Research On Control Strategy Of Three-level Half-bridge LLC Resonant Converter For Wide-output Range

LLC resonant converter can achieve the zero voltage switching of primary main switches and zero current switching of rectifiers in entire operating load range at high switching frequency,which makes it a favorable topology for electric vehicle battery charger,distributed photovoltaic system,fuel cell power system and data center power system.When faced with the requirement for wide-range operations,the traditional LLC resonant converter reduces the inductance ratio,or increasing the working frequency range,which will reduce the system efficiency.Therefore,control strategies of LLC resonant converter for wide range need to be researched in order to achieve a very high overall efficiency.There is the tradeoff between the wide range and the efficiency.It is very meaningful for the LLC converters to achieve high dc gain,while maintaining a very high overall efficiency.In order to increase the operating range,this paper introduces the phase shift control to the three-level half-bridge LLC resonant converter.The circuit operation modes under phase shift control are analysed and divided into six modalities.The time-domain analysis method is used to establish the precise gain curve of the LLC resonant converter under the phase shift control.The conditions of the soft switching under the phase shift control are analysed.The simulation and experiments verify the accuracy of the gain curve and the soft switching conditions.Because the soft switching range of the phase shift control is not satisfactory,this paper presents an Asymmetrical Voltage-Cancellation control strategy in the LLC resonant converter,and gives the scope of application.Then the soft switching conditions under AVC control for the LLC-VDR?voltage doubler rectifier?resonant converter are analysed,and an optimal control trajectory is proposed.An accurate gain relationship between the output voltage and the duty cycle is obtained based on this control trajectory.The AVC control,the phase shift control and the APWM control are compared from the DC gain,the reactive power and the soft switching conditions.Through experiments,it is verified that the soft switching effect of the AVC control is better,and a very high overall efficiency can be realized.In order to solve the problem that the difficulty of decrasing voltage and the inefficiency in the light load The Burst mode is introduced into the LLC resonant converter.Because of the large output voltage ripple in Burst mode,the important parameters such as the frequency of Burst,the voltage increment?(1₄)of each beat are optimized.A Burst mode implementation method based on ripple optimization is proposed.According to the difference of the adoption strategy in the T_on,two different implementation ideas are given.Finally,the feasibility of the proposed Burst method is verified by experiments,and the ouput voltage ripple is improved.