Influence of harmonics on the efficiency of a piezoelectric transformer

The paper considers the influence of high harmonic components of the square wave input voltage on the efficiency of a piezoelectric transformer (PT). The PTs are narrow-range resonant transducers designed to operate with a sinusoidal input of a given frequency. Their common applications are however the DC/DC converters, incorporated in the general class of switching power supply circuits, which are by definition square wave-driven. The paper presents the results of an efficiency analysis of a 40 W ring-shaped step-down prototype PT design. The analysis includes comparison of PT operation efficiency with several different input voltage harmonics combinations ranged between pure sinusoidal and full-square signals. A simple, cost- and space-effective method of selective harmonic elimination is introduced as an alternative to series inductance choke used as input matching circuit in most state-of-the-art PT-based converter designs.