[SI-LIST] Re: The case of 3 planes
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <rtso1@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 8 May 2004 19:10:02 -0400
xsirfr,
At frequencies where the conductor thickness is more than the
skin depth, you can assume that the current does not fully
penetrate the plane, therefore the middle plane behaves
effectively as two layers: one paired up with the plane above,
one paired up with the plane below. In this case you can
ignore the coupling between the outer two planes.
At lower frequencies, say at a few MHz and below for
a usual one ounce copper plane, the current loops will
interact.
Regards,
Istvan Novak
SUN Microsystems
----- Original Message -----
From: "xsirfr" <rtso1@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Saturday, May 08, 2004 2:47 PM
Subject: [SI-LIST] The case of 3 planes
> Suppose I have 3 planes. I wish to create a LCR mesh element model for
> a square section of the 3 planes. Now the inductance per square of
> plane is the loop inductance per square, which can be expressed as a sum
> of Partial inductances: self inductances - mutual inductances,
> assuming currents flow
> in opposite directions, which for a return path is a valid assumption.
> Total Effective Inductance: L_loop = L11 + L22 - 2M (where M=L12=L21)
> For a section comprised of 2 planes, the loop inductance is split in
> half and applied to each "layer" of the schematic for the LCR model
> section. By
> doing this, we don't need to use "K" factors for coupled inductors - we
> simply model the net loop inductance as isolated inductors.
>
> Now what is the correct inductance value for modeling plane#2 in a 3
> plane stack? Intuitively, the sandwiched layer should be modeled as
> having
> a reduced value of inductance/sq, IF it uses both plane#1 and plane#3 as
> return paths. But for a circuit that is powered between plane#1 and
> #2 only,
> it seems the inductance in plane#2 would be too small, since the circuit
> does not use plane#3. My conclusion is that for a 3-plane system, I
> CANNOT use
> isolated inductors, but have to use "K" factors to represent the
> physical effect
> of coupled mutual inductance. This way, the total effective loop
> inductance of either
> adjacent pair (#1, #2) or (#2, #3) can be affected by the direction of
> current in the other
> plane (#3 or #1). I believe I need to model K12 and K23, but can
> ignor K13.
>
> Is there a modeling expert out there that would concur with my conclusion?
>
> Thanks,
> -xsirfr
>
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