1. Graduate School of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060‐8628, Japan
2. Advanced Power Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1‐8-31 Midorigaoka, Ikeda, Osaka 563‐8577, Japan
3. Sensing System Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 807‐1 Shuku-machi, Tosu, Saga 841‐0052, Japan
4. Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060‐8628, Japan