Many embedded systems that cannot connect to the power grid typically use battery power. However, when the battery runs out, the maintenance cost of replacing the battery is relatively high and causes considerable inconvenience. If we can use energy harvesting technology to provide sustainable power for the system, this problem can be solved. This article will introduce how to use energy harvesting technology to establish permanently operating embedded systems, as well as the relevant solutions launched by Silicon Labs (Silicon Labs).

Pursuing Permanent Operation of Energy Harvesting Systems

Energy harvesting technology is rapidly becoming a viable power option for embedded system designers, enabling wireless sensors to be used in applications that traditional battery-powered designs could not achieve. For example, energy harvesting power allows system designers to easily build ultra-thin wireless sensors with a range of over 100 meters and a lifespan of over 20 years.

The ultimate goal of energy harvesting systems is to achieve permanent operation. Energy harvesting systems can achieve permanent operation by ensuring that the energy collected meets or exceeds the energy consumed by the system during operation. Energy management is a key aspect of designing energy harvesting systems. The first step is to determine the available power output of the harvester, which can convert solar energy, mechanical energy, or thermal energy into electrical energy. Solar energy harvesters have the highest power density, capable of harvesting 15 mW/cm2 of surface area, maximizing the output power of the energy harvester, which is crucial for building a robust energy harvesting system.

When designing energy harvesting systems, it is most important to provide sufficient functionality while minimizing the power consumption of the embedded system. By selecting components with low leakage specifications and using ultra-low power microcontrollers (MCU) (for example, Silicon Labs‘s Si10xx wireless MCU), low power consumption can be achieved. Most technologies used to achieve low power operation in battery-powered systems can also be used to minimize power consumption in energy harvesting systems.

Let’s look at an example of a solar wireless sensor node that transmits data every 20 minutes at an average current of 10 µA. The system is equipped with a solar panel that provides a continuous current of 50 µA during the day. The net current available for charging the battery during the day is 40 µA, and the battery discharges at a rate of 10 µA at night. As long as the system is exposed to at least 4.8 hours of sunlight each day, the energy harvesting system can achieve permanent operation.

Thin Film Batteries Balance Harvested and Consumed Average Power

There are two types of energy harvesting systems that can achieve permanent operation, each with different energy storage mechanisms. The first type requires long periods of energy collection and accumulation and uses low leakage, high-capacity energy containers, such as thin film batteries. Permanent operation is achieved by balancing the average harvested energy and the average consumed power. This type of energy harvesting system is the most flexible and usually experiences high power bursts in short periods. Most of the time, these systems are in low-power sleep mode, always powered on and ready to harvest energy. An example of this type of system is a solar wireless sensor node.

The second type of energy harvesting system remains powered off until it detects an energy pulse, collects energy, and stores it in a low-impedance energy container (such as a capacitor). After a brief power-up reset, the system uses the limited energy collected from the energy pulse to perform necessary system functions. Permanent operation is achieved by balancing the total energy consumed during task execution and the energy harvested in a single pulse. An example of this type of system is a wireless light switch that uses energy generated by a mechanical switch to transmit RF signals to a receiver located at the light fixture.

Button batteries, AA lithium batteries, and lithium thionyl chloride batteries have been used in embedded systems requiring long lifetimes for many years. The introduction of thin film batteries creates a new option for system designers to balance cost, size, and safety. As developers are constantly under pressure to reduce system costs, economical button batteries seem to be the best solution to reduce manufacturing costs and bring products to market quickly. However, there are hidden costs associated with replacing button batteries. If you consider that the total lifetime energy storage capacity of a thin film battery exceeds 30 CR2032 button batteries, you will quickly conclude that compared to the cost of replacing 30 button batteries, the initial cost of thin film batteries is negligible and exceeds the lifecycle of the embedded system several times.

When considering battery size, thin film batteries are the thinnest of all battery types (as thin as 0.17 mm). The total lifetime capacity of thin film batteries is equivalent to four lithium “AA” batteries or one “C” type lithium thionyl chloride battery, making thin film batteries very suitable for space-constrained embedded systems that require ultra-thin form factors and long battery life.

Additionally, thin film batteries do not pose safety issues associated with large traditional batteries, such as flammability and explosion hazards. Because thin film batteries are rechargeable, they only store a portion of their total lifetime capacity at any given time. This makes them safer if the battery accidentally shorts out or is exposed to extreme heat or open flames. The waste generated by thin film batteries is also much less than that of large traditional batteries, which typically end up in landfills rather than being recycled.

Energy Harvesting Reference Design Accelerates Product Development

The power consumption of battery-powered IoT devices has always been a key issue affecting the operation of IoT devices. Various organizations behind different wireless standards are committed to helping meet consumer expectations for lower power consumption in this field. Zigbee Green Power is a good example of considering energy harvesting when designing wireless communication.

Silicon Labs and Avnet jointly developed an energy harvesting reference design based on the Silicon Labs EFR32MG22 system-on-chip (SoC), which pairs a Zigbee Green Power light switch with energy harvesting power management. MG22 is designed for the Zigbee protocol, compact in size, and has advanced security features, making it an ideal choice for ultra-low power end devices. Silicon Labs also offers energy-efficient power management ICs, such as EFP0111, to provide better power management capabilities. Silicon Labs also provides MCUs, wireless starter kits, and Simplicity Studio, which offers a robust development and debugging environment to assist customers in quickly developing energy harvesting systems.

The core component of this design is the energy harvesting generator, and this reference design uses the ZF monostable generator module. This is a bidirectional switch generator, meaning energy is generated both when the switch is pressed and released. The switch has magnets with two poles, and pushing the switch down generates a magnetic field that passes through the core and returns to the other pole. Then, when the user releases the switch, the magnetic field changes and passes through the core in the opposite direction, generating current from this changing magnetic field, which is the energy we can harvest. When pressing or releasing the ZF generator, it generates alternating voltage, and the system can use this energy generated from mechanical energy to turn on the light fixture, with the ultimate goal of being able to turn the light fixture on and off without wiring between the switch and the light fixture.

Powering IoT devices is an energy-intensive task, and innovative battery-free powering methods will simplify development and help create a cleaner environment. For example, the energy required to make an LED blink once is enough to transmit multiple RF signals. Low-power silicon chip designs combined with networks optimized for low-power applications will lay the foundation for a new era of power management, significantly reducing costs and waste for manufacturers and consumers.

High-Performance, Low-Power Power Management Solutions

Silicon Labs launched the EFR32MG22 (MG22) series SoC, which is optimized for Zigbee solutions, providing industry-leading energy efficiency for IoT applications such as smart home sensors, lighting control, and building and industrial automation.

The EFR32MG22 and EFR32MG22E Zigbee SoC solutions are part of the Wireless Gecko Series 2 platform. MG22 series offers optimized Zigbee SoC solutions that integrate a high-performance, low-power 76.8 MHz ARM® Cortex® -M33 core with TrustZone. MG22 allows you to create energy-efficient applications, while MG22E (“E” stands for energy-efficient) further enhances energy-saving advantages by extending battery life and supporting completely battery-free designs. MG22 SoC combines ultra-low transmit and receive power (+6 dBm at 8.2 mA TX, 3.9 mA RX), 1.4 µA deep sleep mode power, and low-power peripherals to provide industry-leading energy-saving solutions for applications using Zigbee protocol (including Green Power).

Silicon Labs‘s EFP0111GM20 energy-efficient power management IC (PMIC) is a flexible, efficient, multi-output power management IC that provides complete system power for EFR32 and EFM32 devices, three output voltage rails, and main battery charge measurement functionality. EFP0111 boost converter PMIC operates in a voltage range of 1.7 to 5.2, with static current as low as 150 nA. EFP0111GM20 supports a variety of batteries from 1.5 to 5.5 volts, providing flexibility for various battery technologies while improving EFR32 and EFM32 power efficiency.

Silicon Labs’s Si10xx Sub-GHz wireless MCU combines high-performance wireless connectivity with ultra-low power microcontroller processing in a small package of 5 x 6 mm. The device supports frequency ranges from 142 to 1050 MHz, including an integrated advanced packet processing engine with a link budget of up to 146 dB. By reducing TX, RX, active, and sleep mode currents, as well as supporting fast wake-up times, the device is optimized to lower power consumption in battery-supported applications. Si106x MCU is pin-compatible with Si108x devices, with flash memory capacity adjustable from 8 to 64 kB, and provides robust analog and digital peripherals, including ADC, dual comparators, timers, and GPIO. All devices are designed to meet the 802.15.4g smart metering standard and support global regulatory standards, including FCC, ETSI, and ARIB specifications.

Conclusion

Energy harvesting technology has become quite popular, and due to the many benefits it offers for embedded system design, it is expected to become even more prevalent in the coming years. A well-designed energy harvesting system can achieve permanent operation once it overcomes the initial power-up reset. With careful system design, the lifespan of energy harvesting systems can be extended to more than 20 years. Thin film batteries are commonly used in energy harvesting systems due to their ultra-thin form factor and low leakage characteristics, eliminating the need for a main power supply or traditional replaceable batteries, allowing for the flexibility of self-sustaining embedded system designs, creating new application possibilities, and opening up new areas for embedded system development.Silicon Labs’s MG22 series Zigbee SoC solutions, EFP0111GM20 energy-efficient power management IC, and Si10xx Sub-GHz wireless MCU will provide excellent energy consumption control capabilities for energy harvesting systems, ensuring that embedded systems can operate for long periods without the hassle of battery replacement.

This article is reprinted from the Avnet WeChat public account, original link:https://mp.weixin.qq.com/s/T_ULhgeEj6-x1N029ftzJQ

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