"Zero-power" devices, which use energy from ambient sources, present significant power-management challenges. Circuits capable of harvesting energy efficiently must respond to changing power demands while consuming negligible amounts of energy. Engineers can quickly gain experience designing energy harvesting applications and accelerate new product development by using development kits. Energy harvesting development kits often provide complete zero-power sample application designs and are available from suppliers such as Cymbet, EnOcean, Microchip, Silicon Labs, and Texas Instruments.
Energy-harvesting designs require careful attention to standby and peak power demands because the ambient sources they rely on provide mere microwatts of instantaneous power (See Figure 1). An effective design in standby mode must provide sufficient power for the application and also typically must charge a power-storage device with low self-discharge such as a thin-film battery, supercapacitor, or both.
During high-demand periods, the storage device must deliver sufficient capacity to handle peak loads and still retain enough power to safely return to standby mode. Peak power demands can be substantial in some energy-harvesting applications. For example, during periods when a device must sample sensors and transmit data simultaneously, a zero-power wireless sensor typically requires over 10 mA, which is well above power levels directly available from ambient sources.
Figure 1: Energy harvesting design is primarily an exercise in energy management and requires efficient circuitry capable of handling energy input, power output and energy storage using thin-film devices or supercapacitors. (Source: SiLabs.)
Engineers are familiar with the challenges of designing an effective power budget. However, energy harvesting creates new questions about how to balance energy availability with power utilization. In particular, engineers need to focus on system initialization demands, especially during cold boot, when an energy-harvesting system starts or restarts with no stored power. This situation can occur because the device was turned off or because its primary energy source was unavailable for a prolonged period. For example, light-powered wireless sensors must cold boot after being in the dark for extended periods.
To successfully manage cold boot, engineers must design applications with startup procedures capable of delaying power-hungry initialization procedures until power levels reach the required threshold. For example, a design may need to immediately switch to a low-power state when first powered on, and then wait for an energy-harvesting subsystem to accumulate sufficient power for initialization to safely continue to completion, or continue to the next step in initialization sequence.
As energy becomes available, engineers can switch on different subsystems in the required sequence to sustain further energy-harvesting and application processing. For example, many semiconductor devices intended for energy-harvesting applications include mechanisms to signal circuitry that available energy has reached a certain threshold. In turn, microcontroller units (MCU), or other control circuitry, can use these signals to enable more complex sequences of activity with individual levels of power utilization.
Energy-harvesting design kits provide proven hardware and software solutions to these critical power management problems. Software development resources provide engineers with mechanisms to monitor energy production and experiment with alternatives for balancing energy usage with collection. Most kits are preconfigured with a particular application or provide detailed design information to build one.
For example, Texas Instruments (TI) pre-configures its eZ430-RF2500-SEH Sensor Monitor as a zero-power wireless sensor node. Engineers can monitor the node’s activity on a personal computer. Along with a full microcontroller-based hardware complement – including solar panels and a pair of EnerChip thin-film rechargeable storage devices – TI preloads the kit with firmware for a programmable wireless temperature sensor node. The node can measure its own temperature, receive data from other nodes, and transmit data to a PC through a USB connection provided with the kit. TI's kit addresses specific power management needs of energy harvesting with a charge control block, which continuously monitors the output of the boost converter and manages battery charge and discharge cycles (See Figure 2).
Figure 2: A solar energy harvesting module is at the core of the Texas Instruments eZ430-RF2500-SEH development kit and relies on a power management block to prevent storage devices from discharging too deeply in low-light conditions. Source: Texas Instruments
Microchip offers a pair of energy-harvesting development kits: the TPWR001 kit for RF energy harvesting and the DV164133 kit for solar energy harvesting. Designed as a complete wireless application using ambient RF energy, the TPWR001 serves low-power applications such as RF sensors, temperature sensors, building automation and security. The kit includes a 3-W Powercaster transmitter as the power source, with two P2110 Powerharvester receiver evaluation boards, two custom-designed wireless sensor boards, an IEEE 802.15.4 transceiver, and Microchip PIC 16-bit MCU development boards preconfigured with wireless sensor application software. The kit supports software development and programming through Microchip’s free MPLAB Integrated Development Environment (IDE), and the PICkit 3 programmer and debugger.
Microchip includes two Cymbet EnerChip devices with its DV164133 kit, as well as a solar panel suitable for indoor or outdoor light. The kit relies on its PIC24F MCU to monitor power conditioning and capacity based on the Energy Aware software algorithms developed by Microchip and Cymbet. Using the kit's USB connection to transfer data to a PC, engineers can explore alternatives for balancing power collected through energy harvesting and optimize power usage.
Along with its own EDK 300 baseline development kit, EnOcean offers a series of add-ons for different types of energy, including ambient solar and thermal sources. Engineers can start with an EnOcean EDK 300 or 300C basic development kit and extend a design with the EDK 310 Solar Developer's Kit upgrade or EDK 312 Thermo Developer's Kit upgrade. EnOcean's kit includes application programming interface, which provides access to EnOcean Dolphin software stack and a library of support functions (See Figure 3).
Figure 3: EnOcean's development kits include application programming that interfaces with its Dolphin platform and software stack of routines ranging from board support to power management. (Source: EnOcean.)
Cymbet designs a CBC-EVAL-09 as a universal energy harvesting evaluation kit that engineers connect to a variety of energy transducers, such as the solar cell provided with the kit. The kit includes Cymbet's EnerChip EP CBC915-ACA Energy Processor and EnerChip CBC51100 100 µAh solid-state battery module, which comprises a pair of parallel-connected 50 µAh EnerChip devices.
The Silicon Labs (SiLabs) ENERGY-HARVEST-RD kit is a reference design for an ultra-low-power wireless sensor based on SiLabs' wireless devices. Along with a wireless dongle for data communications, the kit includes a wireless sensor node based on the company's Si1012 wireless MCU. The design uses solar energy harvesting and requires only 50 lux to operate in standby mode. This allows engineers to experiment with designs requiring long-term operation in low-light conditions.
Energy harvesting has tremendous potential for zero-power designs, such as wireless sensor nodes, temperature sensors and security devices. Designing an effective energy-harvesting solution requires overcoming familiar power management challenges. Energy-harvesting development kits provide an effective method to rapidly gain experience designing energy harvesting systems, including semiconductor devices.