Research On LLC Converter Technology

When the input of power system is three-phase voltage,high DC bus voltage after three-phase retification result in high voltage stress on the switches of the DC/DC converter,which lead to large switching loss.High switching loss makes it difficult to increase the switching frequency,which limits the power density of the converter.Soft-switching technology can solve this problem and increase the power density.Thesefore,in this thesis,LLC resonant converter is chosen as the DC/DC stage topology in case of three-phase input to increase power density.In this thesis,the circuit structure of the LLC converter as well as the principle of implementing soft switching is introduced at first.The working status of the LLC converter is analyzed.Then optimal operating point and operating range of the resonant network are determined when the gain of the converter is changed by analysing the gain characteristics of the LLC converter.Secondly,LLC converter is designed according to the design requirements of the subject,including the main circuit and control circuit of the converter.Saber simulation is used to assist the design of the resonant network in order to achieve the gain,while the component stress,volume and circuit loss are taken into account.A set of design parameters that meet the actual hardware production requirements are selected and optimized from several sets of the original design paraments.In order to cope with the high voltage of the switches and reduce the on-state loss,Si C MOSFETs are selected to form the switch network.Synchronous rectification technology is used to reduce the rectification loss caused by the large current of the secondary-side rectification.The frequency conversion control method is adopted in the control system.For the sake of parallel operation requirements of the converter,a parallel current sharing control circuit is designed.Finally,the determined parameters are verified by simulation.After the performance of the converter is verified in various working conditions,the prototype test is carried out.Experiments prove the converter can stabilize the output voltage under various operating conditions.The efficiency and vllume of the converter meet the design indicators.The experimental test results verify the feasibility of the scheme.