Switching power supply for enabling switching element to be soft-started in burst mode, image forming apparatus and soft start method of switching element

An image forming apparatus, comprising:

• a transformer comprising a primary winding, a secondary winding, and an auxiliary winding;
• a switching element configured to enable the secondary winding and the auxiliary winding to generate a voltage through switching operations for turning on and turning off an input voltage to the primary winding;
• a power supply control circuit to which an output voltage of the auxiliary winding is input configured to output a first gate voltage of a first pulse width consecutively for plural times at the time the switching element is driven in a burst mode in which the switching element is intermittently driven;
• a drive time control circuit configured, based on the first gate voltage input consecutively for plural times at the time the switching element is driven, to output a second gate voltage to the switching element consecutively for plural times and gradually increase a pulse width of the second gate voltage to the first pulse width from a second pulse width narrower than the first pulse width;
• a switching circuit to which an output voltage of the secondary winding is input as a feedback voltage configured to switch an input destination of the first gate voltage output by the power supply control circuit from the switching element to the drive time control circuit based on the feedback voltage in the burst mode;
• an image forming unit to which a voltage is input through a line branching from a line between the primary winding and an alternating current power supply for supplying electric power to the primary winding configured to comprise a motor;
• and
• a processor to which the output voltage of the secondary winding is input configured to control the image forming unit.

A soft start method of a switching element executed by a switching power supply including a transformer equipped with a primary winding, a secondary winding and an auxiliary winding and the switching element in a burst mode in which the switching element is intermittently driven, comprising:

• outputting a first gate voltage of a first pulse width consecutively for plural times at the time the switching element is driven in the burst mode;
• outputting a second gate voltage to the switching element consecutively for plural times based on the first gate voltage input consecutively for plural times at the time the switching element is driven and gradually increasing a pulse width of the second gate voltage to the first pulse width from a second pulse width narrower than the first pulse width;
• and
• switching an input voltage to the switching element from the first gate voltage to the second gate voltage based on an output voltage of the secondary winding in the burst mode.