New components and tools make it easier than ever to design electronic devices with ultra-lean power requirements.
As electronic components become more sophisticated and less costly, the demand for smart sensors, wearable devices and ubiquitous communication tools continues to grow, increasing the need for electricity to power them. According to the Intergovernmental Panel on Climate Change, information and communication technology (ICT) could account for more than one-fifth of the world’s energy consumption by the end of this decade. At the same time, the world is trying to cut down on energy consumption in order to curtail climate change, decrease the number of spent batteries in the waste stream, and reduce the cost of replacing batteries in remote sensors. In the U.S. alone, more than three billion batteries make their way into landfills every year. Over 90 percent of all lithium-ion batteries—considered hazardous waste, due to their lead, cobalt, copper and nickel content—are thrown away rather than recycled at the end of their usable lives.
Fortunately, the electronics industry is providing a plethora of ultra-low-power components, longer-lasting batteries, and innovative devices that generate electricity by harvesting free ambient energy. With this hardware and a little ingenuity, engineers can design high-tech products with minimal energy requirements.
Amplifiers Go Digital…
In audio applications, engineers can minimize power consumption by designing with class D amplifiers. These highly efficient power boosters convert the incoming analog waveform into a PWM signal whose overall duty cycle is proportional to the signal’s amplitude, with the width of each pulse representing the instantaneous signal level at the time of sampling. Since these are switching amplifiers, they consume power only during the brief instant that the digital output changes state, and use virtually no power when the output pulse is in the high or low state. Unlike their analog counterparts, whose efficiencies struggle to reach 70 percent, class D amplifiers can easily achieve 90 percent efficiency or higher, making them ideal for audio applications with ultra-low-power requirements.
Just as the op-amp provides a customizable building block for analog amplifiers, an IC-based class D amplifier gives engineers a functional amplifier that’s already optimized for low power consumption, minimal distortion and EMI, good thermal performance, and a tiny footprint. Some even offer the added benefit of an integrated DSP, which lowers the cost, size and weight of a product.
…While Computers Go Analog
Just when you thought everything was going digital, analog designers say, “Not so fast.” Digitizing makes for a more efficient amplifier, but certain computations are better suited to the analog world. The human brain, the world’s most proficient neural network, draws less than 20 watts of power while processing analog information. And unlike its silicon counterpart, carbon-based gray matter is very effective at pattern recognition, having the ability to identify objects and sounds with little effort.
Devices that rely on pattern recognition, such as smart doorbells, voice-assistant speakers and security cameras, spend most of their lives in standby mode, waiting for an appropriate stimulus to wake them. In some cases, like a smart speaker, the device is designed to react to a human voice and ignore other random noises. Nonetheless, all audio waveforms must be analyzed, and that requires waking the MCU every time a sound is detected and running a machine learning (ML) algorithm to perform pattern recognition, all of which expends computing capacity and electrical energy.
A number of IC makers are now producing programmable analog chips with ML capabilities that do preliminary computations on analog quantities prior to the data being digitized. These use a tiny fraction of the power used by digital microcontrollers performing the same ML functions, significantly increasing battery life. The chips can be programmed to accommodate different sensors and identify certain features of the waveform using their version of an analog neural network. When an appropriate signal has been identified, the analog device brings the digital microcontroller out of standby mode for processing.
Ultra-Low-Power Microcontrollers
Given that ICT products nearly always have a microcontroller inside, it’s not surprising that a 2022 report by KVB Research predicted that the ultra-low-power microcontroller market will reach $7.9 billion by 2027 with a CAGR of 10.3 percent. The report cites many reasons for this growth, including climate change, wearable electronics, remote controls, building automation systems, and wireless sensors.
Every microcontroller maker offers a series of ultra-low-power MCUs, and some take it to the next level by incorporating energy-harvesting control units, which extend battery life or eliminate the need for a battery altogether. These draw minimal current in both active and standby modes, making them suitable for wearable electronics, IoT applications, smart buildings and remote sensing applications. Look for a model whose evaluation kit features an energy harvesting device (like a solar panel) and a supercapacitor.
Energy Harvesting
Now that you’ve minimized the power consumption requirements, let’s look at a few ways to harvest the energy needed to operate the device without the need for batteries or a power outlet.
Back in the 1980s, long before the term “energy harvesting” was coined, this engineer purchased a solar-powered scientific calculator (it still works today.) A 4cm2 solar array converts even a modest amount of light into enough power to perform engineering calculations without a battery. Solar cells continue to be a popular way to power devices by harvesting ambient light, but engineers have many more transducers and energy sources to choose from these days.
Which type of energy you harvest and how it’s converted into electricity both depend on the application. Solar is reliable, stable and relatively efficient, assuming there’s a source of ambient light available. A small solar cell may generate a few milliwatts under indoor light and a watt or more in direct sunlight. Radio frequency energy can produce a few microwatts under most conditions, but in industrial settings where there’s a lot of EMI, tens of milliwatts can be harvested. Thermoelectric generators (TEGs) convert a temperature differential into usable power. The human body radiates about 5.7mW/cm2, so a TEG could be used to power a flashlight or other low-power handheld device. TEGs can also harvest wasted heat from generators, CPUs, motors or exhaust systems, offering the added benefit of cooling these components. As solid-state devices, TEGs are durable and long-lasting. In fact, NASA’s Voyager I and II spacecraft are still operating on power from 55-year-old TEGs.
Mechanical movement can be converted to electricity using electromagnetic induction, piezoelectricity, or the triboelectric effect. A small electromagnetic generator can produce several watts, while piezo and triboelectric devices deliver power in the microwatt to milliwatt range. The motion can come from human or animal limbs, machine vibrations, floor tiles, objects swaying in the wind or water, etc.
Regardless of which energy harvesting component you choose, you’ll need to condition the power using a DC-DC converter, power management integrated circuit (PMIC), power management unit (PMU), and/or a low-dropout (LDO) regulator. Many circuit design and simulation tools include models for energy harvesting devices and their associated components.
Since energy harvesting transducers are relatively inexpensive, you may decide to put more than one type in your system. You can also include a small rechargeable battery as a primary or backup power source, and allow the energy harvester to trickle charge the battery. In this case, you should add a charge controller and a battery-monitoring circuit. Fortunately, those battery management features are built into many PMUs.
Tools for Engineers
Just getting started in low-power electronic design? Go to your favorite electronics supplier and search for “energy harvesting,” “power management,” or “battery management.” You’ll find enough components, systems, development kits, daughterboards, design and simulation models, and application notes to help you create your latest high-tech project with uber-lean power requirements.
