Design And Implementation Of LLC Resonant Converter With Wide Input Voltage Range

In power conversion systems such as photovoltaics,wind power,and energy storage equipment and electric vehicle charging piles,active power converters are often required to operate over wide input voltage range.However,the traditional LLC resonant converter is difficult to apply to wide input voltage range due to factors such as its own gain characteristics.Therefore,it is great practical significance and engineering value that research the LLC resonant converter and its control strategy to adapt to wide input voltage range.In order to solve the problem of adapting to wide input voltage range,many literatures have studied the circuit structure and control strategy of traditional LLC resonant converters.Some improve the adaptability of the LLC resonant converter over wide input voltage range through multi-level transformation.Some broaden the input voltage range of the LLC resonant converter by changing the primary side circuit structure.Others adapt the wide input voltage range by mixing the variable frequency control strategy with the phase shift control strategy.These methods either significantly increase the cost and loss,resulting in reduced system efficiency,or the topology transition is not smooth,resulting in output voltage spikes,Therefore,the problem of adapting to a wide input voltage range is not well solved.Based on the full-bridge LLC resonant converter,a hybrid control strategy for switching topology by adjusting frequency and duty cycle is proposed,which enables the topology to smoothly switch between the full-bridge and the half-bridge and output a stable voltage,allowing the converter to operate over wide input voltage range while maintaining high operating efficiency.The main content of this article is arranged as follows:Firstly,the research background and significance of the subject are expounded,and the research status of LLC resonant converters under wide input voltage range is described.The circuit structure and working principle of the LLC resonant converter are introduced.The circuit of the full-bridge and half-bridge LLC resonant converter is analyzed and modeled,and the reason why the LLC resonant converter under thetraditional control strategy is difficult to apply to the wide input voltage range is dissected.Secondly,the operating mode and gain characteristics of the LLC resonant converter under the hybrid control strategy for topology switching are studied,and the optimal switching gain point is determined within its gain range.The frequency characteristics of the LLC resonant converter under the hybrid control strategy are analyzed,and the parameters of the compensation loop are calculated accordingly.Thirdly,the structure of the LLC resonant converter system with digital chip TMS320F28335 as the control core is designed.The hardware part introduces the design and selection of magnetizing inductance,resonant inductor,resonant capacitor,and the design of sampling circuit,driving circuit and auxiliary power circuit of the digital control module.The software part gives the control system program flow chart,the main control chip system initialization and register configuration,and the software principle analysis of the hybrid control strategy.Finally,an experimental prototype of LLC resonant converter with output power of800 W was built and tested by the proposed hybrid control strategy.The experimental data was analyzed in detail.The experimental results show that the proposed hybrid control strategy enables the LLC resonant converter to output stable voltage over wide input voltage range and maintain high efficiency,which verifies the effectiveness of the proposed hybrid control strategy.