Ferrite desaturation in slow motion...

[Piotr Wyderski]

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

 

[Bill Sloman]

On Saturday, September 26, 2020 at 7:40:42 AM UTC+10, Piotr Wyderski wrote:

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?

Back when I was a graduate student in inorganic chemistry - I bailed out after 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 interest. 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 little).

Ferromagnetism didn\'t come up. The very small energy differences involved meant that room temperature thermal excitation could the flip nuclei very easily.

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 rotational 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
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.

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

 

[Piotr Wyderski]

Bill Sloman wrote:

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

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

 

[Tim Williams]

\"Piotr Wyderski\" <peter.pan@neverland.mil> wrote in message
news:rkmtnj$3u1fd$1@portraits.wsisiz.edu.pl...

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

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.

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.

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
Website: https://www.seventransistorlabs.com/

 

[Piotr Wyderski]

Tim Williams wrote:

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

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

 

[legg]

On Sun, 27 Sep 2020 20:43:09 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

Tim Williams wrote:

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

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

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

 

[George Herold]

On Friday, September 25, 2020 at 5:40:42 PM UTC-4, Piotr Wyderski wrote:

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

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)

 

[George Herold]

On Sunday, September 27, 2020 at 6:07:42 PM UTC-4, George Herold wrote:

On Friday, September 25, 2020 at 5:40:42 PM UTC-4, Piotr Wyderski wrote:
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

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)

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

 

[Tim Williams]

\"Piotr Wyderski\" <peter.pan@neverland.mil> wrote in message
news:rkqmfv$1gad0$1@portraits.wsisiz.edu.pl...

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.

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
Website: https://www.seventransistorlabs.com/

 

[Piotr Wyderski]

George Herold wrote:

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.

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

 

[Flyguy]

On Sunday, September 27, 2020 at 11:28:00 PM UTC-7, Piotr Wyderski wrote:

George Herold wrote:

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.

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

Here is a magnetic field sensor with a bandwidth of 200 MHz:
https://tinyurl.com/y364ubhy

 

[Piotr Wyderski]

Flyguy wrote:

Here is a magnetic field sensor with a bandwidth of 200 MHz:
https://tinyurl.com/y364ubhy

Access denied.

Best regards, Piotr

 

[three_jeeps]

On Sunday, September 27, 2020 at 6:07:42 PM UTC-4, George Herold wrote:

On Friday, September 25, 2020 at 5:40:42 PM UTC-4, Piotr Wyderski wrote:
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
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)

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

 

[Piotr Wyderski]

three_jeeps wrote:

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

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

http://web.mit.edu/6.101/www/s2016/projects/farita_Project_Final_Report.pdf

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

Best regards, Piotr

 

[whit3rd]

On Sunday, September 27, 2020 at 11:28:00 PM UTC-7, Piotr Wyderski wrote:

George Herold wrote:

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.
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

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

 

[Piotr Wyderski]

whit3rd wrote:

> Why would you want a fluxgate (very accurate) measurement?

Would I?

> For 1 MHz and above, you can detect changing fields quite well with a coil.

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

 

[whit3rd]

On Wednesday, September 30, 2020 at 10:42:05 AM UTC-7, Piotr Wyderski wrote:

whit3rd wrote:

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

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.

 

[George Herold]

On Monday, September 28, 2020 at 2:28:00 AM UTC-4, Piotr Wyderski wrote:

George Herold wrote:

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.

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

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.

 

[Clifford Heath]

On 1/10/20 3:41 am, Piotr Wyderski wrote:

whit3rd wrote:
Why would you want a fluxgate (very accurate) measurement?
Would I?
For 1 MHz and above, you can detect changing fields quite well with a
coil.

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.

Directional coupler built from a bit of coax?

CH

 

[Tim Williams]

\"Piotr Wyderski\" <peter.pan@neverland.mil> wrote in message
news:rl2g15$1usdb$1@portraits.wsisiz.edu.pl...

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.

\"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
Website: https://www.seventransistorlabs.com/