RE: [SI-LIST] : skin effect

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From: Tsuk, Michael (Michael.Tsuk@compaq.com)
Date: Thu Aug 31 2000 - 12:56:00 PDT


Mike Khusid wrote:

> 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.

I'm sorry, I can't think of a *worse* computational technique to solve skin
effect problems than FDTD! In order to model, say, the skin effect in
copper at 1GHz, you'd need cells no more than say a fourth the size of the
skin depth, which is about 0.08 mils. So to do a 6 mil by 1.4 mil line, say
4 mils above a ground plane, you'd need at least 900 cells wide by 400 cells
high. And you'd want to do a significant length of etch, at 50000 cells per
inch. At 25 bytes per cell, minimum, you're talking 450 GB of storage per
inch of etch! (You could use 2D FDTD, reducing the memory requirements
significantly. But then you'd be making the kind of approximations you're
trying to avoid by using a full-wave technique).

But the real problem is the timestep. The Courant condition would force
you, with 0.02 mil cells, to a timestep on the order of a femtosecond. So
to do just one period of a 1GHz sine wave would take about a million time
steps!

Use finite elements. Use the Partial Element Equivalent Circuit method.
Use a method of moments code with an impedance boundary condition (my
favorite). But *please* don't use FDTD for this!

-- 
Michael Tsuk
Compaq AlphaServer Product Development
(508) 467-4621

-----Original Message----- From: Michael Khusid [mailto:mkhusid@sitaranetworks.com] Sent: Thursday, August 31, 2000 3:03 PM To: 'Clewell, Craig W'; 'Muhammad S. Sagarwala'; si-list@silab.eng.sun.com Subject: RE: [SI-LIST] : skin effect

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 > > > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > si-list archives are accessible at http://www.qsl.net/wb6tpu > **** >

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