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The trend toward improved power efficiency in semiconductor devices has opened the door for energy harvesting designs that rely solely on readily available, but weak sources of energy for powering applications. Energy harvesting brings new challenges for power management, requiring high efficiency operating characteristics in power management devices themselves, as well as in the circuits they control.
Through a combination of low-power characteristics and effective operating modes, a growing array of semiconductor devices can help designers achieve effective power management in energy-harvesting designs. Available from Linear Technology, Microchip Technology, Seiko Instruments, Texas Instruments and others, these parts include devices specifically intended for energy harvesting as well as low-voltage DC-DC converters intended for low-power applications.
Energy harvesting methods have emerged in response to an increasing need for "zero-power" solutions for a growing class of embedded applications in automotive, security, medical, and structural health monitoring arenas, where battery replacement is difficult, costly, or even hazardous. Energy harvesting designs leverage the ability of transducers to convert light, vibration, temperature gradients, and RF energy into useable voltage and current but at incredibly small levels (Figure 1). By accumulating the microWatts of energy available from ambient sources, energy harvesting designs can achieve zero-power operation - consuming no more than that scavenged from their environment.
Figure 1: Ambient energy sources provide available power but at miniscule levels, dictating a need for highly efficient power management. (Source: Texas Instruments).
In some cases, transducer output can be sufficient to power an energy harvesting application, entirely eliminating the need for batteries. A sensor node designed to monitor vibration of a motor assembly might gain enough energy from the vibrating source to complete its monitoring objectives and return to a quiescent mode when the vibration energy source is quiet. Such a design typically operates so close to its available power budget that power for additional power management functionality beyond simple resistor dividers and cut-off diodes will likely not be available.
In fact, few ambient sources are sufficiently reliable and powerful enough to sustain circuit operation, particularly if power-hungry wireless transceivers and communications interfaces are required for the application. Energy harvesting applications built to scavenge energy from ambient light or RF sources - such as wireless sensor nodes for building or security monitoring - would likely face transient blackouts and struggle to find sufficient power for essential communications functions.
Indeed, most energy harvesting applications require batteries or supercapacitors to meet basic sensing, processing and communications requirements. An attractive approach combines supercapacitors and batteries in a hybrid approach that relies on the extended capacity of a battery for normal operations and on the rapid charge/discharge characteristics of supercapacitors for power ride-through during peak load periods.
Power management is essential for optimizing transducer output, ensuring safe and reliable battery and supercapacitor operation, and ultimately maintaining voltage and current levels needed for application circuits. For these designs, engineers must pay special attention to the power characteristics of the system as a whole as well as the power management device itself.
In particular, power management for energy harvesting applications requires minimum startup and supply voltage, zero-power standby capability, ultra-low leakage and standby currents, and maximum efficiency while operating with small loads. Ultra-low-power DC-DC converters address most or all of these concerns, ensuring stable voltage and smooth current needed for the application.
In these designs, power management starts at the transducer. Effective management of transducer power output is critical for ensuring voltage regulation, minimal power consumption, and the overall match between the load and the voltage and current profile of the transducer output. With conventional DC-DC converters, however, designers face a significant gap between transducer output and minimum input voltage for the converters. While conventional converters typically require 0.7 V minimum input voltage, individual solar cells deliver 0.5 V or less and vibration or temperature sensors achieve tens of milliVolts at best. Specialized ultra-low-voltage DC-DC converters are designed to operate at these significantly lower input voltages.
The Linear Technology LTC3108 is a highly integrated step-up DC-DC converter that operates at input voltages as low as 20 mV. Intended for wireless sensing and data acquisition applications, the device requires only a few external components to implement an energy harvesting design (Figure 2). The LTC3108 offers multiple regulated power outputs, including a 2.2 V LDO, a primary output programmable to four fixed voltages, a second output, and an output for charging a supercapacitor or battery.
Figure 2: Linear's LTC3108 provides multiple output voltages including VSTORE to charge a battery or supercapacitor as well as control signals such as PGD designed to support burst activity in low-power environments. (Source: Linear Technology).
With this device, Linear paid special attention to the peak load burst requirements of energy harvesting applications. These applications typically operate at a low power state for much of the time, accumulating raw sensor data for seconds, minutes or hours. Periodically, these circuits wake up and perform data processing and communications in bursts that require peak power drawn typically from batteries or supercapacitors. The LTC3108 supports this mode of operation with its PDG signal, which can enable standby-/sleep-mode circuitry when VOUT reaches target levels, indicating that enough energy is available for a burst (Figure 3). When VOUT drops below nine percent from its regulated voltage, PGD will return low.
Figure 3: Designed to enable burst operation of energy harvesting designs, the LTC3108 provides a signal (PGD) that goes high when VOUT is within 9 percent of the regulated level set by the engineer. (Source: Linear Technology)
The Texas Instruments TPS6120x series of boost converters operates with input voltages as low as 0.3 V (0.5 V startup into full load). These devices address the need for low power consumption with a quiescent current of 55 μA and leakage current of 0.01 μA. Low-current operation often results in lower efficiency pulse-width modulation (PWM) DC-DC converters and this class of device is no exception (Figure 4).
In this case, however, TPS6120x devices provide a special Power Save mode, which boosts efficiency (Figure 5) for designs with the kind of low load currents expected in energy harvesting applications. These devices also feature a down conversion mode, providing additional flexibility for supporting low-power devices in the primary circuit. The converter automatically changes to its Down Conversion mode whenever the input voltage reaches or exceeds the output voltage.
Figure 5: The Texas Instruments TPS6120x family features a Power Save mode that boosts efficiency in low-current operation (left) compared to efficiency obtained with Power Save disabled (right). (Source: Texas Instruments).
As with many converters, the TPS6120x requires a higher startup voltage than needed during nominal operation. Charge pump ICs such as the Seiko Instruments S-882Z series provide a solution. Intended as the input stage to a two-stage design (Figure 6), the Seiko devices differ from conventional charge pump ICs in supporting input voltage as low as 0.3 V. During operation, the stepped up electric power is stored in a startup capacitor, which is discharged to start the step-up DC-DC converter when the startup capacitor reaches the required voltage level. To improve power-consumption efficiency, the S-882Z devices feature a built-in shutdown function that stops the device when the output voltage of the connected step-up DC-DC converter rises above a given value.
Figure 6: An ultra-low-voltage charge pump such as the Seiko Instruments S-882Z IC can help address low-power startup issues in energy harvesting designs. (Source: Seiko Instruments).
Although devices such as the Linear LTC 3108 and TI TPS6120x are particularly tuned for low-voltage operation, other low-power DC-DC converters can be used in energy harvesting designs if the designer is willing and able to trade an increase in power requirements for an increase in efficiency. Converters designed for use with lithium-ion batteries typically feature pulse-frequency modulation (PFM), discontinuous mode operation and true output disconnect. These and other features are designed to reduce battery discharge rates and address key requirements in common with energy harvesting designs using battery or supercapacitor storage.
The Microchip Technology MCP1640 family of such converters isolates or disconnects the input from the output, resulting in less than less than 1 μA (no load) current drain from the storage device. Microchip also offers its MAX17710 for charge storage and protection in energy-harvesting designs operating at a minimum source input voltage of 0.7 V.
Designers intending to use low-voltage DC-DC converters need to exercise particular caution in energy harvesting designs that employ supercapacitors for storage. A supercapacitor appears as an infinite load when it nears depletion, so conventional approaches using feedback regulation will cause the converter to reduce the current substantially, resulting in an extended charge-time for the supercapacitor. Devices such as the Texas Instruments bq33100 Super Capacitor Manager or Linear Technology LTC3225 Supercapacitor Charger offer capabilities to address the unique requirements of supercapacitor management—but add additional burden to thin power budgets inherent in energy harvesting applications.
For the designer building energy harvesting designs, the problem is less about the availability of suitable devices than the tradeoff between the increased efficiency available with alternative devices and the increased power consumption of more sophisticated devices. In fact, while leakage current and standby power consumption are critical performance characteristics for these applications, engineers need to ensure that startup requirements remain within the power budget or the most efficient circuit will not be able to function at all. By combining both a detailed look at device power requirements and a long look at overall circuit operation, engineers can rely on available low-voltage DC-DC converters in zero-power designs able to run for years using ambient energy as the primary power source.
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