8974 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 32, NO. 12, DECEMBER 2017 Letters New Tunable Piezoelectric Transformers and Their Application in DC–DC Converters Mudit Khanna, Student Member, IEEE, Rolando Burgos, Member, IEEE, Qiong Wang, Student Member, IEEE, Khai D. T. Ngo, Fellow, IEEE, and Alfredo Vazquez Carazo Abstract—This paper introduces a new tunable piezoelectric transformer (TPT) and demonstrates its operation in a dc–dc converter application. Piezoelectric transformers (PTs) have been conventionally used in high- voltage, low-power applications such as electronic ballasts. Recently, radial type PTs have been developed for higher power ac–dc and dc–dc step down applications. Based on the latter, a new TPT has been developed featuring an auxiliary secondary terminal to control the voltage gain of the transformer. This results in some exciting characteristics from a dc–dc converter standpoint, like an adjustable frequency response of the TPT, and fixed frequency control of the converter with no-cross talk between the primary and secondary in the control circuit. This paper introduces the design concept behind TPT-based dc–dc converters, and proposes a control scheme for their implementation. Experimental results with a 30 W, 220:55 V converter unit are shown to validate these concepts. Index Terms—Constant frequency control, dc–dc converter, piezoelectric transformers (PTs), resonant converters, tunable piezoelectric transform- ers (TPTs). I. INTRODUCTION T HE development of Rosen-type piezoelectric transformers (PTs) with high voltage gain became popular during the 1990 s and the early 2000 s in high-voltage low-power applications such as backlight- ing of cold cathode fluorescent lamps used in liquid crystal displays in laptop computers. PTs offer some inherent advantages over tradi- tional magnetic counterparts such as light weight, less electromagnetic interference interference, ability to operate in high magnetic fields, au- tomated manufacturing, etc. This has led to new PT-based applications such as fluorescent ballast [1], [2], LED drives [3], and MRI-based applications [4]. For low voltage, step down applications, radial PT structures, such as the so-called Transoner PT, were developed and used for ac–dc and dc–dc converter applications [5]. When operated in the inductive region, PTs can be used as LCC type resonant tank circuits. Just like resonant converters, most of the con- verters using PTs regulate the output voltage by changing the switching frequency of the inverter bridge (variable frequency control) [6]. Re- cent researches have proposed various control schemes that optimize the operation of PT to achieve zero-voltage switching (ZVS) and high efficiency [7], [8]. Some investigations have suggested control schemes that use a combination of pulse width modulation (PWM) and pulse Manuscript received February 5, 2017; revised March 24, 2017 and April 23, 2017; accepted May 2, 2017. Date of publication May 8, 2017; date of current version August 2, 2017. This work was supported by the Defense Advanced Research Projects Agency (DARPA) under Grant 11874492. (Corresponding author: Mudit Khanna.) M. Khanna, R. Burgos, Q. Wang, and K. D. T. Ngo are with the Center for Power Electronic Systems (CPES) at Virginia Tech, Blacksburg, VA 24060 USA (e-mail: mudit1@vt.edu; rolando@vt.edu; wangq@vt.edu; kdtn@vt.edu). A. Vazquez Carazo is with the Micromechatronics, Inc., State College, PA 16803 USA (e-mail: avc@mmech.com). Digital Object Identifier 10.1109/TPEL.2017.2702124 frequency modulation control to regulate the output voltage for large load ranges [9], [10]. All these control schemes, however, require iso- lation in the feedback circuit. This impacts the circuit design in terms of number of components required, size and weight of the converter and its regulation characteristics. In addition, large variations in load and wide input voltage range can cause large variation in the operating frequency of the converter. This is undesirable as the efficiency of the PT is compromised when operated far away from the resonant point [5]. Constant frequency control methods that operate at the optimized frequency of the PT have also been proposed to overcome this issue as in [11]; however, the latter requires a large filter capacitor at the output. To tackle these issues, a new tunable piezoelectric transformer (TPT) featuring an adjustable voltage gain and frequency response, thus capa- ble of constant frequency control, has been recently developed within the framework of a DARPA project. This paper discusses the basic operation of such TPT unit, and proposes a TPT-based dc–dc converter rated at 30 W, 220:55 V DC, and a fixed frequency control scheme capable of regulating the output voltage under load and input voltage variations. II. TUNABLE PIEZOELECTRIC TRANSFORMERS In a conventional PT, the input and output sections are made of piezoelectric ceramic material layers joined together mechanically. The input section acts as a transducer and converts the electrical energy into mechanical energy. This energy is transferred to the secondary layer, which acts as a generator and converts it back into electrical energy. The detailed design and operation of the Transoner radial PT structure is discussed in [12] and [13]. The proposed Tunable PT uses the same structure; however, an additional piezoelectric section (control layer) is sandwiched between existing primary and secondary layers. The prototype design used to test this concept uses thickness polarized discs made of hard Navy type III piezoelectric material with a diameter of 32 mm and thickness of 1.25 mm (same for all layers). The lumped-element “equivalent circuit” of the TPT is derived using the methods proposed in [1] and [13]. The detailed design and model verification for the TPT, is however, considered out of scope for this letter. Fig. 1(a), (b) shows the structure and circuit diagrams of a con- ventional PT and Fig. 1(c), (d) of the proposed TPT for comparison purposes, where the difference between them is apparent, in particular the effects of the additional the control layer. As seen in Fig. 1(c), the control section in the TPT is sandwiched between the input and output sections of the transformer. Other positions of the control section are possible based on design specifications. Auxiliary sections have been used in the past in PTs as a passive mean of sensing the characteristics 0885-8993 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.