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The proliferation of ultra-low power wireless sensor nodes required for measurement and control, coupled with the use of new energy harvesting technologies, has made it possible to have fully autonomous systems powered by local environmental energy rather than batteries. Utilizing the environment or "free" energy to power wireless sensor nodes is attractive because it complements the battery or wire supply, even without the need for batteries or power leads. This is obviously beneficial when it is inconvenient, expensive or dangerous to replace or repair the battery.
Many wireless sensor systems consume very low average power, making them a major candidate for powering energy harvesting technology. Many sensor nodes are used to monitor slowly changing physical quantities. Therefore, the measurement can be performed infrequently and the measurement data does not need to be sent frequently, so the sensor node operates at a very low duty cycle. Accordingly, the average power demand is also low. For example, if a sensor system requires 3.3V/30mA (100mW) when awake, but only 10ms per second is working, then the average power required is only 1mW, assuming that it does not work during the interval of the transmission burst. The sensor system current drops to a few μA. If the wireless sensor only samples and transmits every minute (rather than every second), the average power will drop to less than 20μW. This difference is important because most forms of energy harvesting provide very small steady-state power (usually only a few mW, and in some cases even a few μW). The lower the average power required for the application, the more likely it is to collect energy for power.
Energy harvesting source
The most common sources of energy available for collection are vibration (or motion), light, and heat. Transducers for all of these energy sources have the following common characteristics:
Their electrical outputs are unregulated and are not suitable for direct use in powering electronic circuits.
They may not be able to provide a continuous and uninterrupted power supply
They tend to produce very low average output power (usually about 10μW to 10mW)
If you want to use such an energy source to power wireless sensors or other electronic circuits, you must perform sensible and prudent power management for these features.
Power management
A typical wireless sensor system powered by collected energy can be broken down into five basic components, as shown in Figure 1. All of these components have been used for a while except for the power management components. For example, microprocessors running only a few μW and low-cost, cost-effective small RF transmitters and transceivers are widely used. Low-power analog and digital sensors are also ubiquitous.
Figure 1: Typical Wireless Sensor System Configuration
An ideal power management solution for energy harvesting should be small, easy to use, and workable, while operating with exceptionally high or abnormally low voltages generated by common energy harvesting sources, and ideally provided and sourced Good load matching of the impedance for optimal power transfer. The power manager itself must only require very small currents to manage the accumulated energy and use very few discrete components to produce a stable output voltage.
Some applications, such as wireless HVAC sensors or geothermally powered sensors, present another unique challenge for energy harvesting power converters. Such applications require the energy harvesting power manager to operate with a very low input voltage of either polarity because the polarity of the ΔT across the thermoelectric generator (TEG) will vary. This is a particularly difficult problem, and diode bridge rectifiers are not an option at voltages of tens or hundreds of mV.
Available in a 4mm x 4mm x 0.75mm 20-pin QFN package or a 20-pin SSOP package, the LTC3109 solves the energy harvesting problem of ultra-low input voltage sources of any polarity. The device provides a compact, simple, highly integrated, monolithic power management solution for operation with input voltages as low as ±30mV. This unique capability allows a thermoelectric generator (TEG) to be used to power wireless sensors and collect energy from a temperature difference (ΔT) as small as 2oC. The device uses two small (6mm x 6mm) off-the-shelf step-up transformers and a small number of low-cost capacitors to provide the regulated output voltage required to power today's wireless sensor electronics.
The LTC3109 uses these step-up transformers and internal MOSFETs to form a resonant oscillator that operates with very low input voltages. Using a 1:100 transformer turns ratio, the converter can be started with inputs as low as 30mV, independent of the polarity of the input. The secondary winding of the transformer is responsible for feeding a charge pump and rectifier circuit, which is used to power the IC (via the VAUX pin) and charge the output capacitor. The 2.2V LDO output is designed to be in an adjusted state first to power a low power microprocessor as quickly as possible. The main output capacitor is then charged to the voltage set by the VS1 and VS2 pins (2.35V, 3.3V, 4.1V or 5.0V) to power the sensor, analog circuitry, RF transceiver, and even a supercapacitor or battery. Charging. When the wireless sensor is in operation and transmitting a signal, the VOUT storage capacitor will provide the burst energy required during the low duty cycle load pulse. In addition, a switch output (VOUT2) that can be easily controlled by the host is provided to power circuits that do not have a shutdown or low power sleep mode. The device has a built-in power-good output that signals the host that the “main output voltage is close to its stable valueâ€. Figure 2 shows the circuit schematic of the LTC3109.
Figure 2: LTC3109 schematic for unipolar output operation
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