1. Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan and Department of Information and Computer Science, Faculty of Engineering, Saitama University, 255 Shimo-okubo, Saitama, Saitama 338-0825, Japan