A Mixed Conduction Mode Control Strategy Based on Fixed Turn-off Time for Totem-Pole PFC Converter

This article proposes a mixed conduction mode (MCM) control strategy based on a fixed turn-off time (FOT) to address the issues of low light-load efficiency, poor power factor and high current stress in the single-mode operation of the totem-pole power factor correction (PFC) converter. This strategy enables hybrid operation between discontinuous conduction mode (DCM) and continuous conduction mode (CCM) within a half-line cycle. By fixing the turn-off time of the transistor, the switching boundary between DCM and CCM is clearly defined, enabling adaptive mode switching in accordance with output power. Furthermore, a DCM/CCM turn-on time prediction model has been developed to unify the DCM/CCM control loops, thereby ensuring stable MCM operation. Finally, a 1500W-rated prototype totem-pole PFC converter has been designed to validate the effectiveness of the control strategy. The experimental results demonstrate that, compared to the ideal critical conduction mode (CRM) operation, the proposed control strategy effectively reduces the current stress of the converter, and the peak value of the inductance current is reduced by 16.1% at full load. In comparison to the traditional CCM control, the proposed control strategy significantly enhances the light-load performance of the converter, achieving a maximum improvement of 11.6% in light-load efficiency, a 0.295 increase in power factor and 7.1% decrease in iTHD.