1. College of Electronic Information and Optical Engineering, Nankai University, Tianjin, China
2. School of Electrical Engineering, Tiangong University, Tianjin, China
3. School of Materials Science and Engineering, Tianjin University, Tianjin, China
4. The Bradley Department of Electrical and Computer Engineering, and the Department of Materials Science and Engineering, Center for Power Electronics Systems (CPES), Virginia Tech, Blacksburg, VA, USA