1. Department of Electrical and Electronic EngineeringRitsumeikan University 1‐1‐1 Nojihigashi Kusatsu 525‐8577 Japan
2. Research Organization of Science and TechnologyRitsumeikan University 1‐1‐1 Nojihigashi Kusatsu 525‐8577 Japan
3. Atsugi Research CenterSolar Frontier K. K. Atsugi 243‐0206 Japan