Fixed Switching Frequency Controls for Integrated Buck PFC Converters in Critical Conduction Mode

Abstract

To eliminate the inherent dead zone of the buck power factor correction (PFC) converter, integrated buck PFC converters, such as buck–flyback PFC converter and buck–buck/boost PFC converter, have been proposed. Traditionally, constant on-time (COT) control is used to regulate these converters in critical conduction mode (CRM). However, the switching frequencies of these PFC converters with COT vary greatly during a line cycle, which complicates the design of the electromagnetic interference filter. In this article, a novel fixed switching frequency control (FFC) strategy and a novel dual fixed switching frequency control (DFFC) strategy are put forward for the CRM buck–flyback PFC converter and the CRM buck–buck/boost PFC converter, respectively. The switching frequency of the CRM buck–flyback PFC converter with FFC remains constant in a line cycle, and that of the CRM buck–buck/boost PFC converter with DFFC is fixed in buck operation and buck/boost operation separately. In addition, the turn ratio of the buck–flyback PFC converter's transformer and the switching frequency of the buck–buck/boost PFC converter are optimized to make the power factor and the total harmonic distortion meet standards, such as ENERGY STAR and IEC 61000-3-2. The experimental results are given to verify the effectiveness of the proposed methods.