| 摘要 |
PROBLEM TO BE SOLVED: To resolve the difficulty that the efficiencies of any switching power supply unit are improved equally in both the cases of it being loaded normally and lightly. SOLUTION: In a switching power supply unit, the series circuit comprising a main switch 4 and a primary winding 3 of its transformer is connected with a DC power supply 1. The main switch 4 is on/off-controlled by PWM pulses. An auxiliary capacitor 30 charged by the voltage of a tertiary winding 25 of its transformer is provided. A control input voltage Vf of the synthetic signal comprising the sensed signal of its output voltage, the sensed signal of the current of the main switch 4, and the voltage of the auxiliary capacitor 30 is created. Two comparators 59, 62 are provided to compare respectively the control input voltage Vf with first and second reference voltages Vth1, Vth2. The on-term of the main switch 4 is completed by the output of the one comparator 59 of the two comparators 59, 62. When it is loaded normally, the voltage of a capacitor 49 for forming the PWM pulses is discharged forcedly by the output of the other comparator 62. When it is loaded lightly, the capacitor 49 is forced not to discharge the voltage thereof by the output of the other comparator 62.
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