Patent Publication Number: US-2007103942-A1

Title: Backlight module, inverter, and DC voltage generating method thereof

Description:
This application claims the benefit of Taiwan application Serial No. 94219335, filed Nov. 8, 2005, the subject matter of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates in general to a backlight module, inverter and DC voltage generating method thereof, and more particularly to a backlight module and inverter, which can supply driving voltages to a display panel, and DC voltage generating method thereof.  
      2. Description of the Related Art  
       FIG. 1  is a block diagram of a conventional driving circuit for a small-scale liquid crystal panel using two DC driving voltages provided from the exterior. Referring to  FIG. 1 , the small-scale liquid-crystal-panel (LCD) driving circuit  100  requires two driving voltages, such as +15V and −10V, provided from the exterior in addition to an operation voltage 5V. A conventional LCD control circuit provides only a driving voltage under 5V. In order to generate the two high driving voltages, an additional DC/DC converter  110 , such as a charge pump or a boost circuit, is disposed for converting the operation voltage 5V to the required driving voltages +15V and −10V. However, using the DC/DC converter  110  will increase the cost for manufacturing the LCD.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the invention to provide a backlight module, inverter, and DC voltage generating method thereof. A secondary coil is added to the transformer of the inverter and coupled to a simple-structure rectification circuit for generating the required positive and negative driving voltages of the display panel. Therefore, the cost for manufacturing the LCD can be effectively reduced.  
      The invention achieves the above-identified object by providing an inverter applied to a backlight module. The inverter includes a power converting device, a transformer, and a rectification circuit. The power converting device is coupled to a first DC voltage for outputting a first AC voltage. The transformer includes a primary coil, and a first secondary coil. The primary coil is coupled to the first AC voltage outputted by the power converting device. The first secondary coil is for outputting a second AC voltage by inducing the first AC voltage. The rectification circuit is coupled to the first secondary coil for rectifying the second AC voltage and outputting a second DC voltage.  
      The invention achieves the above-identified object by providing another inverter applied to a backlight module. The inverter includes a power converting device, a transformer, and a first rectification circuit. The power converting device is coupled to a first DC voltage for outputting a first AC voltage. The transformer includes a primary coil and a first secondary coil. The primary coil is coupled to the first AC voltage outputted by the power converting device. The first secondary coil is for outputting a second AC voltage to drive the lamp by inducing the first AC voltage. The first rectification circuit is coupled to the primary coil for rectifying the first AC voltage and outputting a second DC voltage.  
      The invention achieves the above-identified object by providing a backlight module including an inverter and a lamp. The inverter includes a power converting device, a transformer, and a rectification circuit. The power converting device is coupled to a first DC voltage for outputting a first AC voltage. The transformer includes a primary coil, a first secondary coil and a second secondary coil. The primary coil is coupled to the first AC voltage outputted by the power converting device. The first secondary coil is for outputting a second AC voltage by inducing the first AC voltage. The second secondary coil is for outputting a third AC voltage by inducing the first AC voltage. The rectification circuit is coupled to the first secondary coil for rectifying the second AC voltage and outputting a second DC voltage. The lamp is coupled to the third AC voltage.  
      The invention achieves the above-identified object by providing a DC voltage generating method applied to an inverter of a backlight module. The inverter is for receiving a first DC voltage. The method includes converting the first DC voltage to a first AC voltage; converting the first AC voltage to a second AC voltage by an electromagnetic induction method; and rectifying the second AC voltage to generate a second DC voltage.  
      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 the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a conventional driving circuit for a small-scale liquid crystal panel.  
       FIG. 2A  is a circuit structure diagram of a backlight module of a LCD according to a first embodiment of the invention.  
       FIG. 2B  is a flow chart of the method for generating a DC voltage according to the first embodiment of the invention.  
       FIG. 3A  is a circuit structure diagram of a backlight module of a LCD according to a second embodiment of the invention.  
       FIG. 3B  is a flow chart of the method for generating a DC voltage according to the second embodiment of the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Embodiment One  
      Referring to  FIG. 2A , a circuit structure diagram of a backlight module of a LCD according to a first embodiment of the invention is shown. The backlight module  200  including an inverter  210  and a lamp  220  serves as a light source for a small scale liquid crystal panel,. The inverter  210  provides an AC driving voltage Va 0  for driving the lamp  220 . The lamp  220  is a cold cathode fluorescent lamp (CCFL) as preferred. The inverter  210  includes a power converting device  230 , a transformer  240  and a rectification circuit  250 . As shown in  FIG. 2A , the power converting device  230 , a full-bridge switch device as preferred, is coupled to the first DC voltage DC 1 , such as 5V (the DC voltage DC 1  falls in the range (5V, 24V) as preferred), and converts the DC voltage DC 1  to a first AC voltage AC 1  through high-frequency switching.  
      The transformer  240  includes a primary coil  242 , a first secondary coil  244  and a second secondary coil  246 . The primary coil  242  is coupled to the first AC voltage AC 1  outputted by the power converting device  230 . The first secondary coil  244  induces the first AC voltage AC 1  to generate the second AC voltage AC 2  by electromagnetic induction. The second secondary coil  246  induces the first AC voltage AC 1  by electromagnetic induction to output a third AC voltage Va 0 , up to several hundred to a thousand volts, for lighting up the lamp  220 .  
      Besides, the rectification circuit  250  is coupled to the first secondary coil  244  for rectifying the second AC voltage AC 2  and outputting the second DC voltage DC 2  and the third DC voltage DC 3 , such as the driving voltages +15V and −10V. The rectification circuit  250  preferably includes diodes D 1 , D 2  and capacitors C 1 , C 2 . The positive end of the diode D 1  is coupled to an end E 1  of the first secondary coil  244  while the negative end of the diode D 1  is coupled to an end F 1  of the capacitor C 1 . Another end E 2  of the first secondary coil  244  is coupled to another end F 2  of the capacitor C 1  while the end F 2  of the capacitor C 1  is grounded.  
      Moreover, the positive end of the diode D 2  is coupled to an end G 2  of the capacitor C 2 , the negative end of the diode D 2  is coupled to an inner node E 0  of the first secondary coil  244 , and another end G 1  of the capacitor C 2  is coupled to the end F 2  of the capacitor C 1 .  
       FIG. 2B  shows a flow chart of the method for generating a DC voltage according to the first embodiment of the invention. First, in step  260 , also with reference to  FIG. 2A , use the above-mentioned power converting device  230 , such as the full-bridge switch device to convert the first DC voltage DC 1  (such as 5V) to the first AC voltage AC 1  by high-frequency switching. If the threshold voltages of the transistors A, B, C and D are not considered, the AC voltage AC 1  has an amplitude about 5V in the embodiment. Next, in step  270 , convert the first voltage AC 1  to second AC voltage AC 2  by electromagnetic induction. For example, the primary coil  242  of the above-mentioned transformer  240  receives the AC voltage AC 1  and induces the AC voltage AC 1  to generate the second AC voltage AC 2  through the first secondary coil  244  by electromagnetic induction. Finally, in step  280 , rectify the second AC voltage AC 2  to generate the second DC voltage DC 2  and rectify a portion of the second AC voltage AC 2  to generate the third DC voltage DC 3  by the above-mentioned rectification circuit  250 .  
      In  FIG. 2A , assume the winding number of the primary coil  242  is M, and the winding number of the first secondary coil  244  is N 1 . When N 1  is equal to M×3, the first secondary coil  244  will induce the AC voltage AC 1  (5V) to generate the AC voltage AC 2  of 15V (3×5V). The AC voltage AC 2  is rectified by the diode D 1  to generate a DC voltage drop 15V (DC 2 ) across the capacitor C 1 . That is, the Fl node is +15V. Assume the winding number of the first secondary coil  244  from the end E 0  to E 2  is N 1 ′ and N′ is set to be N 1 ×(⅔). The induced voltage on the first secondary coil  244  between the ends E 0  and E 2  is 15V×(⅔)=10V and the induced voltage is rectified to generate a DC voltage drop 10V (DC 3 ) across the capacitor C 2 . Since the node F 2  is grounded, the voltage at the end G 2  is −10V. Therefore, the positive and negative driving voltages +15V and −10V required by the liquid crystal panel can be generated by the primary coil  242  and secondary coil  244  coupling to the rectification circuit  250 . The extra DC/DC converter is not needed in the inverter  210  of the invention, thereby effectively reducing LCD cost.  
     Embodiment Two  
      Referring to  FIG. 3A , a circuit structure diagram of a backlight module of a LCD according to a second embodiment of the invention is shown. The backlight module  300 , such as served as a light source for a small-scale liquid crystal panel, includes an inverter  310  and a lamp  320 . The inverter  310  provides an AC driving voltage AC 2  for driving the lamp  320 . The lamp  320  is preferably a cold cathode fluorescent lamp (CCFL). The inverter  310  includes a power converting device  330 , a transformer  340 , a first rectification circuit  350  and a second rectification circuit  360 . As shown in  FIG. 3A , the power converting device  330 , a full-bridge switch device as. preferred, is coupled to the first DC voltage DC 1 , such as 15V (the DC voltage DC 1  falls in the range (5V, 24V) as preferred), and converts the DC voltage DC 1  to a first AC voltage AC 1  by high-frequency switching.  
      The transformer  340  includes a primary coil  342 , a first secondary coil  346  and a second secondary coil  348 . The primary coil  342  is coupled to the first AC voltage AC 1  outputted by the power converting device  330 . The first secondary coil  346  induces the first AC voltage AC 1  by electromagnetic induction to output a second AC voltage AC 2 , up to several hundred to a thousand volts, for lighting up the lamp  320 . The second secondary coil  348  also induces the first AC voltage AC 1  to output a third AC voltage AC 3  via an electromagnetic induction effect.  
      In this embodiment, the inverter  310  has the first rectification circuit  350  and the second rectification circuit  360 , respectively coupled to the primary coil  342  and the second secondary coil  348 . The first rectification circuit  350  rectifies the first AC voltage AC 1  to output the second DC voltage DC 2 , such as the positive driving voltages +15V, and the second rectification circuit  360  rectifies the first AC voltage AC 1  and outputting a third DC voltage DC 3 , such as a negative driving voltage −10V. Preferably, the first rectification circuit  350  includes a diode Dl and a capacitor C 1 , and the second rectification circuit  360  includes a diode D 2  and a capacitor C 2 . The positive end of the diode D 1  is coupled to an end E 0  of the primary coil  342  while the negative end of the diode D 1  is coupled to an end F 1  of the capacitor C 1 . Another end F 2  of the capacitor C 1  is grounded. As noted,, the present invention generates a negative driving voltage −10V for the small LCD panel through the inverter  310 .  
      Moreover, the positive end of the diode D 2  is coupled to an end G 2  of the capacitor C 2 , the negative end of the diode D 2  is coupled to a first coil end E 1  of the second secondary coil  348 , and another end G 1  of the capacitor C 2  is coupled to a second coil end E 2  of the second secondary coil  348 , which is grounded.  
       FIG. 3B  shows a flow chart of the method for generating DC voltages according to the second embodiment of the invention. First, in step  360 , also with reference to  FIG. 3A , use the power converting device  330 , such as the full-bridge switch device to convert the first DC voltage DC 1  (such as 15V) to the first AC voltage AC 1  by high-frequency switching. If the threshold voltages of the transistors A, B, C and D are not considered, the AC voltage AC 1  has an amplitude about 15V in the embodiment. Next, in step  370 , use the transformer  340  to convert the first AC voltage AC 1  to a third AC voltage AC 3  by electromagnetic induction. Finally, in step  380 , rectify the first AC voltage AC 1  and the third AC voltage AC 3  to respectively generate the second DC voltage DC 2  and the third voltage DC 3  by the first rectification circuit  350  and second rectification circuit  360 .  
      The AC voltage AC 1  is rectified by the diode D 1  to generate a DC voltage drop 15V (DC 2 ) across the capacitor C 1 . That is, the F 1 -end voltage is +15V. Besides, the winding number of the second secondary coil  348  can be properly arranged such that the third AC voltage AC 3  induced by the second secondary coil  348  has an output amplitude about 10V. The induced voltage AC 3  is then rectified by the diode D 2  to generate a DC voltage drop 10V (DC 3 ) across the capacitor C 2 . That is, the node G 2  is −10V. Therefore, the positive and negative driving voltages +15V and −10V required by the liquid crystal panel can be generated by the primary coil  342  of the transformer  340  coupling to the first rectification circuit  350  and the second secondary coil  348  of the transformer  340  coupling to the second rectification circuit  360 . The extra DC/DC converter is not needed in the inverter  310  of the invention, thereby effectively reducing LCD cost.  
      According to the disclosed embodiments, although the power converting device  230  or  330  of the backlight module  200  or  300  is exemplified by a full-bridge switch device, the backlight module  200  or  300  of the invention can also convert the DC voltage DC 1  to the AC voltage AC 1  by using a half-bridge switch device, a push-pull switch device or other alternative devices. By winding the first secondary coil  244  onto the transformer  240  and coupling the rectification circuit  250  to the coil  244  or coupling the first rectification circuit  350  to the primary coil  342  of the transformer  340 , and coupling the second rectification circuit  360  to the second secondary coil  348 , the positive and negative driving voltages required by the liquid crystal panel can be generated, thus reducing LCD cost.  
      Furthermore, although the rectification circuit  250  of the invention is exemplified to generate the driving voltages +15V and −10V by using the diodes D 1 , D 2  and the capacitors C 1  and C 2 , or the rectification circuits  350  and  360  are exemplified to respectively generate the driving voltages +15V and −10V by using the diode D 1  and capacitor C 1 , and the diode D 2  and capacitor C 2 , the polarity connection of the diode D 1  or D 2  can be reversed and properly arranged in the rectification circuit  250 ,  350 , or  360 . Or the rectification circuit  250  can also purely use a diode and a capacitor to generate a positive (or negative) driving voltage. Or the inverter  310  can generate the driving voltage DC 2  by the first rectification circuit  350 . The winding number N 1  of the first secondary coil  244  can also be larger than the winding number M of the primary coil  242  by any proper relation. The winding number ratio of the first secondary coil  244  of the transformer  240  from the end E 0  to E 2  of the first secondary coil  244  can also be properly adjusted to generate various positive and negative driving voltages as needed. Or the rectification circuit  250 ,  350 , or  360  can also be other rectification circuits of a complex type. Therefore, by using the power converting device  230  (or  330 ) and induction coil of the transformer  240  (or  340 ) coupled to the rectification circuit  250  (or  350  and  360 ), the backlight module  200  (or  300 ) can generate a variety of driving voltages for the LCD to achieve the purpose of reducing LCD cost, without departing from the scope of the invention.  
      The backlight module, inverter and the DC voltage generating method thereof disclosed by the above-mentioned embodiments of the invention has the following advantages:  
      1. The high driving voltages required by the LCD can be directly provided by the backlight module  200  having an extra secondary coil coupled to a rectification circuit on the transformer  240 . Therefore, the extra DC/DC converter is not needed in the invention, and the cost for manufacturing LCD can be reduced.  
      2. Owing that the positive and negative driving voltages required by a small scale liquid crystal panel can have an error about 20%, the inverter according to the present invention can generate the driving voltages satisfying the required accuracy by coupling the rectification circuit to the transformer. Therefore, the LCD cost can be reduced by using the backlight module of the invention without influencing the quality of image display.  
      3. The required current from the positive and negative driving voltages need not to be very large (˜10 mA) on the small scale panel. Therefore, the extra secondary coil wound on the transformer of the backlight module in the invention has not to be large, and thus the volume of the transformer is not increased too much. The DC voltage generating method of the invention can thus be implemented under a limited current consideration. Moreover, winding one more coil on the transformer will not increase the cost and the required driving voltage can be adjusted by directly controlling the winding number of the primary or secondary coil. As a result, the LCD cost can be reduced.  
      While the invention has been described by way of example and in terms of preferred embodiments, 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 the appended claims therefore should be accorded the broadest interpretation so as to encompass all possible modifications and similar arrangements and procedures.