Research On The Digital Controlled LLC Resonant Converter With GaN HEMT

With the emergence of GaN device,the high frequency characteristics of the power device is fully improved,so that the unit switching loss is greatly reduced.The switching frequency increasing,the total switching loss is increased.So the high frequency device GaN is used as the switch tube of the soft switching converter,the whole efficiency of the converter will be greatly improved.At present,resonant converter being a hot research on soft switching,LLC resonant converter,the transformer primary side switches of which can realize zero voltage switching(ZVS),rectifier diodes of which can realize zero current switching(ZCS),low switching loss,high conversion efficiency,thus become the main study object.At the same time,digital control,in the gradual replacement of analog control,has the advantages of convenient control,high precision,simple circuit and so on.In this paper,GaN devices used as the primary switches of LLC resonant converter to study high frequency characteristics of GaN devices,and STM32+FPGA dual core digital control scheme is adopted,resonant parameters is designed to realize soft switching.To realize ZVS in the full operating range,the key is the design of the parameters of the three resonant components.In this paper,using the fundamental approximate analysis method and Mathcad software to obtain the voltage gain curve,analysing the effect of inductance ratio coefficient k and quality factor Q of voltage gain,and the primary switches to achieve ZVS conditions,which is summed up a set of practical method for design of LLC resonant parameters,and use Matlab/Simulink software to verify the method of parameters design.On the basis of universal voltage single loop control,the average current mode control is proposed,and the dynamic response performance of the converter is improved significantly by simulation analysis.LLC in the maximum input voltage and light load,the loss caused by that the switching frequency is too high,is greater than the energy consumption,but also output voltage is not stable because of the existence of the parasitic capacitance.So this paper proposes that LLC get into intermittent mode control in the light load or no-load,and gives the specific implementation plan.For the problem of low accuracy in voltage regulation by only using STM32 or DSP for digital control,proposes STM32+FPGA dual chip control program.STM32 is responsible for calculation,generates a frequency control word transferred by useing the flexible static memory controller(FSMC)which is parallel communication mode to FPGA.The direct digital frequency synthesizer(DDS)of FPGA produce directly square wave pulse,no need for comparison between carrier wave and modulation wave,duty cycle being fixed at 50%,frequency bing variable.Finally,the system hardware design: GaN device selection and drive circuit design,integrated transformer design,etc.The ultimates experimental prototype is built,and the experimental waveforms and data are analyzed to verify the correctness of the GaN device in terms of high frequency characteristics,reasonant parameter design and controller design.