Power Extraction From an Ultra-Low Input Voltage Source in a Battery-less Thermoelectric Harvester

This paper presents a thermoelectic energy harvesting technique by sensing the temperature gradient between the human body and the ambient condition. With a temperature difference of 2-3 °C, the voltage available at the output of the thermoelectric harvester is merely 85 mV. To make it usable to CMOS circuits, this voltage needs to be boosted using a DC-DC boost converter. The key objective of this paper is to replace a motion-activated off-chip mechanical switch that is conventionally used in the boost converter with an integrated NMOS switch for starting-up the voltage conversion process. During start-up operation, the fully integrated control circuit produces a suitable level-converted ON-OFF signal with a swing of nearly 0–300 mV for the NMOS switch from the input voltage of 85 mV. This is achieved by using a start-up assisted ring oscillator topology that produces oscillations at a very low voltage. Subsequently, a series of clock booster stages, switch capacitor-based voltage booster stages and level shifter circuit are used to increase the voltage swing of the gate-drive signal for periodic switching of the NMOS switch. The proposed solution implemented in 180 nm CMOS process is capable of boosting the very low harvested voltage to nearly 1 V without requiring any external power supply such as battery or an off-chip mechanical switch.