A Modulation Technique for Capacitor Reduction in Soft Switched Dual Active Bridge for Single-Phase AC–DC Application

A modulation technique to operate a Dual Active Bridge (DAB) connected to a front-end converter, with reduced intermediate dc bus capacitance, is proposed in this paper. By lowering the capacitance, higher ripple voltage is allowed on the intermediate dc bus. The proposed idea is based on a novel PWM technique called Variable Frequency Phase Shift (VFPS). It can maintain soft switching of the DAB in presence of large ripple on its input dc bus. The VFPS technique helps the DAB to operate with Zero Current Switching (ZCS) during both turn-on and turn-off of all switches in either one of its two bridges while maintaining Zero Voltage turn-on (ZVS on) in the other bridge. Thus, switching loss is substantially reduced. Additionally, the backflow power is eliminated in one of the two bridges thereby reducing conduction losses as well. Simultaneously, the DAB inductor current is free from any double line frequency envelop. Conditions for realizing soft switching despite large double line frequency ripple due to reduced dc bus capacitance are mathematically analyzed. Experimental results show DAB efficiency higher than 98% for a 1kW prototype.