Is ultra-low power the way forward?

IEC e-Tech

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This new emerging field is particularly energy efficient and promising. While some standards for it already exist, more will be required.

The number of devices connected to the internet is projected to reach 40 billion worldwide as we move into the era of the Internet of Things (IoT) and, even more so, what some pundits call the Intelligence of Things. Forecasts suggest this could put a strain on global energy use and count for as high as 25% of all energy consumption as soon as 2030. Currently, connected devices do not compare to power-hungry sectors such as transport or buildings, at an only five to seven estimated percentage of total energy consumption. But with the huge predicted growth, energy requirements are expected to escalate.

This increased demand was anticipated by some researchers theorizing the IoT a couple of decades ago. They foresaw that the exponential rise of the IoT and the concomitant increase in energy used to power billions of microelectronic devices required a new approach. And one of the results of this new approach is the burgeoning field of ultra-low power microcontroller, or ULP MCU, systems.

“It became apparent that for the IoT to be realized, we couldn’t continue to use traditional and in some cases energy-intensive approaches for electronic devices,” says IEC expert Leszek A. Majewski, who chairs the technical committee which prepares standards for printed electronics, IEC TC 119. He is also a lecturer in electrical and electronic engineering at the University of Manchester and has a PhD in the development of low-voltage organic field-effect transistors. “Although lithium-ion batteries and lithium-polymer batteries are currently being developed with smaller form factors and greater energy storage capacity for gadgets like smart home sensors, the increase in demand for new microelectronic applications in industrial monitoring, healthcare and space exploration requires approaches based on new materials and structures where batteries are not necessarily the answer,” he confirms.

ULP MCU systems fit in with this new approach. They are deemed essential to facilitate the growth of the IoT and the success of new applications which require extended operation without frequent charging or battery replacement and are often in discrete form factors.

A booming market and a hot topic

The value of the global ULP MCU market is forecast to hit USD 15,27 billion in 2030, driven by the rising adoption of devices such as consumer wearables, medical monitors and IoT sensors. “It’s a very hot topic and also a pretty wide field in terms of the new materials and techniques being explored,” adds Majewski. “In order for microelectronic devices to work, they need to be on pretty much all the time. Depending on the function, always-on devices would have to draw a minimum amount of power to stay on as long as possible. You don't want to have to change them in the field. You want to minimize maintenance.”

Consequently, ULP MCU devices need to be operated with very low supply voltages of 1 V or lower and consume minimal power, typically measured in milli- or microwatts. This significantly decreases power consumption amid rising energy costs.

Engineering challenges for ULP design

Since a lower-power device cannot generate, store or transmit vast amounts of data, the principal limitation is that its functionality needs to be simple. According to Majewski, depending on use, the design of a ULP device will often have to balance size and reliability against energy efficiency and performance.

“The range of the device is limited since the signals cannot be sent far, but you accept that this is an inherent property of a low-power device that you design for. You do not expect it to deliver a huge output. So, it must be something really basic with one or two parameters. Many of these devices are for new use cases that we would never imagine without low power.”

Ingestible devices are one of the applications

One example are ingestible electrochemical sensors, which can be swallowed to monitor health and detect disease as they pass through the body. Electronics can now be directly integrated into moulded plastic objects and devices. In-mold electronics (IME) is driven by the automotive industry because it “significantly reduces the cost, weight, waste and energy required to produce vehicle interior parts,” according to the group which has standardized its development.

IMEs can include all the surface-mounted devices included in traditional electronics to increase functionality, such as sensors, LEDs and microcontrollers. “You could integrate an array of micro low-power devices, like a matrix of transistor-based sensors, which can increase an application’s sophistication and capacity but will also increase power consumption,” Majewski says.

One avenue of enquiry proving particularly beneficial to wearables is that of e-textiles. Consisting of woven networks of flexible fibres, e-textiles can be readily deformed into stretchable, flexible form factors. This makes them ideal for use in wearables like smart watches which require motion tracking of human body’s physical or mechanical movements.

Rather than using hard substances like silicon to make transistors, organic soft matter is an emerging field of research. “Skin-like soft electronics offer conformal, stable interfaces with biological tissues – including skin, heart, brain, muscle and gut – enabling health monitoring, disease diagnosis and closed-loop therapeutic interventions,” researchers explain.

Energy harvesting works for ULP devices

Energy harvesting is the process of capturing and converting energy from the environment into electrical power, in principle as a perpetual and sustainable power source. It is a particularly energy efficient source of energy, as it can be even derived from body movement or heat. “There are a variety of methods to achieve it, and each one will convert the source power into usable energy in a different way,” explains Majewski. “Energy can be harvested from radio waves via a radio frequency (RF) antenna or heat via an infrared (IR) optical rectenna, for example.”

Thermal sensors on vehicles can harvest the radiant heat from the road surface. Other sensors on moving vehicles could obtain power from the motion energy “if placed in high-vibration locations, such as near the wheels or engine components.”

Similarly, energy for wearable devices can be powered by the kinetic movement of the body (piezoelectric) or from body heat or body fluids. Research shows that body-powered kinetic motion could add 10 mW to the primary power source for ULP MCUs.

“In the design of e-textiles you would take account of energy generation from a variety of mechanisms including thermoelectric generators that harvest body heat or you could use materials that incorporate solar cells. The use of piezoelectric mechanisms show particular potential for wearables since just the basic squeeze of the material will generate energy,” Majewski explains.

“However, all of these technologies are currently limited in terms of the amount of energy they are able to generate and in the consistency of energy generation. Consequently, energy harvesting for ULP MCUs is currently for use in limited applications, such as augmenting [extending] battery life,” he adds.

Wireless charging and data transmission

A number of wireless solutions for power delivery are gaining market adoption. These include the Wireless Power Consortium’s Qi standard (Qi2 and Qi2 25W versions) for wireless charging of mobile, handheld electronic devices and NFC Wireless Charging (NFC WLC) supporting lower power 1 W applications over a distance of 2 cm. Backed by the NFC Forum, the latest NFC wireless charging specification supports Qi induction charging platform, which delivers up to 15 W over a distance of 4 cm.

“Devices can be powered using near field communication (NFC) via an RF type of antenna,” explains Majewski. “The device is dormant, and until it is activated for a short period of time, it turns itself off again.” Technologies like Bluetooth Low Energy (BLE), Wi-Fi 6 and Zigbee are already designed to minimize radio-on time and therefore keep power needs to a minimum.

Existing IEC Standards and new ones required

The IEC is also paving the way for this new technology and has embarked in this field within several of its technical committees. TC 119 standardizes materials, processes, equipment and products for printed electronics. The TC is working on the first edition of IEC 62899-202-13, which contains measures for the conductive layer in IME and tests of printed thin-film transistor-based pressure sensors.

Printed electronics technology is not only low-cost but sustainable, says Majewski. “It is generating a lot of interest in manufacturing circles. TC 119 is therefore a key source of safety and performance standards for such technologies.”

IEC TC 124 publications relate to wearable applications, and there is ongoing work on the standardization of low-power electronics, according to Majewski. Further, TS 60747-19-2 provides a guideline for the specifications of a low-power sensor allowing autonomous power supply operation. It also provides a guideline for specifications of the power supply to drive smart sensors in a smart sensing unit. It is published by IEC TC 47 which, among many things, standardizes discrete semiconductors and sensors. It also publishes the IEC 62830-1 series, which includes methods for evaluating the performance of vibration-based piezoelectric energy harvesting devices.

Standards for piezoelectric technology are also developed by IEC TC 49, which addresses piezoelectric, dielectric and electrostatic devices. This includes IEC TS 61994-5, which gives the terms and definitions for sensors, intended for manufacturing piezoelectric elements, cells, modules and the systems.

ISO/IEC JTC 1/SC 41 is a joint subcommittee established between ISO and the IEC to standardize all aspects relating to the IoT, and therefore offers guidelines on the testing of IoT devices, including networks of sensors. “Standardization at the IEC tends to focus on test methods so that can we can ensure a particular device behaves as intended,” Majewski says. “But we need further research to consider the low-power options of new materials including, for example, human body tissues,” he concludes.

Energy – especially ultra-low-power forms of it – can indeed be sourced anywhere and everywhere. This opens up new opportunities for standardization bodies, but also for our future energy requirements in the race to meet our net-zero targets.