The Principle Analysis And Design Of Step-down Piezoelectric Transformer

Based on the advantages of small size, no electromagnetic interference and high efficiency, Piezoelectric transformer (PT) can replace conventional electromagnetic transformer in more and more fields, and it shows broad application prospects. At present, the study of step-down piezoelectric transformer is behind the step-up piezoelectric transformer and there are no theories about the principle of step-down PT. The structures with good step-down effect focus on multi-layer structures, while there are difficulties in the fabrication process. To solve this problem , the principle of step-down PT on voltage reducing is analyzed and a new disk single layer radial mode piezoelectric step-down transformer with higher step-down ratio is introduced.First, based on the PT lumped-constant equivalent circuit, the expressions of the electrical characteristics of PT are deduced. And the Rosen type piezoelectric transformer which is widely used in commerce is reversed to analyze its voltage reducing character. Combined with the theory, the general rules in designing step-down piezoelectric transformer is summarized, then the unipoled PT with common ground, unipoled PT with floating ground and bipoled PT are put forward. The radial displacement and input impedance of these three kinds of PT are analyzed by COMSOL Multiphysics finite element analysis software.Second, the designed step-down piezoelectric transformers is fabricated, and the equivalent circuit parameters of unipoled PT with common ground are measured by Agilent-4294A precision impedance analyzer. Based on the equivalent circuit models, the character of voltage gain, efficiency and normalized output power is obtained by MATLAB calculation. Further more, the voltage gain and normalized output power versus frequency of four different sizes of PT with common ground are tested, and the relationship between the size and the electrical characteristics of unipoled PT with common ground are obtained. What's more, the electrical characteristics of unipoled PT with floating ground and bipoled PT are also measured, and the results showed the electrical character of unipoled PT with floating ground is consistent with the unipled PT with common ground, all the three kinds of PT can realize ideal voltage reducing character and the unipoled PT which interior diameter is 7 mm can realize gain 0.11, efficiency 92%. The step-down PT with floating ground is connected in series-parallel and series-series. Both the connections are proved to reduce the voltage further by MATLAB simulation and experimental measurement. Finally, as an application, a AC-DC converter with step-down piezoelectric transformer which interior diameter is 7 mm is designed, and 3V～10V output voltage, above 3.8W output power can obtained. It is proved that the designed step-down piezoelectric transformer can be used in the low power supply device.