Author Topic: Triple insulated Litz?  (Read 1021 times)

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Online MagicSmoker

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Triple insulated Litz?
« on: October 01, 2019, 02:23:58 pm »
It's been about 15 years since I've designed a transformer using Litz but I do recall it wasn't much fun sorting through the 1000s of permutations and trying to navigate the regulatory mess getting the final design approved. Well, I'm in the same boat as I am trying to redesign a state of the art planar E-core transformer into something that is actually manufacturable and Litz is going to be necessary due to the power and frequency of operation (>200kHz).

Apparently in the intervening time there is now available triple insulated Litz where the additional insulation layers are (smartly) implemented in the serving rather than the individual wires. For example, Rupalit Safety: https://www.packlitzwire.com/products/litz-wires/rupalit-safety/

This looks really good, but I'm soliciting for more suggestions from the peanut gallery.
 

Offline T3sl4co1l

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Re: Triple insulated Litz?
« Reply #1 on: October 01, 2019, 03:35:34 pm »
Hm, what's wrong with planar?

Afraid I don't have any other sources.  I do recall getting jacketed litz from NEWT, although I don't know if it was specified as "reinforced" or not.  (I've seen nylon-served, tape-served, and neoprene and PFA jacketing.  If nothing else, the tape could simply be wound at a lower pitch.)

If you're getting production quantities (>10kft for ca. 18AWG stuff I think?), I'm sure you can custom order whatever you like.

And, you're always going to get a better price from China, but good luck finding exactly what you're looking for, or negotiating the correct spec in a reasonable time frame (and also doing acceptance tests on receiving, and..).

Tim
« Last Edit: October 01, 2019, 03:37:32 pm by T3sl4co1l »
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Online MagicSmoker

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Re: Triple insulated Litz?
« Reply #2 on: October 01, 2019, 03:57:07 pm »
Hm, what's wrong with planar?

At this point I am more asking what's better with planar, as it seems to bring up a heap of problems. The issue here is that the windings can't handle the necessary power and the copper strip being used is already 2x the skin depth in thickness and the maximum allowed width once the triple-wrap of insulating tape is factored in. The only way to increase the ampacity of the windings then, is to double up on the cores and drop the switching frequency so that the same or fewer turns of thicker copper can be used. It's a rather perverse state of affairs for a state of the art planar core.

And this isn't even mentioning the cost of the core halves or the labor-intensive assembly of the strip windings... which then require a specify lacquer be used to meet UL 1446 qualification as "triple insulated" (required because there is no bobbin)... and then have to be potted in Q-Sil 553 to sidestep needing a 10mm clearance. It's all a f***ing mess, really.

Afraid I don't have any other sources.  I do recall getting jacketed litz from NEWT, although I don't know if it was specified as "reinforced" or not.  (I've seen nylon-served, tape-served, and neoprene and PFA jacketing.  If nothing else, the tape could simply be wound at a lower pitch.)

It turns out the first design used Litz from the same vendor you mentioned but it failed UL testing because said vendor hadn't properly completed the paperwork to have the wire UL Recognized... Lots of unhappy faces that day, I'm sure.

If you're getting production quantities (>10kft for ca. 18AWG stuff I think?), I'm sure you can custom order whatever you like.

Well, yes... once I order 1000kg of the stuff I can get whatever I want, but for making a functional prototype that can presented to the contract manufacturer as a standard/sample I'll need to get, say, a 10kg roll.

And, you're always going to get a better price from China, but good luck finding exactly what you're looking for, or negotiating the correct spec in a reasonable time frame (and also doing acceptance tests on receiving, and..).

The application is not cost-sensitive, per se, but the is the usual rush to get things done more quickly than is wise... you know the drill.

 

Offline T3sl4co1l

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Re: Triple insulated Litz?
« Reply #3 on: October 01, 2019, 05:14:02 pm »
Wow, that sounds all kinds of awful!

Now, that sounds like a hand-made prototype..?  Foil and tape?  Potting?

If it's made with multilayer PCBs, that should all be enclosed in PCB material, which should count as a potted or cemented joint, so no worries of creepage.  And you can always stack more layers to get more ampacity, at some expense to capacitance (which is really to say, you're simply connecting transformers in parallel, so Zo falls proportionally; design the board accordingly).  Better copper utilization should be possible by making pseudo-litz as well (say for a single-turn winding, instead of using one fat ribbon, slice it into finer traces, wired in parallel; flip the order of traces halfway through, so they have equal winding lengths and radial positions).

I do agree with the short time frame concern, though.  It seems to me, planar transformers are something you want to reserve a lot of optimization for.  Having to fab a new PCB for every test, isn't very amenable to an aggressive release cycle.

That said, speaking of rushing -- would it serve well enough to get off the ground, to say, take plain old served litz, and have someone wrap it with tape?  Who, I don't know, there's a lot of other businesses that could potentially do that.  Would take a LOT of calling around to figure out who, though...  Can be done by hand for the one-offs, but I'm guessing for early production, it's got to be automated, or purchased as-is?

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Online MagicSmoker

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Re: Triple insulated Litz?
« Reply #4 on: October 01, 2019, 07:06:39 pm »
Wow, that sounds all kinds of awful!

I am relieved to see I am not alone in this assessment.  :o

Now, that sounds like a hand-made prototype..?  Foil and tape?  Potting?

I'm waiting on getting the assembly details from the contract manufacturer because the engineer that designed it <ahem> only specified the number of turns, RMS current for each winding, frequency, core part number, and total gap. He relied heavily on the wisdom/experience of the contract manufacturer to fill in all the pesky details, including what needed to be done to get through UL. Which I am somewhat sympathetic to - it is very difficult keeping track of all the regulations and directives here - but you need to have at least some idea of what is required otherwise you might find out you only have 30mm of winding width available instead of 36mm, etc.

If it's made with multilayer PCBs, that should all be enclosed in PCB material, which should count as a potted or cemented joint, so no worries of creepage....

Nope - it's a big transformer and uses actual copper strip for the windings. Insulation is... aramid tape? And no bobbin of course - planar core - so plenty of issues with clearance and creepage to "dead metal" (ie - the core itself).

And you can always stack more layers to get more ampacity, at some expense to capacitance (which is really to say, you're simply connecting transformers in parallel, so Zo falls proportionally; design the board accordingly).

I have had terrible experiences with massive circulating currents when I've tried to parallel strip windings before to get around the width vs. thickness dilemma so I hesitate to go that route again. Also, this is a resonant mode converter so there is more need than usual to control the leakage inductance and distributed capacitance.

Better copper utilization should be possible by making pseudo-litz as well (say for a single-turn winding, instead of using one fat ribbon, slice it into finer traces, wired in parallel; flip the order of traces halfway through, so they have equal winding lengths and radial positions).

Interesting and out-of-the-box suggestion. I'll have to think on that a bit.

I do agree with the short time frame concern, though.  It seems to me, planar transformers are something you want to reserve a lot of optimization for.  Having to fab a new PCB for every test, isn't very amenable to an aggressive release cycle.

Would 2 years have been enough time to optimize things???  >:D

That said, speaking of rushing -- would it serve well enough to get off the ground, to say, take plain old served litz, and have someone wrap it with tape?  Who, I don't know, there's a lot of other businesses that could potentially do that.  Would take a LOT of calling around to figure out who, though...  Can be done by hand for the one-offs, but I'm guessing for early production, it's got to be automated, or purchased as-is?

I'm getting a crash course in just how miserable your life can become if you stray from using UL Recognized "electrical insulation systems" - that is, an approved combination of wire insulation, optional margin/layer insulation, and varnish and/or potting - but sleeving standard Litz with UL Recognized sleeving (e.g. - Ex-Flex, Silflex, Varglass, etc...) is something I've been considering. It will worsen both leakage inductance and thermal resistance, though, so it's definitely a last resort.

 

Online mzzj

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Re: Triple insulated Litz?
« Reply #5 on: October 01, 2019, 07:46:41 pm »
 

Offline T3sl4co1l

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Re: Triple insulated Litz?
« Reply #6 on: October 01, 2019, 11:01:59 pm »
I'm waiting on getting the assembly details from the contract manufacturer because the engineer that designed it <ahem> only specified the number of turns, RMS current for each winding, frequency, core part number, and total gap. He relied heavily on the wisdom/experience of the contract manufacturer to fill in all the pesky details, including what needed to be done to get through UL. Which I am somewhat sympathetic to - it is very difficult keeping track of all the regulations and directives here - but you need to have at least some idea of what is required otherwise you might find out you only have 30mm of winding width available instead of 36mm, etc.

Ah, hm... yes, can be tricky.

Reminded of the one transformer design journey I had -- an oddball transformer, knew it was going to be something like, toroidal, stripwound, GOSS, water cooling an option.  1-4kHz I think, 20kVA, modest ratio like 10 or 20:1, with a 650V primary.  Got responses ranging from no-quote to some meter-wide monstrosity to something way too small (~8" across?).

It seems magnetic design knowledge is a bit rarefied, even for those in the business of designing and producing them.  A lot of engineers do encounter it (or if not by magnetics directly, then through good old EMC, say), but it seems few have the knack for it.

We ended up going with a place I think in Connecticut, which quoted a very reasonable design, something like a 10" o.d. core, cooling pipe wrapped around it one turn, reasonable Bpk given the frequency, voltage and number of turns, and the overall result was something like 5-6" tall, 12" wide, and cost $750/ea or something like that.  Didn't have any problems with it, didn't seem to get hot in operation.  As far as I know, it's still cooking away, down in Los Alamos (*what* they're "cooking", I'll uh, leave to your imagination ;D ).

Did notice some quirks with it, which is to be expected from steel, and how we were driving it (H bridge).  It seemed to "flux walk" on its own, despite the series coupling capacitor -- it couldn't possibly be walking due to leakage, the capacitor would have to be abysmal.  Nor due to duty cycle shifting in operation, it was a digital control.  In any case, the behavior was apparent by ear -- you'd turn it on and hear a chirp as it finds its operating frequency, and then the whine intensifies as the waveform goes asymmetrical, just barely saturating to one direction.  I think there's some weird spontaneous magnetization stuff going on there.

It was history-dependent, too: once a transformer had been hard saturated (say due to startup transient), it would move more aggressively towards saturation.  Also, always the same direction towards saturation.  (Perhaps if it were hard-saturated in the opposite direction it would flip, I don't know.  Or, heck, maybe I did test that, I don't remember; this was ~7 years ago.)

We ended up with a dummy resistor across the coupling capacitor, to dampen startup transients.  The tiny bias (timing imbalance * supply voltage / dampening resistor) didn't affect the core, and a virgin transformer behaved perfectly (quiet operation, no slow walk towards angry whine).

Ah, but I digress! :)


Quote
Nope - it's a big transformer and uses actual copper strip for the windings. Insulation is... aramid tape? And no bobbin of course - planar core - so plenty of issues with clearance and creepage to "dead metal" (ie - the core itself).

Hmm, I'm thinking about a similar design for a new 10kW induction heater design; it would be 3 x E100/60/28 cores, and the windings would be water-jetted from copper sheet.  The secondary will be, basically, a stack of split washers, shaped to fit around the core of course, and spaced to fit the primary inbetween.  The secondary terminals (which have to carry 1-2kA) will be parallel copper bars with cooling pipes; the bar ends would be notched to accept the turns, which get soldered in.

I don't see any obvious reason why the current would distribute poorly; more that the current gets very crowded at the end of each turn, both because the metal is turning away from the primary (so all the current flow bunches up along the inside corner) and because the current flow is being turned, from a flat-plane geometry, to a perpendicular, parallel-plate geometry.  As long as the primary turns are the same between each layer, though, the only secondary currents that should flow are the image currents of the primary.  (The primary will have several taps for impedance matching, of course only the active turns carry current and therefore the unused part just idles with whatever residual eddy currents they end up with.)

Another way to think of it is, suppose I started with a super thick plate, and wrapped that around a core.  Then I cut slits crosswise into the bend, making a stack of plates somewhat like a dough cutter.  And into those slits, I place the primary winding.

Now, with such a construction, I do still have reasonably smooth current flow, in that it goes from the bar straight into the stack-of-plates section.  But in a PCB construction, it may be exacerbated.

In a PCB, you usually make the connections with thru-vias/pads, likely reinforced with solid wire, but still the current crowds up near the connection point, which is off to one side.  And if the pads are surrounded by other rigid connections, the current may be forced to flow over each layer, crowding up multiple times, before reaching the circuit.

That could make a difference in EMF as well, as there's more voltage drop (some resistive, hence the power, but mostly inductive) to one side of the stack-of-plates, and there's your eddy currents or however you might like to phrase it.  (Really, eddy currents, proximity effect, current crowding, skin effect and etc. all become somewhat ambiguous or equivalent in a complicated structure like this.)  In that case, yeah, the only thing you can do is keep the stack low, and make it wider and wider.

Which isn't that bad an idea, just using more transformers overall, in parallel.  Depends.  It doesn't scale well, is probably the biggest problem.  Different mounting methods could be handy (planar transformer sticks up vertically from the backplane; a bunch sit in rows, all tied together?).

Or taking that further and using multiple converters in parallel, which can be advantageous (lower input/output ripple?) but also very annoying (N times more components, assemblies..).


Quote
I have had terrible experiences with massive circulating currents when I've tried to parallel strip windings before to get around the width vs. thickness dilemma so I hesitate to go that route again. Also, this is a resonant mode converter so there is more need than usual to control the leakage inductance and distributed capacitance.

(Which I've addressed above, but didn't want to interrupt the segue with this quote. :) )

Ah yes, leakage too; although, if leakage is a useful part of the circuit, you can at least handle some, which gives a lot less pressure towards super-low-Z planar designs, and gives you a lot more leeway to use magnetically-spongy litz cable. :)

You might still be limited to less leakage than a single layer of full-size litz offers, but you could kinda hybridize by using multifilar litz, or multiple windings or layers in parallel.

Also, exiting the bobbin of a more conventional windup ("shell" style, your average ER and such design) is a big pain with wide conductors, foil especially.  But, eh, that just happens.  The bigger pain I guess is going to be ensuring adequate insulation along its path -- which further reduces the available winding area.  (Hm, the winding factor of PCB material doesn't look bad at all when you need big currents like that.)

Or if you were thinking about litz for everything, then, that too; amps are amps, you can just collect cables a bit better than you can fold up a hunk of metal is all.

Oh -- one thing that may be helpful: these guys make a (patented??) flat litz of sorts, that's apparently as good as foil, without the eddy currents / proximity effect.
https://www.wcmagnetics.com/
Got prototypes from them once; didn't move into production, but it seems like they knew their stuff. :-+


Quote
Would 2 years have been enough time to optimize things???  >:D

Heh, well, with competent engineers, perhaps even less than that?  But, therein lies the problem, and as I alluded to before, magnetism seems to be enigmatic to many.


Quote
I'm getting a crash course in just how miserable your life can become if you stray from using UL Recognized "electrical insulation systems" - that is, an approved combination of wire insulation, optional margin/layer insulation, and varnish and/or potting - but sleeving standard Litz with UL Recognized sleeving (e.g. - Ex-Flex, Silflex, Varglass, etc...) is something I've been considering. It will worsen both leakage inductance and thermal resistance, though, so it's definitely a last resort.

Yeah, I can see that being a big problem... if it's not perfectly to spec, it's basically something like: "*throw hands up*, well, you've got to subject it to this and that and this other test, plus regular inspection / retests, and..."

On the upside, fiberglass sleeving should fill nicely with varnish, for certain values of sleeving.  If that's acceptable as a "cemented joint", that should do.  Or failing that, potting.  Hm, maybe a partial-discharge test, to verify no voids..?  (I wonder where those procedures sit, on the fence between UR and "whelp".)

Tim
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Online MagicSmoker

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Re: Triple insulated Litz?
« Reply #7 on: October 02, 2019, 12:27:03 pm »
We're sorta veering off topic, but I don't mind as long as the discussion makes my brain smoke/hurt a bit. I'm digging up more information even as I sink deeper into the quagmire but it looks like NEWT got their paperwork completed on the particular wire my client previously ordered from them and, yes, they bought a 10kft run of the stuff. A pity because while it is the right effective AWG size for one of the windings on the transformer I am redesigning, it doesn't use the right strand size for the frequency (Rac is ~2.9x Rdc). And it's being used for a choke, anyway. Still, I might tell the client to send me a few meters and build one prototype with it.


It seems magnetic design knowledge is a bit rarefied, even for those in the business of designing and producing them.  A lot of engineers do encounter it (or if not by magnetics directly, then through good old EMC, say), but it seems few have the knack for it.

Probably because magnetics is the wild West of engineering: wacky units and scaling factors (Gauss vs. Tesla; 2 vs. 4 vs. 4.44 in denominators, 10 vs. 100 vs. 10000 in the numerators), unclear and overlapping definitions (delta_B? Bpk? Bpp? Bac? WTF?), often comically inaccurate approximations (Steinmetz, anyone?), some of the more difficult concepts to conceptualize (proximity effect, fringing effect and Dowell's transformations), all hamstrung by a mess of safety regulations that could bring a medical device manufacturer to their knees.


Did notice some quirks with it, which is to be expected from steel, and how we were driving it (H bridge).  It seemed to "flux walk" on its own, despite the series coupling capacitor...
It was history-dependent, too: once a transformer had been hard saturated (say due to startup transient), it would move more aggressively towards saturation.  Also, always the same direction towards saturation.
...
We ended up with a dummy resistor across the coupling capacitor, to dampen startup transients.  The tiny bias (timing imbalance * supply voltage / dampening resistor) didn't affect the core, and a virgin transformer behaved perfectly (quiet operation, no slow walk towards angry whine).

Fascinating stuff. I love these anecdotes of the weird and perplexing. Current mode control *without* a coupling capacitor usually does the trick in situations like this, but another trick that especially helps with silicon steel - even moreso with the grain-oriented stuff - is to always use a tiny gap. This drops the remanance flux to near zero and softens saturation ever so slightly, at a corresponding small cost in magnetizing inductance. Adding a gap to a strip wound toroid isn't exactly practical, though...


I don't see any obvious reason why the current would distribute poorly;

Circulating current between nominally similar paralleled but insulated windings is primarily the result of length mismatch, and to a greater or lesser extent from differences in flux coupling and the leakage inductance of each winding (which itself is dependent on the previous two variables). And if there is an air gap - as is the case here - then you have to deal with fringing flux affecting the coupling turn-to-turn as well. As is so often the case in magnetics, making the transformer "better" in some respects (ie - reducing leakage inductance or winding resistance) increases the circulating current if there are independent paralleled turns. In fact, you can even get circulating current between the strands in stranded wire unless it is woven like Litz (that being the point of Litz, after all). EDIT - more specifically, to make sure every wire in a bundle sees the same flux.


Which isn't that bad an idea, just using more transformers overall, in parallel....

This is the first option I suggested to the client, and still one under consideration, it just requires the most mechanical and board changes hence isn't the leading contender. I suspect this might be the way to go in the 3rd or later hardware revision. For the 2nd revision, though, a single transformer is the best solution as it won't require any changes to the power stage (except for some component value tweaking).


Ah yes, leakage too; although, if leakage is a useful part of the circuit, you can at least handle some, which gives a lot less pressure towards super-low-Z planar designs, and gives you a lot more leeway to use magnetically-spongy litz cable. :)

You just managed to stumble upon one of my pet peeves with glorious resonant mode converters that "automatically" incorporate leakage as a useful circuit element rather than a parasitic to be despised and minimized because it turns out There Ain't No Such Thing As A Free Lunch still applies: more leakage means a higher surge impedance which reduces maximum power throughput, and requires a smaller resonant capacitor value to have the same f_res which means a higher AC voltage drop in normal operation, so less throughput again (this assumes a series resonant converter). So it turns out that minimizing leakage is still a desirable objective in resonant converters... go figure.


You might still be limited to less leakage than a single layer of full-size litz offers, but you could kinda hybridize by using multifilar litz, or multiple windings or layers in parallel.

This is also an option I am considering... it will depend on lead time, mostly, and cost secondarily. It would likely be less expensive to use all #12 AWG equivalent Litz with multiple wires allotted to each winding in inverse proportion to turns ratio, and it might make better use of the winding area. If I have to order 10kft of the stuff (I'm surprised/not-surprised that unit is still being used) then this will almost certainly be the way to go.


Oh -- one thing that may be helpful: these guys make a (patented??) flat litz of sorts, that's apparently as good as foil, without the eddy currents / proximity effect.
https://www.wcmagnetics.com/
Got prototypes from them once; didn't move into production, but it seems like they knew their stuff. :-+

West Coast Magnetics is one of my favorite vendors for larger transformers and chokes while I like ICE Components for smaller magnetics like gate driver transformers and such.


Yeah, I can see that being a big problem... if it's not perfectly to spec, it's basically something like: "*throw hands up*, well, you've got to subject it to this and that and this other test, plus regular inspection / retests, and..."

It's even worse than that... if you don't use a specific combination of UL Recognized components for a given insulation system then you have to prove your combination meets the standard via accelerated life tests, where "accelerated" means spending weeks to months in a thermal chamber at some percentage above rated voltage and current. This is so expensive as to not even be remotely practical unless you plan on making hundreds of thousands of units and there is a competitive advantage to the new combination.


On the upside, fiberglass sleeving should fill nicely with varnish, for certain values of sleeving.  If that's acceptable as a "cemented joint", that should do.  Or failing that, potting.  Hm, maybe a partial-discharge test, to verify no voids..?  (I wonder where those procedures sit, on the fence between UR and "whelp".)

Approved sleeving can be used as part of an insulation system, and it is definitely an option, but it significantly worsens the thermal performance so is a last resort. Potting is also acceptable - and sometimes required - as part of an insulation system, and usually voids must be eliminated by design or procedure (that is is, the core is oriented so there won't be any trapped air pockets when pouring the potting compound or else pulling a vacuum on it after pouring to eliminate such air pockets).

It's a lot to deal with and so many ways to get it right... or wrong. Hence why I told the client that spitting out a number of different winding a core configurations wouldn't take much time at all, but ensuring any one of them could actually be built in a reasonable time and get through UL was a whole 'nother matter altogether. Fortunately, the 2+ years of previous struggle has illustrated this point quite well.  :scared:
« Last Edit: October 02, 2019, 03:38:35 pm by MagicSmoker »
 

Offline f4eru

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Re: Triple insulated Litz?
« Reply #8 on: October 03, 2019, 11:34:03 pm »
UL is a real pain in north america, especially when dealing with out of the ordinary insulation concepts.

I worked on some HV stuff, and for some products, we just skipped UL altogether and used more reasonable labs to certify, where you can prove with only half the expense that your product is safe also in USA.

Yeah, sometimes you had to explain to US customers that this is fully approved and legal in US, and that other NRTLs exist since over 30 Years, but it was worth that smaller hassle.
« Last Edit: October 03, 2019, 11:40:01 pm by f4eru »
 

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Re: Triple insulated Litz?
« Reply #9 on: October 05, 2019, 10:40:32 am »
So I heard back from New England Wire about this NE-F1 stuff and it turns out it is, indeed, an approval specific to them, and it is, indeed, for the Litz equivalent of triple-insulated magnet wire. Basically, a transformer or inductor made with NE-F1 rated wire can pass UL testing without needing margin tape, potting, a coil former/bobbin, etc. It's available with different overall thickness of outer insulation that are approved for 600Vpk, 1000Vpk and 1500Vpk, which pretty much covers any "low voltage" mains application (ie - less than 600VAC RMS). It is no overstatement to say this will make building UL-approved high frequency SMPS magnetics much, much easier, and likely less expensive overall (sure, potting and layer tape doesn't cost much from a materials standpoint, but from a labor standpoint... that's another kettle o' fish altogether).

Minimum order quantity is 1kft, which isn't exactly prototype friendly, but it's still a lot more palatable than having to buy 10kft at a time. Everything is custom - not unexpected for Litz - and lead time is 4-6 weeks, but sometimes they have overruns from previous orders that you can buy. In those cases the minimum order is $500 (also not unusual for a business that isn't the least bit set up to do retail sales). So I think my plan as of now is to order some infamous ebay Litz to build a rapid prototype and if it works reasonably well in the existing converter design then order the good stuff from NEWT.
 

Offline T3sl4co1l

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Re: Triple insulated Litz?
« Reply #10 on: October 05, 2019, 04:25:10 pm »
Oh cool :-+

It's a shame they don't have samples available, or if they do I guess they're only a few feet at a time.  The one time I met with a rep, they brought in several very interesting things (one being a rectangular formed piece), but I also wonder if they were simply random offcuts from the most interesting jobs they ran in the last however many years, and the reps have about as much selection of their samples, as any customer does... :-DD

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Offline JohnG

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Re: Triple insulated Litz?
« Reply #11 on: October 05, 2019, 06:23:01 pm »
Litz and planar winding have their own advantages and disadvantages. Planar windings can give you the lowest leakage inductance, but really only if you interleave primary and secondary. Otherwise, the leakage will be dominated by the spacing between primary and secondary windings. Planar transformers tend also to suffer a lot from stray capacitance due to the structure. At lower voltages and high currents, insulation requirements aren't always as stringent and capacitance effects not as important, so planar makes a lot of sense.

At high voltages, insulation requirements sometimes make interleaving primary and secondary a problem because the stackup becomes unmanageable (i.e. too thick), and the capacitance can create huge headaches, so planar windings make less sense.

If you are concerned about winding losses magnetics, it might be worth a look at Charlie Sullavin's site: http://power.thayer.dartmouth.edu/index.shtml. There are some handy calculators and more info than most mortals can use. However, here is one worht considering for your particular problem: http://power.thayer.dartmouth.edu/papers/stranded.pdf. YMMV, as always.

Cheers,
John
 
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Online mzzj

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Re: Triple insulated Litz?
« Reply #12 on: October 05, 2019, 10:11:00 pm »

If you are concerned about winding losses magnetics, it might be worth a look at Charlie Sullavin's site: http://power.thayer.dartmouth.edu/index.shtml. There are some handy calculators and more info than most mortals can use. However, here is one worht considering for your particular problem: http://power.thayer.dartmouth.edu/papers/stranded.pdf. YMMV, as always.

Cheers,
John
Interesting read. Never crossed my mind but kinda obvious after reading the paper.
 

Online MagicSmoker

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Re: Triple insulated Litz?
« Reply #13 on: October 06, 2019, 12:06:34 pm »
Litz and planar winding have their own advantages and disadvantages.

Err... Yes, I know all that. The customer has a (mostly) working planar design that they are coming to the realization is not remotely cost-effective, especially since it had to be derated in power throughput because of thermal issues (all of which stem from choosing too small a planar core in the first place, both in core area and window width). I can't be more specific because of an NDA, and the purpose of this thread was really to ask about the almost-mythical sounding "triple-insulated" Litz that has only recently become available and whether it can actually get you through UL without needing all the normal gobbledygook such as margin/layer tape, etc. In other words, the same benefits which accrue from using triple-insulated magnet wire, except for Litz, and without building the Litz out of triple-insulated magnet wire strands, as that would incur too large a penalty in percent area taken up by insulation - far better to use single-build strands and wrap the entire bundle in multiple insulation layers, which is precisely what NEWT is doing with their Class NE-F1 stuff.

Planar transformers tend also to suffer a lot from stray capacitance due to the structure.

Yep, especially when you alternate between primary and secondary every turn to minimize both leakage and proximity effect. And I should note that they haven't even taken the current design through EMC testing, where the no-doubt prodigious amount of interwinding capacitance will rear it's ugly head. In fact, I rather suspect that EMC testing is going to be just as much of a shocker as the prolonged ordeal of getting through safety testing was (which apparently took 6+ months of battling UL - again, one of those hidden prices you pay for not using UL-approved wire in the first place).

At lower voltages and high currents, insulation requirements aren't always as stringent and capacitance effects not as important, so planar makes a lot of sense.

I would also add that you need to be constrained in height (or total volume, maybe) before planar should be considered. In this case, the design team set just such a restriction upon themselves despite that it was not remotely necessary. In fact, I would go so far as to say that if you can't use a PCB-based winding structure then you probably shouldn't even consider a planar transformer at all; they are just too expensive - like 20x - 50x more expensive in this case! - than a conventional design.

If you are concerned about winding losses magnetics, it might be worth a look at Charlie Sullavin's site...

Yep, I've had that site bookmarked for years. I was already familiar with the use of ordinary stranded wire as "poor man's Litz" (not that it could be used here - safety certification to 155C needed) but there was another IEEE paper on the site I hadn't noticed before but which is proving to be immensely helpful right now: "Cost-Constrained Selection of Strand Diameter and Number in a Litz-Wire Transformer Winding" (http://power.thayer.dartmouth.edu/papers/litzcj.pdf).

EDIT - grammar
« Last Edit: October 06, 2019, 12:09:33 pm by MagicSmoker »
 

Online MagicSmoker

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Re: Triple insulated Litz?
« Reply #14 on: October 06, 2019, 06:56:51 pm »
...another IEEE paper on the site I hadn't noticed before but which is proving to be immensely helpful right now: "Cost-Constrained Selection of Strand Diameter and Number in a Litz-Wire Transformer Winding" (http://power.thayer.dartmouth.edu/papers/litzcj.pdf).

Grrr... the most useful equation in the above paper finds the ratio of AC resistance to DC (Fr) for a given Litz and winding configuration but many of the units aren't defined (a serious problem in magnetics in which multiple systems of units are used) and, worse, the example provided doesn't give sensible values for the not-sure-of parameters when the known parameters are plugged back into the equation. To wit:

               pi2 * w2 * uo2 * N2 * n2 * ds6
Fr = 1 + ------------------------------------
                           768 * p2 * bw2

Where the units and the values from the example are:
pi squared is 9.8696
w is frequency in radians, also squared (e.g. - (2 * pi * 150000)2)
uo usually refers to the permeability of air, 4pi * 10-7, so this should be 1.579 * 10-12
N is the number of turns (total, 60, or per winding, 30?)
n is the number of strands in the Litz wire (e.g. - 1100)
ds is the diameter of the strand copper (units undefined, presumably meters; e.g. - 7.874 * 10-5 for #40 AWG, or 2.383 * 10-25 when... hexed?  >:D)
p is the resistivity of copper (units undefined, but in Ohm-meters at 25C it is 1.73 * 10-8, or 2.993 * 10-16 when squared)
bw is the winding breadth (units undefined, presumably meters; e.g. - 0.0446m, or 1.989* 10-3 when squared)

And when I plug all that in I get 3.593 * 10-15 for the numerator and 4.572 * 10-16 for the denominator which... is very different from what I got the first two times I ran through this equation (just like the old saw: I calculated the numbers 3 times and here are the 3 answers, boss).

So, the answer I get now for Fr is 8.85 which is close enough since I guessed what temperature to use for the resistivity of copper. Well, I'll leave this here in case anyone else finds it useful (and because it took a long time to type/format everything).

I'm putting this equation into my handy quick electronics calculations spreadsheet and will edit this post when I have an estimate of Fr for my initial choice of Litz... I can already tell I'm going to be unpleasantly surprised, though.

EDIT - well, as predicted, things would have been downright ugly if I pulled the trigger on the easily available #38 AWG strand size Litz; the AC resistance for my primary winding would be 17x higher...  :P  Going with the strand size I figured I'd need, #42, drops Fr down to 3.7 which is eminently tolerable (don't believe all the SMPS books that say you need to aim for an Fr of 1.5 - that rule quit working when 50kHz stopped being a "state of the art" switching frequency).

« Last Edit: October 06, 2019, 07:42:51 pm by MagicSmoker »
 

Offline TimNJ

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Re: Triple insulated Litz?
« Reply #15 on: October 22, 2019, 04:53:18 am »
If you're still looking for suggestions from the peanut gallery...

From my experience, you still need to be using a recognized UL approved "insulation system" if you are planning on using any other materials in the construction of the transformer (core not included). While you can forgo the margin tape for sure, are you sure you can make the transformer without anything else?

For my job, we work with a vendor in China who "owns" (i.e. purchases) an insulation system such as this one: https://iq.ul.com/systems/List.aspx?%ULID=103091937

A transformer can only be UL approved if all of the applicable materials can be found on this list. UL also audits their facility every year to make sure they are doing everything right..

There are lots of TIW litz manufacturers:

Totoku (Japan)
E&B Tech (China)
Great Leoflon (China)
Hoi Luen (China)
KBI Cosmolink (Korea)

Some of their websites are a little shoddy (and they barely mention that they make TIW litz, but they do). I would reach out to them and see if they'll send you samples. I'd expect it will be less expensive than NE wire.
 
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Online MagicSmoker

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Re: Triple insulated Litz?
« Reply #16 on: October 22, 2019, 11:51:48 am »
If you're still looking for suggestions from the peanut gallery...

Always! I was mainly designing on-vehicle motor drives from 2008 - 2017 which have very different safety (and EMC) requirements. Basically, I was out of the regulatory loop for that entire time and getting up to speed on all that things that have changed since then has proven to be exactly as painful as I predicted.

From my experience, you still need to be using a recognized UL approved "insulation system" if you are planning on using any other materials in the construction of the transformer (core not included). While you can forgo the margin tape for sure, are you sure you can make the transformer without anything else?

For my job, we work with a vendor in China who "owns" (i.e. purchases) an insulation system such as this one: https://iq.ul.com/systems/List.aspx?%ULID=103091937

Right, so if you compare you vendor's UL IQ page to the NE-F1 page - http://iq.ul.com/systems/List.aspx?%25ULID=103091589 - it shows that both types of wire do not require any additional insulation between layers, other windings or dead metal. Nor are bobbins, varnish, potting, sleeving, etc. required. This is basically the same rule that applied to toroids that were coated with an approved insulation like epoxy or parylene as long as wire that was either triple-insulated enamel or teflon/kynar insulated, or sleeved with teflon tubing (et al.) was used. At least for "low voltage" (ie - under 1000Vpk).

A transformer can only be UL approved if all of the applicable materials can be found on this list. UL also audits their facility every year to make sure they are doing everything right..

Yep, and thanks for the suggestions of the other triple-insulated litz manufacturers. I will definitely be inquiring with each of them!
 

Offline TimNJ

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Re: Triple insulated Litz?
« Reply #17 on: October 22, 2019, 01:11:29 pm »
Right, so if you compare you vendor's UL IQ page to the NE-F1 page - http://iq.ul.com/systems/List.aspx?%25ULID=103091589 - it shows that both types of wire do not require any additional insulation between layers, other windings or dead metal. Nor are bobbins, varnish, potting, sleeving, etc. required. This is basically the same rule that applied to toroids that were coated with an approved insulation like epoxy or parylene as long as wire that was either triple-insulated enamel or teflon/kynar insulated, or sleeved with teflon tubing (et al.) was used. At least for "low voltage" (ie - under 1000Vpk).

I understand what you are saying, but from my perspective, the UL IQ page is just noting that the below type of wire has reinforced insulation and that it does not explicitly require additional measures to meet global safety requirements. However, I think (big emphasis on I think), this still requires that the other materials in the transformer are on the insulation system list.

The rationale is: All of the materials on the approved list have been tested rigorously (with each other) to ensure that there will be no chemical/physical interaction that will cause material to break down and cause a safety hazard. So, even if TIW litz doesn't require additional insulation, if you use some sort of tape to secure the winding, that tape needs to be on the list.

In addition, you might want to check how your company deals with UL certification for something like this. In my three years in the power electronics field so far, you have to actually purchase and own the UL insulation system to be able to sell product using that system. I'm not sure if you can just say "hey look, we made sure all the materials we used were on this list". Many times, it's more cost effective to use a magnetics supplier who already owns a bunch of systems and has all the other quality systems in place. At least, this is what I know from the world of relatively low power SMPS.


 

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Re: Triple insulated Litz?
« Reply #18 on: October 22, 2019, 02:11:03 pm »
I understand what you are saying, but from my perspective, the UL IQ page is just noting that the below type of wire has reinforced insulation and that it does not explicitly require additional measures to meet global safety requirements. However, I think (big emphasis on I think), this still requires that the other materials in the transformer are on the insulation system list.

No disagreement there. I was only saying that you didn't need the other things to comply. If you need the other things - bobbing, tape, tie-down cord, etc. - for other (ie-mechanical) reasons then you have to use materials on the approved insulation system list for the wire concerned.

In addition, you might want to check how your company deals with UL certification for something like this. In my three years in the power electronics field so far, you have to actually purchase and own the UL insulation system to be able to sell product using that system. I'm not sure if you can just say "hey look, we made sure all the materials we used were on this list". Many times, it's more cost effective to use a magnetics supplier who already owns a bunch of systems and has all the other quality systems in place. At least, this is what I know from the world of relatively low power SMPS.

Agreed again, and the goal here is to come up with an alternative design for a transformer that can then be manufactured by a magnetic component manufacturer. However, my approach is to design the magnetic component as fully as possible with all the real-world considerations like insulation taking up valuable bobbin area, etc., so you don't get any thermal (or UL safety testing) surprises *after* spending lots of time and money getting a prototype made.

Oh, and you can pick from a smorgasbord of different materials and put a component through UL testing, it just takes a lot longer and costs way more.  >:D
 

Offline TimNJ

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Re: Triple insulated Litz?
« Reply #19 on: October 22, 2019, 03:33:09 pm »
I understand what you are saying, but from my perspective, the UL IQ page is just noting that the below type of wire has reinforced insulation and that it does not explicitly require additional measures to meet global safety requirements. However, I think (big emphasis on I think), this still requires that the other materials in the transformer are on the insulation system list.

No disagreement there. I was only saying that you didn't need the other things to comply. If you need the other things - bobbing, tape, tie-down cord, etc. - for other (ie-mechanical) reasons then you have to use materials on the approved insulation system list for the wire concerned.

In addition, you might want to check how your company deals with UL certification for something like this. In my three years in the power electronics field so far, you have to actually purchase and own the UL insulation system to be able to sell product using that system. I'm not sure if you can just say "hey look, we made sure all the materials we used were on this list". Many times, it's more cost effective to use a magnetics supplier who already owns a bunch of systems and has all the other quality systems in place. At least, this is what I know from the world of relatively low power SMPS.

Agreed again, and the goal here is to come up with an alternative design for a transformer that can then be manufactured by a magnetic component manufacturer. However, my approach is to design the magnetic component as fully as possible with all the real-world considerations like insulation taking up valuable bobbin area, etc., so you don't get any thermal (or UL safety testing) surprises *after* spending lots of time and money getting a prototype made.

Sounds like you understand this stuff just as well, if not better than I do! Good luck.

Oh, and you can pick from a smorgasbord of different materials and put a component through UL testing, it just takes a lot longer and costs way more.  >:D

To this point, I think this is why the insulation system concept exists. If everyone had to validate material compatibility for every transformer ever made, it would be enormously expensive and tedious.
 


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