Bi-directional high-side current sense circuit for switch mode power supplies

In order to control a power supply using piezoelectric transformer, AC current in the transformer needs to be measured. Due to the control strategy it is necessary to measure amplitude, phase angle and zero crossing of this current. In some applications there is common ground between primary and secondary sides of the transformer which is internally implemented inside the transformer. Therefore, current must be measured from the high voltage line in the presence of high input switching voltage. This paper proposes a resistive current sensing circuit based on discrete components useful for input voltages on the order of 200 V. The bandwidth is at least 200 kHz to allow fundamental frequency detection of piezoelectric transformers in use.