Common Mode EMC problem in offline LED driver - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: February 24, 2018, 11:15:16 am
content: Hello,Please assist us in solving our Common mode noise problem?We have designed an Offline 240VAC LED driver (150W) which is linear regulator based. (86% efficient).It does however contain a 1W Buck converter for its bias supply.The schematic of the salient parts, and the filter components, is as attached in the pdf.For cost reasons, we are not allowing ourselves to use any through hole components on the PCB. We are also not allowed any custom made components (only offtheshelf allowed). We are also not allowed to take an earth wire connection to the PCB, as its more cost due to the greater assembly effort. (the Earth wire does, however, connect to the earthed heatsink on which the PCB sits, and is held onto by metal screws)For the following Conducted EMC scans, we changed the values of some of the filter components shown in the pdf…….changing the Inductor values to try and get it to pass conducted EMC….We started off withscan A. The filter values for this were as in the schematic.Scan Ais a fail around 150Khz.Therefore we changed the damping resistors (R2,R3) to 1k, as inscan B, and then it was still a fail at 150kHz, but less badly failing than A.Therefore we kept the 1k resistors for R2 and R3, and then increased L3 from 47uH to 100uH as inScan C. However, this was worse than scan B at the failure frequency of 150kHz. That is, increasing the filter inductance, made the conducted scan worse. This made us realise that our problem is predominantly a common mode problem. 150kHz is not a usual frequency for a common mode problem, but as you know, the 150kHz is simply the frequency of the” envelope” which comprises the much higher frequency common mode disturbances.Anyway…we then reduced L3 to 22uH and finally got it to pass (hoorah!) as inscan D.However, the readout for scan D strangely says it’s a fail at 150kHz because it recorded exactly 66dBuV and that is exactly what the limit line is at. –So the official readout said that we failed by zero dBuV (!!!).Anyway, in a desperation to get further below the limit line, we then kept L3=22uH, and kept R2,R3=1k, but increased L4 and L5 each from 100uH to 150uH. This is shown inscan E...However, this is worse than scan D. –It fails badly around 500kHz. Again it showed that increasing the filter inductance, made the results worse.However, decreasing L4 and L5 each from 100uH down to 47uH also made the result worse. So therefore we have currently stuck with R2 & R3=1k, L3=22uH, L4 & L5 = 100uH. (ie scan D) The rest is as in the schematic shown.However, as scan D shows, we are too close to the limit line, and need to reduce the noise more….Buck switching node:We believe that much of the problem common mode noise, is emitting from the switching node of the 1W Buck converter. (we know this from previously reducing the switching node copper size and taking EMC scans).Shielding the switching node from the earthed heatsink:For the above scans (A-F), we also shielded the Buck’s switching node copper from the Earthed Heatsink by laying copper pours in the internal and bottom PCB layers below the switching node copper. The shielding layers are connected to circuit ground. –We are now wondering if this is a mistake? In fact, we wonder if we should have used the 13V rail net for the shielding layers? The problem with using circuit ground as the shielding layers , is that there is a large circuit ground copper pour over most of the bottom layer of the PCB, which lies on the earthed heatsink. As such, we fear that having circuit ground as our shield for our switching node, is actually in some way injecting too much switching node noise into the earthed heatsink and exacerbating our common mode noise problem? Should we instead use the 13V rail net as our shielding layer? (to go between the switching node and the earthed heatsink).EMC capacitors coupling to Earth ground:As discussed, we don’t have an earth wire connection to the PCB. However, the PCB is held onto the earthed heatsink by metal screws. We are thereby wondering if we can thereby do the following so as to further decrease our common mode noise problem?......We could plate under the head of one of the screws with bare PCB copper. In this way, we would get some PCB copper which was directly connected to Earth ground. We could then couple the 13V rail net (ie a ‘quiet’ node) to this Earthed copper by using a 100pF capacitor. (4kV,2225,X7R)Do you think that this would help decrease our common mode noise problem?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2018, 12:16:46 pm
content: Oooh, what have we here, a new thread!Quote from: treez on February 24, 2018, 11:15:16 amQuoteQuoteThere are solutions for your problems... sadly, they either require top-down redesign, or custom parts, and more assembly effort, than the things you're supposedly unable to do as it is. Sorry.Tim

author: ocset
author_rank: Super Contributor
timestamp: February 24, 2018, 12:40:23 pm
content: Thanks, though our scan D (attached) is pretty much there. It's one gnat's hair away from being a pass.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 24, 2018, 02:23:27 pm
content: Hi,first off, although I'm not sure about your description, connecting earth to a heatsink which (as I understand, I may be wrong about it, but assumed it since you say that earth is "not connected to the PCB") is floating (not electrically connected to any part of your circuit) doesn't seem to be the greatest of ideas safety-wise. Just ignore this remark if the heatsink is actually connected to your circuit ground through whatever component it is attached to, although I wouldn't necessarily trust the reliability of such indirect connection.Since there is no transformer in your design, this is directly connected to mains and safety is a real concern, so I wouldn't take earthing lightly.I would also put some MOV and a fuse on the mains side, but maybe you already did and just gave us a simplified schematic.Safety considerations aside, and without any more knowledge of your device (PCB routing, integration...), I would suggest trying to replace your inductors (L4, L5) with a proper common-mode power choke. Those are basically two coils that are coupled through a (usually toroidal) core. Eg:https://katalog.we-online.de/en/pbs/WE-LF-SMD?sid=e48e5ce187The inductors coupling improves common mode noise rejection a lot.If this is still not enough, you could also try adding a mains filter between mains and your rectifier bridge. I suspect the direct connection of a diode bridge to mains could make the power lines pretty noisy.

author: ocset
author_rank: Super Contributor
timestamp: February 24, 2018, 02:32:27 pm
content: QuoteThanks, our 8mm height restriction means that all offtheshelf surface mount common mode chokes dont fit under our enclosure.Also, as you know, most common mode chokes are through-hole and we are not allowed through-hole components....However, thankyou very much, because you have hit the nail right on the head.......a proper Common Mode Choke is what i would really like to use here, and know it would solve all issues...but as discussed, we are not allowed to use a proper common mode choke with two coils coupled onto the same ferrite. (They dont fit under our enclosure)QuoteThankyou very much once again, because once again, you have hit the nail smack on......thats exactly what i would like to use...but our tight cost constraints mean that we are not allowed to use a mains filter with through hole components like X2 capacitors and Y capacitors, common mode chokes, etc etc.Everything must be SMD and <8mm in height.It looks like to me that we can possibly crack it by making the R2,R3 damping resistors 100R or so, and then make L4 and L5 equal to 150uH. But any thoughts on the top post much appreciated.Seriously, all the General staff at the place where i am working think that i am a total and utter Gonsch, because i still have't totally solved this EMC issue yet......they don't realise the nature of the constraints that we are working under. Also, we are not allowed any increase in enclosure height, and not allowed any increase in PCB size. I had to remove much of the circuitry (luckily some of it was a little 'superfluous', but thats not me being disrespectful to the guy who designed this LED lamp driver...he, in my view should get an O.B.E.......but we cant recomend it for him till we pass EMC regs) so that i could fit this SMD filter on, because the PCB was crammed packed.On Friday, my Boss asked me to design the equivalent of a Philips Xitanium LED driver, -but do it equal or more efficient than a Philips Xitanium driver...and at equal or less cost.......i told him i doubted i could do it like that, because Philips have their own semiconductor fab plant. He was not impressed. I'm pretty certain that they are going to just sack me out the door next week. First of all, they want a report on what should be done to slightly reduce the conducted emissions. Then i reckon i'll be walking.The attached shows what the conducted EMC scan looked like before i started working on this, some months ago. As you can see from the above, there has been a massive improvement, but we just need that little bit more.In truth, i doubt if the attached would do any harm to the mains supply system....even if every pub garden lamp in the country was like this, it would surely have no deleterious effect

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 24, 2018, 03:00:55 pm
content: I found this:https://www.lairdtech.com/products/cm3421y600r-10If this doesn't fit your specs, take a look at Laird products anyway, you may find something that fits. They have great stuff.

author: ocset
author_rank: Super Contributor
timestamp: February 24, 2018, 04:00:35 pm
content: Thanks, its the right profile, but a bit low on impedance.We previously put one of these in each of go and return just downstream of the mains diode bridge. -And to be honest, they didnt help with the conducted scans.http://www.farnell.com/datasheets/1910968.pdf?_ga=2.9114556.845751712.1519485949-287702491.1506211291

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 24, 2018, 04:21:21 pm
content: Quote from: treez on February 24, 2018, 04:00:35 pmI think that may still be worth a shot. You don't really have many options anyway.Quote from: treez on February 24, 2018, 04:00:35 pmThose are not common mode chokes but just single ferrites, so I'm not too surprised they didn't help much.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 24, 2018, 04:39:57 pm
content: As a last (but simple) resort, you may also try adding a snubber network right before the diode bridge, just as you added one (R1, C1) after the bridge.Be careful to select all those capacitors so they can withstand those voltages.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 24, 2018, 04:56:00 pm
content: Use the LEDs themselves as a voltage dropper for a linear regulator.

author: ocset
author_rank: Super Contributor
timestamp: February 24, 2018, 05:22:14 pm
content: QuoteThanks yes i appreciate they are not common mode chokes, but they do provide a similar impedance to common mode noise as the Laird common mode chokes in post #5 above. (when put in both the go and return as we did).

author: Niklas
author_rank: Frequent Contributor
timestamp: February 24, 2018, 08:25:46 pm
content: The is a fundamental difference between a common mode choke and two separate inductors. Hint: the magnetic core and what currents it interacts with. The common mode choke's core is interacting with both the power input and the return currents. Which current(s) will the magnetic core of L4 or L5 interact with and how is the function affected by the individual tolerances of these inductors? There is no guarantee that two inductors next to each other on the same reel will have identical tolerances, so there might be a mismatch of actual inductance values on L4 and L5.Talking about component tolerances and your previous statement about almost passing the conducted emissions test on test D. Have you checked the inductance of the parts that are assembled on the PCBA that you tested? If the tolerance on those tested parts are +20% and you still have 0 margin to a fail, how will a PCBA, where the inductors have -20% inductance, behave? You would like to have a safety/engineering margin between the limit lines and measured emissions.

author: ocset
author_rank: Super Contributor
timestamp: February 24, 2018, 11:59:31 pm
content: QuoteThanks, i couldnt agree more. We aren't allowed to have common mode chokes because those with a significant amount of common mode inductance are either thru-hole, or too high in profile for our use. silicon Wizard kindly sent details of a low profile one, but those have little common mode inductance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2018, 12:36:21 am
content: Ok, I'll bite:https://www.digikey.com/product-detail/en/pulse-electronics-power/PE-53914NL/553-1584-5-ND/2266668Meets rated voltage, but no certs, you'd have to ask Pulse for a (gasp) custom certification. Might still be easier than getting a new custom part.Again, easily avoided with a comprehensive design approach: you don't agree to build things that are impossible. Step 2 of feeling out a project is shopping for parts and seeing what kind of things are and aren't possible.Tim

author: ocset
author_rank: Super Contributor
timestamp: February 25, 2018, 09:16:02 am
content: Thanks, that Pulse Common Mode Choke is very good, probably the best fit i've seen in SMD. Still 0.9mm over the 8mm height constraint. -But one for us to bear in mind. I am actually thinking of the possibility of putting one of SiliconWizard's really small common mode chokes just in the go/return of the 1W Buck converter.Also, the questions regarding shielding and capacitors to earth ground as in the first post are still of interest...in trying to actually reduce the common mode noise problem like that.

author: CopperCone
author_rank: Super Contributor
timestamp: February 26, 2018, 05:40:45 pm
content: I wonder if using a higher order filter on the input of the dcdc converter would help.the datasheet says it runs at 66khz with a 4 khz spread spectrum. Also where is the 13v going? You can probobly filter the output of the dcdc converter to allow a shorter loop path and less effective radiator. The pi filter can be expanded.This means the dcdc converter might be making a mess at 132khz theoretically.Also using something with a higher switching frequency would allow you to utilize a clip on ferrite bead on the cable. 13khz is low for most ferrite beads. That could give you a few db of cm attenuation with the right bead.Also have you tried disconnecting various subcircuits tosee gow the emi profile changes? It is often bizzare because the resonances change (especially on complicated equipment) but it may provide some insight. Desoldering that resistor before the dcdc converter to disable the 13v rail may offer concrete insight.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 26, 2018, 07:19:28 pm
content: I am anything but an expert, but...The design seems to be a fairly straightforward rectifier, but without any reservoir caps to reduce the ripple of the rectified DC, instead it looks like you're using just a bunch of filters to try for the same effect, but for the current draw of the unit, could it be just that it's not sufficient capacitance to get rid of any nonlinearities introduced through the rectifier bits, and not necessarily extra introduced through the switcher? If you haven't already, I'd consider just testing like 2-10x the capacitance at C4 and C25. I figure they're being kept low because of the cost of the caps rated for these voltages, but if a bit more at either place could fix it, then maybe it's worth the expense.While it's not common mode filtering, could a cap directly across the live and neutral be a solution (though maybe not an affordable one/one that can be done through small SMD parts only)?Otherwise, maybe there's a common mode choke that's just a little too thick that you could make a board cutout and embed it in the board. If not or if the choke's just aren't a suitable shape, maybe adding a one with fairly low impedance at the frequencies you're looking at may actually still be enough to take the edge off and get you to nudge under the requirement, could be an option if other stuff doesn't work out, at least.