TIDUF23 may 2023
The NTC temperature sensor built into the power module is sensed and fed back to the controller via an isolated digital signal. This signal is a 50% duty cycle square wave with varying frequency. The temperature sensor is positioned as close as possible to the power devices while remaining electrically isolated from them and therefore provides an approximate baseplate temperature. The temperature reported by the NTC differs largely from the junction temperature of the SiC MOSFETs and must not be used as an accurate junction temperature measurement. There are two ways to measure the NTC feedback signal for the three XM3 modules with the controller. The first method is using the enhanced capture (eCAP) peripheral to digitally measure the frequency of the signal coming directly from the differential receivers. The relationship of the NTC signal frequency to the NTC temperature is given in Figure 3-9 and Table 3-4. For the second method, the frequency signal is filtered and converted into an analog signal which can be measured by ADC on the controller. The analog voltage measures 0.38 V when the frequency is 4.6 kHz and 2.5 V when the frequency is 30.1 kHz.
NTC Temperature (°C) | NTC Resistance (Ω) | Frequency Output (kHz) |
---|---|---|
0 | 13491 | 4.6 |
25 | 4700 | 10.3 |
50 | 1928 | 17.1 |
75 | 898 | 22.8 |
100 | 464 | 26.4 |
125 | 260 | 28.3 |
150 | 156 | 29.5 |
175 | 99 | 30.1 |
The mapping between the NTC resistance (RNTC in Ohms) of the CAB450M12XM3 module and the virtual junction temperature (TVJ) is shown in Figure 3-10. It is the calculated using the following equation:
One additional temperature sensor is installed on the controller PCB to provide a measurement of the ambient temperature inside the reference design case. This temperature sensor consists of a 10 kΩ NTC surface mount thermistor and a 10 kΩ fixed resistor forming a voltage divider. As the temperature increases so will the voltage at the midpoint of the voltage divider. This voltage is low-pass filtered to remove any high-frequency noise from the slowly changing temperature. The conversion between this voltage signal, VT, and the temperature of the thermistor (in Kelvin) can be done with the following: