1. Field of the Invention
The present invention relates to a power supply device for a backlight and to a liquid crystal device using the same, and more particularly, to a power supply device comprising an oscillation control unit that controls the output of a direct current source input, a power driving unit that converts DC power output from the oscillation control unit into AC power, a power transforming unit that transforms the converted AC power, a sensing unit connected in series to one end of a lamp to sense a change of a power applied to the lamp, and a detection control unit that detects a difference between voltages of both ends of the sensing unit and provides a detected signal to the oscillation control unit.
2. Description of Related Art
Since a liquid crystal panel itself in a liquid crystal display device cannot emit light, it cannot be used in a place where there is no light. Therefore, a plurality of fluorescent lamps are used as a backlight for uniformly transferring light from the rear side of the liquid crystal panel to the entire liquid crystal panel. In addition, a high voltage of several hundred volts or more is required for lighting the fluorescent lamps, and an inverter is used as a power supply device for lighting.
In order to drive the fluorescent lamps with constant luminance, there is a need for control of an electric current so that a predetermined constant current flows to the fluorescent lamps. To this end, feedback control is performed for detecting a current flowing through the fluorescent lamps, causing the current to be fedback, and comparing the fedback current with the predetermined current. As shown in FIG. 1, in a conventional backlight inverter 30, a resistor is generally connected in series to a cold electrode 11 of each of fluorescent lamps 10 and one end of the resistor is connected to a common ground A so as to sense a current flowing through each of the fluorescent lamps. Therefore, one end or the cold electrode 11 of each of the fluorescent lamp 10 is connected to the common ground A through the resistor with a resistance value of about several hundred Ω (i.e. a voltage of almost zero is applied), and the other end or the hot electrode 12 of each of the fluorescent lamps 10 is connected to the backlight inverter 30 which supplies AC power having a high voltage (i.e. when an output voltage of the backlight inverter 30 is Vo, a voltage of almost Vo is applied to the other end of the fluorescent lamp 10) (hereinafter, referred to as “grounding method”).
In each of the fluorescent lamps 10 of which the one end 11 is connected to the common ground A through the resistor with a resistance of about several hundred Ω as described above, since a fluorescent lamp reflector 20 and the one end or electrode 11 (generally referred to as “cold electrode”) of the fluorescent lamp 10 are grounded to the common ground A as shown in FIG. 1, a stray capacitance B exists between the fluorescent lamp 10 and the grounded fluorescent lamp reflector 20. As a result, a leakage current path is formed through the stray capacitance B and the common ground A so that asymmetry is caused between the amount of current flowing toward the cold electrode connected to the common electrode A and the amount of current flowing toward the hot electrode which is opposite to the cold electrode. Accordingly, there is a problem in that luminance unbalance of the fluorescent lamp occurs since the side of the electrode 11 grounded to the common ground A is a little darker than the side of the electrode 12 which is opposite to the electrode 11.
This problem becomes more severe as the area of the liquid crystal panel becomes greater. Further, since the life of the side of the fluorescent lamp through which more current flows is rapidly shortened, there is a problem in that the life of the entire fluorescent lamp is shortened.