Power management considerations for wearables
A possible obstacle for their full deployment in these markets is their energy autonomy: in order to reinforce the concept of “wearable”, small batteries are necessary and improvements should be made in energy efficiency and power management.
The integration of electronics has generated many applications and multifunctional scenarios by offering new control opportunities to improve our living environment. Technological advances are travelling at a high rate, and the possibilities offered by wearable devices embrace the field of medicine. Health monitoring wearables can allow a firm control of vital signs, all in real time, offering industry experts the possibility to access monitoring data through the cloud.
The amount of information managed by a normal wearable device, the visual LED interface and the BLE communication protocol require efficient power management solutions for long-life products, but also provide new opportunities for recharging by using energy harvesting solutions. Wearables prove to be a fertile ground for the use of energy harvesting techniques, where one can exploit the kinetic energy of the wearer to produce electricity and directly recharge the battery of the worn device – see figure 1.
Power mode
As a first consideration, designers should optimize energy savings, making the most of low-power solutions in sleep mode for a definite period of time, relying on physical interrupt options to awaken the wearable, for example, a vibration or a sudden movement. A fitness device may work in active situations for data collection, but it might save energy when no motion is detected, remaining in sleep or ultra-sleep mode indefinitely.
Currently, compact lithium-ion battery technology is the predominant source of energy in wearables, however, capacity is closely linked to the size of the battery and capacity largely degrades within a few years of use.
One way to efficiently increase the life of the battery is to decrease the power consumption of the many sensors placed inside a wearable device. All the sensors play an important role and operate at different voltages from that of the battery, hence they requires a DC/DC converter – see figure 2. Improving conversion efficiency directly impacts the lifetime of a battery.
The choice of a switching regulator is also a key factor to maximize the efficiency and to determine the levels of power consumption for each working step. A low quiescent current can give a maximum yield of 80% at 1mA depending on the input voltage and output values.
Inductor-based DC-DC switching converters are the preferred choice for their superior efficiency compared to linear regulators, however, adding up multiple inductor-based switching regulators to address each voltage requirement may prove too costly. Multi-rail DC/DC or switched-capacitor converters should also be considered as a possible alternative to linear regulators to improve overall efficiency and extend battery life.
The TPS82740A module from Texas Instruments is specifically designed to deal with the power requirements of wearable devices such as smart watches. The module comes in a “MicroSIP” technology (System-in-Package) that includes the switching capacitors and the input/output inductor in a package only 6.7 mm².
The classic configuration as displayed in figure 3 does not require external components. The operating principle used by TI is based on Direct Control with Seamless Transition into Power Save Mode (DCS-Control). The device runs on a rechargeable lithium-ion and Li-primary battery such as Li-SOCl2, Li-MnO2, and two or three alkaline batteries. Input voltage up to 5.5V also allows operation from a USB port or thin-film solar modules if one wants to rely on energy harvesting.
Thermal energy harvesting
A heat flux can be converted into electrical energy by using a thermoelectric generator (TEG), whose core is a thermopile. From the theory of thermodynamics, the heat flux on a human skin cannot be effectively converted into electrical energy, even if a human being generates on average more than 100W. If we suppose a low conversion of about 1 to 2%, the amount of available power is sufficient to operate a low power wearable device.
The thermal circuit of a TEG wearable placed in direct contact with the skin can be described by a thermal resistance of the body and that of the environment. These resistors are connected in series and represent the thermal resistance of the thermoelectric generator.
We constantly produce heat as a side effect of our metabolism. However, only part of this heat is dissipated in the environment as a flow of heat and infrared radiation, the remainder being rejected in the form of water vapour.
What’s more, only a small fraction of the heat flow can be collected and stored as energy. The magnitude of the voltage V generated between the two layers depends on the material and the temperature, following a linear relationship as a function of the Seebeck coefficient S.
The energy optimization, as we have seen, necessarily brings with it the need for accurate choices, not only of the various components, but also of a power supply and an intelligent management system, capable of supplying power only when necessary.
There are many aspects to consider when designing a low power system: power consumption, required cycles, voltage and total power consumed. All design scenarios will require careful planning.
About the author:
Electronic Engineer Maurizio Di Paolo Emilio is the author of reference book “Microelectronic Circuit Design for Energy Harvesting Systems”
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