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Wearables become more wearable


Graeme Clark, principle engineer, Renesas Electronics investigates how energy harvesting is helping wearables reach the next level


T


he worldwide market for wearable products has been growing strongly


for the last decade, as the semiconductors used to implement these products become faster and more powerful, while consuming less energy in ever-smaller packages. Despite a slowdown in the growth of some sectors, such as smart and sports watches, the market for wearable products is forecast to reach almost 60 billion dollars by 2022. Today’s new generation of efficient,


flexible, solar cells can provide enough energy from a small area of the wearable to power it. Also, today’s electronic assemblies, using the latest chip scale packaging, are becoming ever smaller and can comfortably fit into a hat or other clothing without causing discomfort to the user. Almost always, the biggest obstacle to making the smallest possible product is the battery, both in terms of size and also often the shape. Many wearable products do not have a form factor that can easily absorb a typical AAA or coin cell battery. Today energy harvesting solutions can


provide a surprisingly large amount of energy, and a standard 25cm2


solar cell


will provide around 150µW in typical indoor low light conditions such as is found on a cloudy day (a illuminance of around 200 Lux). One of the new generations of optimised indoor solar cells can provide as much as 60 µW/cm2


while the latest thermo electric generators (TEGs) can provide similar energy levels from a temperature difference of a few degrees µC. If


24 DECEMBER/JANUARY 2021 | ELECTRONICS


managed correctly, these energy levels are enough to power a complete system that for instance can intermittently measure temperature and skin moisture content (detecting sweating). It could also run algorithms to detect oxygen saturation in the blood, make complex calculations and provide power for a low power radio to share the resulting information.


HIGHER DEMANDS Larger and more complex systems may require more energy, however we can easily imagine products such as a wearable electrocardiograph that could be powered by a TEG, as worn on the body. A significant difference between the ambient air temperature and body temperature could be enough to power the electronics, resulting in a much more flexible system capable of logging the behaviour of the patient’s heart for a considerable time without a bulky battery, resulting in a light, easily wearable and maintenance free product. To support these applications, Renesas


,


has developed a new family of embedded controllers aimed specifically at enabling applications that will use energy harvesting to either supplement and recharge a secondary battery or even replace it. The RE01 family has been implemented on Renesas’s unique Silicon on Thin Buried Oxide ultra-low power process and offers a combination of ultra-low active and standby power, while maintaining the high levels of processing performance often required in wearable applications. The RE01 achieves


the EEMBC ULP-CP benchmark score of 705. A key requirement of many of the


next generation of wearable devices is to try to supplement the small amounts of energy from the size constrained batteries by using energy harvesting to continuously charge the battery or even to replace it. The RE01 integrates an energy harvesting controller that enables easy management of secondary batteries as well as providing power to external sensors and radios, making a complete design fit a small footprint. While we may not see solar cells or


other energy harvesting power sources in the near future, energy harvesting can provide some significant benefits to the next generation of wearable devices. Using energy harvesting can help keep the battery charged, can provide a power source for a range of basic functions when the battery is discharged, and can also help reduce the form factor and the weight of any wearable device. The RE microcontroller can enable


Figure 1:


Graeme Clark, principle engineer, Renesas Electronics


wearable applications which use energy harvesting, providing a compact and ultra-low power solution that can manage the small amounts of energy typically supplied by an energy harvesting power source and keep a secondary battery charged, or even remove the need for a battery entirely. The first members of the RE family are available now in mass production quantities.


Renesas Electronics www.renesas.com/SOTB


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