Research On Wide Range LLC Resonant Converter

Due to ZVS and ZCS characteristics of LLC resonant converter,especially in the field of small and medium power,it is one of the currently preferred DCDC topologies.In a wide range of applications,such as new energy field,a lot of research has been done on the wide gain range of DCDC converters.The wide gain range of LLC resonant converters is also becoming a research hotspot.This paper first analyzes wind power,photovoltaic,electric vehicles and other wide gain range of new energy power generation and applications,summarizing and classifying LLC resonant converters in the wide gain range of fields.And the problems and challenges of LLC resonant converter in a wide gain range of applications are summarized.Some variable modal strategies for wide-gain LLC resonant converter are proposed.A modified gain calculation method that combines accuracy and intuitiveness is proposed.The main research contents of this paper are as follows:In the calculation of the gain of the wide-gain LLC resonant converter,there is a problem that the time domain analysis method is more complicated and less intuitive,and the first harmonic approximation(FHA)is not accurate.This paper combines the advantages of time domain analysis and FHA to propose a more accurate and more intuitive gain algorithm.When moving away from the resonant frequency,the gain curve is judged according to the boundary between the PON and PN modes,and the PON waveform is approximated to obtain the gain in the mode.The PN mode has a closed solution.And equal or slightly less than the resonant frequency can be seen in other paper.Combining these points,the modified gain curve can be obtained,so that the LLC resonant converter is more accurate when it is larger than the resonant frequency,equal to or slightly smaller than the resonant frequency,and far away from the resonant frequency.It is good for parameter design and optimization of the wide-gain LLC resonant converter.In wind power field,based on the characteristics of wind turbine's Maximum Power Point Tracking(MPPT)curve and wide output voltage range,a flexible variable mode theory is proposed to combine full-bridge variable frequency mode(FB_VF),full-bridge phase shift(FB_PS)and half-bridge variable frequency mode(HB_VF).According to the output power-voltage curve of wind turbine,the FB_VF,FB_PS,and HB_VF modes are sequentially arranged according to the gain size.And in the switching process between FB_VF and FB_PS modes,the seamless switching method is adopted by the unified control variables of the two,so that the process has no dynamic adjustment process;while the FB_PS and HB_VF adopt the hysteresis switching,and the switching process adopts the intermediate mode transition.By the gradual continuous change of the control variables,the gain before and after the switching remains unchanged,and the voltage fluctuations in the intermediate transition process are also small.Moreover,the intermediate mode can realize ZVS of MOSFET.Through this modality strategy,LLC ensures high-efficiency operation over a wide range of applications.Compared to only FB_VF mode,efficiency can be increased by 1%at high input voltage.The efficiency and gain range of different modes are different for different power levels.In this paper,another flexible modal scheme is proposed in combination with full-bridge variable frequency(FB_VF)mode,full-bridge asymmetry(FB_ASY)mode and full-bridge double frequency(FB_DF)mode.Under different loads,the efficiency of different modes at different input voltages are measured firstly,and the gain of the three modes are calculated.Combining the efficiency curve and the gain curve at different power levels,two or three of FB_VF,FB_ASY and FB_DF are used,and are arranged in order of gain size.FB_VF and FB_ASY modes switching is adopted seamless switching method,which has no dynamic adjustment process;while FB_VF and FB_DF modes,FB_ASY and FB_DF modes use hysteresis switching,and adopt intermediate mode transition.The ZVS of MOSFET can also be realized in the middle process.For LLC applications,wide gain range and wide power range basically require a wide frequency range.This paper proposes a dual resonant cavity scheme,combined with full-bridge half-bridge switching of the inverter unit,and transformer ratio change.Make the circuit select the inverter unit,the resonant cavity and the transformer ratio at different power levels,so that it always works near the resonance point,the circuit always works at the best state,and the maximum gain range is reached with the smallest frequency range change.In the switching between different circuit structures,the scheme of first pause and then switch is adopted.