Abstract:
A multi-path constant current driving circuit. In the multi-path constant current driving circuit, a current sharing transformer (T 31 , T 32 , T 33 , T 3 ( n −1)) is provided in power supply circuits which are provided with adjacent rectifier and filter units (Z 31 , Z 32 , Z 33 , Z 3   n ). A first winding of the current sharing transformer (T 31 , T 32 , T 33 , T 3 ( n −1)) is connected between a first terminal of a secondary winding of a first power supply circuit and the rectifier and filter unit (Z 31 , Z 32 , Z 33 , Z 3   n ) of the first power supply circuit. A second winding of the current sharing transformer (T 31 , T 32 , T 33 , T 3 ( n −1)) is connected between a first terminal of a secondary winding of a second power supply circuit and the rectifier and filter unit (Z 31 , Z 32 , Z 33 , Z 3   n ) of the second power supply circuit. In-phase current flows through the dotted terminal of the first winding and the synonym terminal of the second winding of the current sharing transformer (T 31 , T 32 , T 33 , T 3 ( n   −1 )). The current sharing transformer (T 31 , T 32 , T 33 , T 3 ( n −1)) is used for sharing the current between the power supply circuits which are provided with the adjacent rectifier and filter units (Z 31 , Z 32 , Z 33 , Z 3   n ). The driving circuit is provided with high current sharing efficiency, a small size and a low cost.

Description:
[0001]    This application claims the benefit of Chinese patent application No. 200910225966.4, titled “CIRCUIT APPLICABLE TO MULTIPATH LED CONSTANT-CURRENT DRIVING” and filed with the State Intellectual Property Office on Nov. 21, 2009, which is hereby incorporated by reference in its entirety. 
         [0002]    This application claims the benefit of Chinese patent application No. 200920273352.9, titled “CIRCUIT APPLICABLE TO MULTIPATH LED CONSTANT-CURRENT DRIVING” and filed with the State Intellectual Property Office on Nov. 21, 2009, which is hereby incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0003]    The present invention relates to the field of constant-current driving technology, and in particular to a multipath constant-current driving circuit. 
       BACKGROUND OF THE INVENTION 
       [0004]    Currently, the most common solution to constant-current control of multipath light-emitting diodes (LEDs) includes a constant-voltage module and multiple non-isolated DC/DC constant-current modules. 
         [0005]      FIG. 1  shows a constant-current control circuit for multipath LEDs in the prior art. In this circuit, an input voltage Vac goes through a constant-voltage module and then inputs to multiple non-isolated DC/DC constant-current modules. Each of the non-isolated DC/DC constant-current modules performs constant-current control separately. However, there is normally a significant disparity between the voltage of the constant-voltage module and the voltage of an LED load; therefore none of the non-isolated DC/DC constant-current modules that follow the constant-voltage module has a high efficiency. In addition, the structure of multiple non-isolated DC/DC constant-current modules is complex and costly. 
         [0006]    According to the Chinese patent application No. 200810085227.5, a balanced-current power supply circuit for multiple groups of LEDs is provided. As shown in  FIG. 2 , a first inductor Lca 1  of a coupled inductor Lca is connected in series to a Direct current (DC) loop after the rectification by diodes D 1  and D 2 , and a second inductor Lca 2  of the coupled inductor Lca is connected in series to a DC loop after the rectification by diodes D 3  and D 4 , so that the coupled inductor Lca can balance the two LED loads. However, according to the circuit shown in  FIG. 2 , each of the two coils of the coupled inductor Lca is connected in series to a DC loop, causing a DC current, but the magnetizing current in the coupled inductor is unidirectional; therefore when the voltages of the two load branches are not balanced, the difference between the currents of the two load branches is large, resulting in poor current balancing. Moreover, the presence of a DC current in the coupled inductor may cause saturation of the magnetic core, which requires air gaps to be created; therefore when the inductance is large, the size of the coupled inductor is large and hence costly. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of this, a technical problem to be solved by the present invention is to provide a multipath constant-current driving circuit, which provides good current balancing, and can reduce the size of the current-balancing transformer and lower the cost. 
         [0008]    Therefore, the embodiments of the present invention provide the following technical solutions. 
         [0009]    According to an embodiment of the present invention, it is provided a multipath constant-current driving circuit, including: a DC/AC converter, a main transformer and at least two rectification and filtering units, wherein 
         [0010]    the DC/AC converter is adapted to provide an alternating current (AC) voltage for a primary winding of the main transformer; 
         [0011]    each of the at least two rectification and filtering units forms a power supply loop with a secondary winding of the main transformer; each of the power supply loops includes: a first terminal of the secondary winding connected to a first input terminal of a corresponding rectification and filtering unit, and a second terminal of the secondary winding connected to a second input terminal of the corresponding rectification and filtering unit; and 
         [0012]    a current-balancing transformer is arranged between power supply loops where adjacent rectification and filtering units are in; a first winding of the current-balancing transformer is arranged between a first terminal of a secondary winding and a rectification and filtering unit in a first power supply loop; a second winding of the current-balancing transformer is arranged between a first terminal of a secondary winding and a rectification and filtering unit in a second power supply loop; currents in the same direction flow through a dotted terminal of the first winding of the current-balancing transformer and a non-dotted terminal of the second winding of the current-balancing transformer, and the current-balancing transformer is for current balancing between the power supply loops where the adjacent rectification and filtering units are in. 
         [0013]    The dotted terminal of the first winding of the current-balancing transformer is connected to the first terminal of the secondary winding in the first power supply loop, and a non-dotted terminal of the first winding of the current-balancing transformer is connected to a first input terminal of the rectification and filtering unit in the first power supply loop; and 
         [0014]    the non-dotted terminal of the second winding of the current-balancing transformer is connected to the first terminal of the secondary winding in the second power supply loop, and a dotted terminal of the second winding of the current-balancing transformer is connected to a first input terminal of the rectification and filtering unit in the second power supply loop. 
         [0015]    The main transformer is: 
         [0016]    a transformer including one primary winding and one secondary winding; or 
         [0017]    a transformer including one primary winding and at least two secondary windings; or 
         [0018]    a transformer including at least two primary windings and at least two secondary windings, where there is a one-to-one correspondence between the primary windings and the secondary windings. 
         [0019]    The rectification and filtering unit includes a first diode, a second diode, a first capacitor and a second capacitor, wherein 
         [0020]    the first diode and the first capacitor are connected in series sequentially between a first input terminal and a second input terminal of the rectification and filtering unit; the second diode and the second capacitor are connected in series sequentially between the first input terminal and the second input terminal of the rectification and filtering unit; and an anode of the first diode is connected to the first input terminal of the rectification and filtering unit, a cathode of the second diode is connected to the first input terminal of the rectification and filtering unit. 
         [0021]    The rectification and filtering unit includes a third diode, a fourth diode, a third capacitor and a fourth capacitor, wherein 
         [0022]    the third capacitor, the third diode and the fourth capacitor are connected in series sequentially between a first input terminal and a second input terminal of the rectification and filtering unit; and a cathode of the fourth diode is connected to an anode of the third diode, an anode of the fourth diode is connected to the second input terminal of the rectification and filtering unit. 
         [0023]    The rectification and filtering unit includes a fifth diode, a sixth diode, a first inductor, a second inductor and a fifth capacitor, wherein 
         [0024]    the fifth diode and the sixth diode are connected in series sequentially between a first input terminal and a second input terminal of the rectification and filtering unit, and an anode of the fifth diode is connected to an anode of the sixth diode; the first inductor and the second inductor are connected in series sequentially between the first input terminal and the second input terminal of the rectification and filtering unit, and a first terminal of the fifth capacitor is connected to the anode of the fifth diode, a second terminal of the fifth capacitor is connected to a point where the first inductor is connected to the second inductor. 
         [0025]    The rectification and filtering unit includes a seventh diode, a eighth diode, a ninth diode and a tenth diode, wherein 
         [0026]    the seventh diode and the eighth diode are connected in series between a first input terminal and a second input terminal of the rectification and filtering unit, and a cathode of the seventh diode is connected to a cathode of the eighth diode; the ninth diode and the tenth diode are connected in series between the first input terminal and the second input terminal of the rectification and filtering unit, and an anode of the ninth diode is connected to an anode of the tenth diode. 
         [0027]    The rectification and filtering unit includes an eleventh diode, a twelfth diode and a sixth capacitor, wherein 
         [0028]    the eleventh diode and the twelfth diode are connected in series sequentially between a first input terminal and a second input terminal of the rectification and filtering unit, and an anode of the eleventh diode is connected to an anode of the twelfth diode; the sixth capacitor is connected in series between the first input terminal of the rectification and filtering unit and the anode of the twelfth diode. 
         [0029]    The DC/AC converter is any one of a bridge circuit, a push-pull circuit, a flyback circuit, a forward circuit, a series resonant circuit, an LLC-type resonant circuit and a soft-switched circuit. 
         [0030]    According to an embodiment of the present invention, it is also provided a multipath constant-current driving circuit, including: a DC/AC converter and a main transformer, wherein the multipath constant-current driving circuit further includes a power supply loop of at least two stages that corresponds to each secondary winding of the main transformer, and wherein 
         [0031]    the DC/AC converter is adapted to provide an AC voltage for a primary winding of the main transformer; 
         [0032]    each of the stages of a power supply loop that corresponds to a secondary winding of the main transformer includes: a first terminal of the secondary winding connected to a second terminal of the secondary winding via second windings of all current-balancing transformers arranged in previous stages of the power supply loop, a first winding of a current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, and a rectification and filtering unit in the current stage of the power supply loop sequentially; and 
         [0033]    currents in the same direction flow through a dotted terminal of a first winding and a non-dotted terminal of a second winding of each of the current-balancing transformers, and the current-balancing transformer is for current balancing between two adjacent power supply loops. 
         [0034]    The rectification and filtering unit includes a first diode, a second diode, a first capacitor and a second capacitor, wherein 
         [0035]    the first diode and the first capacitor are connected in series sequentially between a first input terminal and a second input terminal of the rectification and filtering unit; the second diode and the second capacitor are connected in series sequentially between the first input terminal and the second input terminal of the rectification and filtering unit; and an anode of the first diode is connected to the first input terminal of the rectification and filtering unit, a cathode of the second diode is connected to the first input terminal of the rectification and filtering unit. 
         [0036]    The rectification and filtering unit includes a third diode, a fourth diode, a third capacitor and a fourth capacitor, wherein 
         [0037]    the third capacitor, the third diode and the fourth capacitor are connected in series sequentially between a first input terminal and a second input terminal of the rectification and filtering unit; and a cathode of the fourth diode is connected to an anode of the third diode, an anode of the fourth diode is connected to the second input terminal of the rectification and filtering unit. 
         [0038]    The rectification and filtering unit includes a fifth diode, a sixth diode, a first inductor, a second inductor and a fifth capacitor, wherein 
         [0039]    the fifth diode and the sixth diode are connected in series sequentially between a first input terminal and a second input terminal of the rectification and filtering unit, and an anode of the fifth diode is connected to an anode of the sixth diode; the first inductor and the second inductor are connected in series sequentially between the first input terminal and the second input terminal of the rectification and filtering unit, and a first terminal of the fifth capacitor is connected to the anode of the fifth diode, a second terminal of the fifth capacitor is connected to a point where the first inductor is connected to the second inductor. 
         [0040]    The rectification and filtering unit includes a seventh diode, a eighth diode, a ninth diode and a tenth diode, wherein 
         [0041]    the seventh diode and the eighth diode are connected in series between a first input terminal and a second input terminal of the rectification and filtering unit, and a cathode of the seventh diode is connected to a cathode of the eighth diode; the ninth diode and the tenth diode are connected in series between the first input terminal and the second input terminal of the rectification and filtering unit, and an anode of the ninth diode is connected to an anode of the tenth diode. 
         [0042]    The rectification and filtering unit includes an eleventh diode, a twelfth diode and a sixth capacitor, wherein 
         [0043]    the eleventh diode and the twelfth diode are connected in series sequentially between a first input terminal and a second input terminal of the rectification and filtering unit, and an anode of the eleventh diode is connected to an anode of the twelfth diode; the sixth capacitor is connected in series between the first input terminal of the rectification and filtering unit and the anode of the twelfth diode. 
         [0044]    The main transformer is: 
         [0045]    a transformer including one primary winding and one secondary winding; or 
         [0046]    a transformer including one primary winding and at least two secondary windings; or 
         [0047]    a transformer including at least two primary windings and at least two secondary windings, where there is a one-to-one correspondence between the primary windings and the secondary windings. 
         [0048]    The DC/AC converter is any one of a bridge circuit, a push-pull circuit, a flyback circuit, a forward circuit, a series resonant circuit, an LLC-type resonant circuit and a soft-switched circuit. 
         [0049]    According to an embodiment of the present invention, it is also provided a multipath constant-current driving circuit, including: a DC/AC converter, a main transformer and at least two power supply branch groups, wherein 
         [0050]    the DC/AC converter is adapted to provide an AC voltage for a primary winding of the main transformer; 
         [0051]    the main transformer includes at least one secondary winding; 
         [0052]    each of the power supply branch groups forms a main power supply loop with the secondary winding of the main transformer; and 
         [0053]    a current-balancing transformer is arranged between two adjacent main power supply loops; a first winding of the current-balancing transformer is arranged in one of the two main power supply loops, and a second winding of the current-balancing transformer is arranged in the other one of the two main power supply loops, for current balancing between the two main power supply loops. 
         [0054]    At least one of the at least two main power supply loops includes: 
         [0055]    at least two power supply loops each of which is formed by a rectification and filtering unit and a corresponding secondary winding, and each of the power supply loops includes: a first terminal of a secondary winding of the main transformer connected to a first input terminal of a corresponding rectification and filtering unit, and a second terminal of the secondary winding connected to a second input terminal of the corresponding rectification and filtering unit; and, a current-balancing transformer is arranged between power supply loops where two adjacent rectification and filtering units are in, a first winding of the current-balancing transformer is arranged between a first terminal of a secondary winding and a rectification and filtering unit in a first power supply loop, a second winding of the current-balancing transformer is arranged between a first terminal of a secondary winding and a rectification and filtering unit in a second power supply loop; currents in the same direction flow through a dotted terminal of the first winding of the current-balancing transformer and a non-dotted terminal of the second winding of the current-balancing transformer, and the current-balancing transformer is for current balancing between the power supply loops where the adjacent rectification and filtering units are in. 
         [0056]    At least one of the at least two main power supply loops includes: 
         [0057]    a power supply loop of at least two stages, and each of the stages of the power supply loops includes: a first terminal of a corresponding secondary winding of the main transformer connected to a second terminal of the secondary winding via second windings of all current-balancing transformers arranged in previous stages of the power supply loop, a first winding of a current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, and a rectification and filtering unit in the current stage of the power supply loop sequentially; currents in the same direction flow through a dotted terminal of a first winding and a non-dotted terminal of a second winding of each of the current-balancing transformers, and the current-balancing transformer is for current balancing between two adjacent power supply loops. 
         [0058]    Technical effects of the technical solutions above are discussed below. 
         [0059]    In a multipath constant-current driving circuit as described above, each of the first winding and the second winding of the current-balancing transformer is connected in series in an AC power supply loop; hence there is no DC current. Therefore, even if the difference between the voltages across respective loads is large, good current balancing can be provided. Moreover, the current-balancing transformer dose not need to be provided with air gaps, therefore the transformer can be made at a small size, which further lowers the cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0060]      FIG. 1  is a structural diagram illustrating an LED constant-current control circuit in the prior art; 
           [0061]      FIG. 2  illustrates a balanced-current power supply circuit for multiple groups of LEDs in the prior art; 
           [0062]      FIGS. 3 and 3   a  are structural diagrams illustrating a multipath constant-current driving circuit according to the present invention; 
           [0063]      FIGS. 4 and 4   a  are structural diagrams illustrating a second multipath constant-current driving circuit according to the present invention; 
           [0064]      FIGS. 5 and 5   a  are structural diagrams illustrating a third multipath constant-current driving circuit according to the present invention; 
           [0065]      FIGS. 6   a  and  6   a  are structural diagrams illustrating a fourth multipath constant-current driving circuit according to the present invention; 
           [0066]      FIGS. 7 and 7   a  are structural diagrams illustrating a fifth multipath constant-current driving circuit according to the present invention; 
           [0067]      FIGS. 8   a  to  9   e  are structural diagrams illustrating a multipath constant-current driving circuit according to the present invention with different main transformer structures; 
           [0068]      FIGS. 10   a  to  10   d  are structural diagrams illustrating a sixth multipath constant-current driving circuit according to the present invention with different rectification and filtering unit structures; 
           [0069]      FIGS. 11   a  to  11   i  are structural diagrams illustrating a sixth multipath constant-current driving circuit according to the present invention; and 
           [0070]      FIGS. 12   a  to  12   f  are structural diagrams illustrating a multipath constant-current driving circuit according to the present invention with different DC/AC converter structures. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0071]    At first, the operating principle of a current-balancing transformer is described below. 
         [0072]    Assuming the currents required by loads A 1  and A 2  have I 1 :I 2 =1:1, the turns ratio of the current-balancing transformer for current balancing between the loads A 1  and A 2  is W 1 :W 2 =1:1. 
         [0073]    If the voltage Uo 1  across the load A 1  and the voltage Uo 2  across the load A 2  are not equal, the currents I 1  and  12  that flow through respective loads are not equal, and the currents iw 1  and iw 2  that flow through respective windings of the current-balancing transformer are not equal; hence the magnetizing current in the current-balancing transformer is not zero. The magnetizing current generates an AC voltage across the current-balancing transformer, which automatically balances the difference between the voltages Uo 1  and Uo 2  across respective loads, making the currents iw 1  and iw 2  in respective windings of the current-balancing transformer equal and the load currents I 1  and  12  equal. 
         [0074]    Assuming the currents required by the loads A 1  and A 2  have I 1 :I 2 =m:n, the turns ratio of the current-balancing transformer for current balancing between the loads A 1  and A 2  is W 1 :W 2 =n:m, then, 
         [0075]    if the ratio between the currents iw 1  and iw 2  that flow through respective windings of the current-balancing transformer is not m:n, the magnetizing current in the current-balancing transformer is not zero. The magnetizing current generates an AC voltage across the current-balancing transformer, which automatically balances the difference between the voltages across respective loads, balancing the ratio between the currents I 1  and I 2  in respective windings of the current-balancing transformer into m:n, and realizing balancing between the load currents. 
         [0076]    Implementation of the multipath constant-current driving circuits according to the embodiments of the present invention will be described hereinafter in conjunction with the accompanying drawings. 
         [0077]    A multipath constant-current driving circuit according to an embodiment of the present invention includes: a DC/AC converter, a main transformer and at least two rectification and filtering units. 
         [0078]    The DC/AC converter is adapted to provide an AC voltage for a primary winding of the main transformer. 
         [0079]    Each of the at least two rectification and filtering units forms a power supply loop with a secondary winding of the main transformer. Each of the power supply loops includes: a first terminal of the secondary winding connected to a first input terminal of a corresponding rectification and filtering unit, and a second terminal of the secondary winding connected to a second input terminal of the corresponding rectification and filtering unit. 
         [0080]    A current-balancing transformer is arranged between power supply loops where adjacent rectification and filtering units are in. A first winding of the current-balancing transformer is arranged between a first terminal of a secondary winding and a rectification and filtering unit in a first power supply loop; a second winding of the current-balancing transformer is arranged between a first terminal of a secondary winding and a rectification and filtering unit in a second power supply loop. Currents in the same direction flow through a dotted terminal of the first winding of the current-balancing transformer and a non-dotted terminal of the second winding of the current-balancing transformer, and the current-balancing transformer is for current balancing between the power supply loops where the adjacent rectification and filtering units are in. 
         [0081]    The DC/AC converter may be any one of a bridge circuit, a push-pull circuit, a flyback circuit, a forward circuit, a series resonant circuit, an LLC-type resonant circuit and a soft-switched circuit. 
         [0082]    The main transformer may be: a transformer including one primary winding and one secondary winding; or a transformer including one primary winding and at least two secondary windings; or a transformer including at least two primary windings and at least two secondary windings; or the like. 
         [0083]    The rectification and filtering unit is not limited to a specific implementation. 
         [0084]    The multipath constant-current driving circuit above supplies electric power to a load, e.g., an LED load, via a power supply loop. 
         [0085]    In a multipath constant-current driving circuit above, e.g., the multipath constant-current driving circuit shown in  FIG. 5 , in comparison with the prior art, each of the first winding and the second winding of the current-balancing transformer is connected in series in an AC power supply loop; hence there is no DC current. Therefore, even if the difference between the voltages across respective loads is large, good current balancing can be provided. Moreover, the current-balancing transformer dose not need to be provided with air gaps, therefore the transformer can be made at a small size, which further lowers the cost. 
         [0086]    Implementation of the multipath constant-current driving circuits according to the embodiments of the present invention will be described hereinafter in detail in conjunction with  FIGS. 3 to 9   e.    
         [0087]      FIG. 3  is an implementation structure of a multipath constant-current driving circuit according to an embodiment of the present invention. As shown in  FIG. 3 , the driving circuit includes: a DC/AC converter, a main transformer Ta 3 , n−1 current-balancing transformers T 31  to T 3 ( n− 1), n rectification and filtering units Z 31  to Z 3   n , and  n  load branches A 31  to A 3   n.    
         [0088]    Specifically, the main transformer Ta 1  includes one primary winding and one secondary winding. The primary winding is connected to the output terminal of the DC/AC converter, to receive an AC voltage output by the DC/AC converter. 
         [0089]    A first terminal of the secondary winding of the main transformer is connected to a first input terminal t 1  of each rectification and filtering unit, and a second terminal of the secondary winding of the main transformer is connected to a second input terminal t 2  of each rectification and filtering unit, so that the secondary winding forms a power supply loop with each rectification and filtering unit. 
         [0090]    A current-balancing transformer is arranged between power supply loops where two adjacent rectification and filtering units are in. For example, a current-balancing transformer T 31  is arranged between the rectification and filtering unit Z 31  and the rectification and filtering unit Z 32 . The dotted terminal of a first winding of the current-balancing transformer T 31  is connected to the first terminal of the secondary winding, the non-dotted terminal of the first winding of the current-balancing transformer T 31  is connected to a first input terminal t 1  of the rectification and filtering unit Z 31 . The non-dotted terminal of a second winding of the current-balancing transformer T 31  is connected to the first terminal of the secondary winding, the dotted terminal of the second winding of the current-balancing transformer T 31  is connected to the first input terminal t 1  of the rectification and filtering unit Z 32 . Then, currents in the same direction flow through the dotted terminal of the first winding and the non-dotted terminal of the second winding of the current-balancing transformer T 31 ; and currents in opposite directions flow through the dotted terminal of the first winding and the dotted terminal of the second winding of the current-balancing transformer T 31 , thereby realizing current balancing between the two power supply loops where the rectification and filtering unit Z 31  and the rectification and filtering unit Z 32  are in. 
         [0091]    In  FIG. 3 , each rectification and filtering unit includes: a first diode D 1 , a second diode D 2 , a first capacitor C 1  and a second capacitor C 2 . Specifically, the first diode D 1  and the first capacitor C 1  are connected in series sequentially between the first input terminal t 1  and the second input terminal t 2  of the rectification and filtering unit; the second diode D 2  and the second capacitor C 2  are connected in series sequentially between the first input terminal t 1  and the second input terminal t 2  of the rectification and filtering unit. The anode of the first diode D 1  is connected to the first input terminal t 1 , and the cathode of the second diode D 2  is connected to the first input terminal t 1 . In the rectification and filtering unit, the cathode of the first diode D 1  and the anode of the second diode D 2  are used as the first output terminal O 1  and the second output terminal O 2  of the rectification and filtering unit respectively, each of which is connected to a corresponding load branch and supplies electric power to the corresponding load branch. 
         [0092]    As shown in  FIG. 3   a , the minimum number of branches of the multipath constant-current driving circuit above may be 2. In this case, the multipath constant-current driving circuit shown in  FIG. 3  has a structure shown in  FIG. 3   a , including only two rectification and filtering units and two load branches. The detailed connections are similar to  FIG. 3 , which are therefore omitted here. 
         [0093]    Furthermore, the rectification and filtering unit shown in  FIG. 3  may be implemented with another structure, for example, 
         [0094]    in a multipath constant-current driving circuit shown in  FIG. 4 , most components have a structure the same as  FIG. 3 , except for the rectification and filtering unit, which includes: a third diode D 3 , a fourth diode D 4 , a third capacitor C 3  and a fourth capacitor C 4 . Specifically, the third capacitor C 3 , the third diode D 3  and the fourth capacitor C 4  are connected in series sequentially between the first input terminal t 1  and the second input terminal t 2  of the rectification and filtering unit. The cathode of the fourth diode D 4  is connected to the anode of the third diode D 3 , and the anode of the fourth diode D 4  is connected to the second input terminal t 2  of the rectification and filtering unit. In the rectification and filtering unit, the cathode of the third diode D 3  and the anode of the fourth diode D 4  are used as the first output terminal O 1  and the second output terminal O 2  of the rectification and filtering unit respectively, each of which is connected to a corresponding load branch and supplies electric power to the corresponding load branch. 
         [0095]    When the number of branches in the multipath constant-current driving circuit shown in  FIG. 4  is 2, the multipath constant-current driving circuit may have a structure shown in  FIG. 4   a , which is omitted here in the interest of clarity. 
         [0096]    Alternatively, in a multipath constant-current driving circuit shown in  FIG. 5 , most components have a structure the same as  FIG. 3 , except for the rectification and filtering unit, which includes: a fifth diode D 5 , a sixth diode D 6 , a first inductor L 1 , a second inductor L 2  and a fifth capacitor C 5 . Specifically, the fifth diode D 5  and the sixth diode D 6  are connected in series sequentially between the first input terminal t 1  and the second input terminal t 2  of the rectification and filtering unit, and the anode of the fifth diode D 5  is connected to the anode of the sixth diode D 6 . The first inductor L 1  and the second inductor L 2  are connected in series sequentially between the first input terminal t 1  and the second input terminal t 2  of the rectification and filtering unit. The first terminal of the fifth capacitor C 5  is connected to the anode of the fifth diode D 5 , and the second terminal of the fifth capacitor C 5  is connected to the point where the first inductor L 1  is connected to the second inductor L 2 . In the rectification and filtering unit, the anode of the fifth diode D 5  and the point where the first inductor L 1  is connected to the second inductor L 2  are used as the first output terminal O 1  and the second output terminal O 2  of the rectification and filtering unit respectively, each of which is connected to a corresponding load branch and supplies electric power to the corresponding load branch. 
         [0097]    When the number of branches in the multipath constant-current driving circuit shown in  FIG. 5  is 2, the multipath constant-current driving circuit may have a structure shown in  FIG. 5   a , which is omitted here in the interest of clarity. 
         [0098]    Alternatively, in a multipath constant-current driving circuit shown in  FIG. 6 , most components have a structure the same as  FIG. 3 , except for the rectification and filtering unit, which includes: a seventh diode D 7 , a eighth diode D 8 , a ninth diode D 9  and a tenth diode D 10 . Specifically, the seventh diode D 7  and the eighth diode D 8  are connected in series between the first input terminal t 1  and the second input terminal t 2  of the rectification and filtering unit, and the cathode of the seventh diode D 7  is connected to the cathode of the eighth diode D 8 . The ninth diode D 9  and the tenth diode D 10  are connected in series between the first input terminal t 1  and the second input terminal t 2  of the rectification and filtering unit, and the anode of the ninth diode D 9  is connected to the anode of the tenth diode D 10 . In the rectification and filtering unit, the cathode of the seventh diode D 7  and the anode of the ninth diode D 9  are used as the first output terminal O 1  and the second output terminal O 2  of the rectification and filtering unit respectively, each of which is connected to a corresponding load branch and supplies electric power to the corresponding load branch. 
         [0099]    When the number of branches in the multipath constant-current driving circuit shown in  FIG. 6  is 2, the multipath constant-current driving circuit may have a structure shown in  FIG. 6   a , which is omitted here in the interest of clarity. 
         [0100]    Alternatively, in a multipath constant-current driving circuit shown in  FIG. 7 , most components have a structure the same as  FIG. 3 , except for the rectification and filtering unit, which includes: a eleventh diode D 11 , a twelfth diode D 12  and a sixth capacitor C 6 . Specifically, the eleventh diode D 11  and the twelfth diode D 12  are connected in series sequentially between the first input terminal t 1  and the second input terminal t 2  of the rectification and filtering unit, and the anode of the eleventh diode D 11  is connected to the anode of the twelfth diode D 12 . The sixth capacitor C 6  is connected in series between the first input terminal t 1  of the rectification and filtering unit and the anode of the twelfth diode D 12 . In the rectification and filtering unit, the cathode of the eleventh diode D 11  and the anode of the eleventh diode D 11  are used as the first output terminal O 1  and the second output terminal O 2  of the rectification and filtering unit respectively, each of which is connected to a corresponding load branch and supplies electric power to the corresponding load branch. 
         [0101]    When the number of branches in the multipath constant-current driving circuit shown in  FIG. 7  is 2, the multipath constant-current driving circuit may have a structure shown in  FIG. 7   a , which is omitted here in the interest of clarity. 
         [0102]    Moreover, in the multipath constant-current driving circuits shown in  FIGS. 3 to 7   a , the main transformer includes one primary winding and one secondary winding. However, in practice, 
         [0103]    the main transformer may include one primary winding and at least two secondary windings. Preferably, the number of secondary windings is the same as that of rectification and filtering units or load branches, where there is a one-to-one-to-one correspondence between the secondary windings, the rectification and filtering units and the load branches.  FIGS. 8   a  to  8   e  illustrate the multipath constant-current driving circuits shown in  FIGS. 3 to 7  with their main transformers replaced with a transformer including one primary winding and at least two secondary windings. In this case, the first terminal of each of the secondary windings is connected to the first input terminal of a rectification and filtering unit corresponding to the secondary winding, and the second terminal of the secondary winding is connected to the second input terminal of the rectification and filtering unit corresponding to the secondary winding, so that a power supply loop is formed. 
         [0104]    Alternatively, the main transformer may include at least two primary windings and at least two secondary windings. Preferably, there is a one-to-one-to-one-to-one correspondence between the primary windings, the secondary windings, the rectification and filtering units and the load branches.  FIGS. 9   a  to  9   e  illustrate the multipath constant-current driving circuits shown in  FIGS. 3 to 7  with their main transformers replaced with a transformer including at least two primary windings and at least two secondary windings. In this case, the first terminal of each of the secondary windings is connected to the first input terminal of a rectification and filtering unit corresponding to the secondary winding, and the second terminal of the secondary winding is connected to the second input terminal of the rectification and filtering unit corresponding to the secondary winding, so that a power supply loop is formed. 
         [0105]    According to an embodiment of the present invention, it is also provided a multipath constant-current driving circuit. Specifically, the multipath constant-current driving circuit includes a DC/AC converter and a main transformer, and further includes a power supply loop of at least two stages that corresponds to each secondary winding of the main transformer. 
         [0106]    The DC/AC converter is adapted to provide an AC voltage for a primary winding of the main transformer. 
         [0107]    Each of the stages of a power supply loop that corresponds to a secondary winding of the main transformer includes: a first terminal of the secondary winding connected to a second terminal of the secondary winding via second windings of all current-balancing transformers arranged in previous stages of the power supply loop, a first winding of a current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, and a rectification and filtering unit in the current stage of the power supply loop sequentially. 
         [0108]    Currents in the same direction flow through a dotted terminal of a first winding and a non-dotted terminal of a second winding of each of the current-balancing transformers, and the current-balancing transformer is for current balancing between two adjacent power supply loops. 
         [0109]    Specific examples are given below. 
         [0110]    As shown in  FIGS. 10   a  to  10   d , a current-balancing transformer is arranged between two adjacent stages of the power supply loop. In this specification, the current-balancing transformer between a stage and the next stage of a power supply loop is seen as a current-balancing transformer corresponding to both stages of the power supply loop. Each stage of the power supply loop includes not only a first winding of the current-balancing transformer corresponding to both the current stage and the next stage of the power supply loop, but also second windings of current-balancing transformers arranged in all previous stages of the power supply loop. Specifically, for example, the second stage of the power supply loop includes: the secondary winding of the main transformer Ta 3 , the second winding of the current-balancing transformer T 111 , the first winding of the current-balancing transformer T 112 , and the rectification and filtering unit corresponding to the stage of the power supply loop. Specifically, the first terminal of the secondary winding of the main transformer Ta 3  is connected to the second terminal of the secondary winding via the second winding of the current-balancing transformer T 111 , the first winding of the current-balancing transformer T 112 , and the first input terminal and the second input terminal of the rectification and filtering unit Z 112  sequentially, so that the second stage of the power supply loop is formed. The third to the Nth stages of the power supply loop are similar. For example, the (N−1)th stage of the power supply loop includes: the first terminal of the secondary winding connected to the second terminal of the secondary winding via the second winding of the current-balancing transformer T 111 , the second winding of the current-balancing transformer T 112  . . . , the second winding of the current-balancing transformer T 11 (N−2), the first winding of the current-balancing transformer T 11  (N−1), and the rectification and filtering unit Z 11 (N−1) sequentially. The Nth stage of the power supply loop includes: the first terminal of the secondary winding connected to the second terminal of the secondary winding via the second winding of the current-balancing transformer T 111 , the second winding of the current-balancing transformer T 112  . . . , the second winding of the current-balancing transformer T 11 (N−1), and the rectification and filtering unit Z 11 N sequentially (there is no current-balancing transformer T 11 N because the Nth stage of the power supply loop is the last one). 
         [0111]    The differences between  FIGS. 10   a  to  10   d  lie in the structure of the rectification and filtering unit, which is illustrated in  FIGS. 2 to 9   e  and therefore omitted here in the interest of clarity. 
         [0112]    Moreover, in the multipath constant-current driving circuits shown in  FIGS. 10   a  to  10   d , the main transformer includes one primary winding and one secondary winding. However, in practice, the main transformer may be: a transformer including one primary winding and at least two secondary windings; or a transformer including at least two primary windings and at least two secondary windings, where there is a one-to-one correspondence between the primary windings and the secondary windings. In this case, each of the secondary windings may correspond to a power supply loop of multiply stages, to form a constant-current driving circuit. In addition, different rectification and filtering units may be used in power supply loops corresponding to different secondary windings, for rectification and filtering. The present invention is not limited to any specific implementation. 
         [0113]    Furthermore, in practice, the power supply loop of multiple stages may be implemented in such a way that: each of some (at least one) of all the secondary windings corresponds to a power supply loop of at least two stages, and the other secondary windings form a power supply loop to supply electric power to the loads and realize current balancing using a circuit structure in the prior art or any of the various power supply loops described in conjunction with  FIGS. 3 to 9   e . The present invention is not limited to any specific implementation. 
         [0114]    For example, the circuit structures shown in  FIGS. 3 to 10   a  may be combined in the same multipath constant-current driving circuit. 
         [0115]    For the driving circuit shown in  FIG. 3 , all the power supply branches (the number of which is set to be M, where M&gt;=2) that are connected to the secondary winding and are connected in parallel to each other to form the constant-current driving circuit can be seen as a first power supply branch group; and for the driving circuit shown in  FIG. 10   a , all the power supply branches that are connected to the secondary winding to form the multi-stage constant-current driving circuit can be seen as a second power supply branch group. 
         [0116]    Then, as shown in  FIG. 11   a , the main transformer includes one primary winding and one secondary winding. The input terminals of each of the first power supply branch group and the second power supply branch group are connected to the two terminals of the secondary winding of the main transformer. That is, the first power supply branch group and the second power supply branch group are connected to the same secondary winding in parallel. The first power supply branch group and the secondary winding form a first main power supply loop, and the second power supply branch group and the secondary winding form a second main power supply loop. In this case, in order to balance the total current Im in the first main power supply loop and the total current In in the second main power supply loop, a current-balancing transformer may be arranged between the two main power supply loops. Specifically, a first winding of the current-balancing transformer is arranged between the secondary winding and the input terminals of the first power supply branch group in the first main power supply loop, and a second winding of the current-balancing transformer is arranged between the secondary winding and the input terminals of the second power supply branch group in the second main power supply loop. 
         [0117]    Correspondingly, such a circuit can be extended to various cases where the multipath constant-current driving circuit according to the present invention includes at least one first power supply branch group and at least one second power supply branch group, or where the multipath constant-current driving circuit according to the present invention includes at least two first power supply branch groups and at least two second power supply branch groups. For example,  FIGS. 11   b  to  11   c . Moreover, the circuit shown in  FIG. 11   a  may be extended to the case where the main transformer includes one primary winding and multiple secondary windings, or where the main transformer includes multiple primary windings and multiple secondary windings, as shown in  FIGS. 11   d  to  11   i.    
         [0118]    Preferably, the DC/AC converter in the multipath constant-current driving circuit above may be implemented as any one of a bridge circuit, a push-pull circuit, a flyback circuit, a forward circuit, a series resonant circuit, an LLC-type resonant circuit and a soft-switched circuit. 
         [0119]    For example, as shown in  FIGS. 12   a - 12   f , the DC/AC converters in respective multipath constant-current driving circuits are a bridge circuit, a push-pull circuit, a flyback circuit, a forward circuit, a series resonant circuit, an LLC-type resonant circuit and a soft-switched circuit, respectively. 
         [0120]    Preferred embodiments of the present invention are described above for illustrative purposes only. It should be noted that modifications and alternations may be made by those skilled in the art without deviation from the scope of the present invention. These modifications and alternations shall fall within the scope of protection of the present invention.