| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 92 === To start with the equivalent circuit to model the behavior of the piezoelectric transformer, it was derived that the loading effect influences the step-up ratio, power efficiency, phase differences, input impedance and output impedance of the piezoelectric transformer. To explore further about the equivalent circuit, it was found that once the output impedance is equivalent to the load resistor, named as impedance-matched resistor, the piezoelectric transformer operate in optimal condition. In this optimal condition, the system has the maximum power efficiency. Due to the limitation of the electronic circuit analysis, the equivalent circuit of the piezoelectric transformer can not predict the operating frequency of the optimal condition. Therefore, an experimental set-up is needed to discuss this topic.
The measurement of RLC equivalent circuit parameters of piezoelectric transformer was accomplished by an impedance analyzer under low power condition. It was found by the experiments that the piezoelectric material is still in the linear region under 5.0W high input power. Hence, the equivalent circuit model measured under low power condition is available to predict the electric characteristics of the piezoelectric transformer under 5.0W input power. It was also discovered that the operating frequency of the maximum power efficiency occurs after the resonant frequency. Once the phase difference between input and output voltage is 45°, the power efficiency of the piezoelectric transformer is higher than 95% to its maximum value. The above conclusions can be supported by the theoretical derivations of the equivalent circuit. Based on the requirement of applications, the cold cathode fluorescent lamp requires high voltage to light up and high power efficiency in steady state. According to this requirement, the concept of 45° phase angle becomes the best the choice to design the control strategy of the driving circuit for piezoelectric transformer.
The driving circuit of piezoelectric transformers which presented in past decades is fail to make sure the piezoelectric transformer operate in optimal condition. A dual-mode control strategy was presented to solve this problem. Additionally, a design flow was summarized by the theoretical predictions and experiment results in this thesis. Once the piezoelectric transformer can be simplified to a second order vibration system, the system can be preserved in the most efficient condition by following this design flow.
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