ANALYSIS AND PREVENTION OF POWER MOSFET ANOMALOUS OSCILLATIONS.

When power MOSFETs are connected in a resistively loaded inverter circuit, the devices often oscillate. Experimental results indicate that the oscillation is a small-signal instability caused by a greater than unity loop gain. Using a small-signal model and the Routh-Hurwitz criterion, analytic expressions were derived to determine the conditions for oscillation. The resulting expressions indicate that the maximum allowable gate inductance for which the MOSFET will not oscillate scales roughly as the inverse of the device area. Numerical evaluations of the Routh-Hurwitz criterion demonstrate that the addition of resistance at the gate of the MOSFET decreases the tendency to oscillate. An anomalous behavior is observed when the power MOSFET is turned off, where the MOSFET turns off and then turns back on for several microseconds. This behavior is caused by the parasitic clamping diode of the power MOSFET and is not related to the cause of the oscillation.