Theory and Experimental Verification of a Disk Type Piezoelectric Transoformer

The main thrust of this dissertation is to develop innovative piezoelectric transformers. A newly developed Uni-poled disk type piezoelectric transformer inverter for T5 was proposed in this thesis. On the other hand, crescent input electrode disk type piezoelectric transformer was used to light the LCD monitor backlight. As single-layer piezoelectric transformers have the chance to lower the manufacturing cost of piezoelectric transformer inverter and increase the competitive edge when compared to traditional magnetic coil transformer, single-layer piezoelectric transformers were used throughout this thesis. As the fastest way to understand the piezoelectric transformer electric characteristics was using the equivalent circuit of piezoelectric, MATLAB was first used to analyze the equivalent circuit so as to understand all piezoelectric transformer characteristic such as step up ratio, impedance match, max efficiency, output current, change of input impedance, etc. It was verified that these parameters vary with frequency and load. To design piezoelectric transformer system that can reach optimal state, finite element software was adopted to simulate the modal stress and voltage distributions. As a piezoelectric transformer varies its step up ratio with the load impedance, it fit the lighting lamp need. The simulation model developed provides us with a way to achieve improvement in efficiency and cost.
Traditional limit of piezoelectric transformer power was defined by the vibration speed limited by the piezoelectric property. In that condition, piezoelectric structure quality factor will begin to decrease, which indicates that small power equivalent model is completely different from that of the high power operating condition. In this dissertation, the piezoelectric input impedance associated with high power condition will be detailed. Electromechanical coupling factor is yet another important factor reason that imposes the high power limit form various piezoelectric transformers, where the input and output coupling coefficient relation can be obtained by taking the correspoinding input impedance into considerations within the above-mentioned simulation model.
Soft switching techniques were adopted to reduce the switching loss and to lower the waste heat in this thesis. The switching dead time between the on and off states was adjusted to make the end voltages of MOSFETs being charged and discharged to reach zero voltage right before switching, i.e., to drive the MOSFET in the so-called zero voltage switching (ZVS) condition. Furthermore, to simplify the condition that could make MOSFET reach zero voltage switching condition, a small inductance was connected in front the piezoelectric transformer. The Disk piezoelectric transformer filters out the higher order harmonics while keeping the output voltage and current a sine wave in order to achieve high driving efficiency. Combining all design parameters mentioned above, the dissertation developed innovative piezoelectric transformers based inverters that possess performance higher than traditional inverter.