08055C datasheet(15/20 Pages) AVX

15 / 20 page

Effects of Mechanical Stress – High “K” dielectric

ceramic capacitors exhibit some low level piezoelectric

reactions under mechanical stress. As a general statement,

the piezoelectric output is higher, the higher the dielectric

constant of the ceramic. It is desirable to investigate this

effect before using high “K” dielectrics as coupling capaci-

tors in extremely low level applications.

Reliability – Historically ceramic capacitors have been one

of the most reliable types of capacitors in use today.

The approximate formula for the reliability of a ceramic

capacitor is:

where

Lo = operating life

Tt = test temperature and

Lt = test life

To = operating temperature

Vt = test voltage

in °C

Vo = operating voltage

X,Y = see text

Historically for ceramic capacitors exponent X has been

considered as 3. The exponent Y for temperature effects

typically tends to run about 8.

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

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 = 1⁄2CV

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:

ideal =

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

L = Inductance

Rs = Series Resistance

Rp = Parallel Resistance

Reactance – Since the insulation resistance (Rp) is normal-

ly very high, the total impedance of a capacitor is:

Z =

S + (XC - XL )

where

Z = Total Impedance

s = Series Resistance

C = Capacitive Reactance

π fC

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

identical in value. In a “perfect” capacitor the current in the

capacitor will lead the voltage by 90°.

General Description

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