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A power management system that powers itself


Shaking hands, going out for a walk, cycling to work. For several years, activities like these have been the target of a specific class of device manufacturer – those working on wearable technologies that harvest ‘wasted’ thermal and kinetic energy and deploy it elsewhere. The ultimate goal is to make them self-powered, eliminating the need for batteries. These small-scale generators are much smaller than traditional energy harvesters, and because they’re worn on the body, changes in weather have a minimal impact. Some of the most promising to date have been triboelectric nanogenerators (TENGs), which are cheap, simple, and robust, and they offer high voltage output and high power density. But the intermittent nature of harvested energy means that TENGs require a power management system (PMS) to make their output compatible with other electronic devices. While several PMS designs have been successfully developed, they cannot work without their own independent power supply.

A team at Georgia Tech aim to change that. Writing in Nano Energy [DOI: 10.1016/j.nanoen.2020.104642], they have proposed a design for a new PMS that is powered by the TENG itself, opening the door to truly self-powered devices. The authors say that the key to this has been “employing the unique properties of discrete semiconductor devices.” Namely, a silicon-controlled rectifier (SCR) to act as a switch, and Zener diodes, which when exposed to a sufficiently high voltage, allow current to flow in both forward and reverse directions. This combination allows them to extract the maximum amount of energy from the TENG, while also controlling the power flow from the TENG to the connected device.

They started by modelling an ideal circuit design based on a buck converter, but replaced the switch with the SCR and Zener diodes. This works because once the voltage through the input capacitor exceeds the breakdown voltage of Zener diode, the diode starts to reverse conduct, triggering the SCR which responds instantly by dropping its voltage to zero – like the close of a switch. This drops the input capacitance voltage to nearly zero, which allows current to flow through another diode, transferring the energy to the output capacitor directly connected to the load.

As in any practical device, this PMS experienced some power loss. The team found that in the SCR, it was closely related to the inductance value, and it occurred in its ‘turn on’ interval. Power loss on the diode mostly occurred when it was forward biased. They used this analysis to build a prototype PMS circuit, connected to a contact separation-mode TENG activated by finger tapping.

The PMS was shown to be fully functional, successfully converting the “pulse-like output of a TENG to (a) steady voltage that can be used to supply power for conventional electronics and sensors.” Its overall energy conversion efficiency was found to be 84.3%, when comparing the maximum output power of the TENG with and without the PMS. The authors conclude that “This innovation will be a breakthrough in the development of self-powered mobile devices, IoT devices and sensors.”

This story appeared in Materials Today on 9 April 2020:

Research paper ($): William Harmon, David Bamgboje, Hengyu Guo, Tingshu Hu, Zhong Lin Wang. “Self-driven power management system for triboelectric nanogenerators”, Nano Energy 71 (2020) 104642. DOI: 10.1016/j.nanoen.2020.104642