Abstract:
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.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     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.  
         [0003]     2. General Background  
         [0004]     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.  
         [0005]     Referring to  FIG. 2 , a typical driving circuit for LCD backlight lamps includes a PWM (Pulse Width Modulation) controller  10 , a first transistor Q 1 , a second transistor Q 2 , 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 .  
         [0006]     The PWM controller  10  sends a first PWM control signal to the first transistor Q 1 , and a second PWM control signal to the second transistor Q 2 . The first transistor Q 1  and the second transistor Q 2  are turned on alternately. Then the first transistor Q 1  and the second transistor Q 2  generate two low-voltage pulse signals. The two low-voltage pulse signals are sent to primary windings of the center-tapped transformer Q 2 . 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 R 8  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 .  
         [0007]     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 R 8  is high. The voltage of the resistor R 8  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.  
         [0008]     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.  
         [0009]     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  
       [0010]     A 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.  
         [0011]     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.  
         [0012]     Other advantages and novel features will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a circuit diagram of a driving circuit for LCD backlight lamps of a preferred embodiment of the present invention; and  
         [0014]      FIG. 2  is a circuit diagram of a typical driving circuit for LCD backlight lamps. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0015]     As shown in  FIG. 1 , in a preferred embodiment of the present invention, a driving circuit for LCD backlight lamps includes a PWM controller  100 , an inverter  110 , two transistors  12 ,  14 , a first illuminator like a first lamp  120 , a second illuminator like a second lamp  130 , a first overvoltage protection module  140 , a second overvoltage protection module  150 , a voltage feedback module  160 , a double-bridge circuit  170 , and a current feedback module  180 .  
         [0016]     The PWM controller  100  includes two voltage sources V 6 , V 7 . The voltage sources V 6 , V 7  send PWM control signals to the two transistors  12 ,  14  respectively. The two transistors  12 ,  14  are MOSFETs (metal-oxide-semiconductor field-effect transistors). Gates of the two transistors  12 ,  14  are connected to anodes of the two voltage sources V 6 , V 7  respectively. Sources of the two transistors  12 ,  14  are connected to cathodes of the two voltage sources V 6 , V 7  respectively. The two transistors are turned on alternately. The inverter  110  includes a transformer TX 1 . The transformer TX 1  is a center-tapped transformer. Drains of the two transistors  12 ,  14  are coupled to two ends of primary windings of the transformer TX 1  respectively. A center tap of the primary windings is coupled to a voltage source V 5 . The voltage source V 5  provides a 12V voltage. Anodes of the first lamp  120  and the second lamp  130  are connected to two ends of secondary windings of the transformer TX 1  respectively. A cathode of the first lamp  120  is grounded via a diode D 24 , a resistor R 6 , a diode D 31 , a resistor R 9 , a diode D 32 , and a resistor R 14 . A cathode of the second lamp  130  is grounded via a diode D 27 , a resistor R 12 , and the resistor R 14 .  
         [0017]     The first overvoltage protection module  140  is connected to the anode of the first lamp  120  for detecting a voltage of the first lamp  120 . The first overvoltage protection module  140  includes a MOSFET Q 3 . A gate of the MOSFET Q 3  is coupled to a cathode of the diode D 24 . A source of the MOSFET Q 3  is coupled to the anode of the first lamp  120 . A drain of the MOSFET Q 3  is grounded. The second overvoltage protection module  150  is connected to the anode of the second lamp  130  for detecting a voltage of the second lamp  130 . The second overvoltage protection module  150  includes a MOSFET Q 4 . A gate of the MOSFET Q 4  is coupled to a cathode of the diode D 27 . A source of the MOSFET Q 4  is coupled to the anode of the second lamp  130 . A drain of the MOSFET Q 4  is grounded.  
         [0018]     The voltage feedback module  160  includes an amplifier U 1 . A cathode of the diode D 21  is connected to a non-inverting input terminal of the amplifier U 1 . An anode of the diode D 21  is coupled to the anode of the first lamp  120 . A cathode of the diode D 22  is connected to the non-inverting input terminal of the amplifier U 1 . An anode of the diode D 22  is coupled to the anode of the second lamp  130 . An inverting terminal of the amplifier U 1  receives a voltage reference V 1 .  
         [0019]     The double-bridge circuit  170  is coupled to the cathode of the first lamp  120  via the diode D 24  and the resistor R 6 , and grounded via the resistor R 14 . The double-bridge circuit  170  includes a first branch circuit  172 , and a second branch circuit  174 . The first branch circuit  172  includes the diode D 31 , the resistor R 9 , and the diode D 32 . A cathode of the diode  31  is connected to the resistor R 6  and the resistor R 8 , and an anode of the diode  31  is connected to the resistor R 9 . A cathode of the diode  32  is connected to the resistor R 9 , and an anode of the diode  32  is connected to the resistor R 12  and the resistor R 14 . The second branch circuit  174  includes a diode D 30 , a resistor R 13 , and a diode D 33 . An anode of the diode  30  is connected to the resistor R 6  and a resistor R 8 , and a cathode of the diode  30  is connected to the resistor R 13 . An anode of the diode  33  is connected to the resistor R 13 , and a cathode of the diode  33  is connected to the resistor R 12  and the resistor R 14 .  
         [0020]     A node N between the diode D 30  and the resistor R 9  is connected to the current feedback module  180 . The current feedback module  180  includes an amplifier U 2 . A non-inverting terminal of the amplifier U 2  is connected to the node N. An inverting terminal of the amplifier U 2  receives a voltage reference V 2 . The voltage reference V 2  is 0.55V.  
         [0021]     The PWM controller  100  controls the two transistors  12 ,  14  in generating two low-voltage pulse signals. The transformer TX 1  converts the two low-voltage pulse signals to a high-voltage AC power. The high-voltage AC power is sent to the first lamp  120  and the second lamp  130 . The first overvoltage protection module  140  provides overvoltage protection to the first lamp  120 . The second overvoltage protection module  150  provides overvoltage protection to the second lamp  130 . The voltage feedback module  160  receives a voltage of the first lamp  120  and a voltage of the second lamp  130 . And then the voltage feedback module  160  controls the PWM controller to be turned on or turned off. The current feedback module  180  receives a voltage from the node N, and then controls the PWM controller  100  to be turned on or turned off.  
         [0022]     When both the first lamp  120  and the second lamp  130  work normally, the double-bridge circuit  170  is not operating.  
         [0023]     When either the first lamp  120  or the second lamp  130  ceases to function normally, such as having a defective connection or an open-circuit, the double-bridge circuit  170  provides defective connection protection or open-circuit protection. Should the second lamp  130  have 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 lamp  120  is great. The current of the first lamp  12 O flows through the diode D 24 , the resistor R 6 , and then is divided into two parts. One part of the current of the first lamp  120  flows through the resistor R 8  and then to ground. The remaining part of the current of the first lamp  120  flows through the first branch circuit  172 , the resistor R 14 , and then to ground. When the voltage of the node N is lower than 5.5V, the current feedback module  180  outputs a low voltage. As a result, the PWM controller  100  is turned off and the first lamp  120  is turned off. When the voltage of the node N is higher than 5.5V, the current feedback module  180  outputs a high voltage. As a result, the PWM controller  100  continues functioning normally and the first lamp  120  is still lit. Similarly, when the first lamp  120  has a defective connection, and the second lamp  130  is functioning normally, the second lamp  130  will be protected. And if either the first lamp  120  or the second lamp  130  has an open-circuit, the voltage of the node N is zero and the PWM controller  100  is turned off, and then the second lamp  130  or the first lamp  120  is turned off.  
         [0024]     When the current of the first lamp  120  is greater than the current of the second lamp  130 , and a voltage of the diode D 32  is greater than a turn-on voltage thereof, the diode D 32  is turned on. Part of the current of the first lamp  120  flows through the first branch circuit  172 , the resistor R 14 , and the ground. Then the current of the first lamp  120  goes down, and the current of the second lamp  130  increases. Contrarily, when the current of the second lamp  130  is greater than the current of the first lamp  120 , part of the current of the second lamp  130  flows through the second branch circuit  174 , the resistor R 8 , and then to ground. Then the current of the second lamp  130  goes down, and the current of the first lamp  120  increases. Therefore the double-bridge circuit  170  regulates the current, maintaining balance between the current of the first lamp  120  and the current of the second lamp  130 .  
         [0025]     In the illustrated embodiment, because the double-bridge circuit  170  provides two branches between the first lamp  120  and the second lamp  130 , the current of the first lamp  120  or the second lamp  130  is divided into two parts. When one of the first lamp  120  and the second lamp  130  has 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 module  180  outputs a low voltage, and the PWM controller is turned off. Correspondingly, the one of the first lamp  120  and the second lamp  130  having the defective connection or the open-circuit is turned off. Similarly, when the current of the first lamp  120  is not equal to the current of the second lamp  130 , the branches of the double-bridge circuit  170  regulate current flow to equalize the current of the first lamp  120  and the current of the second lamp  130 .  
         [0026]     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.