Novel Control Methods Of CRM Boost PFC Converter To Reduce The Switching Frequency Variation Range

The Boost Power Factor Correction converter occupies the important position in numerous PFC converters with good performance.Depending on the inductor current to be continuous or not,there are three basic operating modes:continuous current mode?CCM?, critical current mode?CRM?and discontinuous current mode?DCM?.CRM Boost PFC converter operates in critical current mode,so it has the advangtages of the switching zero-current turn-on,no diode reverse recovery,relatively simple control circuit and the high PF which theoretically can be one.And the converter has been widely used for low to medium power conversion applications.However,because the switching frequency of CRM Boost PFC converter is greatly influenced by the input voltage and load,the variation range is very large,and the ratio of the maximum and minimum value can reach several times to more than ten times.This not only complicates the design of EMI filter but also multiplied the turn-off loss of switching tube and the core loss of inductor,leading to low efficiency.Under the condition of high input voltage and light load,the unusual high frequency may cause the converter can not work normally.For CRM Boost PFC converter,this paper puts forward two new control methods to overcome the defect of wide switching frequency variation range.One is the optimal 3^rd harmonic control strategy,which minimizes the variation range of switching frequency in line cycle by injecting the optimal 3^rd harmonic into the input current.The other one is the constant switching frequency control strategy by injecting further 5^th and 7^th harmonic into the input current,which is on the basis of he optimal 3^rd harmonic control strategy.It is worth noting that the input harmonic current of both new control methods meets the IEC 61000-3-2 Class D standards.Compared with the traditional control,the proposed two novel control methods slash the variation range of switching frequency,simplify the design of EMI filter and inductor,improve the effect of input fliter,decrease the output voltage ripple and also reduce the switching loss and core loss to increase the efficiency of converter.The effectiveness of the proposed methods is verified by the experimental results from two prototypes of 120W.