Research On Optimal Control Strategy Of CLLLC Single-phase DAB Converter

High-power isolated DC/DC converters can realize the power conversion between different DC voltages isolated from each other.It has the advantages of large capacity,high efficiency,high energy density,high performance,etc.,and is widely used in industries,transportation,power systems.LLC resonant converter has become one of the research hotspots because of its excellent soft switching characteristics.In this paper,the original secondary-side symmetrical CLLLC resonant converter is selected as the research object,and its operating principle and soft switching characteristics under a wide input voltage range are analyzed.Firstly,the working principle and soft switching range of CLLLC out-of-phase control are analyzed.The soft switching range curve of CLLLC out-of-phase control is obtained by using the state space equation and extended description function.Load soft switching,but limited by the value of the resonant inductor should not be too small,its full-load soft switching range is small;then the working principle and soft switching range of synchronous PWM control are analyzed.Equivalent series resistance is used for energy transmission.Through analysis,it is found that the full-load soft switching range of which the voltage gain is not equal to 1 is also small,but its advantages in reducing the reflow power are obvious.Aiming at the above two methods,this paper proposes a dual phase shift control strategy.On the premise of ensuring power transmission,based on the external phase shift,the higher voltage side is added to the internal phase shift,and the current harmonics are used to widen the soft switching Range and by selecting the optimal combination of double phase shift angles,the peak value of the primary and secondary currents can be reduced,thereby improving the efficiency of the converter.The control method proposed in this paper improves the converter efficiency while greatly increasing the range of soft switching.When the input voltage fluctuates greatly,it can also achieve soft switching in the full load range.It plays a prominent role in areas with large grid voltage fluctuations and when multiple modules are connected in series.