Abstract:
A drive circuit for realizing accurate constant current of multiple LEDs is disclosed. The drive circuit comprises a high-frequency impulse Alternating Current (AC) power carrying N circuit units with same structure. Each of the circuit unit comprises a rectifier filter circuit, a blocking capacitor C 1  and two LED loads. The rectifier filter circuit comprises two independent half-wave rectifier circuits, and two filter capacitors. Each of the two half-wave rectifier circuits comprises two diodes connected in series to supply power for the corresponding LED load. The filter capacitor is connected in parallel with the two ends of an LED load respectively, and the blocking capacitor C 1  is connected in series with the input end of the rectifier filter circuit. The circuit also comprises N−1 equalizing transformers, each of which connects in series between two adjacent circuit units. A drive circuit for constant output current of multiple LEDs with high efficient, low cost and great flow equalization is provided in the embodiment of the invention. When the differential voltage of the two LED loads is large, high efficiency can also be achieved.

Description:
This application is a National Stage application of international application PCT/CN2010/079600 filed on Dec. 9, 2010, which claimed the benefit of Chinese patent application No. 200910155848.0 filed on Dec. 28, 2009. Both the international application and the Chinese application are incorporated herein by reference in their entireties. 
     FIELD OF THE INVENTION 
     The present invention relates to a driving circuit for precise constant-current control of multiple LED branches, and in particular to a circuit for current balancing between LED loads with a balancing capacitor. 
     BACKGROUND OF THE INVENTION 
     Generally, a multipath constant-current control driver for LEDs can be implemented with: 1. a constant-voltage module together with multiple non-isolated DC/DC constant-current circuits (e.g., BUCK circuits); or, 2. a voltage-adjustable voltage regulating module together with multiple linear regulating constant-current circuits. 
     As shown in  FIG. 1 , in the first scheme, the output of the constant-voltage module is inputted to the constant-current circuits, and each of the constant current circuits performs constant-current control independently, which can easily ensure the balancing between output currents. However, there is normally a significant disparity between the voltage of the constant-voltage module and the voltage across an LED load; therefore none of the DC/DC constant-current circuits after the constant-voltage module has high efficiency. In addition, the cost of the multipath constant-current control circuit is high. 
     As shown in  FIG. 2 , in the second scheme, MOS transistors or triodes are used to carry out linear regulation and hence multipath constant-current control. The output voltage of the voltage regulating module follows the linear regulating constant-current circuits after it, so that the output voltage of the voltage regulating module remains slightly higher than the highest one of the output voltages of the linear regulating constant-current circuits; as a result, power consumption of each of the linear regulating constant-current circuits remains close to the minimum while precise constant-current control is achieved. This scheme has the advantages including low cost of the circuit and good current balancing between the LED loads. However, as one of the most common ways for an LED to fail, short circuits may cause a significant disparity between the voltages across the LED loads, which leads to high power consumption at the linear regulating devices and hence a large amount of heat generated by the LED driver. 
     SUMMARY OF THE INVENTION 
     In view of the problems above, the present invention provides a multipath output, constant-current driving circuit for LEDs with high efficiency, low cost and good current balancing, which can achieve high efficiency even when the difference between the voltages across LED loads is large. 
     According to an embodiment of the present invention, a driving circuit for precise constant-current control of multiple LED branches is provided, including: a high-frequency pulse Alternating Current (AC) current source, and a circuit unit provided for the high-frequency pulse AC current source, wherein the circuit unit includes a rectification and filtering circuit, a balancing capacitor C 1  and two LED loads; 
     the rectification and filtering circuit includes two independent half-wave rectification circuits and two filter capacitors; each of the half-wave rectification circuits includes two diodes connected in series, for supplying electric power to one of the LED loads; each of the LED loads is connected in parallel with one of the filter capacitors; the balancing capacitor C 1  is connected in series with an input terminal of the rectification and filtering circuit. 
     Preferably, the rectification and filtering circuit includes a diode D 1 , a diode D 2 , a diode D 3 , a diode D 4 , a filter capacitor C 2  and a filter capacitor C 3 ; 
     a first input terminal of the rectification and filtering circuit is common to both an anode of the diode D 1  and a cathode of the diode D 2 , and a second input terminal of the rectification and filtering circuit is common to both an anode of the diode D 3  and a cathode of the diode D 4 ; the balancing capacitor C 1  is connected in series with one of the input terminals of the rectification and filtering circuit; 
     a positive terminal of a first LED load is connected with a cathode of the diode D 1 , and a negative terminal of the first LED load is connected with an anode of the diode D 2 ; a positive terminal of a second LED load is connected with a cathode of the diode D 3 , and a negative terminal of the second LED load is connected with an anode of the diode D 4 ; 
     the anode of the diode D 2  is connected with the anode of the diode D 4 ; the filter capacitors C 2  and C 3  are connected in parallel with the two LED loads respectively. 
     Preferably, the rectification and filtering circuit includes a diode D 1 , a diode D 2 , a diode D 3 , diode D 4 , a filter capacitor C 2  and a filter capacitor C 3 ; 
     a first input terminal of the rectification and filtering circuit is common to both an anode of the diode D 1  and a cathode of the diode D 2 , and a second input terminal of the rectification and filtering circuit is common to both an anode of the diode D 3  and a cathode of the diode D 4 ; the balancing capacitor C 1  is connected in series with one of the input terminals of the rectification and filtering circuit; 
     a positive terminal of a first LED load is connected with a cathode of the diode D 1 , and a negative terminal of the first LED load is connected with an anode of the diode D 2 ; a positive terminal of the second LED load is connected with a cathode of the diode D 3 , and a negative terminal of the second LED load is connected with an anode of the diode D 4 ; 
     the cathode of the diode D 1  is connected with the cathode of the diode D 3 ; the filter capacitors C 2  and C 3  are connected in parallel with the two LED loads respectively. 
     According to an embodiment of the present invention, a driving circuit for precise constant-current control of multiple LED branches is provided, including: a high-frequency pulse AC current source, and N circuit units provided for the high-frequency pulse AC current source, wherein all the circuit units have the same structure, and each of the circuit units includes a rectification and filtering circuit, a balancing capacitor C 1  and two LED loads, with N being an integer greater than 1; 
     the rectification and filtering circuit includes two independent half-wave rectification circuits and two filter capacitors; each of the half-wave rectification circuits includes two diodes connected in series, for supplying electric power to one of the LED loads; each of the LED loads is connected in parallel with one of the filter capacitors; the balancing capacitor C 1  is connected in series with an input terminal of the rectification and filtering circuit; 
     the driving circuit further includes N−1 current-balancing transformers, each of which is connected in series between two adjacent circuit units. 
     Preferably, the rectification and filtering circuit includes: a diode D 1 , a diode D 2 , a diode D 3 , a diode D 4 , a filter capacitor C 2  and a filter capacitor C 3 ; 
     a first input terminal of the rectification and filtering circuit is common to both an anode of the diode D 1  and a cathode of the diode D 2 , and a second input terminal of the rectification and filtering circuit is common to both an anode of the diode D 3  and a cathode of the diode D 4 ; the balancing capacitor C 1  is connected in series with one of the input terminals of the rectification and filtering circuit; 
     a positive terminal of a first LED load is connected with a cathode of the diode D 1 , and a negative terminal of the first LED load is connected with an anode of the diode D 2 ; a positive terminal of a second LED load is connected with a cathode of the diode D 3 , and a negative terminal of the second LED load is connected with an anode of the diode D 4 ; 
     the anode of the diode D 2  is connected with the anode of the diode D 4 ; the filter capacitors C 2  and C 3  are connected in parallel with the two LED loads respectively. 
     Preferably, the rectification and filtering circuit includes: a diode D 1 , a diode D 2 , a diode D 3 , a diode D 4 , a filter capacitor C 2  and a filter capacitor C 3 ; 
     a first input terminal of the rectification and filtering circuit is common to both an anode of the diode D 1  and a cathode of the diode D 2 , and a second input terminal of the rectification and filtering circuit is common to both an anode of the diode D 3  and a cathode of the diode D 4 ; the balancing capacitor C 1  is connected in series with one of the input terminals of the rectification and filtering circuit; 
     a positive terminal of a first LED load is connected with a cathode of the diode D 1 , and a negative terminal of the first LED load is connected with an anode of the diode D 2 ; a positive terminal of the second LED load is connected with a cathode of the diode D 3 , and a negative terminal of the second LED load is connected with an anode of the diode D 4 ; 
     the cathode of the diode D 1  is connected with the cathode of the diode D 3 ; the filter capacitors C 2  and C 3  are connected in parallel with the two LED loads respectively. 
     Preferably, the high-frequency pulse AC current source is connected directly with the N circuit units; 
     or, 
     the high-frequency pulse AC current source is connected with the N circuit units via a transformer T 2 ; the high-frequency pulse AC current source is connected with a primary winding of the transformer T 2 ; the transformer T 2  has N secondary windings, each of which is connected with one of the circuit units. 
     According to an embodiment of the present invention, a driving circuit for precise constant-current control of multiple LED branches is provided, including: a high-frequency pulse AC current source, and N+1 circuit units provided for the high-frequency pulse AC current source, wherein N of the circuit units have the same structure, and each of the N circuit units includes a rectification and filtering circuit, a balancing capacitor C 1  and two LED loads; the (N+1)th circuit unit includes a rectification and filtering circuit and one LED load; N is an integer greater than or equal to 1; 
     the rectification and filtering circuit includes two independent half-wave rectification circuits and two filter capacitors; each of the half-wave rectification circuits includes two diodes connected in series, for supplying electric power to one LED load; each LED load is connected in parallel with one of the filter capacitors; the balancing capacitor C 1  is connected in series with an input terminal of the rectification and filtering circuit; 
     the driving circuit further includes N current-balancing transformers, each of which is connected in series between two adjacent circuit units. 
     Preferably, the rectification and filtering circuit includes a diode D 1 , a diode D 2 , a diode D 3 , a diode D 4 , a filter capacitor C 2  and a filter capacitor C 3 ; 
     a first input terminal of the rectification and filtering circuit is common to both an anode of the diode D 1  and a cathode of the diode D 2 , and a second input terminal of the rectification and filtering circuit is common to both an anode of the diode D 3  and a cathode of the diode D 4 ; the balancing capacitor C 1  is connected in series with one of the input terminals of the rectification and filtering circuit; 
     a positive terminal of a first LED load is connected with a cathode of the diode D 1 , and a negative terminal of the first LED load is connected with an anode of the diode D 2 ; a positive terminal of a second LED load is connected with a cathode of the diode D 3 , and a negative terminal of the second LED load is connected with an anode of the diode D 4 ; 
     the anode of the diode D 2  is connected with the anode of the diode D 4 ; the filter capacitors C 2  and C 3  are connected in parallel with the two LED loads respectively. 
     Preferably, the rectification and filtering circuit includes a diode D 1 , a diode D 2 , a diode D 3 , a diode D 4 , a filter capacitor C 2  and a filter capacitor C 3 ; 
     a first input terminal of the rectification and filtering circuit is common to both an anode of the diode D 1  and a cathode of the diode D 2 , and a second input terminal of the rectification and filtering circuit is common to both an anode of the diode D 3  and a cathode of the diode D 4 ; the balancing capacitor C 1  is connected in series with one of the input terminals of the rectification and filtering circuit; 
     a positive terminal of a first LED load is connected with a cathode of the diode D 1 , and a negative terminal of the first LED load is connected with an anode of the diode D 2 ; a positive terminal of the second LED load is connected with a cathode of the diode D 3 , and a negative terminal of the second LED load is connected with an anode of the diode D 4 ; 
     the cathode of the diode D 1  is connected with the cathode of the diode D 3 ; the filter capacitors C 2  and C 3  are connected in parallel with the two LED loads respectively. 
     Preferably, the high-frequency pulse AC current source is connected directly with the N+1 circuit units; 
     or, 
     the high-frequency pulse AC current source is connected with the N+1 circuit units via a transformer T 2 ; the high-frequency pulse AC current source is connected with a primary winding of the transformer T 2 ; the transformer T 2  has N+1 secondary windings, each of which is connected with one of the circuit units. 
     The present invention can bring the following benefits: 
     1. Current balancing between multiple LED loads with one conversion stage, and hence low cost; without additional control circuits, which leads to high reliability. 
     2. High-precision current balancing regardless of the difference between the voltages across LED loads. 
     3. High-efficiency current balancing due to the balancing capacitor, and less loss even when the difference between the voltages across LED loads is large. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described in detail in conjunction with the accompanying drawings and embodiments. 
         FIG. 1  is a schematic diagram illustrating a first multipath constant-current control driver in the prior art; 
         FIG. 2  is a schematic diagram illustrating a second multipath constant-current control driver in the prior art; 
         FIG. 3  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a first embodiment of the invention; 
         FIG. 4  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a second embodiment of the invention; 
         FIG. 5  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a third embodiment of the invention; 
         FIG. 6  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a fourth embodiment of the invention; 
         FIG. 7  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a fifth embodiment of the invention; 
         FIG. 8  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a sixth embodiment of the invention; 
         FIG. 9  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a seventh embodiment of the invention; 
         FIG. 10  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to an eighth embodiment of the invention; 
         FIG. 11  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a ninth embodiment of the invention; 
         FIG. 12  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a tenth embodiment of the invention; 
         FIG. 13  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to an eleventh embodiment of the invention; and 
         FIG. 14  is a schematic diagram illustrating a driving circuit for precise constant-current control of multiple LED branches according to a twelfth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 3  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a first embodiment of the invention. The circuit shown in  FIG. 3  is used for current balancing between two LED loads. 
     Specifically, the driving circuit for precise constant-current control of multiple LED branches includes a high-frequency pulse AC current source, a rectification and filtering circuit, a balancing capacitor C 1  and two LED loads. 
     The rectification and filtering circuit includes two independent half-wave rectification circuits and two filter capacitors. The two half-wave rectification circuits have the same structure, and each includes two diodes connected in series. Each of the half-wave rectification circuits is used for supplying electric power to one of the LED loads. Each of the LED loads is connected in parallel with one of the filter capacitors. The balancing capacitor C 1  is connected in series between an input terminal of the rectification and filtering circuit and the high-frequency pulse AC current source. 
     As shown in  FIG. 3 , one of the half-wave rectification circuits includes a diode D 1  and a diode D 4 , for supplying electric power to an LED load  1 ; and the other of the half-wave rectification circuits includes a diode D 3  and a diode D 2 , for supplying electric power to an LED load  2 . 
     An output terminal of the high-frequency pulse AC current source is connected in series to the balancing capacitor C 1 , which is then connected to one of the two input terminals of the rectification and filtering circuit. 
     A first input terminal of the rectification and filtering circuit is common to both an anode of the diode D 1  and a cathode of the diode D 2 , and a second input terminal of the rectification and filtering circuit is common to both an anode of the diode D 3  and a cathode of the diode D 4 . A positive terminal of the LED load  1  is connected with a cathode of the diode D 1 , and a negative terminal of the LED load  1  is connected with an anode of the diode D 2 . A positive terminal of the LED load  2  is connected with a cathode of the diode D 3 , and a negative terminal of the LED load  2  is connected with an anode of the diode D 4 . 
     The anode of the diode D 2  is connected with the anode of the diode D 4 . The filter capacitors C 2  and C 3  are connected in parallel with the two LED loads respectively. 
     As can be seen from  FIG. 3 , in the first embodiment, the negative terminal of the LED load  1  and the negative terminal of the LED load  2  are connected with each other, then both to a terminal common to the anode of the diode D 2  and the anode of the diode D 4 . This can be referred to as a “common cathode” arrangement of the two LED loads. 
       FIG. 4  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a second embodiment of the invention. The circuit shown in  FIG. 4  is used for current balancing between two LED loads. 
     The circuit of the second embodiment differs from that of the first embodiment in that the arrangement of the two LED loads is “common anode”. 
     As shown in  FIG. 4 , an output terminal of the high-frequency pulse AC current source is connected in series to the balancing capacitor C 1 , which is then connected to one of the two input terminals of the rectification and filtering circuit. 
     A first input terminal of the rectification and filtering circuit is common to both an anode of the diode D 1  and a cathode of the diode D 2 , and a second input terminal of the rectification and filtering circuit is common to both an anode of the diode D 3  and a cathode of the diode D 4 . 
     A positive terminal of the LED load  1  is connected with a cathode of the diode D 1 , and a negative terminal of the LED load  1  is connected with an anode of the diode D 2 . A positive terminal of the LED load  2  is connected with a cathode of the diode D 3 , and a negative terminal of the LED load  2  is connected with an anode of the diode D 4 . 
     The cathode of the diode D 1  is connected with the cathode of the diode D 3 . The filter capacitors C 2  and C 3  are connected in parallel with the two LED loads respectively. 
     As can be seen from  FIG. 4 , in the second embodiment, the positive terminal of the LED load  1  and the positive terminal of the LED load  2  are connected with each other, then both to a terminal common to the cathode of the diode D 1  and the cathode of the diode D 3 . This can be referred to as a “common anode” arrangement of the two LED loads. 
     The driving circuits for precise constant-current control of multiple LED branches according to the embodiments of the invention realize current balancing between two LED loads by a balancing capacitor C 1 , and the arrangement of the two LED loads may be “common cathode” or “common anode”. 
     The circuit includes two independent half-wave rectification circuits, each of which consists of two diodes connected in series for supplying electric power to one of the two LED loads, and realizes filtering by a filter capacitor. Due to the presence of the balancing capacitor C 1 , when the voltage drops across the two LED loads are different, the difference between the voltages across the two LED loads can be balanced by the balancing capacitor C 1 , so that the average currents through the two LED loads are equal. In an ideal case where the voltage drops across the two LED loads are the same, the voltage across the balancing capacitor C 1  is zero. 
       FIG. 5  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a third embodiment of the invention. The circuit shown in  FIG. 5  is used for current balancing between three LED loads. 
     As shown in  FIG. 5 , the high-frequency pulse AC current source is connected with a primary winding of a transformer T 2 , and the transformer T 2  has two secondary windings WT 1  and WT 2 . The first secondary winding WT 1  carries two LED loads, and the second secondary winding WT 2  carries one LED load. Current balancing between the first secondary winding WT 1  and the second secondary winding WT 2  is realized by a current-balancing transformer T 1 . 
     The current-balancing transformer T 1  includes two current-balancing windings W 1  and W 2 . A dotted terminal of the first secondary winding WT 1  is connected to a dotted terminal of the first current-balancing winding W 1 ; and a non-dotted terminal of the first current-balancing winding W 1  and a non-dotted terminal of the first secondary winding WT 1  are connected in series to a balancing capacitor C 1 , as well as two rectification and filtering circuits and two LED loads. A dotted terminal of the second secondary winding WT 2  is connected to a non-dotted terminal of the second current-balancing winding W 2 ; and a dotted terminal of the second current-balancing winding W 2  and a non-dotted terminal of the second secondary winding WT 2  are connected to a third rectification and filtering circuit and a third LED load. 
     Current balancing between the two LED loads carried by the first secondary winding WT 1  may be implemented in the manner shown in  FIG. 3  or the manner shown in  FIG. 4 . However, balancing between the total current of the two LED loads and the current through the third LED load carried by the second secondary winding WT 2  is realized by the current-balancing transformer T 1 . This is because currents in opposite directions flow through the two current-balancing windings W 1  and W 2  of the current-balancing transformer T 1 , and the voltage difference generated the winding automatically balances the two currents flowing through the current-balancing windings. 
     In the circuit shown in  FIG. 5 , the high-frequency pulse AC current source supplies electric power to three LED loads via the transformer T 2 . In each of the secondary windings of the transformer T 2 , a current which is in phase with the high-frequency pulse AC current source is generated; hence, the currents in the two secondary windings of the transformer T 2  are in phase. This is equivalent to two high-frequency pulse AC current sources that are in phase supplying electric power to the circuits carried by respective secondary windings. 
       FIG. 6  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a fourth embodiment of the invention. The circuit shown in  FIG. 6  is used for current balancing between three LED loads. 
     The circuit shown in  FIG. 6  differs from that shown in  FIG. 5  in that: there is no transformer T 2 , instead, the high-frequency pulse AC current source supplies electric power directly to the three LED loads. 
     As shown in  FIG. 6 , a terminal of the high-frequency pulse AC current source is connected to a dotted terminal of the first current-balancing winding W 1  of the current-balancing transformer T 1 , and a non-dotted terminal of the first current-balancing winding W 1  and the other terminal of the high-frequency pulse AC current source are connected to two rectification and filtering circuits and two LED loads; a terminal of the high-frequency pulse AC current source is connected to a non-dotted terminal of the second current-balancing winding W 2 , and a dotted terminal of the second current-balancing winding W 2  and the other terminal of the high-frequency pulse AC current source are connected to a third rectification and filtering circuit and a third LED load. 
     The current balancing principle of the circuit shown in  FIG. 6  is similar to that shown in  FIG. 5 . 
       FIG. 7  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a fifth embodiment of the invention. The circuit shown in  FIG. 7  is used for current balancing between four LED loads. 
     As shown in  FIG. 7 , the high-frequency pulse AC current source is connected with a primary winding of the transformer T 2 , and the transformer T 2  has two secondary windings, each of which carries two LED loads. For each secondary winding, current balancing between the two LED loads may be implemented in the manner shown in  FIG. 3  or the manner shown in  FIG. 4 . Current balancing between the two secondary windings is realized by the current-balancing transformer T 1 . 
       FIG. 8  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a sixth embodiment of the invention. The circuit shown in  FIG. 8  is used for current balancing between four LED loads. 
     The circuit shown in  FIG. 8  differs from that shown in  FIG. 7  in that: There is no transformer T 2 , instead, the high-frequency pulse AC current source supplies electric power directly to the four LED loads. The current balancing principle is similar to that shown in  FIG. 7 . 
       FIG. 9  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a seventh embodiment of the invention. The circuit shown in  FIG. 9  is used for current balancing between an odd number of LED loads, and is an extension based on the circuit shown in  FIG. 5 . The high-frequency pulse AC current source is connected with a primary winding of a transformer T 0 . 
     Assuming that the number of LED loads is 2N+1, the transformer T 0  has N+1 secondary windings. Each of N of the N+1 secondary windings is connected with a circuit unit. The circuit units have the same structure, and each includes a rectification and filtering circuit, a balancing capacitor C 1  and two LED loads. This structure is the same as that of the first embodiment, and detailed description is therefore omitted. The (N+1)th secondary winding is connected with a rectification and filtering circuit and one LED load. 
     It is noted that in the circuit of the seventh embodiment, current balancing between two LED loads connected in parallel with the same secondary winding may be implemented in the manner shown in  FIG. 3 , or the manner shown in  FIG. 4 , or a manner combining both. Current balancing between the N+1 secondary windings is realized by N current-balancing transformers. 
     In the circuit shown in  FIG. 9 , the output currents of the secondary windings of the transformer T 0  are in phase. A current-balancing transformer is arranged between every two adjacent circuit units, and each of the two adjacent circuits is connected in series with a current-balancing winding of the current-balancing transformer; hence, currents that are in phase flow through a dotted terminal of a current-balancing winding and a non-dotted terminal of the other current-balancing winding. When the transformation ratio of the current-balancing transformer is n:m and the ratio between the currents flowing through a dotted terminal of a current-balancing winding and a non-dotted terminal of the other current-balancing winding is not m:n, the magnetizing current of the current-balancing transformer is not zero. The magnetizing current generates an voltage across the current-balancing transformer which automatically balances the difference between the voltages of the two circuit units, making the ratio between the currents in the two windings of the current-balancing transformer to be m:n, thereby realizing balancing control of the currents of the two circuit units, especially when m=n, realizing current balancing between the two circuit units. 
     Therefore, current balancing between the N circuit units shown in  FIG. 9  is realized in the same manner by N−1 current-balancing transformers, with N being a positive integer greater than or equal to 2. 
     In the driving circuit for precise constant-current control of an odd number of LED branches according to the invention, each of the N−1 circuit units with the same structure includes a balancing capacitor for current balancing between the two LED loads in the circuit unit. Moreover, balancing control of the total currents of two adjacent circuit units is realized by a current-balancing transformer; hence, by N−1 current-balancing transformers, balancing is achieved between the total currents of every two adjacent circuit units, thereby realizing current balancing between all the circuit units. 
       FIG. 10  illustrates a driving circuit for precise constant-current control of multiple LED branches according to an eighth embodiment of the invention. The circuit shown in  FIG. 10  is used for current balancing between an even number of LED loads, and is an extension based on the circuit shown in  FIG. 6 . The high-frequency pulse AC current source is connected with a primary winding of the transformer T 0 . 
     Assuming that the number of LED loads is 2N, the transformer T 0  has N secondary windings, each of which is connected with a circuit unit. The circuit units connected with respective secondary windings have the same structure, and each includes a rectification and filtering circuit, a balancing capacitor C 1  and two LED loads. This structure is the same as that of the first embodiment, and detailed description is therefore omitted. 
     It is noted that in the circuit of the eighth embodiment, current balancing between two LED loads connected in parallel with the same secondary winding may be implemented in the manner shown in  FIG. 3 , or the manner shown in  FIG. 4 , or a manner combining both. Current balancing between the N secondary windings is realized by N−1 current-balancing transformers. 
     In the circuit shown in  FIG. 10 , the output currents of the secondary windings of the transformer T 0  are in phase. A current-balancing transformer is arranged between every two adjacent circuit units, and each of the two adjacent circuits is connected in series with a current-balancing winding of the current-balancing transformer; hence, currents that are in phase flow through a dotted terminal of a current-balancing winding and a non-dotted terminal of the other current-balancing winding. When the transformation ratio of the current-balancing transformer is n:m and the ratio between the currents flowing through a dotted terminal of a current-balancing winding and a non-dotted terminal of the other current-balancing winding is not m:n, the magnetizing current of the current-balancing transformer is not zero. The magnetizing current generates an voltage across the current-balancing transformer which automatically balances the difference between the voltages of the two circuit units, making the ratio between the currents in the two windings of the current-balancing transformer to be m:n, thereby realizing balancing control of the currents of the two circuit units, especially when m=n, realizing current balancing between the two circuit units. 
     Therefore, current balancing between the N circuit units shown in  FIG. 10  is realized in the same manner by N−1 current-balancing transformers, with N being a positive integer greater than or equal to 2. 
     In the driving circuit for precise constant-current control of an even number of LED branches according to the invention, a balancing capacitor realizes current balancing between the two LED loads in each of the circuit units. Moreover, balancing control of the total currents of two adjacent circuit units is realized by a current-balancing transformer; hence, by N−1 current-balancing transformers, balancing is achieved between the total currents of every two adjacent circuit units, thereby realizing current balancing between all the circuit units. 
     In  FIG. 9  and  FIG. 10  above, instead of via a transformer T 2 , the high-frequency pulse AC current source supplies electric power directly to the 2N+1 or 2N LED loads, resulting in extensions of  FIG. 6  and  FIG. 8 , respectively. In addition, the circuits shown in  FIG. 5  and  FIG. 6  may be used in combination, as well as the circuits shown in  FIG. 7  and  FIG. 8 . 
       FIG. 11  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a ninth embodiment of the invention. The circuit shown in  FIG. 11  is used for current balancing between two LED loads where the high-frequency pulse AC current source is based on an LLC resonant circuit as an example. 
     The high-frequency pulse AC current source includes a DC voltage Vdc, a switching tube S 11 , a switching tube S 12 , an inductor L 11  and a capacitor C 11 . Specifically, a positive terminal of the DC voltage Vdc is connected to a first terminal of the switching tube S 11 ; a second terminal of the switching tube S 11  is connected to a first terminal of the switching tube S 12  and a terminal of the inductor L 11 ; a second terminal of the switching tube S 12  is connected to a negative terminal of the DC voltage Vdc and a terminal of the capacitor C 11 ; the other terminal of the inductor L 11  is connected to a dotted terminal of a primary winding of a main transformer T 2 ; and a non-dotted terminal of the primary winding of the main transformer T 2  is connected to the other terminal of the capacitor C 11 . 
     A dotted terminal of a secondary winding of the main transformer T 2  is connected to an anode of a diode D 1  and a cathode of a diode D 2 ; a non-dotted terminal of the secondary winding is connected to a terminal of a balancing capacitor C 1 ; the other terminal of the balancing capacitor C 1  is connected to an anode of a diode D 3  and a cathode of a diode D 4 ; a cathode of the diode D 1  is connected to a positive terminal of an electrolytic capacitor C 4  and a positive terminal of an LED load  2 ; a cathode of the diode D 3  is connected to a positive terminal of an electrolytic capacitor C 3  and a positive terminal of an LED load  1 ; and an anode of the diode D 2  is connected to an anode of the diode D 4 , a negative terminal of the electrolytic capacitor C 3 , a negative terminal of the LED load  1 , a negative terminal of the electrolytic capacitor C 4  and a negative terminal of the LED load  2 . 
       FIG. 12  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a tenth embodiment of the invention. The circuit shown in  FIG. 12  is used for current balancing between two LED loads where the high-frequency pulse AC current source is based on a full-bridge circuit as an example. 
     The high-frequency pulse AC current source includes a DC voltage Vdc, a switching tube S 21 , a switching tube S 22 , a switching tube S 23 , a switching tube S 24  and an inductor L 21 . Specifically, a positive terminal of the DC voltage Vdc is connected with both a first terminal of the switching tube S 21  and a first terminal of the switching tube S 23  via the inductor L 21 ; a second terminal of the switching tube S 21  is connected to a first terminal of the switching tube S 22  and a non-dotted terminal of a primary winding of a main transformer T 2 ; a second terminal of the switching tube S 23  is connected to a first terminal of the switching tube S 24  and a dotted terminal of the primary winding of the main transformer T 2 ; and a second terminal of the switching tube S 22  is connected to a negative terminal of the DC voltage Vdc and a second terminal of the switching tube S 24 . 
     A dotted terminal of a secondary winding of the main transformer T 2  is connected to an anode of a diode D 1  and a cathode of a diode D 2 ; a non-dotted terminal of the secondary winding is connected to a terminal of a balancing capacitor C 1 ; the other terminal of the balancing capacitor C 1  is connected to an anode of a diode D 3  and a cathode of a diode D 4 ; a cathode of the diode D 1  is connected to a positive terminal of an electrolytic capacitor C 3  and a positive terminal of an LED load  2 ; a cathode of the diode D 3  is connected to a positive terminal of an electrolytic capacitor C 2  and a positive terminal of an LED load  1 ; and an anode of the diode D 2  is connected to an anode of the diode D 4 , a negative terminal of the electrolytic capacitor C 2 , a negative terminal of the LED load  1 , a negative terminal of the electrolytic capacitor C 3  and a negative terminal of the LED load  2 . 
       FIG. 13  illustrates a driving circuit for precise constant-current control of multiple LED branches according to an eleventh embodiment of the invention. The circuit shown in  FIG. 13  is used for current balancing between two LED loads where the high-frequency pulse AC current source is based on a push-pull circuit as an example. 
     As shown in  FIG. 13 , a main transformer T 2  has two primary windings W T1  and W T2 , and a non-dotted terminal of the first primary windings W T1  is connected with a dotted terminal of the second primary windings W T2 . 
     The high-frequency pulse AC current source includes a DC voltage Vdc, a switching tube S 31 , a switching tube S 32  and an inductor L 31 . Specifically, a positive terminal of the DC voltage Vdc is connected with the non-dotted terminal of the first primary winding W T1  (i.e., and the dotted terminal of the second primary winding W T2 ) via the inductor L 31 ; a dotted terminal of the first primary winding W T1  is connected to a first terminal of the switching tube S 31 ; a second terminal of the switching tube S 31  is connected to a negative terminal of the DC voltage Vdc; a non-dotted terminal of the second primary winding W T2  is connected to a first terminal of the switching tube S 32 ; and a second terminal of the switching tube S 32  is connected with the negative terminal of the DC voltage Vdc. 
     A dotted terminal of a secondary winding of the main transformer T 2  is connected to an anode of a diode D 1  and a cathode of the diode D 2 ; a non-dotted terminal of the secondary winding is connected to a terminal of a balancing capacitor C 1 ; the other terminal of the balancing capacitor C 1  is connected to an anode of a diode D 3  and a cathode of a diode D 4 ; a cathode of the diode D 1  is connected to a positive terminal of an electrolytic capacitor C 3  and a positive terminal of an LED load  2 ; a cathode of the diode D 3  is connected to a positive terminal of an electrolytic capacitor C 2  and a positive terminal of an LED load  1 ; and an anode of the diode D 2  is connected to an anode of the diode D 4 , a negative terminal of the electrolytic capacitor C 2 , a negative terminal of the LED load  1 , a negative terminal of the electrolytic capacitor C 3  and a negative terminal of the LED load  2 . 
       FIG. 14  illustrates a driving circuit for precise constant-current control of multiple LED branches according to a twelfth embodiment of the invention. The circuit shown in  FIG. 14  is used for current balancing between two LED loads where the high-frequency pulse AC current source is based on a forward circuit as an example. 
     The high-frequency pulse AC current source includes a DC voltage Vdc, a switching tube S 41 , a switching tube S 42 , an inductor L 41  and a capacitor C 41 . Specifically, a positive terminal of the DC voltage Vdc is connected to a terminal of the inductor L 41  and a terminal of the capacitor C 41 ; the other terminal of the inductor L 41  is connected to a dotted terminal of a primary winding of a main transformer T 2 ; the other terminal of the capacitor C 41  is connected to a first terminal of the switching tube S 41 ; a second terminal of the switching tube S 41  is connected to a non-dotted terminal of the primary winding of the main transformer T 2  and a first terminal of the switching tube S 42 ; and a second terminal of the switching tube S 42  is connected to a negative terminal of the DC voltage Vdc. 
     A dotted terminal of a secondary winding of the main transformer T 2  is connected to an anode of a diode D 1  and a cathode of a diode D 2 ; a non-dotted terminal of the secondary winding is connected to a terminal of a balancing capacitor C 1 ; the other terminal of the balancing capacitor C 1  is connected to an anode of a diode D 3  and a cathode of a diode D 4 ; a cathode of the diode D 1  is connected to a positive terminal of an electrolytic capacitor C 4  and a positive terminal of an LED load  2 ; a cathode of the diode D 3  is connected to a positive terminal of an electrolytic capacitor C 3  and a positive terminal of an LED load  1 ; and an anode of the diode D 2  is connected to an anode of the diode D 4 , a negative terminal of the electrolytic capacitor C 3 , a negative terminal of the LED load  1 , a negative terminal of the electrolytic capacitor C 4  and a negative terminal of the LED load  2 . 
     It is noted that the specific embodiments of the present invention described above are for illustrative purposes only. As a matter of course, the present invention is not limited to the embodiments above, but may include various variations. All the variations that those skilled in the art can make or derive directly from the disclosure of the present invention shall fall with the scope of protection of the present invention.