The design of DC-DC converters mainly relies on the experience of the designer who is usually aided by simulation programs such as SPICE. Since it is very difficult to design highly efficient and reliable converters, even for the experienced designer, we developed a design automation tool for isolated DC-DC converters based on a rule-based expert system. We define a set of rules specifying the decisions that need to be made for selecting the design topology and circuit elements. The rule-based system was implemented and verified in JESS.