Osaka University researchers have found an early-stage method for testing future failures of electric power conversion devices using acoustic emission counts, Science Daily reported.

“In a study recently published in IEEE Transactions on Power Electronics, researchers from Osaka University monitored in real time the propagation of cracks in a silicon carbide Schottsky diode during power cycling tests. The researchers used an analysis technique, known as acoustic emission, which has not been previously reported for this purpose,” Science Daily noted.

Power electronics regulate and modify electric power. They are in computers, power steering systems, solar cells, and many other technologies. Researchers are seeking to enhance power electronics by using silicon carbide semiconductors. However, wear-out failures such as cracks remain problematic. To help researchers improve future device designs, early damage detection in power electronics before complete failure is required.

Figure 1. Acoustic emission (AE) was applied to monitor wear-out failure in discrete SiC Schottky barrier diode (SBD) devices with a Ag sinter die attachment, to successfully monitor the real-time progress of failure of Al ribbons for the first time. (a) Optical image of a SiC-SBD device. (b) Cross-sectional SEM image. (c) Experimental apparatus for power cycling tests and real-time AE monitoring. (d) Waveform of collected AE signal and its characteristic, including counts and amplitude. (e) Generation, propagation, and collection of AE signals (i.e., elastic waves) in power electronics during a power cycling test. Credit: Osaka University

Monitoring the resulting damage to the diode over time when the researchers repeatedly turn the device on and off, they found  progressive damage to aluminum ribbons affixed to the silicon carbide Schottsky diode and that increasing acoustic emission corresponded to progressive damage to the ribbons.

"A transducer converts acoustic emission signals during power cycling tests to an electrical output that can be measured," lead author ChanYang Choe stated in the report. "We observed burst-type waveforms, which are consistent with fatigue cracking in the device."

Figure 2. After eliminating background AE noise—including power on-off switching and ambient noise—via noise-filtering, AE signals were successfully collected for the SiC devices during a power cycling test. (a) Lift-off failure analysis results of failed discrete SiC-SBD devices after a power cycling test. (b) Cross section of one pre-failure Al ribbon where many cracks were observed at the interface. (c) AE single monitoring was compared with the traditional failure monitoring method: using the forward voltage during the power cycling test. The results indicate that AE monitoring can be used to understand fatigue propagation in Al ribbons (i.e., the failure mechanism) and also as an early warning before catastrophic lift-off fracture for power electronic devices. Credit: Osaka University

Researchers found  an abrupt increase in the forward voltage, but only when the device was near complete failure. This traditional method checks damage to a power device by monitoring anomalous increases during power cycling tests. Acoustic emission counts were found to be much more sensitive. This method showed trends in the acoustic emission counts during power cycling tests.

"Unlike forward voltage plots, acoustic emission plots indicate all three stages of crack development," says senior author Chuantong Chen. "We detected crack initiation, crack propagation and device failure, and confirmed our interpretations by microscopic imaging."

No prior sensitive early-warning system for monitoring fatigue cracks that can cause complete failure in the silicon carbide diodes had been available. But, according to the research, acoustic emission monitoring could be the way. Using this method will assist research into why silicon carbide devices fail which will help common and advanced technologies improve future designs.