1W Buck converter with faster diode has worse conducted EMC profile. - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: October 07, 2017, 04:37:22 pm
content: Hello,We are using a 1W , LNK302 based High voltage buck converter in our offline 230VAC, 150W linear LED driver which uses linear mode led drivers (they successively switch in/out banks of leds to keep efficiency high). The 1W Buck supplies the bias power for the LED current controllers.So, to summarise, the only SMPS on the 150W PCB is the 1W Buck converter.From scoping, we know that the Buck converter is operating in Burst mode, and when it is switching, it is in continuous mode.We recently changed the Buck diode from a US1J (trr=75ns) , to a STTH1R06A (trr = 25ns).However, since this change, the conducted EMC scan has gotten worse as the attached shows.Do you think this worsening could have been caused by the faster Buck diode?The only other changes made were that we are now using a 220nF ceramic capacitor downstream of the mains rectifier bridge, whereas before we were using a 220nF X2 capacitor upstream of the mains rectifier bridge.Also, the other change is that we now have no “quiet node” copper under the switching node of the Buck converter, whereas before we did….so I suppose in the previous PCB there might have been better shielding of the switching node from the earthed heatsink on which the whole PCB sits.STTH1R06A datasheet:http://www.st.com/content/ccc/resource/technical/document/datasheet/c7/d5/fe/c4/56/21/41/02/CD00005135.pdf/files/CD00005135.pdf/jcr:content/translations/en.CD00005135.pdfUS1J diode datasheet:https://www.vishay.com/docs/88768/us1.pdfLNK302 datasheet:http://www.mouser.com/ds/2/328/lnk302_304-306-179954.pdf

author: ocset
author_rank: Super Contributor
timestamp: October 07, 2017, 08:17:10 pm
content: We are thinking of adding the shown RC network to the LNK302 based Buck converter in order to slow up the dv/dt of the switching node a little.......as well as using the slower diode...what do you think?We dont have an AC line filter as the cost woul kill the product, we do have a little SMD CLC filter just upstream of the Buck

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 07, 2017, 08:25:15 pm
content: So you're changing several things at once, per test?So, you don't know which one has caused which changes.Tim

author: ocset
author_rank: Super Contributor
timestamp: October 07, 2017, 08:35:16 pm
content: Thanks yes we must, as the EMC place is expensive.

author: Trafi1415
author_rank: Newbie
timestamp: October 07, 2017, 08:54:19 pm
content: The reverse recovery current and especially the cutoff of the reverse recovery current can be a significant source of EMI. However the measurements with the new configuration are failing in such a wide frequency range that I would bet on common mode. The spectrum of common mode noise caused by the du/dt of the switching node starts to attenuate 20db/dec only after the frequency of 1/(pi*tr). In the spectrum you posted the knee seems to be at the frequency of 5 Meg which would correspond to a rise time of about 60 ns which seems plausible. This combined with the changes made to the layout would make the CM noise caused by the removal of the shielding of switching node the most probable culprit in my opinion.In my experience problems with conducted emissions above 1MHz are almost every time caused by common mode noise and they are almost always cured by better layout or better positioning of Y-caps so that is a good quess anyway.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 07, 2017, 09:05:30 pm
content: How sure are you this is caused by the diode, and not by the placement of the diode?

author: ocset
author_rank: Super Contributor
timestamp: October 08, 2017, 08:14:03 am
content: QuoteI am sure the diode placement was good...we did it to minimise current loop area of the power switching current path, and we avoided power switching currents flowing through lengths of control ground.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 08, 2017, 12:59:47 pm
content: Mr. treez,Since test lab time is so expensive, would you be interested in some consultation and review to fix this problem once and for all?Tim

author: znroot
author_rank: Contributor
timestamp: October 08, 2017, 01:08:41 pm
content: This is the worst thing you can do, maybe this will fix your EMI problems but performance and efficiency of the buck converter will be worst.Try only with the capacitor as a snubber or change the PCB layout first.But IMHO you should be able to choose a different diode with better performance which better suite your application

author: Gyro
author_rank: Super Contributor
timestamp: October 08, 2017, 01:17:33 pm
content: In terms of worsened conducted, I would think that the change of position and type of X-cap has possibly had the most effect.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 08, 2017, 02:23:58 pm
content: Quote from: treez on October 07, 2017, 04:37:22 pmA couple of suggestions for you:1) There is no standard way of measure Trr. If you read the datasheets you will see that the two devices are measured under different test conditions.2) In order to compare two diodes, you have to a build a test fixture that can control:a) Forward currentb) Reverse currentc) -di/dtd) TemperatureAll of these will impact Trr3) It is not only the Trr, it the shape of the recovery current. Some fast diodes will 'snap' this generates a fast edge and lots of noise. Soft recovery diodes will not snap and generate lower noise.To avoid the cost associated with the EMC lab, I would buy either a used Spectrum Analyzer, the HP 859x family or a new SA Rigol, Sigilent etc. and either buy or build a LISN. This way you will be able to do pre-scans.Alternatively, as T3sl$co1l (Tim) suggested, have a consultant help you through the process.Conducted emissions, especially at low frequencies, is very predictable.Regards,Jay_Diddy_B

author: ocset
author_rank: Super Contributor
timestamp: October 08, 2017, 05:31:48 pm
content: QuoteThanks, we'd love to, but the cost, even for a simplest setup that can do both quasi peak and average, is £4022, its too much for us.Post #29 of this shows the cost....http://www.edaboard.com/showthread.php?t=366369&page=2

author: ocset
author_rank: Super Contributor
timestamp: October 08, 2017, 05:40:07 pm
content: QuoteThanks, though i am hoping we can clinch it with the next scan session.I am not sure what my company could pay, we have had terrific bad luck with ESD failures recently, due to modified boards, and so we are well in the red.We have passed EMC with all sorts of filters, but they used through hole components, and cost pressure disallows this, so we are trying to get it passed with just SMD components, and not many of those because we have very little board space....cost pressure means we cannot increase the board size. We dont have an earth connection to the board, and would rather not have it, as it means more assembly cost. (there is an earth connection to the heatsink on which the baord sits).

author: Siwastaja
author_rank: Super Contributor
timestamp: October 08, 2017, 05:47:29 pm
content: I'm sorry to say, but I think your company culture doesn't quite work out, or I don't understand it.Are these power supplies your main business? If yes, then I think investments around $5k-$10k needed are not that high at all, especially as they would probably be saved back very quickly as you would have less failures at EMC labs - also quicker time-to-market. I understand that $1M investments are difficult for small companies, but a few thousands, really? How can you run this business at all, I mean, how do you source the components, how do you pay for PCBs, manufacturing, etc.? In this case, I'd consider thinking about a business you can commit to, or finding an investor, or a loan.If this is a side business, you should first concentrate on the main business so that it'd make you enough money for this basic investment.Now it sounds like you are burning cumulative money because you don't have money, so that you'll end up wasting more money than you don't have, and in addition your product will be late, preventing you from getting the money you desperately need. Am I wrong?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 08, 2017, 06:52:00 pm
content: Quote from: treez on October 08, 2017, 05:31:48 pmYou really don't need to spend this kind of money.To start with you don't need QP at all.There is a relationship:Average <= QP <= PeakQP was 'invented' to normalize the readings obtained between a spectrum analyzer and measuring receiver. The measuring receiver used a panel meter. The QP filter has a 1ms rise time and 300ms fall time. You know by the nature of your noise source whether you will get any benefit from QP.For pre-scan you do not the Spectrum Analyzer to be calibrated. You work on your product till you have a 5dB margin. This margin should take into account any errors in your pre-scan and uncertainty in the instruments used.I use an HP8595E. This has the EMI filter bandwidth of 9kHz. This filter has a different shape than the standard 10 kHz filter. But for pre-scan this is not required, the standard 10 kHz RBW filter will be fine.LISNI built my own LISN, total cost less than $100.00 for a two channel LISN.Modelling EMCHere is an LTspice model of the LNK302 circuit. There is no model for the LNK302 so a simplification was used. The model uses a single LISN. When you use two LISNs you will get a 6dB improvement.The gain stage, E1, is included to scale the FFT output by 120dB to convert from dB to dbuV.The model includes the filter shown in Power Integrations test report. Note the capacitors have ESR.You run the model in the normal way.Examine the lisn_out signal between 30ms and 50ms and take the FFT.You can explore your circuit using this LTspice model.I have attached the model.Regards,Jay_Diddy_B