Research On Wide Input High Order CL-LLC Resonant Converter

With the energy and environmental problems becoming serious,electric vehicle is in the stage of rapid development.The on-board power supply DC/DC converter is powered by lithium battery pack,and its input voltage range is relatively wide.Therefore,DC/DC converter with wide input,high efficiency and high reliability,which applies to on-board power supply,has gradually become a research focus.In this paper,the research of high-order high-frequency DC/DC resonant converter with wide input is carried out.LLC resonant converter is widely used in small and medium power supply because of its simple structure,high level of integration and good soft switching characteristics.However,since there are only three resonant elements in LLC converter,the degree of freedom of the resonant tank is relatively low,and it is difficult to improve its gain characteristics.In this paper,the shortcomings of the conventional LLC resonant converter are analyzed.By adding resonant elements,a novel high order CL-LLC resonant converter is proposed.The CL-LLC resonant converter introduces a zero voltage gain point,which improves the drop speed of its gain curve,thus improving the ability to adjust the input voltage change.At the same time,the third harmonic is introduced to transmit power,which can reduce the loss of switch and the reactive power loss in the system,so as to improve the efficiency.In this paper,the three resonant frequencies of the CL-LLC resonant converter are obtained by the fundamental harmonic analysis method,the voltage gain characteristics are analyzed,and the simplified voltage gain curve is obtained.According to the gain characteristics of CL-LLC converter,the rationality of its application in wide input field and the realization theory of soft start and overcurrent protection are analyzed.The efficiency improvement theory of third harmonic injection is analyzed from the view of circuit loss.The switch of the CL-LLC resonant converter can still realize soft switching,which reduces the frequency adjustment range and improves the efficiency of the system.In this paper,the influence of resonant parameters and parasitic parameters on CL-LLC resonant converter is analyzed from the angle of switching loss and converter voltage gain.A novel parameter design method refer to multi-resonant converter is presented,and the feasibility and accuracy of the parameter design method are verified by a 500 kHz prototype.In this paper,the CL-LLC converter is modeled and analyzed by the extended description function method,and the system is compensated according to the Bode diagram.The digital control strategy based on DSP is given,and the functions of the closed-loop control,soft start and over-current protection of the system are realized.This paper has designed a DC/DC converter prototype with input voltage range of 250V-450 V,rated output of 48V/400 W,and rated working frequency of 500 kHz.The system can achieve the highest efficiency of 95.7% under rated input voltage.