PFC inductor has been designed wrong? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 08, 2022, 10:09:49 am
content: Hi,Do you agree, the PFC inductor in the following App Note (pg17) has been designed totally in error? They have violated the datasheet. It’s a 2 stacked 77083A7 torroid(s) from Mag-Inc. They wound it with 64 turns.PFC App Note:https://www.infineon.com/dgdl/Infineon-ApplicationNote_EVAL_2.5KW_CCM_4PIN-ApplicationNotes-v01_00-EN.pdf?fileId=5546d4624fb7fef2014fd65081616257The PFC is 2500W from 230VAC input. Therefore, peak inductor current will be in excess of 15.3A.However, 64 turns x 15.3A = 979.2The 77083A7 torroid datasheet clearly only permits Ampere.Turns up to 800.This is a clear violation of datasheet. Presumably they just assumed the “AL VS NI” graph continued as a straight line after NI=800? They are totally in error in making this assumption?77083A7 datasheet:https://www.mag-inc.com/Media/Magnetics/Datasheets/0077083A7.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 08, 2022, 10:55:16 am
content: Where does the data sheet say "limiting values" or "absolute maximum ratings"?Tim

author: Circlotron
author_rank: Super Contributor
timestamp: April 08, 2022, 11:52:17 am
content: Quote from: Faringdon on April 08, 2022, 10:09:49 amTwo stacked toroids has twice the cross sectional area of a single toroid of course, so wouldn't this arrangement tolerate double the ampere turns of a single core?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 08, 2022, 12:52:58 pm
content: Same amp-turns, twice the inductance.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: April 08, 2022, 02:50:57 pm
content: QuoteThnaks, i see your point, thats what i wondered too....but if the datasheet stops at 800, then it seems a bit off to use beyond 800....i mean, surely if it was ok to use above NI=800then the datasheet would have upp'd the scale to include there.?As you know, the Ampere turns isnt changed by adding another core stacked.....so i dont see why two stacked cores should be able to handle more ampere turns

author: Circlotron
author_rank: Super Contributor
timestamp: April 09, 2022, 05:16:26 am
content: The core in question has a certain maximum ampere / turns rating and that's that.It might be useful if the particular core material also had a published ampere / turns per mm^2 or similar of cross section. That way when you stack several cores it's obvious that the maximum allowed ampere / turns increases accordingly.

author: Faringdon
author_rank: Super Contributor
timestamp: April 09, 2022, 11:32:14 am
content: Well thanks you have raised a very interesting point, ie, why would maximum allowed N.I increase because of adding another core stacked?Its interesting, if you keep the turns the same, but add another core, then the B stays the same, because B=ur.uo.H and N.I = H.dl.....and ur doesnt change.Only thing that changes by adding another core is reluctance of the overall inductor...it goes down......which makes L go up, for the same number of turns.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2022, 05:41:52 pm
content: The only thing "allowed", is how much YOU can accept inductance to decrease by, on the peaks.Bloody hell, it's a lump of iron, the core doesn't give a damn. It doesn't heat up from DC. AC losses don't increase with saturation (as far as I know*).*I'm sure a material can exist, which does this. Magnetism is weird, pretty much anything is possible. But I'd just as likely guess most materials go the other direction, simply because the lower incremental mu (in saturation) means deeper skin effect, means lower losses for given frequency and particle size.Indeed I've seen that with my own eyes, on bulk steel: put a couple thousand amperes at 10kHz around a big steel bar and it'll heat less on the (outer, visible) surface because the surface is completely  saturated; meanwhile, peak heating occurs somewhat under the surface (where hysteresis loss is still active). Almost like a shockwave, but not in motion, just how the fields work out in the bulk material as it magnetically saturates, and also passes curie temperature.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: April 09, 2022, 07:40:59 pm
content: QuoteThanks, thats a very good point......and since the AL vs NI graph that they supply stops at NI=800, its not actually possible to get an AL value for anything greater than ni=800, and therefore, its not possible to be sure what the inductance will be above NI=800.Which makes you wonder why the App Note chooses to work above NI=800?......not only that, but way above NI=800.This is surely a mistake of the App Note?The single only 2kW+ PFC App note on the entire internet, and its in error.

author: bdunham7
author_rank: Super Contributor
timestamp: April 09, 2022, 09:31:05 pm
content: Quote from: Faringdon on April 09, 2022, 07:40:59 pmI'm sure Infineon is capable of testing and characterizing the inductor themselves. And I'm sure they did, considering they used a not-in-the-datasheet pair of stacked toroids. Presumably their demo board actually functions correctly?

author: Faringdon
author_rank: Super Contributor
timestamp: April 09, 2022, 10:45:48 pm
content: QuoteThanks thats interesting......the only scope shots in the App Note show 1kW operation at low line.The App Note claims to be able to do 2.5kW at 230VAC.....there are no scope shots whatsoever of 230VAC operation with 2.5kW load.Not a single scope shot.There is a table of efficiencies, which may have been invented, or taken from some other power supply.On the inductor page of page 17 of the App note, it says the inductor is "600uH at 100kHz"...this doesnt make sense....its nonsense.If you examine the attached LTspice and inductor investigation doc of this App Note, you can see that at 2.5kW and 230VAC, their inductor value would have dropped to zero at the mains peak voltage...........if you follow on the gradient of that graph, then AL value goes to zero at the NI value for 22A.This is the only App Note on the web showing a Boost PFC of more than 1.5kW off single phase high mains.Its unlikely to be real, since Boost PFC's are virtually always split up into "chunks" of 1.5kW maximum.....and they are paralleled at 1.5kW and below, to get higher power.The EMC scans they give are also pretty meaningless as this is a Boost PFC, and would normally need to be with a downstream PWM stage, which is not included here.They also dont show how the load was wired up for the EMC scans, so its pretty meaningless anyway.Also, the App Note is advertising the ICE3PS01G controller, and what controller you use, doesnt particularly impact on EMC.....unless its got dithering, but ICE3P doesnt...even then, ditheringdoesnt help with common mode emissions, and its common mode that are the biggest challenge for EMC.The App note also shows a 5mm by 8mm SMD sense resistor on the board underside.....this is never done on 1kW+ power boards that have big heatsinks and fans and inductors also on that board...as the board would bend in handling and when mounted and such big thin SMD components would tend to snap or be stressed.......PFC's 1kw+ usually use Axial power resistors, and they just RC filter the ringing off them.Also, all the controller circuitry is SMD and not on a daughter board, whcih for the same reason, is not done with 1kw+ PFC's.Also, the App Note schem shows the gate drive has not been done optimally....they have not got turn off diodes.......all Boost PFC gate drives would have turn off diodes.....they actually use the 1ED60N driver which in fact, doesnt need turn off diodes if you connect it up in the requisite way........but they havent done that....they have just connected pins 6 and 7 together.It does look like this App Note has possibly been put together by a contractor working for some distributor, as theyre looking to sell ICE3P chips...and can probably get some money off Infineon to put the App Note together....Distributors doing demo boards for chipmakers is actually quite common.......i was interviewed by a well known disti for such a job.........they had shelves full of demo boards that they had done for this and that.

author: bdunham7
author_rank: Super Contributor
timestamp: April 09, 2022, 11:35:31 pm
content: Quote from: Faringdon on April 09, 2022, 10:45:48 pmOK, so you're saying their demo boarddoes notwork, or at least doesn't perform as advertised. Hmmmm.QuoteReally? I get about 650uH at low DC bias currents just going by the numbers. 642x 80 x 2 (nH) ?QuoteIs extrapolating off the right end of the graph meaningful after all?I'd think you'd hit a sharp downturn at saturation at some point,Edit: actually not, with that core typebut I don't know how those cores actually behave in detail nor what the effect would be at the 65kHz switching frequency. Again, I'd have to assume Infineon at least looked at that unless they're just complete con artists as you seem to be implying. Where did you get those files? If I assume you drew the LTSpice one and made the Excel sheet yourself from some sort of extrapolation, then I'd ask why you used a simple 160uH inductor and why you are discussing 22 amps. The inductor is obviously going to be more complex than a simple ideal simulation. The board will have less than 11 amps input at 2.5kW/230VAC and the peaks will be what you calculated earlier, 15-ish.

author: Faringdon
author_rank: Super Contributor
timestamp: April 09, 2022, 11:56:16 pm
content: Thanks, The attached shows the PFC at exactly 2500W output and 230vac and 65khz......at 15A the inductor they spec would be 232uH.......but that assumes the AL graph just carries on at the gradient it left off at (just up to) NI=800.Because as you know, NI is 960 at 15A with 64 turns.But at 232uH in the PFC.....then the peak current is actually 18.1A....which means an inductor value of 133uH....so in fact, the peak current would be more than 18.1A.QuoteYes, i agree, but the "600uH at 100kHz" that they state doesnt make sense....as you say, its the bias current changing that changes the inductance, not the frequency.

author: bdunham7
author_rank: Super Contributor
timestamp: April 10, 2022, 01:06:17 am
content: Quote from: Faringdon on April 09, 2022, 11:56:16 pmI don't know if any of those assumptions are valid without more actual data on the core. Or pair of cores. If you look at the scope shot of the low-line 1kW startup--which is going to involve nearly maximum input current and may be the toughest operating condition there is--you'll see that the inductor current is peaking at 40 amps or so in the beginning. Also, the tops of those peaks show more and more amplitude from the switching, as you'd expect. I think I sort of understand that you are saying that at some point the circuit will not be able to keep up in CCM as even switching from zero to max current will be insufficient. Or something like that. But as you can see from that scope shot, even at 30-40A peaks (meaning on the 50/60Hz timescale) it is still able to keep up without anything particularly ugly happening. I don't think it could go much further, but looking at that and Infineon's claims that the board operates efficiently at 2.5kW continuous and at least tolerably well at 3kW for a short time, it looks to me that there's no reason to think that there's some fatal flaw here.