The last two decades of research into piezoelectric transformer (PT) based power converters have led to some extensive improvements of the technology, but it still struggles to get its commercial success. This calls for further research and has been the subject of this work, in order to enable the utilization of the PT technology advantages, reduce cost and increase competitiveness. First of all an overview of the basic PT technology used in general power converters is given, including the basic piezoelectric nature, converter topologies and control methods. Compared to traditional magnetic technology based power conversion, the PT technology has some obvious advantages, being the electromechanical energy conversion, low EMI profile, a compact and low profile design, as well as a high potential of high efficiency and power density. The utilized inductor-less half-bridge topology is investigated in detail, revealing its strong points, as well as some shortcomings. As a result of this investigation, a soft switching factor (ZVS factor) is derived, which describes the maximal achievable soft switching capability of the PT, as well as it is related to the structure of the PT, through the effective electromechanical coupling factors. In order to exploit the advantages of the inductor-less half-bridge, research into soft switching optimized PT's has been conducted. Several innovative PT solutions have been proposed, simulated and optimized, using Finite Element Modeling (FEM) tools, all with the main goal of achieving soft switching capabilities. The proposed designs have been manufactured, tested and evaluated. The main achievement has been the development of an Interleaved interdigitated electrode (IDE) PT, which retains some of the easy manufacturing advantages, combined with the high efficiency of the thickness mode vibration. The main focus of this research has been control methods, due to the high control requirements of PT based power converters and the inductor-less half-bridge, as well as the shortcomings of the prior-art solutions, and has led to several innovative solutions. A self-oscillating control method is proposed that has a very tight and precise frequency control, which ensures optimal and soft switching operation at all times. Furthermore a forward conduction mode control method is proposed, which resembles a PLL control and ensures a constant and optimal operation, as well as having the advantage of being purely primary side based. A revolutionary bi-directional control method is proposed, which utilizes active phase shift of the output rectifier that enables bi-directional power flow. Soft switching operation is maintained over the full power flow modulation range, ensuring optimal and efficient operation. Furthermore, it enables line and load regulation.
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Bruun, Erik, Andersen, Michael A. E.
Technical University of Denmark, Department of Electrical Engineering, 2012