[SI-LIST] Re: Interference from planar magnetics
- From: Larry Smith <larry.smith@xxxxxxx>
- To: den@xxxxxxxxxxxx
- Date: Mon, 27 Jan 2003 18:50:43 -0800
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
>
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