A unified practical approach to analyze the stability of the pre-regulator and complete two-stage PFC power supplies under average-current-mode control

Power Factor Correction (PFC) AC-DC converters can present some undesirable nonlinear phenomena that cause subharmonic oscillations at the line frequency characterized by a power factor less than 0.7 and high Total Harmonic Distortion (THD). In this paper, the stability boundary for boost PFC converter is obtained. A unified analytical and practical approach for stability analysis of both pre-regulator and two-stage PFC converters is presented. The advantage of this approach over previous analysis is taking into consideration the practical output ripple that is regularly ignored in previous works about this subject. Moreover, in the new approach the loading effect which is ignored in two-stage PFC previous models appears in the expressions of the stability boundary. This is obtained by studying the dynamics of the first harmonic component in the harmonic state space. By performing a harmonic analysis, it is found that the real system presents period doubling phenomenon causing sub-harmonic oscillations. The steady state values of harmonics of interest are obtained by making some practical assumptions. This approach is applied to the boost PFC pre-regulator and to the complete two-stage PFC boost converter loaded by a buck converter. The performances of both systems are discussed in terms circuit parameters, working at nominal and non-nominal conditions. To validate the theoretical predictions, numerical simulations and experimental measurements are also presented with a good matching.