Closed-Loop Current Control of a SiC-Based Power Converter via Galvanically Isolated Electroluminescence Sensing

Electromagnetic interference poses enormous challenges for feedback-controlled systems, especially at medium-voltage levels. As power converters are increasingly utilized for medium- and high-voltage applications, optically-based measurement methods must be explored. This paper investigates current measurement via electroluminescence from silicon carbide semiconductor devices. A SiC half-bridge MOSFET module, provided by Powerex, is manufactured with fiber optic cables placed against the semiconductor junctions. The new fiber ports are employed to characterize the light spectrum as a function of conducted current and junction temperature for the body diodes. Most of the light energy is concentrated around peaks at 390 nm and 500 nm. It is observed that although the energy at the 390 nm peak increases with rising temperature, the energy at the 500 nm peak decreases. Therefore, the total light output is seen to only slightly vary with temperature and mostly depends on conducted current. A function is fitted to the light transducer output as a in relation to the on-state current. This function is utilized in a microprocessor that implements feedback current control in a buck converter. This type of control forms the basis of torque regulation in a motor drive or an “inner-current loop” of motor drive speed control or converter voltage control. The new electroluminescence control was demonstrated in the laboratory on a prototype system where the current command is stepped from 0 to 25A, showcasing the effectiveness of the new optical sensing method.