Transformers with low-losses are required in isolated high-to-low voltage, low-power applications, with strong constraints regarding the solution’s size and cost. A design approach based on a Design-of-Experiments that allows to study the influence of the main key design parameters of such a passive device. It leads to select few solutions whose performances are measured in a 400 V-to-12 V, 1 W prototype. The best transformer design allows reaching a 85% peak efficiency for a core volume of 2100 mm3 at an operating frequency of 275 kHz.