Generally in high power industrial smps, the PFC and PWM sections are powered using separate isolated auxiliary power supplies which are derived from mains input.So naturally the PFC or PWM circuit ground point and the test equipment ground point will be different. It is observed that, PFC and PWM ground point voltage is much higher(in my case it is around 120V in AC mode or 145V in DC mode using DMM) with respect to my digital oscilloscope ground probe. If i want to make gate drive waveform measurements of PFC and PWM using my normal digital storage oscilloscope, how to choose a suitable ground point?