1. Department of Electrical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, South Korea
2. Korea Institute of Energy Research (KIER), Daejeon, South Korea
3. Centre d'Innovacio Tecnologica en Conver- tidors Estatics i Accionaments, Departament d'Enginyeria Electrica, Univer- sitat Politecnica de Catalunya, Barcelona, Spain