Thermal analysis and thermographic measurements of piezoelectric transformers

Abstract

This paper deals with the problem of thermal effects in piezoelectric transformers. With the development of piezoelectric technology, these high efficiency transformers are soon going to be used in a wide range of electronic equipment. With the rated power of piezoelectric transformers getting higher, despite their high efficiency (above 90%), thermal effects caused by energy loss are becoming to be a great concern due to small size of the transformer. The paper discusses the results of thermal behaviour investigations of a ring-shaped piezoelectric transformer for step-down applications. The analysis consisted of two parts: finite element modelling and laboratory tests including thermal camera measurements. IntroductionPiezoelectric transformers (PTs) are electrical energy transducers, whose working principle is based on mechanical resonance coupling of very high quality factor (Q>1000) [1]. Being much smaller, lighter, safer and more efficient, they constitute an interesting alternative to traditional magnetic-core transformers of comparable power. In optimal working conditions PTs reach up to 98% efficiency, which is unattainable for any standard transformer. The low power PTs (1-2W) for step-up applications have already been introduced to mass production and are currently a standard component of LCD monitors [5]. Much larger application scale is going to be achieved with the forthcoming introduction of PTs to general purpose AC/DC adaptors, present almost everywhere in consumer electronics. To fit this particular application a PT must be more powerful (at least 10W of rated power) and suited for step-down operation. A prototype ring-shaped multi-layer PT fulfilling the above conditions has been developed by Danish piezoceramics specialist Noliac [8]. The mentioned PT operates at relatively high frequency range (approx. 420-470kHz), where it should be able to transform up to 10W with more than 90% efficiency [3]. The results of experimental verification of these declared values are presented in the second part of the paper.The laboratory tests were preceded with a finite element analysis performed in ANSYS [9] modelling environment. The considered piezoelectric transformer structure was modelled as a multi-layer ring (3D with axial symmetry). To determine the main resonant modes of the transformer a modal analysis was performed first. Through the dynamic analysis performed for a set of near-resonance frequencies, the spatial distributions of maximum stress, displacement and electric field were obtained. For the modelling of energy loss, appropriate damping factors were introduced.The main aim of the finite element analysis was to determine the distribution of energy loss within the piezoelectric material in order to find the source of heat generation in a working transformer. The resulting temperature distribution is then computed on the basis of volumetric heat generation rate introduced to the model.The work presented is a part of a preliminary research related to the development process of piezoele...

Piezoelectric Transformer Efficiency Tests in a Digitally Controlled Converter Circuit

“…The mentioned PT operates at relatively high frequency range (approx. 420-470kHz), where it should be able to transform up to 10W with more than 90% efficiency [3], which was discussed earlier by the authors [2], with the assumption of optimal sinusoidal input voltage conditions. In a real converter, however, the transformer is driven with a square voltage containing higher order harmonics, potentially lowering the efficiency.…”