Power-supply device

A power-supply device including a step-down DC-DC converter, comprising:

• power semiconductor switching components,
• driving means for driving said power semiconductor switching components,
• a pulse-width modulation oscillator to supply said driving means with a driving signal,
• an error amplifier to supply said pulse-width modulation oscillator with an error signal indicating a comparison result between a reference value and an output power,
• wherein a control loop of said power-supply device includes a filter provided independently of a power output filter through which said output power passes, an output signal corresponding to said output power being fed back to said error amplifier after having passed through said independently provided filter, and
• wherein said power output filter is an LC filter comprised of an inductor and a capacitor, said independently provided filter being a CR filter comprised of a capacitor and a resistor, said CR filter being connected to both ends of said inductor of said power output filter, and said output signal being fed back to said error amplifier after having passed through said CR filter, and
• a transient variation detecting circuit, said transient variation detecting circuit:
• detecting said output power from an output terminal of said CR filter newly provided at both ends of said inductor of said power output filter, and, if said output power has been found to exceed a predetermined upper-limit value,
• outputting a signal for setting the duty α of said pulse-width modulation oscillator at 0%, and, if said output power has been found to be lower than a predetermined lower-limit value, outputting a signal for setting said duty α of said pulse-width modulation oscillator at 100%.