RE: [SI-LIST] : skin effect

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From: Michael Khusid (mkhusid@sitaranetworks.com)
Date: Thu Aug 31 2000 - 12:02:51 PDT


Craig,

The formula Ray and you are referring to
skin depth = sqrt(2/(omega * mu * sigma))
indeed seems to be geometry independant, but that's where the catch is.

This formula is derived from a current/electric field penetration if an
electromagnetic wave is normally incident on the half-infinite slab of
homogenious material (metal in this case). This would normally be a
viewpoint of microwave textbooks.

There are two gotchas going to high speed digital world. First, in the
stripline the propagation of field is parallel to the metal, not
perpendicular. Second, the thickness of metal comparable to field
penetration depth changes the current distribution, and thus, the skin
effect. What a digital designer ultimately cares for is an increased
resistance of the stripline at high frequencies, and that can be determined
from the current distribution.

My favorite way to solve a skin effect problem is an FDTD (finite difference
time domain) solver which can actually use Maxwell equations directly and
avoid approximations done to derive the aforementioned skin effect formula.
Practically any FDTD solver can solve for current distribution inside a real
conductor and calculate the resulting skin effect loss.

Mike Khusid

> -----Original Message-----
> From: Clewell, Craig W [mailto:ClewelCW@bergelect.com]
> Sent: Thursday, August 31, 2000 2:24 PM
> To: 'Muhammad S. Sagarwala'; si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : skin effect
>
>
> Muhammad,
>
> Funny thing....I was under the impression that the skin depth was
> "frequency" and "material" dependant not geometry dependant.
> I would be
> curious to know what data your boss has that says different.
>
> Craig
>
> -----Original Message-----
> From: Muhammad S. Sagarwala [mailto:msagarwa@san-jose.tt.slb.com]
> Sent: Thursday, August 31, 2000 12:45 PM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : skin effect
>
>
> Hello Si Gurus,
>
> I just had an interesting discussion with my boss on "skin
> effect on pcb
> traces".
> I was of the opinion that increasing the trace thickness from
> 1/2 oz. to
> 1oz. would help reduce the
> skin effect but according to him skin effect does not reduce
> significantly
> with the increase in trace thickness.
> He was of the opinion ( and also had some data to back him
> up) that skin
> effect is more dependent on the
> width of the trace.
>
> I always thought that if one increases the overall perimeter
> of the pcb
> trace - regardless of whether it is done by
> increasing the width or increasing the thickness - the skin
> effect would
> reduce. I would appreciate if somebody
> could come up with a better explanation...
>
> Muhammad
>
> p.s.: when we talked about pcb traces we were talking about
> striplines in
> particular...
>
>
>
>
>
> Muhammad S. Sagarwala
> Design Engineer
> Schlumberger SABER
> Ph. (408) 586 7065
> Fax (408) 586 4668
>
>
>
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