PURPOSE: To detect excess current in response to main current only, by dividing a switching transformer into a main transformer and a multi-transformer.

CONSTITUTION: A multi-output DC-DC converter is composed of a switching circuit 1, a transformer, commutation smoothing circuits 2, 5∼6, a pulse width control circuit 3, an excess current detection circuit 4, and voltage stabilization circuits 7∼8. In this case, the transformer is formed by division into a main transformer T1 and a multi-transformer T2. Primary current I1 flowing through the switching circuit 1 is shunted into main current I11 and multi-current I12, and the excess current detection circuit 4 responds to the secondary current of the main current I11. Then, the output of excess currrent detected by the excess current detection circuit 4 is directed to a pulse width control circuit, and in response to the output, a switching transistor in the switching circuit 1 is put in an interruption state.