Winding a PFC inductor coil too close to fringing fields from a core gap? - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 18, 2020, 03:59:23 pm
content: Do you know why Power Integrations are making no recommendations to move the coils away from the 1.3mm gap of their 300W PFC Ferrite inductor?The fringing fields from this gap will surely fry the coil?DER484 (below) on page 18, shows this PFC inductor to comprise a TDK PQ3230 Ferrite core gapped to AL = 145. The MDT design software tool shows that this means a 1.3mm gap in the centre leg, giving it an inductance of 221uH.Pages 19-21 of DER484 clearly show the winding of this inductor, with coils being layed down right over the centre leg gap! Are they trying to fry the coil?Page 66 of DER484 shows the high ripple current in this inductor…at 115VAC the ripple is showing a peak-to-peak value of some 4A almost right across the 10ms half sine. This represents a delta B of 177mT. Surely with the 1.3mm centre leg gap, this would cook the coil?The coil actually uses 60*#38AWG “Served Litz Wire” which appears to be made by mwswire.com. It makes you wonder if this 60 strand Litz wire reduces eddy currents such that the coil’s close proximity to the centre leg gap doesn’t matter?DER484https://ac-dc.power.com/sites/default/files/PDFFiles/der484.pdfInterestingly, the PFS7528H (PFC controller) datasheet discusses PFC “inductor design” features on page 13, and mentions skin effect, core loss, proximity loss, saturation, but makes no mention whatsoever about coil heating due to fringing fields in the vicinity of a core gap. Do you know why not?PFS7528H datasheethttps://ac-dc.power.com/sites/default/files/product-docs/hiperpfs-3_family_datasheet.pdf

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 18, 2020, 04:23:27 pm
content: If FEM is not available, the selection of the copper strand diameter in litz can be estimated.Firstly, estimate the penetration depth of the current in one copper strand.Delta  = 500 * sqrt( rho /frequency) rho : resistivity of copper , approx 22 e-9 Ohm.meter at 100 Celsius., frequency in HzThen the strand diameter [ metre] of the litz should be less than 2 times Delta.That means, the "electrical size" of the strands should be less than 2.Another comment is that the conductor heating near the gap is greater in a single winding inductor.It is relatively less in say, a double wound transformer-reactor with a gap.Simplistic radial flux distribution plots will show the effect, bu the accurate way is a FEM analysis.I am going to model some pcb planar transformers and inductors using QuickField later.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 18, 2020, 09:37:20 pm
content: Or a few times less than delta, near the gap. The normal skin effect formula is for current flow along an infinite plane; proximity effect (wires carrying current in proximity), and the very inhomogeneous field near the gap, require the wire size to be dropped by several factors. (Note this is true of the strands in a litz cable already, and true again inside a multilayer inductor.)I don't think they went to that degree; probably, it gets toasty in the middle, just not enough to mind. Impregnation with a nice (thermally) conductive varnish might be enough.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 19, 2020, 12:08:48 am
content: Hi Tim,Yes (considering a vertical core) there is a X and Y component of H above and below the gap.While they are both orthogonal to the axis of the strands H_rms is heating the cylindrical Litz strand more by transverse flux.The Bessel type solution does not apply, but for transverse flux heating, it is still inefficient if electrical size is around 2.0.In both the Bessel and the transverse solutions, the heating drops of steeply below electrical size of 2Actually Bessel roll-off is about 2.9 and transverse is about 1.9, so 2 is just an easy number to remember.The "Delta" function I gave above is same in both cases. Edit **It would be difficult to model Litz in FEM beacuse the mesh needs to be smaller than the diameter.So a practical model might be a little chunk of litz near to a gap, compared to say a solid copper conductor.Edit **And should add that the if the delta function is used for the bulk heating of ferrite or steel laminations, the full form of this function should be used:Delta = 500 * sqrt( rho/ (Ur*frequency) ) where Ur  is the relative permability of the conductor in the H field.For copper and aluminum, Ur =1

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2020, 01:08:14 am
content: Yeah, to be specific we're using wire, for which an infinitely long cylinder has skin effect according to Bessel functions rather than exponential, which gives a slightly different delta figure (and uneven in terms of delta/d, because the Bessel function has a decaying oscillatory form, not an exponential cutoff). I didn't note this, because the factor from this is small (<50% I think?).It's the hard geometry stuff (proximity etc.) that's far more significant. Proximity is roughly scaled by the number of layers; and fringing flux can be a factor of 5 or more* on top of that.*But only in the local area, not overall. So, maybe a situation is acceptable for overall losses -- but fails on peak wire temp.Yeah, FEM usually uses an approximation to litz -- assuming anisotropic current flow or something like that. Which might capture the general behavior, but it doesn't say anything about the actual losses in such a material. It might be modeled behaviorally rather than using a physical model, and then you have to trust (or preferably, have some way to verify) that the model is accurate for the conditions used.Regarding your planar sims -- when foil thickness is comparable to or less than delta, field can at least partially penetrate it, which is to say, current can have some chance at distributing across the width. That's nice, but it leads to another effect which tends to dominate: all the currents, summed up starting from the inside edge, again act to shield the middle of the conductor. You have skin effect as usual, but it's two-dimensional, where the conductor resistivity and thickness (resistance/square) equates to a... not a skin depth, that implies depth inside a 3D volume -- more like a line depth inside a 2D boundary. Edge effect?So, point is, when you have currents concentrated in this way, it's not so much thickness as trace width that matters, and there's a characteristic length corresponding to it, all the same.So it makes planar inductors somewhat shite, the edge effect plus proximity effect squeezes the inner turns, intensely. Planar transformers -- where the image current is plane to plane -- can do a bit better, so you'll tend to prefer forward over flyback in planar designs, I think. (Where the traces enter and exit the winding, or where turns cross over other turns unevenly, expect higher intensity by those edges.) On the upside, planar windings can have all the distance from airgap they need.So, expect to find results in line with this. Will be nice to get real numbers from a simulation.You can even make 2D litz, or... try, anyway, though maybe it tends not to be worthwhile due to the complexity of wrangling all those layers, as you can't simply weave PCB traces, you need a free layer or two, and vias between them.Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 19, 2020, 02:01:18 am
content: Hi Tim,Yes the wanted current in the turns of the inductor causes Joule losses defined by Bessel.The purpose of using Litz is to reduce those losses, to be for example, close to I ^2 * RdcThe title of this thread is related to the extra losses where the turns are being heated by transverse flux (TFX)near to an airgap in the core.It is apparent o/p opampsmoker is aware of this. And I surely know this is a real problem from 48 years in the game!Litz also reduces TXF losses near the airgap. As you note, FEM can only do an approximation.It is interesting to me that power electronic threads on here usually show pure inductance in the models.That is , all losses including the bessel defined, and the airgap defined losses...are ignored... ?I saw at least 3 such models over past 2 days including an answer to my query about an open loop transient response.I am wondering if this is normal among power electronics engineers in general - to neglect inductor losses?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2020, 05:12:28 am
content: Well, simple enough -- losses are messy to calculate, and the effort required is typically less than building and testing a part, so who cares. And for power switching circuits, it's typically enough to assume that an inductive current is present, not that its phase angle be precisely such-and-such.Alternately, losses can be quantified by Q curve, given the user applies them correctly -- Q varies with frequency so needs to be applied via Parseval's theorem or the like. Also depends on amplitude (specifically core loss), which isn't well appreciated or specified I think, but tends to be close enough to linear that we don't run into too much trouble assuming it is.And when detailed modeling is needed, it can be calculated via FEM, or measured and approximated behaviorally. Typical good inductor SPICE models (linearized, AC steady state or transient) use a diffusion element, or something approximating that, to account for skin effect and core losses.FEM can even include nonlinear transient response of saturable materials -- I've done that before, you can get some very interesting things at  fluxes where T --> Tc and B ~ Bsat, like greater heating (in bulk steel) under the surface. Albeit at some length to get there, as you have to draw the geometry, and (usually) buy the sim tools, etc. Good ones aren't cheap, QF being one of the more affordable options.I've created nonlinear transient models of ferrite beads before, e.g.https://www.seventransistorlabs.com/Modeling/SPICE/HI0603P600R_NL.cktThe parameters I think aren't quite right for the part, but the method seems promising, and the curves aren't too far off thepublished curves(compare model (thin) to datasheet (blurred)).Tim

author: trobbins
author_rank: Frequent Contributor
timestamp: December 19, 2020, 06:28:04 am
content: Even with sim tools, I'd expect the only way to confirm what the consequence of winding layout/location/layers/litz... etc is to prepare a few samples that measure the same inductance, and test them in a converter that maintains the same operating conditions, and compare the power loss from input to output with high enough accuracy such that the difference in power loss is due to the inductor itself and is quantitively measurable. That was the technique used 35 years ago for class E inductor optimisation when comparing ferrite cores and padding height.