Design of maintenance-free battery-less microcontrollers enabled by tantalum capacitors and supercapacitors are discussed in a technical paper written by Ron Demcko, Daniel West and Ashley Stanziola, KYOCERA AVX Components Corporation.
Ultra-low-power microcontroller families now exist with such low power requirements that they can be powered by energy harvesting rather than battery-operated or conventional mains. These powerful MCUs enable maintenance-free control systems in applications ranging from structure/soil/water/air monitoring applications to industrial point controllers (such as smart faucets) to wearable electronics, location tracking, and even BLE beacons.
Ultra-low-power microcontroller families now exist with such low power requirements that they can be powered by energy harvesting rather than battery-operated or conventional mains. These powerful MCUs enable maintenance-free control systems in applications ranging from structure/soil/water/air monitoring applications to industrial point controllers (such as smart faucets) to wearable electronics, location tracking, and even BLE beacons. Examples of end applications such as battery-less Air Quality Monitoring and Battery-less LoRaWAN Remote Sensors are below:
This paper is based around a Renesas Evaluation board RE01-1500KB that utilizes an RE Family ultra-low power MCU built using Silicon On Thin Buried Oxide Technology – SOTBTM.
The low-power MCU is powered by a modest capacitance value Tantalum capacitor which provides a start-up voltage function. Supercapacitors provide longer-term processing power. Once the charge depletes from the supercapacitor, the Tantalum start-up capacitor provides voltage and powers the MCU in a maintenance state until the supercapacitor becomes fully charged and processing functionality returns to a high level. The process repeatedly loops until the energy harvesting power source stops. At that point, the MCU shuts down and awaits the Tantalum capacitor to flag the MCU that the energy harvesting power source has returned.
The start-up capacitor charges, the MCU is prepared for full processing power to be provided by the supercapacitor, then the tantalum / supercapacitor process repeats. This evaluation documents tantalum capacitor and supercapacitor device characteristics and in-circuit performance.
Energy harvesting has successfully powered less complicated ICs such as IoT modules through small, cost-effective, efficient PMICs matched to power inherently low power IoT loads1. Semiconductor technology advances have continued, and it is now important to recognize the progress in low-power complex ICs. In particular, low-power MCUs capable of operating through small, low-cost energy harvesting/energy scavenging sources. An industry-leading device is the Renesas RE microcontroller family, where SOTBTM Process Technology enables low power use.
SOTBTM technology enables MCUs to have Ultra-low power consumption in both active and standby mode. An example of the typical performance of a 32bit CPU Arm®Cortex®-M core shows its high-speed operation could be up to 64MHz at 1.62V. Typical current consumption of this family could be:
25uA/MHz active (internal LDO mode)
12uA/MHz (external DCDC mode)
400nA Standby with 32KB Ram retention
100nA Deep Standby
Low current consumption at low voltages is why this family of MCUs has the option for battery-less – energy harvesting power sources. A comparison of just how far SOTBTM reduces power consumption relative to other competing process technologies2 is shown in Figure 1.
SOTBTM exhibits significant performance advantages over FDSOI processed devices. Further explanation of the RE01 microcontroller energy harvesting operation shows that each RE01 contains an Energy Harvesting Controller – EHC. A link to the devices landing page can be found here.
The EHC accepts energy generated from solar, piezo, Thermal Electric Generators (TEGs), micro turbine, pressure – etc. It then manages harvested power by channeling and balancing energy through one of the two capacitors to powering the MCU. To achieve this, the EHC contains sub-level PMIC, charge controller, and power management functionality. On a more functional basis, the EHC can be viewed in many ways. It provides functions as basic as reverse current protection. However, it provides more complex functionality when viewed as the direct energy-harvesting link—controlling voltage regulation, quick start-up control, autonomous and reliable start-up sequencing, start-up current control, energy storage charge management selection of capacitor power sources.
The configuration of an energy powered RE01 MCU is shown in Figure 2. Temporary energy storage is provided by a tantalum capacitor and secondary storage is provided by much larger capacitance value supercapacitor.
Selection of Start-up and Storage Capacitors
As previously mentioned, when the RE01 MCU is configured to operate from an energy harvesting power source, the EHC relies upon a start-up capacitor, C-SU, to charge quickly and provide the low-level power for MCU power up initiation. Long-term power comes from batteries (or in the case of this study – supercapacitors).
Figure 3 outlines the relationship between power management states and the EHC interaction with C-SU and the storage supercapacitors. We will concentrate on powering the MCU from voltage stored in a smaller capacitor (C-SU) and maintaining MCU operation with voltage stored in the large supercapacitor.
To summarize: Once energy harvesting power is apparent, the EHC charges the start-up capacitor C-SU. When C-SU charges to 3.0 volts – power on reset initiates at the MCU, and the secondary supercapacitor’s charging starts.
While the supercapacitor is charging, C-SU power is being used by the EHC to initiate operations of various stages of the MCU. Once the supercapacitor is charged, the MCU power transitions to the supercapacitors for longer-term operation. During that time, C-SU is recharged and ready to maintain various active computing functions while the supercapacitors are isolated from the MCU by the EHC and recharged. Once the supercapacitors are recharged, the MCU power transfers from C-SU to the supercapacitors. At that point, the MCU gains more powerful functionality.
The whole process continues in a loop until the energy harvesting power source is no longer available and the system shuts down. At that point, the MCU waits for C-SU to get charged up & the use cycle continues. This study concentrates on the selection and operating characteristics of C-SU and the supercapacitors.