Analysis and Design of Integrated CMOS Energy Harvesting Systems

One activity within the HE4T project has been the development of an integrated energy harvesting system designed for low-power energy sources and ultra-low-power applications. Designed during an M.Sc thesis project at LTU, the system was developed to offer support for an ultra-low-power micro controller from the Texas Instruments MSP430 series. The project also included a comprehensive literature review of recent advancements in capacitive and inductive DC-DC converters, as well as other essential system blocks for energy harvesting systems. Building on insights from the literature review, implementations of integrated designs of several key system blocks using a 180 nm process were proposed. These blocks included a boost converter, level shifters, and a pulse-width modulation (PWM) network consisting of a comparator and an oscillator. While time constraints prevented the design of all system blocks, references to existing market solutions and research were provided for those omitted.

Block diagram for the proposed energy harvesting system.

Simulations revealed significant losses in the boost converter stage due to parasitic effects from the non-ideal inductor. This issue was mitigated by increasing the switching frequency, which reduced inductor parasitics by allowing a smaller inductor value. The boost converter achieved an efficiency of 88.36% when boosting from 500 mV to 2 V with a 120 μW load, and it maintained high efficiency down to input voltages as low as 300 mV. For the PWM circuit, losses were noted due to current spikes in the buffering stage. These were addressed by incorporating higher voltage threshold transistors and cascaded buffers with smaller widths. Simulations at 1 MHz showed a power consumption of 5 μW, with the duty cycle adjustable between 28% and 91%. The final system, composed of the designed blocks and estimated performance for the omitted blocks, achieved an overall efficiency of 71.3%. Future work involves designing the remaining blocks, completing the layout, and performing parasitic extraction to assess the full system's performance under real-world conditions.

Diagram showing the power distribution to load and main parts of the energy harvesting system.

You can read the whole thesis from here: https://ltu.diva-portal.org/smash/record.jsf?pid=diva2%3A1879972&dswid=-5791