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

Thanks Wolfgang and all,
Yes, indeed I haven't thought of fringe capacitance. I don't know if there is 
an easy analytical way to find out what it will be. The area of the polygon is 
1400 sq.mm. The perimeter is 236 mm. The dielectric is FR-4, and the thikness 
is about 5 mil (0.127mm). The top layer is not a plane and there is not much 
copper in it, mostly pads and vias. The gap around this polygon separating it 
from the rest of the plane is 15 mil (0.381mm) wide. The board dimensions are 
233x160 mm.

Thanks again,
/Mikhail


  ----- Original Message ----- 
  From: wolfgang.maichen@xxxxxxxxxxxx 
  To: Mikhail Matusov 
  Cc: si-list@xxxxxxxxxxxxx ; si-list-bounce@xxxxxxxxxxxxx 
  Sent: Thursday, September 09, 2010 2:11 PM
  Subject: Re: [SI-LIST] 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> 
        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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