Optimal Control Strategies For Dual Active Bridge DC-DC Converter

Dual Active Bridge(DAB)DC-DC converter,as the key interface equipment of new generation and distribution systems such as micro-grid,has good application prospects due to its good soft-switching characteristics,high power density,electrical isolation and other advantages.As one of the hotspots of DAB converter research,optimization control has been widely discussed,however,there are still some shortcomings in multiobjective optimization,dead-time effect analysis,DC bias suppression and so on.As a result,the good control performance of the converter has not been fully exploited and utilized.This paper takes DAB converter as the research object,and analyzes its working principle and basic characteristics.Based on the fundamental characteristics,this paper proposes a series of optimal control strategies for the above problems,and verifies their effectiveness through experimental results.The detailed arrangements are as follows:Firstly,the principle of phase-shifting control strategy of DAB converter is introduced.The basic switching state and operation mode of DAB converter are analyzed based on triple-phase-shifting(TPS)control.The soft-switching conditions of each operation mode are obtained by analyzing the soft-switching characteristics of DAB converter.According to the power transmission characteristics of DAB converter,the transmission power model and current stress model are established.Based on these models,a minimum-current-stress control scheme which satisfies the soft-switching condition is proposed.The optimization algorithm is based on Lagrange Multiplier Method(LMIM)and Karush-Kuhn-Tucker(KKT)conditions.It can guarantee the soft-switching and minimum-current-stress operation,thus reducing switching loss and on-state loss at the same time,and further improving overall efficiency.Secondly,the distortion effects of dead-time on voltage and current waveforms of high-frequency and high-power DAB converters are summarized,such as voltage polarity reversal,voltage sag and duty cycle abnormality.Phase-shift errors caused by these effects are comprehensively analyzed,including output voltage sag,soft-switching characteristic failure and optimization algorithm failure.Considering the dead-time effects,the operational modes of DAB converter are divided,and the precise power model and current stress model including dead time are established.An optimal control strategy is proposed to make DAB converter operate under the minimum current stress,which can avoid the phase-shift errors and improve the whole efficiency.Thirdly,the mechanism of DC bias in power regulation of DAB converter is analyzed,and the DC bias model under TPS control is established.A Fast Transient Control(FTC)strategy is proposed,in which the switching state of DAB converter is fixed and the setting time is directly calculated from DC bias model,thus simplifying the control.Therefore,The complexity of transient control can be simplified,the transient speed can be further accelerated,and DC bias can be suppressed.Finally,the hardware and software design of the experimental prototype is introduced,including the selection of switching devices,parameters calculation and design of high frequency transformer,design of auxiliary circuits such as detection,drive and protection,design of software control flow,etc.A 3kW 10-50kHz DAB converter platform based on SiC devices is built,and the performance of optimization strategy is verified based on DSP control system.