The invention relates to the circuit of a nonsymmetrical half-bride forward converter, more suitable to be applied in a low-profile system and the circuit can not only achieve reducing switching noise and losses of semiconductor switch and simultaneously raise the efficiency of the whole converter by synchronous rectification control technique and improve the circuit design for the known technique, but also is easier to achieve zero-voltage switching by external resonance inductance; besides, the external saturated inductance can effectively reduce the EMI.