A flexible control method for a single-stage dual-active-bridge(DAB) based ac-dc converter is proposed. In order to avoid look-up tables and extend the achievable transfer power range, the proposed control scheme utilizes light and heavy load operation modes and adopt the suitable mode depending on the output power level. Meanwhile, unity power factor and zero-voltage switching(ZVS) can be achieved during one full-range mains cycle. Simulation results and experimental results are presented to validate the proposed method and theoretical analysis.