Ultra-low voltage converters advance thermal energy harvesting

　　Figure 3: LTC3109 output current versus input voltage

　　TRANSFORMERS: Transformers

　　Thermoelectric Generator

　　A thermoelectric generator (TEG) is a thermoelectric module that uses the Seebeck effect to convert a temperature difference across the device (and the heat flowing through the device due to the temperature difference) into a voltage. The reverse process of this phenomenon (called the Peltier effect) is to generate a temperature difference by applying a voltage and is commonly used by thermoelectric coolers (TECs). The polarity of the output voltage depends on the polarity of the temperature difference across the TEG. If the hot and cold ends of the TEG are reversed, the output voltage will change polarity.

　　TEG consists of a pair or couple of N-type doped and P-type doped semiconductor chips connected electrically in series and sandwiched between two thermally conductive ceramic plates. The most commonly used semiconductor material is bismuth telluride (Bi2Te3). Figure 4 shows the mechanical construction of TEG.

　　Figure 4: Typical mechanical structure of TEG

　　CERAMIC SUBSTRATE: Ceramic Substrate

　　NEGATIVE -: Negative (-)

　　CONDUCTOR TABS: Conductor tabs

　　POSITIVE (+): positive (+)

　　N-TYPE SEMICONDUCTOR PELLETS: N-type semiconductor chips

　　P-TYPE SEMICONDUCTOR PELLETS: P-type semiconductor chips

　　Some manufacturers distinguish TEGs from TECs. When sold as TEGs, it usually means that the solder used to assemble the thermocouples inside the module has a higher melting point, so it can operate at higher temperatures and temperature differentials, thereby providing higher output power than a standard TEC (which is usually limited to a maximum temperature of 125°C). Most low-power energy harvesting applications do not encounter high temperatures or high temperature differentials. TEGs come in a variety of sizes and electrical specifications. The most common modules are square, with lengths ranging from 10mm to 50mm on each side and thicknesses ranging from 2mm to 5mm.

　　How much voltage a TEG will produce for a given ΔT (proportional to the Seebeck coefficient) is controlled by many variables. Its output voltage is 10mV to 50mV per K of temperature difference (depending on the number of thermocouples) and has a source resistance of 0.5Ω to 10Ω. In general, for a given ΔT, the more series thermocouples a TEG has, the higher its output voltage. However, increasing the number of thermocouples will also increase the series resistance of the TEG, resulting in a larger voltage drop when loaded. Manufacturers can compensate for this by adjusting the size and design of individual semiconductor chips to maintain low resistance while still providing a higher output voltage. The thermal resistance of the TEG is another factor to consider in the process of selecting a TEG and matching it to a heat sink.

　　Load Matching

　　To extract maximum power from any voltage source, the load impedance must match the internal resistance of the source. This is illustrated in Figure 5, where a voltage source with an open circuit voltage of 100mV and a source impedance of either 1Ω or 3Ω drives a load resistor.

　　Figure 5: Simplified schematic of a voltage source driving a resistive load

　　LOAD OR POWER CONVERTER: Load or power converter

　　Figure 6 shows the power delivered to the load as a function of the load resistance. In each curve, it can be seen that the power delivered to the load is maximum when the load resistance matches the source resistance.

　　Figure 6: Power supply output power as a function of load resistance

　　The LTC3109 presents a minimum input resistance of 2.5Ω to the input supply. (Note that this is the input resistance of the converter , not the input resistance of the IC itself.) This resistance value falls in the middle of the range of most TEG source resistances, providing a good load match for near-optimal power transfer. The LTC3109 is designed so that as VIN decreases, the input resistance increases. This feature allows the LTC3109 to adapt well to TEGs with different source resistances.

Choosing TEG for power generation

　　Most thermoelectric module manufacturers do not provide data on output voltage or output power versus temperature difference, which is what thermal energy harvester designers want to see. Some other TEG parameters that may be useful are electrical (AC) resistance and thermal resistance. Manufacturers also do not always provide these parameters. Two parameters that are always provided are VMAX and IMAX, which are the maximum operating voltage and maximum operating current of a particular module (when driven by a heating/cooling application). VMAX divided by IMAX will give an approximation of the module's resistance.

　　If a large amount of heat flux is available, sufficient heat dissipation can be provided on one side of the TEG. A good rule of thumb when selecting a thermoelectric module for power generation is to select the module with the largest product of (VMAX * IMAX) for a given size. This selection generally provides the highest TEG output voltage and the lowest source resistance. One caveat when using this rule of thumb is that the heat sink must be sized to the size of the TEG. Larger TEGs require larger heat sinks for best performance. Note that if resistance is given, it is given as an AC resistance because it cannot be measured using traditional methods with DC current, which would generate a Schiebeck voltage that would result in an erroneous resistance reading. For applications where the available heat flux is limited and/or a smaller heat sink must be used, it is best to select a TEG whose thermal resistance matches the largest available heat sink.

　　Figure 7 shows the output voltage and maximum output power of a 30mm2 TEG over a ∆T range of 1°C to 20°C. Over this temperature range, the output power varies from hundreds of microwatts to tens of milliwatts. Note that the power curves assume an ideal load match and no conversion losses. Ultimately, after being stepped up to a higher voltage by the LTC3109, the available output power becomes less due to power conversion losses. The LTC3109 datasheet provides several available output power curves for several different operating conditions.

　　Figure 7: Open circuit voltage and maximum power dissipation using a 30mm2 TEG

　　OPEN-CIRCUIT: Open circuit

　　MAX, IDEAL: maximum value, ideal situation

　　127 COUPLES: 127 couplers

　　For a given application, the size of the TEG required depends on the minimum available ∆T, the maximum average power required by the load, and the thermal resistance of the heat sink used to keep one side of the TEG at ambient temperature.

　　Thermal Considerations

　　When a TEG is placed between two surfaces at different temperatures, the “open circuit” temperature difference before the TEG is added is higher than the temperature difference across the TEG after the TEG is placed. This is because the TEG itself has a fairly low thermal resistance between its two panels (typically 1°C/W to 10°C/W).

　　For example, consider a large machine operating at a surface temperature of 35°C and an ambient temperature of 25°C. When a TEG is mounted to the machine, a heat sink must be added to the cooler (ambient) side of the TEG, otherwise the entire TEG will heat up to nearly 35°C, eliminating any temperature difference. Remember that output electrical power is generated by the heat flow through the TEG.

　　In this example, the thermal resistance of the heat sink and the TEG determines how much total ∆T exists across the TEG. A simple thermal model of this system is shown in Figure 8.

　　Figure 8: Thermal resistance model of TEG and heat sink

　　AMBIENT TEMPERATURE: ambient temperature

　　RTHERMAL OF HEATSINK: RTHERMAL OF HEATSINK

　　RTHERMAL OF TEG: RTHERMAL OF TEG

　　RTHERMAL OF HEAT SOURCE: RTHERMAL of heat source

　　HEAT SOURCE: Heat source

　　Assuming the thermal resistance of the heat source (RS) is negligible, the thermal resistance of the TEG (RTEG) is 6°C/W, and the thermal resistance of the heat sink is 6°C/W, the ∆T across the TEG is only 5°C. Only a very low output voltage can be produced from a TEG with only a few degrees of temperature difference across it, highlighting the importance of the LTC3109’s ability to operate from very low input voltages.

　　Note that larger TEGs generally have lower thermal resistance than smaller TEGs due to their larger surface area. Therefore, in an application, if a relatively small heat sink is used on one side of the TEG, the ∆T across the larger TEG will be smaller than that of the smaller TEG, and thus may not necessarily provide more output power. In any case, using a heat sink with the lowest thermal resistance will maximize the electrical output by maximizing the temperature difference across the TEG.

　　For applications where a larger temperature difference (i.e., higher input voltage) is available, a transformer with a smaller turns ratio (e.g., 1:50 or 1:20) can be used to provide a higher output current. As a general rule, if the minimum input voltage is at least 50mV under load, a turns ratio of 1:50 is recommended. If the minimum input voltage is at least 150mV, a turns ratio of 1:20 is recommended.

　　Ultra-low power applications with battery backup

　　Some applications run continuously. Such applications are traditionally powered by a small primary battery (e.g., a 3V Li-Ion coin cell). If the power requirements are low enough, such applications can be powered continuously by thermal harvesting, or thermal harvesting can be used to greatly extend battery life, thereby reducing maintenance costs. When all electronics consume less power than the energy harvesting source can provide, the LTC3109 can continuously power the load as long as there is a temperature difference across the TEG. In this case, there is no load on the battery. When the harvested energy is not enough, the backup battery seamlessly steps in and powers the load.

　　in conclusion

　　The LTC3109 can uniquely operate with input voltages as low as ±30mV, providing a simple and efficient power management solution that enables wireless sensors and other low power applications to be powered by thermal energy harvesting from common thermoelectric devices. Available in 20-pin QFN or SSOP packages, the LTC3109 offers unprecedented low voltage capability and high integration to minimize solution size. The LTC3109 seamlessly interfaces with existing low power building blocks to support autonomous wireless sensors and extend battery life in critical battery backup applications.