Shielding under isolated offline flyback SMPS transformer - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: August 21, 2023, 08:37:16 pm
content: Hi,Just checking, are there any regs against "drawing the curtains" (with copper pours) under an SMPS transformer to shield it from an underlying earthed heatsink?ie, using layer 2 on pri, and layer 3 on sec , to totally cover it underneath with "quiet node" copper pours, eg pri and sec ground pours....IE inner layers.

author: Gyro
author_rank: Super Contributor
timestamp: August 22, 2023, 01:55:07 pm
content: Do you haveanyconcept of how multi-layer PCBs are constructed, which layers are separated by part thickness board and which by just a thin layer of pre-preg?

author: Faringdon
author_rank: Super Contributor
timestamp: August 22, 2023, 06:54:46 pm
content: Thanks, for the 1.6mm PCB, ayk, the prepreg will be some 0.35mm thickness....thats good for many kV.Ayk, in air, flashover distance is 160um per kV......so going through pre-preg it will be much more forgiving.Page 53 of the below IPC-2221A, ayk, the bastion of all things PCB layout, confirms that for internal tracking on PCBs, you can get 301-500V with just 0.25mmon the same internal layer.As such, if you are on two different internal layers, then the clearance will not need to be high at all....it certainly wont need to be 6mm as is needed for external layer primary to secondary clearance.I do believe that IPC-2221A does not specify an "internal layer to different internal layer clearance" for primary to secondary clearance.........but it is up to the maker to use due diligence.IPC-2221A PCB standardhttps://www-eng.lbl.gov/~shuman/NEXT/CURRENT_DESIGN/TP/MATERIALS/IPC-2221A(L).pdf

author: Gyro
author_rank: Super Contributor
timestamp: August 22, 2023, 07:32:14 pm
content: ... and the primary - secondary voltage that you are required to withstand?

author: floobydust
author_rank: Super Contributor
timestamp: August 22, 2023, 07:52:08 pm
content: Thanks but ayk IPC is not a safety standard, OP are you following them because it's convenient for your undersize design?What are you talking about "drawing the curtains"? Can you explain a little better. It makes no sense to add capacitive coupling as you propose.In air breakdown voltage of "160um/kV" is of course fiction due to ozone, moisture, pressure etc. and would never be used by anyone other than a bonehead.As well, spacings between PCB inner layers and traces are specified in safety standards.In solid insulation, spacings give consideration for long-term aging and mechanical stress, and on an exposed surface- pollution degree. They are never the absolute minimum you desire and falsely justify.Inner-layer trace spacing has a minimum spec. as well for the inter-layer insulation thickness, pic related. The PCB pre-preg and laminate's manufacturer's rating must not be exceeded. You can't get so cute there.

author: Gyro
author_rank: Super Contributor
timestamp: August 22, 2023, 08:15:13 pm
content: It really is scary considering the number of years he claims to have been designing mains SMPSs in his Linkedin profile. Either inability to reason or inability to learn and retain.

author: Faringdon
author_rank: Super Contributor
timestamp: August 22, 2023, 08:25:56 pm
content: Thanks, The following claims 54kV/mm for pre-preghttps://www.isola-group.com/wp-content/uploads/data-sheets/185hr-laminate-and-prepreg.pdf?t=1005241780Thanks, We need to withstand the standard 3.5kV flash test.

author: floobydust
author_rank: Super Contributor
timestamp: August 22, 2023, 08:41:06 pm
content: Quote from: Gyro on August 22, 2023, 08:15:13 pmHe's been on this 20W SMPS design for months. #5 topic starter on eevblog and every thread comes to no conclusion. 'It's my want and belief I need confirmed'.I expect the Luna-25 experience because the Power Integrations IC is used at max rating and the extra heatsink daughter board was pretty much clown car for design. Then there's the tiny transformer.Spacings are too tight, no regard for safety standards, using parts at their max rating - even china does better most times. I guess some people never learn. All this for 20WQuote from: Faringdon on August 22, 2023, 08:25:56 pmYou don't understand- solid insulation ages, and it's not a UL number the "54kV/mm". The safety standards add allowances for things that your version of insulation physics is not including. They are already onto people trying to thwart/circumvent/minimize the required spacings. Is Stockton Rush your hero?You do realize adding a mm here or there would make your life easier and the PSU would already be finished.

author: Faringdon
author_rank: Super Contributor
timestamp: August 23, 2023, 05:47:25 am
content: QuoteThanks, yes i agree, but ayk, with pri to sec insulation, the 3.5kv test is about resistance to transients....and transients just arent common enough to give significant ageing effects....a typical transient lasts some 100us, ,if that, and they occur seldomly.

author: Gyro
author_rank: Super Contributor
timestamp: August 23, 2023, 10:58:41 am
content: As you haven't specified the usage environment, industrial etc then we don't know how frequently transients will occur... or the predicted service life of the alledged SMPS. You would do well to look at the datasheet of a digital isolator, eg the ADUM3160 which has a good section on isolation MTBF vs voltage stress. Your isolation is completely at the mercy of some cheap Chinese PCB manufacturer anyway, that's no form of UL / VDE etc. certified isolation.You seem determined to do whatever you want to do anyway, so there is really no point in you coming here seeking some flimsy 'agreement' or validation of your assertions. It is just a waste of peoples' time.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2023, 09:18:16 pm
content: OP you give no details of the safety standard you are designing to. Best to refer to it. I haven't seen that 3,500V value you refer to. Is that 1 minute or impulse?Reinforced insulation? it's a 0.4mm minimum thickness between layers which is 2+ layers of sheet incl. prepreg, and likely a requirement for routine testing of the finished PC boards for dielectric strength. One strict standard considers PC board layer solid insulation as a composite and wants 6x the usual separation...The understanding is there are always gaps and voids in solid insulation. The working voltage stress as well as that due to transients are considered. What's the lifetime of your product? Do you want 400VAC across say 0.2mm for 5 years? AC at frequencies over 30kHz also is considered additional stress.One production problem was a design where the transformer core shorted to the pour/vias under it, on the top layer, when it was warped a bit in the bed of nails tester. Nasty to figure out why some boards worked fine on the bench but not in the tester. They painted the via's or Kapton tape to get around that. It was not my PCB design.

author: jonpaul
author_rank: Super Contributor
timestamp: August 24, 2023, 01:18:48 pm
content: Just another FTTS = Farrington Treez Time Sink,Suggest to ignore any of his useless posts.To Farrington: GET A LIFE AND STOP YOUR NAGGINGLife is too shortj

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 28, 2023, 12:01:49 pm
content: Quote from: Faringdon on August 23, 2023, 05:47:25 amIf there's one thing you take away from this thread, it should be that you are wrong about this. Those 60 second ~4kv dielectric tests are specifically to test for degradation over time, which is why they are always type tests and not production tests.

author: Faringdon
author_rank: Super Contributor
timestamp: August 28, 2023, 04:36:20 pm
content: QuoteThanks, do you mean degradation over the total time at which the equipment was exposed to ~3500V transients?......or just time exposed to normal pri to sec voltage?.......As you know, supposing secondary ground is at approx earth potential, (which it probably is) then the most the pri to sec insulation would see under normal conditions would be the mains peaking at 339V, 100Hz...presumabaly the insulation hipot test (~3500vac) for 60 secs is not about testing the withstand-over-time to this?

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2023, 07:00:19 pm
content: stop grasping at straws

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 29, 2023, 12:32:28 pm
content: Quote from: Faringdon on August 28, 2023, 04:36:20 pmThe latter. It's a form of HAL test, where a brief exposure to elevated voltages (3.5-4kvac for 60s) is roughly equivalent to long term exposure to working voltage (220Vac or whatever for years). Supposedly this is based on lots or R&D on ageing processes for solid insulation, though I don't know where that literature is off-hand.Depending on your particular standard, the elevated voltage in the dielectric strength test might also cover some transient surge conditions. But to my knowledge that's not the purpose of the dielectric strength test; surge and transient conditions have their own tests.I'm basing this on IEC 60601-1 8.8.3, which I believe is similar to other standards (though the test voltages required will change between standards).

author: dmills
author_rank: Super Contributor
timestamp: August 31, 2023, 09:58:37 am
content: UL at least for some of the standards require multiple layers of prepreg or core, never just one where safety isolation in concerned, makes for some fun stackups, and no the vendors rating for insulation strength has very little to do with it, this is about avoiding single points of failure. Sort of shit I trust Wurth to get right and do as specified, JLC less so.I have a square foot of board (32A single phase power distribution board) going thru that at the moment, interesting times, and some completely wild shit turns up when talking to UL, cannot say they don't have justification, but they flag stuff you would never have thought of, totally worth someone elses money....Regards, Dan.

author: Faringdon
author_rank: Super Contributor
timestamp: August 31, 2023, 06:57:23 pm
content: QuoteThanks, i know a lighting company that does 2kV isolation hipot test to every unit of a particular type on production...so you would say they are wrong?

author: Faringdon
author_rank: Super Contributor
timestamp: August 31, 2023, 07:24:37 pm
content: QuoteThanks, as you know, the barrier that will take the 3500vac is that between triple insulated wire and enamelled copper wire in the transformer....i wouldnt expect this to see any degradation at all under 3500vac for one minute.The 3500VAC ....the reason i thought it was for transients, was because it must be for a dodgy brand of the insulation say...which might not hack the (up to) 6kv mains transients..........i would expect TIW/ECW in a offline transformer to withstand normal mains useage for 100 years +?...........

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 01, 2023, 12:40:36 am
content: Quote from: Faringdon on August 31, 2023, 06:57:23 pmNo, it's definitely not unusual to do production hipot tests, but it's always at a much lower voltage (and usually a shorter duration) than a proper dielectric strength type test.Quote from: Faringdon on August 31, 2023, 07:24:37 pmWhether or not it would degrade the insulation on a given device depends entirely on that particular insulation. I'm just explaining the motivation behind the tests (for the standards I'm most familiar with, that is), I didn't come up with it.

author: Faringdon
author_rank: Super Contributor
timestamp: September 09, 2023, 05:17:22 pm
content: QuoteThanks i am sure you are right here, but would be great to get the doc name that decrees this?Also, creepage and clearance distances on PCBs, are, ayk, all about both working voltage and overvoltages (ie transients)...eg 1:46 of this......so i wonder , regarding the 3500VAC hipot test (pri to sec) that all offline SMPSs have to go through for EN62368 compliance.......so we are saying that the 3500VAC is all about long term exposure to the normal working voltage , and not the transients?....so if we do 3500VAC for 1 minute from pri to sec.....then for how long does that assure you that the product will not suffer insulation breakdown....?...eg, does 3500VAC for 1 minute equate to say, 2 years at normal working voltage?I wonder if the answer to this is known?

author: Gyro
author_rank: Super Contributor
timestamp: September 09, 2023, 06:40:36 pm
content: Really? You're taking your professional guidance from a hobby magazine (elektor) now?By the time you've got through the first minute of disclaimers... one man's interpretation, no responsibility for compliance, approval, or safety, etc. you don't get the slightest hint that you should be referring to the actual standard, rather a YouTube video?... or forum(s) come to that, I notice that you have cross-posted on edaboard as always.That aside, I can't see how the video (incl part 3) relates in any way to your multi-layer isolation in Chinese PCBs of unknown quality!

author: floobydust
author_rank: Super Contributor
timestamp: September 09, 2023, 08:00:58 pm
content: OP wishes to justify his unsafe designs with inadequate spacings as being perfectly fine in his mind.These endless posts about spacings are just to rile us up. For years now.