Step-Down Piezoelectric Transformer for AC-DC Converters

In this paper, a new piezoelectric transformer for AC-DC converters is presented. This piezoelectric transformer, with a multilayered construction in the thickness direction, operates in a fundamental contour-extensional vibration mode. Output impedance is designed to be low, approximately several tens of ohms. First, we simulated the design of the transformer using equivalent circuit method and finite element method (FEM) analyses. We calculated that the transformer could work with an eficiency higher than 95% and a gain of 0.4 at a resonant frequency of 140 kHz. Second, we fabricated a transformer of 14 mm length, 14 mm width and 5.8 mm thickness and examined it. It was found that the transformer exhibited a 0.4 gain and 96.3% efficiency at 135 kHz, when the temperature increase was 30°C. We applied the transformer to the fabrication of an AC-DC converter using a half-bridge zero voltage switching circuit, and examined the converter. It was found to achieve good line and load regulation, and the highest efficiency of 90% was obtained for Vin=80–120 Vac, Vout=13 Vdc, and Iout= 0.8–1.5 A.