Investigation on Ultra-low Turn-off Losses Phenomenon for SiC MOSFETs with Improved Switching Model

Traditional SiC MOSFET switching models can not predict the turn-off losses precisely in the condition of small driver resistance and small load current, because they have not considered a special case in which the SiC MOSFET closes its channel before the SiC diode turns into the freewheeling-state during turn-off transient. To solve this problem, this paper presents an improved SiC MOSFET turn-off model that considers the special case. The parasitic elements of circuit and the nonlinear characteristics of SiC MOSFET are also included in the proposed model to improve its accuracy. Experimental results show that the improved model can predict the turn-off transient of SiC MOSFET precisely. Moreover, the investigation of turn-off behavior shows that SiC MOSFET can achieve ultra-low turn-off loss in the special case, because the channel current of SiC MOSFET can fall to 0 before the drain-source voltage rises to a high value. Driver resistance and load current are the decisive factors of achieving the special case, and this paper gives the criterion of them. Besides, further studies show that lower common-source inductance, larger drain-source capacitance of MOSFET, and larger capacitance of diode can help to achieve ultra-low turn-off loss. Experimental results verify the analysis results.