Efficient Solar Power Management System for Self-Powered IoT Node
An efficient micro-scale solar power management architecture for self-powered Internet-of-Things node is presented in this paper. The proposed architecture avoids the linear regulator and presents a complete on-chip switched capacitorbased power converter in order to achieve higher end-to-end efficiency. Unlike traditional architectures, where the harvested energy processes twice, the proposed architecture processes the harvested energy only once before it reaches to the load circuit, irrespective of the ambient conditions. The system efficiency has been improved by ∼12% over the traditional architecture. 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 photovoltaic cells, to validate that the proposed power management system is practically realizable.
One of the key issues with this architecture is that the overall efficiency varies with ambient condition. Because the first switched capacitor DC-DC converter has a fixed voltage up-conversion ratio and its output voltage varies with ambient condition. As a result, the drop across LDO varies with ambient condition and much of the extracted power may simply be dissipated within the LDO, and hence, the overall efficiency will degrade accordingly. Another key issue is that often it utilizes two DC-DC converters even if there is sufficient ambient energy to power the load circuit. In order to address these issues, an efficient architecture has been proposed with following silent features: 1) the overall efficiency varies minimally with ambient condition, 2) unlike traditional architecture, input energy processes once before it reaches to the load circuit, and hence, overall efficiency has been improved by ∼ 12%, and 3) avoid inductive and linear converters, but presents a completely on-chip switched capacitor based architecture.
The proposed power management architecture and the associated control signals are shown in Fig. 2. It comprises a switched capacitor boost converter to bring the output voltage several times higher than the input voltage, a current-starved voltage control oscillator (CS-VCO) to generate switching frequencies for the boost converter, a control unit (CU) to regulate the load voltage, a buffer stage to store the excess energy for future references, and an application stage to deliver the load. Due to the high power density and ubiquitous nature of light, a PV cell has been chosen as an energy source for powering the IoT node. The buffer stage has two modes: one is storage mode and other is converter (DC-DC) mode. The storage mode will be activated once there is more than enough ambient energy to supply the load and converter mode will be activated once there is insufficient ambient energy to supply the load. During the storage mode, it will store the excess energy into the buffer capacitor CB and during the converter mode, it will act as a linear charge pump circuit, which will pump the stored energy into the application stage. The CU comprises a reference generator associated with a start-up circuit, analog comparators, and few logic gates to generate the required control signals for the system.
An efficient on-chip power management architecture for solar energy harvesting system is presented, which utilizes a single stage DC-DC converter when there is enough ambient energy for maintaining regulation at both the input and load. The proposed architecture utilized the stored energy to maintain regulation when there is insufficient ambient energy to supply the load requirement. The proposed architecture avoids linear regulator and utilizes simple charge pump concept in order to maintain regulation. By utilizing a switching converter instead of a linear regulator, the proposed scheme achieved higher end-to-end efficiency. Simulation, as well as experimental result, are reported to validate the proposed idea.
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