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　　Technologies to solve industrial energy harvesting problems

　　Around the world, engineers are providing new and innovative ways to harness non-traditional energy sources to solve real-world problems. Increased safety and accessibility, lower maintenance costs, improved energy efficiency and system flexibility are just a few of the many benefits that can be achieved with energy harvesting, wireless detection and monitoring/control systems. The high cost of energy, new government regulations, and environmental concerns have led to a dramatic increase in the need for more efficient power usage in a variety of applications. Emerging alternative energy technologies and improvements in power utilization have the potential to deliver performance breakthroughs in many different markets. Additionally, new products that take advantage of these new technologies represent excellent growth opportunities, both in the short and long term.

　　Many low-power industrial sensors and controllers are transitioning to alternative energy sources as their primary or secondary power supply. Ideally, this harvesting of energy would completely eliminate the need for wired power sources or batteries. Transducers that generate electricity using readily available physical power sources such as thermoelectric devices [thermoelectric generators or thermopiles], mechanical vibrations [piezoelectric or electromechanical devices], and light [photovoltaic devices] are becoming suitable power sources for many applications. Numerous wireless sensors, remote monitors, and other low-power applications are evolving into near-zero-power devices (often referred to by some as nanopower) that use only harvested energy.

　　Although energy harvesting has been around since the early 2000s (when it was in its infancy), only recent technological developments have pushed it to the commercial stage. In short, in 2010 we will usher in its growth phase. Building automation sensor applications using energy harvesting technology have been promoted in Europe, indicating that its growth phase may have begun.

　　Existing applications demonstrate commercial viability

　　Although the concept of energy harvesting has been known for many years, implementing a system in a real-world environment is cumbersome, complex, and expensive. However, examples of markets where energy harvesting methods are being used include transportation infrastructure, wireless medical devices, tire pressure monitoring and, of course, building automation. In the case of building automation, systems such as occupancy sensors, thermostats and light switches can eliminate the normally required power or control wiring and replace it with a mechanical or energy harvesting system. In addition to eliminating the need for wiring installation in the first place (or periodic battery replacement in wireless applications), this alternative reduces the routine maintenance costs that often exist with wired systems.

　　Similarly, wireless networks using energy harvesting technology can connect any number of sensors within a building to reduce heating, ventilation, and air conditioning (HVAC) by cutting power to non-critical areas when no one is in the building. and lighting costs. In addition, the cost of energy harvesting electronic circuits is often lower than the operating costs of detection circuits, so the use of energy harvesting technology can obviously bring economic benefits.

　　A typical energy harvesting configuration or system (represented by the four main circuit system blocks shown in Figure 1 below) usually includes a free energy source, such as a thermoelectric generator connected to a heat source (such as an HVAC duct). generator (TEG) or thermopile. These small thermoelectric devices can convert small temperature differences into electricity. This electrical energy can then be converted by an energy harvesting circuit (the second module in Figure 1) and changed into a usable form for powering downstream circuitry. These downstream electronics will typically include some type of sensor, analog-to-digital converter, and an ultra-low-power microcontroller (the third module in Figure 1). The above components can take this harvested energy (now in the form of electrical current) and wake up a sensor to obtain a reading or measurement, then make this data available through an ultra-low-power wireless transceiver (in the circuit chain shown in Figure 1 represented by the fourth module) for transmission.

　　Figure 1: Four main modules of a typical energy harvesting system

　　Each circuit system module in the link (with the possible exception of the energy source itself) is unique to a set of constraints that have so far undermined its commercial viability. Low-cost and low-power sensors and microcontrollers have been available for quite some time; however, ultra-low-power transceivers have only become commercially available in the past few years. However, the laggards in the link have been the energy harvesters and power managers.

　　Existing power manager module implementations are low-performance discrete structures that typically include 35 or more components. Such designs have low conversion efficiency and high quiescent current. These two shortcomings lead to performance losses in the end system. Low conversion efficiency will increase the time required to power up the system, which in turn increases the time between taking a sensor reading and transmitting that data. High quiescent current limits how low the energy harvesting power supply can go, as it must first exceed the current level required for operation before any remaining energy can be used to power the output. Finally, it requires very advanced analog switch-mode power supply expertise, which is in short supply!

　　The missing link, if you will, has been highly integrated DC/DC converters capable of harvesting and managing residual energy from very low input supply voltages. However, this is about to change.

　　missing link

　　Linear Technology recently introduced its LTC3108, an ultra-low voltage boost converter and power manager designed to greatly simplify the acquisition and management of power supplies from very low input voltages (e.g., thermopiles, thermoelectric generators [TEG], and even small solar cells) are designed for the task of residual energy. Its boost topology operates from input voltages as low as 20mV. This is important because it enables the LTC3108 to harvest energy from a TEG with a temperature change as small as 1°C, something that would be difficult to do due to the high quiescent current of discrete implementations.

　　The circuit shown in Figure 2 uses a small step-up transformer to increase the input voltage supply to an LTC3108, thus providing a complete power management solution for wireless sensing and data acquisition. It captures small temperature differences and generates system power without using traditional battery power.

　　Figure 2: The LTC3108 used in a wireless remote sensor application is powered from a TEG (pelTIerCell)

　　The LTC3108 utilizes a depletion-mode N-channel MOSFET switch to form a resonant boost oscillator (using an external boost transformer and a small coupling capacitor). This allows it to boost an input voltage as low as 20mV to a level high enough to provide multiple regulated output voltages for powering other circuits. The frequency of oscillation is determined by the inductance of the secondary winding of the transformer, usually in the range of 20kHz to 200kHz.

　　For input voltages as low as 20mV, a primary-to-secondary turns ratio of approximately 1:100 is recommended. For higher input voltages, a lower turns ratio can be used. These transformers are standard commercially available components and are readily available to order from magnetics suppliers. Low voltage operation of 20mV is achieved with our composite depletion mode N-channel MOSFETs.

　　As can be seen in Figure 3, the LTC3108 takes a system-level approach to solving complex problems. It is capable of converting low voltage power and managing energy between multiple outputs. The AC voltage developed on the secondary winding of the transformer is boosted and rectified using an external charge pump capacitor (connected between the secondary winding and pin C1) and a rectifier internal to the LTC3108. This rectifier circuit feeds current into the VAUX pin and delivers charge to the external VAUX capacitor and then to the other outputs.

　　The internal 2.2VLDO can support a low-power processor or other low-power IC. The LDO is powered by the higher of VAUX and VOUT. This allows it to enter operation immediately when VAUX charges to 2.3V (while the VOUT storage capacitor is still charging). If there is a step load on the LDO output, current can be taken from the main VOUT capacitor if VAUX drops below VOUT. The LDO output is capable of delivering up to 3mA.

　　Figure 3: LTC3108 block diagram

　　The main output voltage on VOUT is charged from the VAUX supply and can be set by the user to one of four regulated output voltages using the voltage select pins VS1 and VS2. 4 fixed output voltages are: 2.35V (for supercapacitors), 3.3V (for standard capacitors), 4.1V (for Li-ion battery terminals) or 5V (for higher energy storage) and a main system The power rail (used to power a wireless transmitter or sensor) eliminates the need for external resistors with values of several megaohms (MΩ). Therefore, unlike discrete designs that require very large value resistors, the LTC3108 does not require special circuit board coatings to minimize leakage.

　　The second output (VOUT2) can be turned on and off by the main microprocessor using the VOUT2_EN pin. When enabled, VOUT2 is connected to VOUT through a p-channel MOSFET switch. This output can be used to power external circuits such as sensors or amplifiers that do not have low-power sleep or shutdown capabilities. One such example is the power-up and power-down of MOSFETs that are part of the built-in detection circuitry of the building thermostat.

　　VSTORE capacitors can have very large capacitance values (thousands of F or even F) to provide retention in the event of possible loss of input power. Once the power-up operation is completed, the main output, backup output and switching output are available. If the input power fails, operation can still be continued with power from the VSTORE capacitor. The VSTORE output can be used to charge a large storage capacitor or rechargeable battery after VOUT reaches regulation. After VOUT reaches regulation, the VSTORE output is allowed to charge up to the VAUX voltage, which is clamped to 5.3V. The electrical energy storage element on VSTORE can not only be used to power the system when the input power is lost, but also can be used to supplement the current required by VOUT1, VOUT2 and LDO output when the energy of the input power is insufficient.

　　A power-good comparator monitors the VOUT voltage. Once VOUT charges to within 7% of its regulated voltage, the pGOOD output will go high. If VOUT drops more than 9% from its regulated voltage, pGOOD will go lower. The pGOOD output is designed to drive a microprocessor or other chip I/O and is not intended to drive higher current loads such as LEDs.

　　in conclusion

　　In conclusion, the LTC3108 thermal energy harvesting, DC-DC boost converter and system manager is a revolutionary device that can obtain energy from solar cells, thermoelectric generators or other similar heat sources. The device's unique resonant power converter topology enables startup at extremely low input voltages of 20mV. Among the solutions currently on the market that form a complete energy harvesting chain, its high level of integration (including power management controllers and commercially available external components) makes it the smallest, simplest and easiest to use. One.

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