 # TDK Disappearing Capacitance

When looking at the frequency characteristics graph for capacitance, the capacitance value suddenly disappears after a certain frequency is reached. Does this mean that there is no more capacitance? We do not measure capacitance directly. To get the capacitance we measure the ESR (equivalent series resistance) and Xs (synthetic impedance or reactance) and the capacitance values are then calculated with an approximation formula. Below is the approximation formula. The reason it looks like the capacitance has suddenly disappeared on the graph is the formula only works until the self-resonant frequency is almost reached, but cannot be used in the frequency region where the self-resonant frequency is reached or exceeded. Don’t worry as this does not mean that there is no more capacitance.

With an ideal capacitor, impedance decreases as frequency rises. However, in an actual MLCC, in addition to the capacitance component, there is an inductance component and resistance component. Because this inductance component becomes dominant when the self-resonant frequency is exceeded, the impedance increases with the frequency, as shown below. The approximation formula of Xs≒-1/(2πfC) works when the frequency is sufficiently lower than the resonant frequency because the capacitance component is much greater and the inductance component can be disregarded because it is so small. The capacitance value is calculated backward from this formula. In other words, C=-1/(2πfXs). At self-resonant frequencies and above, impedance 2πfESL from inductance components is dominant with total impedance Zs, the C=-1/(2πf*Xs）formula cannot be used, and capacitance cannot be calculated. This is why there are sudden changes in values and the apparent disappearance of capacitance in frequencies near self-resonant frequencies and above.

Information was provided by TDK FAQ and Digi-key Electronics.

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