3.11 Thermal Design

As mentioned earlier, Peltier plates are a thermoelectric heat pump that moves heat from one side to another in response to current flowing through the plate. However, Peltier plates cannot support infinite temperature gradients across the surface – the larger the gradient is, the less heat is pumped by the Peltier. Figure 3-9 shows the amount of heat pumped as a function of the temperature differential for the Peltier element (P/N: CP85435) used in this application.

Figure 3-9. Typical Peltier Performance Curve of CP85435 at 27°C
Note: Figure 3-9 is used with permission from CUI Devices, cuidevices.com.

To minimize the thermal gradient, the Peltier element is connected to a large CPU heat sink (a Noctua NH-D15S) with a 140 mm cooling fan. This fan is crucial – despite the size of the heat sink, the heat sink temperature quickly rises without active airflow. A basic model of the system elements is shown below in Figure 3-10.

Figure 3-10. Simplified Diagram of the System

One other source of heat is the power stage for the Peltier plates. A MOSFET is used to turn ON and turn OFF the power to the Peltier modules. At almost DC-level switching, the primary source of heating in the MOSFET is from RDS (resistance from drain to source). Another source of power dissipation is from the current shunt resistor.

A future improvement for this system is to implement the OTP from the power-switching circuit. A possible method of implementing the OTP in the microcontroller without using any I/O is discussed in the Current Regulator OTP with NTC Sensing chapter.