In certain applications, engineers need to cool a component to a constant temperature or to an operating temperature below ambient. A thermoelectric module, also known as a Peltier module, can enable a compact, lightweight, and power-efficient solution; but in order to create an optimized thermoelectric system, some design work is needed to properly integrate and power the device.
Peltier module basics
Thermoelectric modules leverage the Peltier effect named after the French scientist, Jean Peltier, who observed that passing a current through electrically joined dissimilar conductors creates a temperature difference between the two. A modern Peltier module is typically supplied as a component comprising two ceramic outer plates and internal conductive layers, separated by P-N semiconductor pellets. These P-N pellets are arranged to be connected electrically in series and thermally in parallel.
After applying a dc voltage to the Peltier module, these positive and negative elements work to absorb heat from one surface and expel the heat to the other side. This causes the side where heat absorption took place to become cold and the side where the heat was released to become hot.
Note that the Peltier effect can be used to heat or cool an object. Although this article will focus on a cooling application, the design considerations for heating are identical except that the polarity of the applied voltage, the direction of the current, and the direction of heat flow through the module are reversed.
Designing a Peltier module system
Figure 1: The Peltier module transfers heat from the source to the heatsink. (Image source: CUI, Inc.)
Placing a powered Peltier module between a heat source such as the surface of an IC and a heatsink, as indicated in Figure 1, enables the IC to be actively cooled. The Peltier module’s cold side is attached to the heat source and the heatsink is attached to the hot side. Note that the module transfers heat from the cold side to the hot side but does not absorb heat. The system can be designed to extract heat at a constant rate into the heatsink or, by controlling the power applied, can be varied to ensure the surface in contact with the component remains at a constant temperature. If required, this temperature can even be set at a point below ambient.
Figure 2 shows the basic elements of a system for cooling a component such as an IC. The Peltier module extracts heat from the object to be cooled, while the heatsink must not only dissipate the heat from the IC, but also the heat generated in the Peltier module due to the flow of electric current. An external feedback loop connected to a temperature sensor at the IC controls the power applied to the Peltier module to keep the temperature of the object stable.
Figure 2: Peltier module system with feedback loop for temperature control. (Image source: CUI, Inc.)
Selection of the Peltier module is guided by the thermal requirements of the application. These include the heat to be transferred across the module, the maximum temperature across the module, and the maximum hot-side temperature. After selecting a suitable module, the power-supply requirements can be determined.
The Peltier module is a current-driven device optimally powered by a controlled current source, although a voltage source can be used. If the module is intended to provide continuous maximum cooling, a constant voltage can be applied (Figure 3). In this case, the load current and input voltage for a given cooling requirement can be read directly from the characterization graphs in the datasheet. This is described in detail in CUI’s article, “Choosing and Using Advanced Peltier Modules for Thermoelectric Cooling.”
Figure 3: Simple Peltier system powered by a voltage source. (Image source: CUI, Inc.)
On the other hand, if the module is required to keep the component at a constant temperature throughout changes in the thermal load and/or ambient temperature, a temperature sensor and feedback loop are needed. This was shown in Figure 2.
The relatively low loop bandwidth allows flexibility in the way feedback is implemented. The temperature sensor could be a thermocouple, or a solid-state or infrared sensor, with the data fed back to the power source being used to adjust the applied voltage. Voltage adjustment can be accomplished using an external PWM circuit if the power supply is unable to provide a wide enough adjustment range. Filtering the PWM output so that ripple remains below about 5% is recommended (Figure 4). This ensures the module operates at a high Coefficient of Performance (COP) and minimizes interference with nearby components.
Figure 4: Peltier system for constant-temperature control. (Image source: CUI, Inc.)
In addition to carrying heat away from the component to be cooled, the Peltier module also generates heat internally due to the current flow. This self-heating can be an issue if it causes the module to operate at a lower COP than is desired and will be an issue if it exceeds the thermal transfer capability of the module.
Therefore, both heat sources must be considered when designing the system in order to choose a suitable module and heat sink as well as determine the operating voltage and current requirements. With proper component selection, a Peltier module system can provide an excellent solution to achieve the desired thermal transfer or target operating temperature for the cooled component.
A Peltier module can provide an extremely effective basis for electronic temperature control. Compact, lightweight and efficient when operated at a high COP, the module can be controlled using a current or voltage source. In addition to the module, only a small number of standard components are needed to build an effective temperature-controlled solution capable of maintaining a device’s operating temperature at or below the ambient temperature. Understanding how to use these devices is a valuable skill for addressing a wide range of projects. CUI’s Peltier modules feature a variety of performance ratings and sizes, offering designers multiple options when designing their next thermal management system.