【Introduction: Wireless sensor nodes are increasingly being applied in our daily lives because they are suitable for use in a variety of diverse and difficult to reach environments. They don't need to be powered on because they are usually connected to batteries.

Wireless sensor nodes are increasingly being applied in our daily lives because they are suitable for use in a variety of diverse and difficult to reach environments. They don't need to be powered on because they are usually connected to batteries. However, the battery's endurance is limited, and batteries that have run out of battery must be replaced or charged. However, replacing batteries requires time and effort, and may incur very high costs. If the battery life can be extended, this problem can be avoided, and it also means that long-life applications can use independent sensor nodes. This can be achieved through energy harvesting. Energy collectors can collect available energy from the environment, such as mechanical energy, thermal energy, or photovoltaic energy, and convert it into electrical energy. This article will demonstrate different energy harvesting techniques and the circuits required to effectively store the collected energy.

Energy harvesting technology

Wireless sensor nodes (WSNs) can perceive, process, and transmit specific parameters. They are of great significance for environmental and structural monitoring, and are also applied in the medical field to monitor human health. They are usually powered by batteries and often used for long periods of time, so battery life is crucial for them.

These sensor nodes are usually used in difficult to reach areas, and charging or replacing batteries when the battery runs out can be a very expensive task. There are currently multiple methods to reduce WSN energy consumption and significantly extend battery life, including adjusting the internal power consumption of WSN, and controlling its operation through programming based on busyness, so that it can continue to run in low-power mode (deep sleep) under normal circumstances, only activated for a short period of time to perform tasks such as data collection, calculation, measurement, and communication.

Many emerging applications require decades of network lifespan, and relying solely on batteries is no longer sufficient to meet these demands. If wireless sensors are to be deployed in long-term applications where the running time exceeds the battery life, energy collectors can be used to extend the battery life, ensuring that WSN can self power and ultimately achieve the required battery life. The specific implementation method is shown in Figure 1.

Figure 1: Schematic diagram of wireless sensor nodes powered by energy collection

To use an energy collector to power WSN, the first step is to investigate the available environmental energy sources, mainly including light, heat, mechanical vibration, radio frequency (RF), and wind. To convert the energy collected from these energy sources into electrical energy, corresponding energy exchange devices need to be used, such as collecting indoor lighting energy through photovoltaic cells, collecting vibration energy through piezoelectric components, and collecting temperature difference energy through thermal conductivity generators (TEGs). Subsequently, the collected energy needs to be stored in the battery or supercapacitor through a power management circuit.

Power management

The purpose of the power management circuit is to connect the energy collector with the sensor node, while converting the collected energy as efficiently as possible. In power management circuits, the first thing to consider is the output voltage of the collector, because the output voltage of different types of energy collectors is different. For example, a heat conduction generator outputs a millivolt level DC voltage, while a piezoelectric generator outputs an AC voltage ranging from a few volts to tens of volts. For the latter, the power management circuit must rectify the output of the energy collector and convert the voltage to between 1.8V and 3.6V, which is the standard operating voltage of the sensor node. In addition, the internal impedance of the power management circuit must match the impedance of the piezoelectric generator (usually in the range of thousands of ohms to several megaohms) in order to transmit energy as efficiently as possible.

rectification

The types of energy collectors that generate AC voltage include electromagnetic (RF), electromagnetic (mechanical), electrostatic, and piezoelectric. The AC voltage they generate must be rectified before it can be used by WSN. Rectification is the first part required for power management circuits, as shown in Figure 2, which is a full wave bridge rectifier circuit connected to piezoelectric components.

Figure 2: Rectifier circuit connected to piezoelectric components

Compared to single diodes, bridge rectifiers are a more popular choice because they can provide full wave rectification, converting alternating voltage into DC pulsating voltage. For true diodes, forward voltage must be considered. The forward voltage for silicon diodes is 0.7V, for germanium diodes it is 0.3V, and for Schottky diodes it is only 0.1V. The capacitor Cr (Figure 2) is used as a smoothing capacitor. In this example, the rectified voltage is stored in the form of electrical energy in the capacitor and supplies power to the load RL.

DC/DC converter

Another major function of the power management circuit is to regulate the voltage generated by the energy collector. If the voltage is too low, it must be increased; On the contrary, if the voltage is too high, the voltage must be reduced. To ensure the normal operation of WSN, it is necessary to stabilize the working voltage between 1.8V and 3.6V, and conduct it in the most efficient way possible to minimize energy loss. In addition, the power management circuit is also responsible for charging the WSN battery. To simplify the design process, there are various products available for handling power management of WSNs powered by energy collectors.

E-PEAS AEM10941 is an energy harvesting power management integrated circuit that can obtain direct current from up to 7 solar panels. This product uses an ultra-low power boost converter, which can charge lithium-ion (Li ion) batteries, thin-film batteries, or supercapacitors. AEM10941 has an ultra-low power cold start function, which can operate at an input voltage as low as 380mV and an input power of only 3 μ Start the work under the condition of W. It has two types of power supply voltages:

LVOUT (low voltage), 1.2V or 1.8V, used to power microcontrollers.

HVOUT (High Voltage) can provide power to the radio transceiver, and the voltage can be configured between 1.8V and 4.1V.

The products with both power supply voltage options are driven by efficient Low Voltage Difference (LDO) regulators to ensure low noise and high stability. The typical application of AEM10941 is shown in Figure 3.

Figure 3: Schematic diagram of AEM10941 application example

Analog Devices LTC3331 is another multi in one power management IC circuit that can be connected to energy collectors using energy sources such as piezoelectric, solar, or magnetic. This product integrates a full wave bridge rectifier and combines a step-down switch regulator and a step-down boost switch regulator. In addition, the product is equipped with an on-chip prioritizer, which can select the appropriate converter based on the collected energy or the available battery power. LTC3331 can handle energy harvesting input voltages between 3V and 19V. This product also includes a low static current parallel battery charger, which can use the collected energy to charge lithium-ion batteries. The circuit diagram of a typical application of LTC3331 is shown in Figure 4.

Figure 4: Typical Application Circuit Diagram of LTC3331