Nonlinear composite dielectrics can function as smart materials for stress control and field grading in all fields of electrical insulations. The percolation process is a significant issue of composite dielectrics. However, the classic percolation theory mainly deals with traditional composites in which the electrical parameters of both insulation matrix and conducting fillers are independent of the applied electric field. This paper measured the nonlinear V-I characteristics of ZnO microvaristors/silicone rubber composites with several filler concentrations around an estimated percolation threshold. For the comparison with the experiment, a new microstructural model is proposed to simulate the nonlinear conducting behavior of the composite dielectrics modified by metal oxide fillers, which is based on the Voronoi network and considers the breakdown feature of the insulation matrix for near percolated composites. Through both experiment and simulation, the interior conducting mechanism and percolation process of the nonlinear composites were presented and a specific percolation threshold was determined as 33%. This work has provided a solution to better understand the characteristics of nonlinear composite dielectrics.

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Understanding the Percolation Characteristics of Nonlinear Composite Dielectrics

Wide bandgap semiconductors enable high voltage (10 kV and more) switches. As a consequence, new packaging solutions are required to prepare the ground for such devices. The metallized ceramic substrate is a well-known and established technology for voltages up to 3.3kV, but it exhibits some weaknesses at higher voltages: due to its manufacturing process, the profile of the metallization is sharp and induces a reinforcement of the electric field at the “triple point” area (where the ceramic, the conductor and the encapsulating material meet), which can lead to Partial Discharges (PD), eventually causing a failure of the module. In this paper, we present a new substrate structure, where the triple point is moved away to an area where the electric field is lower. In this structure, the ceramic is machined to form protrusions, and round-edge metallizations are brazed on top. The design of the substrate, based on finite-elements is described, and calculations show that a 1 mm-thick AlN layer should be sufficient to withstand 10 kV. The manufacturing process of this substrate is presented. The test results demonstrate the superiority of this new solution, with a partial discharge inception voltage increased by 38 %.

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Protruding ceramic substrates for high voltage packaging of wide bandgap semiconductors

Increased voltage blocking capability and the development of packaging technology for IGBTs can enhance the local electric field that may become large enough to increase partial discharges (PDs) within the module. The study presents a survey on (i) simulation the electric field within an IGBT module; (ii) current standards for evaluation of the insulation systems of IGBTs; (iii) PD detection and localisation methods as well as other diagnostic and quality control test methods about IGBTs; and (iv) various methods for PD control in an IGBT module. The survey shows remarkable technical gaps in all four areas. More sophisticated numerical and theoretical techniques are needed to model complicated geometries, e.g. extremely sharp edges of the copper metallisation and protrusions in the substrate, and composite non-linear field grading materials. There is no model to take into account defects in the gel and on the ceramic substrate. IEC 61287-1 cannot sufficiently assess the behaviour of PDs on IGBT module under the actual operating conditions exposing fast rise pulse-width modulation-like voltages. There is no agreement on the exact origin and location of PDs in the module with relying on measured phase-resolved PD patterns. PD control methods using non-linear grading materials are not mature enough.

PD measurements, failure analysis, and control in high-power IGBT modules

This manuscript critically reviews recent research on electrical insulation materials and systems used in power electronics devices and focuses on electrical treeing in silicone gel, PD modeling, and mitigation methods. For mitigation methods, electric field grading techniques, such as 1) various geometrical techniques, and 2) applying nonlinear dielectrics are discussed. Alternatives for silicone gel, such as liquid dielectrics, are also highlighted. The drawbacks of reported research and technical gaps are identified. In particular, we show that the investigations carried out to date are in their infancy regarding the working conditions targeted for next-generation WBG power devices. This review will provide a useful framework and point of reference for future research.

Insulation Materials and Systems for Power Electronics Modules: A Review Identifying Challenges and Future Research Needs

Wide bandgap (WBG) power devices with voltage ratings exceeding 10 kV have the potential to revolutionize medium- and high-voltage systems due to their high-speed switching and lower ON-state losses. However, the present power module packages are limiting the performance of these unique switches. The objective of this article is to push the boundaries of high-density, high-speed, 10-kV power module packaging. The proposed package addresses the well-known electromagnetic and thermal challenges, as well as the more recent and prominent electrostatic and electromagnetic interference (EMI) issues associated with high-speed, 10-kV devices. The module achieves low and balanced parasitic inductances, resulting in a record switching speed of 250 V/ns with negligible ringing and voltage overshoot. An integrated screen reduces the common-mode (CM) current that is generated by these fast voltage transients by ten times. This screen connection simultaneously increases the partial discharge inception voltage (PDIV) by more than 50%. A compact, medium-voltage termination and system interface design is also proposed in this article. With the integrated jet-impingement cooler, the power module prototype achieves a power density of 4 W/mm3. This article presents the design, prototyping, and testing of this optimized package for 10-kV SiC MOSFETs.

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Design and Experimental Validation of a Wire-Bond-Less 10-kV SiC MOSFET Power Module

The trend towards increasing blocking voltage of high power Insulated Gate Bipolar Transistor (IGBT) modules is challenging for their electric insulation system. In particular, the electric field at the edges of the substrate metallization increases accordingly and can exceed the dielectric strength of the gel commonly used as encapsulation, leading to partial discharges or even breakdown of the insulation. This paper reports on the use of nonlinear resistive field grading to mitigate the electric field within power electronic modules. Finite element simulations show how the field enhancements can be significantly reduced by applying a functional coating with suitable nonlinear resistive characteristic. Coatings made of polyimide filled with ZnO microvaristors were manufactured and tested.

L. Donzel

Papers

Nonlinear resistive electric field control for power electronic modules