Integrated Reciprocal Conversion With Selective Direct Operation for Energy Harvesting Systems

Abstract

Abstract-Energy harvesting IoT systems aim for energy neutrality, i.e. harvesting at least as much energy as is needed. This however, is complicated by variations in environmental energy and application demands. Conventional systems use separate power converters to interface between the harvester and storage, and then to the CPU system. Reciprocal power conversion has recently been proposed to perform both roles, eliminating redundancy and minimizing losses. This paper proposes to enhance this topology with 'selective direct operation', which completely bypasses the converter when appropriate. The integrated system, with 82% bidirectional conversion efficiency, was validated in 65nm CMOS with only the harvester, battery and decoupling capacitors being off-chip. Optimized for operation with cm 2 photo-voltaic cell and a 32-bit sub-threshold processor, the scheme enables up to 16% otherwise wasted energy to be utilized to provide >30% additional compute cycles under realistic indoor lighting conditions. Measured results show 84% peak conversion efficiency and energy neutral execution of benchmark sensor software (ULPBench) with cold-start capability.

E-Textile Technology Review–From Materials to Application

“…A limited number of subthreshold devices are now available commercially. Energy harvesting circuits may be integrated with these ultralow power microcontrollers, reducing the component count and hence the spatial dimensions of the system [104], although this integration has not seen significant commercial use. Another recent innovation is in transient computing: removing the need for energy storage by allowing systems to use power when it is available, and by making use of non-volatile memory to permit computation to span several power cycles [105], at a potential spatial cost.…”

Energy-driven computing

“…Greater end-to-end efficiency is possible for specialized circuits, e.g. 82% for a reciprocal converter with selective direct operation that tailors to the specific characteristics of subthreshold CMOS processors and tiny energy harvesters [31].…”

Energy Neutral Sensor System With Micro-scale Photovoltaic and Thermoelectric Energy Harvesting

“…For such conditions, SDO exploits the ultra-low operating voltage of subthreshold CPU systems to extend battery capacity by 1) direct operation of logic from harvester thereby eliminating conversion overheads and 2) avoiding battery discharge during these periods. SDO has been successfully demonstrated [8] with the MinE system executing an industry standard sensor benchmark software with ability to cold start at indoor conditions and charge a 1.2V NiMH battery when sufficient ambient energy is available.…”

“…Note that conversion losses increase with conversion ratio and prior works aim to eliminate conversion stages under low input power conditions [6] [7]. A related technique, Selective Direct Operation (SDO), has been demonstrated [8] with Photovoltaic (PV) cells to exploit the low operating voltage of MinE systems and minimize power conversion losses. In this work, fabrication details of the high-efficiency PV cell are provided and the proposed SDO technique is extended to thermoelectric generators (TEGs) as an alternate source of energy harvesting.…”