Design Article
Energy harvesting from thermoelectric sources gets a boost
Tony Armstrong and Dave Salerno, Power Products, Linear Technology Corporation
9/4/2012 9:11 AM EDT
Power management
A typical wireless sensor system powered by harvested energy can be broken down into five fundamental blocks, as illustrated in Figure 1. With the exception of the power management block, all of these blocks have been commonly available for some time. For example, microprocessors that run on microwatts of power, and small, cost effective RF transmitters and transceivers that also consume very little power are widely available. Low power analog and digital sensors are also ubiquitous.
An ideal power management solution for energy harvesting should be small, easy to apply and perform well while operating from the exceptionally high or low voltages produced by common energy harvesting sources, ideally providing a good load match to the source impedance for optimal power transfer. The power manager itself must require very little current to manage the accumulated energy and produce regulated output voltages with a minimal number of discrete components.
Some applications, such as wireless HVAC sensors or geothermal powered sensors present another unique challenge to an energy harvesting power converter. These applications require that the energy harvesting power manager be able to operate not only from a very low input voltage, but one of either polarity as the polarity of the ∆T across the thermoelectric generator (TEG) changes. This is a particularly challenging problem, and at voltages in the tens or hundreds of millivolts, diode bridge rectifiers are not an option.
The LTC3109, available in either a 4mm × 4mm × 0.75mm 20-pin QFN or 20-pin SSOP package, solves the energy harvesting problem for ultralow input voltage sources of either polarity. It provides a compact, simple, highly integrated monolithic power management solution for operation from input voltages as low as ±30mV. This unique capability enables it to power wireless sensors from a thermoelectric generator (TEG), harvesting energy from temperature differentials (∆T) as small as 2°C. Using two small (6mm × 6mm), off-the-shelf step-up transformers and a handful of low cost capacitors, it provides the regulated output voltages necessary for powering today’s wireless sensor electronics.
The LTC3109 uses these step-up transformers and internal MOSFETs to form a resonant oscillator capable of operating from very low input voltages. With a transformer ratio of 1:100, the converter can start up with inputs as low as 30mV, regardless of polarity. The transformer secondary winding feeds a charge pump and rectifier circuit, which is used to power the IC (via the VAUX pin) and charge the output capacitors. The 2.2V LDO output is designed to be in regulation first, to power a low power microprocessor as soon as possible. After that, the main output capacitor is charged to the voltage programmed by the VS1 and VS2 pins (2.35V, 3.3V, 4.1V or 5.0V) for powering sensors, analog circuitry, RF transceivers or even charging a supercapacitor or battery. The VOUT reservoir capacitor supplies the burst energy required during the low duty cycle load pulse when the wireless sensor is active and transmitting. A switched output (VOUT2), easily controlled by the host, is also provided for powering circuits that don’t have a shutdown or low power sleep mode. A power good output is included to alert the host that the main output voltage is close to its regulated value. Figure 2 shows the circuit schematic for the LTC3109.
Figure 2. LTC3109 schematic for unipolar input operation
A typical wireless sensor system powered by harvested energy can be broken down into five fundamental blocks, as illustrated in Figure 1. With the exception of the power management block, all of these blocks have been commonly available for some time. For example, microprocessors that run on microwatts of power, and small, cost effective RF transmitters and transceivers that also consume very little power are widely available. 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 apply and perform well while operating from the exceptionally high or low voltages produced by common energy harvesting sources, ideally providing a good load match to the source impedance for optimal power transfer. The power manager itself must require very little current to manage the accumulated energy and produce regulated output voltages with a minimal number of discrete components.
Some applications, such as wireless HVAC sensors or geothermal powered sensors present another unique challenge to an energy harvesting power converter. These applications require that the energy harvesting power manager be able to operate not only from a very low input voltage, but one of either polarity as the polarity of the ∆T across the thermoelectric generator (TEG) changes. This is a particularly challenging problem, and at voltages in the tens or hundreds of millivolts, diode bridge rectifiers are not an option.
The LTC3109, available in either a 4mm × 4mm × 0.75mm 20-pin QFN or 20-pin SSOP package, solves the energy harvesting problem for ultralow input voltage sources of either polarity. It provides a compact, simple, highly integrated monolithic power management solution for operation from input voltages as low as ±30mV. This unique capability enables it to power wireless sensors from a thermoelectric generator (TEG), harvesting energy from temperature differentials (∆T) as small as 2°C. Using two small (6mm × 6mm), off-the-shelf step-up transformers and a handful of low cost capacitors, it provides the regulated output voltages necessary for powering today’s wireless sensor electronics.
The LTC3109 uses these step-up transformers and internal MOSFETs to form a resonant oscillator capable of operating from very low input voltages. With a transformer ratio of 1:100, the converter can start up with inputs as low as 30mV, regardless of polarity. The transformer secondary winding feeds a charge pump and rectifier circuit, which is used to power the IC (via the VAUX pin) and charge the output capacitors. The 2.2V LDO output is designed to be in regulation first, to power a low power microprocessor as soon as possible. After that, the main output capacitor is charged to the voltage programmed by the VS1 and VS2 pins (2.35V, 3.3V, 4.1V or 5.0V) for powering sensors, analog circuitry, RF transceivers or even charging a supercapacitor or battery. The VOUT reservoir capacitor supplies the burst energy required during the low duty cycle load pulse when the wireless sensor is active and transmitting. A switched output (VOUT2), easily controlled by the host, is also provided for powering circuits that don’t have a shutdown or low power sleep mode. A power good output is included to alert the host that the main output voltage is close to its regulated value. Figure 2 shows the circuit schematic for the LTC3109.
Figure 2. LTC3109 schematic for unipolar input operation
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anne-francoise.pele
9/4/2012 10:57 AM EDT
Click on the link below to check out the collection of the Design Articles, Case Studies, Product How-To articles, Teardowns, etc... related to energy scavenging that have been published on Smart Energy Designline.
Click here: http://www.eetimes.com/design/smart-energy-design/4372778/Energy-harvesting---Design-archive
Check back frequently. The list will be updated as new articles arrive.
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Infinergia
9/5/2012 3:42 AM EDT
How about TEG manufacturers names? Which one can you recommend?
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Pituch
2/8/2013 5:59 PM EST
Global Thermoelectric manufactures TEGs that convert heat into electricity. Using a burner you can generate continuous DC electricity with no moving parts. Generators range in output size from 15 to 550 Watts.
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William Miller
1/25/2013 9:58 AM EST
"3) They generally produce very little average output power, usually in the range of 10µW to 10mW."
So, what's the point in this energy harvesting??
William - http://www.carid.com/
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Habanero
4/11/2013 3:27 AM EDT
No maintenance for battery powered nodes, or no power distribution in difficult environments (plus, no cabling cost)
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