The performance of inductors at high frequencies and small sizes is one of the largest limiting factors in the continued miniaturization of dc-dc converters. Piezoelectric resonators can have a very high quality factor and provide an inductive impedance between their series and parallel resonant frequencies, making them a promising technology for further miniaturizing dc-dc converters. In this paper we analyze the impact of resonator parameters on the performance of the piezoelectric resonator based dc-dc converter, derive the optimal load impedance and efficiency limits, and analyze the impacts of varying conversion ratio and load impedance. This work is accompanied by a prototype dc-dc converter using a piezoelectric resonator fabricated from lithium niobate. The piezoelectric resonator has a quality factor of 4178 and a coupling coefficient, k 2 t , of 29%. The converter is able to achieve high efficiency zero voltage switching and a continuously variable conversion ratio without the use of any discrete inductors. It achieves a maximum power output of 30.9 W at an efficiency of 95.2% with a power density of 6.76 W cm 3 .

Inverters operating at high frequency (HF, 3-30MHz) are important to numerous industrial and commercial applications such as induction heating, plasma generation, and wireless power transfer. A major challenge in these applications is that the load impedance can vary dynamically in both real and complex components over a wide range, making it difficult to maintain high-efficiency soft-switched operation. The constraints that a variable load impedance place on high frequency inverter design results in systems that are often bulky, expensive, and inefficient. This paper presents the design, physical prototype, controller, and experimental results of a high-frequency variable load inverter that is able to directly drive widely variable loads with high efficiency. The prototype can deliver 1kW into a 22 ohm load at 95.4% efficiency as well as deliver significant power to a wide range of both capacitive and inductive loads.

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