Patent ID: 7075032

Claim:
A power supply apparatus comprising: a DC power supply for converting a first AC voltage from an AC power supply to a main DC voltage; a DC-to-AC converter for converting said main DC voltage to a second AC voltage alternating between positive and negative polarities, said second AC voltage being adapted to be applied to a load via first and second output terminals of said power supply apparatus; a superposition voltage supply for generating a negative voltage to be applied to said load via said first and second output terminals, said negative voltage being generated at a time of transition of said second AC voltage from positive to negative and lasting for a period shorter than a period during which said second AC voltage is negative, said negative voltage having a negative peak value larger than a negative peak value of said second AC voltage and changing rapidly from said negative peak value; said superposition voltage supply comprising an auxiliary voltage supply generating a DC voltage having a negative peak value larger than said negative peak value of said second AC voltage, switching means rendered conductive for said period shorter than said period during which said second AC voltage is negative, and a current-limiting resistor connected in series with said switching means; said superposition voltage supply causing a current to flow from said auxiliary voltage supply through said switching means, said current-limiting resistor and said first and second output terminals when said switching means is conductive; and a differentiating circuit connected in parallel with said current-limiting resistor, said differentiating circuit including a resistor having a smaller resistance value than said current-limiting resistor.