April/May 2021

32 ENERGY HARVESTING www.renesas.com Issue 2 2021 Power Electronics Europe www.power-mag.com Powering Low-Power Sensors for the Internet of Things Low-power sensors for the Internet of Things will be embedded into streets, offices and factories to collect data for years and share it with the cloud, most likely wirelessly. The use of batteries as a power source is one of the key areas developers have to address. It comprises many issues around limited lifetimes, product sustainability, manufacturing materials, shipment issues and the disposal of the battery at the end of its life. However, we can look on the bright side as there is a solution that would allow us to reduce the size of the battery or at least remove it. Graeme Clark, Principle Engineer, Renesas Electronics Europe, UK We can use the energy that’s all around us, whether in the form of light, motion, heat or some other form to power our products. This is becoming achievable for a wider range of products with the latest generation of energy harvesting power sources. These are capable of harvesting useful amounts of electrical energy from ever smaller amounts of energy in the environment, and these systems are now capable of using these small amounts of energy more effectively to power products when needed. To allow developers to easily design products that harness energy in our surroundings as a power source, Renesas has implemented an energy harvesting controller on the RE01 family of embedded controllers. These devices are implemented on the new Silicon on Thin Buried Oxide semiconductor process, providing active current consumptions as low as 10 µA/ MHz, making them ideal for intelligent sensors powered by energy harvesting (Figure 1). Start-up power problem The biggest problem that designers have is the start-up current of the circuitry, especially the microcontroller (MCU). When power is applied to an MCU, the power-on reset circuit will release the reset line once there is a sufficient voltage level on the supply pin. The MCU starts to initialize, so clocks start to run, registers are initialized, and any boot application runs. This can take a significant amount of current, often many mAs which most small energy harvesting sources are unable to supply. At this point, the microcontroller will fail to operate correctly as the supply collapses and the start-up will fail. The EHC is designed to remove the start-up problem and allow the developer to manage both the start-up cycle of the microcontroller – without taking too much current – and the external power reservoirs. These comprise storage capacitors and secondary batteries or super caps. This allows the developer to carefully manage the small amounts of energy generated by the energy harvesting power supply and store that energy until needed. The EHC is designed to be flexible enough to work with a wide range of energy harvesting power sources, such as solar cells, thermoelectric generators, vibration harvesters and many other types of power generators. For example, a 25 cm 2 solar cell with a light level of 200 Lux, comparable to the light levels indoors on a rainy day, might provide 40 – 50 µA. The latest generation of thermoelectric generators with a temperature difference of 2 – 3 K will produce a similar current level. So the EHC has to manage these small amounts of energy and store them to be used when required by the microcontroller or by other components in the system. Optimized energy harvesting architecture Figure 2 shows a simplified block diagram of the EHC. It illustrates the solar cell that provides the power to the device, the external storage capacitor that’s used as the original energy reservoir to support the start-up of the device, and an optional secondary battery which can be charged when enough energy is available. The EHC can optionally use energy stored in the system to power external devices, such as sensors or radios. The RE01 microcontroller has a unique power supply design with four separately powered internal power domains and 6 external power domains. Each of these can be independently switched on or off depending on the application’s requirements. This allows the user to optimize the energy consumed by the device depending on the requirements of the application at any time. Each of the I/O peripheral and pin functions is allocated to a separate domain that can be individually powered when required. The RE01 I/O power domains are shown in Figure 3. When the energy harvesting controller detects that voltage has been applied to Figure 1: Energy harvesting controller on the RE01 family of embedded controllers