The influence of external conditions on the physical characteristics of Press-Pack IGBTs

In order to solve the problem of unclear factors affecting the physical characteristics of Press-Pack IGBT devices, this paper explores the influence characteristics of the two main external conditions of fixture pressure and heat dissipation capacity on the physical parameters of Press-Pack IGBTs by building IGBT models with different chip structure layout considering various physical characteristics of materials. The results show that the effect of fixture pressure on the stress characteristics of the device is more obvious than the heat dissipation ability of the device, and the water-cooling heat flux has more influence on the electrical-thermal characteristics of the device. The research results provide a reference for the external conditions and structure selection of Press-Pack IGBTs.


Introduction
With the wide application of high-voltage and large-capacity Press-Pack (PP) converter devices in HVDC transmission system and high-speed railway, etc., the reliability of PP-IGBTs and other PP devices has become a key problem for stable operation in soft and direct transmission projects.
Previous studies at the State Grid Anhui Institute of Electrical Research have shown that electrical overstress is the main cause of chip-level failure [1].Professor Yang Tongguang's team also confirmed that the misalignment of physical parameters and internal structural deformation caused by the deterioration of IGBT modules will seriously affect the transmission performance of the whole soft and direct transmission project [2].Research [3] shows that the main failure cause of commonly used rigid crimped devices is fretting wear failure.In addition, the research team of North China Electric Power University proposed that the accuracy of the physical parameters of the chip surface has a significant impact on the simulation accuracy of the crimped IGBT model [4].T. Poller's research [5][6] shows that the results of the physical characteristics analysis of single-chip PP devices cannot be directly applied to multi-chip PP models.Bin Ren's paper [7] does not take into account the coupling effect of thermal properties on electrical properties.
The above research shows that it is particularly important to analyze the physical characteristics of IGBTs with different chip structures, and it is also significant to find the main factors that affect the physical characteristics of PP devices.In this paper, the main external factors affecting the physical parameters of the crimp type device are explored through simulation modeling and control variables.

The basis and selection of boundary conditions
In the finite element method (FEM) simulation experiment of Multiphysics coupling, it is very important to establish the fully coupled model of Multiphysics field, in which many factors need to be considered.Details to be considered are shown in Figure.

Distribution the FEM Model of PP-IGBTs
By considering physical phenomena such as electromagnetic heat loss, electromagnetic excitation, thermal expansion, contact thermal resistance and dynamic conductivity which in the previous section, and adding corresponding physical parameters to different materials [8], a steady-state PP-IGBTs FEM model was established in the finite element numerical calculation software, as shown in Figure.

Test plan
Firstly, based on the VCEsat, slope resistance and threshold voltage of CRRC TG1500 IGBT, the physical parameters of the finite element model are modified.Then, the Multiphysics coupling model was set up by the control variable experiment method, and the heat dissipation capacity and fixture pressure were respectively limited, and the influence rules of the two on the device VCEsat, junction temperature and IGBT chip surface stress were obtained.Finally, by changing the heat dissipation capacity of the device and the pressure of the fixture at the same time, the boundary conditions of different physical parameters of PP-IGBTs in the coupling field are more seriously affected.

The influence of physical characteristics on PP-IGBTs
By applying different fixture pressure, water-cooling heat flux to the model, the contact stress, junction temperature and current distribution are influenced by external conditions.

The influence law of contact stress characteristics 3.1.1 Effect of fixture pressure on stress distribution on chip surface
By applying different fixture pressures to the PP-IGBTs of different structures as shown in Figure .3(a) to (d), the reliable fixture pressure ranges of the PP-IGBTs of different structures as shown in Figure .3 can be obtained according to the reasonable contact stress on the surface of the IGBT chip described in [9].Through this result, we can find that the surface stress of PP-IGBTs chip and the fixture pressure show a positive correlation when the fixture pressure is greater than a certain value.Specifically, for the single-chip period, this characteristic is present when the pressure is greater than 995N, while for the 7, 9, and 36 chip models, the positive correlation is present when the pressure is greater than 7000N, 11600N, and 60000N, respectively.

Effect of heat dissipation capacity on stress distribution on chip surface
By applying different heat fluxes to PP-IGBTs with different structures, it is clear that the stress distribution uniformity of the device will be improved when the heat dissipation capacity is sufficient, as shown in Figure.

The influence law of junction temperature characteristics
As shown in Figure .6,considering that the IGBT model of 36 chips has the richness of chip samples in the simulation experiment results, and the IGBT model of other chip structures has almost the same conclusion, this section still takes the results of 36 chips as an example, it is found that there is basically an exponential correlation between junction temperature and heat flux, but it has little effect on junction temperature difference between chips at different positions.The fixture pressure has obvious influence on the stability of the junction temperature of the device.When the fixture pressure reaches the reliable range, the junction temperature difference between different chips of the device reaches the minimum and tends to a stable value.Through the experimental results of the influence of the above single variable on the junction temperature, it should be clear that for PP-IGBTs, the optimal selection sequence of external conditions is to first select the appropriate pressure of the fixture, and then select the appropriate heat flux.As shown in Figure .7, the 36-chip model results are again taken as an example in this section, and it is found that the current distribution characteristics are limited by the heat dissipation capability, and the current density of devices at different locations will have a certain influence, but the amplitude is not obvious.In contrast, the effect of fixture pressure on the device flow equalization is similar to that of junction temperature.When the fixture pressure reaches a reliable value, the device current density uniformity is improved well.
In addition, the cross-sectional current density of the central-chip surface was extracted, and the 2D distribution diagram of the influence of fixture pressure on the chip surface current density was obtained, as shown in Figure .8. It can be found that for the single-chip model, when the fixture pressure reaches more than 1000N, the chip surface has good flow sharing, and the chip can maintain the normal operation state under the working condition.Based on the comparison between different chips and the current density distribution on the surface of a single-chip, the importance of fixture pressure on PP-IGBTs current sharing is further demonstrated.

Coupling effects of external conditions
Paper [10] points out that the ideal heat dissipation capability in engineering is to keep the upper and lower plate temperature of the crimped double-sided heat dissipation device below 60℃.The reasonable range of water cooling can be determined by combining the IGBT models with different structures.At the same time, according to the results of previous results, it is clear that there is a suitable clamp pressure range for crimped IGBT devices.Taking 36-chip model as an example, considering the influence of fixture pressure and heat dissipation capacity on the electrical-thermal-mechanical performance of the device, the coupling influence relationship of external conditions as shown in Figure .9 is obtained.According to Figure.9(a)(c), it can be found that the junction temperature and saturation pressure drop VCEsat of PP-IGBTs are more significantly affected by heat dissipation capacity than fixture pressure, and the two physical parameters have a monotonically decreasing relationship with watercooling and heat flux, while fixture pressure has a limited effect on junction temperature and VCEsat.The increase of fixture pressure has a relatively good effect on maintaining device junction temperature, but when the heat dissipation capacity is insufficient, the excessive fixture pressure may lead to the increase of device junction temperature.By observing Figure.9(b), it can be found that although the junction temperature of the device changes significantly with the water-cooling and heat flux, and the stress changes caused by the difference in the coefficient of thermal expansion between materials are also prominent, the contact stress on the chip surface is still mainly constrained by the fixture pressure.
Therefore, combined with the relevant parameters of TG1500 IGBT device developed by Zhuzhou CRRC, it can be obtained that the crimped IGBT of 36-chip can achieve the best physical properties when the fixture pressure is 50-60kN and the water cooling and heat flux on both sides of the device is greater than or equal to 7000W/(m 2 •K).The reliability data obtained from the fatigue and power cycle tests under these external conditions are more valuable for reference.In addition, there are similar rules for IGBT models of other size structures except single chip, which will not be described in this paper.

Conclusion
Based on theoretical derivation and simulation experiments, a FEM simulation model of electricalthermal-mechanical field coupling for PP-IGBTs considering dynamic conductivity is presented.The results show that the fixture pressure has a significant effect on the stability of current, stress and junction temperature of the device, while the heat dissipation ability of the device only has a certain effect on the uniformity of stress and junction temperature distribution.In addition, the range of reliable fixture pressure and heat dissipation capacity of PP-IGBTs is defined.The research results can provide some help for the finite element model construction and the actual pressure and heat dissipation conditions of PP-IGBTs. 1.

Figure. 2
(a) to Figure.2(d) are the IGBT FEM models of single-chip, 7-chip, 9-chip and 36-chip, and the sectional view structure of the chips was shown in Figure.2(b).

Figure. 2
Figure. 2 FEM model of PP-IGBTs with different chip structure.

Figure. 3
Figure. 3 Change curve of pressure contact stress on surface of IGBT chips with different structure.

Figure. 4
Figure. 4 Reasonable pressure range of different structure PP-IGBTs.

5 .Figure. 5
Figure. 5 Surface contact stress curves of IGBTs with different structures subjected to water cooling fluxes.

Figure. 8
Figure. 8 2D distribution diagram of the relationship between fixture pressure and chip surface transversal current density of single-chip.

Figure. 9
Figure. 9 3D distribution diagram of the influence of external conditions on the physical characteristics of PP-IGBTs.