“By optimizing wireless protocols, low-power microprocessor design, low-power sensors, and improving the efficiency of micro-energy collection, collecting environmental energy can help reduce or eliminate battery usage and extend the working life of IoT terminals. When fusing specific micro-energy harvesting technologies, the latest technological advances of EH PMIC can make the size, cost, and complexity management of the system design more flexible.

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Author: Huw Davies

As companies are pursuing digital transformation, various types of smart homes have become the key to improving the quality of life and sustainability, and the deployment of the Internet of Things is constantly developing.

The common IoT endpoints are mostly sensors, and they may also be less common actuators, which are wirelessly connected to aggregation devices or Internet gateways. Usually these devices are deployed in large numbers in larger scenarios such as smart cities, smart factories, or smart agriculture. Therefore, the cost of on-site maintenance (such as battery replacement) for these devices will be very high. In addition, the replaced waste batteries will also bring an increasingly unbearable burden to the environment.

When designing the endpoint, engineers can maintain the life of the device by supplying energy, thereby avoiding battery replacement. But this may take several years. Due to size limitations, devices usually require button batteries. And if the power is not up to the system requirements, you need to choose a larger battery.

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Another method is to redesign the circuit to reduce the power demand of the entire system to within the power storage capacity of the available battery. But using any of the above methods, or a combination of the two, may not achieve our goals.

Microwatts or milliwatts of micro energy can be harvested from the surrounding environment, helping to provide useful and inexhaustible electrical energy. This electrical energy can be used as a supplement or replacement for primary batteries, depending on the application and the environmental energy available. The collected and converted energy can directly supply power to the circuit, or it can be stored in a buffer for emergencies.

In any case, we need suitable environmental energy to meet application requirements. Among the various subsystems of the IoT endpoints, the wireless part has the largest energy demand. Our analysis here can provide useful information for the design and integration of energy harvesting systems.

Power consumption of wireless subsystem

It is important to choose the most suitable wireless technology and use the lowest possible power consumption to provide the required data rate and communication range.

If the sensor is located very close to the aggregator or gateway (such as hubs or routers that are connected to the Internet or through local telecommunications switches), then technologies such as Bluetooth, Zigbee or Wi-Fi may be appropriate, depending on the requirements Data rate and cost limitations. In other cases (for example, endpoints are distributed in a large geographic area), LPWAN or cellular wireless connections may be required. Figure 1 compares the power consumption, data rate, typical maximum range, and relative cost of major wireless technologies in IoT applications.

Figure 1. Comparison of several common IoT wireless communication technologies. (Source: BehrTech)

Coverage, data rate and power consumption can all be expressed in data, which can be directly compared. As shown in Figure 2, the power consumption of the wireless subsystem can be as low as 150µW to 400mW.

Figure 2: Comparison between data rate, bandwidth and power consumption. (Source: Voler Systems)

In order to fully understand the impact on the overall energy demand of the system, we also need to consider the duty cycle. Applications such as smart meters need to send small data packets several times a day or every few days. Other devices (such as security cameras) may need to send large amounts of data more frequently or continuously. Depending on the application, you can perform local filtering inside the system before data transmission to reduce the duty cycle. For example, you can install a motion sensor on the camera to start recording only when it detects activity, or use embedded image processing to discard Some meaningless data. Of course, the energy required to filter the data must be less than the energy saved by reducing the duty cycle, so that it is cost-effective.

Environmental energy

After understanding the energy and power required by the wireless subsystem, the appropriate environmental energy and micro-energy harvesting technology can be evaluated.

Mainstream micro-energy harvesting technologies suitable for powering these systems include solar cell arrays, piezoelectric or electrostatic converters activated by vibration, and Peltier devices that can convert temperature gradients into electromotive force (EMF). The RF energy collected through patch or coil antennas is often not suitable for most IoT applications. Figure 3 compares the specific energy densities of these technologies. With this information, the size and performance of the components used can be evaluated to select technology and begin to develop specifications.

Figure 3: Power density of collected environmental energy.

Suppose the efficiency is 20% and the area is 35~40 cm²The solar cell can generate about 0.5 watts of electricity. The cost of this type of product is less than $1, and the cost of piezoelectric collectors is usually at least an order of magnitude higher and generates less energy. We know that solar cells are less efficient when used indoors. However, some indoor solar collectors recently launched claim to provide enough energy for low-power wireless devices.

Fusion use

With these technological advancements, micro-energy harvesting can be used to reduce or eliminate battery usage on IoT terminals. When IoT devices need to transmit or receive data, the energy itself is usually irregular and not necessarily available, so we usually need an energy buffer or storage device, which can be a rechargeable battery or a capacitor (or super capacitor). As shown in Figure 4, we also need an energy harvesting power management IC (EH PMIC) to process the energy collected by the subsystem, manage the power supplied to the energy buffer, and supply power to the load when needed. Various energy harvesting technologies have different electrical characteristics. The thermoelectric harvester generates continuous DC current at low voltage, so it has low impedance. Although solar cells also generate low DC voltage, their current and impedance will vary with the intensity of light.

Figure 4: EH PMIC is used to manage the charging of the energy buffer and supply power to the application load.

The typical EH PMICs on the market today have a fixed architecture and input voltage range. This is for use with specific types of collectors. If a single power supply cannot meet the requirements of the system, it is impossible to use this alternative collector to collect additional environmental energy. Therefore, if we need a combination of multiple energy sources, and each energy source needs a dedicated EH PMIC. This will increase system cost, size and power consumption, and will also make the design more complicated.

Some EH PMICs can be modified using external circuits to adjust the output of the energy harvester. However, in order to simplify the system design, Trameto’s EH PMIC (called OptiJoule) provides input that can automatically adapt to various types of connected collectors and maximize the power supply of the buffer without the need for external circuits. The product has multiple versions and models, which are used for single input or multiple (up to four) inputs. The multi-input version has strong flexibility and can be connected to similar or different types of collectors. Therefore, with the help of OptiJoule, the ability of micro-energy harvesting can be improved, a single PMIC can also be used for multiple applications, and energy harvesting technology can even be selected later in the product development (if needed).

in conclusion

By optimizing wireless protocols, low-power microprocessor design, low-power sensors, and improving the efficiency of micro-energy collection, collecting environmental energy can help reduce or eliminate battery usage and extend the working life of IoT terminals. When fusing specific micro-energy harvesting technologies, the latest technological advances of EH PMIC can make the size, cost, and complexity management of the system design more flexible.

(Reference original text: Making energy harvesting work for edge IoT devices)