“The heat released by a component or system requiring a TEC moves from the heat sink side of the TEC element and exchanges heat with the heat sink side, which is passively cooled by air cooling of the heat sink, or actively cooled by forced air cooling or water cooling of the heat sink . Bismuth telluride is the more commonly used semiconductor in Peltier elements. To generate large temperature differences, these semiconductors need to have high electrical and low thermal conductivity.
At present, the intelligent analog front end (AFE) and its representative closed-loop control have covered a wide range of applications in the automotive and industrial markets.
In this article, we will discuss in detail one industrial application, thermoelectric cooling (TEC) control. The paper also includes some examples of how TECs can be used and how smart AFEs can improve TEC implementation in systems that require TECs.
What is TEC?
TEC exploits a phenomenon called the Peltier effect, named after Jean Charles Athanase Peltier, who discovered it in 1834. He found that passing an electric current through two different conductors resulted in an increase in heat exchange, making one conductor warmer than ambient temperature and one conductor cooler than ambient temperature. Typically, with a Peltier element (also called a cell), a heat sink side (hot side) and a heat sink side (cold side) are formed when current flows. This phenomenon has a very practical use in heating or cooling the material in contact with the Peltier element in a controlled manner by controlling the current flow between the two conductors. Examples of applications requiring temperature monitoring and control include industrial laser markers, in vitro diagnostic (IVD) equipment, and automotive interior temperature control.
Although the Peltier effect has been discovered for more than 100 years, it was not until 1954 that H. Julian Goldsmid discovered and published that replacing the original conductive material with a semiconductor produced a higher temperature gradient.
Figure 1 shows how a junction is formed between an electrical conductor and a semiconductor element to create a temperature gradient for the TEC using a semiconductor-based approach.
Figure 1: Semiconductor-based Peltier element
The heat released by a component or system requiring a TEC moves from the heat sink side of the TEC element and exchanges heat with the heat sink side, which is passively cooled by air cooling of the heat sink, or actively cooled by forced air cooling or water cooling of the heat sink . Bismuth telluride is the more commonly used semiconductor in Peltier elements. To generate large temperature differences, these semiconductors need to have high electrical and low thermal conductivity.
Where and how to use TEC?
Typically, one side of a TEC element acts as the hot side and the other side acts as the cold side, but interestingly, the two sides can be interchanged if the direction of the original current flow is reversed. Changing the polarity of the power supply (as shown in Figure 1) reverses the direction of the current flow in the Peltier element, causing the hot side to drop in temperature and eventually become the cold side, with gradual heating making the cold side or junction the hot side. This is useful for rapidly heating and cooling coupons in industrial medical applications.
Active control of the direction of the current through the Peltier element improves accuracy and speed, so that by actively controlling the heating and cooling of the material, the material can be adjusted to a set temperature. Many applications in IVD require this type of thermal cycling, such as the polymerase chain reaction. The medical devices responsible for these types of tests use TECs to heat swatches of genetic material to about 85°C and then cool the swatches to about 30°C.
Smart AFE and TEC control
By now you have an idea of what kind of systems can use a TEC, and if we were to integrate these functions into a single integrated circuit, we would need some sense input, memory or processing, and a control output. Most designers choose a discrete implementation, choosing an analog-to-digital converter (ADC) to sense the analog input from the environment, a microcontroller (MCU) or memory to process or address the ADC input, and then the MCU or memory will The corresponding digital information is sent to a digital-to-analog controller (DAC) to output a specific voltage or current. The reason I mention this is that a smart AFE like the AFE539A4 provides a total closed loop solution for the TEC because it integrates these components inside. After meeting these basic functions, what else can the AFE539A4 do?
The AFE539A4’s four outputs can be reconfigured as DAC outputs or ADC inputs for monitoring, allowing the flexibility to specify the function of a channel for a specific application or system. The integrated DAC provides voltage, current or pulse width modulated output.
Figure 2 illustrates the versatility of the AFE539A4, including an integrated reference, nonvolatile memory, DAC or analog output, and ADC or analog input, enabling closed-loop control without an MCU. These integrated devices support TEC current sensing and compensation (shown on the right side of the figure below), as well as direct negative temperature coefficient interfacing. The AFE uses this input data in the proportional-integral control loop to regulate the load to the temperature set point.
Figure 2: AFE539A4 smart AFE configured for TEC control
For connection and communication with the smart AFE, users can choose between an I2C interface, a serial peripheral interface, or a general-purpose input/output (GPIO) interface. The GPIO latch feature can also be used to latch to a value in the event of a fault, such as in high temperature conditions, a value may be specified to protect the system during faults or environmental factors that cause overheating.
As we all know in the past, TEC required many discrete components to achieve the necessary closed-loop control, the smart AFE has a full industrial temperature rating of C40°C to 125°C, and is available in a 3mm x 3mm package to provide the input in a single chip , processing and control functions.