Study on the methods to measure the junction-to-case thermal resistance of IGBT modules and press pack IGBTs
Introduction
It is well known that the junction-to-case thermal resistance is the most important thermal parameter of a power semiconductor device and also the criterion to evaluate its heat dissipation ability [1]. Thus, accurate measurement is notably important for manufacturers to optimize the internal structure in order to improve its reliability and for users to take full advantage of the devices.
Currently, the most commonly used method to measure the thermal resistance is the traditional thermocouple method, which was proposed by IEC, MIL standard [2], and JEDEC51-1 standard [3], and it is referred to as the steady-state method in this paper. The junction-to-case thermal resistance can be calculated using Eq. (1) the junction temperature Tj, case temperature Tc, and power dissipation P of the device under test (DUT) are known [4].
The junction temperature Tj can be indirectly measured using an electrical method. The relationship between the voltage drop caused by a notably small constant current and the junction temperature, also called the K factor, is the most commonly used method, [5]. Although this method is notably simple and easy to perform, its accuracy is significantly affected by the thermocouple position, clamping force for fixture, thermal grease, etc. Thus, this method is often not sufficiently reproducible and accurate [6]. In 2010, the JEDEC51-14 standard [7] specified a new method (the transient dual interface method, which is referred to as the transient method in this paper) to measure the junction-to-case thermal resistance of power semiconductor devices without measuring the case temperature. The transient method defined in this paper is different from the traditional transient method used to measure the transient thermal impedance, which is included in the datasheet. This method considerably improves the accuracy and reproducibility because a thermocouple is not required to measure the case temperature.
The steady-state method is the most commonly used method to measure the junction-to-case thermal resistance of wire-bonded Insulated Gate Bipolar Transistor (IGBT) modules, and the transient method has been recently introduced by some manufacturers, such as Infineon [8]. The differences between the packaging styles (the internal structure is shown in Fig. 1) and working conditions of press pack IGBTs (PP IGBTs) and wire-bonded IGBT modules render the method suitable for wire-bonded IGBT modules, but not for PP IGBTs. Furthermore, an external clamping pressure of approximately 1.2 kN/cm2 is required to maintain the normal operation of PP IGBTs [9], which is much higher than the recommended clamping pressure of 10 N/cm2 for the fixture during the thermal resistance measurement [7]. Therefore, the thermocouple located between the case and the heatsink will be damaged under such a high clamping force, and the heat path will also be influenced by the inserted thermocouple. Consequently, the accurate measurement of junction-to-case thermal resistance of PP IGBTs remains a significant challenge. Only the junction-to-heatsink thermal resistance (but not the junction-to-case thermal resistance) is provided in the datasheet provided by the manufacturers of PP IGBTs such as Westcode [10] and Toshiba [10].
In this paper, the junction-to-case thermal resistances of IGBT modules and PP IGBTs are measured using both the steady-state method and transient method. The applicability of these two methods to IGBT modules and PP IGBTs is compared based on the experimental results and packaging styles. This paper is structured as follows. The steady-state and transient method to measure the junction-to-case thermal resistance of power semiconductor devices are introduced in Section 1. The principle of the transient method and the determination of thermal resistance are described in Section 2. A half-bridge IGBT module manufactured by Infineon is introduced to measure the thermal resistance using the two aforementioned methods and both the experimental results are compared with the values provided in the datasheet in Section 3. Furthermore, the junction-to-heatsink thermal resistance of PP IGBTs is measured using the steady-state method as the thermocouple cannot be placed in the interface between the case surface and heatsink and the value of junction-to-heatsink thermal resistance thus obtained is compared with the value provided in the datasheet. The junction-to-case thermal resistance of the PP IGBTs is measured using the transient method in Section 4. Section 5 concludes the paper and describes the applicability of the steady-state method and transient method to IGBT modules and PP IGBTs according to the experimental results.
Section snippets
Measurement principle
Between the two aforementioned methods, the steady-state method is relatively simple and the most commonly used method to measure the junction-to-case thermal resistance of power semiconductor devices. Therefore, in this paper, only the principle of the transient method is presented.
The transient method requires two measured transient thermal impedance (Zth) curves of a power semiconductor device in contact with a temperature-controlled heatsink. The first test is performed with no thermal
Experiments on IGBT modules
In this section, the thermal resistance tester Phase 11 is used to measure the IGBT chip junction-to-case thermal resistance of the half-bridge module manufactured by Infineon (FF100R12RT4). Both the steady-state method and transient method are applied to measure the thermal resistance and the experimental results are compared.
Test bench
The fixture of the commercial thermal resistance tester is not suitable for PP IGBTs because of their special packaging style and working conditions. In this paper, we designed two fixtures for PP IGBTs and used the thermal resistance tester Phase 11 to measure the junction-to-case thermal resistance and the Zth curve of the PP IGBT.
According to the principle of the steady-state method and transient method, the K factor [5], which accounts for the relationship between the junction temperature
Conclusions
According to the experimental results above, it can be observed that the methods used for IGBT modules may not be suitable for PP IGBTs because of the differences in their packaging styles and working conditions. The applicability of the steady-state method and transient method to the measurement of junction-to-case thermal resistance of IGBT modules and PP IGBTs is summarized in Table 4.
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Both the steady-state method and transient method are suitable for the measurement of junction-to-case
Acknowledgements
The work presented in this paper has been supported by the National Natural Science Foundation of China (51477048), National Key R&D Program of China (2016YFB0901800), and Fundamental Research Funds for the Central Universities 2016XS04.
Erping Deng was born in Hunan province, China, in 1989. He received the bachelor degree in electrical engineering from Harbin Institute of Technology, Harbin, China, in 2013. Currently, he is a Ph.D. student of State Key Laboratory of Alternate Electrical, North China Electric Power University. His main research interest is the packaging and reliability of high voltage power electronics devices.
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Erping Deng was born in Hunan province, China, in 1989. He received the bachelor degree in electrical engineering from Harbin Institute of Technology, Harbin, China, in 2013. Currently, he is a Ph.D. student of State Key Laboratory of Alternate Electrical, North China Electric Power University. His main research interest is the packaging and reliability of high voltage power electronics devices.
Zhibin Zhao was born in Hebei province, China, in 1977. He received the Ph.D. degree in electrical engineering from North China Electric Power University, Baoding, China, in 2005. Currently, he is a Professor of State Key Laboratory of Alternate Electrical, North China Electric Power University. His main research interest is computational electromagnetics and electromagnetic compatibility in power electronic.
Yongzhang Huang was born in Guangxi province, China, in 1962. He received the B.S. degree from the Department of Engineering Physics, Tsinghua University, Beijing, China, in 1984, and the Ph.D. degree in physics from the Chinese Academy of Sciences in 1991.He is currently a professor in the Department of Electrical Engineering at North China Electric Power University, Beijing, China. He is also a Chinese distinguished expert of “thousand talents program” and the deputy director of the State Key Laboratory of new energy power system.