Switching power supply unit having delay circuit for reducing switching frequency

A switching power supply unit comprising a DC power supply, a transformer having a primary winding, a secondary winding and a feedback winding, a main switching element connected in series to the primary winding, and a control circuit connected between the feedback winding and the control terminal of the main switching element so that a DC output can be obtained, the main switching element having an ON time, an OFF time and a switching frequency;

• wherein there are provided a voltage generating circuit coupled to the transformer to output a voltage in accordance with a load power and a delay circuit for reducing the switching frequency by delaying turning on of the main switching element according to a voltage output from the voltage generating circuit thereby prolonging the OFF time, and further wherein an auxiliary feedback winding connected between the control circuit and the control terminal of the main switching element is disposed in the transformer, and the voltage generating circuit comprises a rectifying and smoothing circuit connected to the auxiliary feedback winding.

A switching power supply unit comprising a DC power supply, a transformer having a primary winding, a secondary winding and a feedback winding, a main switching element connected in series to the primary winding, and a control circuit connected between the feedback winding and the control terminal of the main switching element so that a DC output can be obtained, the main switching element having an ON time, an OFF time and a switching frequency;

• wherein there are provided a voltage generating circuit coupled to the transformer to output a voltage in accordance with a load power and a delay circuit for reducing the switching frequency by delaying turning on of the main switching element according to a voltage output from the voltage generating circuit thereby prolonging the OFF time, said delay circuit including a switching device coupled between the transformer and the control terminal of the main switching element to couple a control voltage through said control circuit from the feedback winding to the control terminal of the main switching element.