850 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 51, NO. 4, AUGUST 2004 A New Control Strategy for an AC/DC Converter Based on a Piezoelectric Transformer Juan Díaz, Member, IEEE, Fernando Nuño, Member, IEEE, Juan Manuel Lopera, Member, IEEE, and Juan Antonio Martín-Ramos, Member, IEEE Abstract—Piezoelectric transformers (PTs) are an attractive so- lution to reduce size and weight in ac/dc converters. In this paper, a PT-based topology is presented together with a control method. As an application, an 8-W ac/dc adapter (110 V input, 12 V output) is presented operating at frequencies around 500 kHz. Index Terms—Battery charger, control strategy for piezoelectric transformer (PT) converters, piezoelectric transformers (PTs). I. INTRODUCTION P IEZOELECTRIC transformers (PTs) are an attractive solution for ac/dc and dc/dc converters, since they allow working at high frequencies with good efficiency in a reduced size. These devices, composed of ceramic materials, ensure electric isolation, which makes them suitable for applications such as ac adapters. Fig. 1 shows the equivalent circuit for a typical PT. In the primary side of the PT, a relatively large input capacitance can be seen followed by an R–L–C net. The secondary side is modeled as a capacitor. In the case illustrated, the operating frequency range must be around 500 kHz, for the equivalent circuit is only valid in the frequency range set by resonance and antiresonance frequencies. Further resonance frequencies appearing at higher values are considered to be undesirable. As far as the converter is concerned, there are several points that should be kept in mind. • It would be advantageous if the selected topology could cope with all the PT parasitics, and even include them. • The PT gain varies as a function of frequency, defining a very narrow optimum operating range due to its gain and efficiency characteristics. • Since the operating frequency is relatively high, it is nec- essary to get soft-switching condition in the switches in order to minimize losses. • The lower the number of additional components, the better. In the following sections, a topology suitable for a dc/dc converter driving a PT with a very simple control method is presented. As an application, an ac/dc adapter has been built (110 V /12 V , 8 W; 500 kHz). Manuscript received December 19, 2001; revised November 25, 2003. Ab- stract published on the Internet May 20, 2004. This work was supported by the European Union. This paper was presented at the IEEE Applied Power Elec- tronics Conference and Exposition, Anaheim, CA, March 4–8, 2001. The authors are with the Área de Tecnología Electrónica, Universidad de Oviedo, Gijón 33204, Spain (e-mail: viti@ate.uniovi.es). Digital Object Identifier 10.1109/TIE.2004.831739 II. TOPOLOGY SELECTION AND CONTROL As can be seen in Fig. 1, the equivalent circuit of the PT con- stitutes a bandpass filter [1]. The optimum way to drive it is by means of sinusoidal waveforms, so that high current spikes at its input are avoided. The topology proposed in this paper (Fig. 2) is formed by two switches in a half-bridge configuration, one in- ductor, and the PT. This topology is completed in the secondary side with the simplest possible output stage: a full-bridge recti- fier and a filter capacitor [8]. The operation mode is very simple (see Fig. 2). A 50% duty cycle, square waveform is generated by the inverter. This wave- form is applied to the inductor ( ) plus PT set. The inductor, together with the PT input capacitance ( ), forms a filter that results in a sine plus a dc level applied to the PT input [3]. Since the dc level has no influence on the PT behavior, only the sine is transferred to the secondary side, where it is rectified and filtered. As far as the PT is concerned, it should be noted that a ring-shaped PT 1 operating in thickness-vibration mode has been used. The PT has been chosen to be multilayer in order to obtain a suitable conversion ratio (approximately 8 : 1). Since the operating frequency is high, the piezoelectric transformer is a low-size device. With the topology chosen, the desired converter performance is achieved by including a few additional components around the PT (a couple of switches, an auxiliary inductor, and a simple output stage). It should be noted that the auxiliary inductor ( ) acts not only as a filter, but also as a way to enable soft switching [2], [6]. As can be seen in Fig. 2, choosing the right value for this inductor, it is possible to handle only the reactive energy needed to charge/discharge the switches output capacitance (together with ). Moreover, the inductor helps reduce electromagnetic interference (EMI) [9]. Regarding output voltage control, it can be carried out by mod- ifying the switching frequency, which is a traditional way to per- form this control in resonant converters. However, the response of PTs (in terms of gain and efficiency) is so dependent on fre- quency, and the optimum frequency range for the device is so narrow, that it would be desirable to have the converter working at a fixed frequency rather than varying it. Thus, we have chosen to drive the PT using a constant frequency [4] (the optimum one), and control the topology in quantum-resonant mode. In this control method, the switches in the power stage are so operated that a constant-frequency constant-duty-cycle voltage waveform is applied to the circuit formed by and the PT 1 Patent owner: Limiel, Klaus Brebol 0278-0046/04$20.00 © 2004 IEEE