• Buttay, Cyril

Abstract

Silicon carbide (SiC) power devices can operate at much higher temperature than their silicon counterparts. This enabled the design of power electronics converters for environments in which the ambient temperature can reach or exceed 200°C. For example, jet engine controls or brake actuation for aircrafts, or the down-hole converters for geothermal or gas exploration. Another way of taking advantage of this high junction temperature capability of SiC devices is to save on their thermal management system: using a less efficient (but smaller and lighter) cooling system, and let the junction temperature reach elevated levels, even with a moderated ambient temperature. As a seminconducting material, SiC can in theory operate up to 1000°C. Many additional limitations, however, must be taken into account : melting of the contact metalizations, degradation of the secondary passivation layer... Furthermore, for unipolar devices, the thermal run-away phenomenon must be considered: with these devices, the conduction losses increase dramatically with their junction temperature. At some point, they can exceed the cooling capability of the associated thermal management system, and the device becomes unstable. To build a functional converter, many other aspects must be investigated: passive devices, gate drivers, packaging... High temperature has a strong influence on the converter design, as not all technologies are compatible with this requirement: very few capacitors or encapsulating materials can be found to operate above 200°C. For more ambitious goals (more than 250°C), the design alternatives are even fewer. In many cases, the increase in the maximum operating temperature is associated with wider temperature swings (e.g. for aircraft applications, the ambient temperature can go from -55°C to more than 200°C). This increased thermal cycling generates higher mechanical stresses in the packaging (because of the difference in the coefficient of thermal expansion between the various materials of the package). This has a detrimental consequence on reliability.

Topics

• impedance spectroscopy
• theory
• laser emission spectroscopy
• carbide
• thermal expansion
• Silicon