Partial Discharge Inception in Ceramic Substrates Embedded in Silicone Liquid, Silicone Gel, and Mineral Oil at Fast Voltage Rise and Sinusoidal Voltage

This article summarizes experiments that were carried out on insulating ceramic substrates and a needle-plane arrangement with the purpose of characterizing the inception of partial discharges (PDs) in the insulation of high-voltage semiconductors. PD measurement results obtained on substrates are correlated with the high-field preinception currents measured in a needle-plane electrode arrangement in liquids and gel. The tested substrates were embedded in either silicone liquid, silicone gel, or mineral oil. The PD inception was measured at sinusoidal and switched voltage of different polarities and rise times. For all test objects, the PD inception voltage (PDIV) is significantly lower under switched voltage compared to the sinusoidal voltage. Silicone gel and silicone liquid as insulating media surrounding the substrate are similar in terms of the PDIV. The PDIV is lower for substrates placed in mineral oil, particularly under switched voltage. Silicone gel and liquid are characterized by very low preinception currents measured at a high electric field. In contrast, a higher preinception current is observed in mineral oil. These results are consistent with the hypothesis that the high predischarge current in the more conductive mineral oil leads to space charge build-up in the high-field region that, by deforming the electric field, affects the PD inception probability.

I. Introduction

In order to utilize the improved electrical and thermal properties of wide bandgap semiconductors such as SiC and GaN, a reliable packaging technology for power modules must be developed for higher electric and thermal stresses. Currently, the high-voltage insulation of power modules consists of three main components: insulating ceramic substrates, silicone gel filling, and the insulating housing [1], [2]. Additionally, polymeric passivation materials can be applied [3]. The use of nonlinear field grading materials in the triple point has been actively discussed [4], [5]. Aluminum nitride is used for insulated gate bipolar transistors (IGBTs) with the highest power rating. The production of the substrates includes a step where parts of the copper conductors are removed by chemical etching. This procedure creates very sharp metallic protrusions along the contour of the copper conductors. During operation, electrical discharges may start at such protrusions due to very high local electric fields [6], [7]. Such discharges may propagate deeper into the insulation along the interface between the ceramic and silicone gel.