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

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> 
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09/09/2010 10:50 AM

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[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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