HITACHI, ABB, LoPak1 module lifetime and performance improved using new Thermal Interface Material (TIM)

Article source: ABB Library

Improvements in the lifetime and electrical performance of Hitachi ABB Power Grids’ 1700 V LoPak1 modules will be available for orders starting in June 2020, with the introduction of an optional preapplied thermal interface material (TIM) which is deposited on the outside module base plate surface that contacts the heat sink.

Benefits using TIM 

The use of the TIM improves the thermal conduction at the module baseplate/heatsink interface ensuring more stability over long-term operation than commonly used conventional heat conductive pastes. It can increase the amount of current that can be safely switched by the IGBT and improve the module’s lifetime, since the TIM’s thermal resistance does not increase with the number of switching cycles.

The challenge is high thermal resistance (or low conductivity) through a standard power module

Figure 1 below shows that heat generated by power losses during the operation of IGBT switches must be transported from the chip, through the module, to the heatsink to prevent the junction temperatures of the chips from rising beyond maximum allowable limits. The interface between the module baseplate and the heat sink can have the highest thermal resistance of all of the interfaces in that path, as seen in the chart on the right.

Properties of TIM

The use of the TIM improves the thermal conduction across this interface during long term operation supporting a more stable during long term operation than conventional heat conductive pastes, which are commonly used for this application. (Figure 2). The use of the TIM can increase the amount of current that can be safely switched by the IGBT and also improve the module’s lifetime, since the TIM’s thermal resistance does not increase with the number of switching cycles.

Improved performance using TIM

Significant improvement in the thermal conductivity when TIM is used is shown in figure 4. Both for the case to ambient which is also reflected through the entire flow from the chip to the ambient. This is also due to the spreading of the TIM across the interface when the module is installed and run at operation temperature. 

The use of the TIM can increase the amount of current that can be safely switched by the IGBT because of the improvement it creates for heat transfer allows the virtual junction temperature of the IGBT to be lower for a given current level.

Increased module lifetime using TIM

The ability of TIM to extend the lifetime tive paste stability of a module compared to standard paste can be seen in the cycling test data in Figure 5. While the initial thermal conductivity of pre-applied TIM is comparable with the heat conductive paste, the modules using TIM show no increase in thermal resistance with cycling, while those using the heat conductive paste show increasing thermal resistance during the test. 

These results imply the better longterm stability of the TIM as opposed to the degradation of the thermal interface using the heat conductive paste.

About LoPak

LoPak are medium power IGBT modules which enable extra transient over-current capability by taking advantage of the IGBT module’s maximum operating junction temperature of 175°C, compared to the typical 150°C. The current configuration is a 1700 V phase-leg (half-bridge) IGBT module with copper base plate, using ABB’s uniquely designed SPT++ IGBT and diode devices and offered with 450 A, 300 A and 225 A current ratings. 

This combination provides outstanding safe operating area (SOA) and over-temperature capability. Within its product class, LoPak benefits from Hitachi ABB Power Grids’ knowhow in robust electrical performance and high reliability. 

The detailed design and virtual prototyping used by Hitachi ABB Power Grids makes the LoPak module’s current distribution well-balanced during switching and especially controlled under overload conditions. Excellent field feedback has confirmed that the LoPak modules carries the same DNA for high reliability and robustness as the entire family of Hitachi ABB Power Grids high-power semiconductors. 

The LoPak module is a key part in Hitachi ABB Power Grids’ expanding catalog of products to support the renewable energy and industrial marketplace.

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