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:
C=C2+C1−C21+a1(|V|−VsV0)2+a2(|V|−VsV0)4+a3(|V|−VsV0)6where
C= instantaneous cell edge capacitance
V= instantaneous cell edge voltage
C1= capacitance at the cutoff voltage
C2= infinite |V| capacitance
Vs= voltage magnitude
V0= cutoff voltage. If |V|≤V0 then C=C1.
a1 , a2 , a3 are weighting coefficients