PURPOSE: To get the efficient switching circuit of a piezoelectric transformer with no feedback of higher harmonics by equipping it with an inverter circuit, which converts DC voltage into rectangular waves, and a filter circuit, which converts the rectangular waves into sine waves, and connecting the output part of the filter circuit to the input part of the piezoelectric transformer.

CONSTITUTION: An inverter circuit 2 converts DC input power 1 into rectangular waves, and a filter circuit 3 converts these rectangular waves into sine waves, and inputs them into a piezoelectric transformer 4. If the frequency of the sine waves is set to the resonant frequency of the piezoelectric transformer 4, the sine waves can be sent to the secondary side of the piezoelectric transformer 4 with the amplitude of sine waves not attenuating.