Study On The Isolated Dual Active Bridge DC-DC Converter

With the development of power electronics technology,the application of isolated bidirectional DC-DC converters is increasing in applications such as electric vehicles,DC microgrids and energy storage systems.The isolated dual active bridge(DAB)DC-DC converter has the advantages of electrical isolation,fast dynamic response and high power density,and has gradually become the mainstream research object of current high-power bidirectional DC-DC converters.In this paper,based on the three-phase shift modulation method,the working state of the dual-active bridge DC-DC converter in one switching cycle is introduced.The current and power expressions of DAB are derived by using the state space averaging method according to the working state.Then compare the power transmission characteristics of DAB under three different modulation modes: single phase shift,double phase shift and three phase shift.It is concluded that both double phase shift and three phase shift can effectively reduce the backflow power of DAB and improve the overall efficiency of the system.Based on the harmonic analysis method,the inductance voltage and the inductor current of the DAB converter are decomposed by the Fourier series,and then the active power and reactive power generated between the fundamental wave and the different harmonics are compared with the theoretical value.The relationship is derived by using the fundamental power to describe the unified power model of the DAB converter.The fundamental wave reflow power model and the Lagrangian multiplier method can be used to calculate the phase shift angle which minimizes the DAB reflow power.Using the phase shift angle to modulate the DAB,the fundamental wave reflow power is optimally modulated.For high-voltage applications in DC microgrids,DAB converters usually use a multimodule combination of Input Series Output Parallel(ISOP)to improve the withstand voltage range and power transmission range.In the ISOP-DAB converter,since there are differences in parameters between different modules,it is necessary to implement input voltage equalization and output current sharing control for different modules.Based on the conventional double-closed control strategy of output voltage and current,adding an input voltage equalizing loop can achieve the goals of input voltage equalization and average output current.In order to facilitate the analysis of the stability of the DC microgrid,the input impedance model of the three-closed-loop controlled ISOP-DAB converter is derived,which proves the input stability of the combined system.Then,a model is built in SIMULINK to verified the analysis and control strategy proposed in this paper.Finally,the calculation process of various devices and selection methods and parameters in the physical prototype is further introduced,and the correctness of the analysis and simulation is verified by the prototype.