Constant Switching Frequency Control for CRM Buck PFC Converter

Critical conduction mode (CRM) buck power factor correction (PFC) is widely used in low-power applications because of its several advantages. However, the switching frequency is dependent on the electrical angle of the input voltage, and the variation range of the switching frequency is large, especially at high input voltages. This makes the conducted electromagnetic interference (EMI) spectra appearing great differences and complicates the design of the EMI filter. For this issue, a novel constant frequency control scheme is proposed by varying the on-time of the switch. Compared with the original control, the proposed method realizes a constant switching frequency operation in a line cycle. Consequently, the design of the power devices especially the magnetic components can be simplified. Meanwhile, a higher efficiency and a lower output voltage ripple are achieved. The proposed strategy also yields a higher PF at low input voltages and a lower PF at high input voltages. A 120-W prototype with 90-264-VAC input and 90-V output is built in the lab. The experimental waveforms of input voltage, input current, inductor current, and output voltage ripple, the measured input current harmonic contents, the PF and efficiency curves regarding the input voltage, and the common-mode (CM) and differential-mode (DM) noises' spectra of the converter are presented to show the validity of the proposed scheme.