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?

 

Such measurements are not easy to make with a standard scope.
You likely need a high-voltage, differential probe or differential (difference) amplifier such as this, so that you can measure the voltage with respect to the scope ground.

Warning: Be very careful when you do those measurements, since contacting the power line can be lethal.

Edit: The difference amp I referenced above may be too slow, depending upon he frequency of the PWM waveform and how fast a rise-time you want to measure.