Wide-range Input LLC Resonant Converter And Its Application

In recent years,switching power supply is developing towards high efficiency and high-power density.The effective measure to improve the power density is to increase the switching frequency of the converter.With the increase of switching frequency,the volume of magnetic components is constantly reduced.However,the increase of switching frequency will lead to a significant increase of switching loss,which is not conducive to the improvement of converter efficiency.The LLC resonant converter can realize zero voltage switch(ZVS)of the primary side switch and zero current switch(ZCS)of the secondary side diode by variable frequency control.All the switches of LLC resonant converter realize soft switching,which can reduce the switching loss and improve the efficiency of the converter.However,the traditional LLC converter with variable frequency control has a narrow range of gain with frequency,which is not suitable for wide range input voltage applications.Therefore,it is of great significance to study the wide range input LLC converter.Firstly,the gain characteristics of variable frequency control LLC converter and phase shifted controlled LLC converter are analyzed in detail,and the influence of different parameters(quality factor Q and inductance ratio λ)on the gain range of the converter is studied.In view of the narrow input voltage range of the LLC converter controlled by frequency conversion,this paper proposes a three-stage hybrid control strategy to broaden the voltage input range of the LLC converter.According to the different input voltage range,the converter adopts the segmented control mode to achieve the high efficiency and wide range input of the converter.The voltage gain characteristics of phase-shift control and frequency conversion phase-shift hybrid control are derived respectively.The gain variation law of LLC converter with variable frequency control and phase-shift control is analyzed in detail,and the mode switching point of three-stage hybrid control strategy is obtained.Finally,the correctness of the theoretical analysis is verified by the experimental results.Secondly,in view of the narrow input voltage range of the traditional LLC converter,this paper proposes an improved LLC converter which changes the diode at the bottom of the rectifier bridge at the secondary side of the traditional LLC converter into a switch to adapt to the characteristics of a wide range of input voltage.Taking the improved LLC converter as the research object,the working principle of the converter is analyzed in detail,and the gain characteristics and soft switching characteristics of the converter in boost mode and buck mode are derived by using the state trajectory model combined with the hybrid control strategy of the primary side shift control and the secondary side shift control.Finally,based on the practical application,the parameter design process of the improved LLC converter with hybrid control is given.The results show that the converter can achieve a wide range of voltage input and has a high efficiency.The experimental results verify the correctness of the theoretical analysis.Finally,according to the characteristics of wide range of output voltage of photovoltaic cells and high DC bus voltage,this paper studies the improved LLC converter as the intermediate DC-DC converter of photovoltaic system.In this converter,the resonant tank of the traditional LLC converter is moved to the secondary side,and the double voltage rectifier circuit is used at the secondary side.The hybrid control strategy of variable frequency control and phase-shift control is applied to the converter to adapt to the wide range output voltage characteristics of photovoltaic cells.At the same time,the control algorithm to realize the maximum power point tracking(MPPT)of photovoltaic cells was studied.This paper analyzes the principle of MPPT algorithm and the working characteristics of the improved LLC converter with variable frequency phase-shift hybrid control and derives the gain characteristics and input impedance of the improved LLC converter with variable frequency control,phase-shift control and hybrid control.The MPPT control strategy based on dynamic impedance matching and the digital realization method are adopted to improve the speed and precision of the MPPT control.The experimental results verify the correctness of the theoretical analysis.