| LLC resonant converters are widely used in high-power applications due to their advantages such as low efficiency,low electrical stress,and wide range of soft switching.However,the traditional fundamental approximation design method adopted by the LLC resonant converter brings about problems such as large gain error and wide frequency modulation range,which makes the magnetic component difficult to design and high system loss,and its ultra-low voltage output is difficult to realize.The above problems make it difficult to implement a wide output voltage range for a half-bridge LLC resonant converter,thereby limiting the application of such a converter in the field of vehicle charging.In this paper,a topology of a cascaded high-frequency DC converter with a parallel-connected Cuk circuit of the transformer's primary-side series LLC+ output is proposed.The pre-stage LLC of the cascade converter operates in a fixed-frequency and open-loop mode to achieve electrical isolation and step-down;the rear-stage interleaved Cuk circuit adopts the same duty cycle and closed-loop operation mode to achieve output regulation and natural current sharing.For the former stage LLC,the equivalent circuit model is established by the fundamental wave analysis method,and the corresponding working mode is analyzed.The parameter design scheme is optimized according to the actual situation.The analysis of the soft switching characteristics ensures that the switching tube can realize ZVS in the wide output range,and also realizes the implementation of the secondary side diode ZCS of the transformer.The efficiency of the system can be guaranteed at all times under open-loop conditions.In order to increase the voltage adjustable range,the rear stage structure adopts an interleaved parallel Cuk circuit,which has a buck-boost function and can also reduce the output current ripple.In this paper,a brief analysis of its working characteristics is made.At the same time,the influence of the inconsistency of high-frequency transformer turns ratio and Cuk circuit duty ratio on the natural current sharing is also analyzed.Finally,in order to ensure the service life of the battery and the efficiency of charging,a detailed analysis of the control strategy of the latter stage was carried out.The research of Maas curve provides a good reference for the charging scheme,and proposes six charging pile control schemes.According to the comparison of the current output waveform and the optimal curve of each control scheme,the most suitable control method is selected.On this basis,the commonly used double closed-loop cascade control scheme is proposed.By comparing with other control schemes,the advantages of the scheme are determined,and the system can achieve efficient and stable work.Finally,the corresponding simulation model is built by Matlab software.The parameter index is input voltage is 600 V,output voltage is 160V-240 V,and the operating frequency of the front-stage LLC resonant converter is 400 kHz.The latter Cuk converter adopts PWM control.Ultimately,a 5kW high-power output is required,and the system efficiency operates at up to 95%.The theoretical analysis results are verified by analyzing the output waveform of each module. |
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