A transformer (10) with high sustain voltage includes a first bobbin (125), a second bobbin (126), a core assembly (127), and an insulating frame (128a). The first bobbin defines a first hollow therein. The second bobbin is coupled to the first bobbin, and defines a second hollow therein. The core assembly includes a first iron core (127a) partly received in the first hollow and a second iron core (127b) partly received in the second hollow. The insulating frame is disposed between the first bobbin and the second bobbin, for insulatively isolating at least one part of the first iron core from the second iron core.