The heat dissipation circuit of the power module is through the chip DCB、 The copper substrate, heat sink, welding layer, and thermal grease layer are connected in series. Each layer has a corresponding thermal resistance, which is connected in series. The total thermal resistance is equal to the sum of each thermal resistance. This is because in the process of heat transfer, each thermal resistance needs to be overcome sequentially, so the total thermal resistance is the accumulation of each thermal resistance.

Each chip has multiple thermal conduction layers on its conduction path. When simulating losses and heat, the basic simulation always focuses on a single IGBT or diode. Therefore, the shell temperature that needs to be known refers to the temperature directly below the chip, and the heat sink temperature also refers to the temperature directly below the chip.
The specific testing method is:
Tc： The shell temperature is measured as Tc through small holes penetrating through the heat sink and thermal interface material under the power switch (chip).
Ts（Th）： The temperature of the radiator is measured through a specified blind hole that stops 2mm ± 1mm below the surface of the radiator (type test characteristics should be specified).
Tsx： The temperature of the radiator can also be taken from the hottest touchable point closest to the power switch (chip).
In order to measure Tc, small holes were drilled through the heat sink and thermal interface material. Inserting the sensor will affect the heat transfer from the module shell to the heat sink. Fortunately, for modules with a substrate, heat will diffuse laterally on the substrate, and the measurement error of the holes and probes can be controlled at a level of 5%.
For modules without a substrate, the lateral conduction of heat is very limited, and the effective area of heat transfer is equivalent to the chip size. Drilling holes to measure the shell temperature has a significant impact on module heat dissipation, and the measurement changes the working conditions. Such measurements should not be advocated. The reference temperature for thermal impedance is Ts (Th) instead of TC, which means RthJH is directly defined.
Engineering measurement method for module shell temperature:
Measuring the shell temperature at the bottom of the chip is a type testing method used for power platform development. In practical applications, the power module comes with an NTC and a negative temperature coefficient thermistor as temperature measuring elements.
NTC is installed near the silicon chip to achieve a relatively tight thermal coupling. Depending on the module, NTC can be installed on the same DCB as the silicon chip or on a separate substrate. NTC measurement values need to be corrected based on experience or calibrated for heat dissipation.
Empirical method:
NTC can be used for steady-state overheating protection, with a time constant of approximately 2 seconds. On the transient thermal resistance curve, the thermal time constant of the chip can be read, which is about 0.2 seconds. However, the time constant of the entire heat dissipation system is very large, such as around 20 seconds. Therefore, NTC can detect slow temperature changes and slow overload situations, and is powerless for short-term junction temperature overheating protection, let alone for short circuit protection.
From experience, it is known that for power electronic devices, the difference between the temperature of the radiator and the temperature of the NTC is about 10K. This method is only used for estimation. It is recommended to use the following calibration method and thermal simulation to obtain more accurate values.
Calibration method:
For the power system with completed structural design, we can measure the surface temperature of the chip and the Tvj~TNTC curve under specific heat dissipation conditions. This curve can help you use NTC to monitor the chip temperature under steady-state conditions.
The temperature of the chip is measured using an infrared thermal imager, and the shell temperature is measured using a thermocouple below the chip. The NTC resistance value is recorded by a data collector and converted into the corresponding temperature value based on the NTC resistance temperature curve of the IGBT module.

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