Apparatus and method find leakage due to tissue load or transients and has active and return electrodes for RF energy sensed by transformers for a circuit that finds leakage at more than two thousand times per second or a frequency dependent on the phase shift between voltage and current. Software and feedback manage V.sub.rms to reduce the voltage or increase the crest factor, which is V.sub.rms divided by peak voltage, by pulse width modulation of the RF drive. Voltage and current phase shift changes, angle .theta., are compared to a threshold, and when greater, the frequency at which the differences between the active and return current signals are examined is increased. The differences between the active and return currents are examined and when greater than maximum leakage while in coagulation mode, the RF drive pulse width is reduced to maintain voltage wave peaks at a preset value and reduce V.sub.rms thereby reducing leakage to the allowable maximum. Reducing the duty cycle or the pulse width of the output wave increases the crest factor. When the differences between active and return currents are greater than a maximum for leakage while in cut or bipolar mode, the V.sub.rms is reduced to limit leakage to a maximum level. When the phase angle .theta. is less than a threshold, the frequency with which leakage is examined is reduced.