This paper presents a single-stage inverter with variable frequency modulation (VFM) based on LLC resonant converter. And LLC converter is a common topology of dc/dc conversion. LLC resonant converter can achieve high efficiency and soft-switching performance. Since the dc gain curve of the single-resonant LLC converter is flat when the switching frequency is larger than the resonant frequency, namely fs>fr, an additional L-C series resonant branch is paralleled to the original resonant tank to introduce higher-order-harmonic resonant current and a zero-gain point to the gain curve. Higher-order-harmonics help to deliver power and the zero-gain point enlarges the gain range which improves output THD and reduces the switching frequency range. A 1.2 kW prototype is built to demonstrate the performance of the proposed inverter. Zero-voltage-switching (ZVS) and zero-current-switching (ZCS) are achieved on the primary side and secondary side, respectively. And 97.3% efficiency and 2.17% voltage THD are achieved at full load condition, while 97.2% efficiency and 3.2% voltage THD are achieved at half load condition. / M.S. / The inverter is widely used to connect renewable energy into the grid by converting dc to ac waveform, like photovoltaic (PV) technology. Basically, the two-stage topology is usually used. The inverter would consist of two stages working in high frequency, the first stage is dc/dc converter which can regulate the input voltage to the desired bus voltage for the second stage, and the second stage is dc/ac converter. The first stage works at a specific switching frequency, so it can be designed to achieve higher efficiency in dc/dc conversion. The second stage also works at high switching frequency and converts dc to ac commonly by using SPWM which changes the duty cycle ratio in a sinusoidal pattern. The single-stage inverter only has one stage working in high frequency while the second stage works at twice line frequency. The first stage converts dc to rectified ac waveform and the second stage unfolds it to ac. The topology of LLC resonant converter being applied for the first stage of the single-stage inverter has been proposed. This topology uses variable-frequency-modulation (VFM) which varying switching frequency on the primary side to output different voltage levels. And it achieves zero-voltage-switching (ZVS). However, LLC converter can hardly output very low voltage due to the flat voltage gain curve at high frequency. Also, LLC converter only transfers the fundamental harmonic component to the load. If the higher-order harmonic components help transfer power when the switching frequency equals the resonant frequency, the current shape will be more like a square wave and the peak of resonant current can be reduced. This thesis proposes a topology that has two L-C resonant branches in parallel for the resonant tank in the converter. And the paralleled resonant branches produce a zero-gain frequency point into the gain curve so that the gain range is enlarged within the reduced switching frequency range and 3rd harmonic component of the resonant current helps to transfer power so that the rms value of resonant current can also be reduced.

The development of self-powered wireless sensor networks for structural and machinery health monitoring has attracted considerable attention in the research field during the last decade. Since the low-duty-cycle wireless sensor networks have significantly reduced the power requirements to the range of tens to hundreds of microwatts, it is possible to harvest environmental energy as the power supply instead of using batteries. Vibration energy harvesting using piezoelectric materials has become the most popular technique, which has a good potential to generate adequate power. However, there is a limitation for the conventional beam-shaped harvester designs in real applications due to their limited bandwidth. In order to overcome this limitation, the essential objective of this thesis is to develop harvesters with multi-resonance structures. The multi-resonance harvester with good broadband performance can achieve close resonance frequencies and relatively large power output in each vibration mode. The main tasks and contributions of this thesis are summarised as follows: • A parametric analysis is presented to determine how the modal structural and electromechanical performances of cantilevered beam harvesters are affected by two modal factors designated as mass ratio and electromechanical coupling coefficient (EMCC). The modal performance of using rectangular, convergent and divergent tapered configurations with and without extra masses are systematically analysed by geometric variation using the finite element analysis (FEA) software ABAQUS. • A modal approach using the two modal factors to evaluate the modal performance of harvesters is introduced and a configurational optimization strategy based on the modal approach is developed to pre-select the configurations of multi-resonance harvesters with better modal structural performance and close resonance frequencies in multiple modes. Using this optimization strategy obviates the need to run the full analysis at the first stage. • A novel two-layer stacked harvester, which consists of a base cantilevered beam that is connected to an upper beam by a rigid mass, is developed. By altering the dimensions and the locations of the masses, the two-layer harvester can generate two close resonance frequencies with relatively large power output. The effects of using rectangular, convergent and divergent tapered beam configurations are systematically analysed. • Multi-layer stacked harvesters with up to five layers are developed. The three-layer harvesters with different mass positions, which can generate three close resonance frequencies, are optimized using the configurational optimization strategy. • A novel doubly-clamped multi-layer harvester, which is able to generate five close resonance frequencies with relatively large power output, is developed and thoroughly analysed. • An experimental study of the multi-layer stacked harvester is presented to validate the simulated results and the configurational optimization strategy. • An experimental study of the two-layer stacked harvester using high performance single crystal piezoelectric material PIMNT is presented. The harvester using PIMNT can generate nearly 10 times larger power output and 3.5 times wider bandwidth than using PZT. Besides, by modifying the location of the piezoelectric layer, anti-resonances between two adjacent modes can be eliminated.