[SI-LIST] Re: The current distribution and partial self-inductance

KC-

First, keep in mind that partial self inductance is a mathematical concept. 

You are correct that the H field density at some fixed external point around
a wire, carrying a current, depends only on the enclosed current, not the
diameter of the rod. 

With the partial inductance being the total number of rings of magnetic
field lines around the rod, to count the field lines, you have to integrate
the field line density cutting through an area which as long as the rod in
question and extends from infinity down to the outer dimension of the rod
(for just the external partial self inductance.)

The closer to the center of the rod you are counting field lines
(integrating the flux density), the more field lines you count and the
higher the partial self inductance. 

The narrower the rod, the closer to its center you can integrate, which is
where the high field line density is, and the higher the partial self
inductance. 

A large diameter rod will not have as many external field lines because you
will start counting only at the outer surface, farther from the current.

I find the concept of partial inductance to be the most useful concept there
is to understand how the physical features of interconnects, such as
transmission lines, return paths, package leads, solder balls and vias,
affects the various kinds of inductance and leads to switching noise and
ground bounce.

This topic is covered in my Essential Principles of SI class, which is
coming up in Dallas in March and San Jose in April.

Hope this helps.

--eric



**************************************
Dr. Eric Bogatin, President
Bogatin Enterprises, LLC
Setting the Standard for Signal Integrity Training
26235 w 110th terr
Olathe, KS 66061
v: 913-393-1305
f: 913-393-0929
c:913-424-4333
e:eric@xxxxxxxxxxxxxxx
www.BeTheSignal.com 

Spring 2008 Signal Integrity Training Institute
EPSI, BBDP, HAI
April 7-11, 2008, San Jose, CA
**************************************** 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Kyoungchoul Koo
Sent: Monday, January 21, 2008 11:46 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] The current distribution and partial self-inductance

Hi all,


I have a question about "partial self inductance".

which is introduced in "Signal integrity : Simplified" by Eric Bogatin.



By the definition of partial self inductance, (partial self inductance is
the # of field line loops around a "conductor" per current)

I thought the partial self inductance has only a relationship with the
amount of current enclosed by the field line loops.



But this book says("Signal integrity : Simplified" by Eric Bogatin p.160)
that the partial self inductance varies with the cross-sectional area of
conductor.

It means if we make the radius of round rod wire larger, the current will
spread out more and the partial self-inductance will decrease.



Does the size of cross-sectional area of conductor(the current distribution)
affects the # of field line loops around a "conductor", if the amount of
current is fixed?



Does this make sense with the definition of partial self inductance?

Or do I misunderstand something?



Thanks in advance



K.C Koo


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