IEEE trans. transp. electr. IEEE trans. transp. electr. Printed circuit board coils of multi-track litz structure for 3.3 kW inductive power transfer system Narvaez Alexis Universidad de Zaragoza Carretero Claudio Universidad de Zaragoza Lope Ignacio Universidad de Zaragoza Acero Jesús Universidad de Zaragoza 2023 research-article