08055C datasheet(16/20 Pages) AVX

16 / 20 page

General Description

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

f or Sine

Dissipation Factor (D.F.) = tan

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.

Equivalent Series Resistance – The term E.S.R. or

Equivalent Series Resistance combines all losses both

series and parallel in a capacitor at a given frequency so

that the equivalent circuit is reduced to a simple R-C series

connection.

Dissipation Factor – The DF/PF of a capacitor tells what

percent of the apparent power input will turn to heat in the

capacitor.

Dissipation Factor =

E.S.R.

= (2

π fC) (E.S.R.)

The watts loss are:

Watts loss = (2

π fCV2) (D.F.)

Very low values of dissipation factor are expressed as their

reciprocal for convenience. These are called the “Q” or

Quality factor of capacitors.

Parasitic Inductance – The parasitic inductance of capac-

itors is becoming more and more important in the decou-

pling of today’s high speed digital systems. The relationship

between the inductance and the ripple voltage induced on

the DC voltage line can be seen from the simple inductance

equation:

V = L di

The

seen in current microprocessors can be as high as

0.3 A/ns, and up to 10A/ns. At 0.3 A/ns, 100pH of parasitic

inductance can cause a voltage spike of 30mV. While this

does not sound very drastic, with the Vcc for microproces-

sors decreasing at the current rate, this can be a fairly large

percentage.

Another important, often overlooked, reason for knowing

the parasitic inductance is the calculation of the resonant

frequency. This can be important for high frequency, by-

pass capacitors, as the resonant point will give the most

signal attenuation. The resonant frequency is calculated

from the simple equation:

fres =

Insulation Resistance – Insulation Resistance is the

resistance measured across the terminals of a capacitor

and consists principally of the parallel resistance R P shown

in the equivalent circuit. As capacitance values and hence

the area of dielectric increases, the I.R. decreases and

hence the product (C x IR or RC) is often specified in ohm

faradsor more commonly megohm-microfarads. Leakage

current is determined by dividing the rated voltage by IR

(Ohm’s Law).

Dielectric Strength – Dielectric Strength is an expression

of the ability of a material to withstand an electrical stress.

Although dielectric strength is ordinarily expressed in volts, it

is actually dependent on the thickness of the dielectric and

thus is also more generically a function of volts/mil.

Dielectric Absorption – A capacitor does not discharge

instantaneously upon application of a short circuit, but

drains gradually after the capacitance proper has been dis-

charged. It is common practice to measure the dielectric

absorption by determining the “reappearing voltage” which

appears across a capacitor at some point in time after it has

been fully discharged under short circuit conditions.

Corona – Corona is the ionization of air or other vapors

which causes them to conduct current. It is especially

prevalent in high voltage units but can occur with low voltages

as well where high voltage gradients occur. The energy

discharged degrades the performance of the capacitor and

can in time cause catastrophic failures.

I (Ideal)

I (Actual)

Phase

Angle

Loss

Angle

E.S.R.

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