1 Tianjin Key Laboratory of Advanced Electrical Engineering and Energy Technology, School of Electrical Engineering & Automation, Tianjin Polytechnic University, Tianjin 300387, China;
2 School of Electronics and Information Engineering, Tianjin Polytechnic University, Tianjin 300387, China
3 Engineering Research Center of Ministry of Education on High Power Solid State Lighting Application System, Tianjin 300387, China