Fully integrated ultra low voltage cold start system for thermal energy harvesting . . .

Abstract

Wireless sensor networks used in various monitoring and sensing applications rely on energy harvesting for battery-less operation, as it minimizes the need for human intervention, and offers long term monitoring solutions. Typical energy harvesters use high efficiency boost converters, which are able to step-up voltages from as low as 10 mV. However, they often need > 200 mV in order to start up initially. Current solutions for achieving a low voltage start up require the use of bulky off-chip transformers, leading to undesired area overhead. This research work presents proof-of-concept for a fully integrated start-up system, which can cold-start from < 50 mV using on-chip magnetics, and also be used as an energy harvesting charger for ultra low power applications. The use of lossy on-chip transformers in a Meissner Oscillator compared to high-quality off-chip transformers pose new design and optimization challenges. Hence, we have derived intuitive analytical expressions that are well-suited for use with the on-chip magnetics, and used them to co-optimize the oscillator components. An optimized depletion mode MOS transistor was fabricated and tested with an off-chip transformer, to exhibit oscillations from <3 mV DC input voltage. An optimized on-chip transformer, 36x smaller in area than the off-chip transformers, is currently awaiting layout and fabrication. A switched capacitor DC-DC circuit has also been designed, which can rectify and boost up the oscillator's output voltage to 1.2 V, to have a complete start-up system for energy harvesting.