Bidirectional DC-DC Converters For Aerospace Application
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This thesis deals with soft-switching bidirectional converters for aerospace applications. The modern More Electric Aircrafts (MEA) have a great demand for efficient, compact, and lightweight converters to interconnect their conventional low voltage (28V) and new high voltage (270V) utility buses. Fulfilment of these requirements is quite challenging, especially from no-load to full-load with wide voltage variations of the buses. Dual active bridge (DAB) converters have been proposed for high power bidirectional DC-DC conversion, but still suffer from shortcomings such as limited soft switching range, bulky filter requirements, and low utilization of converter capability for maximizing efficiency. In this thesis new modulation techniques and new topologies are proposed for full-range soft switching and optimal efficiency without auxiliary circuits. Two converter topologies are explored for use in single or modular high power converters. First, the dual half-bridge converter with a new duty cycle plus phase shift control is presented to obtain a fully soft-switched and efficiency optimized converter. The soft-switching zones in the control space are found and a design method to get full range soft-switching is presented. The minimum loss points inside the soft-switching zones for all conditions are also obtained and presented. The converter is operated at minimum loss using lookup tables containing the optimal points. To obtain higher power and more flexibility, a new current-fed dual active full-bridge converter with 3 control degrees of freedom is also proposed and its operation under different combinations of control parameters is analysed. Also an interleaving modular design is presented to use the aforementioned converters as building units to construct higher power modular converters. Finally, a dynamic modeling technique based on state space averaging is proposed to obtain an approximate full order dynamic model of the converter in continuous time such that classic control design can be applied to the multivariable control system. Simulation and experimental results are provided. Experimental results are obtained from a 1kW dual half-bridge and a 2kW dual full-bridge converter operating at 250kHz. Despite the common imperfections in lab prototypes, efficiencies as high as 96% are measured, which demonstrate the effectiveness of the proposed approaches.