Thermoelectric cooling has quickly become a practical proposition for many types of electronic equipment. Devices on the market today are compact, efficient and – with the benefit of advanced internal construction – overcome the traditional reliability challenges that have restricted opportunities for this type of device in the past.
Keeping electronic components like laser diodes or image sensors at a stable temperature is vital to ensure instruments such as high-power lasers, laboratory references, spectroscopes or night-vision systems can function correctly. In some cases, cooling to below ambient temperature may be required. Simple passive cooling, using a combination of a heat sink and forced-air, can struggle to satisfy either of these demands; response to changes in thermal load can be slow and imprecise, and cooling relies on a thermal gradient where the heat source temperature is higher than ambient.
As an alternative to commonly used passive cooling techniques, thermoelectric cooling can offer numerous advantages. These include accurate temperature control and faster response, the opportunity for fanless operation (subject to heat sink performance), reduced noise, space savings, reduced power consumption and the ability to cool components to sub-ambient temperatures.
Peltier elements: principles and structure
The internal structure of the Peltier element comprises semiconductor pellets fabricated from N-type and P-type Bismuth Telluride materials. The array of pellets is electrically connected in series, but thermally arranged in parallel to maximize thermal transfer between the hot and cold ceramic surfaces of the module (Figure 1).
Figure 1: Internal structure of a generic Peltier element (Image source: CUI, Inc.)
Thermoelectric cooling takes advantage of the Peltier effect, which is observed as heat being either absorbed or emitted between the junctions of two dissimilar conductors when a current is passed. A thermoelectric module comprising a Peltier element sandwiched between two ceramic plates of high thermal conductivity, with a power source, is effectively able to pump heat across the device from one ceramic plate to the other. Moreover, the direction of heat flow can be changed simply by reversing the direction of current flow.
Applying a DC voltage causes the positive and negative charge carriers to absorb heat from one substrate surface and transfer and release it to the substrate on the opposite side. Therefore, the surface where energy is absorbed becomes cold and the opposite surface, where the energy is released, becomes hot.
Constructing a cooling unit
To create a practical thermoelectric cooling unit, the Peltier module is built into a system that usually comprises a metal block of high thermal conductivity, such as an aluminum alloy, and a finned heat sink (Figure 2). The metal block is used to attach the device to be cooled – such as the laser diode or image sensor – to the cold side of the cooling element. The thickness of the block is selected to maintain flatness and so ensure consistent thermal connection with the cold plate of the Peltier element, noting that excessive thickness will introduce unwanted thermal inertia. The heat sink is attached to the opposite side, or hot plate, of the Peltier element, to dissipate the extracted heat into the ambient environment. A thin layer of thermal grease, or other thermal interface material (TIM), is applied to each surface.
Figure 2: The Peltier element, aluminum block and heat sink are assembled to create the cooling system (Image source: CUI.)
Module and controller selection
A complete thermoelectric cooling system comprises the Peltier element and heat sink assembly, temperature sensors to monitor the hot and cold plates and a controller unit to ensure the correct current is supplied to maintain the desired temperature difference across the module.
The controller and Peltier module are chosen to ensure the heat from the cooled component combined with the joule-heating effect of the supplied current can be dissipated without exceeding the maximum thermal capacity (Qmax) or maximum temperature difference (ΔTmax) indicated in the Peltier module datasheet. The maximum temperature difference and maximum current should also be considered, to ensure the chosen Peltier module can maintain the desired temperature difference when operating at a suitable current. This should typically be less than 70% of the maximum rated current, to ensure that joule heating remains within manageable limits and the system can respond to short-term increases in the cold plate temperature without encountering thermal runaway.
Calculating current and thermal absorption
If the desired temperature difference and operating voltage of the power supply are known, the thermal dissipation and operating current can be calculated from the module using function diagrams as presented in the datasheet.
As an example, the function diagrams shown in Figure 3 can be used to find the heat pumped and supplied current, for hot plate temperature (Th) 50°C, cold plate temperature 10°C, and supplied voltage 12 V.
Figure 3: Calculation of setting using datasheet function diagrams (Image source: CUI.)
To determine the operating current and thermal absorption:
- Find ΔT:
ΔT = Th – Tc – 50°C – 10°C = 40°C
- Use the function diagram for Th = 50°C to find the current to maintain ΔT = 40°C, at the supplied voltage:
From the diagram, I = 3.77 A
- Find the heat pumped from the function diagram, at I = 3.77 A and ΔT = 40°C:
From the diagram, Qc = 20.75 W
Thermal fatigue in Peltier modules
Thermoelectric coolers can be susceptible to thermal fatigue. Conventionally manufactured units contain ordinary solder bonds between the electrical interconnect (copper) and the P/N semiconductor elements, as well as solder or sinter bonds between the interconnect and ceramic substrate (Figure 4). While these bonding techniques normally create strong mechanical, thermal and electrical bonds, they are inflexible, and can degrade and eventually fail when subjected to the repeated heating and cooling cycles that are typical of normal Peltier module operation.
Figure 4: Solder and sinter bonds of a conventional Peltier module (Image source:CUI.)
CUI conceived the arcTEC™ structure for Peltier modules to combat the effects of thermal fatigue. The arcTEC structure replaces the conventional solder bond between the copper electrical interconnect and the ceramic substrate on the cold side of the module with a thermally conductive resin. This resin provides an elastic bond within the module that allows for the expansion and contraction that occurs during repeated thermal cycling. The elasticity of this resin reduces stresses within the module while achieving a better thermal connection and a superior mechanical bond, and shows no marked drop-off in performance over time.
In addition, a special SbSn (antimony-tin) solder replaces the BiSn (bismuth-tin) solder typically used between the P/N semiconductor elements and the copper interconnect (Figure 5). The SbSn solder has a higher melting point of 235°C, compared to 138°C for BiSn, and so offers superior thermal-fatigue performance and better shear strength.
Figure 5: arcTEC structure enhancements boost reliability and thermal performance (Image source: CUI.)
Improving reliability and thermal performance
To deliver an additional boost to reliability, the P/N elements of arcTEC structure modules are made from a premium silicon and are up to 2.7 times larger than those employed by other modules. This ensures a more uniform cooling performance, avoiding the uneven temperatures that contribute to the risk of a shorter working life. Figure 6 illustrates the effect on temperature distribution by comparing infrared images of a conventional Peltier module (top) and an arcTEC structure module (bottom). The superior P/N elements of arcTEC structure modules also help to improve cooling time by more than 50%.
Figure 6: Improved temperature distribution in arcTEC structure modules (below) compared to conventional modules (above) (Image source: CUI.)
The enhanced life expectancy of arcTEC structure modules can be demonstrated by analyzing the change in internal resistance of Peltier modules exposed to thermal cycling. Because resistance change within Peltier modules is closely linked to bond failure, analyzing the trend provides a useful indication of lifetime. The results shown in Figure 7 further demonstrate the significant improvement in life expectancy made possible by the arcTEC structure.
Figure 7: Assessing reliability by monitoring resistance change (Image source: CUI.)
Although the physics of thermoelectric cooling have been understood for many generations, the arrival of suitable Peltier modules, ready to be designed-in to commercial electronic products, is a relatively new phenomenon. There are numerous advantages offered, including faster response, improved temperature stability and greater flexibility to control the temperature of critical devices such as ICs, laser diodes or sensors. Many new and innovative applications for Peltier modules are expected to emerge as designers gain familiarity with the products and design techniques.
Care should be taken when selecting Peltier modules and designing control circuitry to operate the modules well within their thermal limits. Today’s most advanced Peltier modules, designed with flexible internal interconnects and high-purity P/N pellets, have unlocked further improvements in thermal response and reliability.