**From:** Mary (*mary@advocate.net*)

**Date:** Thu Aug 17 2000 - 14:38:44 PDT

**Next message:**Jon Powell: "[SI-LIST] : positions available"**Previous message:**Barry Ma: "RE: [SI-LIST] : Decoupling capacitors (again!)"**In reply to:**Larry Smith: "Re: [SI-LIST] : Spreading Inductance?"**Next in thread:**Doug McKean: "Re: [SI-LIST] : Spreading Inductance?"

Larry,

I'm having a little trouble with the concept of "spreading inductance".

Do you have a reference you could provide that defines this concept a

little more precisely? How is spreading inductance calculated and how

does it relate to the total inductance of a loop formed with two vias

and two planes? For example, if I have two vias (0.1 mm dia.) spaced

10 cm apart connecting two infinite planes spaced 0.1 mm apart, how do

I calculate the spreading inductance?

Mary

-----Original Message-----

From: owner-si-list@silab.eng.sun.com

[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Larry Smith

Sent: Thursday, August 17, 2000 11:30 AM

To: si-list@silab.eng.sun.com; bobperl@best.com

Subject: Re: [SI-LIST] : Spreading Inductance?

Bob - spreading inductance is something like spreading resistance for a

sheet of conductive material. Most of us are familiar with resistance

in ohms/square. If you have a strip (sheet) of conductive material

with a length and width, the resistance in the length direction is

simply L/W * Rs. L/W gives you the number of squares. Multiply that

by sheet resistance in ohms per square and you get the resistance in

Ohms. If we have a point source and a point sink with some diameter in

a sheet of material, the resistance between the two points can be found

by counting the number of curvilinear squares in series and parallel

and calculating the resistance. This is often called a spreading

resistance problem. The physics has a lot to do with current traveling

through a conductive material. Well, that is the concept.

It turns out that we can do exactly the same thing for inductance of a

power plane pair and call it spreading inductance. The physics is no

longer related to conductivity but rather permeability. As current

spreads out on the voltage plane (and an opposite current "spreads in" on

the ground or return plane), it creates a magnetic B field between the

planes. Inductance is essentially a measurement of the energy stored

in this B field.

Think of the power plane pair as a bunch of transmission lines with a

width and thickness, divided up into long narrow strips. It is easy to

think of capacitance per inch of these transmission line strips. There

is a velocity associated with the transmission lines strips that is

sqrt(L/C) where L is inductance per inch and C is capacitance per

inch. So if you know the velocity and capacitance, you know the

inductance. Extend these 1 dimensional transmission lines concepts to

two dimensional power planes and you get capacitance per square inch

and inductance per square (a lot like ohms per square). Inductance per

square is the spreading inductance. Attached is another email from

last march on the same subject.

regards,

Larry Smith

Sun Microsystems

*> From: "Bob Perlman" <bobperl@best.com>
*

*> To: si-list@silab.eng.sun.com
*

*> Date: Wed, 16 Aug 2000 16:40:53 -0700
*

*>
*

*> Hi -
*

*>
*

*> I've seen the term "spreading inductance" used repeatedly here.
*

*> Would someone be so kind as to define it?
*

*>
*

*> Thanks,
*

*> Bob Perlman
*

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**Next message:**Jon Powell: "[SI-LIST] : positions available"**Previous message:**Barry Ma: "RE: [SI-LIST] : Decoupling capacitors (again!)"**In reply to:**Larry Smith: "Re: [SI-LIST] : Spreading Inductance?"**Next in thread:**Doug McKean: "Re: [SI-LIST] : Spreading Inductance?"

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