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Integrate power mgmt functions for energy harvesting

Posted: 16 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:DC? AC? energy harvesters? Charger?

In the last few years, various companies have made considerable effort to make "perpetually powered" and battery-free systems which operate from ambient energy. The key integrated circuits (ICs) needed to develop such a system are ultra-low-power microprocessors, radios, and power-management ICs (PMICs).

While considerable progress has been made in the field of low-power microprocessors and radios, only recently have PMICs suited for energy harvesting applications started appearing in the market. This article provides a quick introduction to some of the ambient energy sources available followed by a detailed discussion of factors to be considered when choosing a PMIC for these energy sources.

Ambient energy sources can be broadly divided into direct current (DC) sources and alternating current (AC) sources. DC sources include harvesting energy from sources that vary very slowly with time, such as light intensity and thermal gradients using solar panels and thermoelectric generators respectively. The output voltage of these harvesters does not have to be rectified.

AC harvesters include energy harvesting from vibrations and radio frequency power using piezoelectric materials, electromagnetic generators and rectifying antennae. The output of these energy harvesters must be rectified to a DC voltage before it can be used to power a system.

In this article, only DC energy harvesters are considered as energy harvesters using these sources are easier to obtain in high volume quantities as opposed to AC harvesters

Figure 1: Block diagram of generalized energy-harvesting system.

Figure 1 show a generic architecture of an energy-harvesting system. The overall system consists of the ambient energy source, energy buffer (super capacitor/battery), the PMIC, and the system load. Since the energy available from the energy source is dependent on time-varying ambient conditions, the energy from the source is extracted when available and stored on the energy buffer.

The system load is powered from the energy buffer. This allows the system to work, even if there is no ambient energy available. The power management unit itself consists of a DC/DC power converter with an optimized interface to the energy harvester, battery management circuitry, output regulator, and cold start unit. The function and design considerations for each of these blocks is discussed next.

Charger
The function of the charger is to extract the maximum possible energy from the solar panel or TEG and transfer the energy to the storage element. The primary factors to be considered for the charger include topology, efficiency, maximum power extraction network and complexity. The common charger topologies include linear dropout (LDO) regulators, buck converters, boost converters and buck-boost converters.

For a solar panel, the topology is primarily dependent on the output voltage of the solar panel stack. Typically, the output of a single cell solar panel is 0.5V. Therefore, for systems with single cell and two cell solar panels, a boost converter topology is required, as battery voltages are typically greater than 1.2V for NiMH and 3V for Li-Ion batteries.

For a higher number of series-connected cells, other converters such as a diode rectifier, buck regulator, or an LDO can be used. For the thermoelectric generator (TEG), the output voltage ranges from 10mV to 500mV. Therefore, for a TEG, a boost converter is the primary topology of choice. It is possible to stack a number of TEGs in series to obtain a higher voltage so that an LDO or buck regulator can be used. The disadvantage of such a scheme, however, is the large series impedance of the TEG stack.

Figure 2: a) Model of solar panel and b) thermoelectric generator.

To extract the maximum power from a solar panel or thermoelectric generator, the panel or TEG must be operated at its maximum power point. To understand the need to operate the energy harvester at its maximum power point, consider the solar panel and TEG model shown in figure 2a and figure 2b, respectively.

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