I. Introduction
The piezoelectric effect is a commonly used phenomenon for many actuators and sensors [1], [2]. By combining the piezoelectric and inverse-piezoelectric effects, a low-amplitude input voltage can be converted to a large amplitude output voltage by using a piezoelectric transformer (PT). A PT can be used to step voltages up or down, but, unlike a traditional transformer, no mutual magnetic flux linkage is required [3], [4]. A length-extensional PT uses an applied electric field across the thickness of the crystal to produce a longitudinal mechanical vibration. The mechanical vibration produces a high electric field in the output section of the crystal because of the piezoelectric effect [4], [5]. Transformers of this design have compact size, low mass, and high efficiency [4].