[SI-LIST] Re: Basic question on power plane capacitance
- From: steve weir <weirsi@xxxxxxxxxx>
- To: wolfgang.maichen@xxxxxxxxxxxx
- Date: Thu, 09 Sep 2010 11:32:38 -0700
Wolfgang if it looks like a polygon, then for common materials, he is
guaranteed such a high ratio.
Steve.
wolfgang.maichen@xxxxxxxxxxxx wrote:
> 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
> addition, if your dielectric is a low-cost material (e.g. FR-4) its
> 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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Steve Weir
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