Voltage Balancing Control for Series Connected MOSFETs Based on Time Delay Adjustment Under Start-Up and Steady-State Operations

Recently, with research and development of SiC power devices, 1.2-V SiC MOSFETs have become commercially available. Over 3.3-kV SiC-MOSFETs have also been developed; however, it is difficult to use those devices without any special applications. When the medium-voltage converter circuit is designed using commercial SiC-MOSFETs, the 1.2-kV SiC-MOSFETs have to be connected in series. However, the voltage sharing under turn-off operations may not be balanced because each characteristic of the MOSFET is different, and the propagation time delay of the gate drive circuit is also different. This paper proposes a voltage balancing control strategy for series-connected SiC MOSFETs under steady state and start-up conditions using digital control circuits. To compensate for voltage unbalance conditions, feedback control of each drain-source voltage of the MOSFET under the OFF-state is proposed. The control circuit is based on a digital circuit, where controlling the voltage balancing under any conditions can be achieved. Experimental results using a buck chopper circuit utilizing SiC MOSFETs rated at 1.2 kV are also presented.