Driving method of piezoelectric transformer and driving circuit for the

Abstract

A circuit for driving a piezoelectric transformer, which is suitable for driving a piezoelectric transformer having a load such as a cold cathode fluorescent tube, and capable of coping with a wide range of a power source voltage and operating with high efficiency. The driving circuit includes a series circuit constituted by an equivalent input capacitance of the piezoelectric transformer and an inductance circuit and provided between power source terminals, and a switching transistor having both terminals of a main current path thereof in parallel connected to a part of the series circuit including the equivalent input capacitance. The inductance circuit includes a first inductance element of boosting constitution such as an auto-transformer and a second inductance element of non-boosting constitution such as a coil. The first and second inductance elements are selectable alternately. The driving circuit further includes an amplitude detection circuit for detecting an amplitude of a driving waveform between the terminals of the main current path of the switching transistor, and an inductance selection circuit for selecting the first inductance element when the amplitude is lower than a predetermined threshold value and for selecting the second inductance element when the amplitude exceeds the predetermined threshold value.