Keeping Cool

Thermoelectric coolers offer efficient solid-state heat-management options.

Thermal management is a key factor in optimizing optoelectronic device performance. Excessive heat compromises the wavelength stability of laser output and increases noise levels in detectors. Thermoelectric (TE) coolers, also known as Peltier coolers, offer effective, practical options for heat management in compact optoelectronic devices such as dense-wavelength-division-multiplexing (DWDM) components and charge-coupled-device (CCD) detectors. In many cases, TE coolers offer significant advantages over more conventional cooling methods because they offer active cooling and precise controllability.

What is a TE module?

A TE cooler is a semiconductor-based electronic component that functions as a small heat pump. When low-voltage dc power is applied to a TE module, heat moves through the module from one side to the other in proportion to the applied voltage. One module face, therefore, will be cooled while the other is simultaneously heated. This phenomenon is fully reversible: With a switch in the polarity of the applied voltage, heat moves in the opposite direction. Thus, the same module can function as both a heater and a cooler, permitting very precise temperature stabilization.

Thermoelectric modules are not ideal for every cooling application, and there are many situations in which a simpler cooling device, such as a heat sink, is more appropriate. They also won’t cool to as low a temperature as mechanical coolers, e.g. stirling coolers. There are situations, however, in which thermoelectric cooling is the only suitable solution or for which it presents significant advantages over other cooling methods. Unlike a heat sink, for example, a thermoelectric cooler can cool to below ambient temperature. The devices can be more efficient than chillers or fans, and they are generally more compact.

Thermoelectric cooling is particularly effective for cases requiring precise temperature control, such as laser cooling applications. With an appropriate temperature control circuit, TE coolers can stabilize temperatures to better than ±0.1°C. Aside from their performance benefits, TE modules are often used when system design criteria include factors such as high reliability, small size, low cost, and low weight. In the optoelectronics industry, they are often used for spot cooling (the direct cooling of one component rather than a whole system) or for testing scenarios that require temperature cycling of test components or systems.

It should be noted that the thermoelectric module is only one element in the overall cooling system. Its function is to continuously pump the heat away from a component or assembly. This heat then needs to be transferred from the thermoelectric module to the environment using a heat sink or similar device. Condensation issues must be addressed.

Principles of operation

Thermoelectric coolers are based on the Peltier effect, a phenomenon discovered by Jean Peltier in 1834. In a system with two dissimilar metal junctions, heat can be absorbed at one junction and discharged at the other when an electric current flows in the closed circuit.

A thermoelectric cooler generally consists of two or more semiconductor elements, usually made of bismuth telluride (Bi2Te3), that are connected electrically in series and thermally in parallel. These thermoelectric elements and their interconnects typically are mounted between two thin metallized ceramic substrates, which provide structural integrity, insulate the elements electrically from external mounting surfaces, and provide flat contact surfaces.

Both n-type and p-type Bi2Te3 materials are used in a thermoelectric cooler. This arrangement causes heat to move through the cooler in one direction only while the electrical current moves back and forth alternately between the top and bottom substrates through each n- and p-type element. The n-type material is doped so that it will have an excess of electrons while the p-type material is doped so that it will have a deficiency of electrons. The extra electrons in the n material and the «holes» resulting from the deficiency of electrons in the p material are known as carriers. These carriers move the heat energy through the thermoelectric material.

Most thermoelectric cooling modules are fabricated with an equal number of n-type and p-type elements where each n and p element pair forms a thermoelectric couple. The module  has two pairs of n and p elements and would be termed a two-couple module. Heat flux—the heat actively pumped through the thermoelectric module—is proportional to the magnitude of the applied dc electric current. By varying the input current from zero to maximum, one can adjust and control the heat flow and temperature differential.

Typical thermoelectric modules generally have between seven and 128 couples and maximum operating current (Imax) ratings from 1.2 to 36 A, although larger and smaller modules are available. Modules can be mounted in parallel to increase the heat-transfer capacity, or they can be stacked in multistage cascades to increase the temperature differential.

Thermoelectric modules have no moving parts, so they are virtually maintenance free. They are also smaller and lighter than comparable mechanical cooling systems. Their solid-state construction ensures high reliability, which is an advantage when they are to be used in a system that is not easily accessible after installation. Operation is acoustically silent, and electrical interference is negligible.

Choosing a module

To select the correct TE cooler for a specific application, one must evaluate the total system in which the device will be used. For most applications, a standard module will be satisfactory, but custom designs are readily available.

Before attempting to select an appropriate thermoelectric module, you should ask yourself the following questions:

  • At what temperature must the cooled object be maintained, and to what precision?
  • How much heat must be removed from the cooled object?
  • What is the expected ambient temperature range?
  • What is the thermal resistance of the heat sink (hot side)?
  • What is the allowable footprint and height for the module?
  • What dc power is available? What voltage and current restrictions exist?
  • What is the expected approximate temperature of the heat sink during operation? Is this temperature steady or variable?

These answers should yield enough information about the system in which the module is to be installed to permit module selection. Since module performance is often presented graphically, it is important to know how to use these graphs to determine which module is appropriate for your system.

There are three key graphs that are important to understand: heat capacity versus current (Qc vs. I ), input voltage versus current (Vin vs. I ), and coefficient of performance (COP).

Heat sinks

It is important to remember that TE coolers require a heat sink or similar device to dissipate the heat pumped by the module. This heat sink must be capable of removing both the heat pumped by the module and the joule heat from the electrical power supplied to the module. Generally forced-convection or liquid-cooled heat sinks are used, although a natural-convection heat sink may be sufficient for applications requiring minimal heat removal.

A perfect heat sink would absorb an unlimited quantity of heat without exhibiting any increase in temperature. Since this is not possible in practice, a heat sink must be selected that can handle the total heat flow from the device while maintaining an acceptable tem- perature rise. Generally a heat-sink temperature rise of 5°C to 15°C is acceptable. Many optoelectronic systems that use TE cooling are designed such that the chassis acts as the heat sink. When this occurs, particular care must be taken to select a module with operating parameters that minimize the temperature increase to the chassis.

Surface flatness is another issue to consider when selecting a heat sink for use with TE modules. Many off-the- shelf units do not have adequate surface flatness. For satisfactory thermal performance, a surface flatness deviation of 1 mm/m (10-3 in/in) or better is needed. Special attention also should be given to the installation of the module and the heat sink. It is important to ensure that correct mounting techniques and interface compliant materials are used.

Compact and economical, TE coolers provide efficient thermal management solutions with solid-state reliability. For demanding optoelectronic or electro- optic applications, the technology is often the best choice.

(By Robert Otey and Barry Moskowitz, Ferrotec America Corp,.
SPIE,  OEmagasine, March, 2001)

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