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The boost power-factor-correction (PFC) converter with average-current-mode control is a nonlinear system due to the effects of the multiplier and a large variation of the duty ratio. Although its stability analysis must be studied depending on a nonlinear model, most prior research attempted to make some assumptions to force this nonlinear system to be linear. As a result, the practical dynamics and the nonlinear phenomena were disregarded. In this paper, two kinds of nonlinear phenomena are detected under the conditions that are considered to be stable by the prior criteria: one is period-doubling bifurcation and the other is chaos. Stability maps and phase-plane trajectories are introduced at different loads to clarify the power factor and also the instability regions. It is clear that the output storage capacitor is a main contributing parameter on the system stability, therefore, bifurcation maps are developed to determine the accurate minimum output capacitance value that assures the system stability under all operating conditions. Also, PFC converter stability is explained and judged by comparing the consequence of charging and discharging energy in the output capacitor. Index Terms--Bifurcation, boost converter, chaos, instability phenomena, nonlinear analysis, power-factor correction (PFC), stability.