Maker Pro
Maker Pro

permeability

I bought some N41 ferrite material surplus pot cores from BGMicro.
References I dug up on the net give N41 initial permeability of 2800 to
3000.

looking at this:
http://www.ee.surrey.ac.uk/Workshop/advice/coils/mu/
in the section "ferromagnetic materials" B versus H graph, I noticed
the significant variation in permeability (slope of the graph) as flux
density goes from small to medium values.
And then I saw in the following link that the initial permeability of
N41 seems to be measured at the extrememly low flux density, 0.25 mT.
http://www.epcos.com/web/generator/Web/Sections/Components/Page,locale=en,r=263282,a=263370.html
which would seem to indicate that the permeability in practical use
like a SMPS would be quite a bit higher than the listed initial
permeability of about 3000. Should I use the listed ui or some other
value?
 
J

John Popelish

Jan 1, 1970
0
I bought some N41 ferrite material surplus pot cores from BGMicro.
References I dug up on the net give N41 initial permeability of 2800 to
3000.

looking at this:
http://www.ee.surrey.ac.uk/Workshop/advice/coils/mu/
in the section "ferromagnetic materials" B versus H graph, I noticed
the significant variation in permeability (slope of the graph) as flux
density goes from small to medium values.

This is a result of the BH loop having significant width.
And then I saw in the following link that the initial permeability of
N41 seems to be measured at the extrememly low flux density, 0.25 mT.
http://www.epcos.com/web/generator/Web/Sections/Components/Page,locale=en,r=263282,a=263370.html
which would seem to indicate that the permeability in practical use
like a SMPS would be quite a bit higher than the listed initial
permeability of about 3000. Should I use the listed ui or some other
value?

If you are either using an ungapped structure (toroid) for
low level filtering, or a gapped structure like a ferrite
rod antenna that will see only small excitation, the lower,
initial permeability at least has to be included in your
design. If the core will be gapped and be operated at high
flux levels, like most power transformers and energy storage
inductors, then the maximum permeability is more useful, but
you also have to keep track that you are not getting too
near saturation.

A better reference might be:
http://www.epcos.com/web/generator/...izing_20splitter_20design/Page,locale=en.html
 
A

amdx

Jan 1, 1970
0
I bought some N41 ferrite material surplus pot cores from BGMicro.
References I dug up on the net give N41 initial permeability of 2800 to
3000.

looking at this:
http://www.ee.surrey.ac.uk/Workshop/advice/coils/mu/
in the section "ferromagnetic materials" B versus H graph, I noticed
the significant variation in permeability (slope of the graph) as flux
density goes from small to medium values.
And then I saw in the following link that the initial permeability of
N41 seems to be measured at the extrememly low flux density, 0.25 mT.
http://www.epcos.com/web/generator/Web/Sections/Components/Page,locale=en,r=263282,a=263370.html
which would seem to indicate that the permeability in practical use
like a SMPS would be quite a bit higher than the listed initial
permeability of about 3000. Should I use the listed ui or some other
value?
I once ask a similar question regarding ferrite cores in radio antenna
impedance
transformers. The antenna signals are measured in microvolts and the
permeability
is measured with much larger signals. Also when you test the transformer the
test
equipment uses larger signals. So, I wondered what the transformer really
looks
like at the puny power levels.
The group didn't think there was anything to the observation, and told me to
find
something else to do. But I still wonder!
Mike
 
K

kell

Jan 1, 1970
0
John said:
This is a result of the BH loop having significant width.


If you are either using an ungapped structure (toroid) for
low level filtering, or a gapped structure like a ferrite
rod antenna that will see only small excitation, the lower,
initial permeability at least has to be included in your
design. If the core will be gapped and be operated at high
flux levels, like most power transformers and energy storage
inductors, then the maximum permeability is more useful, but
you also have to keep track that you are not getting too
near saturation.

A better reference might be:
http://www.epcos.com/web/generator/...izing_20splitter_20design/Page,locale=en.html

I was looking at making a switch mode power supply to convert 12 volts
to 400 volts at 50 or 100 watts. If there's no DC in the primary, then
driving it with a square wave drive at 100% duty cycle the flux density
should be B = E / (N Ae 4 f) where
B = flux density in teslas
E = drive voltage
N = turns
Ae = core cross section in square meters

If that's right then I may be able to get started on the transformer
design without reference to the permeability of the core material. To
determine Ae
I compared the dimensions of the surplus cores I bought against the
cores in some Amidon literature and it closely matches the dimensions
of their pot core number PC-3622-77, Ae = .0002 m^2.
That tells me I can get away with as few as two turns on the primary if
I drive the tranformer at several tens of kiloherz.
The Amidon table lists the PC-3622-77 as capable of 90 watts at 20 kHz.
It is made of #77 material, not N41 like the cores I have, but I'll
take it as a ballpark figure.
 
J

John Popelish

Jan 1, 1970
0
kell said:
I was looking at making a switch mode power supply to convert 12 volts
to 400 volts at 50 or 100 watts. If there's no DC in the primary, then
driving it with a square wave drive at 100% duty cycle the flux density
should be B = E / (N Ae 4 f) where
B = flux density in teslas
E = drive voltage
N = turns
Ae = core cross section in square meters

If that's right then I may be able to get started on the transformer
design without reference to the permeability of the core material.

I think that's right. The permeability will just affect the
magnetizing current. It is flux swing that supports the
winding's volt seconds.

In this sort of application, you may have to keep an eye on
power lost per volume of core material.
To determine Ae
I compared the dimensions of the surplus cores I bought against the
cores in some Amidon literature and it closely matches the dimensions
of their pot core number PC-3622-77, Ae = .0002 m^2.
That tells me I can get away with as few as two turns on the primary if
I drive the tranformer at several tens of kiloherz.

At that frequency, you have to account for the skin effect
and proximity effect on the losses of that heavy primary
conductor. Twisting 7 or 19 strands of magnet wire to make
that heavy conductor will lower those losses. You should
use about half of the window area for the primary and also
for the secondary.
The Amidon table lists the PC-3622-77 as capable of 90 watts at 20 kHz.
It is made of #77 material, not N41 like the cores I have, but I'll
take it as a ballpark figure.

Is yours a pot core?
 
K

kell

Jan 1, 1970
0
John said:
I think that's right. The permeability will just affect the
magnetizing current. It is flux swing that supports the
winding's volt seconds.

In this sort of application, you may have to keep an eye on
power lost per volume of core material.


At that frequency, you have to account for the skin effect
and proximity effect on the losses of that heavy primary
conductor. Twisting 7 or 19 strands of magnet wire to make
that heavy conductor will lower those losses. You should
use about half of the window area for the primary and also
for the secondary.


Is yours a pot core?
It's a pot core with the same dimensions as the Amidon PC-3622-77 but
made of N41 material instead of the #77.
BGMicro had these cores a while back with plastic bobbins and windings
on them, something like 50 cents apiece.
 
J

John Popelish

Jan 1, 1970
0
kell said:
It's a pot core with the same dimensions as the Amidon PC-3622-77 but
made of N41 material instead of the #77.
BGMicro had these cores a while back with plastic bobbins and windings
on them, something like 50 cents apiece.

Material N41 appears to be a "power" material (good
saturation flux and fairly low losses) so you should be able
to get a t least 90 watts through it. Do you need a
constant power flow, or will the power have high peaks and
longer periods of lower power? If so, you can use more
turns of thinner copper and raise the copper losses at high
power, but lower the core losses all the time.
 
K

kell

Jan 1, 1970
0
It's a pot core with the same dimensions as the Amidon PC-3622-77 but
Material N41 appears to be a "power" material (good
saturation flux and fairly low losses) so you should be able
to get a t least 90 watts through it. Do you need a
constant power flow, or will the power have high peaks and
longer periods of lower power? If so, you can use more
turns of thinner copper and raise the copper losses at high
power, but lower the core losses all the time.

I understand you're saying with more turns there's less flux swing and
less core losses.
Possibly this core's window will be big enough that I can get some
extra turns in there without compromising with the wire thickness.

By the way, is there an established way to quantify core losses based
on the properties of the transformer, frequency, voltage etc etc?
About all I know so far is that as the frequency goes up so do the core
losses, which may call for reducing the flux density. But it's all
pretty vague.
 
J

John Popelish

Jan 1, 1970
0
kell said:
By the way, is there an established way to quantify core losses based
on the properties of the transformer, frequency, voltage etc etc?
About all I know so far is that as the frequency goes up so do the core
losses, which may call for reducing the flux density. But it's all
pretty vague.

Generally, the manufacturer supplies a graph of loss per
cubic cm versus flux swing and frequency. I am having
little luck finding this graph for this material, but it
should be pretty similar to other power materials of the
same permeability.

Fair-rite supplies such graphs in its catalog. Perhaps you
can find one of their materials that is close to type N41.

But regardless, running your transformer at no load produces
about the same core loss as running it at full load. Just
the copper losses are missing. So you can check the
temperature rise from core loss before you add in the copper
losses.
 
K

kell

Jan 1, 1970
0
John said:
Generally, the manufacturer supplies a graph of loss per
cubic cm versus flux swing and frequency. I am having
little luck finding this graph for this material, but it
should be pretty similar to other power materials of the
same permeability.

Fair-rite supplies such graphs in its catalog. Perhaps you
can find one of their materials that is close to type N41.

But regardless, running your transformer at no load produces
about the same core loss as running it at full load. Just
the copper losses are missing. So you can check the
temperature rise from core loss before you add in the copper
losses.

Thanks, John.
 
Top