DV/DT related spurious gate turn-on of bidirectional switches in a high-frequency cycloconverter

We identified a failure mode in a two stage dc/ac converter, comprising a high-frequency dc/ac inverter followed by an ac/ac cycloconverter, both operating at the same switching frequency. The failure-mode is a short-circuit condition, which is a combined effect of the reverse recovery of the MOSFET body diode and simultaneous spurious turn-on of the bidirectional switches of the cycloconverter, owing to a significantly high dv/dt (>2×10⁸V/ns). A high dv/dt causes appreciable current to flow through the gate-to-drain (Miller) capacitance, thereby producing a significant amount of voltage drop across the external gate resistance. Consequently, the gate-to-source voltage of the power MOSFET may exceed the threshold voltage of the device, which turns the device on. We explain the mechanism for the dv/dt-related gate turn-on and present experimental results to validate the explanation. We also demonstrate, how a two-fold increase in the value of external gate resistance of the inverter switches (to reduce the dv/dt applied to the cycloconverter) reduces the periodicity of the short-circuit condition.