Abstract: As a popular application of DC converter technology, dual active bridge（DAB） has received more and more attention from scholars. The reflow power is one of the research hotspots. Excessive return power will seriously affect the operating efficiency of the DAB and reduce the soft-switching range of the DAB. In view of the above problems, on the basis of the extended phase-shift control, this paper determines the boundary conditions that can realize the soft switching of all switches, which effectively reduces the switching loss. Even zero return power can be achieved under certain operating conditions, and the operating efficiency of DAB has been improved. For on-line control, a closed-loop control system is designed, and the effectiveness of the method is proved by the continuous switching simulation of the load. Finally, the DAB experimental platform is built, and the correctness of the theory is verified by hardware.