Analytical parasitic estimation

[xpigreco]

Hello,
I hope it is not a too much weird question…
We all know that if you have two metal layer with a distant d and thickness th1, th2, we could calculate the capacitance for area unit with C = (Er*Eo)/d. My question is, in order to get merely an estimation, does someone know if exists such as similar formula for calculating the capacitance for perimeter unit between the edge th1 of the first layer with the surface (with infinite extension) of the second?

Thank you very much!

xpigreco

 

[Comcokid]

The capacitance off the edge is sometimes refered to as the fringing capacitance. This adds a little to the overall capacitance. You are already pretty close to the capacitance and it is sometimes ignored in approximations. You could google "fringing capacitance".

A quick check in a book I have only gave references to articles and no information.

Langton, N.H. "The Parallel-Plate Capacitor with Symmetrical Placed Unequal Plates," Journal of Electrostatics, Vol. 9, 1981, pp 289-305

Lin, Weigan, "Computation of the Parallel-Plate Capacitor with Symmetrically Placed Unequal Plates, " IEEE Transactions on MIcrowave Theory and Techniques, Vol. MTT-35, No. 4, April 1987, pp. 403-408

 

[jimkirk]

What you're describing is sort of one half of a microstrip.

microstrip:

--
----------------------

half of a microstrip:

-
-----------

So if you calculate the capacitance of a microstrip with the appropriate dimensions and dielectric constant, by symmetry divide the results by two for a first order approximation...

Here are a couple on-line microstrip calculators:

http://www.emclab.umr.edu/pcbtlc/index.html

http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=util_pcb_imp_microstrip