GlassWare Audio Design 7
Here’s an extreme example.Fearing ultra-high-
frequency noise on the power supply’s
output, an audiophile places a small-
valued capacitor across the power transformer’s
secondary. Now, since the power supply yields a nominal 400-volts, a 400-volt high
frequency-shunting capacitor is soldered in place, say 0.1-
µF. But as the power supply
comprises a full-wave center-tapped-
transformer arrangement, the capacitor sees not
400-volts, but 800-peak-
volts! Even worse, if a large choke connects directly to the
diodes (choke loading), then the peak voltage across the leads is 1130-volts, not 400
volts. Considering that the wall voltage may hold momentary peaks of much higher
voltages, a safe rating might be 1600-volts, quite a distance from 400-vdc.
Temperature LimitsThere has to be
a temperature limit to every capacitor, as no
capacitor can
survive on the Sun’s surface for example. With film capacitors, such as
Mylar or polystyrene, the temperature-
limit is seldom printed on their bodies,
although this limit can be found in the manufacture’s spec sheet for the capaci
Because plastic films melt
at a much lower temperature than do glass, mica, porcelain,
or ceramic,care should be taken not to momentarily overheating film capacitors
during soldering, particularly with polystyrene capacitors. For example, a
can easily be thrown off a few percent by such an overheating, as the heat causes the
plastic to change in shape. In contrast, e
lectrolytic capacitors always (well, almost
always) display a temperatur
e limit. This limit should never be approached let alone
exceeded, as the capacitor can be easily damaged and it just might rupture.
addition, an electrolytic capacitor’s useful
life expectancy is shortened drastically with
excess heat, as the wet part
(the electrolyte) evaporates with heat, leaving the capacitor
greatly reduced in capacitance.
Equivalent Series Resistance (ESR) Ideally, a capacitor should hold no ESR,
ideal capacitor dissipates no heat and loses no voltage across its leads, but m
capacitors do both. Real capacitors
are made with metal, thin metal at that, so we
should expect some resistance due to the metal,
from the leads, the electrodes, and the
slight resistance from the solder that binds the leads to the electrodes, but not
dielectric effective residual resistance. Plastic film-foil capacitors display a lower ESR
than metalized-
film capacitors, as foil has a lower resistance. Placing multiple
capacitors in parallel lowers the effective ESR.
Heat is the capacitor’s enemy
. So keeping a capacitor cool is important and ESR
works against us. High ripple current causes a capacitor to self-heat, just as a
would facing the same currents. A capacitor’s construction, unfortunately, limits its
heat transfer to its surroundings. The heat dissipation in a capacitor is equal to I2
ESR, where I is the RMS ripple current.
Equivalent Series Inductance (ESL)
Like ESR, ESL represents the inductance in
series with the capacitance in the capacitor. This is an important considerati
high-frequency power supply filtering and other high-
frequency applications as well.
Placing multiple capacitors in parallel lowers the effective ESL, but the best plan
might be to use specially-designed high-frequency electrolytic capacitor instead.
Dissipation Factor (DF) DF is equal to ESR/XC
. DF is much more important in
power supply applications than it is in signal-coupling applications.
predicting the amount of heat a capacitor will dissipate.
Quality Factor (Q) The inverse of DF is the quality factor (Q). Thus, Q= ESR/XC.
