DAB is popular for energy storage systems such as solar power and electric vehicles. However, if the input-to-output voltage conversion ratio is not equal to the transformer turns ratio, power efficiency will decrease at light loads. To solve this problem, MSC (Masked-Switching Control) has been proposed. MSC problems include that some control variables, insufficient theoretical analysis, and changes in input-to-output voltage conversion ratio are not considered. In this paper, in order to cope with the decrease of control variable and the change of input-to-output voltage ratio, theoretical analysis of MSC by the extended state space averaging method was performed.averaging method was performed.