Finding Inductance and core loss of a powdered iron inductor in a "2TFC" SMPS? - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 19, 2020, 06:05:40 pm
content: HiOn page 35 of DER484 by Power Integrations (below), there is a torroid inductor (Made of "kool mu" powered iron) for the output of a Two Transistor Forward converter.This torroid is 77930-A7 by Mag-Inc. (as below) It is wound with 75 Turns.Finding saturation flux densityLooking at the B vs H curve here..https://www.mag-inc.com/Products/Powder-Cores/Kool-Mu-Cores/Kool-Mu-Material-Curves#kmdcmagnet...Kool Mu appears not to saturate at 600mT....but its not possible to find out from the website what is the saturation flux density of Kool Mu?Find Inductance at 4.6AI wish to first find the Inductance at its average current level of 4.6A.I believe I have to do this by finding the new permeability at the current level of 4.6A, then find the Reluctance at this permeability level {using Reluctance = l/(uo.ur.A) }, then find inductance at 4.6A by L = (N^2)/Reluctance.This is done by looking at the “Permeability vs DC Bias” curve of this…https://www.mag-inc.com/Products/Powder-Cores/Kool-Mu-Cores/Kool-Mu-Material-Curves#kmpermvsdc…in fact , better still, there is an equation here stating “Percentage of initial permeability” is 1/(a+b.H^c) [H in Oersteds]The result is that Inductance at 4.6A is 283uH.Would you agree?Finding Core LossWe then have to find the core loss of this inductor. I take the peak and trough of inductor current as being 5.6A and 3.6A respectively. We then find the B at the peak and trough currents using B=uo.ur.HThe result is that the Bpkpk value is 0.013T. Is this correct? It seems kind of bizarre because the Flux density at 3.6A is greater than the flux density at 5.6A (!)Then, taking this Bpkpk of 0.013T, we use it with the graph of “Core Loss Density curves” shown here…https://www.mag-inc.com/Products/Powder-Cores/Kool-Mu-Cores/Kool-Mu-Material-Curves#kmcoreloss..to find core loss..The switching frequency is 66kHz, and from the above graph, the core loss is seen to be well under 30mW/cm^3 at 66kHz. (I.e 0.03W/1E-6 m^3) The torroid volume is 4150e-9 m^3. Therefore, the core loss is less than 120mW.This seems ridiculously low for a powered iron core, which is known for having high core losses.Do you know what’s wrong here?DER 484 document:https://ac-dc.power.com/sites/default/files/PDFFiles/der484.pdfTorroid is 77930-A7 by Mag-Inc:https://www.mag-inc.com/Media/Magnetics/Datasheets/0077930A7.pdfMy workings are as attached

author: uer166
author_rank: Super Contributor
timestamp: December 19, 2020, 08:45:10 pm
content: If you define "saturation flux density of Kool Mu" of that specific core as, 80% inductance loss, then it saturates at 1A. Look at the specific datasheet for that core and find the amp-turns vs. DC bias permeability loss. Kool Mu has no sudden saturation like ferrite, so you can't really define one number.As for everything else, Magnetics gives you the curve fit functions for pretty much everything with coefficients, use them to calculate core losses (*around 0 DC bias), magnetization, delta B, etc etc.

author: uer166
author_rank: Super Contributor
timestamp: December 19, 2020, 08:53:09 pm
content: I've put your core into my quick and dirty calculator and get B swing of 0.3 Tesla and loss of 5W. I think your magnetization calculation was off.

author: uer166
author_rank: Super Contributor
timestamp: December 19, 2020, 09:10:31 pm
content: I see where the mistake is. You can't simply use the "percentage of initial permeability", multiplying that by permeability at 0 DC current. Instead, use their curve fit function todirectlycalculate B, given H, without extra weird steps in between, then B will rise with H as it should.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 20, 2020, 01:13:56 am
content: QuoteThanks, i must admit i'm still a little phased by the Power Integrations calculations for this inductor in their DER484 document...https://ac-dc.power.com/sites/default/files/PDFFiles/der484.pdf..on page 44, they get an inductance value of 229.4uH at 4.6A....Again on page 44, they get a DC flux of 0.2385T in this inductor....Also pg 44, they get an AC flux of 0.0575T in this inductor...on page 55, they get a total core loss of 0.62W

author: uer166
author_rank: Super Contributor
timestamp: December 20, 2020, 01:30:19 am
content: Okay? You specified: "I take the peak and trough of inductor current as being 5.6A and 3.6A respectively". My calculation was based on that. Looking at the document, I can't find any info on the peak AC current swing in output choke, but it seems to be much less than 2A postulated by you.To not go in circles, what do you think the specific issue is?

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 20, 2020, 11:24:30 am
content: Thanks, i am trying to work out how Power Integrations worked out the values in pages 44 and 45 of their DER484 ...as in my post #4 above.Power integrations (in DER484) come up with an inductance of 229.4uH at 4.6A.......i calculated it should be 283uH.But if i put in 229.4uH into the 2 tran forward design, then i get a peak inductor current of 5.7A, and trough" of 3.4A.(as in my excel attachement of top post)...(this supposes that the inductance stays at 229.4uH between 3.4A and 5.7A, which it obviously doesnt, since its powered iron type, so i was being a little approximative, to save time)Either Power integrations in DER484 are wrong, or i have , in my top post, badly violated the laws of physics, i dont know which it is yet.