Nishit Harshad Shah

From Murmann Mixed-Signal Group

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Research: Energy Harvesting for CMOS Imagers

Email: nishits AT stanford DOT edu

With a swarm of portable devices being deployed ubiquitously, there is an emerging need to power these devices without the aid of bulky batteries and long copper cables. Energy harvesting from various alternative sources can dramatically reduce the size and improve the life of the battery.

This project focuses on harvesting energy from ambient light. It finds applications in all image sensors that are not actively capturing images. The photodiodes of image sensors can work as miniature solar cells and deliver photoelectric energy [1-2]. This harvested energy is initially stored in on-chip capacitors and later boosted to battery voltage or to on-chip power supply voltage as shown in Fig.1.

Murmann Webpage 2014 Fig1.JPG

A test chip had been taped out and energy harvesting capability of different photodiodes has been characterized. The energy harvested is proportional to the incident light energy. The test-chip layout and the measured IV-curve under office lighting are shown in Fig. 2. The measured harvested power is around 1µW/mm2 at an efficiency of 2%. 
Murmann Webpage 2014 Fig2.JPG

The current focus in the project is towards building an integrated DC-DC converter that can harvest incident energy and deliver power at required voltage level. Various different topologies are being evaluated to get the best power conversion efficiency.

[1] A. Fish, S. Hamami and O. Yadid-Pecht, “CMOS Image Sensors With Self-Powered Generation Capability”, IEEE Trans. Circuits Syst. II, Express Briefs, Vol. 53, No. 11, November 2006.
[2] S. U. Ay, “A CMOS Energy Harvesting and Imaging (EHI) Active Pixel Sensor (APS) Imager for Retinal Prosthesis”, IEEE Trans, on Biomedical Circuit and Systems, Vol.. 5, No. 6, December 2011.

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