Research & Development Ultra-low power technology for battery-less IoT sensors
To reach its full potential, Internet of Things (IoT) devices needs battery-free and maintenance-free endpoints. Only smart wireless modules can be used problem-free in all situations and environments.
Battery-free devices with smart sensors that use energy harvesting to charge themselves with energy from the environment have been available for some time now. Current developments in extremely power-saving ICs sensors and radio technologies, for example, Bluetooth Low Energy (BLE), simplify the construction of battery-free IoT nodes with a compact form factor.
How much energy a device requires depends on its system performance and the time it takes it to capture and transmit data. Bluetooth Low Energy and similar protocols, such as ZigBee Green Power, are optimized for short frame duration and low transmission power with sufficient security. The device can to transmit a complete data frame in approximately ten milliseconds or less with each of its protocols.
If the sensor node subsystem is, for example, 10 mA at 1 V, then the sensor’s required energy budget can be calculated like following: 1 V x 10 mA x 10 milliseconds = 100 µJ per process. It is necessary that the energy harvesting subsystem is reliable in its energy generation throughout its entire activity cycle. If this activity interval is between one and ten seconds, the harvester source must operate extremely effective. It’s a tough, but not an impossible, task. Standard solar cells with a size of 6.5 or 13 cm2 and a conversion efficiency of only four percent can meet this requirement.
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The prerequisite is that the system design already includes sensors with extremely low power consumption. Sensors based on the latest MEMS technology meet this requirement through a combination of advanced mechanical design and high functional density. Two examples are Bosch’s sensors BME280 and BMA400. The BME280 environmental sensor combines pressure, temperature, and humidity sensors in an all-in-one device with low power consumption. In addition, the BMA400 is the industry's first three-axis accelerometer to provide extremely low-power operation without having to make compromises in performance. By combining such devices, it is possible to develop a highly energy-efficient multi-sensor platform with additional inertial sensors for applications such as air conditioning, process monitoring, product tracking, or intrusion detection.
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To meet the requirements of data processing and radio subsystems, ON Semiconductor has combined a combination of ultra-low power technologies in its RSL10 System-in-Package (SIP). The device includes a wireless SoC, an integrated antenna, integrated power management, and all necessary passive components. The RSL10’s energy consumption is extremely low, only 62.5 nW in deep sleep mode and 7 mW in receive mode.
The SiP device operates from a supply voltage of 1.1 V and has integrated Flash and RAM. In addition, it supports firmware over the air (FOTA) updates with memory protection. It’s also certified to worldwide standards, including CE/EU and FCC/US. ON Semiconductor has combined its RSL10-SiP with the low-power BME280 and BMA400 sensors and the NCT203 digital temperature sensor with temperature monitor to form a solar cell multi-sensor platform (image 1). This battery-less sensor node connects to a hub, such as a gateway or smartphone, via Bluetooth Low Energy. Source code, circuit diagrams, and Gerber files are supplied by the supplier so that they can be adapted to your own application.
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The ultra-low power components in the RSL10 multi-sensor platform enable the acquisition and transmission of environmental and inertial data in less than ten milliseconds with an average power consumption of approximately ten mA. Energy harvesting from the environment is usually limited. As a result, the system must be designed to have sufficient time for "energy harvesting" before its next activity cycle. The sensor’s activity cycle and its energy generation are related by the so-called amplification factor. Storing energy over a period of, for example, one second and operating the sensor for ten milliseconds results in a gain factor of one hundred. Energy harvesting for ten seconds and recording and transmitting for five milliseconds results in a necessary gain factor of 2,000.
Based on these figures, the energy harvesting system of the RSL10 solar cell multi-platform must provide a power source of 10 mA/100 = 100 µA or 10 mA/1000 = 10 µA to transmit at one or the second intervals. This information helps select a suitable solar module to power the platform. The module can then connect to the platform via an integrated two-port connector.
A suitable solar module is, for example, Ribes Tech’s FlexRB-25-7030. This solar module meets the requirement almost perfectly by providing 16 µA at 200 lux or 80 µA at 1000 lux. 200 lux is a typical natural light intensity in Northern Europe in the afternoon during a cloudy winter day. Brighter sunlight, additional artificial light sources, outdoor installation, or a nearby window can increase the light intensity by several hundred lux.
The ambient energy gained is usually stored in a capacitor or a rechargeable battery. Capacitors with their lower energy density store less energy per volume than rechargeable batteries (image 2). Therefore, a secondary cell is preferred in a solar-powered application. Then no ambient light is expected, they need to remain active for long periods. How long it takes for different batteries to charge and discharge must be taken into account when choosing what right energy storage for a device.
A battery needs protection against overcharging and deep discharge. This increases the switching effort and requires the use of components such as switching regulators, integrated circuits, and additional external components. A capacitor with a suitable rated voltage requires no charging circuit or protective components. That said, reliable regulation of the output voltage is always required.
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The RSL10 sensor platform features a flat 47µF storage capacitor. The voltage is regulated by ON Semiconductor's NCP170, a simple low dropout linear regulator (LDO) with an extremely low quiescent current that minimizes power loss. Today, developers must device weather the integrated devices should have either a low minimum input voltage or a wide supply voltage range to support simple control.
The storage capacitor can be used in places where ambient lighting is strong and lengthy dark periods aren’t expected. Continuous operation is possible under these circumstances. The module is equipped with a pre-installed beacon firmware that transmits sensor and system status data such as the voltage level of the capacitor and uses the beacon mode of Bluetooth 5. The firmware is compatible with iOS or Android BLE scanner applications.
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The gap between the power requirements of embedded systems and the energy generated by energy harvesting systems is closing. Harvesting technologies are more effective today and can harness ambient energy better than ever before. At the same time, new semiconductor technologies with extremely low power consumption are entering the market regularly. Wireless transmission protocols such as Bluetooth Low Energy are also being further optimized. Used together, these technological innovations enable battery-free IoT endpoints to become reality.
This article was first published in German by Elektronik Praxis