I. Introduction
Fully controlled power devices, such as insulated gate bipolar transistor (IGBT) and metal–oxide–semiconductor field-effect transistor (MOSFET), have boosted the development of power electronics technologies over the past few decades [1], [2]. In recent years, wide bandgap (WBG) power devices are promising to improve the power density and the efficiency of power conversion systems due to their superior features [3], [4], such as higher breakdown electron field and shorter transient time. Furthermore, higher band-gap energy of WBG devices leads to higher voltage ratings of single devices or power modules, which is beneficial for medium voltage (MV, 2–35 kV) applications [5], [6], [7]. The voltage rating of the available discrete silicon carbide (SiC) MOSFET and SiC MOSFET power modules do not exceed 3.3 and 15 kV, respectively. Thus, cascaded converter modules and series-connected devices are common and effective methods used to increase blocking voltage capability [8]. The series connection of the devices can reduce the complexity of control strategies and the cost of converters, compared with the stacking submodules of common topologies. Compared with a single high-voltage device, the specific ON-resistance of series-connected devices decays as the breakdown voltage increases with a coefficient that is related to the breakdown voltage as expressed in the following equation [9]: \begin{equation*} R_{\textrm {on}} =k_{1} BV^{2.43}+R_{\textrm {channel}} +R_{\textrm {ohmic}} \tag{1}\end{equation*} where is the coefficient of 4H-SiC. BV is the breakdown voltage. and are the resistance of the drift region and internal channel of a single device. When a high-voltage chip is divided into several low-voltage chips, their ON-resistances will reduce by . Therefore, the series-connected SiC MOSFETs could achieve lower ON-resistance when the breakdown voltage exceeds a certain value.