High-to-low voltage, low-power isolated DC/DC converters with strong requirements on size, cost and efficiency are required for some specific applications. The existing solutions with the same power/input-voltage ranges are usually based on the Flyback architecture and yield low performances. To overcome these limitations, this work summarizes an approach to design such converter, going through several aspects of the converter design such as the topology, the transformer and an integrated circuit. The experimental setup allows to verify the converter functionality. The peak efficiency measured for a 4W-prototype is 84% in the case of an 400V-12V converter and 82% for an 800V-12V one.