Research On Bi-directional Half Bridge LLC Resonant Converter And MPPT Control Strategy

With the consumption of the fossil fuel,human are facing of a serious crisis that energy shortage.To improve the structure of energy supplied,the development and utilization of renewable energy attracts more attention.Solar energy is considered to be one of the most effective resource,but the PV power generation has a characteristic of intermittent,an energy device is required to guarantee the power supplied continuously.The Bi-directional DC/DC converter which can change the direction of power flow plays a important role in the energy storage device.Because of the high power density and high efficiency,the LLC resonant converter has been widely applied.In this paper,the Bi-directional DC/DC converter with LLC resonant and the maximum power tracking algorithm based on frequency modulation have been studied.To meet the household PV system with energy storage device application,an Bi-directional half bridge LLC resonant was chose for the subject.The principle of the converter's forward and reverse operation has been analyzed by the means of fundament harmonic analysis,and summarized the asymmetry which provides the basis for the design.The main waveforms of different direction are analyzed by simulation.In order to work in the low input voltage of the forward mode,the influence of the duty ration for the voltage gain of resonant tank has been studied.The hybrid modulation control strategy based on duty ratio and frequency for the current sharing is proposed.The output voltage could be regulated by the frequency modulation,while the resonant current between the converters as a variable has been used to achieve current sharing through the duty ratio modulation.The simulation results show that the hybrid modulation can effectively solve the problem of the different current in the interleaved parallel and realize the equal power output between the converters.To make the photovoltaic cells output at maximum power all the time,the relationship between the output characteristics of the PV cells and the resonant characteristics of the converter has been analyzed.The maximum power tracking algorithm of the median frequency conversion is adopted,with the switching frequency range divided for four equals,the next frequency interval could be determined by comparing with the power of PV cells when the converter works in median switching frequency.The MPPT of median frequency modulation responses quickly and the error is small.An experiment prototype has been built,and the results verify the correctness of the theory analysis and effectiveness of the MPPT,realizing the current sharing between converters.