This paper presents a lossless power flow model and an improved power flow model of the Partial Parallel Dual Active Bridge (P2DAB) dc-dc converter with single-phase-shift modulation (SPSM). The improved model considers the dead time and the parasitic elements. A GaN based P2DAB converter prototype is built to verify the models. The lossless model is more accurate than the other at small phase shift region, while the improved one is more accurate at medium and large phase shift region. The cause of the errors are discussed, and the solution to improve the accuracy is provided. Moreover, the improved model provides more details about the power flow characteristics than the lossless model.