# Copper and Ferrite loop inductance and skin depth

#### engr_david_ee

Joined Mar 10, 2023
25
The permeability of ferrite is in the range of 1500 to 3000 where as the permeability of copper is close to 1. How do we compare the loop inductance and skin depth at low and high frequencies of these two materials.

#### DickCappels

Joined Aug 21, 2008
10,067
What is the motivation for seeking an answer to this very unusual question? Ferrite is usually not very conductive and would not be most people’s first choice fro making into an inductor.

Not understanding what you want to do, I can only imagine that this might be of some help to you -note the formula: https://www.pasternack.com/t-calculator-skin-depth.aspx

#### engr_david_ee

Joined Mar 10, 2023
25
Posting copyrighted material to AAC without reasonable fair use is prohibited.
I have attached some pages from book chapter. I am trying to compare, how the skin depth of ferrite at higher frequencies is different then copper, it is more than copper or less and how their loop inductance at a given geometry at low frequencies and at high frequencies are related in both materials.

The PDF attached to this post was from Signal and Power Integrity—Simplified, 2nd Edition by Eric Bogatin, a copyrighted work not freely available.

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#### BobTPH

Joined Jun 5, 2013
7,539
I don’t understand why you would compare the properties of ferrite and copper. They would never be used for the same thing. One is used as an inductor core, the other as a conductor.

#### engr_david_ee

Joined Mar 10, 2023
25
I have seen the use of ferrite on the DC power rails on the FPGA boards. What is the purpose of using ferrite on the DC rails and how to select their values given the voltage and the maximum current is known.

#### DickCappels

Joined Aug 21, 2008
10,067
Ferrite on power rails are either to filter out noise or to assure that noise returns to its source rather than is radiated or conducted to a “victim”.

#### Janis59

Joined Aug 21, 2017
1,718
RE: BobTPH - yet indeed their role is another but both are suffering because of Foucault effect. That is the true reason why ferrites are grinded in to so small particles (fixed in ceramic glue mass) like PM10 and even smaller, let the whole grain contains one single swirl and all the volume be utilized effectively. And if one exploits that specific ferrite at higher frequency, then loss factor skyrocketing. Meanwhile another ferrite with smaller grinded particles will allow more high frequency and ferrite what is roughly ground permits rather low frequency. Something so on....

#### RoofSheep

Joined Mar 7, 2023
36
I have seen the use of ferrite on the DC power rails on the FPGA boards. What is the purpose of using ferrite on the DC rails and how to select their values given the voltage and the maximum current is known.
These are probably "lossy" ferrites that are used as EMI filters. High-frequency currents give rise to high-frequency magnetic fields. The ferrites are lossy at high frequencies. So the busbar looks like a resistor at high frequencies.

#### Janis59

Joined Aug 21, 2017
1,718
Re:RoofSheep
Absolutely for sure. I met this nonpleasant fact when I tried to use the ferrite cyliders from PC cabling for symmetrizing the 156 MHz antenna cable. It simply didnt worked. Then I bought a fresh ferrites designed for such aim and they worked brilliantly. Later I tried the same (bad ones) ferrite to apply for stop the power supply demolishing by it load, consumpting 48 V 50 A by small "portions" - at 40.8 MHz. Result was catastrophic - those ferrite became red-hot and melted. Only then I realized why it wasnt effective in my antenna.

#### nsaspook

Joined Aug 27, 2009
11,758
These are probably "lossy" ferrites that are used as EMI filters. High-frequency currents give rise to high-frequency magnetic fields. The ferrites are lossy at high frequencies. So the busbar looks like a resistor at high frequencies.
+1
Always use the right type of component for the job.

Notice the very high R value for a inductor.
https://www.murata.com/en-us/products/emiconfun/emc/2011/06/14/en-20110614-p1
Fig. 3 shows an example of the impedance frequency characteristics of a chip ferrite bead. The basic principle involved is as follows: The impedance increases in proportion as the frequency rises, as in the case of inductors, so by connecting these beads in series in a circuit, they function as a low-pass filter. With regular inductors, the main characteristic among the impedance (Z) values is the reactance component (X). On the other hand, since chip ferrite beads use ferrite materials with a high loss in the high frequencies, the main characteristic in the high-frequency range is the resistance component (R). The reactance component is not accompanied by loss, but the resistance component is. This means that, compared with regular inductors, chip ferrite beads have better properties for absorbing noise energy, providing a higher noise-suppression effect.

#### MrAl

Joined Jun 17, 2014
10,599
The permeability of ferrite is in the range of 1500 to 3000 where as the permeability of copper is close to 1. How do we compare the loop inductance and skin depth at low and high frequencies of these two materials.
Hi,

Just a thought...
You might want to compare copper wire to iron wire and skin effect and all that. You can then apply this to resistors that have steel leads instead of copper.