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The gate capacitance of MOS-gated power devices during the on state is analyzed by experimental measurements and simulations. Negative gate capacitances are found during turn-on transients and could lead to strong oscillations under short-circuit conditions. The physical origins of negative capacitance have been explained, and also, a simplified small-signal model is presented to describe its mechanism. It is concluded that a proportion of the anode current feeds back into the gate circuitry and causes gate oscillation and instability.