Tetsumi Takano

Dynamic potential distributions of surface discharge in silicone gel

Silicone gel is widely used to encapsulate power electronic circuits. It is known that the weakness of this insulation system is the surface discharges between gel and substrate. These discharges give rise to the growth of cavities in gel, which may lead to eventual failure of the insulation. Despite the fact, surface discharges in gel has not yet been studied enough. In order to clarify the nature of streamers in the cavity and relation between streamer propagation and charge accumulation at the interface of the cavity, the relation between apparent charge, light emission, motion of the cavities, and dynamic potential distribution of surface discharge in silicone gel was investigated. The results showed that conductive streamers initiate at the electrode, propagate through the cavity and accumulate charges on the cavity surface. In addition, it was confirmed that conductivity of the cavity surface is low.

Surface discharges in silicone gel on AlN substrate

AlN substrate is widely used along with silicone gel to encapsulate power electronic circuits. It is known that the weakness of the insulation system is surface discharges propagating at the gel-substrate interface. In this research, the nature of surface discharges in gel, on various substrates was investigated. The results are as follows: the maximum stopping length of cavities on AlN substrate was more than twice than that on other substrates, and light emission due to discharges in cavities on AlN substrate was different from that on other substrates; for AlN substrate, tip of the cavity emitted light while other part did not emit light, and for other substrates, all the path along the cavity emitted light. These results indicated that the surface of the AlN substrate is degraded by discharges and becomes conductive. We have confirmed this assumption by measuring the conductivity of the cavity path: 5 kΩ/100 μm for that on AlN and above 1 MΩ/100 μm for Al2O3 and glass substrates. The high electric field at the tip of conductive path elongates the cavity stopping length on AlN substrate compared with others. In order to investigate the degradation process of AlN substrate surface, energy dispersive X-ray spectroscopy analysis was carried out. Reduction of nitrogen component on the cavity path was seen, indicating that generation of Al increases the conductivity of cavity path. We have experimentally shown that oxidation treatment of the AlN substrate significantly reduces the cavity stopping length.

Viscoelastic analysis of cavity propagation in gel with electrical discharge

Quantitative information on the charge and stress profile during partial discharges (PDs) is required in order to discuss and clarify the process of electrical degradation in solids. Such PDs cause cavities to grow. Therefore, we have proposed a procedure for analysing discharges within cavities, based on motion observation in viscoelastic material. The proposed procedure was applied to surface discharges in silicone gel. Spherical sub-cavities (voids) which form along the cavities were analysed. The temporal response of calculated stress exerted on the void surface depends on the location of the void: whether it is at the tip of the cavity or at the root of the cavity. The results indicated that streamers propagate through the tree structure of the cavity and accumulate electrical charges at the void surface at the tip of the cavity. The temporal response of surface charges had a strong asymmetry according to the polarity of the applied field in contrast to the symmetry of the apparent charge. The amount of charge accumulated in a void was calculated to be approximately 0.1–1%. For voids near the electrode an increase in surface conductivity was observed, whereas voids at the tip of the cavity were non-conductive.

Degradation process of silicone-gel by internal surface discharges

Silicone gel is widely used to encapsulate power electronic circuits. It is well known that surface discharge in the module is one of the weakest points in insulation. These discharges cause the growth of cavity in silicone gel. We have observed the cavity dynamics in order to investigate the degradation process. The results indicated that streamers propagate through the gaseous phase in filamentary channels from the electrode and reaches the tip of the cavity, and leaves charges until the next discharge occurs. Moreover it was revealed that the propagation of the cavity was caused by the positive discharges. The self-healing ability of gel determined whether the propagation succeeds or not.

On the nature of surface discharges in silicone-gel: Prebreakdown discharges in cavities

Silicone gel is used to encapsulate power electronic circuits. The weakness of this insulation system is surface discharges which degrade silicone gel. In this paper, various features of surface discharge in gel are evaluated by analyzing the motion of cavities induced by discharges and by measuring dynamic potential distributions of surface discharges in gel. The results indicated that charges accumulated at the cavity surface contribute to retaining the cavity path. Temporal response of the potential at the cavity surface is explained using a classical capacitance model.

Void-free encapsulation technique for semiconductor devices using silicone gel

Silicone gel is widely used to encapsulate power electronic circuits. Voids in silicone gel have been pointed out as one of the weakest points in insulation where partial discharges (PDs) may easily occur. Degassing process is generally utilized to eliminate voids, but it depends largely on empirical know-how and it is not foolproof. In this paper, an effective encapsulation technique is discussed quantitatively. The theoretical analysis is also experimentally verified. The measured results were in good agreement with the theory. The proposed encapsulation technique reduced the amplitude and number of PDs.