Re: [SI-LIST] : Is a magnetic field influenced by trace width?

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From: Neven Pischl (npischl@cisco.com)
Date: Thu May 03 2001 - 16:41:35 PDT


Patrick,

A) Differential-Mode

Inductance of the line (in its differential mode) will decrease if you make it wider (keeping the same gap). Since the inductance is

L= Flux/Current,

the magnetic flux will decrease with increase in the trace width. Thus, you will have less noise coupling if you make the traces wider. Of course, that also assumes that by making the traces wider you didn't decrease spacing to the other traces and components.

BTW, you say that the traces only carry DC voltage, but also switching currents - that can't be. I guess what you meant is that they are supposed to carry DC current/voltage, but they also have superimposed switching currents/voltages.

B) Common-Mode

A different situation is with common-mode noise (generated by the sw. mode power supply e.g.) propagating on these traces. By increasing the width of traces, it may or may not result in increase of noise coupling. That will depend on the path for common-mode noise in your system, and you should investigate it. You basically have to treat both traces as if they were in parallel, and look inductive and capacitive coupling to the system, which may not be a trivial task. However, if the RELATIVE change in width compared to the distances that the common-mode return would take in your system (e.g. to a nearby return planes, chassis etc.) I would say that most likely it would not really increase noise coupling in the system.

Neven

At 02:53 PM 5/3/01 -0700, Patrick Lawler wrote:
>I have a question concerning the magnetic field generated by
>switching currents in a pair of PWB traces.
>
>The situation arises when our PWB designers make traces in our
>switch-mode power supply extra wide. I'm concerned that the extra
>trace area is generating a larger magnetic field, helping to
>couple pulse currents into other parts of the circuit, causing
>EMI problems.
>
>Given conditions:
>1) A trace pair is on one layer of a PWB.
>2) A trace pair carries both outgoing and return currents.
>3) The traces carry only DC voltage - there are no switching
>voltages.
>4) The currents are typical of switch-mode power supplies: a
>100kHz fundamental of 0-6Apk, with rise & fall times of 100ns.
>5) The air gap between the two traces in each configuration is
>the same - only the trace width is different.
>
>Will Configuration 1 generate a larger magnetic field than
>Configuration 2?
>
>Configuration 1:
>----------------------------
>copper copper copper copper
>copper copper copper copper
>copper copper copper copper
>----------------------------
> gap
>----------------------------
>copper copper copper copper
>copper copper copper copper
>copper copper copper copper
>----------------------------
>
>
>Configuration 2:
>----------------------------
>copper copper copper copper
>----------------------------
> gap
>----------------------------
>copper copper copper copper
>----------------------------
>
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