In this paper, we study the transient characteristics of 4H-SiC DMOSFETs with different interface charges to improve the turn-on rising time. A physics-based two-dimensional mixed device and circuit simulator was used to understand the relationship between the switching characteristics and the physical device structures. As the $SiO_2$/SiC interface charge increases, the current density is reduced and the switching time is increased, which is due primarily to the lowered channel mobility. The result of the switching performance is shown as a function of the gate-to-source capacitance and the channel resistance. The results show that the switching performance of the 4H-SiC DMOSFET is sensitive to the channel resistance that is affected by the interface charge variations, which suggests that it is essential to reduce the interface charge densities in order to improve the switching speed in 4H-SiC DMOSFETs.