Piezoelectric transformers (PTs) are solid-state devices that transform electrical energy into electrical energy by means of a mechanical vibration. These devices are manufactured using piezoelectric materials that are driven at resonance. With appropriate design and circuitry, it is possible to step up and step down the voltages between the input and output sections of the piezoelectric transformer, without making use of magnetic materials and obtaining excellent conversion efficiencies. The initial concept of a piezoelectric ceramic transformer was proposed by Charles A. Rosen in 1954. Since then, the evolution of piezoelectric transformers through history has been linked to the relevant work of some excellent researchers as well as to the evolution in materials, manufacturing processes, and driving circuit techniques. This paper summarizes the historical evolution of the technology.

https://www.mdpi.com/2076-0825/5/2/12/pdf

Piezoelectric Transformers: An Historical Review

As a viable alternative to magnetic transformers in the power supply for portable electronics, this paper presents a miniaturized off-line travel adapter or charger for cellular phones using the piezoelectric transformer. Various design considerations in the design of ac PT adapters are investigated before coming up with the proposed pseudo-resonant topology. A prototype hardware design is also presented and verified by simulations and experiments

Design of half-bridge piezo-transformer converters in the AC adapter applications

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.

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

Piezoelectric transformers (PZT) can be used advantageously in high output voltage dc-dc converters. In such applications the output section includes a voltage doubling rectification scheme to help increase the output voltage. This topology was modeled and analyzed by considering the expected voltage and current waveforms under the first harmonics approximation. The results were then used to build a linear ac equivalent circuit that emulates the ac-dc stage. The proposed model was verified against simulation and experimental results.

/pdf/analysis-and-modeling-of-a-voltage-doubler-rectifier-fed-by-3ipephr8rd.pdf

Analysis and modeling of a voltage doubler rectifier fed by a piezoelectric transformer

Usually, the performance of piezoelectric transformers (PTs) is optimum when they are sinusoidally driven. The PT can also be driven with a quasi-square voltage waveform. Although PT performance is not optimized with a squared driving, corresponding topologies can be simpler and the whole converter can be improved. Those alternatives that keep an interesting trade-off between complexity and PT efficiency are compared, helping to select the best approach for different applications. All the analyzed alternatives are based on the half bridge topology plus an input matching network.

Comparison of different alternatives to drive piezoelectric transformers

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.

T. Minomiya

Papers

Efficiency improvement of piezoelectric-transformer DC-DC converter