Specifications

1.8 Three-element equivalent circuit and sophisticated component models

The series and parallel equivalent circuit models cannot serve to accurately depict impedance char-

acteristics of components over a broad frequency range because various parasitics in the compo-

nents exercise different influence on impedance depending on the frequency. For example, capaci-

tors exhibit typical frequency response due to parasitic inductance, as shown in Figure 1-18.

Capacitance rapidly increases as frequency approaches the resonance point. The capacitance goes

down to zero at the SRF because impedance is purely resistive. After the resonant frequency, the

measured capacitance exhibits a negative value, which is calculated from inductive reactance. In the

aspect of the series Cs-Rs equivalent circuit model, the frequency response is attributed to a change

in effective capacitance. The effect of parasitic inductance is unrecognizable unless separated out

from the compound reactance. In this case, introducing series inductance (Ls) into the equivalent

circuit model enables the real impedance characteristic to be properly expressed with three-element

(Ls-Cs-Rs) equivalent circuit parameters. When the measurement frequency is lower than approxi-

mately 1/30 resonant frequency, the series Cs-Rs measurement circuit mode (with no series induc-

tance) can be applied because the parasitic inductance scarcely affects measurements.

Figure 1-18. Influence of parasitic inductance on capacitor

3-element equivalent circuit

model

SRF

Frequenc

–

Capacitive

Inductive

CsLs

(Negative Cm value)

Equivalent L = = Ls (1 -

)

CsLs

–

Effective range of

Log f

1-13