Micro-scale power management interface circuits for IoT node

There is a great interest in powering Internet-of-Things (IoT) nodes by scavenging ambient energy, such as solar radiation, thermal gradient, vibration, or radio frequency (RF) waves. Based on the applications and area of deployment, appropriate energy sources are utilized for powering the wireless-sensor node. As the ambient conditions are changing, one can not directly connect the harvester to the load as the power is not regulated. Moreover, the available scavenged energy may not be sufficient for powering the sensor nodes all the time. Apart from that, unlike a battery, a harvester has a continuous source of power without depletion, but if it is not extracted continuously, the same would be unutilized and lost. Therefore, an interface circuit in between the load and the harvester is needed, in order to maintain regulation both at the input and load. The first part of this work focuses on the development of on-chip switched capacitor based DC-DC boost converter for micro-scale energy harvesting systems. We present a novel idea of single-phase tree-topology charge pump (SPT-QP), which provides better charge transfer capability, reduced charge sharing time, less complexity, and minimum power transfer loss. A systematic analysis is presented to estimate the optimal parameters for maximizing the system efficiency. The proposed interface circuits have been designed using 0.18-μm CMOS technology node and the circuit simulations demonstrate that the proposed strategy offers peak efficiency 70%, under wide range of light intensities. The second part of the thesis deals with an on-chip photovoltaic power harvesting system with a low-overhead adaptive maximum power point tracking scheme for IoT nodes. The proposed scheme tracks the maximum power points within 12 μs by utilizing an inherent negative feedback loop, within a tracking error of 0.6%. The tracking range has been improved by ∼57% using a current-starved voltage controlled oscillator (CS-VCO) instead of a polynomial VCO. The overhead area and power consumed by this tracking scheme are approximately 0.013% and 0.1%, respectively. Using commercially available solar cell of area 11.3 cm, the proposed system can provide 833 μW of power with a light intensity of 600 lux. The proposed energy TH-1750_11610219 processing circuit has been designed using 0.18-μm CMOS technology node and the circuit simulations demonstrate that the proposed scheme can track maximum power point (MPP) under rapidly changing atmospheric conditions with a peak tracking efficiency of 99%. The final part of the thesis focuses on improving the overall system efficiency by a suitable architectural change. An efficient power management architecture for solar energy harvesting system is suggested. The proposed architecture utilizes a single DC-DC converter when there is enough ambient energy and two DC-DC converters when there is insufficient ambient energy, to supply the load requirements. By regulating the intermediate voltage, system efficiency has been improved by 9% when two DC-DC converter are used. This work also addresses the startup issue which is a major bottleneck encountered in any self-powered system. The entire power management system has been designed using 0.18-μm CMOS technology node and the circuit simulations demonstrate that the proposed architectural changes bring in a system efficiency of 82.4% under different light conditions. In addition to that, a hardware setup is created using commercially available ICs and solar cells, in order to validate the proposed power management system. The measurement results indicate that the system is practically realizable. xii TH-1750_11610219