2188 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 8, AUGUST 2010 An Efficient AC���DC Step-Up Converter for Low-Voltage Energy Harvesting Suman Dwari, Student Member, IEEE, and Leila Parsa, Member, IEEE Abstract���The conventional two-stage power converters with bridge rectifiers are inefficient and may not be practical for the low-voltage microgenerators. This paper presents an efficient ac- to-dc power converter that avoids the bridge rectification and di- rectly converts the low ac input voltage to the required high dc output voltage at a higher efficiency. The proposed converter con- sists of a boost converter in parallel with a buck���boost converter, which are operated in the positive half cycle and negative half cy- cle, respectively. Detailed analysis of the converter is carried out to obtain relations between the power, circuit parameters, and duty cycle of the converter. Based on the analysis, control schemes are proposed to operate the converter. Design guidelines are presented for selecting the converter component and control parameters. A self-starting circuit is proposed for independent operation of the converter. Detailed loss calculation of the converter is carried out. Simulation and experimental results are presented to validate the proposed converter topology and control schemes. Index Terms���AC���DC conversion, boost converter, energy harvesting, low power, low voltage, power converter control. I. INTRODUCTION S ELF-POWERED devices harvest the ambient energies by microgenerators and can perform their operations without any continuous external power supply. Many types of micro- generators, used in the self-powered devices, are reported in the literature for harvesting different forms of ambient ener- gies [1]���[8]. The inertial microgenerators, which harvest me- chanical energy from the ambient vibrations, are currently the focus of many research groups [2]���[6], [8]���[13], [16]���[21]. The power level of the inertial microgenerators is normally very low, ranging from few microwatts to tens of milliwatts. Based on the energy conversion principle, the inertial microgenerators can be classified mainly into three types: electromagnetic, piezo- electric, and electrostatic [5]���[10], [12]���[16]. Among them, the electromagnetic microgenerators have the highest energy den- sity [8], [9], [20]. In this research, the electromagnetic micro- generators are considered for further study. The electromagnetic generators are typically spring-mass- damper-based resonance systems (see Fig. 1) in which the small amplitude ambient mechanical vibrations are amplified into larger amplitude translational movements and the mechanical Manuscript received June 26, 2009 revised October 27, 2009. Date of current version July 16, 2010. This work was supported in part by the National Science Foundation. Recommended for publication by Associate Editor S. Y. (Ron) Hui. The authors are with the Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180 USA (e-mail: dwaris@rpi.edu parsa@ecse.rpi.edu). Digital Object Identifier 10.1109/TPEL.2010.2044192 Fig. 1. Schematic diagram of a resonance inertial microgenerator. energy of the motion is converted to electrical energy by elec- tromagnetic coupling [9]. The output voltage of an electromag- netic microgenerator is ac type, but the electronic loads require dc voltage for their operation. Therefore, the ac voltage of the electromagnetic microgenerator output has to be processed by a suitable power converter to produce the required dc voltage for the load. One of the challenges with the electromagnetic microgen- erators is that, due to the practical size limitations, the output voltage level of the generators is very low (few hundreds of millivolts), whereas the electronic loads require much higher dc voltage (3.3 V) [9]. The conventional power converters, re- ported for energy harvesting [2]���[7], [10], [11], [14]���[18], [20], [22], mostly consist of two stages: a diode bridge rectifier and a standard buck or boost dc-to-dc converter [see Fig. 2(a)]. However, there are major disadvantages in using the two-stage power converters to condition the outputs of the electromag- netic microgenerators. First, for very low-voltage electromag- netic microgenerators, rectification is not feasible by the use of conventional diodes. Second, if the diode bridge rectification is feasible, the forward voltage drops in the diodes will cause a large amount of losses and make the power conversion very inefficient. To address the problems of the conventional two-stage con- verters, direct ac-to-dc converters are proposed [10], [13], [15]. In these converters, bridge rectification is avoided and the micro- generator power is processed only in a single-stage boost-type power converter [see Fig. 2(b)]. A dual-polarity boost converter topology for direct ac-to-dc power converter is reported in [10]. In this converter, the output dc bus is split into two series- connected capacitors and each of these capacitors is charged only for one half cycle of the microgenerator output voltage. As the time periods of the resonance-based microgenerators��� output voltages are normally in the order of milliseconds, very 0885-8993/$26.00 �� 2010 IEEE