Reviewing my CCM PFC core loss estimation - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: November 18, 2020, 07:49:49 pm
content: Hi everyone,I've spent that last few days trying to come up with a reasonable core loss estimation for a CCM PFC design I'm working on. My predecessor left me some design spreadsheets, but they seem a bit oversimplified, maybe inaccurate. On the other hand, complicating it leaves lots of room for error. I found this paper from 2002, which was useful in understanding a general approach, although I found it a little lacking in describing the actual implementation:http://drumknott.simplistechnologies.com/resources/supplementary/pfc_inductor_losses.pdfI've attached a spreadsheet, if you care to look. Left half handles the converter input parameters and initial design constraints. Right half computes the core loss based on selected design. Items in red are things I'm not so sure about.---1.)The above paper breaks down the hysteresis core loss into the on-time and off-time of the switch. The equations they describe are below. I think it makes sense. The 2 accounts for the fact that the loss @ flux density is described for +/- Bpk, whereas the delta-B, I calculate is the full +/- swing. Equations seem to match up with what I know.2.)I started athreada while back with a similar question, regarding the Steinmetz coefficients. Once again, I'm having a hard time figuring out what's right. My own curve fitting shows quite different than what Ferroxcube publishes. But Ferroxcube publishes an extended equation which tries to estimate effects of temperature. If I follow their coefficients, I should use 'alpha' = 42.36, 'm' = 2.8, and 'n' = 1.16. If I do my own curve fitting, I get 'alpha' = 26.3, 'm' = 2.73, and 'n' = 1.63. This gives a vastly different core loss, and I think 'n' = 1.63 is definitely wrong, as I only see values 1.0-1.4 for just about every other material. I think I will stick the Ferroxcube numbers, assuming they also apply for the "simple" Steinmetz equation. My intuition says that core loss <100mW for this ~27x20mm core @ ~100KHz, ~100W design, is about right. Anyone have any input? Anything stand out as being grossly wrong/bad idea?----As I've only been involved in magnetics design for <2 years, each time I work on a new design, I learn something "new". A few takeaways:a.)For a CCM design, flux swing is relatively small, so core loss is perhaps not as relevant compared to something with a much larger flux swing, say CrM/BM PFC. For CCM, the dominating loss is the switching loss of the transistor.b.)For a given inductor, it seems that hysteresis core loss does not actually go down as switching frequency is reduced. Seems counter-intuitive to the data-sheet plot, but makes sense if you consider that delta-B will go up if the switching frequency is reduced. However, it still may be advantageous to lower the switching frequency since MOSFET switching loss may tend to go down more than the core loss may go up.c.)When running conducted a EMI scan, itseemsthat the delta-B (flux swing) is correlated to low frequency (150-500KHz) EMI level, but delta-I (inductor current ripple) is not. The reason is not totally clear to me. My assumption was that greater peak-to-peak inductor current would make EMI higher, but there was actually no effect on EMI between a 1050uH and 1200uH version of the choke,with the same number of turns. On the other hand, increasing the switching frequency, thereby reducing delta-B, significantly improved EMI. Why is it so? Am I mistaking this for something else?Anyway, seems like I've barely asked a question. Really, I'm just looking for any feedback on the core loss estimation that you might have.Thanks,Tim

author: jbb
author_rank: Super Contributor
timestamp: November 18, 2020, 08:13:57 pm
content: Hi TimI can’t speak too much to precise core loss calculations. I’ve had OK results just using the delta B vs f plots (at 100 deg C). I do know that core loss is quite complex but how close do you need to get?As it comes to overall converter behaviour, there are trade offs all over. It’s necessary to look at semiconductors, inductors and filter together. We typically found that inductor losses went down (similar delta B applied to smaller cores, and shorter winding lengths) as frequency went up, but switching losses increased and the result was a gradual reduction in efficiency.Additionally, the obvious target is efficiency at full load but the market for the product might not reflect that. For example, I worked on solar inverters which used weighted average efficiency as the benchmark, and efficiency at 50% load was more important.On the EMI front: maybe flux leakage from the inductor is coupling into something? Are you using a gapped core?

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 18, 2020, 09:17:09 pm
content: Snelling (1969) only had properties up to 3C8At 100 kHz, 0.2 T, 85C, Core loss = 510,000 Watt/metre^3( using your constant names)n=1, m=2.5, alpha = 285He added eddy current loss separately so that is not in the above coefficients.To handle temperature he added another line at 25C.The alpha here is 10 times your alpha from curve fitting.What units did you use for core loss?

author: TimNJ
author_rank: Super Contributor
timestamp: November 18, 2020, 09:25:40 pm
content: Quote from: jbb on November 18, 2020, 08:13:57 pmThank you. How close do I need it? Absolute accuracy is probably not critical, as the ferrite material probably doesn't even adhere to the model so closely anyway. But, I'd like to know, generally, what changing certain parameters might to do the overall performance. i.e. Will the loss go up by 5mW or 50mW?Indeed, optimizing inductor loss on its own might lead to a sub-optimal solution, depending on the design. Decreasing switching frequency forces higher turns count to limit flux swing. The inductor loss goes up a little, but the semiconductor switching loss generally goes down more. So, it's an overall win.This is a gapped core. I also wondered if it was magnetic field coupling into nearby traces, or bypassing the EMI filter. I think that's still the best explanation. On the other hand, the inductor is shielded with copper foil, in both the direction of the core, and in the direction of the coil. If you look at the attached image, you can see the EMI signature without any shield (light blue), and with the shields added (dark blue). It makes an enormous difference at about 400KHz and above, but down at the 2nd harmonic (around 200KHz), the levels are much the same. My initial presumption was that the 100-300KHz noise wasnotfrom coupled EMI fields but due to the switching currents being drawn by the converter. On the other hand, perhaps copper's shielding effectiveness is not so good at the lower frequencies, and it's simply that the shield does not provide sufficient attenuation.Thanks!

author: TimNJ
author_rank: Super Contributor
timestamp: November 18, 2020, 09:34:27 pm
content: Quote from: mag_therm on November 18, 2020, 09:17:09 pmI used W/m^3.....I think.If I exchange 42.6 -> 285, 2.5 -> 2.5, and 1.16 -> 1.00, the calculated core loss is only about 2x the core loss I calculated with modern 3C97 material. Doesn't seem too crazy given the 50 years in between?Interestingly, the published Ferroxcube coefficients are all over the place, even with relatively similar materials. For example, 3C95 has alpha = 92, and 3C96 has alpha = 5.1. Obviously, the exponents compensate for this.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 18, 2020, 10:09:37 pm
content: The core loss versus temperature is U shaped with a lowest loss between 80 to about 100 C as I recall.Then Curie temp at about 180 C. If the core gets near Curie, it goes into thermal runaway.One time I had to go down a zinc mine where some frequency converters were failing.It was hot as H*** down there and the line voltage was high.Some of my transformers (probably 3E6 or so) became so hot they melted the solder at the leadout terminals!So I had to re-do them with double the core area.

author: TimNJ
author_rank: Super Contributor
timestamp: November 18, 2020, 11:04:47 pm
content: Quote from: mag_therm on November 18, 2020, 10:09:37 pmInteresting. Thanks for sharing. Doubling the core area to get the core loss down?

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 19, 2020, 12:56:10 am
content: Well... no, ...that was recovering from bad engineering.That was a long time ago. These days we have powerful computers and a lot more tools.I now do magnetics design including 2D finite element modelling (FEM), usually with the magnetics model in frequency domain and coupled to a transient thermal model.These two models have to be solved iteratively.For my own coupled DE/PDE solvers, I use the Curie-Weiss function ( for example ref Bozorth Fig 10-4 page 431) to couple temperature to progressive magnetic saturation.More relevant to work you are doing in this thread, I see Spice4qucs magnetic core libraryI am about to move up to Fedora 32 so I can try this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 19, 2020, 01:01:34 am
content: All else equal, for example having a copper tape shield around the core, or middle air gap only -- I would not expect the core or winding to make any difference.Things not being equal, like differing external gap (core gap by shimming the whole thing, rather than by grinding the middle peg), could have some difference and I would need to see more of the system to make any kind of claim about it.Also, the magnitude of your change isn't enough to show up (~1dB of inductance, can you measure 1dB of EMI response consistently?). Even if we expect a nonlinear regime here (possible I guess, quadratics show up in SMPS calculations from time to time), that's still only 2 or 3dB, hardly anything to worry about. It would take a very steep response, like a hard threshold, to do anything more; and we probably have no reason to suspect there is a threshold in play here. (An example that might apply: depending on line and load condition, changing the inductance might cause a quasi-resonant operating mode to fail timing, and EMI increases suddenly and steeply in certain ranges. This would probably be obvious on the oscilloscope, having done your due diligence checking these points in the operating range.)Meanwhile, EMI is proportional to frequency, at low frequencies, inversely so, due to the DM filter caps having less reactance. At high frequencies, it may tend to increase, because there are more frequent switching edges, and lower reactance on the series capacitances carrying noise out of the supply; or maybe it stays constant because the equivalent circuit up there is more of a capacitive divider; or perhaps the more widely-spaced harmonics manage to dodge a particularly large transmission peak, who knows. Random stuff like that can happen, the frequency response from switch nodes to the outside world isn't exactly well designed.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: November 19, 2020, 01:42:22 am
content: Quote from: T3sl4co1l on November 19, 2020, 01:01:34 amThanks. I should add that the first revision of this inductor had 64 turns. The EMI at the 2nd and 3rd harmonic was very high, failed by about 1-2dB IIRC. For the second revision, the turns count was increased to 72. Inductance was held the same. Switching frequency also the same (100KHz). The EMI at the 2nd and 3rd harmonic came down considerably, to the levels shown in Reply #3. The core is only gapped on the center leg.With the second revision inductor I, out of curiosity, reduced the switching frequency from 100KHz to 95KHz, 5% change. This caused the EMI to increase by 4-5dB at the low end (200KHz-400KHz). On the other hand, the reactance of the filter capacitors, should only be about 0.5dB higher at this slightly lower frequency. If I changed from 100KHz to 75KHz, then this might make more sense to me. As you noted, maybe something else is at play.But so far, my two observations show that increasing the turns count and increasing the switching frequency both (independently) improve the low frequency EMI profile "in a similar way". To me, the factor tying those two together is the flux swing...Obviously, correlation does not equal causality, so who knows.I don't think it's anywhere near saturation at the load condition it's being tested at, but maybe something "non-linear" is happening. Don't think so though, the inductor current ramps look nice and straight. The Bmax is designed around 80Vac input and 150W overload condition. The EMI test condition is 115Vac and 120W load.Thanks for your input, as always.

author: TimNJ
author_rank: Super Contributor
timestamp: November 19, 2020, 01:47:20 am
content: Quote from: mag_therm on November 19, 2020, 12:56:10 amThanks for the recommendations. For Bozorth, are you talking about his book 'Ferromagnetism'?

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 19, 2020, 03:19:38 pm
content: Yes. It is a good (but expensive) reference.Mainly for the steels, as ferrites were new in 1951 when the first edition was published.R.M. Bozorth "Ferromagnetism" ISBN 0-7803-1032-2By the way, what is part number of your core?

author: TimNJ
author_rank: Super Contributor
timestamp: November 19, 2020, 03:45:46 pm
content: Ooof $200USD for a 70 year old book...That's rough, even if the writing and knowledge is excellent. I'll put it on my wish list.The core I am using is ATQ27-16.3, manufactured by Guangdong Sunshine. The actual material is called SSP-96. It is listed as equivalent to 3C97.http://www.chn-sunshine.com/index_en.html

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 19, 2020, 04:10:01 pm
content: I would like to model it in QuickField 2D to see the magnetic field outside the core.But there are two impediments.Firstly it usually takes me a day or so to create the model and get it working with a new B-H curve.Secondly (more importantly)to model using the Ordinary Diff Eqn solver in frequency domain is not accurate with your waveform, as that solver uses a fundamental sinewave.So it would be better to use the Partial Diff Eqn solver, representing the waveform piecewise in time. This solver takes a long computational time (20 to 30 hours) when the core is non-linear and usually necessary to run it a few times as usually I make a screw-up which only becomes apparent after the solution.

author: TimNJ
author_rank: Super Contributor
timestamp: November 19, 2020, 08:23:18 pm
content: Wow...thanks for considering it. This is something I would totally like to get into, but am lost as to where to start. Could we expect similar field pattern/directionality, with a sine wave source?

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 19, 2020, 09:24:51 pm
content: The shape of your core and your waveform is really needing 3D and Partial DE in time.It could be modelled as 2D with the openings facing toward and away from the user.But then the only information we would get is of the field out from each side of the return paths.It is likely that there will be a stronger field from the cutaways where the airgap is exposed.But in 2D , we can't model that.Also using the Partial DE solver does allow a FFT of the magnetic waveform harmonic content.So I think not worthwhile to spend the time on 2D model.