Here, a power converter is used with a feedback configuration which regulates the output voltage. Commonly, this is realized by a resistor divider between the output pin and ground. The mid tap is then fed to an error amplifier. Storage Interaction is needed to charge and discharge energy storages properly and prevent under or over-charging.
Battery-aware relay selection for energy-harvesting relays with energy storage
Due to power fluctuations this scheme cannot be followed completely, but at least voltage limiting for under and over-charge protection must be implemented. The feedback configuration of the converter is then a combination of controlling the output current and output voltage. The Energy Supervisor handles energy and control signals which are interwoven and thus illustrated by green and orange merging into one another. The energy supervisor covers all higher-order functions of a power management and guaranties the interplay of the aforementioned blocks.
Energy supervision functions here simplified as only one block can be used for the following tasks: 1 to sense internal states, 2 to process data and 3 to control power. In practice, the following devices are used: current and voltage sense circuits, or-ing diodes and load switches. Common names for control signals are chip enable , power good , and undervoltage lockout. More examples are listed in Table 1 , where the structure is applied to six state-of-the-art systems. In the function-oriented structure in Figure 2 , power converters the green blocks only have the purpose to transform voltage and current from one level to another.
The term power converter comprises of dc-dc converters, charge pumps and low drop-out regulators LDOs. The term PMIC refers to a wide range of chips commercially available for electric power conversion or energy supervision or a combination of both.
State-of-the-art dc-dc converters, such as the BQ from Texas Instruments, integrate more functions in one IC than just the power conversion. Furthermore, a voltage comparator and a load switch are implemented.
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All these functions Functions which belong structurally to the energy supervisor. Revising the structure in Figure 2 , the focus on power converters without supervising functionality leads to a good system understanding. However, the structure cannot represent the use of PMICs. This issue will be addressed in Section 3. Our solution general hardware blocks , and the implementation as a reconfigurable testbed is presented in Section 3.
The question is relevant as there is a huge variety of how to realize a micro-power management. This variety starts with the structure of the management and is followed by the implementation using discrete electronic components and PMICs. Regarding PMICs, numerous pins need to be connected correctly; external components like inductors, need to be selected carefully; and layout constrains need to be followed.
All of this non-standardized work can be very cumbersome for the designer and distracts from the overall system design. Furthermore, there are numerous storage technologies available such as: electric double-layer capacitor EDLC, short: supercaps , thin-film batteries and various types of secondary batteries.
Therefore a researcher, a design engineer or a student faces the challenging task of designing the optimal power management. Furthermore, an optimal solution is subjective and may mean the cheapest or the smallest or the most robust or the most energy-efficient solution. The motivation and application of our work are WSN. However, the concept and the reconfigurable testbed can be used during the design of any embedded system, preferably of self-powered ones.
The remainder of the paper is organized as follows: First we present in Section 2 related work. Then we present our solution concept in detail within Section 3.
Table of contents
Afterwards, in Section 4 we show the influence of parasitic effects and demonstrate the versatility of the testbed. Finally, we discuss our solution with respect to other modular platforms in Section 5 , and conclude the paper in Section 6. This documents the various components and architectures which are possible when designing a self-powered WSN. For a better overview all systems are summarized in Table 1. Afterwards, commercial and scientific platforms are presented which are designed for testing and prototyping.
How our system outperforms state-of-the-art modular systems and sets new standards is shown in the discussion section. To achieve the high system efficiency, dynamic voltage scaling for different application tasks is applied and three power switches to override the dc-dc converter in certain situations are used.
This observation complies with the fact, that all the following designs use dc-dc converters to increase the system efficiency:. Ambimax presented in [ 18 ], utilizes wind and solar harvesting to power a WSN. The energy is extracted by two LTC dc-dc converter from each harvester. The maximum power point is tracked by a pilot element a light sensor and a speed sensor, respectively which controls the dc-dc converter utilizing a comparator. Energy from these stages are buffered by super capacitors and merged using diodes.
The control is entirely analog without involving a microcontroller. The control of the power management is implemented in a complex programmable logic device CPLD. Internal voltages and currents are sampled using a four-channel analog-to-digital converter ADC. The authors realized a hybrid storage with a super capacitor and a Li-ion battery. A custom H-bridge bidirectional buck-boost converter was built to transfer energy between the storages.
The Hybrid Storage System described in [ 20 ] proposes a central 3. Thus, a hybrid storage comprising of a super capacitor and a Li-ion battery is realized. The authors used a microcontroller for energy supervision to control the charging direction between the storages depending on their voltage.
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Wang et al. The purpose of the WSN is to measure temperature, humidity, illumination and motion in buildings. The intermediate storage is a supercap array. A regulated output voltage for the load is provided by a TPS boost converter. Li et al. For accessing the piezo harvester, a rectifier and a step-down converter LTC are used. Both energy outputs are merged using Schottky diodes to charge a 25 mF supercap. Excess energy is transferred to a LiPo battery which is controlled by a pMOS transistor and a resistor divider. To increase the usable energy from the supercap, a buck-boost converter is used to supply a regulated output voltage.
State-of-the-art power converters LT and BQ are used to extract energy. The harvested energies are merged by diodes power or-ing and stored on a small LiPo battery. A constant voltage for the wristband electronics is provided by using the additional dc-dc buck converter of the BQ The modular plug-and-play platform presented in [ 24 ] is based on a multiplexer module.
Several energy components, such as a supercap, a battery or a PV module, are connected as boards via RJ45 cables. The design aims for a system which can be changed during runtime. Thus, each energy module includes a suitable dc-dc converter, protection circuitry and an EEPROM electrically erasable programmable read-only memory with electrical data. A microcontroller interrogates the modules using the multiplexer to control states and read out voltage levels.
Battery-aware relay selection for energy-harvesting relays with energy storage
Bader et al. Supercaps and thin-film batteries are compared in fixed combination with a step-up regulator and an LDO. For energy extraction, an additional dc-dc converter LT can be activated. Prototyping boards mentioned in [ 26 ] form a modular platform to evaluate different dc-dc converters and loads. The boards are connected side-by-side with pin connectors to form one wide bus. Considering commercial evaluation boards for PMIC, one will notice that through all well-known manufacturers the boards are not standardized by any means.
However, some companies offer evaluation kits for their PMICs including a solar cell and an energy storage. The individual functions can be chosen by jumpers. The successor-board called Gleanergy [ 29 ] focuses on ICs offering energy backup with primary batteries.
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