Abstract:
The invention provides a current balancing circuit, which includes a plurality of light-emitting diode assemblies; an AC power generator for providing currents required by the light-emitting diode assemblies; and a plurality of current-equaling elements connected to the AC power generator, each of which is connected to a common mode connecting two light-emitting diode assemblies for balancing currents of the light-emitting diode assemblies.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a current balancing circuit, and more particularly to a current balancing circuit for balancing currents flowing through a plurality of light-emitting diode (LED) assemblies. 
       BACKGROUND OF THE INVENTION 
       [0002]    Recently, with the breakthrough advancement of the manufacturing technique of light-emitting diodes (LEDs), the luminance and efficiency of the light-emitting diodes are greatly improved. The LED has replaced old fluorescent lamps as the illuminating device of the next generation. Nowadays, the LED has been widely employed in the applications of home illuminating appliances, car illuminating devices, handheld illuminating devices, the backlight source of LCD panels, traffic signal indicators, and billboards. In order to increase the luminance of the LED, a number of LEDs are connected in series to form a LED assembly. 
         [0003]    LEDs are generally DC loads. In the application where a number of parallel-connected LED assemblies are employed, the current flowing through the LED assemblies are different from each other as the characteristics and impedance of each LED are different from each other. If the currents flowing through the LED assemblies are not balanced, the luminance will not be uniform and the longevity of respective LED will be shortened. This would further impair the electronic device. 
         [0004]    In order to tackle the problem that the currents of the LED assemblies are not uniform, several current balancing techniques have been proposed to address this problem. One of such techniques is to employ independent drivers to individually drive each LED assembly. However, such independent driver will complicate the circuitry and increase the manufacturing cost. More disadvantageously, the current balancing effect is bad as the driver has tolerance. Another state-of-the-art current balancing technique is to use a common choke to equal the currents flowing through the LED assemblies. Nonetheless, using a plurality of common chokes will increase the number of magnetic elements in the circuitry, and thus the manufacturing cost is elevated and the size of the circuitry is expanded. More disadvantageously, the current balancing effect is also bad as the common choke will induce a magnetizing current. 
         [0005]    Hence, it is needed to develop a current balancing circuit for addressing the aforementioned problems encountered by the prior art. 
       SUMMARY OF THE INVENTION 
       [0006]    The primary object of the invention is to provide a current balancing circuit for balancing the currents flowing through a plurality of LED assemblies, thereby addressing the problem encountered by the prior art. 
         [0007]    To this end, the invention provides a current balancing circuit, including a plurality of light-emitting diode assemblies, which includes a first light-emitting diode assembly; a second light-emitting diode assembly connected to the first light-emitting diode assembly in parallel in reverse order, and connected with the first light-emitting diode assembly through a first common node; and a third light-emitting diode assembly connected to the second light-emitting diode assembly in parallel in reverse order, and connected with the second light-emitting diode assembly through a second common node. The current balancing circuit also includes an AC power generator for providing currents required by the first light-emitting diode assembly, the second light-emitting diode assembly, and the third light-emitting diode assembly. The current balancing circuit also includes a plurality of current-equaling elements, which includes a first current-equaling element connected between the AC power generator and the first common node for balancing the current of the first light-emitting diode assembly and the current of the second light-emitting diode assembly, and a second current-equaling element connected between the AC power generator and the second common node for balancing the current of the second light-emitting diode assembly and the current of the third light-emitting diode assembly. 
         [0008]    Now the foregoing and other features and advantages of the invention will be best understood through the following descriptions with reference to the accompanying drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates the circuit block diagram of the current balancing circuit according to a preferred embodiment of the invention; 
           [0010]      FIG. 2  illustrates the partial circuitry of the current balancing circuit of  FIG. 1 ; 
           [0011]      FIG. 3  illustrates the circuitry of the current balancing circuit of  FIG. 2  with an additional LED assembly incorporated in the circuitry; 
           [0012]      FIG. 4  illustrates a modified circuitry of the current balancing circuit of  FIG. 2 ; and 
           [0013]      FIG. 5  illustrates another modified circuitry of the current balancing circuit of  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    Several exemplary embodiments embodying the features and advantages of the invention will be expounded in following paragraphs of descriptions. It is to be realized that the present invention is allowed to have various modification in different respects, all of which are without departing from the scope of the present invention, and the description herein and the drawings are to be taken as illustrative in nature, but not to be taken as a confinement for the invention. 
         [0015]    Referring to  FIGS. 1 and 2 , in which  FIG. 1  illustrates the circuit block diagram of the current balancing circuit according to a preferred embodiment of the invention, and  FIG. 2  illustrates the partial circuitry of the current balancing circuit of  FIG. 1 . As shown in  FIGS. 1 and 2 , a current balancing circuit  1  is applied to various illuminating devices, such as home illuminating devices, car illuminating devices, handheld illuminating devices, backlight source for LCD panels, traffic signal lights, and billboards. The current balancing circuit  1  includes a plurality of LED assemblies, a plurality of current-equaling elements, and an AC power generator. 
         [0016]    In this embodiment, the number of the LED assemblies is N, where N is a positive integer and is larger than or equal to 3. The number of the current-equaling elements is less than the number of the LED assemblies by one. That is, the number of the current-equaling elements is N−1. In this embodiment, the current balancing circuit  1  includes a first LED assembly  10 , a second LED assembly  11 , a third LED assembly  12 , a first current-equaling element  14 , and a second current-equaling element  15 . The first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12  are powered by an AC voltage V AC  provided by the AC power generator  13 . The first LED assembly  10  and the second LED assembly  11  are connected in parallel with each other in reverse order. Also, the second LED assembly  11  and the third LED assembly  12  are connected in parallel with each other in reverse order. In other words, as shown in  FIG. 2 , the negative terminal of the first LED assembly  10  is connected to the negative terminal of the AC power generator  13 . The positive terminal of the first LED assembly  10  and the negative terminal of the second LED assembly  11  are connected to a first common node A. The positive terminal of the second LED assembly  11  and the negative terminal of the third LED assembly  12  are connected to a second common node B. The positive terminal of the third LED assembly  12  is connected to the positive terminal of the AC power generator  13 . 
         [0017]    In alternative embodiments, the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12  may include a single LED or a plurality of serially-connected diodes, respectively. Also, in alternative embodiments, the connecting relationship of the positive terminals and the negative terminals of the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12  may be opposite to the connecting relationship of the positive terminals and the negative terminals of the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12  shown in  FIG. 2 . 
         [0018]    The first current-equaling element  14  is connected between the positive terminal of the AC power generator  13  and the first common node A for balancing the current of the first LED assembly  10  and the current of the second LED assembly  11 . The second current-equaling element  15  is connected between the negative terminal of the AC power generator  13  and the second common node B for balancing the current of the second LED assembly  11  and the current of the third LED assembly  12 . 
         [0019]    In the foregoing embodiments, the first current-equaling element  14  and the second current-equaling element  15  may include a capacitor, respectively. As the capacitor has the Amp-Second balance characteristic, i.e. The charge balance characteristic of the capacitor, the average current flowing through the first current-equaling element  14  and the average current flowing through the second current-equaling element  15  will both be zero. In other words, the average current flowing through the first current-equaling element  14  in forward direction will be equal to the average current flowing through the first current-equaling element  14  in reverse direction, and the average current flowing through the second current-equaling element  15  in forward direction will be equal to the average current flowing through the second current-equaling element  15  in reverse direction. Hence, the first current-equaling element  14  can balance the current of the first LED assembly  10  and the current of the second LED assembly  11 , and the second current-equaling element  15  can balance the current of the second LED assembly  11  and the current of the third LED assembly  12 . Thus, the current of first LED assembly  10  and the current of the second LED assembly  11  and the current of the third LED assembly  12  will be balanced simultaneously by the first current-equaling element  14  and the second current-equaling element  15 . As the invention employs the physical characteristics of the capacitor to attain the current balance for a plurality of LED assemblies, the current balancing effect of the invention is much better compared to the conventional current balancing techniques of using drivers or common chokes. More advantageously, the current balancing circuit  1  of the invention has a simple circuitry, a small size, and low manufacturing cost. 
         [0020]    In the foregoing embodiments, the AC voltage V AC  outputs its positive half-cycle voltages and its negative half-cycle voltages through the first current-equaling element  14  to drive the first LED assembly  10  and the second LED assembly  11 , respectively. Also, the AC voltage V AC  outputs its positive half-cycle voltages and its negative half-cycle voltages through the second current-equaling element  15  to drive the third LED assembly  12  and the second LED assembly  11 , respectively. Hence, the AC voltage V AC  can alternately drive the first LED assembly  10  and the second LED assembly  11  to illuminate, and can alternately drive the second LED assembly  11  and the third LED assembly  12  to illuminate. 
         [0021]    In alternative embodiments, the AC power generator  13  may include a commercially available power source or a generator for directly outputting the AC voltage V AC , as shown in  FIG. 1 . In alternative embodiments, the AC power generator  13  may be implemented by a full-bridge resonant DC-AC converter, as shown in  FIG. 2 . The full-bridge resonant DC-AC converter  13  of  FIG. 2  is used to receive an input voltage V IN  and convert the input voltage V IN  into a sinusoidal AC voltage V AC . The full-bridge resonant DC-AC converter  13  of  FIG. 2  includes a switch circuit  130 , a resonant tank  131 , a transformer T, and a stabilizing capacitor C f . The switch circuit  130  is used to receive the input voltage V IN  and includes a plurality of switch elements Q 1 -Q 4 . The switch elements (Q 1 , Q 2 ) and the switch elements (Q 3 , Q 4 ) respectively form two rectifier arms. The driving signals received by the switch elements (Q 1 , Q 4 ) and the switch elements (Q 2 , Q 3 ) (not shown) are set to drive the switch elements to conduct switching operations with their duty ratio being approximate to 50%, and the switch elements Q 1 -Q 4  are set to conduct zero-voltage switching operations. The resonant tank  131  and the primary winding N p  of the transformer T are connected to the intermediate nodes of the rectifier arms in the switch circuit  130 . The resonant tank  131  may include a resonant capacitor C r  and a resonant inductor L r  connected in series with each other. The energy received by the primary winding N p  of the transformer T is transformed and the transformed energy is outputted by the secondary winding N s  of the transformer T. The filtering capacitor C f  is connected across the secondary winding N s  of the transformer T for filtering and stabilizing the energy outputted by the secondary winding N s , thereby generating the AC voltage V AC . 
         [0022]    Certainly, the number of the LED assemblies in the current balancing circuit  1  of the invention may not be limited to three as shown in  FIGS. 1 and 2 . As shown in  FIG. 3 , the current balancing circuit  1  includes an fourth LED assembly  16  in addition to the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12 . The fourth LED assembly  16  is connected in parallel with the third LED assembly  12  in reverse order. That is, the positive terminal of the fourth LED assembly  16  is connected to the negative terminal of the AC power generator  13 , and the negative terminal of the fourth LED assembly  16  and the positive terminal of the third LED assembly  12  are connected to a third common node C. In order to balance the current of the third LED assembly  12  and the current of the fourth LED assembly  16 , the current balancing circuit  1  further includes a third current-equaling element  17  which is connected between the positive terminal of the AC power generator  13  and the third common terminal C and may include a capacitor C. It can be understood that the LED assemblies in the current balancing circuit  1  may be three or more, in which each LED assembly is connected to another LED assembly in parallel in reverse order and both are set to illuminate alternately. Also, the number of the current-equaling elements is less than the number of the LED assemblies by one, thereby allowing a plurality of current-equaling elements to balance the currents of a plurality of LED assemblies. 
         [0023]    In the alternative embodiment of  FIG. 4 , the current balancing circuit  1  may include a plurality of rectifying diodes, such as a first rectifying diode D 1 , a second rectifying diode D 2 , and a third rectifying diode D 3  which are respectively corresponding to the LED assemblies. The current balancing circuit  1  may include a plurality of filter circuits, such as a first filter circuit  18 , a second filter circuit  19 , and a third filter circuit  20 . The first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  may include a capacitor C 1  and may be connected in parallel with the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12 , respectively. The first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  are used to filter the abnormal pulse voltages and store the energy supplied by the AC voltage V AC . In case that the first LED assembly  10 , the second LED assembly  11 , or the third LED assembly  12  are put out during the positive half-cycle or the negative half-cycle of the AC voltage V AC , the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  are respectively set to supply the stored energy to the first LED assembly  10 , the second LED assembly  11 , or the third LED assembly  12 , thereby preventing the first LED assembly  10 , the second LED assembly  11 , or the third LED assembly  12  from being put out. Therefore, the LED D in the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12  can be free from the repetitive alternate dimming operations by the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20 . Thus, the longevity of the LED D is prolonged. 
         [0024]    The first rectifying diode D 1  and the first LED assembly  10  are connected in series with each other in forward order. The second rectifying diode D 2  and the second LED assembly  11  are connected in series with each other in forward order. The third rectifying diode D 3  and the third LED assembly  12  are connected in series with each other in forward order. The first rectifying diode D 1 , the second rectifying diode D 2 , and the third rectifying diode D 3  are used to prevent the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12  from being infiltrated by the bidirectional current when the AC voltage V AC  is outputting positive half-cycle voltages or negative half-cycle voltages. Thus, the current balance among the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12  can be ensured. 
         [0025]    Certainly, the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  of  FIG. 4  are not limited to be implemented by the capacitor C 1 . In the alternative embodiment of  FIG. 5 , the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  may be implemented by an inductor L. In case that the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  are implemented by the inductor L, the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  are connected in series with the first LED assembly  10 , the second LED assembly  11 , and the third LED assembly  12 , respectively. Under this condition, the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  of  FIG. 5  can achieve similar filtering effect with the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  of  FIG. 4 . Also, in case that the first filter circuit  18 , the second filter circuit  19 , and the third filter circuit  20  are implemented by the inductor L, as shown in  FIG. 5 , the first rectifying diode D 1 , the second rectifying diode D 2 , and the third rectifying diode D 3  may be kept in the circuitry, as shown in  FIG. 4 , or removed from the circuitry, as shown in  FIG. 5 . 
         [0026]    In conclusion, the current balancing circuit of the invention employs a plurality of current-equaling elements made up of capacitors to balance the currents flowing through the LED assemblies. Therefore, the invention is advantageous over the prior art in terms of simplified circuitry, low manufacturing cost, and small size. 
         [0027]    While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be restricted to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the invention which is defined by the appended claims.