An apparatus for receiving and modifying a PWM trigger signal such that consecutive ON and OFF periods have durations at least as long as a critical dwell time where the critical dwell time is the minimum time required to avoid greater than twice overvoltage. The trigger signals are modified by, each time a trigger signal changes from low to high, providing a minimum pulse signal which remains high for the greater of a critical dwell time or the time over which the trigger signal remains high, when the minimum pulse signal changes from high to low, providing a maximum pulse signal which remains low for a critical dwell time, adding the maximum and minimum pulse times using a logical AND gate and providing the output of the AND gate as a modified trigger signal.