Research On Phase Shift And Variable Frequency Control Of LLC Single-phase DAB Converter

With the rapid development of communication systems,computers and various electronic products,the power supply for them has become a current research hotspot.There are many types of power supplies,such as server power supplies,LED lighting power supplies,switching power supplies,UPS uninterruptible power supplies,etc.They are used in different places,but they all need high power density and efficiency to reduce the volume and loss of the equipment in order to save energy.LLC resonant converter can achieve high switching frequency and soft switching of the switch itself,so it can achieve high power density and high efficiency.Because of the two advantages of LLC resonant converter,it is very popular in many related fields,such as power systems,microgrids,servers,communications,lighting,etc.At present,the main control methods of LLC resonant converter are: variable frequency control,phase shift control and PWM control.The phase shift control methods are divided into single phase shift control,double phase shift control,extended phase shift control and three phase shift control.Due to the limitation of the requirement of the relationship between the switching frequency and the resonant frequency of the LLC resonant converter,most of the LLC phase shift control methods is single phase shift control currently.The LLC resonant converter cannot achieve a wide gain range under a single control method.For example,under frequency control,in order to ensure that the primary and secondary sides can achieve soft switching,the gain can only be greater than 1,and due to the influence of the system quality factor,The maximum voltage gain of frequency modulation control will also be limited;under phase shift control in the primary side,its gain can only be less than 1,so this paper proposes a hybrid control method,and introduces model predictive control method,which can make LLC Resonant converters have a wider range of voltage gains and have better dynamic response characteristics.The specific control principle is as follows:When the gain of the converter is less than 1,the control method of phase shift inthe primary side is adopted,and model predictive control is introduced.The optimal shift ratio is selected for control by MPC in each control cycle,so PI adjustment is not required Compared with the regulator to adjust the shift,the time to reach steady state is greatly reduced;when the converter gain is greater than 1 but less than the maximum limit of the LLC gain curve,the frequency modulation control method is used;At the maximum extreme value,the control method of phase shift in the secondary side is adopted,and model predictive control is introduced.This can ensure that the LLC resonant converter has a wide range of gain variation,fast dynamic response,and can achieve soft switching of all switching tubes.In this paper,the parameter design of LLC resonant converter is carried out,and the principle analysis,mathematical model derivation,simulation and experiment is performed each for control method.The dynamic and steady-state effects of the traditional phase shift control and model predictive control are compared on the dual active bridge converter and LLC resonant converter.Finally,the working principle of the proposed hybrid control,the switching points of the control methods and the optimal working mode of each control method are introduced in detail,and the correctness and feasibility of the method are verified by Matlab/simulink.