1. Field of the Invention
The present invention relates to a driving circuit for LCD (Liquid Crystal Display) backlight lamps, and particularly to a driving circuit for providing defective connection protection and open-circuit protection to LCD backlight lamps, and proportionately adjusting current flow through the lamps.
2. General Background
Typically, a transmission type LCD uses a backlight lamp to provide light for illuminating pixels to display data or information. In addition, because a high voltage of about 1000-1500V is required to drive a backlight lamp, low-voltage power supplied from a main supply should be converted. For satisfying this requirement, a driving circuit for backlight lamps such as FIG. 2 is used.
Referring to FIG. 2, a typical driving circuit for LCD backlight lamps includes a PWM (Pulse Width Modulation) controller 10, a first transistor Q1, a second transistor Q2, a center-tapped transformer 20, a first lamp 30, a second lamp 40, a first overvoltage protection module 50, a second overvoltage protection module 60, a voltage feedback module 70, and a current feedback module 80.
The PWM controller 10 sends a first PWM control signal to the first transistor Q1, and a second PWM control signal to the second transistor Q2. The first transistor Q1 and the second transistor Q2 are turned on alternately. Then the first transistor Q1 and the second transistor Q2 generate two low-voltage pulse signals. The two low-voltage pulse signals are sent to primary windings of the center-tapped transformer Q2. The center-tapped transformer 20 converts the two low-voltage pulse signals to a high-voltage AC power. The high-voltage AC power is sent to the first lamp 30 and the second lamp 40. A resistor R8 is connected between a cathode of the first lamp 30 and ground. The first overvoltage protection module 50 provides overvoltage protection to the first lamp 30. The second overvoltage protection module 60 provides overvoltage protection to the second lamp 40. The voltage feedback module 70 receives signals from the first lamp 30 and the second lamp 40, and then controls an output signal of the PWM controller 10. The current feedback module 80 is coupled to the first lamp 30, and then controls the output signal of the PWM controller 10.
When the first lamp 30 works normally, and the second lamp 40 has a defective connection or an open-circuit, the current flowing through the second lamp 40 is zero in theory. However, because of the electric field effect a small current is still flowing through the second lamp 40, and a current flowing through the first lamp 30 is very large. Because the current flowing through the first lamp 30 is very large, a voltage of a resistor R8 is high. The voltage of the resistor R8 is fed back to the voltage feedback module 70. The voltage feedback module 70 outputs a high voltage, and the PWM controller 100 continues to work normally. So the first lamp 30 is still lit. As a result, the first lamp 30 can become disabled prematurely. The driving circuit for the LCD backlight lamps does not protect the first lamp 30 when the second lamp 40 gets a defective connection or an open-circuit. Therefore reliability of the lamps is reduced.
When a current of the first lamp 30 is greater than a current of the second lamp 40, or the current of the second lamp 40 is greater than the current of the first lamp 30, the driving circuit for the LCD backlight lamps does not maintain balance between the currents of the first lamp 30 and the current of the second lamp 40. As a result, one of the first lamp 30 and the second lamp 40, the current of which is larger, can become disabled prematurely.
What is needed, therefore, is a driving circuit for LCD backlight lamps able to provide defective connection protection and open-circuit protection to every lamp, and proportionately adjusting current flow through the lamps.