Research On Three Phase LLC Resonant Converter With Transformers Primary Windings Star Connection

LLC resonant converter has the characteristics of miniaturization,light weight,low switching loss at high frequency,high efficiency,high power density and low EMI due to its natural soft switching feature.However,due to the characteristics of the circuit itself,the output current ripple is large,which requires multiple output filter capacitors to be connected in parallel,increasing the volume of the system and bringing great restrictions to the application of the converter.To solve this problem,interleaving technology can be used,but the inevitable parameter difference of components,make the output current deviation of the interleaving LLC resonant converter is very big,which affects the reliable operation of the converter,also can't achieve the purpose of reducing the output current ripple.For this reason,a interleaved three-phase LLC resonant converter with star connection of transformer primary winding is adopted.The phase difference of each LLC resonant converter is 120 degrees,and the magnitude of one-phase resonant current is the sum of the other two phases at a certain time,and the direction is opposite to the other two phases,so that the LLC resonant converter has the inherent current sharing capability.When the parameters of the resonant component do not match,the phenomenon that a certain phase current drops excessively and other phases exceed the load limit can be well avoided,so that each output current is evenly divided,and the output current ripple is reduced.three-phase LLC resonant converter with star connection can also be very convenient to apply phase shedding technology to reduce circuit loss and improve light load efficiency of the system.Firstly,the working principle of single-phase LLC resonant converter is introduced in detail.the steady-state equivalent model of LLC resonant converter is established by fundamental analysis method,and the gain characteristics and the conditions for ZVS realization are derived,which provide the basis for the design of the converter.Secondly,the influence of the phase numbers of LLC resonant converter and the difference of resonant components on the output current ripple is analyzed,and the solution is given.The parameters of the converter are designed,and the design scheme is verified by theoretical simulation with PSIM software.Finally,a 1.2 kW experimental prototype is made,TMS320F28035 is used as the main control chip of the digital controller,the main circuit,control circuit and software design are completed,the experimental verification including phase shedding is carried out,and the efficiency of the converter was tested.The experimental results show that the three-phase LLC resonant converter with star connection at the transformer primary can effectively solve the problem of large output current ripple caused by the difference of resonant components.