What are creepage requirements for planar transformers? - Page 1
Forum section: electronics / projects

author: jbb
author_rank: Super Contributor
timestamp: March 29, 2021, 09:11:57 pm
content: Hi allFor some time I've been interested in planar magnetics, but haven't designed any myself. There are obviously upsides and downsides to using them, but I'm not sure how to answer one question; what limits apply to creepage and clearance?Clearly, any real design would require proper examination in full detail. This is an initial inquiry to understand what I might be getting into - wouldn't want to go a long way down a design path and find out I stuffed the safety properties!Here's a sketch of what I'm thinking about:An off-line DC - DC converter, e.g. phase shifted full bridgeReinforced insulation required for Safety Extra Low Voltage (SELV) outputOperating from an AC line up to 265V AC RMSPFC rectifier, internal DC rail up to 400V DCPollution Degree 2 (lab/office)Planar transformer integrated into main PCB using PCB windingsPCB uses FR4 material - does that put it in Material Group II?That seems to give a clearance of 4mm and creepage of 5.6mm per IEC60950, which is about what I expected for the outside of the board.Now, what about the hypothetical transformer? (Please forgive the horrible ASCII art)Core: CCCCCCCCCCCCCTop: C CPrepreg: CoooooooooooCMid1 (S): C ------- CPCBcore: CoooooooooooCPCBcore: CoooooooooooCMid2 (P): C - - - - CPrepreg: CoooooooooooCBot (P): C - - - - CCore: CCCCCCCCCCCCCThe secondary side is one the top half of the PCB (could be other way up, whatever).There's no top copper because the ferrite core might rub on it.Secondary (S) winding is on Mid1.Primary (P) winding is on Mid2 and Bot.OK, now here's the question - how do I determine the creepage I need laterally between the edge of the secondary winding and the (PCB cutout for the) core, shown asXXbelow?Prepreg: CoooooooooooCMid1 (S): CXX-------XXCPCBcore: CoooooooooooCIs this considered a creepage path at all (I suspect so, because there's a glue line)?Is it the same as the bare PCB Pollution Degree 2 case above - 5.6mm? If so, that's sacrificing alotof winding area.Or does it move to Pollution Degree 1 (because the glue line is sealed at the PCB fab and can't get debris in it) - which I think is then 2.0mm creepage?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2021, 10:26:36 pm
content: A tip: put SELV on the outer layers, so creepage to core, and insulation in general, is N/A.That leaves two concerns:1. What does the internal layer count as?2. What to do about the primary connections?The answer to #1 I believe is a CEMENTED JOINT (60950-1, uh I think I read the 2013 version to be specific?). The clearance is simply the dielectric rating of the material. Beautiful! You might want to double the distance just in case (and note this applies to inter-layer distance, so make sure those prepregs are adequate), and probably a hipot or other inspection process is desirable.2. Unless you use even more layers, and buried vias, you're going to have vias nearby which reach the surface, and connect to the primary winding. These surface features are either plain old creepage, CTI group III, pollution II; or, if soldermask counts as an insulating coating (I forget which section this is under, but it seems to require an inspection process under 60950-1 and I don't think fabs offer that as such), it may be better.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: March 29, 2021, 10:48:47 pm
content: I've never done anything planar for AC-DC application, but check this out:https://www.ti.com/download/trng/docs/seminar/Topic4LD.pdfSection 4-13:QuoteSo..I guess you can rely on the pre-preg to create essentially seamless insulation, once cured. I'd actually like to know more myself!

author: jbb
author_rank: Super Contributor
timestamp: March 29, 2021, 11:12:27 pm
content: ThanksRe the core: I was under the impression that the core was generally considered live. Is there scope to (with appropriate engineering) argue that the core is insulated from the primary to the extent that the core itself is consistent with Safety Extra Low Voltage?Re coating: there definitely are dielectric coatings but I’d rather avoid wet processes.Does anyone have experience with something like a stick-on film? I know Kapton tape has great dielectric strength but rubs through...Re extra prepreg: that’s an interesting thought. If it counts as a cemented joint, it might be possible for a PCB manufacturer to laminate it in while doing the board stack.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2021, 12:46:15 am
content: Quote from: jbb on March 29, 2021, 11:12:27 pmHmm, I can't think of anything that would necessitate that. I understand the impression -- but I can't think of a reference where I've seen it claimed, either.At least from first principles: if everything is properly insulated (REINFORCED type), what's it matter? It's just another conductive bit of metal (or ferrite semiconductor). I don't recall 60950 making any special cases about it, it should be handled like anything else.There are good reasons to ground the core, of course -- for safety (primary-core insulation can be BASIC type), or for EMI (often the noisy primary is in close proximity to the core, thus some capacitance couples to it). In the latter case, grounding might amount to simply shielding it, which can also be combined with a flux band for example, which offers another kind of EMI shielding too.So, I don't think that's a problem?Heh, speaking of shielding, besides a shield over the board, or like a heatsink plate or something (which might be necessary anyway, given the power density these designs can offer), you could use an EMI spring to touch the core. There's lots of kinds available, I bet there's something to handle horizontal contact in a low profile...QuoteHmm, I wouldn't say it rubs through? It's pretty tough stuff, though tape being tape, it could rub off from an edge. Or, if you mean with enough time and vibration against a loose (i.e. not cemented?) core, yeah, that'll happen eventually.Tape has the problem that it's not likely to sit perfectly flat on the board, and any void is a direct creepage path. The insulation is not the film itself, but the adhesive holding it there. (Polyimide is a magnificent insulator by itself, at least!)Not sure what if anything, standards have to say about tape. It seems a bit dubious to me, but maybe it's acceptable after all.Hm, I forget if there's consideration for hi-potting something at enough of an excess voltage to satisfy concerns about creepage. That is, test it at a voltage where any creepage path is certain to flash over, and if it doesn't, well the tape must be doing its job.Would that even make sense, though? What about exposure to a slightly salty, condensing environment, such that creepage paths might become filled in with water?So, Idunno.Note that, in conventional transformer construction, tape is applied such that it can be assumed to have gaps between layers -- enough tape width, overlap, and extension over the edges of the windings (using margin tape to reserve space if necessary) is used to ensure the worst case creepage path is still long enough. And then it's filled in with varnish, which is probably not a guarantee by itself -- it's effectively potting when it works, but how do you inspect a winding, y'know?Best practice would seem to be, do everything you can, starting with absolute minimum requirements (i.e., overlapped tape, leaving margins), and make it better and better from there (the adhesive will seal to some extent, the varnish even more, etc.).QuoteOh-- I think you can special-order certain kinds of coatings and laminates, uh, who knows how that's going to break down in terms of cost, labor, special ordering -- but that might end up cheaper than doing whole-ass extra layers on things. Just slap down a square of prepreg around the affected area, run it through the press again, then off to the router. Or... well, with all that special ordering, who knows, a semi-proto 6/8 layer board might still be cheaper. In quantity? Who knows.Also a good reason to consider using planar transformer components. You can get hundreds of windings on a single panel, and you can stack them vertically in parallel (given enough core height) for arbitrary ampacity. The assembly can be potted, guaranteeing ratings. And it can be soldered into a much cheaper, larger board in the end. But that's also additional labor making whole-ass components, so, tradeoffs of course.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: March 30, 2021, 02:24:47 am
content: For planar construction, my guess is that given the close proximity of the core to the PCB, it may be hard to practically keep the core floating (in according with 60950-1/62368-1).Regarding insulators, do you work with magnetics vendors professionally? (Or this a personal project?) To understand what materials are allowed to constitute a transformer, you can look to a UL recognized insulation system. Often times, if you are working with a magnetics vendor, they can tell you what systems are available to them (and thus, to you).Here is one system created by a TIW litz manufacturer:https://iq.ul.com/systems/List.aspx?%ULID=103091937. I'm not sure the intricacies of how it all works, but basically a vendor purchases the rights to use this system for products they build.Here is a UL report for a product made by XP Power which uses a planar transformer:https://www.xppower.com/portals/0/pdfs/UL_nanofleX.pdfStarting on Page 13, you can see all the critical components used to build the transformer. Maybe this will give you an idea about what is acceptable/typical. Further down is a complete mechanical stack-up and all layer drawings.Here are two Huawei chargers for reference:https://www.chongdiantou.com/wp/archives/50773.html,https://www.chongdiantou.com/wp/archives/57121.html

author: jbb
author_rank: Super Contributor
timestamp: March 30, 2021, 03:19:13 am
content: Yowza, that’s alotof windings!I’m more at the personal project level and not in contact with magnetic Ema manufacturers. Might eventually feed into something open source.Best solution is probably to find premade standard parts :-)

author: KlausKragelund
author_rank: Regular Contributor
timestamp: June 13, 2021, 06:22:05 pm
content: Quote from: TimNJ on March 30, 2021, 02:24:47 amWow that is an impressive report, shows all the details of the constructionI have long had the idea to do a planar design, but even though the transformer cost is low (for simple designs, probable 1/10th of a standard transformer), there is an added cost of production (needing a special station for assembling the cores, gluing them, and curing the glue). Not to mention no experience of the quality levels obtainableHave you ever done it in volume production?CheersKlaus

author: bjbb
author_rank: Contributor
timestamp: June 13, 2021, 09:10:44 pm
content: The referenced UL safety report has problems, so be careful when using it as a reference.The problem with planar transformers is that annex G of 62368-1 would not, depending on your construction, be scoped for some planar xfmrs. So the Generic tables,with multipliers would also have to be used to determine spacings. Also reference clause 5 to determine the energy source classification which would determine construction and mitigate requirements.For 60950-1, clause 2.10.5 covers almost all xfmr constructions, so would be easier to discern requirements. But as most NRTLs and NCBs are no longer doing new stuff for this deprecated standard, use of this standard is problematic.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 14, 2021, 03:14:51 am
content: There may be a requirement for thickness of any solid insulation used for a particular insulation category.jbb, do you have the latest 60950 version, and is that the only applicable standard for your intended application? Sadly until you have confirmed all the clause conditions for your intended application then yes you run quite a risk that a cursory check misses an important requirement.

author: jbb
author_rank: Super Contributor
timestamp: June 14, 2021, 07:16:03 am
content: Hi allThis was more idle curiosity; I’ve been interested in planar magnetics and wanted to clarify what I’d need to look out for.I don’t have access to an up to date standard, unfortunately.