Driving circuit for LCD backlight lamps

An LCD backlight driving circuit is provided for providing protection to LCD backlight lamps. In a preferred embodiment, the driving circuit for LCD backlight lamps comprises: a PWM controller sending control signals; two transistors receiving the control signals and sending two low-voltage pulse signals; an inverter converting the two low-voltage pulse signals to a high-voltage AC (Alternating Current) power; two lamps; two overvoltage protection circuits connected to the two lamps respectively; a voltage feedback module connected to the two lamps, a current feedback module connected to one of the two lamps; and a double-bridge circuit connected between the two lamps, and connected to the current feedback module for controlling the PWM controller to turn-on or turn-off. The driving circuit is capable of providing defective connection protection and open-circuit protection to LCD backlight lamps, and proportionately adjusting current flowing through the lamps.

BACKGROUND

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 asFIG. 2is used.

Referring toFIG. 2, a typical driving circuit for LCD backlight lamps includes a PWM (Pulse Width Modulation) controller10, a first transistor Q1, a second transistor Q2, a center-tapped transformer20, a first lamp30, a second lamp40, a first overvoltage protection module50, a second overvoltage protection module60, a voltage feedback module70, and a current feedback module80.

The PWM controller10sends a first PWM control signal to the first transistor Q1, and a second PWM control signal to the second transistor Q2. The first transistor Q1and the second transistor Q2are turned on alternately. Then the first transistor Q1and the second transistor Q2generate 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 transformer20converts the two low-voltage pulse signals to a high-voltage AC power. The high-voltage AC power is sent to the first lamp30and the second lamp40. A resistor R8is connected between a cathode of the first lamp30and ground. The first overvoltage protection module50provides overvoltage protection to the first lamp30. The second overvoltage protection module60provides overvoltage protection to the second lamp40. The voltage feedback module70receives signals from the first lamp30and the second lamp40, and then controls an output signal of the PWM controller10. The current feedback module80is coupled to the first lamp30, and then controls the output signal of the PWM controller10.

When the first lamp30works normally, and the second lamp40has a defective connection or an open-circuit, the current flowing through the second lamp40is zero in theory. However, because of the electric field effect a small current is still flowing through the second lamp40, and a current flowing through the first lamp30is very large. Because the current flowing through the first lamp30is very large, a voltage of a resistor R8is high. The voltage of the resistor R8is fed back to the voltage feedback module70. The voltage feedback module70outputs a high voltage, and the PWM controller100continues to work normally. So the first lamp30is still lit. As a result, the first lamp30can become disabled prematurely. The driving circuit for the LCD backlight lamps does not protect the first lamp30when the second lamp40gets a defective connection or an open-circuit. Therefore reliability of the lamps is reduced.

When a current of the first lamp30is greater than a current of the second lamp40, or the current of the second lamp40is greater than the current of the first lamp30, the driving circuit for the LCD backlight lamps does not maintain balance between the currents of the first lamp30and the current of the second lamp40. As a result, one of the first lamp30and the second lamp40, 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.

SUMMARY

An LCD backlight driving circuit is provided for providing protection to LCD backlight lamps. In a preferred embodiment, the driving circuit for LCD backlight lamps includes a PWM controller sending control signals; two transistors receiving the control signals and sending two low-voltage pulse signals; an inverter converting the two low-voltage pulse signals to a high-voltage AC (Alternating Current) power; two lamps, wherein each of the lamps receives the high-voltage AC power and then is grounded; two overvoltage protection circuits, wherein the two voltage protection circuits are connected to the two lamps respectively; a voltage feedback module connected to the two lamps, and controlling power to the PWM controller; a current feedback module connected to one of the two lamps, and also controlling the PWM controller to be turned on or turned off and a double-bridge circuit connected between the two lamps, and connected to the current feedback module for controlling the PWM controller to be turned on or turned off as well.

The driving circuit is capable of providing defective connection protection and open-circuit protection to LCD backlight lamps, and proportionately controlling current flowing through the lamps.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As shown inFIG. 1, in a preferred embodiment of the present invention, a driving circuit for LCD backlight lamps includes a PWM controller100, an inverter110, two transistors12,14, a first illuminator like a first lamp120, a second illuminator like a second lamp130, a first overvoltage protection module140, a second overvoltage protection module150, a voltage feedback module160, a double-bridge circuit170, and a current feedback module180.

The PWM controller100includes two voltage sources V6, V7. The voltage sources V6, V7send PWM control signals to the two transistors12,14respectively. The two transistors12,14are MOSFETs (metal-oxide-semiconductor field-effect transistors). Gates of the two transistors12,14are connected to anodes of the two voltage sources V6, V7respectively. Sources of the two transistors12,14are connected to cathodes of the two voltage sources V6, V7respectively. The two transistors are turned on alternately. The inverter110includes a transformer TX1. The transformer TX1is a center-tapped transformer. Drains of the two transistors12,14are coupled to two ends of primary windings of the transformer TX1respectively. A center tap of the primary windings is coupled to a voltage source V5. The voltage source V5provides a 12V voltage. Anodes of the first lamp120and the second lamp130are connected to two ends of secondary windings of the transformer TX1respectively. A cathode of the first lamp120is grounded via a diode D24, a resistor R6, a diode D31, a resistor R9, a diode D32, and a resistor R14. A cathode of the second lamp130is grounded via a diode D27, a resistor R12, and the resistor R14.

The first overvoltage protection module140is connected to the anode of the first lamp120for detecting a voltage of the first lamp120. The first overvoltage protection module140includes a MOSFET Q3. A gate of the MOSFET Q3is coupled to a cathode of the diode D24. A source of the MOSFET Q3is coupled to the anode of the first lamp120. A drain of the MOSFET Q3is grounded. The second overvoltage protection module150is connected to the anode of the second lamp130for detecting a voltage of the second lamp130. The second overvoltage protection module150includes a MOSFET Q4. A gate of the MOSFET Q4is coupled to a cathode of the diode D27. A source of the MOSFET Q4is coupled to the anode of the second lamp130. A drain of the MOSFET Q4is grounded.

The voltage feedback module160includes an amplifier U1. A cathode of the diode D21is connected to a non-inverting input terminal of the amplifier U1. An anode of the diode D21is coupled to the anode of the first lamp120. A cathode of the diode D22is connected to the non-inverting input terminal of the amplifier U1. An anode of the diode D22is coupled to the anode of the second lamp130. An inverting terminal of the amplifier U1receives a voltage reference V1.

The double-bridge circuit170is coupled to the cathode of the first lamp120via the diode D24and the resistor R6, and grounded via the resistor R14. The double-bridge circuit170includes a first branch circuit172, and a second branch circuit174. The first branch circuit172includes the diode D31, the resistor R9, and the diode D32. A cathode of the diode31is connected to the resistor R6and the resistor R8, and an anode of the diode31is connected to the resistor R9. A cathode of the diode32is connected to the resistor R9, and an anode of the diode32is connected to the resistor R12and the resistor R14. The second branch circuit174includes a diode D30, a resistor R13, and a diode D33. An anode of the diode30is connected to the resistor R6and a resistor R8, and a cathode of the diode30is connected to the resistor R13. An anode of the diode33is connected to the resistor R13, and a cathode of the diode33is connected to the resistor R12and the resistor R14.

A node N between the diode D30and the resistor R9is connected to the current feedback module180. The current feedback module180includes an amplifier U2. A non-inverting terminal of the amplifier U2is connected to the node N. An inverting terminal of the amplifier U2receives a voltage reference V2. The voltage reference V2is 0.55V.

The PWM controller100controls the two transistors12,14in generating two low-voltage pulse signals. The transformer TX1converts the two low-voltage pulse signals to a high-voltage AC power. The high-voltage AC power is sent to the first lamp120and the second lamp130. The first overvoltage protection module140provides overvoltage protection to the first lamp120. The second overvoltage protection module150provides overvoltage protection to the second lamp130. The voltage feedback module160receives a voltage of the first lamp120and a voltage of the second lamp130. And then the voltage feedback module160controls the PWM controller to be turned on or turned off. The current feedback module180receives a voltage from the node N, and then controls the PWM controller100to be turned on or turned off.

When both the first lamp120and the second lamp130work normally, the double-bridge circuit170is not operating.

When either the first lamp120or the second lamp130ceases to function normally, such as having a defective connection or an open-circuit, the double-bridge circuit170provides defective connection protection or open-circuit protection. Should the second lamp130have a defective connection, there will still be a small current flowing through it because of the electric field effect. However, a current of the first lamp120is great. The current of the first lamp120flows through the diode D24, the resistor R6, and then is divided into two parts. One part of the current of the first lamp120flows through the resistor R8and then to ground. The remaining part of the current of the first lamp120flows through the first branch circuit172, the resistor R14, and then to ground. When the voltage of the node N is lower than 5.5V, the current feedback module180outputs a low voltage. As a result, the PWM controller100is turned off and the first lamp120is turned off. When the voltage of the node N is higher than 5.5V, the current feedback module180outputs a high voltage. As a result, the PWM controller100continues functioning normally and the first lamp120is still lit. Similarly, when the first lamp120has a defective connection, and the second lamp130is functioning normally, the second lamp130will be protected. And if either the first lamp120or the second lamp130has an open-circuit, the voltage of the node N is zero and the PWM controller100is turned off, and then the second lamp130or the first lamp120is turned off.

When the current of the first lamp120is greater than the current of the second lamp130, and a voltage of the diode D32is greater than a turn-on voltage thereof, the diode D32is turned on. Part of the current of the first lamp120flows through the first branch circuit172, the resistor R14, and the ground. Then the current of the first lamp120goes down, and the current of the second lamp130increases. Contrarily, when the current of the second lamp130is greater than the current of the first lamp120, part of the current of the second lamp130flows through the second branch circuit174, the resistor R8, and then to ground. Then the current of the second lamp130goes down, and the current of the first lamp120increases. Therefore the double-bridge circuit170regulates the current, maintaining balance between the current of the first lamp120and the current of the second lamp130.

In the illustrated embodiment, because the double-bridge circuit170provides two branches between the first lamp120and the second lamp130, the current of the first lamp120or the second lamp130is divided into two parts. When one of the first lamp120and the second lamp130has a defective connection or an open-circuit, the voltage of the node N is lower than that of the typical driving circuit for LCD backlight lamps. Therefore the current feedback module180outputs a low voltage, and the PWM controller is turned off. Correspondingly, the one of the first lamp120and the second lamp130having the defective connection or the open-circuit is turned off. Similarly, when the current of the first lamp120is not equal to the current of the second lamp130, the branches of the double-bridge circuit170regulate current flow to equalize the current of the first lamp120and the current of the second lamp130.