On the output voltage ripple in the ideal boost and buck-boost dc-dc converters

This paper re-examines the typical textbook analysis of the ideal boost and buck-boost dc-dc converters operating in the continuous conduction mode. It is shown that there is a range of valid average load currents for which the simple equation usually derived to relate output voltage ripple to output capacitor size in both converters is not strictly correct. An equation giving the correct relationship for the range of load currents in question is derived, using basic concepts. It is shown that the error introduced by using the original equation where the new one is appropriate is significant if both the switching duty cycle of the converter is relatively small (similar to0.3 or smaller) and the average load current is close to the continuous conduction cutoff. In such cases the size of the output capacitor needed to meet a maximum output voltage ripple specification will be significantly underestimated. The results are illustrated with an example and PSPICE verification.