Copper and Ferrite

[engr_joni_ee]

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.

 

[Easy peasy]

unless you are running current through your ferrite it doesn't really have inductance in the same sense a coil of wire does

 

[KlausST]

Hi,

permeability is a magnetic measure. Inductance is an electric measure.
Both are completely different things.

In a coil you combine both to get the desired function.

Klaus

 

[FvM]

Loop inductance is a property of conductor geometries, not materials. Ferrite parts rarely act as conductors. Although it's possible to calulate a skin depth as a function of conductivity and permeability, it doesn't make much sense for most applications.

What are you trying to achieve?

 

[engr_joni_ee]

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.

 

[KlausST]

Hi,

You ignore a lot of posts ... but ignoring them does not help

The document talks about permeability of conductors..
But in a coil
* the conductor usually is not designed for permeability. It is designed to carry electrical current i.e. conductivity, not for magnetics
* the core is not designed to be conductive. Not to carry current. Indeed if the core is highly conductive it will lead to eddy currents within the core.

So usually you don't have to care about conductivity of ferrite, nor about permeability of conductors.
Only if you are working with a (rare) application where these have to be considered, but then you should give us more detiails about the exact problem.

Klaus

 

[dick_freebird]

Ferrite is an insulator (or a semiconductor if you treat
it right) but inductor efficiency wants zero core eddy
current so you're going to get a nonconductive, or
extremely high volume resistivity.

 

[FvM]

The book is exclusively talking about ferromagnetic metals with sufficuent conductivity, e.g. nickel, not ferrites.

 

[engr_joni_ee]

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.

 

[Easy peasy]

Show a picture or a sketch.

 

[dick_freebird]

Ferrites on the rails are to keep EMI "in the bottle" and
putting the, means the input supply cannot have any
ability to remove that hash from the rails. So your
local decoupling has to be totally effective and yet not
so high-Q that the rails become a resonator.

I imagine that as an "EMI patch" the value might be
altogether ad-hoc. Informed by some DC current and
AC-component measurements perhaps (as these are
key selection params for ferrite beads).

 

[barry]

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.

Wow, that is a completely different question. I thought you were concerned about skin depth and inductance of materials????

 

[engr_joni_ee]

Hi, I am sorry for making a mess but just to make concept clear about Ferrite in power rails. They are actually inductors made of material ferrite, right ?

As capacitor block the DC or does not work when used in series in DC circuit. Similarly inductors block the AC and they only work in DC circuits, right ? Using ferrite in power rails is actually inductors in DC rails if I understood correct.

 

[D.A.(Tony)Stewart]

Ferrite can be mixed with different magnetic concentrations (L), binders (C), and nickle (Ni) (low R) or Manganese (Mg) (higher R) , or inductance, lower or raise eddy current losses into a brittle ceramic package. Higher mu ferrite might be measured by high insulation properties of core resistance which is best for low frequencies. Losses must be tested with some measurement device.

Generally going to higher frequency, one must transition to lower permeability and lower resistance ferrite until going to air loops.

The impedance is defined below ferrite losses as Z=DCR+ jX(f), X(f) = 2pi*f*L
ζ=R/X(f) or Q = ωL/R and DCR is the DC resistance of wire not incl. skin effects.

Thus if you saw a tuneable inductor at 11 MHz for the FM IF it will be ferrite. But if you need a high current inductor > 300 MHz it would be a small L air coil with L~8nH/cm for straight wire and much higher when coiled in a loop. At even higher frequencies it is likely to be a half-wave stripline.

 

[engr_joni_ee]

This means that we can not use just an inductor in series in the DC power rail but Ferrite should be use instead to to reduce EMI, right ? How do we select which Ferrite can reduce the EMI and how much ?