40
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:
f
res = 1
2 LC
Insulation Resistance – Insulation Resistance is the resis-
tance 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 farads
or 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.
di
dt
General Description
