General Description
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.
Insulation Resistance – Insulation Resistance is the
resistance measured across the terminals of a capacitor and
consists principally of the parallel resistance RP 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 farads or
more commonly megohm-microfarads. Leakage current is
determined by dividing the rated voltage by IR (Ohm’s Law).
E.S.R.
C
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.
Dissipation Factor – The DF/PF of a capacitor tells what
percent of the apparent power input will turn to heat in the
capacitor.
E.S.R.
XC
Dissipation Factor =
= (2 π fC) (E.S.R.)
Dielectric Absorption – A capacitor does not discharge
instantaneously upon application of a short circuit, but drains
gradually after the capacitance proper has been discharged.
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.
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.
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.
Parasitic Inductance – The parasitic inductance of capac-
itors is becoming more and more important in the decoupling
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:
di
dt
V = L
di
dt
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 =
1
2ꢃꢀLC
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