Energy harvesting for sub-0.5-V sources

Energy harvesting is taking off, generating interest for a wide variety of practical applications and embedded systems

BY BOB CHAO
Advanced Linear Devices
Sunnyvale, CA
http://www.aldinc.com

While the practice of energy harvesting has emerged from the laboratory and established legitimacy over the last few years, the electronic circuitry used in energy-harvesting systems has to advance to capture power from ever lower voltage sources to fulfill the promise of powering embedded systems. While potentially useful sources such as thermoelectric generators and single photovoltaic cells can generate relatively high-current, ultra-low-voltage circuitry is needed and can be designed to boost the low output voltages of these devices to useful operating voltages, making these sources practical for system design.

An opportunity

Energy harvesting continues to generate interest for a wide variety of practical applications and embedded systems. Since energy harvesting often requires a multidisciplinary approach in bringing systems to market, the electronics industry has an opportunity to provide the specialized circuitry for this nascent field.

Fig. 1. Energy-harvesting printed circuit board.

“Unlike the general electronics industry that is braced for a negative impact from today’s subprime market conditions, various energy-harvesting technologies and related power management ICs are poised for rapid and profitable growth in 2009,” says Linnea Brush, Senior Research Analyst, Darnell Group. “A convergence of several factors including new government regulations and economic incentives is resulting in a favorable environment for wireless sensor systems incorporating power sources based on energy harvesting.”

Freedom from conventional power sources

One of the primary missions with energy harvesting remains the same enable freedom from batteries and other conventional power sources by accumulating power generated by ambient sources that are readily available at the application site and can eliminate refueling requirements. Piezoelectric elements, for example, are well established in supplying power to embedded systems operating in the 3 to 5-V range when operating in environments where there is bountiful and everlasting vibration.

Circuits that capture, accumulate, and store energy from sub-0.5-V sources require special design considerations as these circuits need to operate below the conventional bipolar VBE or MOSFET threshold voltages. By examining the unique properties of ultra-low-voltage sources, a new benchmark can be established for developing the circuitry that make these sources a practical power supply for a range of embedded systems.

Energy capture

To start with, energy-harvesting circuitry must capture energy from sources producing about 100 to 200 mV. We are expecting this demand as solutions for using thermoelectric generators and single photovoltaic cells and other very-low-voltage energy sources.

Fig. 2. Requirements for an energy-harvesting system.

A single solar cell puts out in the neighborhood of 0.4 to 0.5 V depending on conditions. However, once it starts charging and generating electricity, the optimal power point is at about 50% of that voltage. Through experience, it’s necessary to have from 0.2 to 0.3-V operating voltage to draw useful power from a single solar cell and an energy-harvesting module would have to work in a voltage range to cover the low end of a solar cell’s output.

Thermoelectric energy generators consist of thermal couples that are arranged together by hundreds of strands. It is then possible to generate 100 to 200 mV using TEGs. These devices can put out up to a few hundred microamps in power for relatively low thermal differentials. The very-low-voltage energy that TEGs generate needs to be converted to high voltage and low current. The conversion requires a super voltage booster.

Energy-harvesting modules now on the market operate in the range of ±500 V down to ±4 V, and they burn about 0.9-µW maximum power. The design demands efficiency and typical power consumption will be much less than that. These modules can achieve more than 90% energy efficiency.

Ultra-low-energy considerations

For ultra-low-voltage energy-harvesting sources, the maximum input should be ±4 V and the minimum input should be as low as ±0.1 V. Once operating voltage goes below 1 V, things get interesting because it approaches zero. For example, a circuit operating at 0.1 V and boosting the output voltage to 4 V is a 40-fold voltage improvement, so it becomes apparent why energy boosting is so essential in circuit design.

To draw a comparison, many engineers may remember the old television booster circuits that could provide 5, 10, or 20 V or up to 20,000 V if it was needed. Energy-harvesting circuitry is similar, but everything has to be scaled down to this ultra-low-voltage level, which has never been done before.

One approach to design circuits that are up to the task is to use components that have very precise design specifications and thresholds. The use of precision gate trimming in the manufacture of these devices is one way to achieve these specifications. The level of precision and the level of operating voltage and current are the new frontier in this space. Currently, ultra-low-threshold-voltage MOSFETs are used in energy-harvesting circuits, but now even zero-threshold MOSFETs should be considered for ultra-low-voltage module design.

A key requirement for this ultra-low-voltage booster is to make it internally self-powered. In other words, you can’t work with 0.1 V and expect the circuit to need a 1-V power supply in an energy-harvesting system.

Another key design consideration is very-high-energy capture. With great efficiency, the energy-harvesting module has to retain energy and store it.

To make the module useful for providing output it would have to power CMOS logic circuits such as microprocessors and wireless sensor networks. It also has to have a long, projected operating life so designers should plan for about 70 years.

Even though the circuit will have no wearable parts, it should be designed to outlast the life of the system. Two other design considerations are unlimited charge and discharge cycles and Super Cap storage or battery storage capability.

While ultra-low-voltage energy sources are driving the design, it’s important to remember the primary mission of energy-harvesting modules: the process of capturing, accumulating, and storing energy from a variety of sources that by themselves produce waste energy. These waste energies normally cannot supply adequate power for any kind of useful purpose until a system is designed specifically to capture them.

In that environment, the fuel that drives the system is the waste energy. The old term for this practice is energy scavenging.

The purpose of energy harvesting is to effectively manage this small trace of harvested energy to power a variety of sensor circuits. But since the amount of energy is very small, most of the applications involve intermittent duty cycles that give the energy time to accumulate while the remaining microprocessor and sensory circuits are in sleep mode. The system needs to wake up, perform its duty, and then go back into sleep mode. If the application requires high energy and high duty cycle, energy harvesting probably will not be a suitable power supply. ■

For more on energy harvesting, visit http://www2.electronicproducts.com/Power.aspx.