Doctoral Dissertation
Control and devices for practical piezoelectric power conversion
Power conversion provides the supporting backbone of our electrical energy infrastructure by efficiently converting electricity between its different forms, from hundreds of kilovolts AC in transmission lines to single volt DC in consumer electronics. Many applications within this infrastructure, especially in transportation and aerospace, demand smaller and lighter power converters to realize improved system performance. To meet this demand for high power density, power converters would need to scale from today’s 10 to 100 kHz switching frequencies to MHz switching frequencies to shrink the bulky inductors and capacitors inherent to power electronics circuits. The advent of commercial wide bandgap semiconductor devices, namely Gallium Nitride (GaN) and Silicon Carbide (SiC), has enabled MHz switching frequencies from a switching device perspective. However, the physical loss mechanisms of inductors causes poor scaling to high frequencies and small volumes, creating a bottleneck for increasing power density. Piezoelectric devices, unlike inductors, scale favorably to high frequencies and small volumes through efficient energy storage in mechanical vibration. With coupling to the electrical domain via the piezoelectric and inverse piezoelectric effects, these devices can provide passive energy storage to power converters similar to that of inductors. Power converters designed around piezoelectric devices instead of inductors could theoretically bypass the frequency scaling bottleneck and achieve higher power density. However, the idea of piezoelectric power conversion dates back to the 1960s and has yet to realize high power density power conversion at scale due to a lack of control methods capable of MHz switching frequencies and limitations of commercially available piezoelectric devices. This thesis addresses these two challenges presenting control methods and piezoelectric device designs to enable practical piezoelectric power conversion. To be practical, a power converter must satisfy all the requirements of real world applications including specifications such as efficiency, power density, reliability, and controllability. Solving these challenges enables practical piezoelectric power conversion to begin a new paradigm of high power density power conversion in real-world applications. First, to overcome a reliability issue with resonator spurious modes, we develop a fixed-frequency control method for piezoelectric resonator based DC-DC converters. Spurious modes constrain established control methods to limited operating ranges whereas a practical power converter needs to operate continuously from minimum to maximum output power. Fixed-frequency control enables operation across all output powers by avoiding spurious modes and circulating power within the high quality factor piezoelectric resonator. In a prototype converter with a spurious-constrained operating region, we demonstrate how fixed-frequency control extends the operating range by a factor of 2.7× while keeping efficiency high. Second, we present acoustic designs to eliminate spurious modes altogether. After discussing piezoelectric device parameters and materials relevant to power conversion, we develop custom fabricated lithium niobate thickness mode resonators with a novel spurious-free, ring resonator acoustic design. These devices achieve a record high component power density of 5.7 kW/cm3 when tested in a 3.2 kW electric vehicle on-board charger. Moreover, we introduce lithium niobate radial mode resonators with high quality factors exceeding 20,000 and high 99.3% DC-DC efficiency. Third, to realized closed-loop control of piezoelectric resonator based DC-DC converters at MHz switching frequencies, we present a current mode control method that utilizes the power of modern microcontrollers to ensure efficient zero-voltage-switching at high frequency. An additional feedforward compensator eases control design by accounting for the higher order dynamics of the piezoelectric resonant tank. The control method is evaluated with a prototype DC-DC converter operating at 750 kHz with stable and efficient regulation.