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NTC characterization for IPMs – KBA237584

NTC characterization for IPMs – KBA237584

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1.1 Introduction

The aim of the negative temperature coefficient (NTC) characterization is to measure the relationship between the temperature obtained from the NTC and the actual measured junction temperature. To have direct access to the junction temperature, measurements are done on etched devices. In this case, phase V’s low side is the etched die.

The following parts are considered:

  • IM241-L6T2B
  • IM241-M6T2B
  • IM241-S6T2J
  • IM241-S6T2B (only without heatsink)

The NTC temperature is calculated as follows:

NTC bias circuit.png

Figure 1 NTC bias circuit

Thermistor resistance is calculated using the following equation:equation1.png

 Once Rth is known, the thermistor temperature (Tth) is calculated using the following equation:

equation2.pngHere, B = 4006 (NTC datasheet parameter) and,


Where, Rth,25 = 47 kΩ, NTC resistance at 25°C.

1.2 Measurements conditions

The NTC characterization is performed for the following two conditions:

  • With heatsink (Rthc-amb around 7 °C/W, same heatsink used for the MADK board)
  • Without heatsink

The NTC is characterized by a junction temperature in the range of 85°C to 150°C. The module is connected to a PMSM motor, and the phase current is increased step-by-step according to the desired junction temperature. The NTC readout is performed once the system is in a thermally steady state. The motor is driven using the iMOTION2.0 firmware with three-phase modulation and an 18-kHz switching frequency. The heatsink is DC-calibrated before performing the motor run test. The junction temperature is measured using a thermal camera, and the NTC temperature is calculated with the help of VTH voltage measurements, as mentioned earlier.









Figure 2 Characterization without heatsink

1.3 Results


 Rth,c-amb = 7.21°C/W


 Rth,c-amb = 7.02°C/W


Rth,c-amb = 6.83°C/W

Figure 3 Characterization with heatsink

1.4 Conclusion

Results without heatsink:

IM241-L6T2B and IM241-S6T2J have a very similar equation that describes Tj as a function of Tth. Similarly, IM241-M6T2B and IM241-S6T2B have a very similar Tj as a function of the Tth relation.

The reason could be that IM241-L6T2B and IM241-S6T2J have similar etch size (small etch on phase V low-side IGBT). On the other hand, IM241-M6T2B and IM241-S6T2B have a similar etch size (Phase V low-side IGBT is completely exposed, resulting in a bigger etch size if compared with the other two-part numbers).

Results with heatsink:

IM241-L6T2B and IM241-S6T2J have very similar equations (small differences could be due to slightly different Rthc-amb). In addition, the equation for IM241-M6T2B is slightly different. In this case, a different etching size and a different Rthc-amb could be the cause of this difference.

It is the etching size, not the die size, that seems to influence the relationship between NTC temperature and junction temperature.