The Circuit Designer's Companion - diagramas.diagram...

92 The Circuit Designer’s Companionthe rated capacitance and working voltage. Many manufacturers offer low leakageversions which are usually specified at 0.002CV µA. Alternatively, leakage current isa fairly well-defined function of applied voltage and usually drops to around a tenth ofits rated value at about 40% of rated voltage, so that a low-leakage characteristic can beobtained by under-running the component. Leakage is also temperature-dependent andcan be ten times its rated value at 25˚C when run at maximum operating temperature.It is also a function of history (see later under “lifetime”): when voltage is first appliedto a new component its leakage is higher.Ripple current and ESRFor power supply reservoir applications, leakage current is unimportant and instead twoother factors must be considered, ripple current (I R ) and equivalent series resistance(ESR). The ripple current is the ac current flowing through the capacitor as the reservoircharges and discharges, usually at 100/120Hz for ac mains supplies or at the switchingfrequency for switch-mode supplies. It develops a power dissipation across the resistivepart of the capacitor impedance (ESR) which results in a temperature rise within thecapacitor, and it is this dissipation which limits the capacitor’s I R rating. Published datafor all electrolytics include an I R rating which must be observed. The rating increasesto some extent with increasing frequency and reducing temperature. But note that therating is normally published as an RMS value, and actual ripple waveforms are oftenfar from sinusoidal, so a correction factor must be derived for this difference.Such thermal considerations imply that, particularly for reservoir applications, youmay need to select a capacitor with a higher voltage or capacitance rating than wouldbe expected from the circuit parameters.The ESR value (Figure 3.14) is important both because it contributes to the I R ratingand because it limits the effective high-frequency impedance of the capacitor. ThisZESRmin = ESRfFigure 3.14 Capacitor equivalent series resistancepoint has become of increasing importance with the advent of high-frequency switchingpower supplies where the output ripple voltage is determined by the output capacitor’sESR rather than its absolute capacitance value. Some manufacturers now offer speciallow-ESR versions specifically for these applications. ESR of non-solid electrolyticsincreases dramatically as the operating temperature is reduced below 0˚C, which can bea problem in circuits where actual dissipation is low. The better-quality ranges ofelectrolytics include this factor in their specification of “impedance ratio”, the ratio ofESR at some sub-zero temperature to that at 20˚C, which is usually around 3 or 4 butmay be much worse. Solid electrolytics do not exhibit this behaviour to the same extent.Temperature and lifetimeThe capacitor characteristics as discussed for ceramic and film types are generallyworse for electrolytics. Capacitance/temperature curves are rarely published but nonsolidtypes can vary non-linearly by around ±20% over the operating temperaturerange, capacitance reducing with lower temperatures; solid types are better by a factor