Technical Note 2
Capacitor Basics
Capacitor is a basic storage device to store electrical charges and release it as it is required by
the circuit. In a simple form it is made of two conductive plates (Electrodes) and an insulating
media (Dielectrics) which separate the electrodes.
As the voltage applies between the opposite electrode plates of a
capacitor, electric field takes place between the plates and
charges start accumulating on the surface of the electrodes.
The charges (Q) on the capacitor plates depend on the voltage
(V)and the capacitance value (C) and is as follows:
Dielectrics
Q=C.V
Capacitors charges exponentially versus time and the
Fig 1. Basic single
charging current follows the following formula:
layer capacitor
I=dQ/dt=C.dV/dt
The unit of capacitance measurement is in Farad (F). Due to the
large value of Farad, capacitance is typically mentioned as a fraction of Farad as follows:
microFarad (uF)=10-6 Farad
nanoFarad (nF)=10-9 Farad
picoFarad (pF)=10-12Farad
Electode
There are so many dielectrics in industry. Each dielectric is characterized by its ability
to store energy (dielectric constant K), dielectric strength, Insulation resistance, Temperature
Coefficient (TC), High Frequency performance, etcâ€¦
Multilayer capacitor unlike single layer, is made of so many layers. This would allow
the manufacturer to achieve higher capacitance values. (Fig 2)
The capacitance of a multilayer capacitor is calculated as
Follows:
C=K.A.n/4.44.t
Fig 2: Multilayer Capacitor
C: Capacitance in uF
K: Dielectric Constant
A: Active area (in2)
n: Number of layers
t: Active dielectric thickness (in2)
As one can see, the higher capacitance value can be achieved by increasing the number of
layers (n), decreasing the dielectric thickness (t), larger size or using higher dielectric constant
(K) materials.
Typical ceramic capacitors are made with Barium Titanate (BaTio3) material. The
dielectric constant of some of the common dielectrics used in the industry is as follows:
Vacuum
1.0
Mica
6.0
Air
1.004
Glass
4.0-18.0
Mylar
3.0
Barium Titanate
1500-2000
Paper
5.0
Other formulated ceramics 8000-20000
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