Understanding of suitabily of ferrite grade materials for resonant tuned circuit - Page 1
Forum section: electronics / beginners

author: Sparks_nz
author_rank: Contributor
timestamp: March 20, 2023, 11:59:50 pm
content: HI,I want to wind 2 resonant inductors, with resonant frequencies of 160kHz and 224kHz, and am looking at the various ferrite materials datasheets to see what grade of ferrite are suitable for this use and the freq band of operation.I have some EPCOS N48 and N30 cores that I could use, however the datasheet for N48 says it is suitable for resonant inductors but only up to 100kHz. The N30 datasheet says its suitable for use up to 400kHz, but useage listed as a broadband transformer, so I presume its not suitable for use as a high Q resonant inductor. I cant see any other grades of material that are readily available that would be suitable.Can someone please advise me on whether the N48 will indeed work well (high Q) at the 2 frequencies I need, and whether the N30 will also work well as a resonant inductor with high Q, even though it states use as a broadband transformer. I dont understand how the useage is determined for the various grades of material.thanks and regards,JEFF

author: Jwillis
author_rank: Super Contributor
timestamp: March 21, 2023, 02:02:07 am
content: Here's a PDF that has the various materials and applications.https://www.cwsbytemark.com/CatalogSheets/Ferrite_datasheet_oct06/FR_MATL.pdfHere's some insight on how the composite structure also defines the core characteristics.https://www.ti.com/lit/ml/slup124/slup124.pdfBut generally any ferrite materials should be have frequency ranges of several MHz 160kHz to 224kHz shouldn't be a problem.But your probably looking for a NiZn type 67 ferrite material for high Q.I'm no expert but I hope I helped some.regards

author: jonpaul
author_rank: Super Contributor
timestamp: March 21, 2023, 02:13:01 am
content: Sorry, but The design of a resonant mode soft switching power supply does not begin in this manner.Learn about resonant MODE PSU design de théorie in the many books, papers, seminars openly availableEspecially reccomend the books and papers by our old friends Dr Richard REDL and Nathan Sokol, inventor of class E mode.For core and materials suggest the old Ferroxcube/Philips/ Siemens ferrite core catalogues and applications notesNowadays TDK EPCOS and Fair-Rite have similar.Matériel sélection will be by FIRST what is the thruput power?Then, frequency, losses, operation Hotspot temperature. But Cost and availability drive the selection, as many cores sizes and shapes are available only in a limited choice Finally you need a power matériel not wideband !!!Bon courageJon

author: Sparks_nz
author_rank: Contributor
timestamp: March 21, 2023, 05:00:00 am
content: Thank you both for your replies and the links to material. I will study this.My application is a low power oscillator, where the resonant circuit is tuned to a harmonic of the input drive frequency.In looking at the ferrite material applications, it seems to me that the recommended materials for high Q resonant circuits (such as #67) all have very low permiability (ui=40)and hence low AL value when winding, requiring perhaps several hundred turns to create a 10mH inductor for example, which I presume would have high parasitic capacitance. So it is puzzling for me to see why such a material would have a high Q, compared to a medium permiability core such as N30 (ui=4300), which is designated for broadband transformers, which would only require say 50 turns to create the same10mH coil, but with minimal parasitic capacitance. Can someone please comment on this observation, as it is bugging me, and I presume Im not seeing things clearly.regards and thanks,JEFF

author: Jwillis
author_rank: Super Contributor
timestamp: March 21, 2023, 07:16:08 am
content: I may be way off here but Ni-Zn ferrites seem to be capable of very high Coercive Field (which is defined as "the strength of the electric field at which the macroscopic polarization of the ferroelectric capacitor disappears" and/or "is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized.")At the same time having a low Remanence or residual magnetism which is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed.I'm a little lost on the Coercive Field but seem to understand Remanence or residual magnetism.Maybe being able to hold a stable field from external disruption in combination with the ability to "clear" the magnetic field quickly when a current is removed from the coil determines the Q ?But I'm uncertain as to the difference between one number ferrite to the next. It appears to be the crystalline structure of the material and percentages of the various elemental metals used.Just so much to absorb.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2023, 08:47:58 am
content: Looking for harmonics suggests that the linearity of the material is important, possibly more relevant than a high Q.To get better linearity one could consider having a core with an air gap, even though it reduces the AL. An air gap can also improve the Q, as most of the energy is stored in the air, with not loss there.Looking for a 10 mH inductance suggsets a rather low capacitance and rather high impedance. It could help to look for a larger capacitor and thus a lower impedance.Especially the high µ ferrites can react quite a bit to mechanical stress. Winding directly on a ring core can reduce the AL value quite a bit. Mechanical stress (e.g. handling, thermal) could than also shift the frequency.

author: jonpaul
author_rank: Super Contributor
timestamp: March 21, 2023, 10:56:59 am
content: Impossible to comment without power? voltages? schematic ?j

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2023, 11:19:59 am
content: You've mentioned Q five times, but never once put a number on it.My guess is it doesn't matter and anything will do. Don't worry about it.More important is signal level, and energy storage. Ferrite doesn't store energy very well, but air gap does. But air gapping reduces permeability, necessitating more turns. No free lunch.Also, if you have an underlying application or purpose or function for this, we may be able to comment on the overall suitability of your solution. Perhaps there is an easier way.Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: March 21, 2023, 03:54:57 pm
content: The manufacturer of the driver [still a mystery] will usually recommend the magnetics- its best to stick with their suggestions.You would, as we say in the trade, be bonkers to design your own driver.Theres more to it than the core types- winding patterns and wire types will have a significant effect on the efficency and parasitics.

author: Sparks_nz
author_rank: Contributor
timestamp: March 21, 2023, 07:54:57 pm
content: Thanks. There is mention of linearity, energy storage and air gap in regards harmonics. Does this mean that an air cored coil would be better than ferrite in this regard? Im thinking a twisted multi-filar (single) winding with all the windings in series, so overall turns number is reduced, but winding capacitance would of course be much higher. This brings in the question of the number of filar wires used verses the increase in capacitance, and whether there is a sweet spot in terms of wire numbers used in the winding. I dont know if this is better than a single winding of same inductance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2023, 08:24:22 pm
content: Quote from: Sparks_nz on March 21, 2023, 07:54:57 pmAgain -- how much? Do you know how much linearity you need? At what energy levels? Would an air-core inductor get too large (overall physical size; also resonant modes of the coil, if it has to deal with high frequencies)?There are answers, but without numbers, all we can say is "yep, that's a thing".Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: March 21, 2023, 08:29:21 pm
content: An air core is much better (near perfect) when it comes to linearity, unless you drive the current quite high so that the coil expands or heats up significant.Using a bifilar winding is sometimes used to get an easy / accurate center tap. If the parasitic capacitance is an issue, than it is not a good idea.With a suitable core shape there is not problem winding 100 or more turns.The right size and type of core really depends on the use. This does not sound like a resonant DCDC converter, more like a filter circuit that is planed. Still too much speculation and not much info.

author: Sparks_nz
author_rank: Contributor
timestamp: March 21, 2023, 10:14:36 pm
content: Thanks. Application is a tuned inductor, tuned to a harmonic of drive frequency, tuned to 160kHz as target for example. Primary interest was choice of material for the inductor. I had N30 and N48 potcores to hand, but willl consider other materials. There seems so much technical theory involved and Im not an engineer, so wanted some simple advice as to whether any of the two materials above would be suitable or not, or to find another easily sourced material that would give a suitable high Q. It is mentioned that linearity is perhaps more important than Q in relation to harmonics, so Im now a bit lost as to what is important. I had considered a shielded coil form by Amidon, material #3: 50kHz - 500kHz. Dont know much about these or their performance.

author: Bud
author_rank: Super Contributor
timestamp: March 22, 2023, 12:01:26 am
content: Mind core permeability dependency from temperature. Your resonance ciruit that uses a ferrite core based high Q coil may have an unacceptable drift, given that its design purpose as a harmonic selector.

author: rhb
author_rank: Super Contributor
timestamp: March 22, 2023, 03:45:36 pm
content: The Amidon catalog is very useful. At that low a frequency you want powdered iron cores. Amidon #3 (gray) is recommended for 50-500 kHz. For high Q at low frequency they recommend using larger cores. Amidon gives curves showing Q vs frequency for multiple windings on each of their cores.Have Fun!Reg

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2023, 04:12:30 pm
content: I've made inductors with Q over 400 at those frequencies, using ferrite; litz may be required.Powder tends to be limited, because no matter how much copper you put on it, Q is no more than the intrinsic core loss at that frequency. And the better grades have quite low mu, making copper that much more important. Ferrite you can get better grades and increase airgap to improve Q.But again, anything or nothing may be suitable. Impossible to tell without numbers.Tim

author: rhb
author_rank: Super Contributor
timestamp: March 22, 2023, 06:33:21 pm
content: All I know is what Amidon and others recommend. I've never built a 150 kHz oscillator by intent. Just when my 120 dB DC audio amp misbehaved until I bypassed the electrolytics with ceramics. Also there are a lot of ferrites as there are iron powders. What did you use and at what frequency? Numbers please.I only posted because I remembered the figures from the Amidon catalog. I'll attach the figures later.Edit: Here they are

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2023, 07:02:32 pm
content: To be clear, Q over 100 is generally superlative. It's not that they're bad, just that they can't be arbitrarily good.Think the best I've heard is around 2000 for very fine litz in an open frame "spider" wound configuration. Ooooold school radio loop antenna sort of stuff. Might have trouble increasing that at all with ferrite or powder, but a much smaller part could certainly be made (and with less radiation!).For a complete listing, see Micrometals' catalog (Amidon's numbered powder parts are all Micrometals brand):https://s3.amazonaws.com/micrometals-production/filer_public/7e/d0/7ed096a0-fe6e-4df1-9da9-e129c1ee73d2/q_curve_catalog_issue_h.pdfTim

author: Sparks_nz
author_rank: Contributor
timestamp: March 22, 2023, 07:55:43 pm
content: Thank you everyone for your comments and suggestions. I was looking at the Micrometals Q curves document, and it seems a Q of over 400 is achievable with a T130-2 powdered iron core at my freqs of interest (150-250kHz). I am having trouble understanding a guideline Ive read about, which is the recommendation to operate the inductor at a much lower frequency than the self resonant one. This doesnt make sense to me, from when I used to make crystal sets as a boy. Selectivity was always so much better at the top of the band, where the L/C ratio was high, compared to the bottom of the band, where selectivity was lousy, and L/C ratio was lower. I presumed that operating the coil at the SRF was the ultimate L/C ratio, and provide the best results, but apparently Ive read it doesnt, and articles Ive read say to operate the coil at 50% (or less) of its SRF by padding it down with capacitance. Can someone please help me understand why this is so, as it goes against my logic from my crystal set days, where L/C ratio seemed vital to good selectivity.Thanks for the suggestion of a basket / loop coil. I built a few of these too in my early days, and when linked in to my MW radio, allowed me to listen to distant stations unheard by the radio alone.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 22, 2023, 09:43:32 pm
content: L/C ratio is meaningless in isolation. That's the square of the resonant impedance: Zo = sqrt(L/C).If you have an LC circuit, as a filter, embedded within a system, the ratio of resonant to system impedances gives the loaded Q factor (as opposed to the component Q factor, given by the ratio of reactance to resistance of the component itself).It is critical to know the system impedance.For a crystal set, it will be defined by radiation resistance of the antenna and loss resistance of the earpiece (in turn, the effective or cycle-averaged RF resistance at the detector input).Tim

author: rhb
author_rank: Super Contributor
timestamp: March 23, 2023, 02:55:50 am
content: The Amidon catalog I have (ancient) doesn't show measured Q for ferrites. Not sure why, but I thought I'd add these two pages in case the OP finds them helpful.Have Fun!Reg

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 23, 2023, 01:12:53 pm
content: Well, aside from the lowest mu types, you'd never use ferrite toroids for energy storage; and shapes are almost always used with air gap. Especially spool and rod types, of course. Energy storage goes as Bsat^2 / (2 mu_0 mu_r), so the high permeability makes for terrible inductors. (Even so, I've measured some shockingly high Qs on one modern ferrite, with minimal airgap. You'd still gap it of course, because of energy storage. And the reduced tempco is nice.)More literature to be found onhttps://www.fair-rite.com/for example, including the impedance curves of many bead and ring cores. The complex mu plots (mu = mu' + j mu'') for the material basically give you the Q factor vs. frequency, assuming an ideal winding (and the same core..). Most roll off by 100kHz to 10MHz (roughly inverse with mu_r), that is to say, mu'' dominates at high frequencies; and complex inductance is just another way to say resistance.But do mind that those material curves are themselves made on a typical ring core, and geometry plays a huge role in their shape: compare the impedance of say #43 beads, of various aspect ratios, and to rings, and to the material characteristic (which is measured on a... something around a T80 size I think?).We can explain the diversity of these impedance curves by understanding wave effects: namely that wave velocity is very much slowed in ferrite, because index of refraction goes as 1/sqrt(mu_r e_r), and if mu_r ~ 1000 and e_r ~ 10, that's a good 0.01c propagation velocity, and it doesn't take much to start seeing 1/4 or 1/2 wave reflections (hence peaks or inflection points in the impedance curve) due to the length, diameter or thickness of these parts. Say 100MHz is 3m wavelength, 1/4 wave is 750mm, and 1/100th of that is 7.5mm, a typical scale for beads-on-leads, which have impedance peaks round about this frequency.So it's all a bit hand-wavey up around or above the material cutoff frequency; and, ferrite still has some conductivity so exhibits eddy currents and skin effect (mainly a concern for high power levels (big transformers, like cores 10cm across) at power switching frequencies (~100kHz)). (And, hysteresis loss causes the same phase shift and absorption to the field, so acts equivalently to conductivity for purposes of skin effect.)But we don't mind those effects for board-level components, and most importantly of all, they largely go away when an airgap is introduced, which seems to act to reduce mu_r (that is, if we were to assume the core were solid, what its effective mu_r would be), and the introduced amount is lossless (air!) so greatly increases the Q factor; even mu_r ~ 10k ferrites are usable at 100s kHz with an airgap, since while they might be dissipating relatively high core loss, the reactive power at the winding is just that much more (and Q = VAR / P).(Not to be confused with S = P + jQ, the symbols for apparent, real and reactive power used in power electronics. Since we're using Q for "quality factor", I'll just quietly replace that with VAR (volt-amps reactive).)Tim

author: Sparks_nz
author_rank: Contributor
timestamp: March 24, 2023, 01:59:59 am
content: Thank you all for the replies, and especially to Tim for the detailed response. Ive been looking up the various options, and I see a low mu gapped tunable P26/16 potcore, material M33 (<1MHz) might be suitable. Alternately a powdered iron toroid from the Q curves document, although it would have to have a suitably large inner diameter, so I can get the wire spool threaded through it ok. Another more radical option might be the old fashioned ferrite rod used in MW radios. I presume they would be low mu too, but maybe they have rubbish specs and are not worth considering?One thing I still cant get my head around is why is it recommended to resonate the inductor at a much lower freq than the inductor's SRF? Can someone please help me understand this?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 24, 2023, 07:04:51 am
content: A single ferrite rod, like the AM antennas is not a good idea, as it would couple too much to the environment - the whole idea of using it as an antenna. I don't know what material they use, but it would be OK to have a high µ as most of the magnetic loop resistance is anyway from the air gap and a high µ material would lead to less temperature effect.If planed to use for some harmonic analysis I would question if it makes sense to have a ferromagnetic core at all, because they tend to be somewhat nonlinear.I can make sense to go below the self resonance frequency to get a lower inpedance. At the relatively low frequencies and with a high Q the impedance in self resonance is just impractical higher and even a FET based amplifier would dampen the resonance and in most case make it nonlinear.With a ferrite core one can also get a ferroresonance: that is a mechanical resonance of the ferrite core that couples via magnetistriction to the electrical side. This can get a Q in excess of 1000, but depends on the magentization of the core. So the coupling can change with prior magnetization or an external field. In most cases this is more like nuisance. It is used with some anti-theft tags.

author: jonpaul
author_rank: Super Contributor
timestamp: March 24, 2023, 11:43:20 am
content: spark nz,any inductor has self capacitance in the winding, depending on turns, winding techniques, insulation etc.Calculations or measurements easily reveal the SFR, which is in most commercial inductor specifications,See the excellent parts from Coilcraft or Pulse Engineering.Below the SRF, the inductance predominate.At SRF the inductive reactance is cancelled by the shunt capacitive reactance, thus at resonance, only the resistive losses remain, copper resistive loss and Pfe iron losses.Above the SRF, the part behavior is capacitive.Thus use any inductor below the SRF.Finally reading this long thread, there is some confusion about Q, it's rôle In a filter and filter specification and design.For a few hundred khz filter for a squarewave or similar input, a Q perhaps 5..10 is sufficient.Research Q and inductors design.simple solution isa. stock off the shelf inductors designed for small Signal use, Coilcraft or Pulse Engineeringb. a gapped pot core, using a high permiability ferrite.c. perhaps powdered iron core from Micrometalsdo not use ferrite rod.Bon courageJon

author: Sparks_nz
author_rank: Contributor
timestamp: March 25, 2023, 08:33:03 pm
content: Thank you all for your advice. I will try a gapped potcore first, and see how it goes. Then perhaps a powdered iron toroid, if I can get one a suitable size to put wire bobbin through it. I appreciate all the comments, and have learnt a lot. Still reading material you guys have suggested. This forum is awesome !!

author: jonpaul
author_rank: Super Contributor
timestamp: March 26, 2023, 12:44:55 pm
content: std 22, 26 mm pot core u 1600, 2200 etcgap yourself with paper or G10use bobbinToroids winding is a pain, and have high self C thus lower SRFJ

author: Sparks_nz
author_rank: Contributor
timestamp: March 26, 2023, 08:42:28 pm
content: Thanks for this idea Jon. I was looking at the two grades of gapped P26 potcores which are recommended for resonant circuits. EPCOS N48 and M33.N48 ui = 2,300 (10-100kHz)M33 ui = 750 (0.2-1MHz)From what youre saying, I presume the N48 ferrite can operate well over a much wider freq range than what is recommended above.I never thought about putting an insulating gap between the core halvesthat is brilliant. I do wonder about the exact thickness needed. I presume this critical to getting the correct amount of ue reduction?JEFF

author: Sparks_nz
author_rank: Contributor
timestamp: March 26, 2023, 08:47:35 pm
content: Another thing Im not sure about is I presume I measure the SRF using no added capacitor, and if it is suitably high enough by adjusting turns (say twice the required frequency) then I add a suitable capacitor for resonance at the desired frequency. I have seen using 50% of SRF mentioned as the best percentage for maximum Q?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 26, 2023, 09:15:39 pm
content: Yeah, shimming the cores is the second best way to airgap it; grinding the center peg is the best way. Which can be done with a rotary tool and grinding wheel, but it's a bit messy and dirty. Ferrite is brittle, so go carefully. You have to grind away a bit, check inductance, and back and forth until it's the required value.There are various ways to measure SRF. The most basic is to use the component in a series impedance divider. Adjust frequency until the phase shift goes away: at this point the component has the same phase as the other element. In a divider with a resistor, this means the component is resistive, which means it's at resonance (L and C cancel out).So:Signal generator from Vi to GNDComponent from Vi to VoResistor from Vo to GNDScope probe from Vo to GNDUse a 10x probe, and use a smallish value resistor (a few k max). This minimizes loading from the probe (which is capacitive in the ~MHz range).It could also be tested with a grid-dip method, using a coupling link (i.e., a small coil) and whatever coupling gets through the ferrite core (pot cores are well shielded, but not perfect; or a small loop could be put into the winding window or near the air gap to get more coupling). In this case you're testing for the frequency where the link/loop/antenna sees a dip (notch) in amplitude response. A sweep generator and oscilloscope (set to peak-detect mode) is one way to do this. (This isn't as easy a method overall, so I'm not bothering to get very specific; it used to be more common, with the "grid dip meter" being fairly common equipment. Which would be easy enough to make, it's true, but not so easy to calibrate, is the problem.)As for measuring inductor parameters with limited equipment, I use these for various things:https://www.seventransistorlabs.com/Calc/RLC.htmlTim

author: jonpaul
author_rank: Super Contributor
timestamp: March 27, 2023, 12:02:17 pm
content: Rebonjourgaps....in 55 years we never ground a gap.a. paper or G10 can easily be adjusted, once ground out a gap cannot be reduced.b. certain pot cores have adjusters to trim the inductance eg tune a resonanceSRF..SELF resonant frequency mean the inductance by itself NO added cap! it's the resonance of the L and winding parasitic C.Measurement SRF...any oscillator,audio, function generator RF, that covers the approx frequency range.GEN>>series R 10k..100k>>L under test.observe V across the inductance with 10x scope probe.Sweep F and find maximumAlternative demo revers L and R, observe voltage on resistanceEasy to make a Vi lissajous figure.J

author: Sparks_nz
author_rank: Contributor
timestamp: March 27, 2023, 08:11:02 pm
content: Hi,Thanks. Yes, I see that the EPCOS N48 potcore has an adjustable slug that can be screwed in to adjust inductance, the spec sheet says it has 10% range or so.

author: Sparks_nz
author_rank: Contributor
timestamp: March 28, 2023, 07:51:17 pm
content: One thing that is confusing is that the P26 potcore spec sheet lists various variants of the N48 material type, depending on the gap distance (s). See attached snap.The AL value varies considerably due to this, and Im unsure what or if there is an optimum one to choose from.Does a large gap mean it is more broadband in its overall Q value, compared to a smaller gap, or are there other properties that the gap size will influence in regards a resonant circuit. I know you have mentioned energy storage in relation to the gap.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2023, 11:13:33 pm
content: Q generally goes up with gap, because air has no loss. But at low enough mu_eff, the amount of wire required to take advantage of the core goes way up, and eventually overall Q starts going down again. mu_eff in the 30-120 range is generally best with copper windings.Tim

author: Sparks_nz
author_rank: Contributor
timestamp: March 29, 2023, 05:28:21 am
content: Thanks Tim, I think Ive got a good handle on things now, thanks to yourself and the other guys.J