To provide a switching power supply unit which performs soft start for reducing overshoot of rush current and output voltage when power supply is started and can prevent deterioration and damage of a main switching element due to flowing of current of not less than a maximum allowable current value to the main switching element in a soft start period.
The switching power supply unit includes a blanking period generation circuit 23 inhibiting turn-on of the main switching element 2 until blanking time elapses after the main switching element 2 is turned on. The unit is also provided with a soft start period generation circuit 25 deciding the soft start period until soft start time elapses after the main switching element 2 starts oscillation and a blanking time adjusting circuit 26 for generating a signal for shortening blanking time compared to time after lapse of the soft start period during the soft start period.
Yoshihiro Morimoto
Toshiji Sasahara
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