In this paper, a series input parallel output (SIPO) interleaved flyback converter for 75W AC-DC adapter is designed by three methods. To improve the efficiency of the SIPO interleaved flyback converter, discontinuous conduction mode (DCM) with valley switching technique is applied to reduce the switching loss at high line, and continuous conduction mode (CCM) is applied to reduce the conduction loss at low line. Since the input voltage is divided by the series input structure, the main switch can use the low voltage-rated switch. In addition, the small size of output-filter can be used by the interleaving operation. Therefore, the SIPO interleaved flyback converter with DCM at high line and CCM at low line, achieves high efficiency and high power density for the low-cost 75W (15V-5A) AC-DC laptop adapter application.