| Piezoelectric transformer is a new transformer based on forward and inverse piezoelectric effect. Compared to traditional electromagnetic transformer, this new transformer has many advantages such as higher conversion efficiency, greater power density, less electromagnetic interference, better environment flexibility, thus it becomes the most popular alternative of electromagnetic transformer. The PT efficiency and gain can be influenced by load resistance, working frequency and the input voltage, the driving control circuit is necessary in order to make PT works well in high efficiency.To provide a foundation for step-up piezoelectric transformer control strategy, the efficiency characteristic of a step-up piezoelectric transformer is analyzed using equivalent circuit. Then the input and output power of the PT is measured using an analog multiplier, thus the efficiency is obtained. Finally, the influence of working load resistance, input voltage and frequency on PT's efficiency is analyzed. The results show that, the closer the load resistance matches the optimum load, the greater efficiency PT provides. The efficiency reaches its maximum when the working frequency is slightly larger than the resonant frequency. And the efficiency trend to reduce if the input voltage of PT increases.Efficiency oriented combined control strategy of PWM and PFM was proposed. Using XC2S100E FPGA as controller, full-bridge inverter circuit built of power MOSFET and the feedback network base on measurement circuit of PT input/output power, driving control circuit was designed with calthode fluorescence lamp as its load. The control algorithms and programs were designed. In the end, the working condition when the designed circuit driving a 100kΩconstant load was tested, the result showed that the driving circuit can make PT working at the peak efficiency frequency, and the peak dfficiency was 71.57%. |
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