But the need is most definitely there!

In order to reduce the ripple to a minimum, you want the impedance of the output capacitor (at the switching frequency) to be significantly lower than that of the load. And with a switching frequency of, say, 100kHz, even a modest amount of capacitance will already produce an almost negligible impedance (16mOhm for 100µF, for example). This means, in practise, that ESR is the major factor in determining a capacitor's impedance.

Moreover, the ESR will produce losses (ripple current * ESR) that detract from the PSU's efficiency and also cause capacitor heating, which is not a good thing in electrolytics. Remember that the expected lifetime of an electrolytic halves for every 10°C rise in temperature!


Rob.