Achieving High Input Power Factor For DCM Boost PFC Converters

Discontinuous-current-mode (DCM) boost power-factor-correction (PFC) converter features zero-current turn-on for the switch, no reverse recovery in diode, constant frequency operation, simple control and low cost, and it is suitable for the low-to-medium power conversions. However, the input power factor (PF) is relatively low and the input current contains rich harmonics when the duty cycle is constant over the whole line cycle. This dissertation is dedicated to improving the PF and reducing the output storage capacitor of the DCM boost PFC converters.The expressions of the input current and PF of the single-phase DCM boost PFC converter are derived, based on which, a variable duty cycle control is proposed so as to improve the PF to unity over the whole input voltage range. A method of fitting the duty cycle is further proposed for simplifying the circuit implementation. Besides a higher PF, the proposed variable duty cycle control achieves a lower output voltage ripple and a higher efficiency over the constant duty cycle control. The experimental results from a prototype with 90–264 VAC input and 400 V/120 W output are presented to verify the effectiveness of the proposed method.With constant duty cycle control, the input current of the single-phase DCM Boost PFC converter mainly contains the 3rd harmonic with an initial phase difference ofπin respect to the fundamental component. Therefore, the PF is low and the input power pulsation is large. However, if the 3rd harmonic is in phase with the fundamental component, the input power pulsation can be reduced, leading to a lower output voltage ripple. In other words, the output storage capacitance can be reduced with the same output voltage ripple. Injecting approriate 3rd harmonic which is in phase with fundamental component into the input current is proposed in this dissertation. The proposed method achieves storage capacitor reduction of 63% and 57% over the constant duty cycle control, when the harmonic specification of IEC 61000-3-2 Class D or Energy Star is met respectively. A method of fitting the duty cycle is further proposed for simplifying the circuit implementation. The experimental results from a prototype with 90–264 VAC input and 400 V/120 W output are presented to verify the effectiveness of the proposed method.For the three-phase single-switch boost PFC converter, the operation principle of the converter is analyzed in detail and the expressions of the input current and PF are derived. Based on which, the ideal dutyc cyle which makes the PF of all the three phases be unity is discussed. Since only one duty cycle is available in the converter, the weighting method is applied to get the variable duty cycle control which can improve the PF of all the three phases to an optimal value. The method of fitting the duty cycle is further proposed for simplifying the circuit implementation. Besides a higher PF and lower input current harmonics, the proposed variable duty cycle control achieves a lower output voltage ripple and a higher efficiency over the constant duty cycle control. A prototype with 220±20%/380±20%VAC input and 750V/3kW output has been built and tested in the lab, and the experimental results are presented to verify the effectiveness of the proposed method.