POWER SUPPLY HAVING IMPROVED POWER FACTOR

摘要

A power supply is provided with an input-side rectifier circuit (2) which full-wave rectifies the AC input, and an output-side converter which has voltage transforming means (11) equipped with a primary-side winding (11a) and a secondary-side winding (11b), and is connected to the paired outputs of the input-side rectifier circuit through an inductor (16) and the load of which is connected to the secondary-side winding through an output-side rectifier circuit. The power supply is also provided with a switching element (18) the ON/OFF of which are controlled by a control circuit (20) and a capacitor (19) which is connected to the input-side rectifier circuit (2) and the primary-side winding (11a) so as to be charged through a charging circuit including the inductor (16) during the OFF period of the element (18) discharged through the primary-side winding (11a) during the ON period. The control circuit (20) is provided for controlling the ON/OFF of the switching element (18) with a period shorter than that of the AC input. In addition, a second switching element (31) is connected to the paired outputs of the input-side rectifier circuit (2) in series with the inductance (16) and the capacitor charging circuit of the output-side converter through a diode (17). Moreover, the power unit is provided with a switch (34) which cuts off the conduction of the switching element (31) when the electric current flowing through the element (31) exceeds a prescribed value. Since the power unit is constituted in such a way, excessive voltage of the input-side capacitor when changed can be prevented and, therefore, the power unit can be used without any problem even when the input voltage fluctuates and the load changes.