PROBLEM TO BE SOLVED: To improve the voltage boosting ratio of a center input and both-end output type piezoelectric transformer using a tertiary surface spread vibration mode, by adjusting the coupling coefficient of the piezoelectric transformer.
SOLUTION: The piezoelectric transformer has input electrodes provided on both the top and reverse surfaces of the lengthwise center of a rectangular piezoelectric ceramic substrate, and output electrodes provided at both their ends. Further, the piezoelectric transformer is provided with two input electrodes as auxiliary driving electrodes on both the sides of the center input electrodes as main driving electrodes, and the piezoelectric ceramic substrate has the part of the input electrodes polarized along the thickness and the output electrode side polarized along the length, so that the center input electrodes as the main driving electrodes and the two input electrodes as the auxiliary driving electrodes are driven in opposite-phase relation.
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