Energy Harvesting Systems Create Power Path Challenges
Harvesting energy from the environment is an increasingly popular means of sourcing power for integrated circuits. The technology has advanced sufficiently over the past decade that harvested energy is now commonly used for powering ICs in environments where battery or grid power is impractical. This includes medical implants, wireless sensors, remote monitoring applications, wearable devices, and specialized sensor nodes on the Internet of Things (IoT). The image to the right shows the application for smart labels and packaging.
The use of harvested energy makes perfect sense in an environment filled with ambient energy, both human generated and naturally occurring.
Commonly used sources of harvested energy include:
- ambient electromagnetic waves (RF, microwave),
- heat gradients,
- vibration and motion
Using harvested energy to power ICs comes with it's own set of challenges, many of which fall directly on the circuits in the power path. These circuits must efficiently retrieve energy from the harvesting sensor (e.g. an antenna) and convert it to power that is usable by the IC. They must do this despite the fact that the harvested energy is variable, often of low magnitude, and intermittent.
This creates a number of design problems, including:
- impedance matching between the energy sensor and the power converter can be difficult, given the variable nature of the power supply,
- loads (i.e. applications) that require more power than is currently being harvested must be gracefully accommodated,
- conversely, when the harvested energy exceeds the requirements of the load, the extra power must be utilized or dissipated in a way that avoids overheating the IC,
- power-on-reset and startup require special consideration, especially if the power-on-reset sequence requires more energy that normal operation.
Power paths driven by traditional power sources, e.g. batteries, regulated line voltage, etc. avoid these problems, because the power supply is typically stable under normal conditions. Impedances are more easily matched, which allows operating efficiencies to be achieved in a more straightforward manner. Furthermore, specialized systems to handle variable power are not usually required.
Addressing the Challenges
Intrinsix develops energy harvesting solutions for multiple markets, including biomedical applications, wearable devices, and the Internet of Things. Based on years of experience working with these systems, we have developed specialized IP and techniques for addressing the challenges presented by energy harvesting applications.
At a basic level, energy harvesting systems consist of an energy sensor (antenna, solar cell, piezoelectric transducer), and a power conveyer unit that transforms the energy into useful power that can then be consumed by a application dependent system load. Storage of some sort, either a capacitor or a small battery, is usually included. Fundamentally, energy harvesting systems are power defined sources – the amount of power they can deliver is determined solely by the amount of energy that is being harvested at any point in time, which is typically variable. The throughput level is determined by the harvested input power, not by the demands of the load, as one would encounter in a battery or line powered system. This creates design problems that are discussed below.
For conventional power sources, the power throughput is maximized by reflecting the load impedance back to the power source. However, this won’t work in an energy harvesting system, because the harvested power, and thus the matching impedance is variable. Instead, it is necessary to control the load, including energy storage elements, dissipative elements, and application circuitry, to prevent the demands from exceeding the delivered power. A variety of strategies can be employed to achieve this, depending on the requirements of the application. All involve managing and potentially rationing power consumption in the load.
Excess Power Consumption
Conversely, it is necessary to handle scenarios in which the harvested power exceeds the demands of the load. The excess power must be utilized, while maintaining the output voltage level at the load. This is typically achieved with a shunt regulator that will dissipate excess energy and prevent overvoltage at the energy storage element (capacitor or small battery). If overheating of the IC is a concern, the power level of the shunt regulator can be used to trigger a detuning mode in the harvester to reflect unwanted power instead of dissipating it as heat.
Despite the variable nature of the harvested power, and the need to manage power within the load, it is still necessary to match the impedance of the power sensor with that of the power converter in order to achieve maximum power throughput. Because the conveyed power is variable, a dynamic mechanism called maximum power point tracking (MPPT) is utilized. This will be addressed in more detail in a subsequent blog.
Applications for energy harvesting will continue to grow, driven by advances in low power IC technologies, and the emerging demand for miniaturized self powered devices in remote, inaccessible, or highly dispersed locations. We expect to see increasing demand for these systems, and are aggressively developing IP that will help our customers jump start their energy harvesting projects.
- Smart Bandage photo courtesy of Cymbet Corporation, as presented at the Power Forum Conference (sponsored by Avnet).
- Smart Label image courtesy of Flexible Packaging online article, “Electronic Packaging, Smart Labels, the Internet of Things & the Manufacturing Paradigm,” retrieved from https://www.flexpackmag.com/articles/88018-electronic-packaging-smart-labels-the-internet-of-things-the-manufacturing-paradigm.