Study On High-performance Modulation And Digital Design Of Dual Active Bridge Converter

In order to curb the severe trend of global warming and take the path of sustainable development,“emission peak” and “carbon neutrality” has been incorporated into the national development strategy.The path to carbon neutrality in electricity is building a new power system with energy sources as the mainstay.The DC eco-operation model is a promising and comprehensive solution.The dual active bridge(DAB)converter is considered the key component of the DC eco-operation mode due to its galvanic isolation,bi-directional current capability,flexible control capability and wide soft switching range,is responsible for interconnecting DC buses of different voltage levels and various DC loads.However,the difference between the efficiency of DAB converter at full load and light load over a wide voltage range is more than 10%,which is a bottleneck for their further industrialization.Therefore,how to ensure the highefficiency operation of the DAB converter in the whole working range is a critical problem that needs to be broken.Accordingly,this paper takes the DAB converter as the object of study and addresses its efficiency issues,starting with hardware and software,and focusing on modulation scheme,analysis model and main circuit parameter optimization.Firstly,in terms of modulation scheme,the optimal asymmetric duty modulation(OADM)and optimal five-degrees-of-freedom(O5-DOFs)modulation schemes are proposed to address the low-efficiency of DAB converter at light load due to the presence of reactive power and loss of soft switching characteristics.The OADM scheme is based on a three-degrees-of-freedom(3-DOFs)asymmetric duty modulation scheme,which simplifies the complexity of the optimization algorithm while improving the efficiency of the converter by constructing a low-coupling inductor peak-to-peak current as the optimization target.Further,by systematically investigating the respective transmission characteristics and advantageous ranges of the 3-DOFs symmetric and asymmetric duty modulation schemes,a 5-DOFs modulation approach is proposed that utilises the general form of the phase-shift modulation scheme to achieve unification of all phase-shift modulation schemes.Based on this,the O5-DOFs modulation scheme is formed by considering the conduction loss and switching loss from multiple angles,which can ensure that the power switch achieves soft switching while minimising the inductor root mean square current.Thus,it can achieve the maximum improvement of conversion efficiency from the perspective of the modulation scheme.Secondly,in terms of analytical modelling,a multi-timescale modelling approach that considers both the ideal steady-state process and the transient switching process is proposed.It can address the lack of accurate and practical models of the DAB converter to better guide the optimal design of the main circuit parameters.In order to simplify the analysis without losing accuracy,on the one hand,the non-linear variation of the parasitic capacitor with the drain-source voltage is accounted for in the power device model.On the other hand,the most general five-degrees-of-freedom modulation scheme is used to analyse the current loop in which the power switch is located and to establish a unified equation of state.Finally,a periodic-steady-state analysis model(PSSAM)is developed.The model can accurately predict not only steady-state characteristic quantities such as transmission power and inductor current,but also the residual voltage under incomplete zero-voltage switching.Compared to existing commercial software,PSSAM is able to greatly increase the simulation speed and reduce the calculation time by a factor of approximately 28-61,facilitating the optimal design of the DAB converter.Finally,in terms of optimising the design of the main circuit parameters,to address the lack of a simple quantitative digital optimization method for DAB converter,the safety design space and optimization design space criteria are proposed to constrain the transformer ratio and inductor gradually,and to determine the optimal parameters in conjunction with the prediction results of the loss analysis.Among them,the safe design space mainly includes constraints on transmission power and inductor current stress,and parameters satisfying this region can ensure the normal operation of DAB converter.The performance optimization space includes the optimization of inductor root mean square current,and soft switching range,and the parameters in this area ensure that the DAB converter operates at high performance.The final parameters obtained are well matched to the O5-DOFs modulation scheme and improve the weighted average efficiency of the converter over the entire operating domain.In summary,this paper systematically investigates the modulation scheme,analysis model and main circuit parameter optimization design of DAB converter,which improves the conversion efficiency of DAB converter,and breaks through the technical bottleneck of light load and inefficient operation,providing new theories,methods and technical support for the industrial application of DAB converter.