1. Center for Power Electronics Systems, The Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
2. Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, California 90086, USA
3. The Electrical Engineering Department, San Jose State University, San Jose, California 95192, USA
4. Novel Crystal Technology, Inc., Sayama 350-1328, Japan