The Optimized Modulation Strategy For Three-Level Dual Active Full Bridge DC-DC Converter

In the Energy Internet and electric vehicle areas,DC-DC converters play a more and more important role.Dual active bridge(DAB)converter has the characteristics of high power density,bidirectional transmission capacity,and other advantages,which make it suitable for the new generation of power interface equipment.For high voltage and high power scenarios,the DAB with the three-level structure can effectively meet the voltage demand,reduce the system cost,and facilitate the controller design,which has great development significance.At present,the control strategy about the three-level DAB still needs to be improved,especially in the fields of improving transmission efficiency,reducing switching loss,and optimizing current stress.To solve these problems,an optimal control strategy is designed in this paper.The main arrangements of this dissertation are as follows.Firstly,all kinds of the topology of three-level DAB are summarized,and the principles of topology evolution are obtained.At present,the application voltage of the full-bridge DAB structure is the highest,and this type of DAB is most widely used.Besides,its modulation strategy can be extended and applied to other three-level DAB converters.As a result,the three-level dual-active bridge DC-DC isolated converter with a full-bridge structure is the main research target.To solve the switching loss and low-efficiency problems when DAB switching frequency is high,this paper designs a boundary condition that can ensure all the switches of three-level DAB to operate at soft-switching states.This paper also designs a control algorithm which can both ensure the soft-switching and minimum of the DAB backflow power.Besides,by analyzing the equation of DAB states variables,a phase-shift based optimized control strategy is proposed,which makes DAB can adjust two modulation variables in real-time according to the ratio of DAB input and output voltages.Then,to further optimize the DAB harmonic content and enhance the EMC performance by using the triple modulation mode,the working principle of this modulation strategy is explained in detail in this paper.On this basis,the theoretical implementation of its zero voltage switching state is also analyzed on the mathematical model and the working modes are divided,when the bilateral soft switching realization conditions are obtained.To optimize the current stress in the circuit under this modulation strategy and further depress the demand for the device's current index,the optimization design for the current stress is also unified into the strategy optimization.According to the ratio of actual port voltages,the expected modulation variables are calculated and the soft switching realization is judged.Then a set of control variables with minimum transfer current stress are selected out.This paper mainly verifies the research target through Simulink,and a three-level DAB prototype is built as the research platform.