Patent Publication Number: US-9888536-B2

Title: Circuit of light-emitting elements

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 14/617,229, filed Feb. 9, 2015, which claims priority to Taiwanese Patent Application Serial Number 103125122, filed Jul. 22, 2014, the disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     Field of Invention 
     The present disclosure relates to a circuit of light-emitting elements. More particularly, the present disclosure relates to a circuit in which light-emitting elements share a zigzag conducting line configured to be electrically connected to a power terminal. 
     Description of Related Art 
     In recent years, display devices have become one of the indispensable components of electronic products. In addition, owing to the exponential growth of network transmission rate and high coding efficiency of video compression technology, consumers have increasing demands for display quality. 
     Generally speaking, a backlight module of a display device usually enhances contrast ratio of the display device by light dimming. In greater detail, when the display device displays a dark picture (such as a night scene), the luminance of the backlight module is reduced. When the display device displays a bright picture (such as a sunny day), the luminance of the backlight module is increased. However, since the brightness across the picture is not constant, the backlight module performs a local dimming operation so as to satisfy the desired brightness at all pixels constituting the picture, thus improving contrast ratio of the display device. 
     Conventionally, in order to achieve local dimming, conducting states of light-emitting elements are controlled by adjusting a plurality of negative power terminals.  FIG. 1  depicts a schematic diagram of a circuit of light-emitting elements  100 . The circuit of light-emitting elements  100  comprises a plurality of light-emitting elements  101 - 106 . First terminals of the light-emitting elements  101 - 106  are jointly in electrical connection with a positive power terminal. Second terminals of the light-emitting elements  101 - 106  are respectively in electrical connection with different negative power terminals. With respect to the driving method, the positive power terminal is maintained at a high voltage level, and local dimming function is implemented through adjusting voltage levels at the negative power terminals. 
     Although aforementioned method can achieve the objective of local dimming, paths starting from the positive power terminal, passing through each of the light-emitting elements  101 - 106 , and ending at corresponding negative power terminals do not have identical length, given that a viewpoint following the current flow is taken into account. Moreover, the conducting line itself has a specific resistance value, that is, R=ρ (L/A), wherein R is the resistance value of the conducting line, L is the length of the conducting line, A is the cross-sectional area of the conducting line, and p is the resistivity of the conducting line. The magnitude of the resistivity p relates to the material property of the conducting line itself. Hence, the resistance values of paths passing through each of the light-emitting elements  101 - 106  are different. As a result, under the same driving voltage, the light-emitting intensities of the light-emitting element  101 - 106  differ from each other. For example, since the path in which the current flows through the light-emitting element  101  is much longer than the path in which the current flows through the light-emitting element  106 , the luminance of the light-emitting element  101  is lower than the luminance of the light-emitting element  106 , which makes the local dimming method inefficiently raise the contrast ratio of the display device. In addition, adjusting voltage levels at the plurality of negative power terminals also increases design complexity and power consumption of the external driving circuit. 
     SUMMARY 
     To solve above difficulties, the present disclosure provides a circuit of light-emitting elements so that multiple light-emitting elements all exhibit the same luminance when local dimming is performed. 
     A circuit of light-emitting elements, electrically connected between two power terminals, is provided in one embodiment of the present disclosure. The circuit of light-emitting elements includes a smooth conducting line, multiple light-emitting elements, and a zigzag conducting line. The smooth conducting line is electrically connected to one of the power terminals. One terminal of each light-emitting element is electrically connected at a different position of the smooth conducting line. The zigzag conducting line is connected to the other of the power terminals. The other terminal of each light-emitting element is electrically connected at a different position of the zigzag conducting line. Each shortest path starting from the one of the power terminals, passing through any of the light-emitting elements along the smooth conducting line, and ending at the other of the power terminals along the zigzag conducting line has substantially a same resistance value. 
     A circuit of light-emitting elements, electrically connected between two power terminals, is provided in another embodiment of the present disclosure. The circuit of light-emitting elements includes multiple smooth conducting lines, multiple light-emitting element sets, and a zigzag conducting line. The smooth conducting lines are connected to one of the power terminals in sequence or simultaneously. Each light-emitting element set has a first light-emitting element and a second light-emitting element. The first light-emitting element and the second light-emitting element are electrically connected in parallel to the corresponding smooth conducting line. The zigzag conducting line is connected to the other of the power terminals. Each first or second light-emitting element is electrically connected at a different position of the zigzag conducting line. Each shortest path starting from one of the power terminals, passing through the corresponding first or second light-emitting element along any smooth conducting line, and ending at the other of the power terminals along the zigzag conducting line has substantially a same resistance value. 
     It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. In the drawings, 
         FIG. 1  depicts a schematic diagram of a circuit of light-emitting elements; 
         FIG. 2  depicts a schematic diagram of a circuit of light-emitting elements; 
         FIG. 3  depicts a schematic diagram of a circuit of light-emitting elements according to one embodiment of this disclosure; 
         FIG. 4  depicts a schematic diagram of a circuit of light-emitting elements according to another embodiment of this disclosure; 
         FIG. 5  depicts a schematic diagram of a circuit of light-emitting elements according to still another embodiment of this disclosure; 
         FIG. 6  depicts a schematic diagram of a circuit of light-emitting elements according to yet another embodiment of this disclosure; and 
         FIG. 7  depicts a schematic diagram of a circuit of light-emitting elements according to another embodiment of this disclosure 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various embodiments are described more fully below with reference to the accompanying drawings, which form a part hereof, and which show embodiments for practicing the disclosure. However, embodiments may be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. 
     It will be understood that when an element is referred to as being “connected” to another element, it can be directly connected to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” to another element, there are no intervening elements present. 
     As used herein, “substantially” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “substantially” can be inferred if not expressly stated. 
       FIG. 2  depicts a schematic diagram of a circuit of light-emitting elements  200 . The circuit of light-emitting elements  200  is electrically connected between a power terminal P 1  and a plurality of power terminals N 1 -N 3 . The circuit  200  comprises a plurality of light-emitting elements  201 - 203 , a smooth conducting line  211 , a zigzag conducting line  212 , a zigzag conducting line  213 , a smooth conducting line  214 , and plurality of first connecting line segments  221 - 223 . The power terminal P 1  and the plurality of power terminals N 1 -N 3  are controlled by an external driving circuit (not shown in the figure). Each of the light-emitting elements  201 - 203  comprises a first terminal and a second terminal. 
     The first terminal of the light-emitting element  201  is electrically connected to the smooth conducting line  214  via the first connecting line segment  221 , and the second terminal is electrically connected to the smooth conducting line  211 . 
     The first terminal of the light-emitting element  202  is electrically connected to the smooth conducting line  214  via the first connecting line segment  222 , and the second terminal is electrically connected to the zigzag conducting line  212 . 
     The first terminal of the light-emitting element  203  is electrically connected to the smooth conducting line  214  via the first connecting line segment  223 , and the second terminal is electrically connected to the zigzag conducting line  213 . 
     In the present embodiment, the power terminal P 1  is maintained at a high voltage. The power terminals N 1 -N 3  are configured for receiving different control voltages to control the conducting states of the light-emitting elements  201 - 203  (that is, whether the light-emitting elements  201 - 203  emit light or not) so as to allow the circuit  200  achieve the objective of local dimming. For example, if it is intended to turn on the light-emitting element  202  and turn off the light-emitting elements  201 ,  203 , what is needed is to simply set the voltage at the terminal N 2  to a low level and the voltages at the terminals N 1  and N 3  to a high level. Then, the light-emitting element  202  will emit light alone. 
     The zigzag conducting lines  212 ,  213  are respectively configured for adjusting shortest path lengths of currents flowing through the light-emitting elements  202 ,  203  so that shortest paths respectively corresponding to the light-emitting elements  201 - 203  have substantially the same length. In greater detail, take the light-emitting elements  201 ,  202  for example, and the light-emitting elements  201 ,  202  are regarded as conducting lines. A difference between a length of the zigzag conducting line  212  and a partial length D 1  of the smooth conducting line  211  is used for compensating a partial length D 2  of the smooth conducting line  214 , a partial length D 3  of the first connecting line segment  221 , and a partial length D 4  of the smooth conducting line  211 , which allows the shortest paths of currents flowing through the light-emitting elements  201 ,  202  have substantially the same length. That is, magnitudes of resistance values of paths respectively from the power terminal P 1  to the power terminals N 1 , N 2  are substantially identical. Similarly, the zigzag conducting line  213  corresponding to the light-emitting element  203  is also configured for compensating path differences resulting from a distinct position of the light-emitting element  203  relative to the light-emitting elements  201 ,  202 . 
     Hence, the shortest paths respectively starting from the power terminal P 1 , passing through the light-emitting elements  201 - 203 , and ending at the corresponding power terminals N 1 -N 3  have substantially the same length; that is, the shortest paths have substantially the same resistance value. 
     However, if the light-emitting elements  201 - 203  emit light simultaneously, adjustment of the voltages at the power terminals N 1 -N 3  electrically connected to the circuit  200  may still slightly increase design complexity and power consumption of the external driving circuit, although the light-emitting elements  201 - 203  are able to exhibit the same luminance. In addition, the circuit of light-emitting elements  200  is usually fabricated as a slim light bar when the circuit  200  is applied to an edge-lit light emitting diode (LED) backlight of a liquid crystal display. It is not easy to lay out and dispose within the narrow light bar width a plurality of zigzag conducting lines that occupy more width. If the number of the light-emitting elements is increased, the number of zigzag conducting lines needs to be increased accordingly. As a result, the light bar width needs to be enlarged, which contradicts the trend of thinning backlight modules. 
     In order to resolve the above-mentioned problem, a description is provided with reference to  FIG. 3 .  FIG. 3  depicts a schematic diagram of a circuit of light-emitting elements  300  according to one embodiment of this disclosure. The circuit  300  is electrically connected between the power terminal P 1  and the plurality of power terminals N 1 , N 2 . The circuit  300  comprises a plurality of light-emitting elements  301 - 304 , a smooth conducting line  311 , a zigzag conducting line  312 , a smooth conducting line  313 , a plurality of first connecting line segments  321 - 324 , a plurality of second connecting line segments  331 - 333 , a plurality of switching elements  341 - 344 , and a plurality of control lines  351 - 354 . Each of the light-emitting elements  301 - 304  comprises a first terminal and a second terminal. Each of the switching elements  341 - 344  comprises a first terminal, a second terminal, and a control terminal. The zigzag conducting line  312  comprises a plurality of zigzag segments  312   a - 312   c.    
     The first terminals of the light-emitting elements  301 - 304  are electrically connected at different positions of the smooth conducting line  313  respectively via the first connecting line segments  321 - 324 . The second terminal of the light-emitting element  301  is electrically connected to the smooth conducting line  311 . The second terminals of the light-emitting elements  302 - 304  are electrically connected at different positions of the zigzag conducting line  312 . 
     The zigzag conducting line  312  comprises the plurality of zigzag segments  312   a - 312   c  connected in series. Each of the zigzag segments  312   a - 312   c  comprises a starting end. 
     The switching element  341  is electrically connected between the light-emitting element  301  and the smooth conducting line  311 . The switching elements  342 - 344  are electrically connected respectively between the light-emitting elements  302 - 304  and the different positions of the zigzag conducting line  312 . Specifically, the first terminals of the switching elements  341 - 344  are respectively in electrical connection with the second terminals of the light-emitting elements  301 - 304 . The control terminals of the switching elements  341 - 344  are respectively in electrical connection with the control lines  351 - 354 . The second terminal of the switching element  341  is electrically connected to the smooth conducting line  311 . The second terminals of the switching elements  342 - 344  are electrically connected the starting ends of the zigzag segments  312   a - 312   c  respectively via the second connecting line segments  331 - 333 . In other words, the switching elements  342 - 344  are electrically connected between the second connecting line segments  331 - 333  and the light-emitting elements  302 - 304 . 
     In some embodiments, the switching elements  342 - 344  may be electrically connected between the smooth conducting line  313  and the light-emitting elements  302 - 304 . More specifically, the switching elements  342 - 344  are electrically connected between the first connecting line segments  322 - 324  and the light-emitting elements  302 - 304 . 
     In some embodiments, the first connecting line segments  322 - 324  have a same resistance value, and the second connecting line segments  331 - 333  have different resistance values. 
     It is worth to notice that the zigzag conducting line  312  and the second connecting line segments  331 - 333  are configured for adjusting the shortest path lengths of currents flowing through the light-emitting elements  302 - 304  so that the shortest paths respectively corresponding to the light-emitting elements  301 - 304  have substantially the same length. In greater detail, take the light-emitting elements  302 ,  303  for example, if lengths of the first connecting line segments  321 - 324  are the same, a length of the second connecting line segment  332  is set to be a sum of a length of the zigzag segment  312   a , a length of the second connecting line segment  331  and a partial length D 5  of the smooth conducting line  313  so as to allow the shortest path lengths of currents flowing through the light-emitting elements  302 ,  303  are substantially the same. That is, magnitudes of resistance values of paths respectively from the power terminal P 1 , through the light-emitting elements  302 ,  303 , to the power terminal N 2  are substantially the same. Similarly, the second connecting line segments  333  corresponding to the light-emitting element  304  is also used for compensating path differences resulting from a distinct position of the light-emitting element  304  relative to positions of the light-emitting elements  302 ,  303 . 
     Hence, the shortest paths starting from the power terminal P 1 , respectively passing through the light-emitting elements  301 - 304 , and ending at the corresponding power terminals N 1 -N 2  have substantially the same length, that is, have substantially the same resistance value. The shortest path corresponding to the light-emitting element  301  comprises the smooth conducting line  313 , the first connecting line segment  321 , the light-emitting element  301 , the switching element  341 , and the smooth conducting line  311 . The shortest path corresponding to the light-emitting element  302  comprises part of the smooth conducting line  313 , the first connecting line segment  322 , the light-emitting element  302 , the switching element  342 , the second connecting line segment  331 , and the zigzag conducting line  312 . The shortest path corresponding to the light-emitting element  303  comprises part of the smooth conducting line  313 , the first connecting line segment  323 , the light-emitting element  303 , the switching element  343 , the second connecting line segment  332 , and part of the zigzag conducting line  312  (the zigzag segment  312   b  and the zigzag segment  312   c ). The shortest path corresponding to the light-emitting element  304  comprises part of the smooth conducting line  313 , the first connecting line segment  324 , the light-emitting element  304 , the switching element  344 , the second connecting line segment  333 , and part of the zigzag conducting line  312  (the zigzag segment  312   c ). 
     The control lines  351 - 354  are configured for receiving control signals C 1 -C 4  transmitted from an external driving circuit (not shown in the figure) to allow the switching elements  341 - 344  to determine turning-on/off of the light-emitting elements  301 - 304  according to the above-mentioned control signals C 1 -C 4 . 
     In the present embodiment, the power terminal P 1  stays at a high voltage level, and the power terminals N 1 , N 2  are kept at a low voltage level. If it is intended to turn on the light-emitting elements  301 ,  304 , the external driving circuit (not shown in the figure) would control the control signals C 1 , C 4  to have an enabling voltage and the control signals C 2 , C 3  to have a disabling voltage. Thus, the light-emitting elements  301 ,  304  will emit light, and the objective of local dimming may be carried out. It should be noticed that when performing local dimming, the control signals C 1 -C 4  in the embodiment are low voltage signals as compared with those (high voltage signals) at the terminals P 1 , P 2  of the circuit  200  shown in  FIG. 2 . Using low voltage signals for controlling conducting states of the light-emitting elements  301 - 304  would effectively reduce power consumption and design complexity in the external driving circuit. Additionally, the light-emitting elements  302 ,  303 ,  304  in the embodiment share the same zigzag conducting line  312 , which differs from the prior art adopting multiple zigzag conducting lines occupying larger widths. The embodiment can further lessen width of the light bar in the backlight module, achieving a result of thinning the backlight module. 
     In some embodiments, the zigzag segments  312   a - 312   c  may have different resistance values, different conducting line lengths, different conducting line widths, different conducting line densities, or different shapes. 
     Referring to  FIG. 4 .  FIG. 4  depicts a schematic diagram of a circuit of light-emitting elements  400  according to another embodiment of this disclosure. As compared with the circuit of light-emitting elements  300  in  FIG. 3  which comprises the zigzag conducting line  312 , the circuit  400  according to the present embodiment comprises a zigzag conducting line  401  and a smooth conducting line  402 . The zigzag conducting line  401  comprises zigzag segments  401   a - 401   c  connected in series. Each of the zigzag segments  401   a - 401   c  comprises a starting end. 
     The zigzag conducting line  401  is electrically connected to the power terminal N 2  via the smooth conducting line  402 . The starting end of the zigzag segment  401   a  electrically connects to the second connecting line segment  331 . The starting end of the zigzag segment  401   b  electrically connects to the second connecting line segment  332 . The starting end of the zigzag segment  401   c  electrically connects to the second connecting line segment  333 . By adjusting line widths and lengths of the zigzag segments  401   a - 401   c  as well as lengths of the second connecting line segments  331 - 333 , resistance values of shortest paths passing through the light-emitting elements  302 - 304  and a resistance value of a shortest path passing through the light-emitting element  301  are substantially the same. 
     In the embodiment, based on distances between the light-emitting elements  302 - 304  and the light-emitting element  301 , distances between the light-emitting elements  301 - 304  and the power terminal P 1 , and distances between the light-emitting elements  301 - 304  and the power terminal N 2 , the order of conducting line widths of the zigzag segments  401   a - 401   c  from large to small is: the zigzag segment  401   a , the zigzag segment  401   b , and then the zigzag segment  401   c . With such a configuration, the shortest paths respectively starting from the power terminal P 1 , passing through the light-emitting elements  301 - 304 , and ending at the corresponding power terminals N 1 , N 2  would have substantially the same resistance value. In the embodiment, width design of a conducting line may affect resistance value of the related path under a premise that the length of the conducting line is fixed. For example, adopting a wider conducting line can lower the resistance value per unit length of the conducting line. Likewise, adopting a narrower conducting line can augment the resistance value per unit length of the conducting line. 
     The switching elements  341 - 344  are configured for controlling conducting states of the light-emitting elements  301 - 304 . The driving method of the circuit  400  is similar to that of the circuit  300  in  FIG. 3 . 
     Referring to  FIG. 5 .  FIG. 5  depicts a schematic diagram of a circuit of light-emitting elements  500  according to still another embodiment of this disclosure. As compared with  FIG. 4 , a plurality of zigzag segments  501   a - 501   b  of a zigzag conducting line  501  of the circuit  500  have similar conducting line widths but different conducting line densities. In the present embodiment, the zigzag conducting line  501  is a sinusoidal conducting line; the conducting line density of the zigzag conducting line  501  may thus be defined as the spatial frequency of the sinusoidal wave shown by the zigzag conducting line  501 . Based on distances between the light-emitting elements  302 - 304  and the light-emitting element  301 , distances between the light-emitting elements  301 - 304  and the power terminal P 1 , and distances between the light-emitting elements  301 - 304  and the power terminals N 1 , N 2 , the order of the conducting line densities of the zigzag segments  501   a - 501   c  from small to large is: the zigzag segment  501   a , the zigzag segment  501   b , and then the zigzag segment  501   c.    
     As to the driving method, the driving method of the circuit  500  is similar to the driving method of the circuit  400  in  FIG. 4 . 
     As shown in  FIG. 3  to  FIG. 5 , the proportion of the light-emitting elements to the switching elements in either of the circuits  300 ,  400 ,  500  is 1:1. In other words, it is necessary to supplement one switching element if an extra light-emitting element is added. In a small-sized display device, since only a few lighting segments have to be controlled, it is feasible to increase the number of the switching elements appropriately. In an embodiment of large-sized display device, based on the technique of utilizing the zigzag conducting line to balance resistance values in aforementioned embodiments, the present disclosure further provides extensive embodiments to lower the number of the switching elements needed for increasing lighting segments, and therefore further simplify wire connection. 
     Referring to  FIG. 6  and  FIG. 7  simultaneously.  FIG. 6  depicts a schematic diagram of a circuit of light-emitting elements  600  according to yet another embodiment of this disclosure. The circuit  600  is electrically connected between the power terminal P 1  and the power terminal N 1 . The circuit  600  comprises a plurality of light-emitting element sets  601 - 602 , a zigzag conducting line  611 , a plurality of smooth conducting lines  612 - 613 , a plurality of first connecting line segments  621 - 622 , a plurality of second connecting line segments  631  and  633 , a plurality of third connecting line segments  632  and  634 , a plurality of switching elements  641 - 642 , and a plurality of control lines  651 - 652 . External switches  643 - 644  are connected between the power terminal P 1  and the circuit  600 . 
     The light-emitting element set  601  comprises a first light-emitting element  601   a  and a second light-emitting element  601   b . The first and second light-emitting elements  601   a ,  601   b  are electrically connected in parallel to the smooth conducting line  613 . The first and second light-emitting elements  601   a ,  601   b  are electrically connected at different positions of the zigzag conducting line  611 . The light-emitting element set  602  comprises a first light-emitting element  602   a  and a second light-emitting element  602   b . The first and second light-emitting elements  602   a ,  602   b  are electrically connected in parallel to the smooth conducting line  612 . The first and second light-emitting elements  602   a ,  602   b  are electrically connected at different positions of the zigzag conducting line  611 . 
     In the present embodiment, the switching element  641  is connected in series between the first light-emitting element  601   a  and the zigzag conducting line  611 , and the switching element  642  is connected in series between the first light emitting element  602   a  and the zigzag conducting line  611 . In some embodiments, the switching element  641  may be connected in series between the second light-emitting element  601   b  and the zigzag conducting line  611 , and the switching element  642  is connected in series between the second light emitting element  602   b  and the zigzag conducting line  611 . 
     The first connecting line segment  621  is configured for electrically connecting the first light-emitting element  601   a  and the second light-emitting element  601   b  to the smooth conducting line  613 . The first connecting line segment  622  is configured for electrically connecting the first light-emitting element  602   a  and the second light-emitting element  602   b  to the smooth conducting line  612 . 
     The second connecting line segment  631  is configured for electrically connecting the switching element  641  to the zigzag conducting line  611 . The second connecting line segment  633  is configured for electrically connecting the switching element  642  to the zigzag conducting line  611 . 
     The third connecting line segment  632  is configured for electrically connecting the second light-emitting element  601   b  to the zigzag conducting line  611 . The third connecting line segment  634  is configured for electrically connecting the second light-emitting element  602   b  to the zigzag conducting line  611 . 
     It is worth to note that the second and third connecting line segments  631 ,  633 ,  632 ,  634  are respectively configured for adjusting the shortest paths passing through the first and second light-emitting elements  601   a ,  602   a ,  601   b ,  602   b  to have substantially the same length. As a result, the shortest paths passing through the light-emitting elements  601   a ,  602   a ,  601   b ,  602   b  have substantially the same resistance value. 
     In some embodiments, the second connecting line segments  631 ,  633  and the third connecting line segments  632 ,  634  are respectively configured for adjusting conducting line widths, conducting line shapes, or conducting line densities thereof so that the shortest paths passing through the light-emitting elements  601   a ,  602   a ,  601   b ,  602   b  have substantially the same resistance value. 
     In some embodiments, the second connecting line segments  631 ,  633  and the third connecting line segments  632 ,  634  may be electrically connected to starting ends of different zigzag segments of the zigzag conducting line  611 , so that the shortest paths passing through the light-emitting elements  601   a ,  602   a ,  601   b ,  602   b  have substantially the same resistance value. 
     The switching element  641  is configured for controlling conducting states of the first light-emitting elements  601   a . The switching element  642  is configured for controlling conducting states of the first light-emitting elements  602   a.    
     The external switches  634 ,  644  are configured for determining whether the smooth conducting line  613 ,  612  are electrically connected to the power terminal P 1  so as to control conducting states of the first light-emitting element  601   a , the second light-emitting element  601   b , the first light-emitting element  602   a , and the second light-emitting element  602   b.    
     The control line  651  is configured for receiving a control signal C 5 . The control line  652  is configured for receiving a control signal C 6 . The control signals C 5 , C 6  are respectively configured for controlling conducting states of the switching elements  641 - 642  so as to regulate conducting states of the first light-emitting elements  601   a ,  602   a.    
     As to the driving method, the control signals C 5 , C 6  and the external switches  643 - 644  are configured for controlling conducting states of the light-emitting element sets  601 - 602 . Take the light-emitting element set  601  for example, when the control signal C 5  has an enabling signal and the external switch  643  is turned on, the two light-emitting elements  601   a - 601   b  in the light-emitting element set  601  are simultaneously turned on. When the control signal C 5  has a disabling signal and the external switch  643  is turned on, the light-emitting element  601   b  in the light-emitting element set  601  is turned on alone. When the control signal C 5  has the disabling signal and the external switch  643  is turned off, none of the light-emitting elements  601   a - 601   b  in the light-emitting element set  601  will be turned on. The driving method of the light-emitting element set  602  is similar to the driving method of the light-emitting element set  601 . 
     In summary, each current path in the circuit  600  has substantially the same resistance value so as to allow each of the light-emitting elements  601   a ,  601   b ,  602   a ,  602   b  has roughly the same luminance. In addition, since the external switches  643 ,  644  usually locate on one side of a circuit board or are implemented in an external driving circuit (such as a driver IC chip), the number of the switching elements in the circuit  600  won&#39;t be increased. Hence, the proportion of the light-emitting elements  601   a ,  601   b ,  602   a ,  602   b  to the switching elements  641 ,  642  becomes 2:1. As therefore, wire connection in the circuit  600  shall not be complicated due to excessive switching elements. 
       FIG. 7  depicts a schematic diagram of a circuit of light-emitting elements  700  according to another embodiment of this disclosure. As compared with the circuit  600  in  FIG. 6 , the light-emitting element sets  601 / 602  of the circuit  700  further comprises a third light-emitting element  601   c / 602   c . The circuit  700  further comprises a plurality of fourth connecting line segments  701 ,  702 . In the present embodiment, neither the power terminal P 1  nor the power terminal N 1  has a fixed voltage. 
     The third light-emitting element  601   c , the first light-emitting element  601   a , and the second light-emitting element  601   b  are electrically connected in parallel to the smooth conducting line  613 . The third light-emitting element  602   c , the first light-emitting element  602   a  and the second light-emitting element  602   b  are electrically connected in parallel to the smooth conducting line  612 . 
     The fourth connecting line segments  701 ,  702  are respectively configured for electrically connecting the third light-emitting elements  601   c ,  602   c  to the second connecting line segments  631 ,  633 . 
     As to the driving method, the control signals C 5 , C 6 , the external switches  643 - 644 , and the power terminals P 1 , N 1  are respectively configured for controlling conducting states of the light-emitting element sets  601 - 602 . Take the light-emitting element set  601  for example, when the power terminal P 1  has a high voltage and the power terminal N 1  has a low voltage, the driving method for the first and second light-emitting elements  601   a ,  601   b  in the circuit  700  is similar to that for the first and second light-emitting elements  601   a ,  601   b  in the circuit  600  of  FIG. 6 . Additionally, when the power terminal P 1  has a low voltage, the power terminal N 1  has a high voltage, and the external switch  643  is turned on, the third light-emitting element  610   c  would be turned on. 
     Hence, in the circuit  700 , the proportion of the light-emitting elements  601   a ,  601   b ,  601   c ,  602   a ,  602   b ,  602   c  to the switching elements  641 ,  642  becomes 3:1. In other words, the circuit  700  reduces the number of switching elements corresponding to each light-emitting element. 
     In all preceding embodiments, each of the switching elements may be a bipolar junction transistor (BJT). 
     In every forgoing embodiment, each the light-emitting element may be a light-emitting diode. 
     In terms of prior embodiments, applying the disclosed circuit of light-emitting elements not only makes the shortest path corresponding to each light-emitting element have substantially the same resistance value, but also effectively diminishes design complexity and power consumption of the external driving circuit. In addition, the number of switching elements utilized in the circuit of light-emitting elements is effectively reduced. Consequently, the objective of local dimming is achieved and the contrast ratio of the display device is efficiently enhanced. The present disclosure can further narrow the width of the light bar in the backlight module so as to get the result of thinning the backlight module. 
     Although the present disclosure has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims and their equivalents.