April/May 2021

34 ENERGY HARVESTING www.renesas.com Issue 2 2021 Power Electronics Europe www.power-mag.com producing current, the operation continues with the power supplied from the secondary battery. This process is shown in Figure 8. There is also a reverse current prevention function to stop any damage caused by power flowing back to the solar cell: the circuit inside the EHC is disconnected. The application will continue to run, powered by the energy contained in the secondary battery and storage capacitor, until these are discharged. As the EHC monitors the voltage levels on both these devices, it will generate a warning that the power is failing to allow the system to power down before the energy supply is exhausted. The EHC allows users to usefully manage the energy from an energy harvesting source, generating currents as low as a few µA, and store this energy to be released when required by the application. The typical power supply cycle is shown in Figure 9, including the start-up of the EHC when power is first supplied from the solar cell. The diagram also illustrates how the MCU is released from reset. The voltage on the storage capacitor drops; the MCU then initializes, sets up the harvester and enters a low power state while still executing code slowly and consuming around 1 µA. The storage capacitor is then recharged and the secondary battery starts charging. When the secondary battery is charged and interrupt is generated, there is enough energy available to switch the MCU into full speed, and switch on external sensors and a radio as required. This can be done under the control of the MCU’s application. When the secondary battery is discharged and the voltage falls to a predetermined limit, the system gets a warning to reduce power and the cycle starts again. Conclusion The energy harvesting controller implemented on the RE01 family of microcontrollers allows users to easily implement the hardware required to use many types of energy harvesting power sources. The EHC enables us to both overcome the start-up limitations of normal microcontrollers and manage the energy available to power the complete application. The EHC allows us to look on the bright side of how to power our products in future, if we are expected to populate the Internet of Things with billions of intelligent communicating devices to sense and record the environment around us. Energy harvesting provides the ideal solution to power these devices, either to remove the need for batteries, or at least allow the local recharging of secondary batteries to remove the need for battery replacement and remote recharging. This is really the bright side of the IoT. Figure 6: MCU starts up powered by the energy stored in the storage capacitor Figure 7: All energy reservoirs charged and CPU powered Figure 8: MCU operation continues when power fails from the solar cell Figure 9: Waveform diagram showing power status of storage capacitor and secondary battery during operation