Study On PFM/PWM Hybrid Control Strategy And Optimized Design For Interleaved Single-stage Boost-LLC AC-DC Converter

In an effort to suppress the harmonic current caused by the widely used power electronic equipment and solve the energy crisis,AC-DC converters with high power factor,high efficiency and high power density are becoming an active research topic in power electronic filed.By sharing switches of the Boost PFC cell and half bridge LLC resonant cell,the traditional single-stage Boost-LLC AC-DC converter has advantages in cost,control complexity and efficiency when compared with traditional two-stage AC-DC converter,which makes it quite fit for low power rating applications.Due to the interleaved structure,the single-stage interleaved Boost-LLC AC-DC converter can increase the power rating while decreasing the input current ripple.Therefore,it is chosen as the studied converter in this thesis.Nevertheless,universal input line voltage(85-265Vac)will lead to high bus voltage if only PFM control method is employed,thus limiting the acceptable input voltage range to 85-135 Vac.In addition,wide bus voltage range will cause difficulties in designing the LLC resonant parameters.To solve these issues,the studied converter is analyzed and discussed from terms of modulation,control method and parameter design.Ouput voltage feedback,bus voltage proportional feed-forward and dual-loop control strategies based on PFM/PWM hybrid modulation are prosposed and the respective design procedures are given in this thesis.Firstly,the operation modes of the studied converter under PFM and PWM modulation are explained respectively.Characteristics of the Boost PFC and LLC resonant cell are carefully analyzed.Moreover,the reason and condtion of achieving soft switching for primary switches are also derived in detail.Then,operation principles and control shemes of the proposed three control strategies are detailly stated.By utilizing the digital controller,the implemented algorithm for different control strategies is accomplished and presented.Taking the higer power factor value and acceptable bus voltage both into consideration,the parameters of the converter are optimized based on the power balance principle between the Boost PFC and LLC resonant cell,the parameter calculation and constraint are also obtained in this thesis.A 300 W experimental prototype has been designed and fabricated in the lab.The experimental results verified feasibility and validity of the three different control strategies.The prototype is able to work over universal input line voltage range while maintaining high power factor and effieciency.Finally,the advantages and drawbacks of the three control strategies are compared and summarized.