Dielectrics are materials that can be polarized by an applied electric field. As the essential property for dielectrics, the relationship between electric field and dielectric polarization, has been widely studied and used in the area of electrical engineering. Representative applications are insulators and electrical energy storage capacitors. For some types of dielectrics, the dielectric polarization is not only decided by the applied electric field, but also is affected by mechanical and thermal properties. This work studies the electro-mechanical and electro-thermal energy inter-conversions and proposes the design of power converters for these materials.
Piezoelectric effect is a cross-coupling between mechanical property and electrical property of dielectrics. It is a reversible process where external electric potential can generate internal mechanical force while external mechanical force can also generate internal electric potential. This effect is utilized to build a piezoelectric transformer (PT) by combining two sets of piezoelectric material together. One set is used as the input, to cause a geometric strain by applied electric field, while the other set is used as the output, to generate an electric charge by the coupled mechanical stress. Compared to traditional magnetic transformers, PTs store energy in mechanical inertia and compliance and therefore they do not generate electromagnetic noise. They are suitable for batch mass manufacturing since there is no winding requirement. Among many types of PTs, radial PT and Rosen-type PT are most widely used.
To provide a guide for the design of PT-based converters, the electrical characteristics of PTs are first analyzed. The accuracy and applicability of different levels of models of PTs are compared and discussed. The detailed universal attributes of PTs, which include the gain characteristic, the input impedance characteristic and the efficiency characteristic, are also derived. In addition, with the assistance of additional compensation component(s), PTs can provide better performance. The impacts of the input and output inductors and capacitors on gain and efficiency characteristics of a PT are analyzed.
Tunable PT is a recently developed raidal PT with three ports: input, output, and control ports. When connected with different impedance at the control port, tunable PT has different voltage gain characteristics. It is proposed to use this property for output voltage regulation while keeping constant switching frequency to ensure high efficiency operation of the PT in PT-based power converters. A closed-loop control scheme is proposed, where the regulation is done by a duty cycle controlled switched capacitor at the tunable PT control port. Two types of output filter are also analyzed and compared. Dc-dc converters with power rating ranging from 30 W to 100 W are built to verify the proposed design.
Rosen-type PT features natural mechanisms for high transform ratio in a compact planar form, which provide an alternative solution for dc bus-fed high step-down voltage-ratio auxiliary power supplies in medium-/high-voltage systems without using bulky magnetic transformer with high turns numbers. The design procedure of the Rosen-type PT-based high step-down voltage-ratio dc-dc converter is presented. The proposed design is validated by a prototype with height of 1 cm, whose nominal output power is 5 W, input voltage ranges from 200 V to 1.5 kV, regulated output voltage is 5 V.
Pyroelectric effect is a cross-coupling between thermal property and electrical property in some dielectrics. It is also reversible. The pyroelectric effect refers to the polarization change caused by temperature change, while the reversed pyroelectric effect refers to a temperature change generated by a electric field change. The reversed pyroelectric effect can be used for building a environmentally friendly thermodynamic system. Electrical characterization of the pyroelectric material is executed to facilitate the design of the power converter needed in the corresponding thermodynamic system. Specifically, this work proposes an energy recovery circuit to increase the coefficient of performance of the system since during the thermodynamic cycle, part of the electrical driving work does not pump heat and may therefore be recovered. / Doctor of Philosophy / When a dielectric material is placed in an electric field, electric charges slightly shift from their average equilibrium positions, causing dielectric polarization. In the area of electromagnetism, dielectric material is widely used as an electrical insulator and to build capacitors. For some types of dielectrics, dielectric polarization is not only affected by electric field. Strong couplings between electrical and mechanical characteristics, and between electrical and thermal characteristic also exist and can be utilized in practical applications. Piezoelectric effect is a coupling between electrical and mechanical characteristics. It is a reversible process where external electric potential can generate internal mechanical force and vice versa. It can be utilized to build transformers, which do not require coil winding nor generate electromagnetic interference compared to their magnetic counterparts. This work analyzed the electrical characteristics of piezoelectric transformers and proposed the design of dc-dc converters based on different types of piezoelectric transformers for different applications, which include tunable radial piezoelectric transformer-based power converters and Rosen-type piezoelectric transformer-based step-down converter with high voltage conversion ratio. (Reversed) pyroelectric effect is a coupling between electrical and thermal characteristics in some dielectrics. An adiabatically applied or removal electric field results in an increase or decrease in the temperature of the corresponding material. This effect can be used to build a environmentally friendly thermodynamic system instead of the most prevalent vapor compression method which involves the use of hydro-fluorocarbon gases leading to global warming and ozone depletion. Electrical characterization is executed first to facilitate the design of the power converter needed by the thermodynamic system. In addition, during the thermal cycle, part of the work done to drive representative cycles does not pump heat and may therefore be recovered. This work proposed circuit featuring energy recovery to provide the desired electric field for driving the thermodynamic system and charge recycling to improve the system efficiency.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/109654 |
| Date | 12 April 2022 |
| Creators | Wang, Le |
| Contributors | Electrical Engineering, Burgos, Rolando, Lu, Guo Quan, De La Reelopez, Jaime, Dong, Dong, Wicks, Alfred L. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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