Analysis of output voltage ripple caused by ESR of a smoothing capacitor for a low-voltage high-current buck converter

This paper presents the analytical results of output voltage ripple for a buck converter with synchronous rectifier. In the conventional analysis of the output voltage ripple, it is assumed that the inductor current ripple is equal to the ripple current of the smoothing capacitor and that the equivalent series resistance (ESR) of the smoothing capacitor is sufficiently small or sufficiently large. However, if the load impedance is not much larger than the impedance of the smoothing capacitor including ESR, this assumption is not correct. In this paper, we consider the cases that the inductor current ripple is not always equal to the ripple current of the smoothing capacitor and that there are no restrictions on ESR. From the analytical results, it is shown that the output voltage ripple has four modes which are determined by the relation of load resistance, the capacitance, the ESR, the switching cycle, and the duty ratio. As a result of analysis, we show the lower limit of the ripple rate of the output voltage. The analytical results are confirmed by simulations and experiments. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 143(2): 59–66, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10133