Abstract: Taking the dual-output three-level phase-shift full bridge converter as the research object, firstly, we introduced the topology and modulation mode. Secondly, we analyzed the commutation of the full-bridge inverter after considering the switching function. It is found that the dead time is the direct cause of the primary voltage sag. The total harmonic distortion factor (THD) increases due to the primary voltage sag. Increasing the leakage inductance of the transformer can eliminate the primary voltage sag. Then, the mathematical relationship between the leakage inductance and the time of the primary voltage sag, the time of the duty cycle loss in secondary voltage and the phase shift duty cycle are derived. The optimal value of the leakage inductance is the minimum leakage inductance corresponding to the elimination of primary voltage sag. At last, the experimental results show that the theoretical analysis is correct.