Power Electronics

Effective Energy-Harvesting Requires Appropriate Components

Designers are attracted to energy harvesting solutions that eliminate the need to run power cables to remote locations and to eliminate the need for frequent battery replacement. To be effective, these energy harvesting systems need components that provide the requisite performance, efficiency, and physical size.

Efficient harvesting has been hampered by the many different power management blocks and associated functions. As essential components were assembled, more space was consumed, which defeated the goal of reducing application size and cost. Meanwhile, a larger battery cell was needed for storage because quiescent current was added to the system, and the overall power budget rose to unmanageable levels for low-energy ambient sources.

One component that integrates all the necessary power management functions is Maxim’s recently introduced MAX17710 (Fig. 1). Operating at an ultra-low current level, the MAX17710 accepts energy from a variety of poorly regulated energy harvesting sources with output levels ranging from 1µW to 100mW. Examples include light from photovoltaic cells, vibration from piezoelectric elements, heat from a thermoelectric generator, and RF from near-field communications (NFC).

MEC

Some alternative harvesting approaches use primary batteries. These batteries add bulk to the form factor and can be unwieldy to use in high-volume deployments. Eventually each battery needs to be replaced, a process that costs both time and money. A new component that solves the battery size and replacement issue is the THINERGY® Micro-energy Cell (MEC) from Infinite Power Solutions Inc. (IPS) a solid-state, rechargeable, thin-film battery, shown in Fig. 2. The MECs exhibit high cycle life capabilities and unique metal foil encapsulation enables decades of reliable, maintenance-free operation.

The MAX17710 integrates a programmable input boost regulator and needs no expensive external components to charge an MEC with energy sources as low as 0.8V. It protects the MEC by using a linear shunt-series regulator. An ultra-low-quiescent current, adjustable low-dropout linear regulator (LDO) with selectable voltages of 3.3V, 2.3V, or 1.8V allows the MAX17710 to adapt to a variety of loads. Packaged in a low-profile 0.5mm TQFN, it enables a new class of thin, card-like applications. The IC will also be available in wafer form to enable even thinner form factors. The MAX17710 is targeted for powered smart cards, real-time clock (RTC)/memory backup applications, and wireless sensor networks. Examples of wireless sensor networks include remote applications like irrigation valve control, building energy management, machine monitoring systems, asset tracking, biometric security systems, medical applications, and a myriad of portable consumer electronics.

The MAX17710 provides the power management to maximize, protect, and control the energy stored in MECs. “When combined with ambient energy harvesting, MECs uniquely enable autonomous, perpetually powered solutions for decades of use,” explained David Squires, Vice President of Business Development for Infinite Power Solutions. “In energy harvesting applications, a key enabler is the quiescent current drawn by the power-management IC,” Squires added. “The MAX17710 has an unprecedented 1nA battery current draw when a harvesting source is not present.”

The ultra-low operating current MAX17710 simplifies the design of energy-harvesting systems by integrating an input voltage boost circuit, a programmable output regulator, buffer energy storage management, and the charger and protection for THINERGY MECs.

The MAX17710 has an ultra-low-quiescent linear charger block to safely charge THINERGY MECs. To protect the MEC from overvoltage conditions, the MAX17710 regulates the input voltage and can shunt excess power. An ultra-low-quiescent current, undervoltage protection circuit prevents potentially damaging overdischarge of MECs. The undervoltage protection recovers only when an external energy source raises the voltage of the MEC back into a safe zone.

At very low temperatures, all batteries exhibit increased characteristic impedance, which limits high pulse currents to the application loads. The MAX17710 integrates a unique feature that manages an external storage capacitor to augment the battery output and provide high pulse currents, even at very cold temperatures like -40 °C.

Maxim’s MAX17710, in tandem with THINERGY MECs, overcomes the power- and battery-management limitations posed by the traditional sensor installations. Managing harvested ambient energy from available sources with the MAX17710, and efficiently storing that energy in a THINERGY MEC, provides an autonomous, maintenance-free energy source to power a remote sensor. This solution eliminates the need for expensive wiring or prohibitive labor costs to replace traditional primary batteries.

The MAX17710 also works in power-bridging applications where energy harvesting is not necessarily used. In such applications, infrastructure power (from the grid or a larger battery) is typically available to power the system and can trickle-charge an MEC for memory or RTC backup power. In the event of a loss of grid power, or a system “brownout” during replacement of the larger system battery, the stored energy in the MEC continues to power volatile memory and/or an RTC for hours, days, or even weeks. This solution displaces bulky coin cells and supercapacitors that have high self-discharge currents and limited life, especially at elevated temperatures.

Designed for the low-profile requirements of many energy harvesting applications, the MAX17710 is packaged in a lead-free, 12-pin, 3mm x 3mm x 0.5mm UTDFN.

Evaluation Kit

The IPS-EVAL-EH-01 kit available from IPS is a universal energy-harvesting evaluation kit for autonomous wireless sensors and other sub-100mW applications (Fig. 3). The kit features the company’s award-winning THINERGY® MEC101 solid-state, rechargeable, micro-energy cell, a unique type of thin-film battery with industry-leading performance. The kit also features the recently released MAX17710 from Maxim Integrated Products. This kit enables design engineers to develop and evaluate self-sustaining “green” power supply solutions, displacing the use of conventional batteries and supercapacitors that require frequent replacement. It can also be used to power popular third-party micro-controller and radio module development kits, enabling designers to create and evaluate a variety of self-powered microelectronic applications. The circuit board footprint of the IPS-EVAL-EH-01 is similar to a credit card and consists of three key elements: energy harvesting, energy storage and energy management. Due to the unique and ultra-low quiescent current of the integrated Power Integrated Management IC from Maxim, and the low self-discharge rate of the integrated THINERGY MEC101, the IPS-EVAL-EH-01 Energy Harvesting evaluation kit efficiently accepts charge currents less than one microampere, making it ideal for storing harvested energy. An amorphous silicon photovoltaic solar array is integrated for out-of-the-box functionality and recharges the MEC101 to a 100% state of charge in about 30 minutes in full sun and in about 8 hours in typical office lighting (~1,500 lux). In addition, the board provides an interface to any type of external ambient energy-harvesting transducer.

The kit also features an interface header and 6-pin connector that are compatible with popular micro-controller and low power radio development kits available from a variety of leading semiconductor and module manufacturers such as Anaren, BlueRadios, Energy Micro, Microchip, Silicon Laboratories and Texas Instruments.

Harvested energy is efficiently stored in a THINERGY MEC101-7S, which is a 4V, 0.7 mAh capacity rechargeable battery ideally suited for energy-harvesting applications. The MEC101-7S is a thin, flexible, solid-state and near loss-less energy storage cell the size of a postage stamp. Self-discharge is so low and insignificant that energy can be reliably stored for decades on a single charge. IPS continues to lead the industry in delivering the most efficient and powerful batteries in its size class--outperforming all other energy storage technologies currently on the market, such as supercapacitors, coin cells, printed batteries and other thin-film batteries.

Extremely small, powerful and robust, the MEC101 features a unique, patented, flexible package design that maximizes the active area of the cell to deliver high energy and power density. The MECs can be stacked vertically in a parallel configuration for more power and capacity without consuming additional system footprint. For example, a five-cell MEC101 stack is approximately 1mm in total thickness and delivers an amazing 200mA of continuous discharge current (approximately 0.5W of power). The MEC101 can recharge to 90% in only 15 minutes and can be recharged more than 100,000 times. With an expected lifetime of up to 20 years, THINERGY MECs require no maintenance or periodic replacement compared to other batteries and supercapacitors, allowing for the lowest total cost of ownership. Moreover, THINERGY MECs contain no heavy metals, are eco-friendly, and completely safe. As a result, THINERGY MECs are changing how the power source for micro-electronic systems is being design.


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