Novel Topologies For Inductorless Actuation Of Piezoelectric Transformers

Piezoelectric transformers (PTs) are power delivery devices that convert electrical energy to mechanical energy and back based on inverse and direct piezoelectric effects. The distinct advantages of PTs, which include high efficiency and power density, simple structure and low cost, make them attractive for information processing, communications and miniaturized power equipment. The development of PT converters not only is determined by the improvement of PT’s performance, but also depends on the actuation technology. Usually a matching network should be inserted between the driver circuit and PT to achieve the zero-voltage-switching (ZVS) conditions for optimal working conditions of the whole system. The network must have two fundamental functions:(1) filtering of the driving waveform to actuate the PT in sinusoidal conditions, and (2) compensation of the noticeable parasitic capacitance of the PT observed from the output of the driver circuit. The simplest matching network is an inductor introduced between the driver circuit and the PT. However, the need for a wirewound inductor negates the benefit of the PT. Therefore, the study of this dissertation is to develop a PT-based converter without any auxiliary inductor. First, we proposed a concept of piezoelectric inductor (PL) and utilized a PL successfully in compensating for the capacitive impedance of PTs at the working frequency. Then, piezoelectric filters (PFs) were introduced to filter the harmonics in the waveform before actuating the PT, increasing the total efficiency of PT converters significantly. Finally, together with the matching network of the PF and PL, an inductorless driving topology for the PT is achieved via a specially designed driver circuit, which was aimed to decrease the complexity of the PFs. The main research contents and innovations of this dissertation are as follows:1) Based on the inductive characteristic of piezoelectric ceramics between resonant and anti-resonant frequencies, we developed a concept of piezoelectric inductor (PL) and utilized a PL successfully in compensating for the capacitive impedance of PTs at the working frequency. Traditional wirewound inductors are bulky, flammable, and complicated in mass production, which restrict the development of integrated circuits with utilization of inductors. Whereas the PLs inherit the merits of piezoelectric material in volume, weight and price, and are simple in structure, which can meet the needs of integrated circuits for inductors to some extent.2) To have a deep analysis and understanding about the phenomena of piezoelectric inductor, the detailed admittance equation of a piezoelectric structure was derived based on the model of lumped parameter system. According to the admittance equation, we analyzed the inductive behavior of piezoelectric structure theoretically and found that this behavior indeed occurred between the resonant and anti-resonant frequencies, coincident with the founding in experiments. Simultaneously, the impact factors on the inductive behavior of piezoelectric structure, like electromechanical coupling factor, damping and external impedance, were investigated. These detailed analyses explored the physical explanation of PL, and make us a new understanding to the impedance of piezoelectric structure. Moreover, it can provide guidance for the design of PL in practical applications.3) It is the first time to utilize a PF in the actuation of PT for the improvement of efficiency. Investigations indicated that elimination of high harmonics in the driving waveform of the half-bridge inverter can avoid the excitation of parasitic models in PT, which is essential for the high efficiency of PT and switching loss decreasing of the inverter. However, the filters obtained are usually designed with small inductances and capacitances in parallel resonance, and extraneous inductors are still used. Hence, we presented a new method that introduced a PF as a good substitute for LC filter to eliminate the high harmonics, and improved the whole efficiency significantly. Moreover, a PF has lots of advantages such as low price, wide frequency band, small insertion loss, and has a good potential for device miniaturization in future.4) For a trade-off consideration on the circuit efficiency and driving complexity of a PT, the optimum driving waveform for the PT was analyzed. As we know, sinusoidal driver circuits are optimum for PT, but are usually very complex, expensive, and inefficient. A PT is usually driven directly with a square wave voltage fed by a converter circuit. In terms of the efficiency of PT, high harmonic components in the square wave have a negative influence. Effective narrow-range band-stop filter of PF has already been adopted to eliminate only the exact frequencies of interest, and excellent performance has been observed. However, the necessary square wave driving of a PT complicates the design of harmonic elimination because of the presence of large amounts of harmonics. In this study, efficiency analyses for a simplified push-pull power amplifier (PPPA) model were conducted. The square waveform of the switching signal was imitated with amount-limited frequency components, and calculations found that the output waveform of a(sin(ωt)+(1/6)sin(3ωt)) is optimum to achieve a trade-off effect on the circuit efficiency and driving complexity of PT.5) The presence of an inductor during actuation of a piezoelectric transformer (PT) is a problem that has been investigated for many years. An inductorless driving topology to solve the inductor problem is proposed in this dissertation. This topology consists of a driver circuit and a pure piezoelectric matching network between the circuit and PT. The piezoelectric matching network is a combination of piezoelectric inductor and filter. The driver circuit is constructed to provide a specially designed waveform. The Fourier spectrum of the waveform presents dominant fundamental and third harmonics, with higher harmonics controlled to negligible levels. The third harmonic component is eliminated by a piezoelectric filter (PF), which is a component of the piezoelectric matching network. Another component is a piezoelectric inductor used to compensate for the capacitances of the PT and PF. An experimental setup is constructed to verify the performance of the driving topology. A total efficiency of 76.3% with an input power of 14.5 W is achieved when the whole driving system is considered. Although its performance is not exactly ideal, this topology is a highlight of investigations of the inductorless actuation of PTs. The proposed circuit and pure piezoelectric matching network indicate potential improvements in the performance and miniaturization of PT-based converters.