To provide a switching power supply unit enabling to pick up output power more largely than PWM control of a fixed frequency in a normal load without increasing an L value of a transformer and a coil while preventing a decrease in switching loss and sudden increase in output voltage when a load is light.
A control circuit includes: a feedback circuit 3 for amplifying a feedback signal FB and generating a control signal VEAO; an oscillation frequency adjusting circuit 7 for comparing a reference signal VLIMIT generated from a power supply voltage VDD with the control signal VEAO to determine a frequency; an oscillator 6 operating at the determined oscillation frequency; a drain current detecting circuit 4 for detecting a drain current applied to a switching element 1 and generating a voltage signal VCL in response to the current value of the drain current; and a drain current control circuit 5 for determining the turnoff of the switching element 1 when the voltage signal VCL of the drain current detecting circuit 4 reaches any lower one of the control signal VEAO and the reference voltage VLIMIT.
MURATA ICHIHIRO
MORI YOSHIHIRO
YAMASHITA TETSUJI
FUKUSHIMA TETSUYUKI
Yoshihiro Morimoto
Toshiji Sasahara
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