Third Order Longitudinal Mode Piezoelectric Ceramic Transformer for High-Voltage Power Inverter

Takeshi INOUE  Mitsuru YAMAMOTO  Shingo KAWASHIMA  Seiji HIROSE  

IEICE TRANSACTIONS on Electronics   Vol.E81-C   No.7   pp.1128-1135
Publication Date: 1998/07/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: PAPER
Category: Ultrasonic Electronics
piezoelectric transformer,  liquid crystal display,  impedance matching,  power inverter,  

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Low-profile, miniaturized and highly efficient power inverters are required to light up backlights, which include cold cathode fluorescent lamps (CCFLs), in color liquid crystal displays (LCDs), replacing conventional power inverters with electromagnetic transformers. The object of this study is to actualize a 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 generating part operates in a piezoelectric stiffened mode in order to increase both energy conversion efficiency and power density. This transformer has great advantages. Namely, all the electronic terminals in this transformer can be connected at the vibration nodes of the transformer, which contributes to the guarantee of stable transformer performances at high power operation, and this transformer is superior in impedance matching against the backlight load at steady state, because the output impedance of this transformer is much lower than that of conventional Rosen type transformers. Then a power inverter with the transformer was fabricated. 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. As a result, the fabricated power inverter exhibited more than 90% overall efficiency and 4. 5-W output power, which is enough power to light up a 9. 4 inch color LCD, including the stray capacitance loss resulting from CCFL mounting. The luminance value on a light transmission plate of the backlight was more than 2000 cd/m2.