FAQ1011

Answer

There are a number of factors to consider when fine-tuning a thermal management system for maximum stability. Our specification is based on a 10 kΩ thermistor using an off-ambient temperature setpoint, and a 35 mW load. Closely examine your test set up to verify that you’ve eliminated potential sources of instability. Points to consider:

• The sensor type and bias current should be selected to maximize sensitivity at the target temperature. Thermistors provide the best performance, particularly for applications where a single setpoint temperature must be accurately maintained. For example, at 25°C a 10 kΩ thermistor has a sensitivity of 43 mV/°C, whereas an RTD sensor has a sensitivity of 4 mV/°C.
• Variations in ambient room temperature affect the stability of both the load and the controller
• Varying airflow across the load may cause transient temperature variations
• Changes in the heat load, such as ramping the laser diode power up or down
• Changes in ambient temperature and airflow conditions around the heatsink
• The PID terms (Proportional, Integral, Derivative) of the controller algorithm have a strong influence on stability. If the proportional term is set too high the controller is likely to overshoot and oscillate about the setpoint. A too-high proportional setting also makes the controller very sensitive to minor fluctuations in load temperature, and may cause the temperature to oscillate.
• In a similar manner, the location of the sensor relative to the active heat load will impact stability. If the sensor is placed very near the load the control system will be more sensitive to changes and if the PID gain is too high the temperature may oscillate. Conversely, setting the sensor too far from the heat load will slow the response of the system, and may cause the controller to drive the TEC current rail-to-rail.
• Thermal paste application between the conductive parts of the load will impact stability; the paste should be applied in a thin and uniform layer. Otherwise use a thermal washer material.
• Properly mounting the sensor is important: if the thermistor is placed in a machined hole in the load block, the diameter of the hole should be as close as possible to the diameter of the thermistor. Use thermal paste between the thermistor and the load block to ensure good thermal conductivity.

If you require very high temperature stability, we recommend placing the load in a thermal chamber to provide stable and uniform ambient conditions. If a thermal chamber is not available, isolate the load from ambient variations by placing an insulated enclosure over the load – but be sure to leave the heatsink open. Keeping the ambient conditions stable around the heatsink will help keep the load temperature stable, as well.