Ferrite desaturation in slow motion

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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

Re: Ferrite desaturation in slow motion
On Saturday, September 26, 2020 at 7:40:42 AM UTC+10, Piotr Wyderski wrote:
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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.

https://en.wikipedia.org/wiki/Magnetic_refrigeration

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.

https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance

 >Thermal excitations? What is the time scale? What  
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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


Re: Ferrite desaturation in slow motion
Bill Sloman wrote:

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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

Re: Ferrite desaturation in slow motion
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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.


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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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Re: Ferrite desaturation in slow motion
Tim Williams wrote:

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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

Re: Ferrite desaturation in slow motion
On Sun, 27 Sep 2020 20:43:09 +0200, Piotr Wyderski

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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.

https://static6.arrow.com/aropdfconversion/f054de036df0d70bc5a88a0415bdb3ea4c7af4d4/fr_catalog-14thed_rev3.pdf

RL

Re: Ferrite desaturation in slow motion
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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:
https://www.seventransistorlabs.com/Modeling/Images/HI0603P600R_Overlay.jpg
and the model:
https://www.seventransistorlabs.com/Modeling/SPICE/HI0603P600R_NL.ckt
(Saturation isn't quite right, but it's probably close enough to do a crude  
nonlinear circuit.)

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
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Re: Ferrite desaturation in slow motion
On Sunday, September 27, 2020 at 11:43:17 AM UTC-7, Piotr Wyderski wrote:

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The usual picture of grain magnetization has small domains which grow and
shrink.   A true saturation would make the whole grain ONE domain, and
coming out of saturation means generating new domain boundaries, not just
moving preexisting ones; that's a spontaneous symmetry break, and it always
means entropy, i.e. energy loss to heat.  Saturation might depend on grain sizes.

Re: Ferrite desaturation in slow motion
On Friday, September 25, 2020 at 5:40:42 PM UTC-4, Piotr Wyderski wrote:
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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)

Re: Ferrite desaturation in slow motion
On Sunday, September 27, 2020 at 6:07:42 PM UTC-4, George Herold wrote:
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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

Re: Ferrite desaturation in slow motion
George Herold wrote:

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Yes, fluxgates can be sensitive and accurate, but they are pretty slow.
The BW of those I know of is <1MHz. Here I have quite the opposite  
problem: accuracy can be low, and no linearity is required (a magnetic  
window comparator is what I need), but the time scale of H change is on  
the order of 10ns. I am trying to figure out what the B change would  
then be.

    Best regards, Piotr

Re: Ferrite desaturation in slow motion
On Sunday, September 27, 2020 at 11:28:00 PM UTC-7, Piotr Wyderski wrote:
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Here is a magnetic field sensor with a bandwidth of 200 MHz:
https://tinyurl.com/y364ubhy

Re: Ferrite desaturation in slow motion
Flyguy wrote:

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Access denied.

    Best regards, Piotr

Re: Ferrite desaturation in slow motion
Flyguy wrote:
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Re: Ferrite desaturation in slow motion
On Sunday, September 27, 2020 at 11:28:00 PM UTC-7, Piotr Wyderski wrote:
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Why would you want a fluxgate (very accurate) measurement?   For 1 MHz and above,
you can detect changing fields quite well with a coil.

Re: Ferrite desaturation in slow motion
whit3rd wrote:

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Would I?

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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

Re: Ferrite desaturation in slow motion
On Wednesday, September 30, 2020 at 10:42:05 AM UTC-7, Piotr Wyderski wrote:
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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.

Re: Ferrite desaturation in slow motion
On 1/10/20 3:41 am, Piotr Wyderski wrote:
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Directional coupler built from a bit of coax?

CH

Re: Ferrite desaturation in slow motion
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"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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Re: Ferrite desaturation in slow motion
Tim Williams wrote:

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"Lossless" = not introducing much additional losses beyond what already  
is being dissipated. That limits the methods available to magnetic field  
sensing and busbar voltage drop sensing. 100 amps now, 500A in the  
future, low voltage, loss budget is 200mW. I want to detect that there  
is no significant current flowing, I don't care how high the current is
if it is beyond some threshold. Quite a similar problem to synchronous  
rectification.

The sensor needs to be relatively small, fast and reliable and cost  
under 30 dollars in low volume. Hence my musing about saturable  
ferrites, which have all the required properties, perhaps except for the  
speed: 10ns is my pain threshold, 5ns would be perfect.

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No problems with that.

    Best regards, Piotr

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