PURPOSE: To minimize the whole copper loss of a transformer by a method wherein the raio of the primary side copper loss against the secondary side copper loss is decided by a specific formula in accordance with a rated value of the duty ratio of an ON.OFF of the primary side switch.

CONSTITUTION: A ratio of the primary side copper loss PP of a transformer TR against the secondary side copper loss PS is decided by a formula of PP/PS=[DH/ (1-DH)]1/2 in accordance with a rated value D of the duty ratio DH of an ON.OFF of the primary side switch S. And further, it is made that the raio of the sectional area SP of the primary side coil NP against the sectional area SS of the secondary side coil NS can satisfy the relation of a formula of SP/SS=[DH/(1-DH)]1/2. With this, the full copper loss of a transformer TR of a D/C converter using a circuit as shown in a figure can be minimized.