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High Temperature Anti Short Circuit Function for Normally-on SiC JFET in an Inverter Leg Configuration
Keywords: SiC JFET driver, anti short circuit, safety function
The SiC JFET is commercially available as a normally-on device. In an inverter leg configuration, its high temperature performances are balanced by the induced short circuit in case of driver failure. JFET drivers are not operational at the inverter start-up and can experience failure in many case in aeronautic applications. Typically SiC JFET withstands short circuit energy in the range of 2J to 8J. The short-circuit duration is limited to 1ms before ageing stress is to be considered. A safety circuit is then mandatory to provide as soon as possible an off-state condition to the power switches in the inverter leg, at least the low-side device. The safety function has not been demonstrated yet in literature and even less in harsh environment. The anti-short circuit takes its energy from the 28V DC bus in aeronautic application. Energy can be drawn from the inverter 540VDC bus at the expense of an additional SiC JFET. The anti-short circuit provides a negative voltage to the JFET gate in the shortest possible period of time when driver failure is detected. Most drivers provide a reactive under voltage lock-out signal for that purpose. A discrete demonstrator is demonstrated at 200°C and offers 200µs response time. The discrete demonstrator has been validated against repetitive short-circuit stress test. A CMOS SOI integrated circuit is also demonstrated up to 200°C. Primary results show a response time between 5µs when the low-side JFET current is significantly reduced and 20µs for the JFET off-state. Feasibility is validated but repetitive short-circuit stress tests are still under progress. Failure mechanisms inside the proposed anti-short circuit in SOI CMOS are the ones of the semiconductor technology. They are not addressed in the paper. Details of the circuit, testboard and results will be given.
Khalil El-Falahi, Ph.D. Student
Universite de Lyon, Ampere, INSA Lyon

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