Sunao Hamamura

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.

Combined PWM and PFM control for universal line voltage of a piezoelectric transformer off-line converter

A new control method for an AC/DC converter using a piezoelectric transformer is presented. The combination of pulse width modulation and pulse frequency modulation makes it possible to control the output voltage, and its adaptability to a worldwide range of input voltage variation. The AC/DC converter implemented on a printed circuit board is experimentally evaluated, and as a result, good controllability is successfully achieved. The highest efficiency of 86% was obtained at an input voltage of AC 90-270 V, output voltage of 17 V, load current of 0.57-1.4 A, and operating at frequencies around 107 kHz.

New control method of piezoelectric transformer converter by PWM and PFM for wide range of input voltage

A new control method of a piezoelectric transformer (PT) converter by PWM and PFM is presented. The combination of an active-clamp circuit and a resonant circuit makes it possible to control the output voltage of the PT converter by PWM at a fixed switching frequency and PFM at a fixed duty ratio. The PT converter implemented on a printed circuit board was experimentally evaluated and a good controllability was successfully achieved.

AC-DC Converter Based on Parallel Drive of Two Piezoelectric Transformers

An AC–DC converter based on a parallel drive of two piezoelectric ceramic transformers was investigated. The transformers operate in a fundamental contour extensional vibration mode which has a large electromechanical coupling factor notwithstanding the use of a piezoelectric transversal effect. The converter includes Al electrolytic capacitors for smoothing circuits and metal oxide semiconductor field effect transistors (MOSFETs) for the switching and rectifying circuits. Maximum converter efficiency of 90% was obtained. The prospect of a low profile, highly efficient and high power (more than 50 W) AC–DC converter with piezoelectric transformers has been obtained through these experimental results.