Abstract:
A lamp driving device for driving a lamp. A feedback circuit includes a voltage falling unit and a rectification circuit. The voltage falling unit provides a low voltage signal in response to a first DC power signal, a first AC power signal or a first driving power signal. The rectification circuit rectifies the low voltage signal to generate a feedback signal. The rectification circuit provides the feedback signal. A controller provides a control signal in response to the feedback signal. A first DC-to-AC converter transforms the first DC power signal to the first AC power signal in response to the control signal. A first voltage raising unit raises the voltage of the first AC power signal to generate the first driving power signal. The first raising unit provides the first driving power signal to a first end of the lamp, so that the lamp achieves the desired brightness stably.

Description:
[0001]     This application claims the benefit of Taiwan application Serial No. 93127941, filed Sep. 15, 2004, the subject matter of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates in general to a lamp driving circuit, and more particularly to a lamp driving circuit applied in a backlight module.  
         [0004]     2. Description of the Related Art  
         [0005]     Refer to  FIG. 1A  and  FIG. 1B .  FIG. 1A  is a diagram of a conventional backlight module lamp driving circuit;  FIG. 1B  is a circuit diagram of a conventional feedback circuit. Liquid crystal display uses a fluorescent lamp  102  in a backlight module lamp driving circuit  100  as the backlight source to provide the light source during display. A conventional backlight module lamp driving circuit comprises a feedback circuit  104 , a DC-to-AC converter  106 , a voltage raising unit  108  and a controller  110 . The feedback circuit  104  provides a feedback signal FSi in response to driving power signal PS required to drive the fluorescent lamp  102 , so that the backlight module lamp driving circuit  100  adjusts the driving power signal PS for the fluorescent lamp  102  to achieve the desired brightness and maintain stability in response to the feedback signal FSi. The conventional feedback circuit  104 , which is an ordinary rectification circuit, comprises diodes D 1  and D 2 , a resistor R and a capacitor C, rectifies and filters the AC driving power signal PS, then provides the feedback signal FSi. When the rectification circuit corresponds to a small-sized liquid crystal display, the position of disposition, as illustrated in  FIG. 1 , can only be coupled to the fluorescent lamp  102  and the grounding end, or between the high voltage side coil of the voltage raising unit and the grounding end. Since the single end of the fluorescent lamp  102  is connected to the ground voltage, the feedback circuit  104  is serially connected to a low-voltage node.  
         [0006]     As the size of liquid crystal display becomes larger and larger, the length of the fluorescent lamp  102  becomes longer and longer, and so too the striking and operating voltage of the fluorescent lamp  102  become higher. When the length of the fluorescent lamp  102  is over 900 mm, the required voltage of the fluorescent lamp  102  will be over 1.5 KV. So the lamp driving circuit  100  in the backlight module of a large-sized liquid crystal display has evolved from original single side drive mode to dual side drive mode, so that the two ends of the fluorescent lamp  102  will not have a low-voltage node. However, if a conventional feedback circuit  104  is used to convert the high voltage driving power signal PS into a feedback signal Fsi, the voltage of the feedback signal Fsi will be too high thus cannot be used by the controller  110  directly. Moreover, the elements of conventional feedback circuit  104  is too weak in terms of voltage resistance, so is unable to receive the high voltage driving power signal PS. Therefore, conventional feedback circuit  104  cannot be applied in the floating system backlight module  100 .  
       SUMMARY OF THE INVENTION  
       [0007]     It is therefore an object of the invention to provide a lamp driving circuit, and particularly to a lamp driving circuit of a feedback circuit which can be applied in a floating system backlight module and can receive a high voltage power signal to generate a feedback signal.  
         [0008]     The invention achieves above-identified object by providing a lamp driving device for driving a lamp. The lamp driving device includes a controller, a first DC (direct current) to AC (alternating current) converter, a first voltage raising unit and a feedback circuit. The feedback circuit includes a voltage falling unit and a rectification circuit. The voltage falling unit provides a low voltage signal in response to a first DC power signal, a first AC power signal or a first driving power signal. The rectification circuit rectifies the low voltage signal to generate a feedback signal, and the rectification circuit provides the feedback signal. The controller provides a control signal in response to the feedback signal. The first DC to AC converter transforms the first DC power signal to the first AC power signal in response to the control signal. The first voltage raising unit raises the voltage of the first AC power signal to generate the first driving power signal. The first raising unit further provides the first driving power signal to a first end of the lamp, so that the lamp achieves the desired brightness stably.  
         [0009]     Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1A  is a diagram of a conventional backlight module lamp driving circuit;  
         [0011]      FIG. 1B  is a circuit diagram of a conventional feedback circuit;  
         [0012]      FIG. 2A  is a circuit diagram of a lamp driving circuit according to the first embodiment of the invention;  
         [0013]      FIG. 2B  a circuit diagram of a lamp driving circuit according to the first embodiment of the invention;  
         [0014]      FIG. 3A  is a circuit diagram of a feedback circuit according to the first embodiment of the invention;  
         [0015]      FIG. 3B  is a circuit diagram of a feedback circuit according to the first embodiment of the invention;  
         [0016]      FIG. 4  is a circuit diagram of a feedback circuit according to a second embodiment of the invention;  
         [0017]      FIG. 5A  is a circuit diagram of multi-lamp driving circuit;  
         [0018]      FIG. 5B  is a circuit diagram of multi-lamp driving circuit;  
         [0019]      FIG. 6  is a circuit diagram of a lamp driving circuit according to the second embodiment of the invention;  
         [0020]      FIG. 7  is a circuit diagram of a single end driving circuit of multiple fluorescent lamps; and  
         [0021]      FIG. 8  is a circuit diagram of a preferred lamp driving circuit according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     EMBODIMENT ONE  
       [0022]     Referring to  FIG. 2A  and  FIG. 2B , circuit diagrams of a lamp driving circuit according to a first embodiment of the invention are shown. Lamp driving circuit  200  is applied in a backlight module to drive a fluorescent lamp  202  as a backlight source. Since the drive mode of backlight module in a large-sized liquid crystal display has evolved from original single side drive to dual side drive mode, the circuits disposed on the two sides of the lamp driving circuit  200  are symmetric to the fluorescent lamp  202 . The lamp driving circuit  200  comprises a controller  204 , a first DC-to-AC converter  206 - 1  and a second DC-to-AC converter  206 - 2 , a first voltage raising unit  208 - 1  and a second voltage raising unit  208 - 2 , and a feedback circuit  210 . The controller  204  provides the control signal CS in response to the feedback signal FS. The first DC-to-AC converter  206 - 1  and the second DC-to-AC converter  206 - 2  respectively comprise a switch unit, at least a capacitor such as a first capacitor C 1  or a second capacitor C 2 , a first switch unit  212 - 1  and a second switch unit  212 - 2 . The first DC power signal DC 1  and the second DC power signal DC 2  are respectively provided by corresponding DC power sources. The capacitors C 1  and C 2  respectively store the corresponding voltage of the first DC power signal DC 1  and the second DC power signal DC 2 . The first switch unit  212 - 1  and the second switch unit  212 - 2  in response to the control signal CS respectively provide the first AC power signal AC 1  and the second AC power signal AC 2 . The first AC power signal AC 1  and the second AC power signal AC 2  respectively correspond to the cross-voltage of the capacitors C 1  and C 2 . Both he first voltage raising unit  208 - 1  and the second voltage raising unit  208 - 2  are converters, which respectively raise the voltage of the first AC power signal AC 1  and the second AC power signal AC 2 , then the first voltage raising unit  208 - 1  provides the first driving power signal PS 1  to the first end X 1  of the fluorescent lamp  202 , the second voltage raising unit  208 - 2  provides the second driving power signal PS 2  to the second end X 2  of the fluorescent lamp  202 . The feedback circuit  210  is used to generate a feedback signal FS.  
         [0023]     Due to the property of the component of a rectification circuit in a conventional feedback circuit, the voltage of the power signal received by the conventional feedback circuit cannot be too high, lest the voltage of the rectified feedback signal might be too high to the controller and the feedback circuit as well. Therefore the disposition of a conventional feedback circuit on a backlight module can only be between the fluorescent lamp and the grounding end, or between the high voltage side coil of a voltage raising unit and the grounding end.  
         [0024]     The feedback circuit  210  of the invention comprises a voltage reduction unit  214  and a rectification circuit  216 , wherein the voltage reduction unit  214  is series connected to the circuit, the voltage of the received power signal is appropriately reduced, then the power signal is transmitted to the rectification circuit  216  to be rectified and provided as a feedback signal FS. The disposition of the feedback circuit  210  on the lamp driving circuit is not limited to the position of the disposition of a conventional feedback circuit. The voltage reduction unit  214  can be a transformer or an operation amplifier circuit. The position of the two types is exemplified in  FIG. 2A  and  FIG. 2B , wherein the first position L 1 , the second position L 2  and the third position L 3  represent the position that can be used when the voltage reduction unit is a transformer, while the first position L 1 , the second position L 2 , the third position L 3 , the fourth position L 4 , the fifth position L 5  and the sixth position L 6  represent the position that can be used when the voltage reduction unit is an amplifier.  
         [0025]     Furthermore, when the voltage reduction unit  214  is a feedback circuit transformer, refer to  FIG. 3A  and  FIG. 3B , circuit diagrams of a feedback circuit according to the first embodiment of the invention. The feedback circuit  210  comprises a voltage reduction unit  214  and a rectification circuit  216 . The voltage reduction unit  214  comprises a feedback circuit high voltage side coil  302 , a feedback circuit low voltage side coil  304 , a first impedance unit R 1  and a second impedance unit R 2 . The second impedance unit R 2  and the low voltage side coil  304  are connected in parallel, and so too are the first impedance unit R 1  and the high voltage side coil  302  connected in parallel, wherein the first impedance unit R 1  and the second impedance unit R 2  can be a capacitance, a resistance. Beside that, even the first impedance unit R 1  or the second impedance unit R 2  is omitted, the voltage reduction unit  214  can still work.  
         [0026]     The feedback circuit transformer  214  transmits the received power signal to flow through the first impedance unit R 1  so as to generate a corresponding voltage drop and reduce the voltage to a low voltage signal L. The feedback circuit transformer can only be operated under AC power signal, and can only receive AC power signal, so the power signal received by the feedback circuit transformer  214  can be a first AC power signal AC 1 , a second AC power signal AC 2 , a first driving power signal PS 1  or a second driving power signal PS 2 . The rectification circuit  216  comprises a half-bridge rectification circuit  306  and a filtering circuit  308 , wherein the half-bridge rectification circuit  306  rectifies and provides the low voltage signal L. The filtering circuit  308  comprises a third impedance unit R 3  and a fourth impedance unit R 4 , wherein one end of the third impedance unit R 3  and one end of the fourth impedance unit R 4  are both coupled to a half-bridge rectification circuit  306 , while the other end of the third impedance unit R 3  and the other end of the fourth impedance unit R 4  are both coupled to a constant voltage such as a ground voltage. The third impedance unit R 3  and the fourth impedance unit R 4  can be a resistance, a capacitance. The third impedance unit R 3  or the fourth impedance unit R 4  can also be omitted. The filtering circuit  308  filters the noise of the rectified low voltage signal L then provides a feedback signal FS. The half-bridge rectification circuit  306  can be a full-bridge rectification circuit  310  as shown in  FIG. 3B . In  FIG. 2A  and  FIG. 2B , the voltage reduction unit of the feedback circuit  210  is a transformer and can be disposed at the first position L 1 , the second position L 2  or the third position L 3 . The details are exemplified below.  
         [0027]     The first position L 1  is the position in which some element can be coupled between the first DC-to-AC converter  206 - 1  and the first voltage raising unit  208 - 1  or between the second DC-to-AC converter  206 - 2  and the second voltage raising unit  208 - 2 .  
         [0028]     The second position L 2  is the position in which some element can be coupled between the high voltage side coil end GV 1  of the first voltage raising unit  208 - 1  and the ground voltage or between the high voltage side coil end GV 2  of the second voltage raising unit  208 - 2  and the ground voltage.  
         [0029]     The third position L 3  is the position in which some element can be coupled between the first end X 1  of the fluorescent lamp  202  and the high voltage side coil end GV 1 ′ of the first voltage raising unit  208 - 1  or between the second end X 2  of the fluorescent lamp  202  and the high voltage side coil end GV 2 ′ of the second voltage raising unit  208 - 2 . When a capacitor CX 2  exists between the GV 1 ′ end and the X 1  end, for example, one end of the capacitor CX 2  is coupled to a node N 1  while the other end of the capacitor CX 2  is connected to the ground voltage, the third position L 3  further comprises an L 3 A position in which some element can be coupled between the node N 1  and the high voltage side coil end GV 1 ′ of the first voltage raising unit  208 - 1 .  
         [0030]     When a capacitor CX 1  exists between the GV 1 ′ end and the X 1  end, the third position L 3  further comprises an L 3 B position in which some element can be coupled between the capacitor CX 1  and the GV 1 ′ end, and an L 3 C coupled to the capacitor CX 1  and the X 1  end.  
         [0031]     Similarly, when a capacitor CX 2 ′ or a capacitor CX 1 ′ exists between the second voltage raising unit  208 - 2  and the second end X 2  of the fluorescent lamp  202 , wherein the inter-space can be divided into L 3 A, L 3 B and L 3 C, and the feedback circuit  210  can be disposed at any position among L 3 A, L 3 B and L 3 C of the third position L 3 .  
         [0032]     Furthermore, when the voltage reduction unit  214  is an amplifier circuit, refer to  FIG. 4 , a circuit diagram of a feedback circuit according to the second embodiment of the invention is shown. The voltage reduction unit  214  comprises a first impedance unit R 1 ′, a second impedance unit R 2 ′, a third impedance unit R 3 ′, a fourth impedance unit R 4 ′, a fifth impedance unit R 5 , a sixth impedance unit R 6  and an amplifier  402 . The amplifier  402  has a positive input end, a negative input end and an output end, wherein the positive input end is coupled to one end of the first impedance unit R 1 ′ via second impedance unit R 2 ′, the negative input end is coupled to the other end of the first impedance unit R 1 ′ via the third impedance unit R 3 ′, and the fourth impedance unit R 4 ′ is coupled to the negative input end via the output end and provides a low voltage signal L accordingly. One end of the fifth impedance unit R 5  is coupled to the output end, and the other end is coupled to a first constant voltage such as a ground voltage. One end of the sixth impedance unit R 6  is coupled to the positive input end, and the other end is coupled to a second constant voltage such as a ground voltage. The first impedance unit R 1 ′, which can be a capacitance or a resistance, enables the power signal which flow through the first impedance unit R 1 ′ to generate corresponding voltage drop. The second impedance unit R 2 ′ and the third impedance unit R 3 ′ are both resistance. The fourth impedance unit R 4 ′ can be a resistance, a capacitance or a resistor-capacitance. The sixth impedance unit R 6  can be a resistance or a capacitance. The fifth impedance unit R 5  is a resistance, a capacitance. The fifth impedance unit R 5  can also be omitted.  
         [0033]     The amplifier circuit converts the corresponding voltage of the power signal flowing through the first impedance unit R 1 ′ to a low voltage signal L and transmits the low voltage signal L to rectification circuit  216 . Since the amplifier circuit  214  can be operated under both AC and DC power signals, the power signal flowing through the first impedance unit R 1 ′ can be a first DC power signal DC 1 , a second DC power signal DC 2 , a first AC power signal AC 1 , a second AC power signal AC 2 , a first driving power signal PS 1  or a second driving power signal PS 2 . The rectification circuit  216  receives the low voltage signal L and provides the feedback signal FS in response to the controller  204 . The disposition of the feedback circuit  210  can be a first position L 1 , a second position L 2 , or a third position L 3  as well as a fourth position L 4 , a fifth position L 5  or a sixth position L 6  as shown in  FIG. 2 .  
         [0034]     The fourth position L 4  is the position in which some element can be coupled between the DC power source of the first DC-to-AC converter  206 - 1  and the first capacitor C 1 , or between the DC power source of the second DC-to-AC converter  206 - 2  and the second capacitor C 2 .  
         [0035]     The fifth position L 5  is the position in which some element can be coupled between the first capacitor C 1  and the first switch unit  212 - 1 , or between the second capacitor C 2  and the second switch unit  212 - 2 .  
         [0036]     The sixth position L 6  is the position in which some element can be coupled between the first switch unit  212 - 1  and the grounding end, or between the second switch unit  212 - 2  and the grounding end, wherein the grounding end is coupled to ground voltage.  
         [0037]     Of the seven positions disclosed above, any position can generate a feedback signal FS to the controller  204  by means of a voltage reduction unit  214  and a rectification circuit  216 , so that the controller  204  provides a control signal CS to control the brightness of the fluorescent lamp  202 . Referring to  FIG. 8 , a circuit diagram of a preferred lamp driving circuit according to the invention is shown. The feedback circuit  210  is preferably disposed at the third position L 3 , the closer to the fluorescent lamp the better.  
         [0038]     Referring to  FIG. 5A  and  FIG. 5B , circuit diagrams of multi-lamp driving circuit are shown. The lamp driving circuit  200  can further drive a plurality of fluorescent lamps such as fluorescent lamps  202  and  202 -X. It can be seen from the diagram that, a feedback circuit  210  can be disposed between the ends X 1 ′ and X 2 ′ of the fluorescent lamp  202 -X and the ends GV 1  and GV 2  of the two voltage raising units  208 - 1  and  208 - 2 . Under such circumstance, disposition of the feedback circuit  210  can be any position of L 3 D, L 3 E, L 3 F apart from original L 1 , L 3 A, L 3 B, L 3 C, L 4 , L 5 , and L 6 .  
       EMBODIMENT TWO  
       [0039]     Referring to  FIG. 6 , a circuit diagram of a lamp driving circuit according to a second embodiment of the invention is shown. The lamp driving circuit  200 ′ changes from dual side drive mode to single side drive mode. That is to say, the lamp driving circuit  200 ′ only comprises a controller  204 , a first DC-to-AC converter  206 - 1 , a first voltage raising unit  208 - 1  and a feedback circuit  210 , wherein the first end X 1  of the fluorescent lamp  202  receives the first driving power signal PS 1 , the second end X 2  of the fluorescent lamp  202  is connected to a constant voltage, such as a ground voltage. The drive mode of the fluorescent lamp changes from dual side drive mode to single side drive mode, the principles of the method are the same and are not repeated here.  
         [0040]     However, the spirit of the invention can be used to apply the voltage reduction unit  214  and the rectification circuit  216  to the multiple positions on the lamp driving circuit and use corresponding power signals to generate a feedback signal FS. When the voltage reduction unit  214  is a feedback circuit transformer, as shown in  FIG. 3A and 3B , the position of the disposition of the feedback circuit  210  is the same with position L 1 ˜L 3  in the first embodiment. Furthermore, by connecting the second single end X 2  of the fluorescent lamp  202  to the round voltage, the feedback circuit  210  can further be disposed between the second single end X 2  of the fluorescent lamp  202  and the grounding end, i.e., the seventh position L 7 , wherein the grounding end is coupled to the ground voltage.  
         [0041]     When the voltage reduction unit  210  is an amplifier circuit as shown in  FIG. 4  the disposition of the feedback circuit is the same with the positions L 1 ˜L 6  in the first embodiment as well as and the seventh position L 7  in the embodiment.  
         [0042]     Besides, the lamp driving circuit of the present the embodiment can drive multiple fluorescent lamps such as a fluorescent lamp  202 -X. Referring to  FIG. 7 , a circuit diagram of a single end driving circuit of multiple fluorescent lamps is shown. Similarly, according to the spirit of the invention, the feedback circuit  210  can also be disposed at the first position L 1  and the third to the seventh position L 3 ˜L 7 , wherein the feedback circuit  210  is originally disposed at the second position L 2 , due to the high voltage side coil of the first voltage raising unit  208 - 1 , the grounding end GV 1  of the first voltage raising unit  208 - 1  is coupled to the first end X 1 ′ of the fluorescent lamp  202 -X. Therefore, the third position L 3  has three additional positions, namely, L 3 D, L 3 E and L 3 F.  
         [0043]     When the electrical signal which is nearest to the fluorescent lamp is selected as the feedback signal, the lamp driving circuit disclosed in the above the embodiment of the invention can reduce the corresponding voltage of the first and the second driving power signal via an amplifier circuit or a feedback circuit transformer, so that the feedback circuit  210  can provide a feedback signal FS. Thus the difficulty encountered in obtaining a feedback signal when the driving voltage for the fluorescent lamp gets higher and higher.  
         [0044]     While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.