Battery Boost for Ultra-Low Power Systems

The very recent technical performance advances in energy storage, battery boost, and power management devices have enabled fast growth in the energy-harvesting sector, particularly for wireless sensor network applications. This article will highlight some examples of devices, evaluation boards, and development kits that may help designers exploit this fast moving technology.

The ability to power sensor networks without wiring or battery replacement has opened up a vast range of new applications, such as smart building control, industrial control, security, asset tracking and environmental monitoring. Systems that scavenge energy from ambient sources, including sunlight, vibration or temperature differential in order to power these wireless sensor networks, are becoming increasingly popular, some would say ubiquitous. This is primarily due to technology advances in power management devices, enabling more efficient use of harvested energy.

Some sensor network systems may have access to sufficient ambient energy sources that they can run autonomously and virtually indefinitely. Sensors on industrial machines, trains, or bridges, for example, that can harvest energy from vibration, can be designed such that there will always be enough ambient energy available to power sensors to take readings and transmit data. Others, where energy can only be harvested intermittently (such as photovoltaic cells), or where there are insufficient power levels, will require some form of energy storage device. Primarily, batteries or supercapacitors are the energy storage devices of choice.

How frequently sensor readings need to be taken and transmitted depends very much on the specific application, and this affects energy storage design decisions. Generally, these systems are designed to be predominantly in standby mode, requiring only a few microwatts of power. The sensor circuitry only wakes up periodically to take and send a reading. Some applications may only require sensors to be read a couple of times an hour. Others may be programmed to wake up only once or twice a day or once a week. However, real time monitoring is growing in interest in the transportation sector and for industrial machine monitoring, for example. It is in this area that improvements in energy storage technology, as well as power management, are facilitating growth.

Long live the battery!

One of the initial barriers to the early uptake of wireless sensor networks for monitoring applications was the potential cost and inconvenience of having to replace batteries which may be in remote locations or even totally inaccessible. However, with energy harvesting, and thanks to longer lasting batteries that can be trickle-charged by the energy-harvesting device, systems are now commonly designed to be autonomous for a decade or more – often longer than the expected lifetime of the end application!

Another beauty of energy harvesting is the ability to run ultra-low power systems. However, energy sources for harvesting applications are typically not only low power, but also low voltage – normally below 0.5 V. Typically, each node on the wireless sensor network is going to need a power burst, which could be several hundred milliwatts, to wake up a microcontroller or other electronic device, take a sensor reading, convert it to digital format, and transmit it using a low power RF communications protocol. Most electronic circuits, including some power conversion devices and even ultra-low power microcontrollers, require voltages of at least 1.8 V, and sometimes 3.3 to 5 V or more. Similarly, for communications purposes, while the power output of energy harvesters is typically tens or hundreds of microwatts, the power consumption of RF devices can be as high as tens of milliwatts.

The solution to delivering the necessary energy burst is to first store the harvested energy in a battery or supercapacitor, then boost up the voltage as efficiently and reliably as possible. A growing range of manufacturers is producing battery boost converter devices designed specifically for energy harvesting applications. These include Advanced Linear Devices, Cymbet, Linear Technology, Maxim, Microchip, and Texas Instruments.

It is worth remembering that a battery boost converter can usefully optimize the power efficiency of a system by providing regulated power over the battery’s operating range. In turn, this can make a microcontroller more efficient by enabling it to run at a lower voltage. A further advantage is that the boost converter can provide short circuit protection through current limiting. Discharging a battery at voltages lower than its rated value can also damage the battery, providing further justification for battery boost converters.

Device manufacturers are exploiting a symbiotic relationship between the microcontroller and a booster. In operation, the microcontroller turns on the boost circuit when the system voltage falls below a preset level, and then shuts it off, eliminating any operating current. Yet the microcontroller continues to access the power it needs from the booster’s output capacitor. Meanwhile, the booster circuit can provide a voltage to power an application while the system is in sleep mode.

Battery or supercapacitor

Before reviewing some of the battery boost devices currently available, it is worth considering the features, benefits and constraints of batteries compared to supercapacitors when applied to storing energy harvested from the environment.

The primary advantage of supercapacitors is their capacity. If an application requires fast charging and discharging with a significant burst of energy then a supercapacitor has the advantage over a battery. Supercapacitors can be used in hybrid vehicles, for example, as they can capture and store large amounts of electrical energy generated during braking, and release it quickly for reacceleration. Smaller devices are equally useful in ultra-low-power sensor network applications.

Supercapacitors are generally characterized with a far higher cycle life than batteries ¬– as much as 500,000 charge/discharge cycles compared to around 2,000 for batteries. Furthermore, they operate over a wider temperature range, making them more suitable for use in solar powered systems, location tracking and on machinery.

The downside of the supercapacitor is leakage current, which claims to be proportional to the capacity – that is, the leakage current increases as capacity increases. The operating range of supercapacitors suitable for energy harvesting applications runs from around 2.1 to 5.5 V.

One of the leading manufacturers in this market is NessCap, with its Electric Double Layer Capacitor (EDLC) devices in the UltraCap range. These small-sized EDLC cells range from 3 F to 50 F in capacitance with operating voltages ranging from 2.3 V to 2.7 V. The cells are designed for use in various types of applications, which mainly require smaller amounts of capacitance but immediate pulse power, such as automatic meter reading and portable consumer electronics.

The physically smallest device in the UltraCap range (see Figure 1 below) is just 20 mm long and 8 mm in diameter. It features a 3 F rated capacitance and 2.7 V rated voltage. NessCap quotes an impressive 87,600 h lifetime at 65oC and 500,000 cycles.

Further information on using supercapacitors in energy harvesting applications can be found in an earlier article¹ in the Energy Harvesting TechZone. A brief review of devices from companies including AVX, Cooper/Bussman, Cornell Dubilier, Elna, Panasonic and Taiyo Yuden is also included.

For energy harvesting wireless sensor network applications that require a more moderate power burst and constant power source, batteries can provide an effective solution. However, the disadvantage of batteries is a comparatively low charge/discharge cycle lifetime. All batteries, furthermore, require protection and conditioning for optimum performance.

Popular battery technologies for energy harvesting applications include lithium ion, coin cell, and thin film. Lithium ion/polymer batteries offer high energy density, low self-discharge, low maintenance and wide voltage range. Coin cells are compact and have stable discharge characteristics as well as high energy density. Emerging thin film battery technology is addressing the charge/discharge cycle constraint, and offers format flexibility in that it can be manufactured in virtually any shape. Depending on capacity, thin film batteries can be smaller than coin cells, and can be more easily integrated into electronic circuitry and modules. It is interesting to consider a couple of examples of this innovative approach to energy storage.

Thin film battery innovations

A recognized leader in thin film battery technology (as well as energy harvesting power management solutions) is Cymbet. Its EnerChip™ family of rechargeable thin film batteries is packaged in a convenient and compact surface-mount form factor, smaller than conventional coin cells. They are also longer lasting and self-discharge rates have been greatly reduced, making them ideal for energy harvesting applications. Available in 5, 12, or 50 µAh capacities, these solid-state chips can be incorporated with other electronic components in a single integrated package or multi-chip module. The EnerChip CC range incorporates integrated power management for use in battery back up and secondary power applications. The CBC050-M8C, for example, is the standard Enerchip 50 µAh capacity device, while the CBC3150-D9C is the EnerChip CC version with integrated power management.

Another international leader in thin film micro-energy storage devices is Infinite Power Solutions. Its THINERGY® MEC220 and MEC225 families of paper-thin lithium batteries claim to provide unrivaled cycle life and power performance, with low self-discharge rates, making them ideal for energy harvesting applications. Typical devices available include the MEC220-3P with a capacity of 300 µAh and voltage rated at 4.1 V, and the smaller MEC225-1S with a capacity of 130 µAh. An evaluation board with solar cells and MEC201 thin film battery, plus power management circuitry is also available to give first time designers an idea of how this technology works.

Complementary combination

While some applications will choose between supercapacitors or batteries, some will find the two technologies complementary. There may be times when a supercapacitor cannot store enough energy, such that a battery back up may be necessary for use over an extended time when the ambient source is not available. Conversely, an application may sometimes require a burst of peak power that exceeds the limit the battery can supply. The battery can be designed to charge the supercapacitor for the power burst, which obviates the need to deeply discharge the battery.

Ultra-low power advances

It has only been comparatively recently that significant advances and innovations in the critical components used in energy harvesting systems have been made, dramatically improving the commercial viability of this technology in extended wireless monitoring applications. Ultra-low-power microcontrollers with integrated peripherals such as multiple a-to-d converters and wireless transceivers, for example, have been available for a while. Now they can be supplied with ultra-low-power transceivers for more energy efficient wireless connectivity. However, the greatest advances have been made in the new generation of energy harvester blocks, with reduced component count, improved conversion efficiency and reduced quiescent currents. These improvements have an added benefit of enabling the use of smaller and lower cost batteries and supercapacitors.

Linear Technology introduced one of the earliest of the new generation boost converter devices. The LTC3105 operates from a 225 mW input voltage, and features an integrated maximum power point controller (MPPC), which enables operation directory from low voltage, high impedance alternative power sources. A burst mode operation provides a 250 mV start up capability. Since then, the LTC3108 has emerged, described as a highly integrated DC/DC converter based on the same step-up topology allowing it to operate from input voltages as low as 20 mV. It features a selectable voltage output of 2.35, 3.3, 4.1 or 5 V, to power an external microcontroller. Meanwhile, LDO is quoted at 2.2 V at 3 mA, sufficient to power a wireless transceiver. A storage capacitor is included for when the ambient energy source is unavailable, and it can be used to charge supercapacitors. Meanwhile, the LTC3109 adds autopolarity architecture, operating from inputs down to +/-30 mV, and can be used with a capacitor or battery as a backup.

Another popular offering from Linear are the LTC3588-1/2 piezoelectric energy harvesting power supply solutions, integrating a low-loss full-wave bridge rectifier with a high efficiency buck converter. The LTC3588-1 has an ultra-low quiescent current under-voltage lockout (UVLO) mode with a wide hysteresis window, which allows charge to accumulate on an input capacitor until the buck converter can efficiently transfer a portion of the stored charge to the output. In regulation, the LTC3588-1 enters a sleep state in which both input and output quiescent currents are minimal. The buck converter turns on and off as needed to maintain regulation. With the LTC3588-2, an ultra-low quiescent current under-voltage lockout (UVLO) mode with a 16 V rising threshold enables efficient energy extraction from piezoelectric transducers with high open circuit voltages. This energy is transferred from the input capacitor to the output via a high efficiency synchronous buck regulator.

Interestingly, Midé Technology has an energy harvesting development and evaluation board module, the Volture EHE004, based on the LTC3588. Exploiting energy generated by piezoelectric vibration, the kits are successfully used to power sensors for wireless sensor networks in the defense, avionics, industrial, transportation and factory automation sectors.

Described as an energy harvesting conditioning circuit, it converts the AC output from a piezoelectric energy harvester to a regulated DC output. The kit consists of a full-wave rectifier with integrated charge management and DC/DC conversion, and connects directly to any Volture piezoelectric energy-harvesting product. The DC output can be configured to the following voltage settings: 1.8 V, 2.5 V, 3.3 V, and 3.6 V. The board includes 200 µF of storage capacitance onboard, and more capacitance can be added if required. The kit uses the LTC3588 to maximize total piezoelectric energy harvester output and mechanical-to-electrical conversion efficiency.

Stepping up to low power

Receiving good reviews in the press for some time now are the range of step-up low voltage booster devices and energy harvesting modules from Advanced Linear Devices. The latest devices, the EH4205 and EH4295, together with modules including the EH300 and EH301, are covered in depth in another TechZone article.²

Amongst the wide range of power devices, battery chargers, and specialized energy harvesting components and kits developed by industry giant Texas Instruments, the BQ25504 low power boost converter is proving popular. Targeted squarely at power management solutions for energy harvesting applications, the device is designed to acquire and manage power continuously from ambient energy sources. The boost converter requires an input of 330 mV initially, but can then harvest energy from the source with V_IN down to 80 mV. It can be incorporated in a system to store energy in lithium ion batteries and supercapacitors. It also includes circuitry to protect the energy storage element from over and under voltage conditions, and to kick-start the system when the battery is deeply discharged. Typical quiescent current is quoted at 330 nA. A BQ25504 Evaluation Board is a useful start point for designers keen to experiment in this area.

Another device worthy of investigation from TI is the TPS62120, one of a family of high efficiency synchronous step down DC/DC converters optimized for low power applications. It supports up to 75 mA output current and allows the use of tiny external inductors and capacitors. The wide operating input voltage range of 2 V to 15 V supports energy harvesting and low power, battery powered applications. A power save mode maintains high efficiency over the entire load current range. It consumes only 10 uA quiescent current from V_IN in PFM mode operation. The device has an additional SGND pin, which is connected to GND during shutdown mode. This output can be used to discharge the output capacitor.

Partnerships and solutions-based approaches

A number of vendors in the energy-harvesting sector are combining components to provide more complete solutions, reference designs for specific applications, or development platforms for a range of systems based on the same architecture. These and other vendors, in this fast growing marketplace, are regularly partnering to provide more complete, yet flexible solutions for customers.

Maxim Integrated Products, for example offers the MAX17710 energy harvesting evaluation kit. Self-powered by on-board solar cells, the board integrates power management functions together with circuitry for charging and protecting micro-energy cells. This evaluation board contains a THINERGY thin film battery from Infinite Power Solutions. Using the MAX17710, energy can be input from a variety of energy sources with output levels ranging from 1 µW to 100 mW. Quiescent current is very low and an adjustable LDO offers selectable voltages of 1.8, 2.3 or 3.3 V.

A range of energy harvesting development kits, evaluation boards and reference designs are available from Microchip Technology. The XLP 16-bit energy harvesting development kit, for example, is based on the XLP series of PIC microcontrollers. It is ideal for developing applications with sleep currents down to 20 nA, active mode currents down to 50 µA/MHz, code execution efficiency, and multiple wake-up sources. With the end devices powered only by radio waves (RF energy), the development kit enables rapid prototyping of low power applications such as RF sensors, temperature/environmental sensors, building automation and security sensors.

Another version of the kit, the DM240311, based on ultra-low-power, nano-Watt XLP technology, is suitable for prototyping many low power applications including RF sensors, metering sensors, remote controls, security sensors and other energy harvesting applications.

Cymbet is another example of a supplier offering discrete components, partial solutions and fully integrated reference designs, partnering with other vendors, including Microchip, Silicon Labs and Texas Instruments. In addition to thin film batteries, the company has available specialized power management devices, as well as evaluation kits for battery charging and energy harvesting applications. The CBC-EVAL-09, for example, includes power management circuitry, energy storage via its EnerChip thin film battery, and an energy harvesting processor operating from a range of sources, including EM/RF, solar, thermal and vibration.

An example of Cymbet’s collaboration with Texas Instruments can be seen in TI’s eZ430-RF2500-SEH solar energy harvesting development kit. It is aimed at perpetually powered wireless sensor network applications based on TI’s ultra-low-power MSP430 microcontroller. The Solar Energy Harvester module includes a high-efficiency solar panel optimized for operating indoors under low-intensity fluorescent lights. This provides enough power to run a wireless sensor application with no additional batteries. Inputs are also available for external energy harvesters such as thermal, piezoelectric, or another solar panel. The system manages and stores additional energy using a pair of Cymbet’s thin-film rechargeable EnerChips, capable of delivering enough power for 400+ transmissions. The EnerChips act as an energy buffer that stores the energy while the application is sleeping and recharges while there is light available to harvest.

Conclusion

This article has highlighted some examples of devices, evaluation boards, and development kits that may help designers exploit the energy storage, battery boost and power management technology that is enabling fast growth in the energy-harvesting sector. Designers might also like to reference a number of additional related articles in the TechZone, covering topics including: minimizing power consumption³ in wireless sensor applications; ultra low power MCUs⁴; and power management ICs for micro-harvesting designs.⁵

References:

1. Supercapacitors for energy harvesting article
2. Step-up Micropower Voltage boosters article
3. Minimizing power consumption article
4. Ultra Low power ICs article
5. Power management ICs article