TEGs are simply thermoelectric modules that convert a temperature differential across the device, and resulting heat flow through it, into a voltage via the Seebeck effect. The reverse of this phenomenon, known as the Peltier effect, produces a temperature differential by applying a voltage and is familiarly used in thermoelectric coolers (TECs). The polarity of the output voltage is dependent on the polarity of the temperature differential across the TEG. Reverse the hot and cold sides of the TEG and the output voltage changes polarity.
TEGs are made up of pairs or couples of N-doped and P-doped semiconductor pellets connected electrically in series and sandwiched between two thermally conductive ceramic plates. The most commonly used semiconductor material is bismuth-telluride (Bi2
). Figure 3
illustrates the mechanical construction of a TEG.
Figure 3. Typical mechanical construction of a TEG
A number of variables control how much voltage a TEG will produce for a given ∆T (proportional to the Seebeck coefficient). Their output voltage is in the range of 10 mV/K to 50mV/K of differential temperature (depending on the number of couples), with a source resistance in the range of 0.5Ω to 10Ω. In general, the more couples a TEG has in series, the higher its output voltage is for a given ∆T. However, increasing the number of couples also increases 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 the individual pellets to preserve a low resistance while still providing a higher output voltage. The thermal resistance of the TEG is yet another factor to take into consideration when choosing and matching it to a heatsink.
With its ability to operate at input voltages as low as ±30mV, the LTC3109 provides a power management solution that enables thermal energy harvesting for powering wireless sensors and other low power applications from common thermoelectric devices. Available in either a 20-pin QFN or SSOP packages, this product offers low voltage capabilities and a high level of integration to minimize the solution footprint. The LTC3109 interfaces seamlessly with existing low power building blocks to support autonomous wireless sensors and extend the battery life in critical battery backup applications.
Tony Armstrong is reachable at email@example.com
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