Soft-start switching power converting apparatus with pulse-frequency modulation technique

A switching power converting apparatus comprising:

• a voltage conversion module including a transformer,
• wherein said transformer includes a primary winding adapted to receive an input voltage, a first secondary winding and a second secondary winding,
• wherein said voltage conversion module is operable, based on a switching signal having a switching frequency, to generate a secondary side current flowing through said first secondary winding of said transformer so as to output an output voltage associated with the secondary side current, and to generate a sensing voltage across said second secondary winding, the sensing voltage being associated with a winding turns ratio of said first secondary winding to said second secondary winding, and the output voltage;
• a detecting unit coupled to said voltage conversion module for receiving the sensing voltage therefrom, and operable to generate a detecting signal based on the sensing voltage, the switching signal and a predetermined reference voltage;
• and
• a switching signal generating unit coupled to said detecting unit and said voltage conversion module, and receiving the detecting signal from said detecting unit and an adjusting signal, said switching signal generating unit generating the switching signal based at least on the detecting signal and the adjusting signal so that the secondary side current is gradually increased during a start period of said switching power converting apparatus;
• wherein, during the start period, the switching signal has a switching frequency that increases with increase of a voltage difference between the detecting signal and the adjusting signal;
• and a duty cycle that increases with increase of the voltage difference between the detecting signal and the adjusting signal.