Reducing Storage Capacitor of a DCM Boost PFC Converter

The discontinuous current mode boost power factor correction (PFC) converter automatically achieves PFC when the duty cycle is kept constant in a line cycle; however, there is large third harmonic in the input current, and the third harmonic has the initial phase of π in respect of the fundamental component. Therefore, the input power factor is low, and a large storage capacitor is needed. Injecting appropriate third harmonic with initial phase of zero into the input current could reduce the storage capacitor. This paper proposes the variable duty cycle control to inject the required third harmonic into the input current to meet the requirement of IEC 61000-3-2 Class D or Energy Star. A method of fitting the duty cycle is further proposed for simplifying the circuit implementation. The experimental results from a 120-W universal input prototype are given to verify the effectiveness of the proposed method.