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Analysis of Voltage Sharing of Series-Connected SiC MOSFETs and Body-Diodes

Voltage sharing
Fig. 1. Voltage sharing of series-connected 10 kV SiC MOSFETs with different gate resistor.
  Parasitic components, particularly the parasitic capacitors, surrounding the devices and gate- drivers introduce different voltage unbalance of series- connected SiC MOSFETs under different conditions. The impact of parasitic capacitors needs a detailed investigation. The understanding of such impacts will help design engineers select the proper devices, gate-drivers, and packaging solutions to enable sufficient voltage safety margins when operating the series-connected devices under high-speed switching operations. In this work, the impact of the parasitic capacitor, especially the gate parasitic capacitor, is further analyzed and more experiments are conducted to verify the analysis.
  The gate parasitic capacitor will result in an extra gate current and affect the turn-off miller plateau during the transient. The analysis indicates that the voltage difference will be increased with a higher gate resistor or higher turn- off current. As shown in Fig. 1, voltage sharing of series- connected 10 kV SiC MOSFETs with different gate resistors is measured to verify the analysis.
  On the other hand, the analysis also investigates the impact of drain/source parasitic capacitors on voltage sharing under different conditions. The analysis shows that the drain/source parasitic capacitor has a similar impact on voltage sharing of both MOSFETs and body-diodes. Fig. 2 shows the voltage sharing of series-connected 10 kV SiC MOSFETs under ac load current condition. The voltage sharing during the body-diode period is similar to the MOSFET period, which also verifies the analysis.
Voltage sharing
Fig. 2. Voltage sharing of series-connected 10 kV SiC MOSFETs under ac load current.


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