Abstract:
A multi-path constant current drive circuit includes a DC/AC converter, a main transformer and at least two rectifying and filtering units. The main transformer includes at least one assistant side winding with a tap; together with the assistant side winding of the main transformer, each of the at least two rectifying and filtering units respectively forms a power supply loop; each power supply loop includes a first rectifying loop and a second rectifying loop, which are relatively used for the rectification of the positive and the negative half-cycle alternating voltage; a current-equalizing transformer is arranged between the adjacent first power supply loop and second power supply loop, the windings of the current-equalizing transformer are respectively in the rectifying loops contained in the first power supply loop and the second power supply loop, thus realizing the current equalization between the different rectifying loops in which the adjacent rectifying and filtering units are contained. The multi-path constant current drive circuit performs a good current equalization, and can reduce the volume of the current-equalizing transformer and decrease the cost.

Description:
This application is a National Stage application of PCT international application PCT/CN2010/078714 filed on Nov. 15, 2010 and titled “MULTI-PATH CONSTANT CURRENT DRIVING CIRCUIT”, which claims the benefit of Chinese patent application No. 200910225966.4 filed on Nov. 21, 2009, and claims the benefit of Chinese patent application No. 200920273352.9 filed on Nov. 21, 2009. Both the PCT international application and the Chinese applications are incorporated herein by reference in their entireties. 
     FIELD OF THE INVENTION 
     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 
     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. 
       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. 
     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 
     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. 
     Therefore, the embodiments of the present invention provide the following technical solutions. 
     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, 
     the DC/AC converter is adapted to provide an alternating current (AC) voltage for the main transformer; 
     the main transformer includes at least one secondary winding with a tap, the tap divides the corresponding secondary winding into a first winding and a second winding, with a non-dotted terminal of the first winding being connected to a dotted terminal of the second winding; 
     each of the at least two rectification and filtering units forms a power supply loop with the secondary winding of the main transformer; each of the power supply loops includes a first rectification loop and a second rectification loop for rectification of the positive and the negative halves of an AC voltage, respectively; and the first winding and the second winding are in the first rectification loop and the second rectification loop, respectively; and 
     a current-balancing transformer is arranged between a first power supply loop and a second power supply loop where adjacent rectification and filtering units are in; and the current-balancing transformer includes four windings in respective rectification loops included by the first power supply loop and the second power supply loop, for current balancing between the different rectification loops where the adjacent rectification and filtering units are in. 
     Of the four windings of the current-balancing transformer, currents in opposite directions may flow through a dotted terminal of a first winding and a dotted terminal of a second winding, currents in opposite directions may flow through the dotted terminal of the first winding and a dotted terminal of a third winding, and currents in opposite directions may flow through the dotted terminal of the third winding and a dotted terminal of the fourth winding. 
     The rectification and filtering unit may include a first diode, a second diode and a first capacitor; and 
     the power supply loop may include: a dotted terminal of the first winding of the secondary winding connected to the non-dotted terminal of the first winding of the secondary winding via the first diode and the first capacitor sequentially connected in series, with an anode of the first diode being connected to the dotted terminal of the first winding of the secondary winding; and the dotted terminal of the second winding of the secondary winding connected to a non-dotted terminal of the second winding of the secondary winding via the first capacitor and the second diode sequentially connected in series, with an anode of the second diode being connected to the non-dotted terminal of the second winding of the secondary winding. 
     In a power supply loop where a first rectification and filtering unit is in, a first winding of the current-balancing transformer may be connected in series between the dotted terminal of the first winding of the secondary winding and the first capacitor; and a second winding of the current-balancing transformer may be connected in series between the non-dotted terminal of the second winding of the secondary winding and the first capacitor; and 
     in a power supply loop where a second rectification and filtering unit is in, a third winding of the current-balancing transformer may be connected in series between the dotted terminal of the first winding of the secondary winding and the first capacitor; and a fourth winding of the current-balancing transformer may be connected in series between the non-dotted terminal of the second winding of the secondary winding and the first capacitor. 
     The rectification and filtering circuit may include: a third diode, a fourth diode, a first inductor and a second capacitor; and 
     the power supply loop may include: a dotted terminal of the first winding of the secondary winding connected to the non-dotted terminal of the first winding of the secondary winding via the third diode, the first inductor and the second capacitor sequentially connected in series, with an anode of the third diode being connected to the dotted terminal of the first winding of the secondary winding; and the dotted terminal of the second winding of the secondary winding connected to a non-dotted terminal of the second winding of the secondary winding via the second capacitor, the first inductor and the fourth diode sequentially connected in series, with an anode of the fourth diode being connected to the non-dotted terminal of the second winding of the secondary winding. 
     In a power supply loop where a first rectification and filtering unit is in, a first winding of the current-balancing transformer may be connected in series between the dotted terminal of the first winding of the secondary winding and the first inductor; and a second winding of the current-balancing transformer may be connected in series between the non-dotted terminal of the second winding of the secondary winding and the first inductor; and 
     in a power supply loop where the second rectification and filtering unit is in, a third winding of the current-balancing transformer may be connected in series between the dotted terminal of the first winding of the secondary winding and the first inductor; and a fourth winding of the current-balancing transformer may be connected in series between the non-dotted terminal of the second winding of the secondary winding and the first inductor. 
     The main transformer may include one primary winding and one secondary winding with a tap; and 
     the secondary winding with the tap forms a power supply loop with each of the rectification and filtering units. 
     The main transformer may include one primary winding and at least two secondary windings with a tap, each of the secondary windings corresponding to a rectification and filtering unit; and 
     each of the secondary windings forms a power supply loop with a rectification and filtering unit that corresponds to the secondary winding. 
     The main transformer may include at least two primary windings and at least two secondary windings with a tap, where there is a one-to-one-to-one correspondence between the primary windings, the secondary windings and rectification and filtering units; and 
     each of the secondary windings forms a power supply loop with a rectification and filtering unit that corresponds to the secondary winding. 
     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. 
     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, 
     the DC/AC converter is adapted to provide an AC voltage for the main transformer; 
     the main transformer includes at least one secondary winding with a tap, the tap divides the corresponding secondary winding into a first winding and a second winding, with a non-dotted terminal of the first winding being connected to a dotted terminal of the second winding; and at least one of the at least one secondary winding with a tap corresponds to a power supply loop of at least two stages; 
     each of the stages of the power supply loop corresponding to the secondary winding includes a first rectification loop and a second rectification loop; the first rectification loop includes: a first terminal of the first winding of the secondary winding connected to a second terminal of the first winding of the secondary winding via a rectification and filtering unit, third windings of all current-balancing transformers arranged in previous stages of the power supply loop, and 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 the second rectification loop includes: a first terminal of the second winding of the secondary winding connected to a second terminal of the first winding of the secondary winding via a second winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, fourth windings of all the current-balancing transformers arranged in the previous stages of the power supply loop, and a rectification and filtering unit; and 
     the current-balancing transformer includes four windings, for current balancing between different rectification loops where adjacent rectification and filtering units are in. 
     Of the four windings of the current-balancing transformer, currents in opposite directions may flow through a dotted terminal of the first winding and a dotted terminal of the second winding, currents in opposite directions may flow through the dotted terminals of the first winding and a dotted terminal of the third winding, and currents in opposite directions may flow through the dotted terminals of the third winding and a dotted terminal of the fourth winding. 
     The rectification and filtering unit may include a first diode, a second diode and a first capacitor. 
     The first rectification loop may include: the first terminal of the first winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the third windings of all the current-balancing transformers arranged in the previous stages of the power supply loop, the first diode, the first winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, and the first capacitor sequentially; and 
     the second rectification loop may include: the first terminal of the second winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first capacitor, the second winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, the second diode, and the fourth windings of all the current-balancing transformers arranged in the previous stages of the power supply loop sequentially. 
     The first rectification loop may include: the first terminal of the first winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the third windings of all the current-balancing transformers arranged in the previous stages of the power supply loop, the first winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, the first diode, and the first capacitor sequentially; and 
     the second rectification loop may include: the first terminal of the second winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first capacitor, the second diode, the second winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, and the fourth windings of all the current-balancing transformers arranged in the previous stages of the power supply loop sequentially. 
     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, where there is a one-to-one correspondence between the primary windings and the secondary windings. 
     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. 
     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, 
     the DC/AC converter is adapted to provide an AC voltage for the main transformer; 
     the main transformer includes at least one secondary winding with a tap, the tap divides the corresponding secondary winding into a first winding and a second winding, with a non-dotted terminal of the first winding being connected to a dotted terminal of the second winding; and the secondary winding of the main transformer is connected to at least two power supply branch groups, to form respective main power supply loops; 
     a current-balancing transformer is arranged between two adjacent main power supply loops, with a first winding and a second winding of the current-balancing transformer being arranged in one of the two main power supply loops, and a third winding and a fourth winding of the current-balancing transformer being arranged in the other one of the two main power supply loops, for current balancing between the two main power supply loops. 
     At least one of the two main power supply loops may include at least a power supply loop of at least two stages; each of the stages of the power supply loop may include a first rectification loop and a second rectification loop; the first rectification loop may include: a first terminal of the first winding of the corresponding secondary winding connected to a second terminal of the first winding of the secondary winding via a rectification and filtering unit, third windings of all current-balancing transformers arranged in previous stages of the power supply loop, and a first winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop; the second rectification loop may include: a first terminal of the second winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via a second winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, fourth windings of all current-balancing transformers arranged in the previous stages of the power supply loop, and a rectification and filtering unit; and the current-balancing transformer may include four windings, for current balancing between different rectification loops where adjacent rectification and filtering units are in. 
     At least one of the two main power supply loops may include at least two power supply loops; each of the power supply loops may include a rectification and filtering unit and a corresponding secondary winding of the main transformer, forming a first rectification loop and a second rectification loop; the first winding and the second winding of the corresponding secondary winding may be in the first rectification loop and the second rectification loop, respectively; the first rectification loop and the second rectification loop are for rectification of the positive and the negative halves of an AC voltage; a current-balancing transformer may be arranged between a first power supply loop and a second power supply loop where adjacent rectification and filtering units are in; and the current-balancing transformer may include four windings in respective rectification loops included by the first power supply loop and the second power supply loop, for current balancing between the different rectification loops where the adjacent rectification and filtering units are in. 
     Technical effects of the technical solutions above are discussed below. 
     The first winding and the second winding (or, the third winding and the fourth winding) of the current-balancing transformer are in two different rectification loops for the positive and the negative halves of the AC voltage respectively, with currents in opposite directions flowing through their dotted terminals; hence it is equivalently that a bidirectional AC current flows through the windings of the current-balancing transformer. Therefore there is no DC current in the current-balancing transformer, which eliminates the need for air gaps in the current-balancing transformer, allows large inductance at a small size, provides good current balancing, and lowers the cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a structural diagram illustrating an LED constant-current control circuit in the prior art; 
         FIG. 2  illustrates a balanced-current power supply circuit for multiple groups of LEDs in the prior art; 
         FIGS. 3 to 3   b  are structural diagrams illustrating a two-path constant-current driving circuit according to the present invention; 
         FIGS. 4 to 4   b  are structural diagrams illustrating another two-path constant-current driving circuit according to the present invention; 
         FIGS. 5 to 5   b  are structural diagrams illustrating a third type of two-path constant-current driving circuit according to the present invention; 
         FIGS. 6   a  to  6   e  are structural diagrams illustrating a two-path constant-current driving circuit with different DC/AC converters according to the present invention; 
         FIGS. 7   a  to  7   c  are structural diagrams illustrating a multipath constant-current driving circuit according to the present invention; 
         FIG. 8   a  is a structural diagram illustrating a fourth type of multipath constant-current driving circuit according to the present invention; and 
         FIGS. 9   a  to  9   e  are structural diagrams illustrating a fifth type of multipath constant-current driving circuit according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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, 
     the DC/AC converter is adapted to provide an alternating current (AC) voltage for the main transformer; the main transformer includes at least one secondary winding with a tap, the tap divides the corresponding secondary winding into a first winding and a second winding, with a non-dotted terminal of the first winding being connected to a dotted terminal of the second winding; 
     each of the at least two rectification and filtering units forms a power supply loop with the secondary winding of the main transformer; each of the power supply loops includes a first rectification loop and a second rectification loop for rectification of the positive and the negative halves of an AC voltage, respectively; and the first winding and the second winding are in the first rectification loop and the second rectification loop, respectively; and 
     a current-balancing transformer is arranged between a first power supply loop and a second power supply loop where adjacent rectification and filtering units are in; and the current-balancing transformer includes four windings in respective rectification loops included by the first power supply loop and the second power supply loop, for current balancing between the different rectification loops where the adjacent rectification and filtering units are in. 
     Of the four windings of the current-balancing transformer, currents in opposite directions flow through a dotted terminal of a first winding and a dotted terminal of a second winding, currents in opposite directions flow through the dotted terminal of the first winding and a dotted terminal of a third winding, and currents in opposite directions flow through the dotted terminal of the third winding and a dotted terminal of the fourth winding. 
     The main transformer may be implemented to include one primary winding and one secondary winding; or, one primary winding and at least two secondary windings; or, at least two primary windings and at least two secondary windings; or the like. 
     The DC/AC converter may be implemented to 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. 
     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 the accompanying drawings. 
       FIG. 3  is a structural diagram illustrating a multipath constant-current driving circuit according to an embodiment of the present invention. As shown in  FIG. 3 , the multipath constant-current driving circuit includes: a DC/AC converter, a main transformer Ta 3 , a first rectification and filtering unit Z 31 , and a second rectification and filtering unit Z 32 . The electrical energy output by the first rectification and filtering unit Z 31  and the second rectification and filtering unit Z 32  is supplied to the loads such as an LED. 
     The main transformer Ta 3  includes one primary winding and one secondary winding with a tap. The tap divides the secondary winding into a first winding Wa 31  and a second winding Wa 32 , and a non-dotted terminal of the first winding Wa 31  is connected to a dotted terminal of the second winding Wa 32 . 
     The secondary winding forms a power supply loop with each of the first rectification and filtering unit Z 31  and the second rectification and filtering unit Z 32 . Specifically, the dotted terminal of the first winding Wa 31  of the secondary winding is connected to a first input terminal t 1  of the first rectification and filtering unit Z 31 , the non-dotted terminal of the first winding Wa 31  of the secondary winding is connected to a second input terminal t 2  of the first rectification and filtering unit Z 31 , and the non-dotted terminal of the second winding Wa 32  of the secondary winding is connected to a third input terminal t 3  of the first rectification and filtering unit Z 31 . The connections between the second rectification and filtering unit Z 32  and the secondary winding are similar to that between the first rectification and filtering unit Z 31  and the secondary winding, which are therefore omitted here. 
     A current-balancing transformer T 31  includes four winds, i.e., a first winding W 311 , a second winding W 312 , a third winding W 313  and a fourth winding W 314 . Currents in opposite directions flow through the dotted terminal of the first winding W 311  and the dotted terminal of the second winding W 312 ; currents in opposite directions flow through the dotted terminal of the third winding W 313  and the dotted terminal of the fourth winding W 314 ; and currents in opposite directions flow through the dotted terminal of the first winding W 311  and the dotted terminal of the third winding W 313 . 
     The first winding W 311  and the second winding W 312  are in the power supply loop where the first rectification and filtering unit Z 31  is in; and the third winding W 313  and the fourth winding W 314  are in the power supply loop where the second rectification and filtering unit Z 32  is in. Specifically, the first winding W 311  is connected in series between the dotted terminal of the first winding Wa 31  of the secondary winding of the main transformer Ta 3  and the first input terminal t 1  of the first rectification and filtering unit Z 31 ; the second winding W 312  is connected in series between the non-dotted terminal of the second winding Wa 32  of the secondary winding of the main transformer Ta 3  and the third input terminal t 3  of the first rectification and filtering unit Z 31 ; the third winding W 313  is connected in series between the dotted terminal of the first winding Wa 31  of the secondary winding of the main transformer Ta 3  and the first input terminal t 1  of the second rectification and filtering unit Z 32 ; and the fourth winding W 314  is connected in series between the non-dotted terminal of the second winding Wa 32  of the secondary winding of the main transformer Ta 3  and the third input terminal t 3  of the second rectification and filtering unit Z 32 . 
     In each of the power supply loops formed by a rectification and filtering unit and the secondary winding, two rectification loops are included, for rectification of the positive and the negative halves of the AC voltage, respectively. 
     For example, as shown in  FIG. 3   a , the rectification and filtering unit may include: a first diode D 1 , a second diode D 2  and a first capacitor C 1 . 
     Each power supply loop includes: the dotted terminal of the first winding Wa 31  of the secondary winding connected to the non-dotted terminal of the first winding Wa 31  via the first diode D 1  and the first capacitor C 1  sequentially connected in series, with the anode of the first diode D 1  being connected to the dotted terminal of the first winding Wa 31  of the secondary winding; and the dotted terminal of the second winding Wa 32  of the secondary winding connected to the non-dotted terminal of the second winding Wa 32  via the first capacitor C 1  and the second diode D 2  sequentially connected in series, with the anode of the second diode D 2  being connected to the non-dotted terminal of the second winding Wa 32  of the secondary winding. 
     The first winding of the secondary winding, the first diode D 1  and the first capacitor C 1  form a first rectification loop; and the second winding of the secondary winding, the second diode D 2  and the first capacitor C 1  form a second rectification loop. The positive and the negative halves of the AC power flow through the two rectification loops, respectively. 
     Therefore, the first winding W 311  of the current-balancing transformer T 31  is in the first rectification loop corresponding to the first rectification and filtering unit Z 31 ; the second winding W 312  of the current-balancing transformer T 31  is in the second rectification loop corresponding to the first rectification and filtering unit Z 31 ; the third winding W 313  of the current-balancing transformer T 31  is in the first rectification loop corresponding to the second rectification and filtering unit Z 32 ; and the fourth winding W 311  of the current-balancing transformer T 31  is in the second rectification loop corresponding to the second rectification and filtering unit Z 32 . 
     Alternatively, as shown in  FIG. 3   b , the rectification and filtering unit may include: a third diode D 3 , a fourth diode D 4 , a first inductor L 1  and a second capacitor C 2 . 
     Each power supply loop includes: the dotted terminal of the first winding W 311  of the secondary winding connected to the non-dotted terminal of the first winding W 311  via the third diode D 3 , the first inductor L 1  and the second capacitor C 2  sequentially connected in series, with the anode of the third diode D 3  being connected to the dotted terminal of the first winding W 311  of the secondary winding; and the dotted terminal of the second winding W 312  of the secondary winding connected to the non-dotted terminal of the second winding W 312  via the second capacitor C 2 , the first inductor L 1  and the fourth diode D 4  sequentially connected in series, with the anode of the fourth diode D 4  being connected to the non-dotted terminal of the second winding W 312  of the secondary winding. 
     Then, the first winding of the secondary winding, the third diode D 3 , the first inductor L 1  and the second capacitor C 2  form a first rectification loop; and the second winding of the secondary winding, the fourth diode D 4 , the first inductor L 1  and the second capacitor C 2  form a second rectification loop. The positive and the negative halves of the AC power flow through the two rectification loops, respectively. 
     Therefore, the first winding W 311  of the current-balancing transformer T 31  is in the first rectification loop corresponding to the first rectification and filtering unit Z 31 ; the second winding W 312  of the current-balancing transformer T 31  is in the second rectification loop corresponding to the first rectification and filtering unit Z 31 ; the third winding W 313  of the current-balancing transformer T 31  is in the first rectification loop corresponding to the second rectification and filtering unit Z 32 ; and the fourth winding W 311  of the current-balancing transformer T 31  is in the second rectification loop corresponding to the second rectification and filtering unit Z 32 . 
     Specifically, the operating principle of the current-balancing transformer T 31  is described below. Output currents iw 311  and iw 312  flow through the dotted terminal of the winding W 311  of the current-balancing transformer at the positive half of the AC voltage and the non-dotted terminal of the winding W 312  of the current-balancing transformer at the negative half of the AC voltage, respectively; and W 311 =W 312 . Output currents iw 313  and iw 314  flow through the non-dotted terminal of the winding W 313  of the current-balancing transformer at the positive half of the AC voltage and the dotted terminal of the winding W 314  of the current-balancing transformer at the negative half of the AC voltage, respectively; and W 313 =W 314 . 
     When the turns ratio of the current-balancing transformer is W 311 :W 313 =1:1, if load currents I 1  and I 2  are unbalanced because of unbalanced voltages across respective LED loads A 1  and A 2 , the currents flowing through the dotted terminals and the non-dotted terminals of the current-balancing transformer T 31  are not equal in magnitude; hence the magnetizing current in the current-balancing transformer is not zero. The magnetizing current generates an AC voltage across the windings of the current-balancing transformer, which automatically balances the difference between the voltages across respective LED loads, so that the currents iw 311 , iw 312 , iw 313  and iw 314  in the current-balancing transformer are balanced, thereby realizing balancing between the load currents (I 1  and  12 ). 
     When the turns ratio of the current-balancing transformer is W 311 :W 313 =n:m, if the voltages across respective LED loads A 1  and A 2  are unbalanced, the magnetizing current in the current-balancing transformer is not zero. The magnetizing current generates an AC voltage across the windings of the current-balancing transformer, which automatically balances the difference between the voltages across respective loads, so that the ratio of the current iw 311  (iw 312 ) to the current iw 313  (iw 314 ) is m:n, thereby realizing control of multiple load currents. 
     The current-balancing transformer in fact balances only the AC components of the load currents, but does not affect the DC components. The greater the inductance of the current-balancing transformer, the better the current balancing between the two loads. 
     Based on the analysis above, in the multipath constant-current driving circuit shown in  FIG. 3  to  FIG. 3   b , the first winding and the second winding (or, the third winding and the fourth winding) of the current-balancing transformer are in two different rectification loops for the positive and the negative halves of the AC power supply respectively, with currents in opposite directions flowing through their dotted terminals; hence it is equivalently that a bidirectional AC current flows through the windings of the current-balancing transformer. Therefore there is no DC component in the current-balancing transformer, which eliminates the need for air gaps in the current-balancing transformer, allows large inductance at a small size, provides good current balancing, and lowers the cost. Moreover, when such a circuit is applied to the case where the AC signal is a square wave having a duty cycle of approximately 50%, almost no efficiency is lost; and when the difference between output voltages is large (or even short-circuited), there is no additional stress or peaks to the rectifier, thereby improving the reliability of the device, lowering the cost of the device, and providing better electromagnetic interference (EMI) performance. 
     In the multipath constant-current driving circuit shown in  FIG. 3  to  FIG. 3   b , the main transformer includes one primary winding and one secondary winding, and the secondary winding forms a power supply loop with each rectification and filtering unit. In practice, the main transformer may include one primary winding and at least two secondary windings. Then, each of the secondary windings corresponds to a rectification and filtering unit, and forms a power supply loop with the corresponding rectification and filtering unit. In this case, the circuit structure shown in  FIG. 3  turns into the circuit structure shown in  FIG. 4 . The only difference between  FIG. 4  and  FIG. 3  lies in the structure of the main transformer, which results in the difference between the power supply loops formed. Similarly,  FIG. 4   a  and  FIG. 4   b  show implementation structures of a multipath constant-current driving circuit according to the present invention with different rectification and filtering units.  FIG. 4   a  and  FIG. 4   b  correspond to  FIG. 3   a  and  FIG. 3   b , respectively. The differences are also the structure of the main transformer, which are therefore omitted here. 
     Alternatively, in the multipath constant-current driving circuit shown in  FIG. 3  to  FIG. 3   b , the main transformer may include at least two primary windings and at least two secondary windings, and there is a one-to-one-to-one correspondence between the primary windings, the secondary windings and rectification and filtering units. Then, each of the secondary windings corresponds to a rectification and filtering unit, and forms a power supply loop with the corresponding rectification and filtering unit. In this case, the circuit structure show in  FIG. 3  turns into the circuit structure shown in  FIG. 5 . The only difference between  FIG. 5  and  FIG. 3  lies in the structure of the main transformer, which results in the difference between the power supply loops formed. Similarly,  FIG. 5   a  and  FIG. 5   b  show implementation structures of a multipath constant-current driving circuit according to the present invention with different rectification and filtering units.  FIG. 5   a  and  FIG. 5   b  correspond to  FIG. 3   a  and  FIG. 3   b , respectively. The differences are also the structure of the main transformer, which are therefore omitted here. 
     Furthermore, the implementation of the DC/AC converter is not limited according to the present invention, which 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. For example, in the implementation structure of a multipath constant-current driving circuit according to the present invention shown in  FIG. 6   a  to  FIG. 6   e , the DC/AC converter is implemented as an LLC resonant circuit, a symmetric half-bridge circuit, an asymmetric half-bridge circuit, a full-bridge circuit and a push-pull circuit, respectively. 
     The multipath constant-current driving circuits shown above include two paths. In a practical application, more than two paths may be included, which is illustrated by  FIG. 7   a  to  FIG. 7   c , corresponding to  FIG. 3   a ,  FIG. 4   a  and  FIG. 5   a , respectively. 
     As shown in  FIG. 8   a , 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, 
     the DC/AC converter is adapted to provide an AC voltage for the main transformer; 
     the main transformer includes at least one secondary winding with a tap, the tap divides the corresponding secondary winding into a first winding and a second winding, with a non-dotted terminal of the first winding being connected to a dotted terminal of the second winding; and of the at least one secondary winding of the main transformer, there is at least one secondary winding that corresponds to a power supply loop of at least two stages; 
     each of the stages of the power supply loop corresponding to the secondary winding includes a first rectification loop and a second rectification loop; the first rectification loop includes: a first terminal of the first winding of the secondary winding connected to a second terminal of the first winding of the secondary winding via a rectification and filtering unit, third windings of all current-balancing transformers arranged in previous stages of the power supply loop, and 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 the second rectification loop includes: a first terminal of the second winding of the secondary winding connected to a second terminal of the first winding of the secondary winding via a second winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, fourth windings of all the current-balancing transformers arranged in the previous stages of the power supply loop, and a rectification and filtering unit; and 
     the current-balancing transformer includes four windings, for current balancing between different rectification loops where adjacent rectification and filtering units are in. 
     Of the four windings of the current-balancing transformer, currents in opposite directions may flow through a dotted terminal of the first winding and a dotted terminal of the second winding, currents in opposite directions may flow through the dotted terminals of the first winding and a dotted terminal of the third winding, and currents in opposite directions may flow through the dotted terminals of the third winding and a dotted terminal of the fourth winding. 
     As shown in  FIG. 8   a , the rectification and filtering unit may include a first diode D 1 , a second diode D 2  and a first capacitor C 1 . 
     Specifically, as shown in  FIG. 8   a , the secondary winding corresponds to a multi-stage power supply loop, and each of the stages of the power supply loop includes a first rectification loop and a second rectification loop. 
     In  FIG. 8   a , the first rectification loop includes: the first terminal of the first winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first diode D 1 , third windings of all current-balancing transformers arranged in previous stages of the power supply loop, a first winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, and the first capacitor C 1  sequentially; and 
     the second rectification loop includes: the first terminal of the second winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first capacitor C 1 , a second winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, and fourth windings of all the current-balancing transformers arranged in the previous stages of the power supply loop, and the second diode D 2  sequentially. 
     Alternatively, in each power supply loop, the first rectification loop may include: the first terminal of the first winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via third windings of all current-balancing transformers arranged in previous stages of the power supply loop, a first winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, the first diode, and the first capacitor sequentially; and 
     the second rectification loop may include: the first terminal of the second winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first capacitor, the second diode, a second winding of the current-balancing transformer corresponding to both the current stage of the power supply loop and the next stage of the power supply loop, and fourth windings of all the current-balancing transformers arranged in the previous stages of the power supply loop sequentially. 
     Therefore, the (N−1)th current-balancing transformer balances the total current of the previous N−2 loads and the current of the last load, thereby realizing current balancing between the N loads. 
     For example, in  FIG. 8   a , for the (N−1)th stage of the power supply loop, its first rectification loop includes: the first terminal of the first winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first diode D 1 , third windings of the current-balancing transformers T 81  to T 8 (N−2), a first winding of the current-balancing transformer T 8 (N−1), and the first capacitor C 1  sequentially; and 
     the second rectification loop includes: the first terminal of the second winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first capacitor C 1 , a second winding of the current-balancing transformer T 8 (N−1), fourth windings of the current-balancing transformers T 8 (N−2) to T 81 , and the second diode D 2  sequentially. 
     For the Nth stage of the power supply loop, its first rectification loop includes: the first terminal of the first winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first diode D 1 , third windings of the current-balancing transformers T 81  to T 8 (N−1), and the first capacitor C 1  sequentially; and 
     the second rectification loop includes: the first terminal of the second winding of the secondary winding connected to the second terminal of the first winding of the secondary winding via the first capacitor C 1 , fourth windings of the current-balancing transformers T 8 (N−1) to T 81 , and the second diode D 2  sequentially. 
     In this case, the Nth stage of the power supply loop is not followed by another stage; hence the Nth stage of the power supply loop does not include the first winding and the second winding of the current-balancing transformer. 
     Moreover, in  FIG. 8   a , the main transformer is a transformer including one primary winding and one secondary winding. In a practical application, the main transformer may also 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 these cases, each of the secondary windings of the main transformer may correspond to a multi-stage power supply loop similar to that shown in  FIG. 8   a.    
     In the technical solution above, of all the secondary windings of the main transformer, each secondary winding may correspond to a power supply loop of at least two stages; alternatively, each secondary winding may correspond to multiple power supply loops of at least two stages, which implementation may also achieve the multipath constant-current driving circuit according the present invention. 
     Alternatively, each of some (at least one) of all the secondary windings may correspond to a power supply loop of at least two stages, while the other secondary windings form a power supply loop use a circuit structure in the prior art for supplying electric power to the loads, thereby realizing current balancing, or, using any of the various power supply loops described in conjunction with  FIG. 3  to  FIG. 7  for supplying electric power to the loads, thereby realizing current balancing. The present invention is not limited to any specific implementation. 
     For example, the circuit structures shown in  FIG. 8   a  and  FIG. 7   a  may be combined to the same multipath constant-current driving circuit. 
     For the driving circuit shown in  FIG. 7   a , 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. 8   a , all the power supply branches that are connected to the secondary winding and form the multi-stage constant-current driving circuit can be seen as a second power supply branch group. 
     Then, as shown in  FIG. 9   a , the main transformer includes one primary winding and one secondary winding. The input terminals 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, respectively. That is, the first power supply branch group and the second power supply branch group are connected in parallel to the same secondary winding. The power supply loop formed by the first power supply branch group connected to the secondary winding can be seen as a first main power supply loop, and the power supply loop formed by the second power supply branch group connected to the secondary winding can be seen as a second main power supply loop. In this case, in order to balance the total current Im of the first main power supply loop and the total current In of the second main power supply loop, a current-balancing transformer T 90  may be arranged between the two main power supply loops, with its first winding and second winding arranged between the secondary winding and the input terminals of the first power supply branch group in the first main power supply loop, and its third winding and fourth winding arranged between the secondary winding and the input terminals of the second power supply branch group in the second main power supply loop. 
     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, as shown in  FIG. 9   b  to  FIG. 9   c , the circuit shown in  FIG. 9   a  can also be extended to the cases where the main transformer includes one primary winding and multiple secondary windings, and where the main transformer includes multiple primary windings and multiple secondary windings, which are shown in  FIGS. 9   d  to  9   e , respectively. 
     In addition, the DC/AC converter in the embodiment 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, which are illustrated by  FIG. 6   a  to  FIG. 6   e  and are therefore omitted here. 
     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.