Powder/Alloy Cores Saturation Flux Density vs. Temperature - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 01:17:03 am
content: Greetings (once again),I don't think I've made a thread in about six months and now here I am with two threads in a two days! Go figure.I have an existing CCM PFC design that I've been experimenting with to improve the current harmonics at lower % of its rated output power. (e.g. 25%). Given the space constraints, changing from a ferrite core to an MPP core with ~2x nominal inductance has helped considerably and still fits in the same space.Now, I'm sanity checking the efficiency, current waveform and a few other things @ 100% load. Everything looks pretty much OK, until I really let it soak for 1-2 hours @ 100% load. At some point after the 1 hour mark, the current waveform goes wonky...and if I had to guess, the PFC choke is saturating (progressively move) and the pk-to-peak inductor current is high enough to tap the OCP threshold, which then reacts in a not so pleasant way. Or, the current starts to increase/react in a weird enough way (not linear) to throw off the control loop sufficiently. Something like that.Check the attached (zoomed in) current waveforms (and voltage).Almost 100% certain that the onset is thermally related. It's about 100°C when things start to go south. I sprayed the PFC inductor with freezer spray while running, and the waveform immediately goes back to normal.Looking through many catalogs, I only see:1.) Permeability vs. temperature (measured under very little DC bias)2.) Permeability vs. DC bias (measured @ ~25°C I think)...but I do not see something like saturation flux density vs. temperature. Of course, the saturation characteristic tends to be graceful, so for most applications, maybe who cares?----Firsly, somehow I can't find the standardized definition of saturation flux density of powder cores. Is it the -30% rolloff point?Secondly, and more importantly, is there any data available on how this rolloff point changes with temperature for a variety of materials? Does saturation flux density reduce with temperature? (If I'm connecting the dots correctly, then I think that ought to be the case.)Thirdly, if I sound like a lunatic, let me know.Thanks,Tim

author: uer166
author_rank: Super Contributor
timestamp: December 10, 2022, 05:03:33 am
content: Quote from: TimNJ on December 10, 2022, 01:17:03 amThat is certainly the case with all ferrite I've seen and I recently found with nanocrystalline tape wound cores as well. In ferrite it's a serious reduction in max B and the knee becomes much sharper as well. For N87 it's something like 15% from 25C to 100C.For MPP specifically they give permeability vs. temperature (see attached). The higher the T the higher the temperature, and presumably unless we break physics, the higher permeability implies also a lower saturation point.It's hard to tell from your scope shots that saturation is actually happening, maybe need to zoom into a single switching cycle.

author: uer166
author_rank: Super Contributor
timestamp: December 10, 2022, 05:22:25 am
content: Here's an old document that shows that MPP is quite a bit more stable than ferrite:https://elnamagnetics.com/wp-content/uploads/library/Magnetics-Documents/Magnetics_MPP_THINZ.pdfMaybe it's a control issue due to something else? In any case a zoom would help.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 10, 2022, 06:38:25 am
content: Yes -- think of it this way: ferromagnetism is atoms coordinating en masse to make myriad (full strength) domains, which orient (to some degree, or not) to the applied field; but as you get closer to Tc, the atoms are less and less able to coordinate and hold that orientation, until, *blip*, they stop doing it entirely. It's not so much a phase transition *blip* like melting ice, but the phenomenon degrades hyperbolically as you approach the transition point. Which I believe is why they call it a 2nd order phase transition? Something like that."Able to hold orientation" translates directly to Bsat. So, generally Bsat drops roughly inversely as T approaches Tc. Or maybe it's an exponential, or some other function, I don't recall what precisely.In any case, far from Tc, it's a tiny difference. It can be quite noticeable in circuit with NiZn (low Tc)*, noticeable but easily designed around for MnZn, and essentially irrelevant for metals (Tc's of 500C+!).*Demonstrated this twice, in fact. One case was a higher frequency (~1MHz) proto with nice fast gate drive into the gate (~10ns?), but a fairly long gate lead (because proto reasons). A ferrite bead on the gate solved that fine ...until it heated up to Tc, and the parasitic oscillation weaseled back into being. Then the sizzling touch of a licked finger brought it back down again. That was I think around Qg = 50nC at 12V and 2MHz, so, the ferrite bead could've soaked up, up to a watt I guess, in the process. The other case, was trying to dampen loop ringing in a half bridge; I didn't have the opportunity to do the lick-test in that case as the bridge was at ~400V (the other one was finger-safe at SELV).Powder does degrade quicker at elevated temperature, but that wouldn't be reversible, of course(!).Have you triedYe Oldeheat gun and cold spray to try and identify problem components? Chilling just the inductor would be a pretty clear indication. Or heating or chilling everything else. Would be really weird if the DCR or L or Q was changing enough to do it (MPP tempco is pretty low, as are/should all the others). Electrolytic I guess would be the next strongest suspect, but I'm assuming this is new work?, and besides, they usually work *better* when hot.Tim

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 10, 2022, 10:49:44 am
content: I wonder if you are using average current mode control (ACM) as I have seen this sort of waveform before!!Instability in the control loop because it is resonating with something somewhere most typically a filter.Not very specific I know but I fixed it by changing the frequency response of the ACM feedback loop.In your case I suggest the control loop might already be close to resonating with something and temperature change is enough to tip it over the edge![ Specified attachment is not available ]

author: jonpaul
author_rank: Super Contributor
timestamp: December 10, 2022, 02:11:12 pm
content: Bonjoursuggest you check detailed specs or ask mfg tech support for curves of core loss per unit, vs flux and temperature100 C surface or Hotspot at what ambiance temp?Cooling systems used? Output watts?TDK ferrites had several different materials optimization, perhaps similar for MPP, but no personal experience.other materials can be considered depending on the cost, availability, size constraints.We used powdered iron, tapewound and cut core silicon steel, amorphous.bon chancejon

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 10, 2022, 02:53:41 pm
content: Re TimNJ's comment: "...but I do not see something like saturation flux density vs. temperature. Of course, the saturation characteristic tends to be graceful, so for most applications, maybe who cares?"Yes, I have not seen this relationship in the data sheets of the various providers. Usually just T_curie is stated.And nobody really cares except for induction heating engineers, and those who designed converters with inductors (err...) on the hot side.In my time I have been both of those classes!The function is called the Curie-Weiss Law.It is a curve which traces the the fall in magnetization (Is/Io) Y axis by thermal agitation,as temperature increases T_c/To along the X axis.The function was modified to include quantum spin components. The curves, along with measurements for pure solid Iron,Nickel and Cobalt are graphed in Bozorth "Ferromagnetism" Fig 10.4 page 431.I used the original Curie-Weiss as basis for steel properties as it is simpler and on the conservative side.In ferrites, the Curie-Weiss curve and eventual T_curie are influenced by proportion of Zinc.Curie Weiss curves for ferrites with Zinc as parameter, and some of the classed ferrites are in Snelling Fig 3.3 pages 61 ~ 64Ferrite Initial Permeability verus temperature at Fig 3.8 page 91.I don't have the curves for any of the powered alloys.

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 04:21:46 pm
content: Quote from: uer166 on December 10, 2022, 05:03:33 amQuote from: uer166 on December 10, 2022, 05:22:25 amThank you. Yes, that makes sense to me.Yes, I need to take a better quality scope-shot. I just captured these from an AC power analyzer which only goes down to 500us/div.I agree that it's more likely a control issue than an issue of saturation in the classical sense. By that I mean: The inductor is pretty much incapable of abruptly saturating at some point, but *something* is changing about the inductor enough to cause the control to get wonked out.I'm trying to figure out what the *something* is. My initial guess was saturation based on what other ferromagnetic materials tend to do at higher temperatures. And, when the issue occurs, the peak-to-peak current seems to increase exponentially-ish. But this may very well be due to the controller erroneously making the on-time much longer than it should be.And, uer166, that graph is helpful. At 100C, it doesn't seem like it should be too much different. Drew in a totally made up 100C curve, assuming that the change in permeability will be less than that of ferrite by...I don't know the ratio of the Curie temperatures? Just making things up, really.

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 04:34:44 pm
content: Quote from: T3sl4co1l on December 10, 2022, 06:38:25 amThanks. That helped a lot. It's helpful to understand that saturation point in relation to temperature is related to the Curie temperature of the material. Ferrite is much lower, I think most power materials are like 200°C, and higher permeability materials can do down to 130°C or so. On the other hand, powder materials are typically in the range of 400-700°C! So, the percentage of Curie temperature is much less, as you point out.Quote from: T3sl4co1l on December 10, 2022, 06:38:25 amYes, I sprayed the PFC inductor with freezer spray, just a short blast, and the current waveform was cured almost instantly. I tried spraying on the control circuit itself, and this did not do anything (that I could tell). So, it doesn't seem to be temperature drift in the control IC error amplifier, external compensation components, current sense resistor and so on.There is a DM filter inductor before the diode bridge (160u high flux) which isreallycooking, even more so than the PFC inductor. (I need to fix that.)...but I was pretty careful not to spray that one. I'll try it though.I am cautious to jump to conclusions, but based on how quickly the current waveform corrected itself (< 2 seconds after spraying), I am inclined to think the root cause is a temperature related phenomenon of the PFC inductor, possibly exacerbated by a marginal control loop (or something).

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 04:50:59 pm
content: Quote from: fourtytwo42 on December 10, 2022, 10:49:44 amWell would you look at that! Yes, you are correct about ACM:https://www.ti.com/lit/ds/symlink/ucc28180.pdfYes, the current loop is tuned quite aggressively and Iamdoing something "undocumented" with the current sense signal. I found, to improve zero crossing distortion, applying a small DC bias to the current sense signal can help quite a bit. I guess it helps with gain reduction when input signal is near zero. So, I'll remove this too and see if that's the answer. Currently, I'm just feeding in a little current into the current sense node via high-ish value resistor (300K), which means the whole current sense signal is shifted. I wonder if that's an issue. Maybe it would be advantageous to only apply this offset near the zero crossing. Do-able...but considerably more complicated to do analog.

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 04:58:21 pm
content: Quote from: jonpaul on December 10, 2022, 02:11:12 pmRoughly 100C surface temperature, natural convection cooling (no fan), about 100W.To try to prove whether this is truly a saturation issue (have my doubts), I'm going to try a different material that I have samples of:"Nanodust"Core loss is slightly higher than MPP, but saturation flux density is about 50% higher. On the other hand, the permeability vs temperature relationship is opposite (permeability decreases with temperature) and the percentage change is greater. (About -2.5% @ 100C / 125u)We'll see. Thanks.

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 05:11:02 pm
content: Quote from: mag_therm on December 10, 2022, 02:53:41 pmI really ought to get the Bozorth book, even if it's quite old at this point. Your point is well understood and seems to match Tim's point above.Attached is a normalized curve for iron. It seems that the general shape of this curve is conserved for many different materials. To de-normalize the curve, it is can be stretched on the Y and/or X axis depending on the material's saturation point and curie temperature. Is that generally correct?So, for a material with Tc = 500C, if operated at T0 = 100C, we should see only a few percent shift in saturation point.I understand am painting this with a very broad brush.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 10, 2022, 05:42:59 pm
content: That graph in your reply #11 looks like the Curie-Weiss function which ( in text, sorry) is a trig function ( tanh)Isat/Io = tanh ( {I/I0} /{T/T_curie} )Bozorth called it one of the most important in ferromagnetism.When I used it in model, I converted it to an exponential funtion which closely tracks the tanh.With the steel alloys, the T_curie appears to stay within about +/- 30K, but the I0 varies.In engineering units [Amp/metre] theoretical I0 is in range 1.2e6 to 1.4e6 A/m ( there is an exact one from Bohr for pure Fe I recall)Practical steels appear to heve I0 down around 700000 to 800000 A/mThat was the parameter I varied to make the curve track various alloys.It is difficult to measure all this, in 2011 I spent more than a week just to get values to track with two alloysThe other useful function is an empirical one:This is for the variation of Ur with field strength, under the Curie-Weiss.Ur = 734000 / ( {Hs ^ 0.92} ) where Hs is the magnetic field in the material [A/m]I think that constant 734000 corresponds to IO. This function is published in various forms and I once worked with a steel lamination manufacturer that used a version using log series which closely tracked the above.In the absence of published data , the above 2 functions can provide insight, although the constants would be needed for the material.I have not used the above functions outside of steel alloys.

author: Wolfram
author_rank: Frequent Contributor
timestamp: December 10, 2022, 07:39:29 pm
content: The core parameters change with tempeature as discussed, but not too dramatically at reasonable operating temperatures, so the design is likely operating very close to the OCP limit already.Once the OCP triggers, it operates like peak current mode control if I understand it correctly. This has implications for stability, particularly with regards to subharmonic oscillation at some operating points.Quote from: T3sl4co1l on December 10, 2022, 06:38:25 amAFAIU, this is mainly a problem with iron powder, which uses organic binders, and less of an issue with alloy powder cores which use a ceramic binder.

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 10, 2022, 07:44:22 pm
content: Quote from: TimNJ on December 10, 2022, 04:50:59 pmWayyyyyyyyyy back in 2018 I simulated an ACM controlling a GTI and produced a waveform almost exactly like yours when the compensation was bad (see below).Another issue with ACM that may be part of your problem is sub-harmonic oscillation covered in these two references (again from 2018)https://electronics.stackexchange.com/questions/360545/subharmonic-instability-in-average-current-mode-controlhttps://www.ti.com/lit/an/slua079/slua079.pdfI can't think of a reason for the offset to be a problem but you never know.Of course with control loops it's cause and effect so I might be barking completely up the wrong tree or there again just plain barkingP.S. I found the loop compensation an extremely dark art and had to consult a witch in the backwoods to get the right solution

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 10, 2022, 08:14:57 pm
content: Quote from: Wolfram on December 10, 2022, 07:39:29 pmOh do they? That would be excellent... I only ever saw resin with iron powder and assumed they didn't have anything better for other compositions. Do you have a reference? I can't seem to search anything offhand ("magnetic", "ceramic", etc. just return MUCH more common results..).Then the temp limit would be defined only by the resin coating and loss tempco? Do they ever make them wrapped with fiberglass and/or silicone cement (or vitrified glaze for that matter, if the tempcos work out..?!) for extreme applications? I've never heard of such before, but that would be cool (hot) if so.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 08:30:21 pm
content: Quote from: T3sl4co1l on December 10, 2022, 08:14:57 pmMicrometals has this page with some "200C" materials under it:https://www.micrometals.com/products/materials/200c/Is that hot enough for you?Example datasheet just says "epoxy coating":https://datasheets.micrometals.com/T130-70-DataSheet.pdf

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 08:40:54 pm
content: Quote from: fourtytwo42 on December 10, 2022, 07:44:22 pmThanks a lot! I really like that stackexchange discussion. So, if I understand the explanation, the "inverted downslope" of the inductor current is the period of time when the inductor's magnetic field is collapsing (MOSFET off) and current is ramping down, through the load? If that di/dt is too fast, you might get an "extra crossing".V/L = di/dt...so di/dt increases when either L decreases or V increases. For a "soft-saturating" inductor like this, inductor di/dt will tend to increase near the peak of the AC line as it loses inductance. And then the question still is: What is the thermal phenomenon that would drive it into this situation?Quote from: fourtytwo42 on December 10, 2022, 07:44:22 pmHaha, well you're not alone in that. I can't find a good way (easy way?) to measure the current loop response, so it's all trial and error here. We have about a 20 sheet Excel document with countless iterations to try to dial in the harmonics, comparing waveforms and so on.

author: TimNJ
author_rank: Super Contributor
timestamp: December 10, 2022, 08:47:59 pm
content: Quote from: Wolfram on December 10, 2022, 07:39:29 pmCould be. This IC has two forms of OCP: soft-overcurrent and peak-current limit. Sounds like soft-overcurrent should come first, but if there is a disaster, peak-current limit kicks in...which would probably have those effects.Though, I had the current sense resistor at a 25% higher value (-25% lower current limit), and it handled it rather nicely...flat-topped the current waveform, but did not go crazy beyond that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 10, 2022, 08:52:42 pm
content: Quote from: fourtytwo42 on December 10, 2022, 07:44:22 pmWell, it's pretty clearsomethingis very marginal, so it wouldn't take much bias to shift gm, or DC shifting L, or... to do something, it seems.I wonder why it looks like a step response. Is this one of those controllers with the chonky feedforward compensation? Ah, no, not even any Vin sense on this one. There is still a multiplier block, but as part of the control loop I think that has to be a proper (continuous) multiplier.Would be interesting to do impulse/step response testing on these, to verify phase margin, but it's tricky. Can you sync a signal generator to mains and couple that into VCOMP, ICOMP or the load (switched resistor, say)?Tim

author: jonpaul
author_rank: Super Contributor
timestamp: December 11, 2022, 01:30:51 am
content: thinking deeperSaturation can be measured with a B*H loop lissajousRequires a search winding on the core and integrator RC, scope in XY mode.Very useful to detect SaturationFor iron powder cores, temp rating g and,characteristics ask Micrometals tech supportAbout loop compensation the 2ond order system is characterized by the poles and zeros.This technique is well known since 1920s.In 70' took the course in university, so a bit rusty....Just the ramblings of an old retired EEJon

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 11, 2022, 09:46:10 am
content: Quote from: TimNJ on December 10, 2022, 08:40:54 pmMy solution was extensive modelling & simulation of both the control loop & power path, runtime was horrendous sometimes taking several hours to complete the 1/2 cycle I have shown here but it did in the end allow me to pinpoint the problems and arrive at a workable solution. The difficulty in real life is the wide range of real operating conditions where modelling can really only be used to check the extremes (at least when the runtimes are hours) but it might help.On another note as I am desperate to keep the control loop theory aliveI wonder if some parameter of the control chip is wandering to close to the edge with increased temperature caused by conduction through the pcb although I would agree the whole thing would have to be extremely close to the edge of the stable envelope for that to be true but perhaps it is!I have an simple pulse inductor saturation tester I made and use it to test materials and new designs, though I have never done it, it would be simple to heat the inductor while testing it to verify both it's saturation and inductance at high temperatures, that way you could either set your mind at rest or wonder how it ever worked for sure

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 11, 2022, 07:05:40 pm
content: Oh, since it's not sensing Vin, and, I didn't see anything about line sync or something, it should be perfectly fine with DC input. That ought to make testing simpler.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: December 29, 2022, 10:56:44 pm
content: I was so busy working on this, I forgot to give an update.The above issue was really a secondary effect of a different issue. It's somewhat interesting, but (I'd say) very specific to my DUT. Probably not much value to anyone reading. But here goes.As the DUT heated up, the PFC voltage drifted high, from ~392V nominal, up to around 405-407V at high temperature. This is quite a lot! The PFC is fed into an LLC resonant converter. We have some glue logic which operates on the SNSBOOST pin of TEA19161T (LLC control IC). Basically, the glue logic should put the PFC controller in burst mode when it senses that the LLC controller has pushed up the SNSBOOST voltage beyond a certain threshold. However, coincidentally, SNSBOOST isalsothe sensed PFC rail voltage (LLC input voltage). So, if the LLC input voltage drifts high, then this glue logic is erroneously triggered, because it looks like the LLC is trying to put the PFC into burst mode.So, the PFC is put into burst mode when operating at full power. (Obviously not good.) At the start of the next "burst", the PFC rail voltage has drooped a lot (due to the PFC having been off for some period of time), and pulls a very large current to try to restore the nominal PFC rail voltage.It is this re-start attempt that I captured in the above scope waveform.With the MPP PFC inductor, this re-start attempt is quite nasty, and my best explanation is sub-harmonic oscillation (as fourtytwo42 suggested) possibly because the inductance has dropped so much which in turn causes higher di(L)/dt . MPP has a saturation flux density of around 0.75T.The MPP PFC inductor was replaced by an inductor with the same nominal inductance, but wound on the so-called "Nanodust" core mentioned above. This material has a saturation flux density of around 1.3T. (The inductor was replaced prior to discovering the PFC rail voltage drift issue.) After swapping out the inductor, we found that the erratic behavior (PFC shut-off) still occured, but the re-start attempt waveform was much cleaner. It no longer oscillated in a horrendous way, but it was still not right.The issue was resolved once we found out about the PFC rail voltage drift. It was a compounding issue. Basically: PFC rail voltage drifted high due to temperature --> LLC operating point pushed farther from resonance --> LLC operates at lower efficiency, produces more heat --> PFC voltage drift worsens . And once the PFC voltage drifts to a high enough value, the glue logic was erroneously triggered and put the PFC in burst mode.It turns out that the original designer specified PFC voltage divider resistors about 15x higher than the datasheet recommended values. So, the top resistors in the string equaled around 15MΩ, whereas the datasheet value showed about 1MΩ total. Two issues with this:1. It seems "high" value (thick film) resistors have an inherently higher TCR (??)... or are at least more difficult to find with low TCR. Above 1-2MΩ (0603 - 1206), it seems most standard thick film resistors are 200ppm, or 100ppm at best. And, standardthinfilm has poor availability beyond 1-2MΩ. So, part of the solution was to drop the total resistance to about 2.4MΩ, which allowed use of 3x820K thin film resistors (50ppm), cheap and easy to get.2. But even if the original resistors were 200ppm, all worst case drift and in the same direction, the PFC rail drifted should only have drifted up about 7-8V,not13-15V. (Unless our "200ppm" resistors were more like "400ppm". So, the other issue I think might be related to (something like) the DC gain of the voltage error amplifier of the PFC controller. Somehow, I don't think it could tolerate the high value resistors. Maybe the selected values were just at the fringe of values that would "work".After changing to lower value divider resistors with 50ppm TCR, the PFC rail voltage is rock solid, and no longer does it trigger the werido issue that I originally reported.Thanks everyone.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2022, 02:09:14 am
content: Don't forget to include input pin leakage; it may bias the node up or down (depending on how the ESD protection is done internally). I forget if that chip specifies it well, but many have a bad habit of not specifying input current limits, or over temperature, etc. So it can be a crap shoot whether 100k or 10M is okay for the divider...Tim

author: TimNJ
author_rank: Super Contributor
timestamp: December 30, 2022, 03:31:54 am
content: Quote from: T3sl4co1l on December 30, 2022, 02:09:14 amAh yes, that sounds even more likely than something to do with DC gain or steady state error of the amplifier.