transformer core flux propagation speed

'Stocks'? I think you mean 'Stalks'

Another model shot!

Barkhausen noise used to be a huge problem in hard disk heads, back when the pole pieces were large enough to have more than one magnetic domain. For the past decade or more, the domain walls have been pinned by the geometry, so it's not such a big worry.

Cheers

Phil Hobbs

Hi,

Does Barkhausen noise stop above a certain frequency, maybe proportional to the magnetic domain sizes? Also for an extreme example, for AC core losses, normally they are thought of as caused by eddy currents, but if the frequency is (extremely!) high enough, maybe the eddy current losses will start reducing, but will the transformer core also start to lose its inductance properties so that it is not useful? Just curious maybe 50 years in the future core losses will not matter!

cheers, Jamie

AFAIK it isn't white, but it goes well up into the megahertz. The spectrum depends on everything including what you had for breakfast. (Like so many ferromagnetic things.)

Barkhausen seems to have made a career out of investigating inconvenient effects, so that he got his name associated with a lot of annoyances: Barkhausen noise, Barkhausen oscillations, and the Barkhausen criterion for when your amp becomes an oscillator, or vice versa.

Cheers

Phil Hobbs

Probably on par with the domain relaxation time, which depends on material; most likely this corresponds to the cutoff frequency as well.

More or less. If you model core losses entirely as eddy currents (ignoring hysteresis or material properties), then the cutoff frequency corresponds to the skin depth in the material: when the skin is shallower than the core thickness, less core is utilized and the effective permeability drops. Skin effect, in turn, occurs when the current flow produces a field opposing the applied field. If you can reduce the conductor dimensions (stacks of iron sheets, or powder), or use a higher resistivity material (by adding silicon to the steel, or using a ferrite), the cutoff frequency rises again.

Permeability has the same frequency response / amplitude / phase interplay that any other filter does, so it should be no surprise that, if normal (real) permeability results in an inductive core, a lossy (phase shifted, complex or imaginary) permeability results in a resistive core, and that resistive phase shift necessarily occurs over a frequency range where permeability is dropping with rising frequency. Indeed, all core materials I've seen look almost entirely resistive at or past some frequency.

Some graphs:

If you recall |mu| = sqrt((mu')^2 + (mu'')^2), i.e. the vector sum of real and imaginary components, you might notice that, at the point the curves intersect, the value of each is about 70% the flat range (about 1/sqrt(2)). This means, at the intersection, the value of |mu| doesn't actually dip. As a result, |mu| actually remains fairly constant up to almost 4MHz. The implication is, the magnetizing impedance of an inductor or transformer remains essentially inductive from DC to 4MHz, though the phase shifts substantially past 1MHz. Beyond 4MHz, the impedance remains flat or drops further, because |mu| also drops.

In fact, the roll-off can be calculated from the graph. The tail drops at a rate of about 2 decades in mu for one decade in F, so the impedance Z = j*2*pi*F*mu drops off inversely with frequency past this point. This is a second order (-20dB/decade) drop, which is physically significant: if it were simply resistive, it would be 10dB/decade instead.

Ferrite beads graph this more directly. See for example:

Another material of note, Fair-Rite #43 (I'd guess at some equivalents, but it's actually kind of funky so I won't go out on a limb here), which is typically used for high frequency transformers and ferrite beads.

Something else pertinent to draw from these graphs: mu vs. temp is provided. You can see there's a sort of "stochastic resonance" approaching Curie point, which means the domains are getting easier to align -- more thermal energy makes more flips, which reduces the number of spins available to use (saturation flux density is reduced -- see the B-H curves at different temperatures), but they're easier to use.

FYI, "stochastic resonance" is a big phrase for, if a system has a dead band or hysteresis, adding a low-level stimulus (be it a coherent waveform or random noise) actually improves the noise floor of the system. For example, dithering and averaging an ADC can yield more bits accuracy. Strictly, "stochastic" only applies to systems where random noise is used.

Tim

Was she a stunning beauty, and what camera did you use? ;-)

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trigger happy google!

Anyway, after working with metglas in the micron ranges, I've come to the conclusion that many magnetic material data sheets are STILL showing macro-effects. Refer to the 3C90 data sheet. The data sheet gives information that is important to you the way you use the core material but does NOT reflect the true basic nature of magnetic materials. What has happened is that from reading data sheets, engineers have gotten a mindset of how magnetism works but that mindset is misleading both as one starts to scale smaller AND if one tries to use that mindset's understanding to solve some kind of performance issue. Instead of really solving a problem, you end up optimizing a 'weak' solution.

It's just that when you see losses and permeability roll off for a material, much of those losses and much of that rolloff can be attributed to conductivity and eddy currents minimizing the fields destroying effective permeability, NOT the true nature of the material, but rather its gross effectc because of how it's used. Again, true these parameters are important to the designer, BECAUSE that is the component they're working with, but do not really reflect the nature of the basic material.

For example, every one seems to accept that high permeability material rolls off above some low frequency, like 1MHz, NOT TRUE. the EFFECTIVE permeability rolls off above 1MHz because of the way the material has been configured into the form you're using it! *IF* you can gain the luxury of restructuring how you use the material you will see most materials have extremely high permeability above 100MHz

Sadly, at around 1-2GHz magnetic material is gone due to the moment of inertia of the magnetic molecule.

No, no, Tim, you don't understand science at all. ;) Terms like "stochastic resonance" are beautifully designed to help folks like these

extract $$ from the feds. What's more scientific than that?

Cheers

Phil Hobbs

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I had this inspiration that amorphous magnetics, metglas, might not have Barkhausen noise, so I looked it up. They do.

I understand about half the words they use and still grok no understanding :^)

Kidding, I get what they're getting at... but more to the point, I get what you're getting at :)

Tim

Indeed, the ferrite chips I linked peak well into the GHz. That's going to be due to package L and C as much as material properties, but clearly nothing at all would happen if the ferrite itself were not also peaking out there.

To be fair, the graphs of mu do include the note as to what they were measured on: a typical size toroid and the instrument. It's not obvious from the measurement how it might vary with size.

Ferrite works well into the GHz, but I don't know in that range if it's limited by materials, construction or both. All the ferrites listed for very high frequency use are low permeability, probably because the individual ferrite grains are well insulated with impurities (glass?), making a ferrite "powdered iron".

At school, their standard microwave hardware included a couple watt Gunn diode, followed by an isolator. You don't want to get any diskettes close to the thing as its magnets are strong enough to hold keys on end.

Tim

Hi,

Maybe these magnetic monopoles keep their moment of inertia?!

cheers, Jamie

Embarrassingly, NEVER heard of any of this research!

Thanks for the URL.

Another grandstand play by another idiot scientist, so new? Calling those things monopoles just confuses people, but hey, why not, it brings in $$ and invited-speaker gigs. Why worry about accuracy when you can have tenure instead?

Cheers

Phil Hobbs

Can a true monople come into existance under ANY conceivable circumstances?

For example, the suddent creation of charge particles? I mean creation, too.

If there were really any free magnentic monopoles, they'd eat up magnetic fields the same way that free electric monopoles (e.g. electrons) eat up electric fields. You'd magnetize something, and then within a few minutes it would look as though it were demagnetized, because all its surfaces would be decorated with magnetic monopoles until the field outside went to zero. Just the way ferroelectrics behave in the real world.

Martin is probably much more up on this than I am, but iirc the persistence of intergalactic magnetic fields puts some absurdly low upper limit on the cosmic abundance of magnetic monopoles.

Cheers

Phil Hobbs

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I had this crazy idea (in my youth) that magnetic monopoles would obviously occur in pairs and all be bound up like hydrogen atoms. Bound up magnetic monopoles might then be dark matter... only x-rays or something can break them apart.

George H.