The Research Of Piezoelectric Transformer DC/DC Converter Based On Phase Control

Compared to the traditional electromagnetic transformer, piezoelectric transformers(PT) has advantages of small size, high conversion efficiency, no-electromagnetic interference and high power density, etc. Its electrical characteristic shows that it could replace reactance of DC/DC converter and has big advantage. Maximum voltage gain and maximum efficiency can be achieved only near PT's resonant frequency. Control methods of DC/DC converter Based on PT mainly have PWM, PFM, PWM/PFM mixed control and phase control. PWM and PFM couldn't finish the tracking of PT's resonant frequency and regulate converter's voltage, and weakness of PWM/PFM mixed control is that the working frequency range is wide when load's change is large, so PT's efficiency is not high. Phase control method make PT always work near the resonant frequency, so the efficiency is high, also its working frequency range is narrow. Compared with PWM, PFM and PWM/PFM mixed control, phase control method has higher efficiency, and more stable output.This paper analyzed the PT double-rectifier circuit model's frequency characteristics of efficiency, voltage gain and phase difference of PT output current and input voltage, and pointed out that in the resonance frequency voltage gain and efficiency is near the largest, and phase difference almost change linearly, so we can track resonance frequency by fixing phase difference. This paper built a FPGA based phase control system, designed a new phase discriminator, and measured that relation curve between phase difference and frequency and relation curve between phase difference and duty are consistent respectively under different load, so it put forward the phase control algorithm to realize the resonance frequency tracking and to regulate DC/DC converter's output voltage under narrowband (1 kHz) condition, also the system didn't use inductive elements, so to some extent, it reduce the magnetic interference. By using DC/DC converter(input 3.3V, output-100V) based on MPT300650L0 as an example to illustrate the proposed method and make experimental research, the experimental results show that non-inductive DC/DC converter phase control is feasible and the theoretical analysis is correct. At last, used PT3906BR and different load to verify the generality of phase control method.