Closed loop converters are strongly nonlinear system. Dynamic behavior is difficult to investigate by traditional approach. For some PWM controlled converters, low frequency ripple may occur in the output voltage and results in unsteadiness to the system. In this paper buck converter is taken as an example to investigate the stability feature. A delay in the feedback loop is related with stability closely, so the converters with delay and without delay are analyzed in detail, respectively. The second-order differential equation for output voltage profile is established. Characteristic equations are deduced from the differential equations. Based on Hurwitz Stability Criterion, stability requirement is derived from characteristic equations. It is shown that converters without delay in the feedback loop are steady. On the other hand converters with delay in feedback loop may produce low frequency ripple in some cases. Regulating parameters can avoid the low frequency ripple phenomenon. The frequency and amplitude of the low frequency ripple voltage are determined by using zero-pole analysis method. The proposed approach is verified by simulation and experiment. The simulation and experiment results demonstrate that the analysis is correct.