Design considerations for power conversion using piezoelectric materials
Keywords: Engineering.
Electrical Engineering.

The internal volume of switched-mode power supplies is dominated by passive components, capacitors and inductors. These components are required to store and buffer energy, and both capacitors and inductors are needed for efficient power conversion. However, compared to capacitors, inductors have significantly higher loss and lower power density and consequently comprise a larger portion of the internal volume. The most direct way to reduce component volume and increase power density is a higher switching frequency, which reduces the energy storage requirements for the passive components. However, most magnetic core materials have relatively poor high frequency performance, leading to poor frequency scaling and an even greater disparity between inductors and capacitors as frequency is increased. While advancements in wide-bandgap semiconductors have enabled large increases in switching frequency, the poor performance of inductors has hindered these advancements from translating to large increases in power density. A promising approach to avoiding the limitations of inductors is the piezoelectric resonator. The piezoelectric resonator consists of a low loss mechanical resonator that is electrically coupled through the piezoelectric effect. These resonators provide an inductive impedance around resonance that can be used to replace conventional inductors in power conversion and, compared to inductors, have lower loss, a greater power density, and better high frequency scaling. This work focuses on realizing the theoretical advantages of piezoelectric resonators in power conversion through both circuit design and piezoelectric resonator design. In the circuits part of this work, power converters using piezoelectric resonators are analyzed in the context of a consistent switching cell. This switching cell can achieve bidirectional power transfer with an arbitrary conversion ratio, and the efficiency is derived as proportional to the product of two properties of the piezoelectric resonator: the electromechanical coupling coefficient, k^2, and the mechanical quality factor, Qm. Using the switching cell as a building block, converters with enhanced performance characteristics, such as improved efficiency at low conversion ratio, are synthesized. A prototype converter based on one of these derived topologies operates at 180 V-to-60 V and a maximum power output of 89 W with 97% efficiency. At this operating point the converter has a piezoelectric resonator power density of 1340 W/cm^3, which is an order of magnitude greater than the inductor power density of comparable designs. The piezoelectric resonator focused portion of this work maximizes power density through the use of lithium niobate, a low-loss piezoelectric material. The intrinsic k^2Qm of the material is higher than that of more common PZT ceramics, allowing for a higher loss limited power density. The material also has a large yield strength and coercive field strength, enabling a high material limited power density. The material limited power density increases linearly with frequency and is calculated as 28 kW/cm^3 at 10 MHz. However, one challenge in using lithium niobate is that the dielectric constant is low. As a consequence, a high resonant frequency and resonator geometries with a large cross-sectional area are required to provide a good impedance match and achieve high efficiency operation in most dc-dc conversion applications. Resonators with these geometries are prone to spurious resonant modes that can cause large increases in loss, and the resonator design and fabrication work focuses on methods to suppress these spurious modes. Three generations of resonators are detailed, and the last achieves effective suppression of spurious modes with a perimeter ring structure and a k^2Qm value of 1460, which is above what can be achieved with PZT ceramics. Resonators are further validated under power. While the lithium niobate resonator exhibits little change until mechanical failure at close to the published yield stress, a PZT resonator exhibits significantly increased loss at relatively low stress. These results further validate the high power density of lithium niobate piezoelectric resonators.