Control apparatus of switching power supply for generating drive signal of incremental on-width

A control apparatus of a current resonance type switching power supply that has a switching element and generates an output voltage, the control apparatus comprising:

• a drive circuit configured to generate a drive signal to drive the switching element of the switching power supply;
• a soft-start control circuit connected to the drive circuit, the soft-start control circuit being configured to send a signal to the drive circuit to cause the drive circuit to increment an on-width of the drive signal by a prescribed step per prescribed switching number, thereby to gradually increase a switching period of the switching element from a beginning of a soft start period to an end of the soft start period, to gradually decrease a switching frequency of the switching element from the beginning of the soft start period to the end of the soft start period, and to reduce an overshoot of the output voltage when the switching power supply is started;
• and
• an input voltage rank determination circuit that determines a rank based on input voltages of the switching power supply inputted when the switching power supply is started, and outputs a value corresponding to the rank, wherein
• the soft-start control circuit
• implements a first step of holding the on-width of the drive signal for a predetermined time corresponding to the prescribed switching number,
• implements a second step of increasing the on-width of the drive signal by the prescribed step after the predetermined time, and
• repeats the first step and the second step, alternately.