Small multilayer piezoelectric transformers with high power density - Characteristics of second and third-mode Rosen-type transformers

Low-profile inverter power sources are increasingly required for backlight systems of liquid crystal displays. A great deal of attention has been focused on the application of miniaturized piezoelectric transformers to such power sources. In this study, the output performances of two types of piezoelectric transformers, one operating in the second-order longitudinal vibration mode with dimensions of 20 mm x 6 mm x 0.9 mm, the other operating in a third-order longitudinal vibration mode with dimensions of 27 mm x 4.5 mm x 0.9 mm, are compared by means of equivalent circuit analyses and experiments. The values of efficiency for both reach a maximum when connecting a load resistance of 50 k Omega to the output terminals. In terms of output power, the latter, with seven ceramic layers in the driver section, generates an output power of 3.3 W, which is twice as much as that of the former with nine ceramic layers. It is concluded that the difference is caused by mechanical losses which depend on their vibration levels and multilayer structure and by the variation of impedance at their output terminals.