# [SI-LIST] Re: Basic question on power plane capacitance

• From: wolfgang.maichen@xxxxxxxxxxxx
• To: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>
• Date: Thu, 9 Sep 2010 11:11:34 -0700
```Mikhail,
If the polygon isn't very large (~100x) compared to its distance from the
ground plane then fringe capacitance will contribute significantly to the
total value - i.e. the actual capacitance will be quite a bit higher than
what you'd expect from the simple formula for a planar capacitor. In
dielctric constant can easily be off by ~10% from the specified value.

A 2.5D or 3D field solver can tell you how much total capacitance to
expect. One suitable choice would be Sonnet Lite (i.e. the demo version of
Sonnet). If you can tell me the size/geometry of your polygon, distance to
the ground plane, and dielectric constant of the board material I can run
a quick simulation for you.

Just to rule out an obvious mistakes, you did take into account that
having both a plane below and above the polygon will douple the
capacitance compared to just a single pair (polygon and single ground
plane)? Forgetting that would immediately explain a mismatch factor of 2.

Regards,

Wolfgang

"Mikhail Matusov" <matusov@xxxxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
09/09/2010 10:50 AM

To
<si-list@xxxxxxxxxxxxx>
cc

Subject
[SI-LIST] Basic question on power plane capacitance

Hi all,

I have a small polygon in a plane layer of a multi-layer PCB. There is a
solid ground plane underneath it and the top signal layer above. I have
calculated from the basic plane capacitance equation that the plane
capacitance for this polygon should be in the range of 420 pF. However, I
am
measuring 890 pF with one meter and 1 nF with another. I was wondering
what
am I doing wrong?

Thanks,
/Mikhail

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