Research On LLC Resonant Converter With High Efficiency For Wide Input Voltages

With the increasing attention on energy consumption and environmental protection all over the world,how to use energy and electricity efficiently has become one of the hot research topics.Wind power,photovoltaic and other renewable energy have the advantages of being clean,sustainable and flexible in power generation.However,the output voltage of the latter stage is characterized by randomness and large variation range.Therefore,it is necessary to study DC / DC converter with high efficiency for wide input voltages.Compared with PWM converter,the resonant converter has the advantages of high switching frequency,small switching loss,and wide input voltage range.Among them,the LLC resonant converter has good performance and has attracted the attention of the industry.However,due to its complicated working modes,it is difficult to design and control.Therefore,further research on the working mechanism and control strategy of LLC resonant converter is needed.Usually,the fundamental harmonic approximation(FHA)method is used to analyze the performance of the LLC resonant converter.The voltage and current at both ends of the transformer are equivalent to sine waves and the higher harmonics are ignored for frequency domain analysis.This method does not take into account the discontinuous conduction mode(DCM)of LLC circuit,certain error exists.Usually frequency modulation(FM)control method is adopted in LLC to adjust the voltage gain of the circuit by changing the switching frequency.This method is simple,but the working efficiency of the circuit drops obviously when the switching frequency is higher than the resonant frequency.Aiming at the limitations of the existing analysis methods and control strategies of the LLC resonant converter,the time domain analysis method is used to study the implementation process of zero voltage switching(ZVS)and zero current switching(ZCS),its complex working modalities are analyzed.By studying the discontinuous conduction mode of LLC resonant converter,an improved FHA voltage gain analysis method based on DCM analysis is proposed to conduct parameter design and optimization.The design process is sequential and does not require simulation and iteration.By analyzing the cause of the efficiency reduction caused by traditional frequency modulation,the influence of FM control on ZCS and the influence of PWM control on ZVS are studied,and the critical duty cycle of ZVS achieved by PWM control is studied,an FM + PWM hybrid control method for LLC is proposed.The LLC simulation model is established on the PSCAD,and a 300 W DSP controlled LLC resonant converter prototype is built.Through simulation and experiment,it is verified that the designed LLC resonant converter can meet the voltage gain requirements within the wide input voltage range of 260 V ? 420 V,with maximum efficiency of 96.17%.The proposed improved FHA voltage gain analysis method,based on DCM analysis reduces the error by 80.1% on average compared with the traditional method,and the proposed hybrid control strategy increases the working efficiency by 0.76% compared with the traditional method.