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A unified approach for the stability analysis of both preregulator and two-stage power factor correction (PFC) ac-dc converters is presented. Practical and accurate analytical expressions for the stability boundary are obtained by studying the dynamics of the first harmonic component. The resulting model of this dynamics has an equilibrium point at the origin of the harmonic state space. In the case of a stable equilibrium point, the first-order component converges to zero, and the system oscillates at twice the line frequency. If the equilibrium point loses stability, another equilibrium point depending on high-order harmonics will be reached while the real system will exhibit a period-doubling phenomenon leading to subharmonic oscillations. It is demonstrated that the stability boundaries depend on the power level. Numerical simulations and experimental results in both a preregulator and a complete two-stage PFC converter validate the approach.