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

[Ege Engin <engin@xxxxxxxxxx>]

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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