Research On Multi-mode And Wide Voltage Range Bidirectional Isolation LLC Resonant Converter

With the strong support of governmental policies,the electric vechiles industry is booming.And charging pile technology is essential for the development of electric vechiles industry.Since different types and brands of electric vehicles have their own charging voltage ranges,the charging piles at present can only meet the charging needs of some types,and the charging pile technology with a wider charging voltage is needed,which requires a wider voltage range of DC/DC converter.The most common DC/DC converter is LLC resonant converter,which can use parasitic parameters and rely on resonance to achieve soft switching characteristics,thereby achieving the advantages of high efficiency and power density.However,the traditional LLC resonant converter needs a very wide operating frequency to achieve a wide voltage gain range,which makes the design of magnetic components very difficult.In order to solve this problem,a multi-mode wide voltage range bidirectional isolated LLC resonant converter is proposed in this dissertation.The proposed topology can use its own circuit structure to produce two inverter modes and rectification modes through different control schemes,so that the converter has three working modes when it is in bidirectional operation.Due to the existence of multiple operating modes,the operating frequency range of LLC resonant converters can be shortene d to a quarter compared with the traditional LLC converter,which helps achievie an ultra-wide voltage range output within a narrow operating frequency range.The equivalent resonant tank analysis method will directly affect t he accurate design of the resonant converter,and the analysis methods produce the voltage gain expression.Based on the fundamental harmonics approximation in the below-resonant working state,the current of rectifier side is discontinuous,which has not taken into account and leds to the low accuracy of voltage gain.In this case,a modified fundamental harmonics analysis method is proposed in this dissertation.At the same time,an modified simplified time-domain analysis method is proposed for the shortcoming that the simplified time-domain voltage gain expression is not affected by the load.Through accuracy comparison,these two modifeid analysis methods both show higher voltage gain accuracy.Through two modified analysis design methods of the resonant converter,an experimental prototype was built to verify the three working modes of forward and reverse operation.The experimental waveforms verify that the converter can achieve bidirectional ultra-wide voltage range and soft switching characteristics,which is consistent with the theoretical analysis.In addition,the rationality of the control strategy for mod es transitions is also verified.