3RD-ORDER LONGITUDINAL MODE PIEZOELECTRIC CERAMIC TRANSFORMER FOR HIGH-VOLTAGE POWER INVERTER

Low-profile, miniaturized and highly efficient power inverters are req uired to light up backlights, which include cold cathode fluorescent l amps (CCFLs), in color liquid crystal displays (LCDs), replacing conve ntional power inverters with electromagnetic transformers. The object of this study is to actualize a power inverter to which a novel multil ayered piezoelectric ceramic transformer operating in the third order longitudinal mode is applied. The piezoelectric transformer has a symm etrical structure in the lengthwise direction and its generating part operates in a piezoelectric stiffened mode in order to increase both e nergy conversion efficiency and power density. This transformer has gr eat advantages. Namely, all the electronic terminals in this transform er can be connected at the vibration nodes of the transformer, which c ontributes to the guarantee of stable transformer performances at high power operation, and this transformer is superior in impedance matchi ng against the backlight load at steady state, because the output impe dance of this transformer is much lower than that of conventional Rose n type transformers. Then a power inverter with the transformer was fa bricated. In this power inverter, a separately excited oscillation cir cuit was adopted to drive the transformer with high efficiency, and th e transformer drive frequency was controlled by detecting the backligh t current in order to adjust the backlight luminance properly. As a re sult, the fabricated power inverter exhibited more than 90% overall ef ficiency 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 t he backlight was more than 2000 cd/m(2).