Sometimes cooling needs to be more tightly controlled than a simple heat sink and fan can manage. Rapidly fluctuating loads can make it hard to keep a component at a stable and cool temperature. However, this can often be important when trying to improve signal-to-noise performance or prevent damage to a system.
By Bruce Rose, Principal Applications Engineer at CUI Inc.
On the other hand, reliability calculations may call for a driver or power component to operate at below the normal ambient temperature. Non-electronics applications such as controlling the speed or yield of a chemical reaction also require precision-managed cooling.
Fortunately, there is a solution to help meet difficult challenges like these. Thermoelectric modules, or Peltier modules, are available in a variety of sizes that can be attached to an IC package or other types of components in a similar way to a heat sink.
Powered by a small current, the module actively extracts thermal energy from the attached heat source and dissipates it into the atmosphere. It can be designed to keep the cooled component at a specific temperature, or even below the ambient temperature if required.
A Peltier module comprises two external ceramic plates separated by an array of PN-doped semiconductor pellets that are connected electrically in series. When a current is passed through the array, a temperature gradient is set up according to the Peltier effect.
This causes one of the module’s plates to become cooler, enabling it to absorb heat from a source (such as the surface of a component like a chip or a laser diode), while the other side becomes hotter, and so can dissipate heat into the ambient air or a heat sink. This permits heat to be extracted efficiently from an attached component, provided both the extracted heat and the heat generated internally by the operating current can be dissipated at the opposite surface.
Designing a thermoelectric cooling system
Note that the Peltier element cannot absorb heat. It is purely a transfer mechanism. Hence, dissipating heat from the hot surface is essential. To begin designing a system that will cool a component to a desired case temperature, we need to know a few key parameters:
A Peltier module’s datasheet specifies the temperature difference (ΔT), as measured on the outside surfaces of the module’s external ceramic plates. A thin layer of thermal interface material (TIM) should be inserted where the module attaches to the component, and to the heat sink, as figure 1 shows, so the effects of these must be considered when designing the system.
Figure 1. Constituent parts of the thermoelectric cooling system
The temperature of the hot side needs to be known because Peltier module characteristics change with their operating temperature. CUI’s datasheets specify module performance at several temperatures, to help designers optimise the system.
Even so, it is unlikely that the temperature required for the application will match exactly one of those in the datasheet, so the designer must look at the curves for the closest temperature to understand how the module will perform in practice.
The surface area of a Peltier module needs to be similar to that of the heat source. A large mismatch can be compensated using a heat spreader as shown in figure 2. The spreader is typically made from aluminium or copper.
Figure 2. Using a heat spreader to utilise the module surface area
Like an LED, thermoelectric modules are a current-driven device. The easiest way to achieve the desired performance is to drive a Peltier module with a controlled current source that has enough compliance to provide the required load voltage. This can be determined from the module datasheet and operating constraints.
By way of an example, let us use the CP603315H module to demonstrate the design of a thermoelectric cooling system where the required heat transfer, surface temperatures and object surface area are known.
20W heat transfer through the module
20°C temperature difference across the module
30°C hot-side temperature (use 27°C datasheet graph)
30x30mm object surface area
The heat-energy versus temperature and input-voltage versus temperature curves (figure 3) are used to calculate the operating current and voltage.
Figure 3. Using the datasheet graphs to calculate the required current and voltage compliance.
Running at 2.7A and 7.5V, the module’s power consumption is 20W. Hence the total heat dissipated by the heat sink is 40W (20W heat source + 20W module).
As an alternative to conventional cooling with a directly-attached heat sink, using a thermoelectric or Peltier module can ensure greater temperature accuracy and stability thanks to faster transient response, while the latest semiconductor advancements enable modules to be reliable and cost effective.
By understanding the basics of these powerful little devices, one can better choose the properly sized module with performance ratings to adequately support the thermal management requirements of their design.
View more about CUI’s extensive range of fully characterized thermoelectric modules.
Learn more about Peltier modules and thermal management from CUI’s videos and whitepapers.