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2 Capacitor Characteristics

2.1 Capacitance of a capacitor

2 Capacitor Characteristics

The essential characteristics for a capacitor are presented and explained in detail

in this chapter. These characteristics are crucial in the selection of a capacitor for a

certain application.

2.1 Capacitance of a capacitor

The most important characteristic of a capacitor is its capacitance C . The capaci-

tance C describes the property of a capacitor’s capability to store electrical energy if

a (given) voltage U is applied. Capacitance denotes how many units of charge can be

stored in the capacitor per voltage unit. Furthermore, the capacitance is important for

the AC resistance of a capacitor at certain frequencies, essentially the properties when

used in filters.

The unit in which capacitance is specified is a Farad. This was named after the

English natural scientist and prominent experimental physicist Michael Faraday (1791

to 1867). A capacitor which is charged to one Volt with one Coulomb of charge (this

means that one Ampere of current flows for one second) has a capacitance of one

Farad.

The following relationship applies:

1F = 1

(2.1)

The capacitance of a capacitor is normally specified in pF, nF or µF. The most common

capacitors today lie within this order of magnitude. Values greater than one Farad are

attained in the field of super-capacitors. The trend is towards achieving increasingly

large capacitance values.

The capacitance of a capacitor essentially depends on the area jointly covered by the

electrodes, the separation of the electrodes, the dielectric used and its thickness (see

Chapter 1.8 Capacitor).

The capacitance of a capacitor can be increased by means of the following design

parameters:

• increase in the effective area of the electrodes

• reduction in the separation between the electrodes

• thinner layer of dielectric

• increase in insulation with a suitable dielectric with higher permittivity or improved

dipole formation

Capacitance C

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2 Capacitor Characteristics 2.1 Capacitance of a capacitor 2 Capacitor Characteristics The essential characteristics for a capacitor are presented and explained in detail in this chapter. These characteristics are crucial in the selection of a capacitor for a certain application. 2.1 Capacitance of a capacitor The most important characteristic of a capacitor is its capacitance C .
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Capacitor Characteristics 2.1 Capacitance of a capacitor 2.1.1 Dependence on voltage By applying a voltage, some insulators used in capacitors as dielectric experience a change in permittivity εr and consequently a reduction in capacitance. This can be explained in the case of the dielectrics mentioned here in that once a specific v oltage is applied to the electrodes, a defined polarization of the molecules occurs. This reduces the permittivity εr of the material or dielectric.
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2 Capacitor Characteristics 2.1 Capacitance of a capacitor 0.50 Capacitance change ∆C/C0 (%) 0.30 0.10 –0.10 –0.30 –0.50 –0.70 –0.90 –1.10 1.0 10.0 100.0 Frequency f (kHz) Fig. 2.2: Frequency-dependent capacitance change, example: MKP film capacitor 2.1.3 Dependence on time Changes in capacitance can occur over time or use in the application.