FreeLists / si-list / [SI-LIST] Re: Internal Inductance

[steve weir <weirsp@xxxxxxxxxx>]

George,

I agree that any model that shows inductance going to infinity for a an=20
infinitly large plane is wrong.

Finite flux can result in infinite ( or very large ) inductance, since=20
inductance is the flux density / mmf.

I suspect that what is happening in the model is that the finite mmf is=20
getting improperly approximated as either a very small number or zero.

Regards,

Steve.
At 11:59 AM 8/6/2004 -0700, George Tang wrote:
>I disagree with what you are suggesting.  If you have a wire above a plane
>with a finite current flowing through the loop, even with infinite plane
>size and thickness, the DC inductance cannot be infinite.  You can=
 integrate
>the flux density out to infinity, but the total flux should add up to the
>loop current (enclosed current), which is finite, so the total flux is
>finite.  This will result in a finite inductance (internal and external).
>
>
>George
>
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ege Engin
>Sent: Friday, August 06, 2004 6:16 AM
>To: steve weir
>Cc: scott@xxxxxxxxxxxxx; cclewell@xxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Internal Inductance
>
>
>Steve, the paper cited by Jayaprakesh also demonstrates that there is a
>non-zero internal inductance, which decreases with frequency, associated
>with an infinite ground plane. Actually the given equation also suggests
>that the internal inductance tends to infinity at dc. However, the
>ground plane is assumed to be also infinitely thick, which makes the
>obtained expression less and less reliable at low frequencies. I think
>for an infinitely wide plane having a finite thickness your statement
>might be true though. It might be that the divergent behavior I observed
>is due to the fact that the total loop inductance tends to infinity at
>dc (since the current loops get larger and larger), and not the internal
>inductance of the plane.
>
>Ege
>
>steve weir schrieb:
>
> > Ege, for infinite planes, I think you mean the inductance tends to
> > zero.  Since the current distributes over an infinite area, the flux
> > density tends towards zero as does any current impeding counter-EMF.
> >
> > Steve.
> > At 11:10 AM 8/6/2004 +0200, Ege Engin wrote:
> >
> >> Craig,
> >>
> >> The internal inductance is frequency dependent, and simulations at
> >> multiple frequency points are necessary as Scott explained. L.J.
> >> Giacoletto; "Frequency- and Time-Domain Analysis of Skin Effects", IEEE
> >> Tran. Mag., Jan. 1996, gives some analytical formulas for rectangular
> >> conductors.
> >>
> >> I assume you want to obtain the internal inductance at dc. The method
> >> that we apply to extract the dc internal inductance of conductors is to
> >> simulate at a very high and a very low frequency (if you have an
> >> eddy-current solver such as Ansoft Maxwell 2D). The difference in the
> >> inductances gives the dc internal inductance. Though, I think it is
> >> difficult to obtain a definite internal inductance value for conductors
> >> in the presence of planes. The reason is that if the planes are assumed
> >> to be infinite in extent,  the current should distribute itself
> >> uniformly in the planes at dc. But this should also mean that the
> >> inductance goes to infinity! Although I could not find a supporting
> >> statement in the literature, it seems obvious, and simulations have=
 also
> >> shown this divergent behavior. As a result, this method gives a=
 definite
> >> internal inductance value only if the planes have a finite (and known)
> >> width.
> >>
> >> Regards,
> >> Ege
> >>
> >> Scott McMorrow schrieb:
> >>
> >> >Craig,
> >> >
> >> >I don't believe there is a straightforward analytical formula for
> >> >non-round conductors.  When you start getting into internal inductance
> >> >for trace geometries in the presence of a plane, then it will
> >> definitely
> >> >depend on the shape and proximity to the other conductors.  However,
> >> >this problem is easily solved by several different field solvers.
> >> >
> >> >Commercially, Ansoft Maxwell 2D does a very good job of extracting
> >> >inductance at multiple frequency points.  You just have to be sure to
> >> >use the impedance matrix.
> >> >
> >> >Academic codes such as FastHenry from MIT and Simian from Dean Neikirk
> >> >at the University of Texas can correctly solve for frequency dependent
> >> >inductance and resistance of arbitrary shapes.  Links to these can be
> >> >found at http://www.fastfieldsolvers.com/links.htm
> >> >
> >> >Or, if you are dealing with stripline traces, you can use a boundary
> >> >element method or method of moments field solver to run two problems.
> >> >
> >> >Problem 1) your rectangular trace at the normal thickness centered
> >> >between the planes.
> >> >
> >> >Problem 2) a very very thin trace centered between the planes.
> >> >
> >> >
> >> >
> >> >regards,
> >> >
> >> >scott
> >> >
> >> >
> >> >Craig Clewell wrote:
> >> >
> >> >
> >> >
> >> >>Greetings,
> >> >>
> >> >>Does anybody out there have an equation handy to calculate the
> >> internal
> >> >>inductance of a square, rectangle, or any other object that is not
> >> >>round?  I already know that the internal L of a round wire is
> >> (u*length)
> >> >>/ (8*pi), but I'm not looking at using a round wire.  I've read
> >> that it
> >> >>should be about 20% of the external inductance, however I want to=
 dial
> >> >>it in better than that.  I'm so used to neglecting this, that I know
> >> >>find myself at a "loss".....ok...that wasn't funny :>)
> >> >>
> >> >>Thanks for sharing....
> >> >>
> >> >>Craig
> >> >>
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