Abstract:
A two-phase driver comprises two H-bridge circuits each of them including a transformer and two switch assemblies connected to the opposite terminals of the primary side of the transformer, in which one of the switch assemblies is shared by the two H-bridge circuits. Each of the switch assemblies includes a high-side switch and a low-side switch, and each of the high-side switch and low-side switch is switched by a respective signal so as to modulate the loading currents supplied to two loading loops connected to the two H-bridge circuits, respectively.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to a driver configured with H-bridge circuit, and more particularly, to a two-phase driving circuit and method for cold cathode fluorescent lamps (CCFLs). 
   BACKGROUND OF THE INVENTION 
   CCFL has been applied for the backlight sources of displays, especially for liquid crystal displays (LCDs), which requires a circuit to drive them. Due to the small size of conventional displays, so far one CCFL is enough for one display. However, recent displays tend to be enlarged in their scales, resulting in that two or more CCFLs are required for one backlight source, and there is thus a need of a driving circuit capable of driving two or more CCFLs for the backlight source. 
   A prior art driver to drive two CCFLs is proposed by U.S. Pat. No. 5,892,336 issued to Lin et al., which comprises a transformer having its primary side connected with an AC power supply and secondary side connected with two CCFLs. Since these two CCFLs are connected in series to the driver, the currents flowing through them are identical and therefore limit the CCFLs to be adjusted individually. However, there are always more or less variations between CCFLs once they are manufactured, and thus they have diversified luminescent features. As a result, this driver with CCFLs connected in series and thereby having the same driving current for all of the CCFLs cannot be available for applications that adjustable brightness of individual lamp is required. 
   An alternative driver proposed by U.S. Pat. No. 6,396,722 issued to Lin employs four MOS transistors and one transformer to form a full bridge circuit to drive a CCFL, yet this driver drives only one CCFL. Two independent full bridge circuits are required to drive two CCFLs individually, if this art is utilized. Even the driving currents are adjustable for respective CCFLs for their brightness to be uniformed when two independent full bridge circuits are provided, the cost and volumn of the driver are dramatically increased. 
   Therefore, it is desired a low-cost and small-size driver to adjust the driving current for individual CCFL in a multiple CCFL system to control their respective brightness. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to disclose a two-phase driver configured with H-bridge circuit and its driving method. 
   Another object of the present invention is to propose a two-phase driver configured with H-bridge circuit and the driving method thereof for modulating two loading currents for two loading loops. 
   Yet another object of the present invention is to provide a two-phase driver configured with H-bridge circuit and the driving method thereof for balancing two loading currents of two balance or imbalance loads. 
   In a two-phase driver, according to the present invention, two H-bridge circuits are comprised and each of them including a transformer connected with two switch assemblies at the opposite terminals of its primary side and a CCFL at its secondary side, of which one of the switch assemblies is shared by the two H-bridge circuits and each of the switch assemblies includes a high-side switch and a low-side switch controlled by a respective signal so as to modulate the loading currents flowing through the two CCFLs, and therefore to balance the loading currents or to uniform the brightness of the two CCFLs. Furthermore, only six switches are employed in the two-phase driver, and thus the cost and volunme is reduced due to the less switch componants. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows an embodiment of two-phase driver configured with H-bridge circuit according to the present invention; 
       FIG. 2  illustrates the timing diagram of the two-phase driver shown in  FIG. 1  under imbalanced loading currents; 
       FIG. 3  illustrates the timing diagram of the two-phase driver shown in  FIG. 1  under balanced loading currents; and 
       FIG. 4  shows another embodiment of the present invention under balanced loading currents. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows an embodiment two-phase driver  100  that comprises a first switch assembly  110  composed of a high-side NMOS transistor  112  between input voltage V in  and node  114  and a low-side NMOS transistor  116  between the node  114  and ground GND, a second switch assembly  120  composed of a high-side NMOS transistor  122  between the input voltage V in  and node  124  and a low-side NMOS transistor  126  between the node  124  and ground GND, and a third switch assembly  130  composed of a high-side NMOS transistor  132  between the input voltage V in  and node  134  and a low-side NMOS transistor  136  between the node  134  and ground GND. The primary side of a transformer TX 1  is connected between the switch assemblies  110  and  120  by the nodes  114  and  124  to thereby form an H-bridge circuit in combination with the switch assemblies  110  and  120 , and the secondary side of the transformer TX 1  is connected to first loading loop, i.e., a CCFL  140 . Likewise, a transformer TX 2  has its primary side connected between the switch assemblies  110  and  130  by the nodes  114  and  134  to thereby form another H-bridge circuit in combination with the switch assemblies  110  and  130 , and has its secondary side connected to second loading loop, i.e., another CCFL  150 . In this arrangement, the switch assembly  110  is shared by the two H-bridge circuits, and thus two or more switch componants are saved. Moreover, two current sense circuits  160  and  170  are inserted to the first and second loading loops so as to sense the loading currents I 1  and I 2  of the CCFLs  140  and  150  thereof, respectively. 
   The NMOS transistors  112 – 136  are all serving as switches manipulated by six control signals S 1 –S 6 , respectively, as shown in  FIG. 1 . When the NMOS transistors  116 ,  122  and  132  are turned on at the same time by their respective control signals S 2 , S 3  and S 5 , currents I A1  and I B1  are generated to flow through the primary sides of the transformers TX 1  and TX 2 , respectively, and when the NMOS transistors  112 ,  126  and  136  are turned on at the same time by their respective control signals S 1 , S 4  and S 6 , currents I A2  and I B2  are generated to flow through the respective primary sides of the transformer TX 1  and TX 2  in opposite directions. As a result, the supplied power is transformed by the transformers TX 1  and TX 2  so as to generate the loading currents I 1  and I 2  for the CCFLs  140  and  150 , respectively. In this embodiment, the modulations of the loading currents I 1  and I 2  and thereby the brightness of the CCFLs  140  and  150  are achieved by the timings of the signals S 3 , S 4 , S 5  and S 6  to the other two S 1  and S 2 . 
     FIG. 2  shows the timing diagram of the control signals S 1 –S 6  in the two-phase driver  100  of  FIG. 1 , in which the shaded area A 1  indicates the period when the signals S 2  and S 3  both are turned on, the shaded area A 2  indicates the period when the signals S 1  and S 4  both are turned on, the shaded area A 3  indicates the period when the signals S 2  and S 5  both are turned on, and the shaded area A 4  indicates the period when the signals S 1  and S 6  both are turned on. The areas of the shaded areas A 1  and A 2  indicate the magnitude of the currents I A1  and I A2  flowing through the primary side of the transformer TX 1  and corresponding to the loading current I 1  transformed on the secondary side of the transformer TX 1 , and the areas of the shaded areas A 3  and A 4  indicate the magnitude of the current I B1  and I B2  flowing through the primary side of the transformer TX 2  and corresponding to the loading current  12  transformed on the secondary side of the transformer TX 2 . As shown in  FIG. 2 , the areas of A 1 –A 4  are not all identical, and therefore, the magnitudes of the currents I A1 , I A2 , I B1  and I B2  flowing through the transformers TX 1  and TX 2  are different. To modulating the loading currents I 1  and I 2 , the timings of the control signals S 1 –S 6  are properly selected to adjust the currents I A1 , I A2 , I B1  and I B2  of the transformers TX 1  and TX 2 . Since the brightness of a CCFL is proportional to the current flowing therethrough and the transformed currents I 1  and I 2  are determined by the overlapped areas A 1 –A 4  between the control signals S 1 –S 6 , the brightness of the loading CCFL  140  or  150  shown in  FIG. 1  is increased or decreased by adjusting the duty cycles of the signals S 3  and S 4  or S 5  and S 6  such that the overlapped areas A 1 –A 4  are properly controlled corresponding to the selected signals S 1  and S 2 . This manner the two loading CCFLs  140  and  150  are individually and well controlled to have an uniformed brightness, even they are different in their luminant features. 
   In the same way, referring to  FIG. 3 , the overlapped areas A 1  and A 2  determined by the signals S 3  and S 4  and A 3  and A 4  determined by the signals S 5  and S 6  for a pair of selected signals S 1  and S 2  can be adjusted for the two CCFLs  140  and  150  to have a same magnitude for the loading currents I 1  and I 2  therethrough. In other words, either the brightness or the loading currents can be uniformed or balanced for the two CCFLs  140  and  150  by the same driver  100  under the control signals S 1 –S 6 , no matter these two loading loops  140  and  150  are originally balanced or imbalanced. Furthermore, due to the driver  100  having superior controllability of the loading currents I 1  and I 2 , the two loading loops  140  and  150  are flexible for loading variations. 
     FIG. 4  is another embodiment of the control signals S 1 –S 6  upon balanced loading currents. As in the first embodiment, a driver  200  shown in  FIG. 4  comprises six NMOS transistors  112 – 136  configured in two H-bridge circuits with a shared or common switch assembly composed of switches  112  and  116 , two transformers TX 1  and TX 2  connected in the two H-bridge circuits, respectively, to transform the supplied power to the two loading CCFLs  140  and  150 , as well as two current sense circuits  160  and  170  for the two loading loops, respectively. However, the two transistors  122  and  132  respectively in the two H-bridge circuits are common gated to be controlled by the same signal S 3 , and another transistors  126  and  136  are also common gated to be controlled by the same signal S 4 . By this arrangement, the laoding currents I 1  and I 2  for the CCFLs  140  and  150  are identical or balanced. Due to the six control signals being reduced to four control signals S 1 –S 4 , this embodiment further simplifies the control signal generator for the signals S 1 –S 4 . As such, the cost and volumn of the overall circuit are further reduced. 
   By the proposed driver herewith configured in two H-bridge circuits sharing a common switch assembly, two loading loops can be individually modulated for their loading currents and as a result, the brightness of two CCFLs. With a same driver, either loading currents or brightness of two CCFLs in two loading loops can be balanced, depending on the control signals for the two H-bridge circuits. Moreover, the cost and volumn of the driver are reduced by less switch componants, and the cost and volumn of the whole circuit are further reduced by less control signals. 
   While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.