Power supply having a chopper with an improved power factor

A power supply comprising:

• a fullwave rectifier for coupling to an AC power source to provide a rectified pulsating DC voltage from said AC power source;
• a chopper including a switching element which is connected in series with an inductor across said rectifier, said switching element driven to turn on and off for providing a periodically interrupted voltage which is applied through a blocking diode across a smoothing capacitor to produce a smoothed DC voltage for driving a load, said inductor storing energy from said rectifier in response to said switching element being turned on and releasing said energy in response to said switching element being turned off by flowing a current through said inductor in a positive direction, said inductor cooperating with parasitic capacitance inherently present in said chopper to form an oscillation circuit which admits said current to flow through said inductor in a negative direction opposite to said positive direction;
• current sensing means which senses said current flowing through said inductor and issues a start signal when said current decreases to a certain level;
• a controller which responds to said start signal for turning on said switching element at a timing when said current flows in said negative direction and which subsequently turns off said switching element after a predetermined ON-period so as to release said current from said inductor, thereby causing said current to flow firstly in the negative direction and then in the positive direction through said inductor during said ON-period;
• and
• input current wave shaping means having a current compensator for controlling, in response to said pulsating DC voltage, at least one of the timing of turning said switching element on and off and the current flowing through said inductor so that an absolute value of a time integral of the current flowing continuously through said inductor in said negative direction during a first period is always smaller than an absolute value of a time integral of the current flowing continuously through said inductor in the positive direction during a second period immediately following said first period.