13
Fundamentals of Electric Capacitors
miniaturized electronic devices that require high reliability and extended
lifetimes.
Electrolytic capacitors present some advantages over other types of
capacitors: (1) larger capacitance per unit volume due to their high dielectric constants and storage capabilities; (2) suitability for use in relatively
high-current and low-frequency electrical circuits, especially in power
supply filters to moderate output voltage and current fluctuations in rectifier output; (3) widespread use as coupling capacitors to allow conduction
of AC and halt DC. One drawback of electrolytic capacitors is that the
standard design requires the applied voltage to be polarized; that is, one
specified terminal must always have a positive potential with respect to
the other. Therefore, electrolytic capacitors need a DC polarizing bias to
be used with AC signals. Some other drawbacks include relatively low
breakdown voltages, higher leakage current and inductance, poorer tolerances and temperature ranges, and shorter lifetimes than other types of
capacitors.
1.3.1.4 Paper and Polymer Dielectrics and Their Capacitors
In the past, paper impregnated with wax or oil was used as a dielectric
material as an alternative to glass and mica dielectrics. A major drawback to
using paper was its moisture-absorbing nature because moisture degrades
the capacitive ability and increases internal resistances. Polymer dielectrics are used in applications with low frequency and high stability. A list
of polymers along with their respective features and shortcomings can be
found in Table 1.2. Polymer dielectric-based capacitors include polymerized
organic semiconductor solid electrolyte capacitors and conductive polymer
capacitors. A typical polymer capacitor has an aluminum foil cathode, a
solid state polymer electrolyte, and aluminum foil with an oxide layer as
the anode. In general, they last longer but cost more than standard (liquid)
electrolytic capacitors.
1.3.1.5 Classification of Dielectric Materials
Dielectric materials fall into two classes: Class I used for linear capacitors
and Class II used for non-linear capacitors. In general, Class I materials are
natural dielectrics such as glass and mica and have capacitances within the
range of a couple to several hundred picofarads. When capacitors use Class I
materials, the capacitance will not change with a dynamic operating voltage
and frequency. Class I dielectric materials are more costly.
Typical Class II materials are ferrodielectric ceramics and polymer electrolytes with high dielectric constants. They are used to develop high capacitance capacitors. For example, metalized polymer foils can achieve a few
thousand picofarads to a few microfarads; an electrolytic capacitor can offer
a capacitance in the range of a few to several thousand microfarads; some