April/May 2021

www.renesas.com ENERGY HARVESTING 33 www.power-mag.com Issue 2 2021 Power Electronics Europe the VSC_VCC pin and the supply is capable of generating at least 5 µA, then the energy harvesting cycle is started. The design of the energy harvesting controller power supply is such that the power output from the VCC pin can be supplied both to other power supply pins and to peripheral devices, such as external sensors and a radio. When power output starts from the VCC pin, the VCC pin and the secondary battery are not connected inside the energy harvester. A simplified diagram of the power supply circuit is shown in Figure 4. At this time, if the I/O domains and the external devices are connected directly to the VCC, the power they consume may be larger than the output of the solar cell and the energy held in the storage capacitor will be insufficient. In this case, the MCU will not be able to operate correctly and the start-up cycle will fail. In order to avoid this phenomenon, a mechanism to separate these circuits from the power is necessary. Figure 4 shows a switch installed between the VCC pin and the peripheral devices, and the load switch is controlled on/off by using a general- purpose port. For this reason, one of the I/O domains is always powered. (The load switch should be turned off at start-up and turned on after the secondary battery is fully charged.) When voltage is first applied to the device from the solar cell (as shown in Figure 5), power is supplied from the solar cell to the EHC. However, rather than starting up the microcontroller, switches SW1 and SW3 are closed so the energy available is used to charge the external storage capacitor. The EHC monitors the state of the capacitor to detect whether it contains sufficient energy to power the MCU. The capacitor should be dimensioned correctly to supply enough energy for the MCU to complete its initialisation routines. Power is applied to the MCU inside the RE01 by closing switch SW6 and the reset signal is released. At this point, current is also output from the VCC pin, so care should be taken to minimize any current consumption by external circuitry powered by this pin. This process is shown in Figure 6. Once the MCU has started and has run its initialization software using the energy in the storage capacitor, the MCU changes state into one of its ultra-low power modes to allow the storage capacitor to be recharged. Once this is complete, we can look to charge the secondary battery (or supercap if preferred). In this case, switch SW3 is opened and switch SW2 is closed to divert the available energy to charge the battery. This is done under the control of the MCU, which monitors the state of the battery. The MCU monitors the state of the secondary battery (or supercap) and when it is charged we can release the full power of the device as required. With a charged battery, we can power external sensors or a radio as required, and can increase the MCU speed and peripheral function. This operating state is illustrated in Figure 7. Fully charged The energy harvesting controller is still monitoring all the voltage levels on the device, including the voltage levels on the storage capacitor and on the secondary battery. If the voltage across the capacitor drops below a set threshold, the charging process of the secondary battery is temporarily stopped to recharge the storage capacitor. When the storage capacitor is fully recharged, the secondary battery charging can start again. This cycle can be repeated as many times as required during operation. The energy harvest control circuit also has a function to prevent the overcharging of the secondary battery. When the power supply from the solar cell is too large and the secondary battery is overcharged, the switch SW2 is turned off to protect it. When the power generation element stops Figure 2: RE01 energy harvesting controller Figure 3. I/O Power domains Figure 4: Energy harvesting system with simplified power supply design Figure 5: Power supplied by the solar cell charges the storage capacitor