Please advise on Cheap  Conducted EMC test setup for offline SMPS's - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: October 30, 2018, 09:00:26 pm
content: Please help advise about the very basic conducted EMC test setup for offline SMPS’s, on figure 6, page 3 of AN15 by power.com.This setup is obviously a cheap compromise, since the test setup in figure 6 is not as complete as in an accredited test house, the conducted EMC scans will therefore be discrepant.What will be the main nature of this discrepancy? Up to what frequency will the setup of Figure 6 be accurate?Page 3, figure 6, of AN15 shows a conducted EMC emissions setup for an offline SMPS (figure 6). It says that 6 foot cables should be used between the unit under test and the LISN, and the UUT and the load....but in the picture they dont look like 6 foot cables, they look much shorter...can this be "six foot" be right?Also, it seems to be saying that the table is up against a vertical "reference plane" which is 2m by 2m. Presumably this reference plane is just a floating piece of metal sheet?The LISN must be bonded to this metal plane. I thought that the LISN should be right up against the reference plane, so that the LISN's connection to the reference plane can be as short as possible? –But in Figure 6, the LISN seems to be 40cm away from the reference plane….surely this can’t be right?Also, i thought that the table should have a metal sheet beneath it aswell as the one at the end of it?Also, where does the spectrum analyser go in this setup?..presumably it connects to the LISN via a coaxial cable..and the specturm analyser is at least 2 metres away from the unit under test?AN15https://ac-dc.power.com/design-support/product-documents/application-notes/an-15-topswitch-power-supply-design/

author: coppercone2
author_rank: Super Contributor
timestamp: October 30, 2018, 10:15:44 pm
content: the cheapest way is to hide it in someone else's product

author: TimNJ
author_rank: Super Contributor
timestamp: October 31, 2018, 04:08:37 am
content: First off, the conducted EMI test setup is quite forgiving in comparison to the setup required for radiated EMI. By that I mean, your exact setup probably doesn't mattertoomuch. You will likely get reasonably close results. Just give yourself a little margin. 10dB is a nice number.Where I work, we have a horizontal reference plane on the floor, made of an aluminum sheet, bonded to local earth connection. On top of the reference plane we have a short wooden table, which we place the DUT on. On a separate, normal height wooden table, we have the spectrum analyzer, PC, and LISN. Use a low-inductance connection to bond the LISN to the reference plane below. (Braided ground strap is a good choice.) In our case, the strap is relatively long, but results are still accurate. (Would advise against using long piece of wire, though. Inductance is too high.) You can keep the spectrum analyzer, PC, and LISN relatively close to each other. Just make sure you have some distance between those devices and the DUT, about 80cm recommended. Make sure the cable connecting between the DUT and the LISN does not dangle too close to the floor. Bundle non-inductively, if you must. 1m cable is typical.Also helps to solidify the earth connections between the LISN and spectrum-analyzer frame. You can put clamp-on ferrites on PC cables for the monitor, power, etc.

author: ocset
author_rank: Super Contributor
timestamp: October 31, 2018, 07:55:04 pm
content: ThanksWe believe that the DSA815 specturm analyser is the cheapest in the world for our purposes (rough conducted emissions test for offline SMPS)....DSA815 spectrum analyserhttps://www.rigolna.com/products/spectrum-analyzers/dsa800/Regarding the LISN, we believe that the Hameg HM050 is the cheapest on the market.....Hameg HM6050 LISNhttp://www.farnell.com/datasheets/1851611.pdf?_ga=1.216594710.1679746183.1489787856We find that the total cost is going to be £4022…..DSA815 spectrum analyser = £1300Including the following necessary options for DSA815 spec analyser..-Advanced Measurment kit (AMK-DSA800) = £423-VSWR-DSA800 = £400-EMI-DSA800 = £454S1210 EMI pre-compliance software = £450HAMEG HM6050 LISN = £1000….however, we realise that we are going to also need to buy transient limiters (to protect the spectrum analyser input) and a broadband RF power amplifier for the connection from LISN to spectrum analyser. Does anyone know how much these are and can recommend cheap ones to buy?Also, do you think we can get away with just a one metre square aluminium ground plane?Also, do you know what is the difference between having a vertical ground plane, and a horizontal ground plane? (we appreciate that in official EMC test houses for conducted EMC, the entire room is shielded, but we cant afford that)

author: Niklas
author_rank: Frequent Contributor
timestamp: October 31, 2018, 09:20:07 pm
content: Do you really need the AMK-DSA800 and VSWR-DSA800 packages for your conducted emissions setup?The DSA815 also has a built in pre-amp, so you start with that.It is possible to run the DSA815 in standalone mode without the PC software.

author: chris_leyson
author_rank: Super Contributor
timestamp: October 31, 2018, 10:43:44 pm
content: QuoteWe use the same setup, short wooden table with maybe 3m2aluminium sheet reference plane. The LISN is bonded diectly to the reference plane with a large copper right angle bracket and the test receiver is on an office table about 2m away. The Rohde and Schwarz LISN has an internal 10dB attenuator which helps protect the test receiver.

author: coppercone2
author_rank: Super Contributor
timestamp: November 01, 2018, 02:26:13 am
content: how do you setup your earth grounds for that kind of stuff?Multiple rods? Do you measure the impedance of the soil/ground resistance?

author: chris_leyson
author_rank: Super Contributor
timestamp: November 01, 2018, 04:28:45 am
content: QuoteThe metal reference plane is the earth and the LISN just plugs into a wall socket. Nothing fancy like earth rods needed. Even if you had access to a massive earth rod like on a lightning conductor for example it wouldn't madke any difference to the measurement.

author: coppercone2
author_rank: Super Contributor
timestamp: November 01, 2018, 05:22:22 am
content: so for the purposes of commercial tests so long you meet the requirement for residential you have a ground thats fine for conducted emissions?I think at a test lab I went to they had alot of grounding rods for some reason. My LISN is on hold still need to design the coupler and chassis but at least the big magnetics are assembled

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 01, 2018, 06:13:40 am
content: Galvanic grounding is a safety feature and has absolutely nothing to do with RF.I suppose it's kind of unfortunate that we use the same word ("grounding") to mean two things. They're really the same, but they are applied very differently in practice.Effectively, the LISN measures the differential mode currents in the twin transmission line that is the mains cable: hot with respect to ground, and neutral with respect to ground. (Or if no ground is present, then the LISN ports act in series, i.e., 100 ohms differential).Note that common mode (ground wire with respect to ground plane) is not measured, at least not typically. This is probably not a big deal (the unbalanced cable, and measurement method, makes CM-diff mode conversion very likely, so that such an EMC problem will still be apparent), but nonetheless noteworthy.A LISN typically has quite high ground leakage, making some precautions necessary for operation (if an RCD breaker is present) and safety. This is a separate issue, easily addressed by in-house techs (e.g., bypassing the RCD, using an isolation transformer, etc.).Tim

author: TheNewLab
author_rank: Frequent Contributor
timestamp: December 18, 2018, 08:55:57 am
content: I see that you are explaining an aluminum ground plane on the floor. How about a ground plane (grounded of course to electrical outlet ground) placed under a sheet of plywood?My hobby lab is in the middle of a commercial business district. I have all sorts of WiFi, Radio floating around. When i test for mains electrical EMI.. it also appears around most of this area. My thinking was. if ground planes work so well on PCBs, why not place aluminum sheeting on the worktable, connect to ground with a 1Ohm resistor (just like a ESD ground mat), and place 2 inches of plywood over it.Will this work, or am I creating more problems?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 18, 2018, 07:39:04 pm
content: That's the beginning of several standards...I've seen test labs with metal floor plating, and extended up the wall, for setting up various tests. Otherwise you need a portable (vertical) ground plane, or something like that.Tim

author: tautech
author_rank: Super Contributor
timestamp: December 18, 2018, 11:56:46 pm
content: Quote from: treez on October 31, 2018, 07:55:04 pmWell actually it might not be:SSA3021X GBP 1168https://www.siglenteu.com/spectrum-analyzers/ssa3000x-series-spectrum-analyzers/

author: Cloud
author_rank: Regular Contributor
timestamp: December 19, 2018, 08:16:07 am
content: Maybe these presentations from EMC seminar will help youhttp://lpvo.fe.uni-lj.si/fileadmin/files/Ostalo/Delavnice/Delavnica_EMC_2018/PPT_ECE_R_10.05.pdfhttp://lpvo.fe.uni-lj.si/en/education/seminarji/nacrtovanje-elektronike-za-emc-2018/(I know they are mostly in Slovenian language but maybe some schematics of test would be useful)