Conducted EMC scan with isolated Flyback  SMPS output not Earthed? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: August 18, 2018, 10:11:49 am
content: Hello,We have just hurriedly designed and built a 60W offline Flyback LED driver for a demo at a show. It’s now working well.We have booked in for Conducted and Radiated EMC scan as we don’t have the EMC test kit.(However, in my haste, I forgot to add a 1Meg resistor from Earth to secondary 0V on the PCB).As such we will have to EMC test it with the secondary output just floating.In the real application, the secondary output will have a DALI radio module connected to it, (as well as the LEDs) and the 0V of this radio module is Earthed. As such, in the real application, the secondary of our 60W offline Flyback will be earthed. This obviously means that the conducted EMC test test results regarding common mode noise would be worse in the real application, as common mode noise can flow to earth via the secondary without having to go through the common mode filter of our 60w offline flyback PCB.Anyway,supposing we do Conducted EMC testing on our 60W FlybackwithOUTthe DALI radio module connected, and with its output just just floating and powering the LEDs….then does this give us a valid EMC pass?.......ie, is it valid to test it without the secondary being Earthed? (it will be Earthed in the real application).

author: TJ232
author_rank: Frequent Contributor
timestamp: August 18, 2018, 01:45:11 pm
content: No.

author: ocset
author_rank: Super Contributor
timestamp: August 18, 2018, 02:08:31 pm
content: OK thanks, in that case, must the conducted EMC test be done with the exact make and model of DALI radio module connected to the output that will be connected in the final application?.....also, in the final application, the SMPS will be in a plastic enclosure which itself will be enclosed inside an earthed metal enclosure….must the conducted EMC test be done with the SMPS and DALI radio module inside the metal enclosure aswell?....and with that metal enclosure being Earthed?

author: coppercone2
author_rank: Super Contributor
timestamp: August 18, 2018, 05:10:22 pm
content: you need to test it like it will be used by the customer , as explained by the manual. Significant electrical changes require retesting.

author: ocset
author_rank: Super Contributor
timestamp: August 18, 2018, 11:51:48 pm
content: QuoteThanks, that’s interesting, it means that even if we buy an off-the-shelf offline LED driver, then we will still need to EMC test it in the final system…….along with whatever DALI radio module we buy…..and if it fails then we have to add an external filter box.

author: coppercone2
author_rank: Super Contributor
timestamp: August 19, 2018, 12:38:05 am
content: I have seen things that were independently tested retested when they were used as a part of a subassembly or component of a system. This was medical stuff but even if they took 1 piece of equipment, bolted it to a cart, and connected it to another piece of equipment with a serial connection, they would need to test the whole apparatus, even though one of the equipments was being tested and sold for a decade, I was told because they decided to re-sell it together, because the product they advertised required both pieces of equipment to do what it was meant to do. The only difference was the brackets and 8 attachment screws and the cart, but they still retested the whole thing.I don't know if you can get around it by having two sections on your website, one of power supplies, and one of drivers, and make no real statements as to what the power supply is used for. I would consider it to be unethical and against the spirit of the laws though.* I don't know consumer stuff though, I only saw medical/industrial to big vendors that tended to care.I don't know if it still applies to kits, but I am pretty sure if you make an enclosure that has a power plug on it to mains, and all the crap is wired inside with a harness, you need to test it as a new product. I would like to know where a bunch of distributed crap connected by loose wires that is meant to be mounted all over a wall stands and where the line is drawn. Maybe regulations related to telecom and wired LAN apply? I never even glanced at those though.I am almost certain if you put it in a enclosure and have aharnessits a product that needs to be tested as is with wiring as will be, even if everything is tested independently. Maybe if every subassembly has its own AC mains input you don't need to test so long you meet electrical code requirements? That would be no different then stacking everything in a shelf.. but then again you don't wanna sell a shelf that makes interference problems. It must have something to do with mechanical mounting and wiring harnesses.Also, customers tend to get angry if you say they can use your thing as a subassembly, then they find out its built like shit and gives them EMC problems, so they compare your work to their work and go like WTF is this SHIT? That's when you get invaded by a boarding party that starts making demands to your assembly line, if you wanna play with the big boys.At that time you basically have a parole officer that starts making nonsense demands and throwing their weight around. Best not to attract attention. Because quality. I have also seen the moods and facial expressions of those effected.I have seen it happen (not only once) and its well documented.microwaves101.com"Boarding partyCustomer group on a fact-finding trip to your lab, to see why you are late and over budget (or a group that your company sends to a vendor for similar harassment, which is much more fun.)"And it can be harmful and beneficial to a company, they might offer help but it means more procedures and overhead and sometimes downright retarded horse shit involving cleanliness and organization. Your ISO book will get fatter. I suspect this is why managers that no one knows the reason for start to appear. This tends to hurt the wallet and damper important R&D.Have also heard about this from a friend of mine that started to do government contracts. I don't remember but I think it started because of quality problems with welding and turned into floors not being clean enough.Also with my other friend that had overseas management start appearing and making demands because of some bad shipments. Most were irrelevant to the fault, equipment that worked fine started being replaced.. all sounds good till you realize its wasteful and your employees are pulling longer shifts for no god damn reason replacing furnaces and doing stupid preventative maintenance tasks that don't fit the profile of the factory. I think because of an uncalibrated thermocouple and easily controlled humidity problems. So they had went from a normal schedule that Americans find reasonable to some kind of 11 hour 5 days a week shit with a extra day or two off every two weeks or something ridiculous like that. I heard it was soul grinding.Don't get pinched son

author: TJ232
author_rank: Frequent Contributor
timestamp: August 19, 2018, 04:49:24 am
content: 100%

author: janoc
author_rank: Super Contributor
timestamp: August 19, 2018, 12:41:24 pm
content: Quote from: treez on August 18, 2018, 11:51:48 pmYes, of course you need to test it like that.A pre-tested off-the-shelf module could be fine alone but then you connect a wire from it to another component that will start to act as an impromptu antenna and the entire system will fail the test. Or part of the chassis/enclosure could radiate. Etc. That a component is pre-tested only means that you can have a reasonable expectation to not fail the testing because of it (and perhaps can skip some tests), not that it absolves you from having to re-test the entire setup.Search the forum, this has been discussed at length in the past.

author: coppercone2
author_rank: Super Contributor
timestamp: August 19, 2018, 02:42:20 pm
content: how does it work legally, if you sell a brick and a lamp you made that are both tested. so long you sell them together and specify they are used together you need to test em together?I mean chassisless stuff.Like if you put a barrel jack on your lamp that says 9V 3A, then happen to sell a power supply thats 9v3a on your website that has a barrel jack on it. But its not on the purchase order by default. Are they gonna like run a sting on your sales people to try to incriminate them for recommending the two be used together? Can you run a kind of boiler room ?How about selling computer towers without a power supply? It is just a mechanical organization of parts..How do the laws deal with this kind of behavior? I only worked with things that were sold in a chassis like a real product ready to plug in. I know with communications ports to make device networks you typically had to attach a long coiled up cable, seemingly to simulate the worst case, but the labs seem to trust that the serial port would not go apeshit connected to a load (its rated for like 750uA pulses i think)

author: janoc
author_rank: Super Contributor
timestamp: August 19, 2018, 11:08:59 pm
content: Quote from: coppercone2 on August 19, 2018, 02:42:20 pmAFAIK (but I am not a lawyer!), if you sell separate products (e.g. lamp + a wall wart/power brick), then you need to test only each of the products separately (or sell pre-certified products, e.g. off-the-shelf PSU - saves a lot of money on the EMI and safety testing). If the combination causes EMI, then that will be the customer's problem to deal with (i.e. the regulator will fine them, not you, for operating it, assuming the components are separately OK and have the necessary paperwork/certifications).That customer could obviously go after you if it was you who has recommended/sold that particular power supply and they got fined because of it. But that is a different issue - a trade dispute and not a regulatory issue.The same the computer tower sans power supply. In that case it is even clearer because it is up to thecustomerto put the supply there if they want to actually use the machine and to make sure the combined device is not causing interference. You (the tower vendor) can't meaningfully control what is the customer going to install in it anyway. This case is actually explicitly covered in the regulations and excluded - if you are selling only parts meant for further integration (i.e. not a complete computer in this case), then you don't have to test them. It wouldn't make much sense because the way they are installed could affect their performance and you have no control over that. This the case for the various modules, kits, etc.Many module vendors will pre-test/certify their modules anyway but that is not because they have to do it but as a commercial service for you. If you use such pre-certified module, you can skipsome(not all) parts of the testing (e.g. things related to radios if your gadget uses any radio module or electrical safety if you use an off-the-shelf mains power brick) and will save time and money.On the other hand, if you actually build a device (let's say an IoT toothbrush) out of off the shelf parts (let's say motor, Bluetooth module, power supply) and want to sell this, then you have to test the complete toothbrush, as it is meant to be used, regardless of whether or not the components are certified already (but you may be able to skip some tests, see above). If that includes e.g. a charger too, then you need to have the charger tested with it as well. If there is no charger in the box and the customer is supposed to bring their own (e.g. a common phone charger), then there is no need to test it with a charger.Finally, if you really have some funky case and aren't sure about whether and what needs to be tested, just talk to your test house. They will tell you exactly what and how needs to be tested for the various certifications (CE, FCC, etc.).

author: Neilm
author_rank: Super Contributor
timestamp: August 20, 2018, 07:06:24 pm
content: Quote from: treez on August 18, 2018, 11:51:48 pmCorrect - two CE devices do not mean the final assembly will pass. I know most power supplies are tested with a pure resistor on the output, so it is often completely different when you test the final system. I don't know if the same holds true for LED drivers.