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

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From: Degerstrom, Michael J. (degerstrom.michael@mayo.edu)
Date: Fri May 04 2001 - 07:39:25 PDT


A larger conductor will lower the magnetic field strength not
increase it.

However a common mistake is to increase the size of the
conductor rather than reducing the loop area between the
conductor and another conductor that would carry the
return current for the first conductor.

I assume in your diagrams below that we are looking
down onto the conductors? If so then configuration 1
will generally offer less magnetic field coupling
to other circuits on the board. One possible exception
is that the wider traces are routed too closely to
a victim signal. You would be MUCH better off
if you routed one path on one metal layer and
the other path directly below it. You still would
want to use wider conductors and the wider conductors
are much more effective at lowering the field strength
than by using wider conductors with your original approach.

Mike

_______________________________________________________________
Mike Degerstrom Email: degerstrom.michael@mayo.edu
Mayo Clinic; 200 1st Street SW ; Rochester, MN 55905
Phone: (507) 538-5462 FAX: (507) 284-9171
WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
_______________________________________________________________

> -----Original Message-----
> From: plawler@west.net [mailto:plawler@west.net]
> Sent: Thursday, May 03, 2001 4:53 PM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : Is a magnetic field influenced by trace width?
>
>
> 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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