An insulation bobbin includes a winding portion, an outer stopper portion and an inner stopper portion. The winding portion includes a body and two sidewalls respectively and perpendicularly connected to two first sides of the body. A length of a first side edge of each of the sidewalls is greater than a length of a second side edge, and a connecting edge of each of the sidewalls connects a first bottom edge and the second side edge. The outer stopper portion connects one of the second sides of the body and the first side edge of each of the sidewalls. The inner stopper portion connects the other one of the second sides of the body and the second side edge of each of the sidewalls. There is a height difference between the first bottom edge of each of the sidewalls and a second bottom edge of the inner stopper portion.