CHARACTERISTICS OF RADIAL-MODE PIEZOELECTRIC TRANSFORMERS

Radial-mode piezoelectric transformers of 23.50 mm in diameter and 1 mm in thickness were fabricated. The transformers, which were made of hard piezoelectric materials wit`h high mechanical quality factor, Qm, were poled only along the thickness direction. The input and output electrodes were concentrically placed in the top surface of the ceramic. The input electrode was the external electrode ring and the output electrode was the internal center dot. The electrical properties in terms of voltage step-up ratio, output power, efficiency, and temperature rise were measured under various load resistances. The voltage step-up ratio increased proportionally with load resistance. With the optimum resistive load, RL = 1/ωCd2, the efficiency showed a maximum value of more than 90 % and the voltage step-up ratio was 2. These results were compared to the calculations from the lumped equivalent circuit analysis and they were in good agreement. This transformer, which exhibited approximately ten times higher power density than a rectangular Rosen-type piezoelectric transformer, was successfully applied to operate an 8 W compact fluorescent lamp