Critical Conduction Mode Boost PFC Converter With Fixed Switching Frequency Control

Critical conduction mode (CRM) boost power factor (PF) correction converter features zero-current turn-on for the switch and no reverse recovery in diode, which is suitable for low-to-medium power applications. However, the switching frequency varies in a line cycle, and the wide variation range of the switching frequency is different under different input voltage and load conditions. This makes the conducted electromagnetic interference (EMI) spectra appearing great differences and complicates the design of EMI filter. In this paper, based on the expression of the switching frequency for traditional CRM control strategy, a variable on-time control strategy is proposed, and the implementation circuit is designed to realize a fixed switching frequency in a half-line cycle. The proposed strategy also achieves a lower output voltage ripple and a lower peak current for the power components. In spite of the PF decrease, the input current harmonic values are much lower than the IEC 61000-3-2 Class D limits. A 120 W prototype has been fabricated and tested in the laboratory and the experimental results are presented to verify the effectiveness of the proposed method.