6 (note the optional GND by C6; in this case neither the secondary side GND nor C6 would be present, as your LEDs are completely floating*) is often down to experimentation; it depends on exact circuit topology, and like, winding details, so, is hard to anticipate or model, and easier to just provide the placement options and try them when EMI testing.*Actually let me modify that a bit; we can model the CM impedance (the capacitance of the LEDs and wiring to the surrounding space) as some impedance like C6, though it's going to be a more complicated equivalent circuit once you include all the wiring and stuff, and radiation resistance. So, maybe it's only like 100pF, not the ~nF of an intentional filter cap. And that's where the "reaction mass" comes from that allows noise current to be backfed to the mains side.And note that the CMC has a lot of leakage (typically), so the DM filtering between it and the X caps can be quite reasonable. So that often takes care of CM and DM at the same time.What you can't do, is to put injusta mains filter: if it has Y caps to GND on the load side, well those just bypass all your noise straight to mains/ground and your LEDs are still noisy as hell. So watch out for that. The kind with a CMC facing load should be okay/helpful, but the impedance of that CMC will still provide some reaction against which to radiate via the LED wiring.If you can get some CM impedance to GND on the LED side, then the filter has something to work against, but watch out that the loop between filter, module and GND return path (back to filter GND), is short -- otherwise it'll act as some degree of antenna itself, too. All of which are avoided by cramming the thing in a box... hence that option.And, it doesn't need to be aboxbox, you can open a hole in one side for example, if there's nothing immediately around the hole; even open it up so far that there's little more than a ground plane under the module, with some filters on the connections before the wiring connects. The more open it gets, the more chance there is for induction/radiation, and the more important it is that the circuit sits close to that plane, and the less safe our assumptions are about the box-like-iness shielding situation. (Planes do work quite well, which is why so many things can get away with just being a PCB in a plastic enclosure.)The important part is eliminating AC voltage drop between wires (as in vs. any given wire (normal mode), or within pairs (diff mode), or between pairs (common mode)). Having all the wires group together and get filtered in one common location, minimizes that voltage drop between them due to any other stray induction or voltage drops or whatever. (It's often better to have input and output on a common side of a supply, so that they can be filtered against each other locally, without having to also filter against voltage drops along the length of the board.)Tim

author: bdunham7
author_rank: Super Contributor
timestamp: February 17, 2022, 05:01:20 am
content: Quote from: T3sl4co1l on February 17, 2022, 02:22:08 amIt appears to be line-conducted only, both units work fine on battery with the line cord pulled out of the back. They are also both pretty well shielded, with internal shielding boxes for the sensitive stuff. When they didn't work right on battery is when they were connected via the outputs to certain other line-powered units. It seems, for example, that the old Fluke 8505A is affected by or passes through the interference, but an 8846A seems to shrug it off completely.It will take a bit for me to digest the rest. I think I'm going to dissect this thing for further study, but I'm having some other weird problems, like a BNC cable that seems to generate a consistent 40uVDC somehow....

author: Zero999
author_rank: Super Contributor
timestamp: February 17, 2022, 08:42:18 am
content: Yes, the power supplies lack the necessary filtering.I hope you're treating the secondary side, with the same precautions as the mains. I doubt there's sufficient separation between the primary and secondary. All of the connections should be insulated and any metal casing earthed.

author: bdunham7
author_rank: Super Contributor
timestamp: February 17, 2022, 04:13:21 pm
content: Quote from: Zero999 on February 17, 2022, 08:42:18 amGood point. There isn't any grounding at all and while I haven't tested the leakage current, the whole thing looks none too robust. Certainly no space for creepage and clearance on the tiny board. I'm not in any danger with the current installation, but I can see where there might be issues.