Global Optimization Of Rms Current And Direct Power Control For Dual Active Bridge Converters

Dual active bridge(DAB)converters are widely used in scenarios such as DC microgrids and power electronic traction transformers due to the advantages of galvanic isolation,natural realization of soft switching,high power density and efficiency,and bi-directional energy transfer.In this thesis,three aspects of modeling,efficiency optimization and dynamic performance optimization of a DAB converter are studied,which provides a reference for the engineering application and design of a DAB converter.With triple phase shift(TPS)modulation,classification of operating modes of the converter usually requires enumerating all possible modes and discussing their characteristics separately,which increases the complexity of the optimization process.In this thesis,symmetric transformation and space projection methods based on an AC equivalent model are used to eliminate redundant feasible regions.It is demonstrated that forward and reverse power transfer,Buck and Boost modes can all be unified to Buck mode with forward transfer condition for control,avoiding several times of complex calculations.Meanwhile,the efficiency optimization problem of the converter can be reduced to an optimal control problem that minimizes the rms value of the inductor current.The mathematical form of the problem can be described by a constrained optimization problem in different modes.Aiming at the problem of lacking mathematical rigor and inability to solve all solutions in solving stationary points and Lagrange multiplier method,the thesis uses optimality conditions to solve this problem.By discussing possible local mimimum solutions under various constraints in a classified manner,closed-form global solutions are obtained.On the other hand,the dynamic performance is also an important indicator of the controller performance.In order to further improve response speed,a direct power control strategy based on the minimum inductor rms current model is investigated in the thesis.The modulation parameters are calculated in real time by the instantaneous load power of the converter.This strategy requires only one additional output current sensor to give accurate control output within one switching cycle.Finally,the direct power control algorithm based on the minimum inductor rms current model is implemented with field-programmable gate array(FPGA)on a low-power prototype using SiC devices.The optimized TPS modulation improves the efficiency of the converter in the low and medium power levels and achieves up to 13 % improvement compared to the conventional single phase shift(SPS)modulation.The prototype is tested under the input step and load step conditions in the wide input voltage range from 100 V to 200 V and the full power range from 0 W to 270 W.The direct power control enables the converter to enter the steady state in as fast as 150 μs compared to the voltage loop proportional–integral(PI)control with a response time of at least 60 ms,proving the superior dynamic performance of this control strategy.