In the last few years, there has been considerable effort by various companies 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.
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.
A solar panel can be modeled as a reverse-biased diode that delivers current in parallel with a parasitic capacitance (CHRV). The current output of the diode is proportional to the light intensity. The model of a thermoelectric generator consists of a voltage source in series with a resistor. The resistor models and the internal impedance of the TEG are dependent on the material properties and dimensions of the TEG.
For a typical solar panel and TEG, the current versus voltage and power versus voltage is shown in Figure 3 and 4, respectively. You can see that for a solar panel, the maximum power is obtained at approximately 80 percent of the open circuit voltage (OCV). Similarly, for the TEG, the maximum power point is obtained at 50 percent of the OCV.
Based on the curves presented in Figure 3, it is clear that an interface circuit is needed to extract the maximum power available. The maximum power extraction circuit dynamically adjusts the input impedance of the power converter to extract the maximum power. For solar-energy harvesting, maximum power extraction is done using simple techniques such as input-voltage regulation at a fixed fraction of the open-circuit voltage, input-current regulation at a fixed fraction of the short-circuit current, or using complex microprocessor-based techniques.
Figure 3: Voltage versus current, and voltage versus power, of a solar panel.
Figure 4: Voltage versus current, and voltage versus power, of a thermoelectric generator.
Some of the techniques for extracting maximum power from TEGs include dynamically changing the switching frequency of a DC/DC converter and regulating the input voltage of a DC/DC converter at 50 percent of the open circuit voltage. In all these converters, the output voltage is determined by the energy buffer.
Note that the choice of converter topology is a tradeoff between design complexity, component count, and efficiency. Switching converters typically provide better efficiency than linear regulators, but at the cost of increased components, design complexity and board space.
In energy-harvesting systems, an energy buffer is used to store the intermittently energy available from the energy harvester. The stored energy is then used to power the system. This architecture allows the overall system to operate continuously, even though the energy available is intermittent.
The commonly used energy buffers include rechargeable batteries of different chemistries, as well as super capacitors. The battery-management circuitry has two main functions:
•First, it monitors the voltage across the energy buffer and ensures that the voltage is within the safe operating region determined by the undervoltage (UV) and overvoltage (OV) thresholds.
•Second, it monitors the capacity of the energy buffer and provides an indication to the load regarding the availability of energy to do useful work.
The capacity measurement can be performed using simple techniques such as monitoring the voltage across the energy buffer or using fuel gauging techniques, which measure the voltage and current sourced and sunk by the battery. In cases when a simple voltage-based technique is used to provide an indication of the capacity left in the energy buffer, a user-programmable intermediate-voltage level known as the power-good level can be implemented.
The design considerations of the battery-management section are dependent on the energy buffer used. For rechargeable batteries, the OV and UV thresholds are based on the cell chemistry. For super capacitors, the OV and UV thresholds are determined by the lower value of the absolute max ratings of the IC and the capacitor. Using the optimal settings for the energy buffer maximizes the life time of the system.
Another consideration in the design of the battery-management section is the quiescent current consumed by the battery-management section. The circuitry in the battery-management block includes building blocks such as references, comparators, and digital logic. The current consumed by these circuits must be minimized. This is because any energy used by the battery-management section drains the battery and the energy is not being supplied to the external load.
The cold-start unit is an optional block that may or may not be present in a typical energy-harvesting PMIC. The function of the cold-start unit is to boot strap the system when there is insufficient energy stored in the storage element.
The design of the cold-start unit is application dependent. For solar applications, an input-powered (as opposed to a battery-powered) oscillator can be used to drive the switches of a temporary low efficiency switching converter (Reference 1). Once sufficient energy has been built up in the energy buffer, the highly efficient switching converter can take over.
For thermoelectric generators, the cold start units can be implemented using transformer coupled oscillator topologies or by using mechanical motion of the system (References 2 and 3). The design considerations for this block are minimum startup voltage, startup power, peak inrush-current, and time needed for startup.
The function of the regulator is to provide a regulated voltage from the battery. The topology of this block is dependent on the battery, system-load requirements, and quiescent current.
In this article we discussed several key considerations in the design or selection of a power management IC for DC energy-harvesting applications, including the design considerations for each of the IC building blocks. A PMIC for energy harvesting can have some or all of the functions integrated onto a single IC. The choice of the PMIC is dependent on the energy harvesting source, energy buffer, and system load.
- “Low input voltage step-up converter in 6 pin SC-70 package,” (TPS61220), Data Sheet (SLVS776), Texas Instruments, Jan 2009.
- “A battery-less thermoelectric energy harvesting interface circuit with 35 mV startup voltage,” Yogesh Ramadass, Anantha Chandrakasan, IEEE Journal of Solid State Circuits, Jan 2011.
- “Design of a low input voltage converter for thermoelectric generators,” John Damaschke, IEEE Journal of Industrial Applications, Vol 3. No. 55. pp 1203 – 1208, 1997.
About the Authors
Karthik Kadirvel is a mixed-signal design engineer in the nano-power energy harvesting group at Texas Instruments. He holds a Doctoral degree from the University of Florida.
John Carpenter is the systems engineer for the nano-power energy harvesting group at Texas Instruments. He holds a master’s degree from the University of South Florida.
Brian Lum-Shue-Chan is the design manager for the Battery Management group at Texas Instruments. He holds a BS and MS degrees in Electrical Engineering from Georgia Institute of Technology, Atlanta, Georgia.
These authors can be reached at firstname.lastname@example.org.
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