Development of power supplies for electric vehicle (EV) applications has become increasingly important as the market shifts to greener, emission-free technologies. Common bus systems are 400 VDC and 800 VDC for EV systems that require auxiliary and emergency power supplies. To support the EV power system, these power supplies may need to work with reduced power at 40 VDC when integrity of the main voltage rail is compromised. The goal of designing a power supply with an ultra-wide input voltage range (40 VDC – 1000 VDC) has therefore been a key activity for system level engineers. This paper presents the design of isolated flyback converters that can operate across an ultra-wide range input. To work with wide input voltage, the flyback converter must be able to operate in CCM and DCM modes and incorporate secondary side regulation to achieve better regulation and dynamic response.