Third order longitudinal mode piezoelectric ceramic transformer and its application to high-voltage power inverter

Color liquid crystal displays (LCD) require low-profile, miniaturized and highly efficient power inverters, replacing conventional electromagnetic transformers, to light up their backlights, which include cold cathode fluorescent lamps (CCFL). The object of this study is to actualize the power inverter, to which a novel multilayered piezoelectric ceramic transformer operating in the third order longitudinal mode is applied. The piezoelectric transformer has a symmetrical structure in the lengthwise direction and its output part operates in a piezoelectric stiffened mode in order to increase both energy conversion efficiency and power density. This transformer has a great advantage, in that all the electronic terminals can be connected at the vibration nodes of the transformer, which contributes to the guarantee of stable transformer performances at high power operation. In this power inverter, a separately excited oscillation circuit was adopted to drive the transformer with high efficiency, and the transformer drive frequency was controlled by detecting the backlight current in order to adjust the backlight luminance properly. Using a triple-layered piezoelectric transformer, a fabricated power inverter exhibited the overall efficiency of 92%, the output power of 4.5 W, which is enough power to light up a 9.4 inches color LCD, including the stray capacitance loss resulted from CCFL mounting. The luminance value on a light transmission plate of the color LCD was 2500 cd/m
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while the inverter dimensions are as small as 76×22×6 mm