Considering the characteristics of materials required for piezoelectric transformers, and introducing the maximum vibration velocity vmax that generates a fixed temperature rise, we can express the maximum output power of a piezoelectric transformer as the product of this v2max, the square of the power coefficient on the output side A2 and the load resistance RL. Moreover, the power coefficient on the output side A2 is proportional to the product of the square root of Young's modulus YE, the square root of the electromechanical coupling coefficient k, and the permittivity ϵ33. This is verified by using two types of piezoelectric transformers in an inverter where the output section may be of the Rosen type or the transverse-effect type. It is shown that the contributions made by the structure of the piezoelectric transformer and the load resistance differ in each case. The Rosen structure is used in the piezoelectric transformer for the inverter, and it is shown that a material with large maximum vibration velocity vmax is useful in this case. © 1999 Scripta Technica, Electron Comm Jpn Pt 3, 82(11): 86–92, 1999

The rectangular-bar-shaped multilayer piezoelectric transformer, which employs the length extensional vibration mode and interdigital internal electrodes, has been suggested and developed by us for the application to AC adapters. First, the methods of increasing the power-to-volume ratio of the transformer are investigated experimentally. As a result, it is found that using a high driving frequency and optimizing the positions of input and output parts increase the power-to-volume ratio. Second, to prevent characteristic degradation resulting from spurious vibrations in its driving frequency range, the methods of controlling such spurious vibrations are studied theoretically and experimentally. It is found that controlling the size of the transformer can widen the driving frequency range without spurious vibrations, and that using a low order length extensional vibration mode can decrease the number of spurious vibrations in its driving frequency range. Also, it is found that spurious vibrations can be eliminated or weakened by properly arranging the positions of input and output parts according to the mode pattern of the spurious vibrations.

Piezoelectric-transformers (PT) have a lot of merits in comparison with magnetic transformers. We have previously presented some types of piezoelectric-transformer DC-DC converters (PT-converters) for AC-adapters, but their efficiency was restricted. The maximum efficiency was less than 80%. Furthermore, the frequency characteristics of efficiency are not good due to the power loss generated in the input filter circuit of the PT. In this paper the improvement of the converter efficiency by using a different rectifier circuit topology and a synchronous-rectifier technique is described. Furthermore, the frequency characteristics of efficiency are improved by using a different topology of the input filter circuit and its optimum design. As a result, the converter efficiency has been increased up to 88% and the frequency characteristics of the efficiency is also improved.

A piezoelectric transformer with longitudinal vibration mode in both input and output part using inter-digital internal electrodes is proposed, Equivalent circuit analysis was carried out to obtain typical electric characteristics at first. Then in experiment, output power 20 W, power density 12 W/cc and efficiency 91% were obtained.

A piezoelectric transformer of which the structure differs from the Rosen-type structure was investigated. According to the aim of reducing the heat generation of the piezoelectric transformer, the relationship between the heat generation and vibration velocity of the transformer was investigated. The temperature of the transformer rapidly rises when the vibration velocity of the transformer increases to above a critical value. Based on analysis of the electrical equivalent circuit of the piezoelectric transformer, the vibration velocity of the transformer is proportional to the output current and inversely proportional to the force factor of the output part. As a result, new piezoelectric transformers with a strip electrode at the output part which can minimize the heat generation of transformers have been developed. The inverters with these 3, 6 and 10 W piezoelectric transformers showed more than 85% efficiency at 60–80 kHz driving.

A new piezoelectric transformer (PT) converter which functions as an AC-adapter is proposed. The device's PT operates in longitudinal-vibration mode and features good isolation, incombustibility and compact size. The PT was implemented as a part of the new AC-adapter and a significant reduction in size was achieved as a result. Experimental evaluation of the new adapter confirms that good performance was successfully obtained in spite of the need to comply with strict safety and noise standards.

Multilayer piezoelectric vibrators made from high mechanical quality factor (Q m) PbZrTiO3 ceramic material with internal electrodes have been investigated for application to ultrasonic devices. The existence of internal electrodes activates the ceramic sintering reaction and as a result, cofiring of the the multilayer vibrator at a lower temperature becomes possible. Multilayer vibrators with 2-9 layers of internal electrodes were fabricated, and evaluated. The values of Q m>1500 and k 31=0.36 can be obtained. It was found that high-power ultrasonic devices driven by low voltages.

We have investigated the application multilayer piezoelectric ceramics (MPC) to accelerometers to sense acceleration in the low-frequency range. A method of analysis applying the piesoelectric fundamental equation of bimorph to each layer of the MPC and summing them up has been studied. The propriety of this method was demonstrated by comparing calculated and experimental results.

Destruction mechanisms were investigated in electrostrictor, piezoelectric and phase-change antiferroelectric ceramic multilayer actuators with an interdigital electrode configuration. Simultaneous observations were done by three different methods: visual observation with a charge coupled device (CCD) microscope, field-induced strain and acoustic emission (AE) measurements. During a cyclic electric field application, the crack was initiated from the edge of an internal electrode and propagated obliquely to another electrode in the piezoelectric sample, while in the antiferroelectric, the crack was initiated between a pair of electrodes and propagated parallel to the electrodes. The field-induced strain curve exhibited asymmetric and enhanced strains in all the samples during the crack propagation, probably due to the internal delaminations. The AE count also increased drastically before the destruction.

The destruction process of piezoelectric actuators under bipolar driving was studied using acoustic emission (AE) monitoring and induced-displacement measurement. The effect of floating electrode over the internal electrode's end was also investigated. Floating electrodes can suppress the field concentration and cracking in the actuator.

A high sensitivity accelerometer using multilayer piezoelectric ceramics including internal electrodes has been developed. The sensitivity of this accelerometer is about 900 pC/G at the frequency range from 0.01 to 10 Hz. Because the circuit was structed to cancel pyroelectric voltages, the output voltage changes of this accelerometer were decreased to 1/10 compared with the case of a noncanceling circuit.

An actuator has been developed by multilayer piezoelectric ceramic green sheets with line electrodes in such a way that electrode lines are parallel and slide one-half a pitch in alternate layers. Large displacements were obtained by the multiplication of the longitudinal converse piezoelectric effect occurring between the nearest pairs of electrode lines. A long cofired multilayer actuator with a large displacement can be easily produced, and reliability is improved by eliminating exposure of the opposite-polarity electrodes on the same surface of the actuator to humidity in the air. For example, a 74-mm-long actuator with 106 electrode lines on each of 100 layers provided a displacement of 50 μm/350 V, with a generative force of 350 kg/cm2. At 85°C/90% RH (relative humidity) and 180-V constant loading, a lifetime of over 1000 h was demonstrated