Ferrite materials for PSUs design - how to navigate through them ? - Page 1
Forum section: electronics / projects

author: Adrian_Taylor333
author_rank: Regular Contributor
timestamp: July 28, 2025, 09:33:10 pm
content: I have a hard time figuring out subtle differences.There are tradeofss between permeability, H-field strength, losses frequency etc etc.But napkin-calculation brings me to more or the same result - I can use higher currents, but lower the frequency etc etc.It all ends about the same power at the end.So what's the point of all these material variations ?Also, paging through TDK, Ferroxcube etc portfolio, most of them have INSANE property variations over temperature.How is one supposed to work with that over reasonable temperature span without his design blowing up ?That is, almost all, except baseline material like 3C95.Is that the only thing used nowadays in PSUs and similar stuff that have to work over wide delta T spans ?What is the point of bazzilion of other materials then ? 🙄Where look and what to be wary of ?Also, are there industry standards - material lines that are interchangeable across suppliers globally ?Do Chinese sources keep current with those or do they have their own formulations that differ significantly ?

author: Someone
author_rank: Super Contributor
timestamp: July 28, 2025, 09:57:25 pm
content: Quote from: Adrian_Taylor333 on July 28, 2025, 09:33:10 pmOld timers trick: let the supply heat up the core and run it relatively constant temperature as the losses decrease with core temperature, never let the design roll over into thermal runaway.

author: Adrian_Taylor333
author_rank: Regular Contributor
timestamp: July 28, 2025, 10:06:16 pm
content: Quote from: Someone on July 28, 2025, 09:57:25 pmFew problems with this:* what if material has no distinct decrease in loss over temperature or if it has a peak efficiency point ? If temp exceeds it, you get a runaway ?* what if point of optimal permeability doesn't coincide with that ?Also, many material have so wild permeability/temp curve that the PSU could seriously go bang or into overload/shutdown mode before the core heats up.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 28, 2025, 10:15:41 pm
content: A few things to keep in mind:1. Many ferrite materials are not meant for switchmode power conversion at all. Many are meant for EMI suppression, or RF power applications (like high Q resonant power circuits). For switchmode power, you usually want to ignore there.2. Often the temperature-dependence is intentionally engineered to prevent thermal runaway. It makes analysis a bit messy, but is ultimately serving a good purpose.3. Usually core vendors will have some rough guidelines for selecting material depending on the intended switching frequency. This should be how you narrow down dozens of options to just a frew.4. Last time I checked, there were at least a half dozen definitions of "permeability" to choose from (effective permeability, initial permeability, amplitude permeability, etc). Not sure which one you're referring to.

author: Adrian_Taylor333
author_rank: Regular Contributor
timestamp: July 28, 2025, 10:21:21 pm
content: Quote from: mtwieg on July 28, 2025, 10:15:41 pmAD1: I'm pretty sure I was looking at MnZn materials for power applications. TDK also lists their intended usage in the header IIRC.AD2: Fine, but they state material as high permeability and then you see int eh graph that is some peak of the temp curve. How is one supposed to regulate temp of the core and make sure it never gets into low parts ?AD3: their guidelines are vry hazy, to say the least.AD4: Neither am I.

author: Adrian_Taylor333
author_rank: Regular Contributor
timestamp: July 28, 2025, 10:50:04 pm
content: Also, it is unclear how the f*#!% is one supposed to work with permeability tolerances of 20% on top of all that, especially since many different materials differ by about that or less.So, when factoring all tolerances, uncertainties and unknowables conditions inside the core are approaching it-could-be-anything/"Schroedinger's cat" state.WTF ?Fine, one can go for resonant design that would adapt to core's moods and variations, but what's the use of all those marketed properties if one can't depend on them ?

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 28, 2025, 11:06:04 pm
content: Usually ferrite permeability does not need to be tightly controlled. Maybe you can explain more specifically what your concern is?If you need very tight control of magnetizing inductance, then add an air gap (though keep in mind the air gap has its own tolerance, so if the gap is very small this may actually make tolerance of the inductance worse!).

author: Adrian_Taylor333
author_rank: Regular Contributor
timestamp: July 28, 2025, 11:56:14 pm
content: Quote from: mtwieg on July 28, 2025, 11:06:04 pmAirgap kills the magnetic loop permeability, introduces stray inductance and thus kills winding magnetic coupling.It is used on smal flyback units, not half- or full-bridge...

author: Benta
author_rank: Super Contributor
timestamp: July 29, 2025, 12:23:38 am
content: Quote from: Adrian_Taylor333 on July 28, 2025, 11:56:14 pmIf you're working on half- or full-bridge topologies, why do you care so much about tolerances and temp. coefficients? Sure, your transformer magnetizing current will vary some, but is that really important?

author: TimFox
author_rank: Super Contributor
timestamp: July 29, 2025, 01:00:12 am
content: Quote from: Adrian_Taylor333 on July 28, 2025, 11:56:14 pmYes, the air gap reduces the magnetizing inductance to a value dominated by the gap, which can be controlled by a spacer. The effective path (reluctance) is proportional to the (length through ferrite divided by permeability) + (length through air). The first term should be small compared with the second term, but the permeability depends on the flux density. It’s a trade-off between max inductance and variation in inductance, plus tolerance to DC.

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 29, 2025, 01:09:36 am
content: Quote from: Adrian_Taylor333 on July 28, 2025, 11:56:14 pmSmall air gaps are definitely used in some full/half bridge supplies (to help prevent flux walking), and is also seen in PSFB and LLC converters (to get tighter inductances). Of course it has downsides (compared to an external inductor in parallel with the primary). But it's not too rare to see.Quote from: Benta on July 29, 2025, 12:23:38 amExactly. In most cases, flux swing is the main thing we want to control, and this usually has nothing to do with permeability.

author: Someone
author_rank: Super Contributor
timestamp: July 29, 2025, 01:19:37 am
content: Quote from: Adrian_Taylor333 on July 28, 2025, 10:50:04 pmCompletely normal, just like any other uncontrolled variable, a robust power supply is designed to operate "normally" despite all the possible states/extremes of the part. I've a design here that is happy with anorder of magnitudebetween the high and low, and a factor of 2 is a good economic starting point. If high variability isn't for you then building your own magnetics isn't for you (and you probably haven't measured off the shelf inductors/transformers across a significant working range).

author: TimNJ
author_rank: Super Contributor
timestamp: July 30, 2025, 10:15:28 pm
content: Assuming you are talking aboutpowerferrite materials? As others have noted, ferrite cores are typically used in two distinct applications: power and suppression. Power materials have low core losses to minimize heat generation. Suppression materials have intentionally high core losses (i.e. resistive part of complex permeability) such that EMI energy is dissipated as heat, albeit usually just little tiny bits of heat.For power materials, there are materials optimized for different frequency ranges and there are materials optimized for different temperature ranges or expected operating temperature. In power supply design, thermal performance is usually assessed at the maximum rated ambient temperature per specification. So, it may turn out that you don't care if the core loss is high at 40C, because the core is going to reach 100C anyway. If that's the case, you might pick 3C96. If you are trying to meet a certain efficiency specification at lower temperature, then you might prefer a material like 3C95. A possible issue with 3C96 is that if your design runs hotter than 100C (where core loss is minimum), you could initiate thermal runaway where the increasing core loss heats up the core, which heats up the copper, whose resistivity increases as a result, which heats up the core, whose core loss increases as a result..etc..In this case, you might prefer a material like 3C97 which will be more forgiving to such a scenario due to the flatness of the core loss curve.

author: jonpaul
author_rank: Super Contributor
timestamp: July 31, 2025, 11:55:30 am
content: SMPS magnetics design requires a lot of learning and experience.See our old friends Steve Smith, Magnetic Components Design and the IEEE/PESC/seminars of Dr Richard REDL.See the many texts, seminars and papers, as well aas the extensive app notes and white papers from Ferroxcube, EPCOS, Philips, Magnetics Inc, and TDK.Here is the excellent TDK core matl cat. We used TDK PC40https://product.tdk.com/en/system/files/dam/doc/product/ferrite/ferrite/ferrite-core/catalog/ferrite_mn-zn_material_characteristics_en.pdfpower materias are specified for a temp range optimum and freq range.Al tolernace is not an issue in transformer and inductor design as good practice takes account of the wide (20-50% ) tolerance of Al.Finally see the old but excelletn transformer CAD tool, Magnetics Designer from Intusoft.http://www.intusoft.com/mag.htmBon chanceJon ( power electronics, magnetics since 1970s. )