Circuit for preventing current flowing backwards

The utility model relates to a circuit for preventing current flowing backwards when synchronous rectifying converters are in synchronous rectification and parallel operation. The circuit comprises a synchronous rectifying driving circuit, a power supply control circuit, a sampling circuit and a voltage comparison circuit. The sampling circuit outputs voltage signals to a first input end of the voltage comparison circuit, the power supply control circuit receives the output signals from the voltage comparison circuit and controls the synchronous rectifying driving circuit, the power supply control circuit is further connected with a control end of a synchronous rectifying tube of the synchronous rectifying driving circuit, the sampling circuit is coupled with a main-side winding or a secondary-side winding or an output inductor of a transformer of a second converter in the converters, and a second input end of the voltage comparison voltage receives output voltage from a first converter. The synchronous rectifying converters does not cause output voltage pit and overshoot when in parallel operation, and are high in efficiency and low in loss.