| In the field of AC-DC conversion,power factor correction(PFC)technology should be adopted for the pollution reduction to power grid and meeting the relevant harmonic standards.Based on different topologies and inductor current conduction modes,many kinds of circuits and control methods are available.Buck PFC converter features simple circuit,low voltage stress of power components and comparatively stable efficiency in wide input voltage range.In critical conduction mode(CRM),the switch turns on with zero-current and the diode has no reverse recovery.Therefore,CRM Buck PFC converter is widely used in low power applications.However,the switching frequency is affected by the instantaneous input voltage,and the variation range is large in a half line cycle,especially at high input voltages.This not only makes the design of magnetic components more complex,but also increases the loss of power components and lowers the efficiency.In this paper,the converter with conventional variable switching frequency control(VFC)method is analyzes,and the mathematical relationship between switching frequency and electrical parameters such as output voltage,output power and instantaneous input voltage are derived.For the issue of frequency variation,a new constant switching frequency control(CFC)strategy is proposed,and the corresponding implementation circuit is designed.The performance comparison is made between CFC and VFC in respect of power factor(PF),the peak and root-mean-square(RMS)current of inductor,the output voltage ripple and the power loss.Besides the constant frequency,a higher efficiency,a reduction of current stress and a lower output voltage ripple are also achieved.Meanwhile,the proposed strategy brings about a higher PF at low input voltages and a lower PF at high input voltages.A prototype is built in the lab,and the experiments are carried out on the converter with two control methods.The experimental results validate the proposed scheme. |
