Please note: the figures refered to in the text are those printed in the original article (December 2002 issue of Practical Wireless).
No electronic component, either passive or active, is perfect.
In addition to exhibiting its marked resistance,
a resistor will also have a little series inductance
and a tiny amount of parallel capacitance.
Similarly, an inductor will have both series resistance
and parallel capacitance.
Capacitors also have a small measure of series inductance, but this is usually only of concern at relatively high frequencies. Rather, it is the capacitor's equivalent series resistance (e.s.r.) that can be significant at both high and low frequencies.
A capacitor's e.s.r. can be modeled electrically as a resistance in series with a perfect capacitor. At high frequencies, or when the current through the capacitor is substantial, this resistance must be taken into account. In the case of capacitor C1 in Fig. 2, its e.s.r. and its capacitance, together with the resistance of the load, form a CR network that influences the stability of the circuit.
Real capacitors typically have e.s.r. values ranging from a few milliohms to several ohms. To demonstrate that e.s.r. does make a difference, imagine a capacitor that has a value of 100uF and an e.s.r. of 0.1ohm. At a frequency of 20kHz, the reactance of the capacitor is just 0.08ohm. That's less than its e.s.r.!
Clearly, if this capacitor is subjected to frequencies greater than 20kHz, then its e.s.r. will dominate. This has repercussions in switched-mode power supplies, where ripple frequencies can vary from a few kHz to 1MHz or higher. In fact, at the switching frequencies found in today's switched-mode power supplies, a capacitor's e.s.r. is often greater than its reactance. This is why capacitor manufacturers continue to develop new ranges of capacitors with ever lower e.s.r. values.
There's one more note to add: while doubling a capacitor's value will always halve its reactance, its e.s.r. is seldom halved at the same time. So you're unlikely to get twice the smoothing effect in a switched-mode supply. If more smoothing is necessary, it may be better to parallel two small capacitors rather than use a single, large capacitor. Now you'll understand why two (sometimes more) capacitors are often found wired in parallel inside high-power, switched-mode power supplies.