The growing number of energy-autonomous applications raises the need for reliable DC energy harvesting techniques such as Thermoelectric Generators (TEGs). One key issue, however, is the minimum voltage (40-60 mV) required for start-up in small TEG energy harvesting sources. We review in this paper recent start-up solutions for TEG energy harvesting technologies. Different solutions have been categorized into 5 main approaches: external battery, extra-fabrication-process-based, transformers, multisource energy harvesting, and DC-AC-DC conversion using oscillators. The ”DC-AC-DC conversion ring oscillators” approach is then shown to be the most promising solution in line with DC energy harvesting applications because it offers several advantages over other approaches, such as allowing full integration with good performance, compatibility with regular CMOS technology, and lower cost. Then, its different implementations are discussed and a detailed analysis is provided to identify their respective advantages and limitations.

Implementing fully integrated start-up energy harvesting (EH) systems is very challenging. The most popular start-up circuit implementations use a low-voltage ring oscillator (RO). The characteristics of this RO rapidly deteriorate at V _DD values below the transistor’s threshold voltage. A novel delay cell based on a Schmitt trigger (ST) inverter leveraging dynamic body biasing (DBB) is proposed to mitigate this deterioration. The RO of the proposed cell, implemented in a 22 nm FD-SOI CMOS technology, is compared against conventional and state-of-the-art circuits implemented in the same technology. Simulated results show the proposed RO achieves the highest output swing and DC gain within the 40 to 100 mV supply voltage range. A prototype chip comprising a 27-stage RO was fabricated and tested in a 22 nm FD-SOI CMOS technology. When fed by a 50 mV supply voltage, this RO produces an output swing of 80% of that voltage while oscillating at a 0.8 kHz clock frequency and consuming 300 pW. In reported experiments, oscillations started with V _DD as low as 32 mV. To the authors’ knowledge, the proposed CMOS RO operates with the second-lowest ever reported supply voltage. Combining its low power and low area, this RO is most suitable for EH systems with restricted power and area budget.

This paper proposes an on−chip, ultra−low−power cold−start circuit for a low−voltage, fast start−up of a boost converter for autonomous thermal energy harvesting from slight temperature differences. An improved charge transfer obtained from a Schmitt trigger (ST) clock booster enables voltage multiplication of the low input voltage during a cold start, providing low power consumption and a high boosting ratio. The start−up charge pump operates with an on−chip clock generated by an ultra−low−voltage ring oscillator that comprises modified Schmitt trigger (ST) delay cells, as previously reported by the authors. The proposed start−up architecture combines a previously reported advantageous one−shot control mechanism to start the boost converter efficiently. Theoretical analysis supports the proposed start−up scheme, and it is implemented in 22 nm FD−SOI CMOS technology. Simulation results show that, when combined with an inductive boost converter, the proposed harvesting system starts operating in 1.2 s when harvesting energy from a 48 mV DC source. To the best of the authors’ knowledge, 48 mV is the lowest reported minimum start−up voltage for a simulated fully integrated start−up circuit operating without additional sources of energy. The complete primary boost converter that was simulated requires a standard 82 μH off−chip inductor.