Most engineers know about the solid-state refrigerators called Peltier devices or, more commonly, TECs (thermoelectric coolers) and how they can actively cool temperature-sensitive electronic components, such as optical detectors and solid-state lasers. It’s also common knowledge – although perhaps less so – that TECs are bidirectional heat pumps and can therefore both heat and cool, depending on the direction of the supplied drive current. TECs can therefore serve as the basis for precision microthermostats, maintaining a predetermined temperature against ambient-temperature excursions that range both above and below the setpoint.
|Figure 1.||This plot of temperature versus maximum current shows that
operating a TEC at high currents achieves heating and cooling
from a unipolar drive.
The rub is that bidirectional-TEC drive tends to be an inconvenient design problem. It requires either dual bipolar power supplies or relatively complex H-bridge-drive output circuits involving arrays of power transistors that selectively reverse the TEC excitation as the required direction of heat flow dictates. But an alternative method offers advantages whenever simplicity matters more than efficiency. This Design Idea presents a novel approach to bidirectional-TEC-temperature control that avoids both the inconvenience of dual power supplies and the complexity of bidirectional drive. It works by exploiting a little-known quirk of all TECs: the inherent reversal of net heat flow at unconventionally high levels of drive current.
|Figure 2.||This circuit puts unipolar drive in a PID-feedback loop to stabilize the temperature of the target device.|
The specifications of every TEC include IMAX, the drive current that results in maximum net cooling. Plotting heat transfer versus drive current relative to IMAX results in a typical parabolic curve (Figure 1). The left-hand, gray half of the plot in the figure shows the usual bipolar TEC’s operating region, which confines drive current to the range of –0.5×IMAX. The right-hand half shows the region of interest, in which the same bipolar-temperature excursion results from unipolar-current drive: IMAX. Operation of the TEC in this second operating region thus allows bidirectional temperature control without the complexity of bidirectional-current drive.
Figure 2 shows an implementation of the concept in a high-performance PID (proportional-integral-derivative)-feedback loop. The component count is less than one-fourth that of a comparable bipolar-drive design. Feedback stability is robust, and settling time is short. The downside is a current draw as much as 150% higher than that for a conventional bipolar driver, which limits the technique to applications in which power consumption and heat dissipation aren’t critical priorities and small TECs are adequate.
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