Digital-Based Critical Conduction Mode Control for Three-Level Boost PFC Converter

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Abstract
Conventional critical conduction mode (CRM) control for many boost-derived power-factor-correction (PFC) circuits forms triangular inductor current. For three-level boost (TLB) converter, the conventional method is unattractive because losses increase but waveform quality remains unchanged, compared to other topologies. In order to obtain higher efficiency and improved waveform qualities, this paper focuses on the distinctive structure of TLB and its inherent degree-of-freedom (DOF) in current-slope shaping. Based on the DOF, a new CRM control for TLB PFC is proposed and analyzed. Detailed design and digital implementation method are also provided. Different from conventional methods, the proposed CRM control divides each switching cycle into three parts including common on-time of two switches, additional on-time of one switch and common off-time. Accordingly, TLB inductor current is synthesized in quadrangular forms. The analyses and experimental results confirm that switching frequency and peak input current of TLB can be reduced by the proposed control. Due to the reductions, efficiency, total-harmonic-distortion (THD) and quality of input current are improved with practically unchanged power factor.

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