Variable Mode Control Strategy Of Three-Phase Staggered-Parallel LLC Resonant Converter

In the age of taking energy conservation and environmental protection seriously,people put forward higher requirements for converter efficiency and power density.At the same time,with the continuous expansion of the converter application,how to improve the load carrying capacity and power rating of the converter has become one of the research hot topic.For high-power,low-voltage and high-current applications,the topology of the three-phase half-bridge staggered parallel and LLC resonant cavity Y-connection is considered as the research object,using the staggered parallel principle to solve the shortcoming of resonant converter's short service life due to the large output current ripple;The power balance between the phases is achieved by using star-connected topology.Although the LLC resonant converter improves the operating efficiency of the converter through soft switching technology,the out-of-control phenomenon of the output voltage may occur at light load;On the other hand,there is a problem that the magnetic element is difficult to integrate due to the wide operating range of the high frequency switching frequency.Based on the existing control methods,applicable control strategies are proposed for different load conditions.In the overload and rated load range,the parallel double closed-loop PFM control strategy is designed to achieve constant current control in overload range and constant voltage control in rated load range;In the light load range,symmetrical PWM control strategy is adopted to avoid the high frequency of switching frequency and loss of soft switch during PFM control,and the switching frequency is designed using the characteristics of the load-independent point in the voltage gain curve,which not only achieves soft switching at light loads,but also limits the increase of the switching frequency;Further study about the working principle and gain characteristics of symmetric PWM control,and make in-depth analysis about the adjustable voltage and non-adjustable voltage control area of the converter;In extremely light load and no load conditions,the Burst control method is used to achieve effective control of the output voltage.Finally,a detailed analysis and design of the selection of switching points between different control strategies is carried out to improve the efficiency of the converter when switching control modes.Under the premise of the variable mode control strategy to effectively limit the range of switching frequency variation,the optimization of the converter parameters is designed.Matlab/Simulink simulation software is used to build a simulation model and complete the design and debugging of the resonant converter experimental prototype.Theoretical simulation and experimental analysis verify the correctness and rationality of the transformer performance analysis and the variable mode control strategy.