Analysis and design of radial mode piezoelectric transformers for resonant converter applications

The demand for electronic devices is increasing thus as is the need for power dense, efficient power converters. Piezoelectric transformers (PTs) are an alternative to traditional magnetic transformers, offering several advantages over magnetic devices and are ideal for use in low-to-medium power resonant converters. These advantages include desirable material properties, such as being non-ferromagnetic (thus reducing EMI) and the ability to operate in high ambient temperatures, high efficiency, and an integrated resonant tank circuit. However, to date, these devices have seen minimal commercial adoption, owing to the complexity and multidisciplinary nature of their design. In the first part of this thesis, several mathematical models are developed which allow the characterisation of PTs (Chapter 3), allow the designer to avoid issues with spurious vibrations (Chapter 4) and allow the estimation of electrical equivalent circuit component values for radial mode PT topologies (Chapters 5-8). A full derivation of each model is presented and then subsequently verified against experimental and simulated results. Chapter 9 investigates the use of BSPT (BiScO3-PbTiO3) as a high-temperature piezoelectric material for use in high-temperature PTs. A design is formulated using the mathematical models developed throughout the thesis and the performance of the BSPT PT is characterised using a both impedance analysis and power converter measurements. The results of this analysis are compared against a high performance PZT material, with promising results. Finally, building on the work in the previous chapters, a genetic algorithm approach to PT design is then presented. This approach incorporates the modelling performed in the first part of the thesis to allow optimised designs to be generated based on a converter specification. Unlike previous approaches, this method considers a wide range of metrics, including device efficiency, vibration velocity, zero voltage switching performance and spurious mode avoidance when designing the device. The work in this thesis provides a method of generating optimised designs for radial mode PTs for specific power converter applications, without expert knowledge or FEA tools. This will provide an excellent starting point for PT designs and will help to increase the commercial adoption of PTs.