Another contributor to internal
temperature rise is ambient temperature.
The life expectancy of a capacitor is
reduced signifi cantly as the ambient
temperature increases simply because
this also results in evaporation of the
electrolyte, which becomes worse as
the temperature increases. In practice,
the life expectancy of an electrolytic
capacitor at 60°C is twice that when
operating at 75°C. Beyond that, the life
expectancy of the capacitor is halved
for every 10°C increase in temperature.
It is particularly important to be
aware that high internal temperatures
can result in a signifi cant increase in
the capacitor’s internal pressure. This
situation can be relieved by several
methods, including a controlled
thickness vent in the can, a diaphragm
in large cans, or by expulsion of the
rubber end plug in miniature axial and
radial lead electrolytic capacitors.Working voltage
The voltage rating for a capacitor is
related to the voltage used while forming
the dielectric during manufacture
and normally represents the maker’scapacitance. Some manufacturers quote
insulation resistance as a product of
the insulation resistance (in MΩ) and
capacitance (in μF). The insulation
resistance may thus be specifi ed in MΩ-μF.
To determine the insulation resistance
for a given capacitor it is simply a matter
of dividing the MΩ-μF value by the
component’s capacitance. For example,
if a 2.2μF capacitor is taken from a range
with a quoted IR value of 4.4MΩ-μF, it
will have an insulation resistance of 2MΩ.Leakage current
Leakage current is often specifi ed in
terms of the amount of stored charge
(ie, as a constant multiple of capacitance
(C) and applied voltage, V – typically
0.01CV – or as a maximum current in
μA (whichever is the greater). Leakage
current decreases with insulation but
increases with applied voltage and
temperature. Typical leakage currents for
electrolytic capacitors are in the range
1μA to 10μA, and this is usually not
signifi cant in power supply applications.Having explained some of the
characteristics of electrolytic capacitors
you will now realise that such components
are inherently imperfect and their
operation is affected by internal factors
such as the presence of small amounts
of series resistance and inductance. We
can model a real capacitor using the
equivalent circuit of a capacitor shown
in Fig.6.8. The components shown are:
Effective capacitance, C
Parallel (or ‘shunt’) resistance (RP)
through which a small leakage
current fl ows
Equivalent series resistance (ESR), RE
Effective series inductance (ESL), LS.
It is important to understand that the
components shown in Fig.6.8 have
quite different effects on a capacitor’s
performance in a working circuit.
For example, at low frequencies, ESL
(LS) is insignifi cant, it does become
increasingly important at high and very
high frequencies. RP, on the other hand is
Equi valent circuit of a
capacitor__
of little consequence in low-impedance
equipment (such as in a power supply)
but it does become important in high-
impedance circuit applications (for
example, in a sample and hold circuit).
Conversely, while RE is unimportant in a
high-impedance circuit, this component
becomes critical in low-impedance
situations (such as conventional and
switched-mode power supplies).
Fortunately, for most power supply
applications we can simply ignore the
shunt resistance and series inductance
leaving us with just the ESR (RE) to
consider. Modern high-current switch-
mode power supplies have increased
the demand for low-impedance, high-
frequency capacitors with high ripple
current ratings. These components must
always have a low value of ESR.Capacitor failure
Capacitor failure may be either drastic
or progressive. There are two types of
drastic failure: short-circuit and open-
circuit. The likely consequence of either
condition is that the equipment to which
the component is fi tted will fail to operate
totally or will fail to operate within
specifi cation. A progressive failure, on
the other hand, is when one or more
aspects of the capacitor fall outside their
specifi ed limits. For example, when a
capacitor’s leakage current increases to
a value that is signifi cantly greater than
the maximum rating for the component.
Short-circuit failures normally result
from failure of the dielectric due to
voltage stress. Open-circuits, on the
other hand, are usually attributable to
mechanical failure of the joint between
the foil and capacitor terminals. Such
failures are very much less common
than electrical failures in the dielectric.
Exposure to hydrogenated hydrocarbon
solvent cleaners (which enter electrolytic
capacitors through the pressure seal) can
often result in chemical attacks on the
aluminium foil electrodes of electrolytic
capacitors. This type of failure is now less
common due to the use of no-wash or
water-washable fl uxes while soldering.
When electronic equipment requires
cleaning, it is important to ensure that
electrolytic capacitors are treated very
sparingly, if at all!Temperature effects
Failure of large electrolytic capacitors is
often caused by relatively high values of
working temperature which, in turn, are
usually caused by relatively high values of
ESR. Since ESR increases with temperature
this can become a vicious circle!
The reduction in the working life of a
capacitor working at a high temperature
usually occurs because the electrolyte
starts to evaporate, and its vapour passes
through the end seal. The consequent loss
of electrolyte reduces the capacitance
and increases the ESR (since less
electrolyte is present). In turn, this can
result in early equipment failure where
correct circuit operating conditions are
no longer maintained due to ineffective
power line fi ltering or decoupling.Fig.6.9 In-circuit measurement of the
ESR of a PCB mounted radial lead
electrolytic capacitor.
Fig.6.10. Various capacitor ESR
measurements showing how ESR
varies for different types and sizes
of capacitor.