Theory and experiment of high voltage step-up ratio disk type piezoelectric transformer for LCD-TV

Piezoelectric transformers were known to power the cold cathode fluorescent lamp (CCFL) of the LCD-TV backlight module with high efficiency. Previously, electrode distribution of the input and output electrode of an unipoled disk-type piezoelectric transformer was known to be directly proportion to the voltage step-up ratio. It is detailed that the electrode area ratio also influences the efficiency of piezoelectric transformers as the loading impedance must match the output impedance of the piezoelectric transformer to achieve high efficiency. More specifically, the area ratio was found to be a good design parameter to achieve high operational efficiency. Orthogonally condition among different modes was used to design the surface electrode shape of traditional Rosen type piezoelectric transformers before. Earlier experimental results indicated that this modified type of piezoelectric modal actuator, which was named quasi-modal Rosen type piezoelectric transformer, possessed output waveform with far less high-order harmonics when compared to traditional Rosen type piezoelectric transformers. Furthermore, it had better energy transfer efficiency than the traditional uniformly distributed type piezoelectric transformers. Follow the innovative design thinking mentioned above, this paper used the distribution sensor and actuator concept to design the input and output electrode shape of the unipoled disk-type piezoelectric transformers. With an intention to optimize the input electrode shape, it was designed to match the modal strain field of the piezoelectric structure. In addition, the output static capacitance of the piezoelectric transformer was designed to match the loading impedance as mismatch leads to low energy transfer efficiency. It is to be noted that low efficiency results in rising surface temperature or even thermal breakdown. The design methodology of a disk type piezoelectric transformer, which can achieve high energy transfer efficiency and high step up ratio, was designed to illustrate the above-mentioned purpose. Finite element software ANSYS was used to simulate the vibration of the piezoelectric transformer first. Experimental results were also presented to verify the theoretical predictions as well as to examine the effectiveness of the innovative design rule proposed.