Photovoltaic Energy Harvester with Power Management System

The reduction trend of modern microsystem power consumption is balanced by the constant growth of complexity. Traditional batteries are, therefore, no longer sufficient for many applications, since they cannot guarantee a long enough life time [1]. By harvesting energy from the environment, such systems could work for nearly infinite time without the need of replacing batteries. In this paper we present a photovoltaic energy harvester system with a power management system to handle the collected energy. In the realized solution a couple of integrated miniaturized solar cells are used as energy source. The block diagram of the proposed system is shown in Fig.1 Fig.1 Block diagram of the photovoltaic energy harvester A 1 mm integrated solar cell supplies an oscillator that provides the correct phase signals to eight Dickson charge pumps (only one charge pump is shown for simplicity). The storage capacitor CS is external, thus allowing us to choose the proper capacitance according to the operating time-slot required to supply the load. In order to control the charge status of the capacitor, the voltage across CS is monitored by an hysteresis comparator followed by a level shifter. This charge monitoring system is supplied by the second solar cell (auxiliary). When CS is fully charged, the level shifter turns on the p-MOS switch MS , thus connecting the load, discharging the capacitor and starting a new working cycle. In order to guarantee a stabilized voltage supply to the load we realized on-chip also a voltage regulator (LDO). The LDO, placed between the switch p-MOS switch MS and the load, is supplied only when the load is connected. The photovoltaic energy harvesting elements, implemented on-chip, are based on p-n junctions. Fig. 2 shows the cross-section and the equivalent circuit of the integrated micro solar cell [2]. Fig. 2 Cross-section and equivalent circuit of the integrated photovoltaic energy harvesting element The cell is realized with an n-well enclosing a p-diffusion, implemented with a particular geometry [3], in order to optimize the active area density and, hence, the efficiency. Fig. 3 shows the power curve of the realized cell, obtained with 300 W/m illumination. As the substrate must be shortcircuited with the n-well, the efficiency of the cell is, unfortunately, reduced by the recombination effect in the base of the parasitic pnp vertical transistor. Fig. 3 Output power curves of the integrated micro solar cell The proposed system has been realized in 0.35-μm CMOS technology. Fig. 4 shows the microphotograph of the chip. The chip area is 2.32 mm x 2.54 mm. Fig. 4 Microphotograph and layout of the system Fig. 5 shows the measured voltage VS, acquired over a time-slot of 15 s, obtained for different values of CS (47 nF, 100 nF and 147 nF). The system is illuminated with a light source, delivering about 300 W/m and the load is a 10 MΩ resistor. Fig. 5 shows also the measured curent through the 10-MΩ resistive load over a time-slot of 30 s for different values of CS (47 nF, 100 nF and 147 nF). The different peak values of the curves are due to the acquisition sampling frequency. Depending on the capacitance value, the energy available for the load and, hence, the operation duty-cycle changes. With CS = 147 nF, for example, the system provides the same current value as with CS = 47 nF, but for a longer time-slot. Fig. 4Measurement ofVS and of theload currentwith differentvalues of CS REFERENCES1. C. W. Seitz, “Industrial battery technologies and markets,” IEEE Aerospace and Electronics Systems Magazine,vol. 9, no. 5, pp. 10–15, May 1994.2. J. Ohsawa, K. Shono, and Y. Hiei, “High-voltage micro solar cell arrays of GaAs with output voltage up to 100 V”in IEEE/LEOS International Conference on Optical MEMS Conference Digest, 2002, pp. 103–104.3. M. Ferri, D. Pinna, E. Dallago and P. Malcovati, “Active area density optimization technique for harvesterphotodiodes efficiency maximization,” in Sensors and Microsystems, P. Malcovati, A. Baschirotto, A. D’Amico,and C. Di Natale, Eds., Springer, The Netherlands, 2010, pp. 117–120.