The purpose of this work is to develop a 100 V to 3.3 V, 50 W DC to DC converter which achieves the best compromise between low mass and high efficiency. A particular emphasis is put on using components qualified for the aerospace environment. The work was carried out through the following steps: (1) Explore all possible topologies which lend themselves to efficiently convert a raw 100 V input bus to a tightly regulated 3.3 V output, galvanically isolated from the input. (2) Among all these topologies, select the one which leads to the highest efficiency and the lowest mass. (3) Optimize the chosen topology. (4) Develop a small signal moded of the selected topology.  In order to validate the calculations and the mathematical models developed, a prototype of a 100 V to 3.3 V, 50 W converter was built with Space qualified components and tested. With this prototype, conversion efficiencies in excess of 87% (including control circuit consumption) were measured.