a method and unit for driving a small high - efficiency piezoelectric transformer allowing a cold - cathode tube to have stable luminance by detecting only an active current flowing in the cold - cathode tube based on a phase difference between an output current and voltage of the piezoelectric transformer , removing a reactive current caused by stray capacitance formed between the cold - cathode tube and a reflector , and accurately controlling driving of the piezoelectric transformer so that a constant active current is detected .