SM015A100JAN120 Datasheet, PDF (73/112 Pages) AVX Corporation – AVX Advanced Ceramic Capacitors for Power Supply, High Voltage and Tip and Ring Applications

General Description
A capacitor is a component which is capable of storing
electrical energy. It consists of two conductive plates (elec-
trodes) separated by insulating material which is called the
dielectric. A typical formula for determining capacitance is:
C
=
.224 KA
t
C = capacitance (picofarads)
K = dielectric constant (Vacuum = 1)
A = area in square inches
t = separation between the plates in inches
(thickness of dielectric)
.224 = conversion constant
(.0884 for metric system in cm)
Capacitance – The standard unit of capacitance is the
farad. A capacitor has a capacitance of 1 farad when 1
coulomb charges it to 1 volt. One farad is a very large unit
and most capacitors have values in the micro (10^-6), nano
(10^-9) or pico (10^-12) farad level.
Dielectric Constant – In the formula for capacitance given
above the dielectric constant of a vacuum is arbitrarily cho-
sen as the number 1. Dielectric constants of other materials
are then compared to the dielectric constant of a vacuum.
Dielectric Thickness – Capacitance is indirectly propor-
tional to the separation between electrodes. Lower voltage
requirements mean thinner dielectrics and greater capaci-
tance per volume.
Area – Capacitance is directly proportional to the area of the
electrodes. Since the other variables in the equation are
usually set by the performance desired, area is the easiest
parameter to modify to obtain a specific capacitance within
a material group.
Energy Stored – The energy which can be stored in a
capacitor is given by the formula:
E = ¹â„2CV²
E = energy in joules (watts-sec)
V = applied voltage
C = capacitance in farads
Potential Change – A capacitor is a reactive component
which reacts against a change in potential across it. This is
shown by the equation for the linear charge of a capacitor:
I _ideal
=
C
dV
dt
where
I = Current
C = Capacitance
dV/dt = Slope of voltage transition across capacitor
Thus an infinite current would be required to instantly
change the potential across a capacitor. The amount of
current a capacitor can “sink” is determined by the above
equation.
Equivalent Circuit – A capacitor, as a practical device,
exhibits not only capacitance but also resistance and
inductance. A simplified schematic for the equivalent circuit is:
C = Capacitance
R_s = Series Resistance
L = Inductance
R_p = Parallel Resistance
R_P
L
R
S
C
Reactance – Since the insulation resistance (R_p) is
normally very high, the total impedance of a capacitor is:
ͱ ^{Z =}
R
2
S
+
(X_C
-
X_L )²
where
Z = Total Impedance
R_s = Series Resistance
X_C = Capacitive Reactance = 1
2 π fC
X_L = Inductive Reactance = 2 π fL
The variation of a capacitor’s impedance with frequency
determines its effectiveness in many applications.
Phase Angle – Power Factor and Dissipation Factor are
often confused since they are both measures of the loss in
a capacitor under AC application and are often almost iden-
tical in value. In a “perfect” capacitor the current in the
capacitor will lead the voltage by 90°.
I (Ideal)
I (Actual)
Loss
Angle _␦
Phase
Angle
f
V
IR _s
In practice the current leads the voltage by some other
phase angle due to the series resistance R_S. The comple-
ment of this angle is called the loss angle and:
Power Factor (P.F.) = Cos
Dissipation Factor (D.F.) =
f_tanor␦^Sine
␦
for small values of ␦ the tan and sine are essentially equal
which has led to the common interchangeability of the two
terms in the industry.
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