A Nonlinear Capacitor circuit component definition applies nonlinear behavior to the component based on the instantaneous voltage across its cell edge.

Nonlinear capacitors should be used when only time-domain results are of interest. This is because nonlinear capacitors violate the assumptions of linear system theory, making it incorrect to apply either a Fourier or discrete Fourier transform to the time-domain results. A simulation containing a nonlinear capacitor produces invalid frequency-domain results—including impedances, S-parameters, and steady-state far-zone fields—even if the time-domain results eventually decay to zero. The time-domain results are unaffected, and therefore are valid.

A nonlinear capacitor's behavior is defined by the following equation:

\begin{equation} C = C_2 + \frac{C_1 - C_2}{1 + a_1\left(\frac{ \vert V \vert - V_s}{V_0}\right)^2 + a_2\left(\frac {\vert V \vert - V_s}{V_0}\right)^4 + a_3\left(\frac {\vert V \vert - V_s}{V_0}\right)^6} \end{equation}where

$C = $ instantaneous cell edge capacitance

$V = $ instantaneous cell edge voltage

$C_1 = $ capacitance at the cutoff voltage

$C_2 = $ infinite $ \vert V \vert $ capacitance

$V_s = $ voltage magnitude

$V_0 = $ cutoff voltage. If $\vert V \vert \leq V_0$ then $C=C_1$.

$a_1$ , $a_2$ , $a_3$ are weighting coefficients