Soft Starting Strategy of Cascaded Dual Active Bridge Converter for High Power Isolated DC-DC Conversion

Abstract

Dual active bridge (DAB) converter is promising in high power isolated dc-dc conversion applications. Soft starting of DAB converter is important to avoid current surge during starting and eliminate bulky external pre-charge circuits. A three step soft-starting strategy is proposed in this paper to simplify complex soft starting control, optimize charging speed, and reduce dc bias of transformer. The charging current and current stress are analyzed in detail. A suitable modulation strategy can be chosen that optimize pre-charge current, limit current stress and simplify control. Input voltage balance between cascaded modules can be achieved with the control of charging current. Transformer flux dc bias in soft-starting procedure is analyzed, and a control strategy is used to prevent transformer from saturated. The proposed soft starting strategy is verified by simulation in Matlab/Simulink, and is experimentally verified on a 300kW cascaded DAB converter prototype.