Boost PFC inductor is going to overheat? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 06, 2022, 11:40:03 pm
content: The below App note shows a 2.5kW Boost PFC from 230VAC in.At 65kHz. Inductor is two stacked 77083A7 torroids wound with 64 turns of 1.2mm enamelled copper wire.App note 2.5kW PFChttps://www.infineon.com/dgdl/Infineon-ApplicationNote_EVAL_2.5KW_CCM_4PIN-ApplicationNotes-v01_00-EN.pdf?fileId=5546d4624fb7fef2014fd6508161625777083A7 torroidhttps://www.mag-inc.com/Media/Magnetics/Datasheets/0077083A7.pdfThe App note quotes the zero current inductance as some 600uH. This is correct, but at the peak inductor current of some 17A, the inductor is only 160uH, and so there is a good 6Apkpk ripple in it at mains peak.So why have they wound it with a single strand of 1.2mm diam enamelled copper wire?…..that level of current ripple surely requires Litz?The DC losses alone of that 64 turns would be 10.6W. This is way too much. It would be far more when the 65kHz ripple is factored in.Do you agree this inductor is not practical?..do you agree this app note is in error?Another point, as can be seen, is that the 77083A7 torroid is only spec'd up to an Ampere.Turns of 800.......whereas the above app note has at least 17Apk and 64 turns...thats 1100 Ampere turns....this is way way outside of the datasheet's allowed maximum value for N.I. of 800....Would you agree that this App Note should be closed down?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 07, 2022, 12:27:27 am
content: Hi,it should be noted that Infineon has two fans on the evaluation board:The other thing to look at is the total power lost in the circuit. This can be calculated from the efficiency numbers provided by Infineon:At 85 VACPower out = 1197.6W and efficiency = 93.57% so Power In = 1197.6/0.9357 = 1279.9WPower losses= 1279.9 - 1197.6 =82.3WAt 230 VacPower out = 2500.9 W and efficiency = 97.63% so Power In = 2500.9/0.9763 = 2561.6WPower losses= 2561.6 - 2500.9 =60.7WThis is why they need the fans.Looks okay to me.Jay_Diddy_B

author: uer166
author_rank: Super Contributor
timestamp: April 07, 2022, 02:50:51 am
content: I'm surprised they were able to achieve >98% with plain Kool Mu, that's nothing to sneeze at. I bet with some Litz, Kool Mu Hf, MPP, or plain old ferrite, they can push a couple extra 0.1%. 60W with that size fans and heat sinks seems okay, as JDB said.. Maybe not at 70C ambient, but for normal use.

author: Faringdon
author_rank: Super Contributor
timestamp: April 07, 2022, 08:07:06 am
content: Thanks, yes, even for a 275W PFC inductor, it would be expected to use Litz....as on page 22 of the following.....https://www.power.com/sites/default/files/documents/der547.pdf....40 strands of #38 Litz...and thats for 10 times less power than the infineon app note.The use of enamelled copper wire has been gotten away with by the Infineon app note because they have only run it in low temperatures, in open air, with no enclosure round it, and as you say, with fans....They have also violated the torroid datasheet by operating at an Ampere.Turns way above the maximum recomended in the torroid datasheet.Enamelled copper wire may have been gotten away with here, but is surely a faux pas. Very odd this App Note has been around for so long.--------In the following App Notehttps://www.infineon.com/dgdl/InfineonApplicationNote_PFCCCMBoostConverterDesignGuide-AN-v02_00-EN.pdf?fileId=5546d4624a56eed8014a62c75a923b05..infineon use exactly the same inductor again, but this time for a 1.2kW PFC, switching at 100kHz.........so something seems very amiss.......at half the power and almost double the switching frequency, and they had to use the same size inductor?---------------So the above shows Power Integrations using 40 stranded Litz wire for a 300W PFC inductor...and infineon show single strand enamelled copper wire for a 2.5kW PFC indcutor.........one of them has got things massively wrong. 40 stranded Litz is extremely expensive, and is a custom build........and thats if you can actually find someone to custom build it within your lead time.--------------I have to confess, when ive reverse engineered offline PSU's up to 3kW......i did, in fact, notice that the Boost PFC inductors werent using Litz.....just enamelled copper wire, single strand, or at most , 2 strand. How do they get away with it? Why is the PFC application immune to skin effect?----------------It looks to me that the DC component in a typical CCM PFC is so large, that single strand enamelled copper wire is of some usefulness.............i mean, Litz would be better, but its "gappy", and you end up not being able to wind enough turns of it through a torroid....plus its expensive.....so it looks like a cost/effectiveness thing tells people to use single strand enamelled copper wire for Boost PFC inductors.............i guess with kool mu cores, they are never going to overheat, and the wire may get hot but wont melt......so just make a PFC inductor furnace and be done with it?........white hot wire still conducts electricity...

author: uer166
author_rank: Super Contributor
timestamp: April 07, 2022, 06:17:34 pm
content: Quote from: Faringdon on April 07, 2022, 08:07:06 amI know that at my place of work we have a single ~13kW (I think it's 40kW total, 3 phase) boost inductor PFC on some fist-sized Xflux cores, where the windings are rectangular and made of solid copper. You're generalizing a lot, and if the thermals can be managed, there is not reason why you HAVE to have litz, which is very expensive and a pain in the ass to terminate. This is doubly true when it's a CCM boost with a small ripple fraction.

author: JohnG
author_rank: Frequent Contributor
timestamp: April 07, 2022, 09:32:56 pm
content: Do some calculation of Rac/Rdc for the wire at the ripple frequency, and for the RMS value of the ripple. Then you can estimate the percentage loss due to the ripple and I think the answer will become clear.John

author: Faringdon
author_rank: Super Contributor
timestamp: April 07, 2022, 09:59:55 pm
content: Thanksg=1361/(6641*pi*sqrt(f)*R)where R = radius(m), f = Hzg = Rdc/RacIts a misleading figure though, because if radius = skin depth, then Rac=Rdc...which isnt correct, because Rac is always more than Rdc for any given conductor.And if radius >> skin depth, then Rac >> Rdc, but increasing radius does in fact reduce Rac.So it seems misleading.And if you have a wire of infinite radius, then Rdc/Rac = 0, which suggests that Rac is much larger than Rdc.....Which again is misleading, because a wire of infinite radius would have Rac=0.

author: JohnG
author_rank: Frequent Contributor
timestamp: April 08, 2022, 12:21:06 am
content: R_dc = K/r^2Where K is a function of the dc conductivity (or resistivity, if you prefer), and r is the radius of the conductor.R_ac = K/[r^2 - (r-d)^2]which is an approximation that assumes all current flows uniformly through a layer of one skin depth of thickness d.R_ac/R_dc = r^2/[r^2 - (r-d)^2]For d=0.26 mm (room temperature copper at 65 kHz), and r = 1.2 mm/2 = 0.6 mm, R_ac/R_dc = 1.47. This is an approximation. If you feel the need for an exact answer, brush off your Bessel functions or run an FEA analysis.The RMS current for a 6 A pp triangle wave is 6/2/sqrt(3)=1.7 A. Since loss is proportional to I^2, the approximate loss due to the ripple compared to the line frequency component at the peak of the cycle isRipple loss/line loss = 1.47*(1.7/17)^2=0.0147, or about 1.5%.These are approximations. Possible sources of error:R_ac: current doesn't really all flow in one skin depthLoss ratio of line frequency and switching frequency: neglecting harmonicsOther: designer needs to compute this over one line cycle, which I didn't (and won't) do.John

author: uer166
author_rank: Super Contributor
timestamp: April 08, 2022, 01:17:28 am
content: Quote from: JohnG on April 08, 2022, 12:21:06 amIn my measurements and simulations it seemed that proximity effect dominates over "normal" skin effect. The number of layers in the windup seemed to matter more than whether or not it's litz sometimes. Even 2 or 3 layers of coiled wire is many factors worse than that same wire un-wound..

author: JohnG
author_rank: Frequent Contributor
timestamp: April 08, 2022, 02:07:09 am
content: Quote from: uer166 on April 08, 2022, 01:17:28 amGood catch.In this case I don't expect proximity effect to be dominant because the photos show a single layer winding on the toroid, so most of the length of each turn is a few diameters away from adjacent turns and there is no layer effect (n=1). But, they certainly do increase the loss even in this case. If it's jumble wound or multilayer, my experience is the same as yours.John

author: Faringdon
author_rank: Super Contributor
timestamp: April 08, 2022, 03:04:35 pm
content: QuoteThanks, i think the attached LTspice sim is a good way to find out what is the 60khz ripple in a pfc inductor?And yes, as can be seen, its surprisingly low........as you say, this looks like why single core ECW is OK for PFC inductors

author: Faringdon
author_rank: Super Contributor
timestamp: April 08, 2022, 10:28:03 pm
content: The point here is that this inductor, 2 stacked kool mu 77083A7 torroids, with 64 turns of 1.15mm enam copper wire, is going to dissipate 16 Watts of winding loss alone(!!!)Thats AC and DC losses.DC losses alone is 15.3W(admittedly core loss is only some 0.5w)Its 6.6 metres of wire so thats 2.4W/metre.That wire is going to be very hot...surely 16 Watts in that length of wire is just too much?, even with the fans.

author: uer166
author_rank: Super Contributor
timestamp: April 08, 2022, 11:50:44 pm
content: Why don't you get the kit and try it out? It's a quick enough test and surely high education value.