[SI-LIST] Re: Interference from planar magnetics

Larry, it works out to be just about 1MHz in 1oz Cu.  The formula is depth 
in inches = sqrt( 2.6/F )  At 1MHz it comes out 1.6mils which is close 
enough for government work to 1.4mils.

Regards,


Steve.
At 06:50 PM 1/27/2003 -0800, Larry Smith wrote:

>Actually, the copper planes are a good magnetic shield at high frequency but
>poor at low frequency.  The transition region occurs when skin depth is
>equal to the copper plane thickness.  The skin depth is the depth to which
>magnetic fields penetrate the copper (or at least diminish to 1/e of their
>value on the surface).  At high frequency where skin depth is much less than
>copper thickness, very little magnetic field penetrates the copper.
>
>As I recall, the skin depth of copper is about the same as the plane
>thickness at 1 MHz (but I cannot remember if this is for 1 oz or half
>oz copper).  The copper planes will not shield the fundamental switching
>frequency but will shield the higher harmonics from reaching buried
>transmission lines.
>
>regards,
>Larry Smith
>Sun Microsystems
>
>Dennis Schmitz wrote:
> >
> > "I could, of course put a large copper shape (with plenty of vias tied to
> > ground planes below) beneath the VRM to block the electric fields from
> > penetrating into the lower wiring planes.   While I believe that this would
> > affect the magnetic field (since it affects the electric field), it 
> will not
> > block it!."
> >
> > I think you'll find that copper is a very effective magnetic shield at 
> those
> > frequencies. The principal is that the magnetic fields induce currents that
> > cancel out the field on the far side of the copper plane. The frequency
> > components where the magnetic fields are no longer effectively cancelled by
> > copper are so large that your diffpairs won't even even notice it's there.
> > You'll get some coupling on the single-ended stuff, though.
> >
> > The rule then is that the copper plane needs to be as large as you can make
> > it, preferably the entire top surface of the board (and be a ground plane).
> > The higher the conductivity the better, so use 1 oz (or even 2 oz copper if
> > you can get your board shop to do it). The GHz stuff should be ok too.
> >
> > If you have sensitive analog signals, you'll have trouble, though.
> >
> > Dennis
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Bob Welte
> > Sent: Friday, January 24, 2003 4:31 PM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Interference from planar magnetics
> >
> > Hello (once again)
> > I would like to get some insight, (or references) on the effects of having
> > planar magnetics (from DC/DC converters) on a digital board in close
> > proximity to the digital traces.
> > Specifically, I need to determine the effects that the fields from the
> > planar magnetics of the DC/DC VRM might have on various types of digital
> > signals.
> > The digital signals fall into two classes:  High frequency (1.25Ghz) which
> > are differential, and low voltage (1V amplitude), and low frequency (<200
> > Mhz), unbalanced, but with various voltages (some are 3.3V, 2.5, and 1.8V
> > signals).
> > The VRMs are relatively low voltage/ high current (2.5 and 1.8V at 10-25
> > Amps).  The VRMs typically have switching frequencies not much greater than
> > a couple Mhz.  The magnetics of the VRM are not "part of the board" (ie
> > board traces do not make up any portion of the inductor/transformer
> > windings).
> > Ideally, I would like to be able to route the digital signals near (and
> > even under) the VRM.   I could, of course put a large copper shape (with
> > plenty of vias tied to ground planes below) beneath the VRM to block the
> > electric fields from penetrating into the lower wiring planes.   While I
> > believe that this would affect the magnetic field (since it affects the
> > electric field), it will not block it!.
> > As voltages get lower, and VRMs need to be closer to the ASICs, where board
> > real estate is at a premium, I thought this topic would have wide interest.
> > Any thoughts?
> >
> > Bob Welte
> > IBM Microelectronics
> >
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> > or to administer your membership from a web page, go to:
> > http://www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List archives are viewable at:
> >                 http://www.freelists.org/archives/si-list
> > or at our remote archives:
> >                 http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> >                 http://www.qsl.net/wb6tpu
> >
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >
> > or to administer your membership from a web page, go to:
> > http://www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List archives are viewable at:
> >                 http://www.freelists.org/archives/si-list
> > or at our remote archives:
> >                 http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> >                 http://www.qsl.net/wb6tpu
> >
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>http://www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List archives are viewable at:
>                 http://www.freelists.org/archives/si-list
>or at our remote archives:
>                 http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
>                 http://www.qsl.net/wb6tpu
>


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: