3.6.2 Temperature Monitoring with NTC Thermistors

To measure the temperature of the heat sink and cold plate, Negative Temperature Coefficient (NTC) thermistors were used. These devices provide variable resistance directly correlated to the temperature of the environment in which they are placed. To bias the NTC thermistors, a resistor divider network is implemented where the NTC thermistor forms the lower element, as shown in the schematic below.

Another benefit of using the NTC on the low side of the voltage divider is the ability to implement Over-Temperature Protection (OTP) for the current regulator. This is discussed in the Current Regulator OTP with NTC Sensing chapter. Additionally, a simple low-pass RC filter network reduces input noise and provides a bit of Electrostatic Discharge (ESD) protection. With the resistor divider, the voltage across the thermistor will equal:

With thermistors, the temperature resistance ratio is not linear. For accuracy and computation simplicity, a look-up table was implemented to convert measured voltages into temperatures. The manufacturer of the NTC thermistors provides a spreadsheet of resistance versus temperature. Later, the thermistor resistance versus temperature was measured to further refine the look-up table. By knowing the parameters of the NTC (and the surrounding circuit), ADC values could be converted into temperature directly. To search the look-up table more efficiently than a linear search, a version of the quicksort algorithm was implemented. This algorithm is described in detail in Section 3.6.2.1 Quicksort Algorithm for Table Searching.