High electric fields at the edge of the substrate metallization can give rise to partial discharge within power electronic modules and can lead to eventual failure. This paper examines the use of silicone gels filled with barium titanate to reduce the electric field enhancement at the edge of substrate metallization and therefore increase partial discharge inception voltages. The barium titanate filled gel produces a dielectric in which the relative permittivity is increased over a plain gel and that also exhibits a dependence on electric field. The theoretical electric field reduction that can be achieved in a power electronic module through the use of filled gels is demonstrated and compared against experimental measurements including the trial of the technique in some commercial modules. As promising results are achieved, consideration is also given to the effect of the barium titanate filler on the viscosity of the gel and the thermal conductivity, two key manufacturing issues.

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Partial discharge control in a power electronic module using high permittivity non-linear dielectrics

Although the adverse effects of using power electronic conversion on the insulation systems used in different apparatuses have been investigated, they are limited to low slew rates and repetitions. These results cannot be used for next-generation wide bandgap (WBG)-based conversion systems targeted to be fast (with a dv/dt up to 100 kV/ws) and operate at a high switching frequency up to 500 kHz. Frequency and slew rate are two of the most important factors of a voltage pulse, influencing the level of degradation of the insulation systems that are exposed to such voltage pulses. The paper reviews challenges concerning insulation degradation when benefitting from WBG-based conversion systems with the mentioned dv/dt and switching frequency values and identifies technical gaps and future research needs. The paper provides a framework for future research in dielectrics and electrical insulation design for systems under fast, repetitive voltage pluses originated by WBG-based conversion systems.

Accelerated insulation aging due to fast, repetitive voltages: A review identifying challenges and future research needs

Silicone gels are widely used to encapsulate power electronics modules. The objective of this paper is to study the partial discharge (PD) mechanism in a silicone gel, using electrical measurements and fast visualization. Experiments are carried out in a pointplane geometry, using either impulse voltage or ac. Under impulse voltage, the visualization shows that the first discharges recorded in a new sample are due to the initiation and propagation of "streamers", with features close to those previously reported in viscous liquids. Under ac, a stable PD regime rapidly establishes: a streamer creates a cavity with a long lifetime, and PDs occur in this cavity during the subsequent ac waves. Since slow streamers develop during long times (some ms), classical PD measurements do not provide an adequate evaluation of the actual discharge magnitude: very small PDs (1 pC) are recorded, whereas large cavities with a total charge in excess of 10 pC are actually present in the gel. When PDs are repeated permanent degradations of the gel rapidly occur, showing the limited self-healing capabilities of silicone gels.

Streamers and partial discharge mechanisms in silicone gel under impulse and AC voltages

Increasing operating voltages of insulated gate bipolar transistor (IGBT) modules results in higher demands on electrical insulation capabilities as well as partial discharge resistance. This paper discusses the insulation failure modes observed in high voltage power modules; most critical are water trees, partial discharge (PD) and electrical trees. Besides the review of insulation failure mechanisms, an experimental analysis method is discussed in order to identify possible failure source. Presented is a PD setup that includes a light sensitive CCD camera for optical inspection. Within the optical measurements, electroluminescence maps are also recorded in order to identify critical regions of high electrical fields. This method permits an analysis even before PD or electrical treeing begins. Optical PD inspections allow identifying PD failure root causes like protrusions or irregular edge shapes. In addition, investigations of PD failures on the ceramic substrate level have been performed and the distribution of inception voltages for one substrate type analyzed.

Analysis of insulation failure modes in high power IGBT modules

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

Today, most railway applications use hybrid power modules. These IGBT modules are constituted of several dielectric materials to achieve chip packaging. Due to the presence of high voltage, new phenomena which could be previously ignored for lower voltage ratings have now to be taken into account. This is the case of partial discharges which may appear in this dielectric material stack. Up to now, the only test carried out to detect partial discharges in such modules is the application of a normalised test (IEC270). The proposed test configuration only allows testing of a part of the dielectric insulation. For this reason, we propose and discuss a novel method for determining partial discharges existence in power modules.

Partial discharge detection in power modules

In power electronics, the development of new IGBT power modules allow them to switch kV and kA during /spl mu/s. However, such components need to be cooled and ceramic materials are used as an interface between the electronic devices (the chips) and the cooling. Another key point of the reliability of the packaging of these devices is their encapsulation made of silicone gel to prevent any discharge on the substrate surface and also in the vicinity of the bondings. In order to identify the physical mechanisms involved in these materials, different experiments are performed. The results of conductivity and space charge measurements (using the LIPP method) under HVDC electrical stress at different temperatures are presented and analysed.

Investigations on DC conductivity and space charge in silicone gel

In railways applications high voltage IGBT power modules are used to build power converters Such an approach allows increasing at the same time the reliability, the thermal and electrical performances, to decrease the cost and to achieve an improved design From a material point of view, such modules have to be considered as a stack of different types of materials Among them, the dielectric materials, both organic and inorganic, suffer from a lack of knowledge of their properties This paper describes the step by step method which has been followed to study different ceramic substrates and encapsulation materials in order to get a better understanding of their properties and their ability to be used in such a structure Choice and dimensioning criteria will be presented

Contribution to the dimensioning of the different insulating materials used in high voltage power electronics module manufacturing

500/spl mu/m thickness CVD diamond samples of different grades related to the sp2/sp3 phase ratio and able to be used as substrates in power electronic applications have been characterized. The following conclusions may be drawn: breakdown and resistivity values of thick CVD diamond are far from the ones claimed for thin film (/spl sim/MV/cm). and for the natural materials (/spl sim/10 MV/cm) Samples presenting the largest amount of nitrogen behave from an electrical point of view better than pure "undoped" diamond samples.

http://ieeexplore.ieee.org/iel5/7584/20667/00955593.pdf

Breakdown characteristics of different grades of CVD diamond samples

An increase in the power levels involved, an appropriate integration and good reliability of the whole system are the actual trends in power electronics As regards to this last point, many studies are realized from the semiconductor point of view The key point in today's integration appears to be the passive components in hybrid structures Such systems are built taking into account each function to be realized and the corresponding applied stresses The work presented in this paper deals with the realization of specific materials, mainly based on barium titanate ceramics, whose properties are studied starting from the synthesis of the powder up to the final realization of the layers which is used in the integrated structure

D. Malec

Papers

Partial discharge detection in power modules

Investigations on DC conductivity and space charge in silicone gel

Contribution to the dimensioning of the different insulating materials used in high voltage power electronics module manufacturing

Breakdown characteristics of different grades of CVD diamond samples

Integration of passive components in power electronics: from the material to the system