Switching power-supply device having control circuit controlling switching element to turn on-and-off

A switching power-supply device, comprising:

• a first terminal configured to connect to one end of a DC power source;
• a first winding connected to the first terminal;
• a second winding, which is connected in series to the first winding and is magnetically coupled to the first winding;
• a first capacitor connected in series to the second winding;
• a transformer including a primary winding connected in series to the first capacitor and a secondary winding magnetically coupled to the primary winding;
• a rectifying-and-smoothing circuit connected to the secondary winding for rectifying-and-smoothing a voltage generated in the secondary winding and outputting the voltage to an output terminal;
• a second terminal, which is configured to connect to another end of the DC power source and is connected to an opposite end of the primary winding opposite to a connection end connected to the first capacitor;
• a first switching element, which is connected between a connection point of the first winding and the second winding and the second terminal;
• and
• a control circuit, which receives the voltage from the output terminal and controls the first switching element to turn on-and-off,
• wherein while the first switching element is being turned off, the first capacitor is charged by a voltage of the DC power source, and while the first switching element is being turned on, a voltage of the first capacitor and a voltage caused in the second winding are applied to the primary winding.