Theory and Experimental Verification of Modal Based Piezoelectric Transformer for LCD Backlight Inverters

The main thrust of this dissertation is to develop innovative piezoelectric transformers so as to use these series of newly developed piezoelectric transformers for the development of piezoelectric technology based inverter for LCD monitor backlight. The specifications of the piezoelectric technology based inverter vary whenever the dimension and purpose of the LCD monitor is changed. Various types of piezoelectric transformers for large and mid sized LCD TV and portable LCD monitor were examined and studied. For large size LCD TV backlight module, a single-layer piezoelectric transformer was used as the component to amplify the input voltage to light cold cathode fluorescent lamp (CCFL). The single-layer piezoelectric transformer provides us with an opportunity to lower the manufacturing cost of the piezoelectric technology based inverter, which in turn then increase the competition edge when compared to traditional magnetic coil transformer. To overcome the limitation that the step-up ratio of a single layer piezoelectric transformer is low, internal power factor correction circuit (PFC) was used to convert. the AC voltage derived from regular electric outlet to high DC input voltage for the newly developed inverters. For portable LCD monitor, the DC voltage usually was derived form small battery, which leads to low DC input voltage. Therefore, to increase the step-up ratio of the piezoelectric transformer, a multi-layer piezoelectric transformer was used to complete the piezoelectric technology based inverter for portable LCD monitor.
By introducing the design thoughts of modal sensors and actuators into the design of piezoelectric transformer input electrode, a series of innovative piezoelectric transformers were developed. It was shown that these newly introduced input electrodes can provide modal filtering property needed to filter out unwanted high frequency voltage embedded within the input voltage waveform. To design modal electrode for various piezoelectric transformer, traditional piezoelectric theory was used to analyze the resonant vibration behaviors of the piezoelectric transformers. Traditionally the structural vibration of the piezoelectric transformer was modeled by using a partial differential equation (PDE). Adopting normal mode expansion, this PDE can be decomposed into a series of ordinary differential equations (ODE). Each ordinary differential equation can be used to represent mechanic vibration of the piezoelectric transformer when the driving frequency is near the resonant frequency of the piezoelectric transformer. Both full modal electrode and quasi-modal electrode were developed within this dissertation. By using the quasi-modal electrode, the voltage waveform with odd times piezoelectric transformer resonant frequency existed within the input voltage can be completely filtered out. By using the quasi-modal electrode, both rectified and trapezoidal input voltage can be used to drive the piezoelectric transformer directly without suffering efficiency loss. It was also shown that full modal filtering effect can be achieved by using full modal electrode. The experimental result and the theoretical analysis presented in this dissertation demonstrated that full modal electrode can improve the energy transfer efficiency and expand the optimal operating region of the piezoelectric transformer.
To lower the switch loss of MOSFET adopted in the piezoelectric technology based inverter, soft switching technology that was adopted inside the traditional switching power supply circuit was used to design the driving circuit needed for piezoelectric transformer based inverter. Traditionally, an equivalent circuit model was used to simulate the property of the piezoelectric transformer. To verify whether the MOSFET can reach zero voltage switching (ZVS) condition or not, the equivalent circuit model of the piezoelectric transformer must be used to calculate. The equivalent circuit of the piezoelectric transformer includes a capacitance and an inductance, both of which produce the LC resonant circuit. By controlling the internal inductance of the piezoelectric transfer and the dead time of the MOSFET switch, zero voltage switching condition was shown to be achievealbe and practical. It was also shown that the zero voltage switching condition can decrease the switching loss such tha t energy transfer efficiency was improved.
Furthermore, to simplify the conditions that can make MOSFET reach zero voltage switching condition, a small inductance was connected in front the piezoelectric transformer to make sure that the MOSFET work under the zero voltage switching condition. The quasi modal electrode was used to filter out the high frequency voltage that exists within the input voltage.
Combining all design parameters mentioned above, the dissertation attempts to combine traditional mechanic and electric analysis for piezoelectric transformer, which finally leads us to invent a series of approaches that can be used to circumvent all of the prior art. By using quasi-modal electrode and soft switching technology, a coil less piezoelectric based inverter for LCD TV backlight was completed. A full modal multi-layer piezoelectric transformer was also completed to light portable LCD monitor backlight. Finally, the full modal electrode was shown to improve the overall energy transfer efficiency of the piezoelectric based inverter.