Ferrite desaturation in slow motion

Hi everyone,

The following appears to be more physics than electronics, but is very relevant to the latter and many of you have already amazed me with your knowledge. So here is the question.

A ferrite toroid is saturated by some current defined by the geometry of the core/winding and some material constants. The exact values of I and B(I) are not important, assume they are sufficiently high.

Now, as the current is decreased, B(I) eventually decreases to some B_r. This is a relatively accurate collective description of the underlying phenomena. But what are these phenomena? What causes the domains to lose their alignment? Thermal excitations? What is the time scale? What actually happens in the ferrite when observed at nanosecond resolution? I know what the situation is going to look like after a microsecond, but what is the dynamics of the change?

Could you please suggest me some good reading on the transient phenomena in ferrite ceramic materials? I would like to understand that far better and beyond what the typical magnetics design books have to offer.

Best regards, Piotr

Reply to
Piotr Wyderski
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Back when I was a graduate student in inorganic chemistry - I bailed out af ter two years with a master's degree, and went on to do a Ph.D. in physical chemistry - the magnetic behaviour of transition metal nuclei was of inter est. They were either paramagnetic (the nuclei tended to line up amplifying the field a little ) or diamagnetic (the magnetic axes of adjacent nuclei tended to point in opposite directions, attentuating the external field a l ittle).

Ferromagnetism didn't come up. The very small energy differences involved m eant that room temperature thermal excitation could the flip nuclei very ea sily.

Magnetic refrigeration exploits this down at liquid helium temperatures.

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How fast it could happen is anybodies guess. You are talking about the rot ational inertia of an atomic nucleus which is very small indeed.

Nuclear magnetic resonance in a 20 Telsa field happens at frequencies from

60?1000 MHz, so it can be quite quick.

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None of the books I've read about magnetic phenomena have been in the least helpful. Physicists may have access to better texts. Phil Hobbs or George Herold come to mind.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

People are using pretty typical ferrite toroids in pulse compression circuits, and the time scale is tens of nanoseconds or less. So the underlying physics is fast enough, but I am not sure what the physics is.

At the moment, I am interested in the transition phenomena only ("edges", not "levels"). On a practical note, I would like to know how fast I can desaturate a piece of ferrite without making it explode, what ferrites would be the fastest and how to optimise the process. Most importantly, I want to understand what's going on under the hood, as I am not a big fan of voodoo science.

Thank you, Bill.

Best regards, Piotr

Reply to
Piotr Wyderski

I would propose a figure on the order of the electron paramagnetic resonance. So, some GHz typically. This isn't very noticeable in spinel ferrites (losses dominate), but garnets are useful in the GHz -- using the Faraday effect in circulators/isolators, and the EPR directly in YIG oscillators.

Losses are scale dependent, hence why large ferrite beads have a lower peak resonance than small ones, etc.

Higher loss materials, in relatively large shapes, mask the effect of underlying physics -- there's just so little material participating, there's practically no signal left at the ~GHz where interesting things might be observed.

So, MnZn (high loss, high mu) tends to be... "classical", in the sense that it can be described as a lossy bulk material with all the usual messy hysteresis and saturation properties. NiZn, same thing but lower loss, mu and Bsat. YIG lower still, but finally low enough that quantum effects are perceptible (like EPR).

Likewise for metals, bulk forms are lossy in a classical skin-effect manner (laminated iron, amorphous/nanocrystalline strip). Powder is lossy in a similar way, given a range of particle size and some bulk conductivity (depending on binder fraction and pressing).

I'm not sure offhand if there's a metal powder composition that has particles fine enough, or of the right alloy, such that quantum effects are measurable at high frequencies.

Mind, this is all very hand-wavey, partly because I know very little about the physics itself, and partly because physics itself knows very little about ferromagnetism. There isn't much explanatory value in theoretical studies of such a complex material; empirical studies tend to be more useful.

Also just my rough understanding; I haven't played with a lump of YIG for example.

Under the above assumptions, I think you'll find that, even if you set external fields to zero, the bulk will take some time to "relax" to whatever level it does (remenance), and that time will be determined in essence by the L/R time constant of the bulk material. Which again, depends on size (it's a stretch to call it a "bulk time constant", it's scale dependent).

This can also be understood in terms of wave propagation: the speed of light inside the material is quite low (high mu, modest e_r, modest rho), so the external field change is transmitted through the bulk at a corresponding rate. And because the material is lossy, the field doesn't reach the center intact, it's attenuated and dispersed. (The variable mu and loss with frequency causes velocity to change as well.) So you get some standing waves, but they're largely damped, and there's a tail as the internal field eventually settles out.

I have seen a few articles where magnetic compressors or shock lines were built with stacks of alternating washers, of ferrite and dielectric. Same idea as laminated iron, done at proportionally higher bandwidth, and with proportionally higher frequency material. :-)

Neat fact: standing waves are measurable on ferrite beads. Compare long and thin to short and fat shapes. Most parts have a more-or-less simple resonance (that's reasonably well fit by a single RLC unit, plus some diffusion RL on the LF side, plus DCR), others have an inflection point or even a double peaked response.

Somewhat less useful fact: standing waves occur in metals, too. This is why round wires have skin effect given by Bessel functions. The limit, as delta/R --> 0, does indeed equal the exponential solution found in the infinite-plane case. That is to say, as the curvature of the wire, relative to skin depth, goes to zero, the geometry and solution are equivalent to the plane case. (Less useful, because the difference in AC resistance isn't much, in the end.)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

Thank you very much for your very practical input.

It led me to a follow-up question: how do ferrite losses depend on saturation? If the frequency is sufficiently high, say ~1GHz, and the wire passes through the core, can I turn on/off the losses by saturating the ferrite? Say the attenuation range of interest is 2:1 or more.

I am thinking about a magamp-like structure, but the controlled parameter would be attenuation, not inductance. A HF magnetoresistor, in fact.

Best regards, Piotr

Reply to
Piotr Wyderski

Attenuation/impedance at non-gigahertz frequencies is illustrated in the Fair-Rite catalog for selected commercial part types at varying levels of DC bias. Magnetic fields imposed from external sources on unbiased parts should be expected to have similar characteristics.

Heavily saturated parts show impedance as low, but still increasing with frequency, at 1GHz, for some structures.

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RL

Reply to
legg

I know almost nothing of magnetics. But we made this ~flux gate magnetometer out of an inductor* (kinda over driven) and observing it come in and out of saturation was very interesting to me.

My only thoughts, George h.

*in some external B-field (over-wrapped coil)
Reply to
George Herold

One purpose of the over wrapped coil was to cancel the Earth's B-field... so that gives you some estimate of the fields involved. GH

Reply to
George Herold

Yes; mu falls and, probably losses remain a constant fraction of that, but because the magnetic path is effectively getting more air gap (which is lossless), the Q rises.

legg linked the Fair-Rite catalog that shows some plots with DC bias; and Laird's catalog is even more expansive (if blurry).

Here's a part with curve fitting besides:

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and the model:
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(Saturation isn't quite right, but it's probably close enough to do a crude nonlinear circuit.)

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

Yes, fluxgates can be sensitive and accurate, but they are pretty slow. The BW of those I know of is

Reply to
Piotr Wyderski

Here is a magnetic field sensor with a bandwidth of 200 MHz:

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Reply to
Flyguy

Access denied.

Best regards, Piotr

Reply to
Piotr Wyderski

Hmmm, forerunner to a flux capacitor in a DeLorean? lol

Reply to
three_jeeps

Actually, a very old and ingenious concept, predating the WW2 days:

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It is amazing that something that crude can offer this level of performance.

Best regards, Piotr

Reply to
Piotr Wyderski

Why would you want a fluxgate (very accurate) measurement? For 1 MHz and above, you can detect changing fields quite well with a coil.

Reply to
whit3rd

Would I?

I know of no CT with 100MHz+ bandwidth. Only the Rogowski coil comes close, but even then it is far from trivial, as the output voltage is tiny and the integrator brings its own problems.

I want to detect if current is below some arbitrary threshold (the exact value is not very important, an amp or two), but I want to detect that event fast and in a lossless manner.

Best regards, Piotr

Reply to
Piotr Wyderski

Current transformer offerings aren't intended for that, of course, but a Rogowski coil IS completely acceptable; any radio that tunes UHF has enough bandwidth, and enough sensitivity, with the coil connected as an antenna. The 'output voltage is tiny' isn't an issue if the signal/noise ratio is good and the impedance is low.

Reply to
whit3rd

OK, HF magnetics is beyond my experience... (except for winding transformers/coils on some ferrite) One thing I found interesting with flux gates is how they go in and out of saturation... that's a large field effect as well as being ~DC.

George H.

Reply to
George Herold

Directional coupler built from a bit of coax?

CH

Reply to
Clifford Heath

"Lossless" is a physical impossibility, the real question is how much loss can you tolerate, under what conditions?

Notice it's not cheating to use, for example, a CT with a diode shunting the burden resistor. Fine resolution at low currents, modest dissipation at high currents (if high average current, or pulse load, is a requirement). The burden resistor can be a low value, giving a short time constant with the diode capacitance, or transformer strays. (The transformer does need to be good enough for the bandwidth, typically with winding length much shorter than the wavelength in question, which significantly limits the number of turns.)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

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