Patent Publication Number: US-8970119-B2

Title: Light emitting device and method of controlling light emitting device

Description:
RELATED APPLICATIONS 
     The present application is a National Phase of International Application Number PCT/JP2011/063655, filed Jun. 15, 2011, and claims priority from Japanese Application Number 2010-166261. 
     TECHNICAL FIELD 
     The present invention relates to a light emitting device including light emitting components such as LEDs and a method of controlling the light emitting device. 
     BACKGROUND ART 
     The increased number of light emitting devices including LEDs (light emitting diodes) or other types of semiconductor components as light emitting components is used in recent years. Researches have been conducted on such light emitting devices for application thereof to light sources of backlight for liquid crystal displays because of high initial-driving performances and tolerances to vibration and repeated switching between on and off. 
     An example of light emitting device circuit configuration is disclosed in Patent Document 1. In light emitting devices having similar configurations as the one in Patent Document 1, power losses may increase depending on circuit configurations of the light emitting devices. This is because different control is required for those devices from devices including CCFLs (cold cathode fluorescent lamp) as light sources, which are conventional light sources of backlights. When the power losses increase, the amounts of heat generated during power consumption increase resulting in temperatures increases in the light emitting device. Therefore, some kind of measures to reduce the temperatures of the light emitting device, such as a heatsink, is required. 
     A technology for reducing a power loss in the light emitting device is disclosed in Patent Document 1. According to the technology, the power loss is reduced by the following method. In a circuit for driving sets of light emitting components in which light emitting components are connected in series, forward voltages in the sets are measured and a common voltage applied to the sets is properly adjusted. 
     RELATED ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-242477 
       
    
     Problem to be Solved by the Invention 
     Even if the technology disclosed in Patent Document 1 is used, a power loss due to differences in forward voltage among the sets of light emitting components cannot be reduced. The forward voltages of the sets are different from each other due to differences in forward voltage of each light emitting components. Therefore, the common voltage is determined based on the maximum forward voltage and applied to the sets of light emitting components. If the forward voltage of the set is not the maximum forward voltage, an excessive voltage is applied to a driver connected in series with the set of the light emitting components. Current that flow through respective sets of light emitting components are controlled by drivers to remain constant at a common amount. Certain amounts of currents flow through the drivers connected in series with the sets of light emitting components. Namely, a power loss due to the excessive voltage applied to the driver for the set of light emitting components, the forward voltage of which is not the maximum voltage, cannot be reduced. As a result, the temperature increases in some areas. Therefore, some measures for reducing the temperature increase of the drivers, such as heatsinks, are required. 
     DISCLOSURE OF THE PRESENT INVENTION 
     The present invention was made in view of the foregoing circumstances. An object of the present invention is to reduce a temperature increase in a driver due to differences in forward voltage of sets of light emitting components. 
     Means for Solving the Problem 
     To solve the above problem, a light emitting device according to the present invention includes a light emitting unit, a driver, a current divider, and a power supply unit. The light emitting unit includes a plurality of light emitting components connected in series. The driver is configured to control driving of the light emitting unit and connected in series with the light emitting components so as to form a series circuit. The current divider is connected in parallel to the driver of the series circuit. The power supply unit is configured to apply a voltage to the series circuit. 
     In this light emitting device, the current divider is connected in parallel to the driver of the series circuit. Therefore, some of the current from the light emitting unit can be fed to the current divider and an amount of current in the driver can be reduced in comparison to an amount of current flowing through the light emitting unit. According to the light emitting device, the amount of current in the driver can be reduced even when an excessive voltage is applied to the driver. Therefore, a power loss in the driver can be reduced and a temperature increase in the driver can be reduced. 
     In the light emitting device according to the present invention, the series circuit may include a plurality of series circuits connected in parallel to each other and to the current divider. The power supply unit may be configured to apply the same voltage to the series circuits. The control of the driving of the light emitting unit may be performed such that currents that flow through the light emitting units are adjusted to a common constant amount. If a first forward voltage Vf 1  of a first light emitting unit of a first series circuit is lower than a second forward voltage Vf 2  of a second light emitting unit of the second series circuit, a first divided current Id 1  in a first current divider connected to the first series circuit may be adjusted larger than a second divided current Id 2  in a second current divider connected to the second series circuit. 
     If the series circuits are connected in parallel to each other and the same voltage is applied to the series circuit by the power supply unit, excessive voltages are more likely to be applied to the drivers due to differences in forward voltages between the light emitting units. In one of the series circuits of this light emitting device, the forward voltage of the light emitting unit is relatively low and a relatively high excessive voltage is applied to the driver. The divided currents are set such that a relatively large divided current flows in the current divider connected to the series circuit. Namely, in the series circuit including the driver to which a relatively high excessive high voltage is applied to the driver, the divided current is adjusted such that a relatively small current flows in the driver. According to the light emitting device, a power loss in the driver can be reduced and a temperature increase in the driver can be reduced. 
     At least one of the current dividers may be connected to a regenerator configured to store a power by receiving the divided current in corresponding one of the current dividers. A power that is lost by a driver in a known configuration can be stored by the regenerator and thus a power loss in a generator can be reduced. 
     Each current divider may include a current divider driver configured to perform driving control on the divided current therein. With the current divider driver, the amount of current in the current divider can be adjusted. As a result, the amount of current in the driver can be properly adjusted. 
     The light emitting device may further include a control unit configured to control the driver and the current divider driver. Through the control of the driver and the current divider driver by the control unit, the currents that flow through the light emitting units can be controlled and the currents in the drivers and the current dividers are controlled, respectively. According to the light emitting device, the amounts of current that flow through the light emitting units can be maintained constant and the amounts of currents in the drivers and the current dividers can be properly adjusted. 
     The control unit may be configured to measure a driver voltage Vk applied to the driver of each series circuit, a driver current Ik of the driver, and the divided current Id in each current driver connected to the series circuit, and to control the driver and the current divider driver based on the measurements. With this configuration, the amounts of currents that flow through the light emitting units, the drivers, and the current dividers can be properly adjusted. 
     The control unit may be configured to control the current divider driver to restrict a flow of the divided current in the current divider connected to the series circuit including the light emitting unit, the forward voltage of which is the maximum voltage among the series circuit. 
     In the series circuit including the light emitting unit, the forward voltage of which is the maximum voltage, the voltage applied by the power supply unit is determined based on the series circuit. Generally, an excessive voltage is not produced in the driver or the excessive voltage is reduced. In such a driver, a power loss is small or a measure for reducing a temperature increase in the driver such as a heatsink may not be required. In this light emitting device, the control unit controls the current divider driver to restrict the flow of divided current in the current divider in the series circuit including the light emitting unit, the forward voltage of which is the maximum voltage. According to the light emitting device, the control by the control unit can be simplified. 
     The constant current light emitting components may be LEDs. With this configuration, a temperature increase in the driver configured to drive the LEDs in the light emitting device including the LEDs can be reduced. 
     The light emitting device may be configured for a liquid display device. With this configuration, the light emitting device in which a temperature increase in the driver is reduced can be used for a backlight of a liquid crystal display. Namely, a backlight in which a light emitting amount is adjusted and a local temperature increase is less likely to occur can be provided. 
     Advantageous Effect of the Invention 
     According to the present invention, a temperature increase in the driver due to forward voltage differences between the light emitting components can be reduced. Therefore, a measure for reducing the temperature increases in the driver such as a heatsink is not required or a size of the heatsink can be reduced even in a case that the heatsink is required. The configuration of the light emitting device can be simplified and the manufacturing cost can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an LED backlight  10 . 
         FIG. 2  is a circuit diagram of a parallel circuit H 1 . 
         FIG. 3  is a flowchart illustrating a control process performed by a control unit  24 . 
         FIG. 4  is a waveform diagram illustrating waveforms of a driver current Ik and a divided current Id. 
         FIG. 5  is a waveform diagram illustrating waveforms of a driver current Ik and a divided current Id. 
         FIG. 6  is a waveform diagram illustrating waveforms of a driver current Ik and a divided current Id. 
         FIG. 7  is a circuit diagram of a parallel circuit H 1 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiment 
     An embodiment of the present invention will be explained with reference to the drawings. This embodiment includes an LED backlight system  10  (an example of a light emitting device, hereinafter referred to as an LED backlight) as a backlight for a light emitting unit of a liquid crystal display device. However, a light emitting unit to which the scope of the present invention can be applied is not limited to the LED backlight  10 . The scope of the present invention can be applied to light emitting units used for various kinds of lighting devices and display devices. 
     1. Configuration of LED Backlight 
     The configuration of the LED backlight  10  will be explained with reference to  FIG. 1 . 
     As illustrated in  FIG. 1 , the LED backlight  10  includes a circuit  20 , a power supply unit  22 , and a control unit  24 . The circuit  20  includes four parallel circuits H 1  to H 4 . The parallel circuits H 1  to H 4  are connected in parallel to each other. The power supply unit  22  applies a common supply voltage Vo to the parallel circuits H 1  to H 4 . 
     The parallel circuit H 1  includes a light emitting unit  30 , a driver  32 , a current divider  34 , and a regenerator  40 . The light emitting unit  30  and the driver  32  are connected in series to form a series circuit T 1 . The voltage Vo is applied to the series circuit T 1 . Namely, the power supply unit  22  applies the voltage Vo to the series circuit T 1 . The current divider  34  is connected in parallel to the driver  32  of the series circuit T 1 . The current divider  34  includes a current divider driver  36 . The regenerator  40  is connected to the current divider  34 . 
     The control unit  24  is connected to the power supply unit  22  and configured to control the supply voltage Vo output by the power supply unit  22 . The control unit  24  is connected to the drivers  32  and the current dividers  34  of the parallel circuits H 1  to H 4  via separate lines and configured to individually control the drivers  32  and the current dividers  34 . 
     The parallel circuits H 2  to H 4  have the same configuration as that of the parallel circuit H 1  except for the regenerator  40  and thus the configuration thereof will not be explained. 
       FIG. 2  illustrates a detailed circuit configuration of the parallel circuit H 1 . 
     Light Emitting Unit  30   
     The light emitting unit  30  includes a plurality of white LEDs  42  (an example of a light emitting component) connected in series. Generally, each LED  40  is designed such that light emitting efficiency is at the maximum under constant current control. Therefore, the current that flows through the light emitting unit  30  is regulated to a predetermined constant current Io. In the LED  40 , a forward voltage drop occurs due to the current flowing through the LED  40 , and a forward voltage Vf 1  appears at the light emitting unit  30 . A driver voltage Vk 1  is calculated by subtracting the forward voltage Vf 1  of the light emitting unit  30  from the supply voltage Vo applied by the power supply unit  22  (Vo−Vf 1 ). The driver voltage Vk 1  is applied to the driver  32  and the current divider  36  connected in parallel to the driver  32 . 
     Generally, the forward voltage drops that occur in the LEDs  42  are different from one another. Therefore, the forward voltages Vf 1  appear at the light emitting units  30  are different from one another. Namely, different driver voltages Vk 1  to Vk 4  are applied to the respective drivers  32 . 
     Driver  32   
     Each driver  32  includes a switching component Q 1  (e.g., FET or another type of switching component having a similar configuration) and resistors R 1  to R 3 . The switching component Q 1  and the resistor R 1  are connected in series between a connecting point P and the ground G. The resistors R 2  and R 3  are connected in series between the connecting point P and the ground G. The connecting point P is a point at which the driver  32  is connected to the light emitting unit  30 . The ground G is also a point at which the driver  32  is connected to the power supply unit  22 . The resistances of the resistors R 2  and R 3  are set higher than those of the switching component Q 1  and the resistor R 1 . Therefore, the flow of current through the resistors R 2  and R 3  in the driver  32  is restricted. 
     The switching component Q 1  is connected to a control terminal S 1  of the control unit  24  and controlled by the control unit  24  between open and closed. As described earlier, the current flow through the resistors R 2  and R 3  is restricted in the driver  32 , and a current flows through the switching component Q 1  and the resistor R 1 . When the control unit  24  opens the switching component Q 1 , the driver current Ik 1  flows. When the control unit  24  closes the switching component Q 1 , the driver current Ik 1  stops. Namely, the flow of the driver current Ik 1  in the driver  32  is controlled by the control unit  24  using the switching component Q 1 . 
     A current measurement terminal I 1  of the control unit  24  is connected to the midpoint between the switching component Q 1  and the resistor R 1  for measuring the driver current Ik 1  that flows through the driver  32 . A voltage measurement terminal V 1  is connected to the midpoint between the resistors R 2  and R 3  for measuring the driver voltage Vk 1  applied to the point P based on a resistance ratio between the resisters R 2  and R 3 . 
     Current Divider  34   
     Each current divider  34  includes a switching component Q 2 , a coil L 1 , and a resistor R 4  that are connected in series in this sequence between the point P and the ground G. The switching component Q 2  is connected to a control terminal S 2  of the control unit  24  and is controlled by the control unit  24  between open and closed. When the control unit  24  opens the switching component Q 2 , a divided current Id 1  flows in the current divider  34 . When the control unit  24  closes the switching component Q 2 , the divided current Id 1  stops. Namely, the switching component Q 2  functions together with the control unit  24  as a current divider driver  36  for controlling the divided current Id 1  that flows in the current divider  34 . A current measurement terminal I 2  of the control unit  24  is connected to the midpoint between the coil L 1  and the resistor R 4  for measuring the divided current Id 1  that flows in the current divider  36 . 
     Regenerator  40   
     The regenerator  40  includes at least a coil L 2  and a capacitor C 1  connected to each other. The coil L 2  is held close to the coil L 1  of the current divider  34 . When a current flows through the coil L 1 , the coils L 1  and L 2  are electrically or magnetically connected, and a current flows through the coil L 2 . As a result, energy is stored in the capacitor C 1 . 
     Control Unit  24   
     The control unit  24  controls the parallel circuit H 1  as follows. The control unit  24  adjusts the driver current Ik 1  in the driver  32  by controlling the switching component Q 1  and the divided current Id 1  in the current divider  34  by controlling the switching component Q 2 . As a result, the current (Ik 1 +Id 1 ) that flows through the light emitting unit  30  is controlled. As described earlier, the current that flow through the light emitting unit  30  needs to be controlled at the predetermined constant current Io. During the control of the driver current Ik 1  and the divided current Id 1 , the control unit  24  determines the amounts of the driver current Ik 1  and the divided current Id 1 . The control unit  24  determines the amounts by adjusting the ratio of the driver current Ik 1  to the divided current Id 1  while maintaining the total amount of the currents Ik 1  and Id 1  at the predetermined constant current Io. 
     The control unit  24  measures the driver current Ik 1  and the divided current Id 1  at current measurement terminals I 1  and I 2 . The control unit  24  reflects the measurements on the control of the switching components Q 1  and Q 2 . Therefore, the driver current Ik 1  and the divided current Id 1  are controlled with high accuracies. The control unit  24  measures the driver voltage Vk 1  at a voltage measurement terminal V 1 . The control unit  24  reflects the measurement on the control of the supply voltage Vo from the power supply unit  22 . Therefore, the supply voltage Vo is controlled with a high accuracy according to environmental factors including temperature. 
     Control by Control Unit 
     The control performed by the control unit  24  will be explained with reference to  FIG. 3 . 
     The control unit  24  is connected to the parallel circuits H 1  to H 4 . The control unit  24  measures the driver voltages Vk 1  to Vk 4 , the driver currents Ik 1  to Ik 4 , and the divided currents Id 1  to Id 4  of the parallel circuits. The control unit  24  controls the drivers  32  and the current divider driver  36  of the parallel circuits and the power supply unit  22  with reference to the measurements. The driver currents Ik 1  to Ik 4  and the divided current Id 1  to Id 4  are adjusted to satisfy the following condition.
 
 Ikn+Idn=Io , where  n= 1 to 4
 
     The control unit  24  measures the forward voltages Vk 1  to Vk 4  of the parallel circuits (step S 2 ), and calculates the forward voltages Vf 1  to Vf 4  of the parallel circuits (step S 4 ). The forward voltages Vf 1  to Vf 4  are determined based on the currents Io that flow through the light emitting units  30 . The forward voltages Vf 1  to Vf 4  do not depend on the supply voltage Vo. The forward voltages Vf 1  to Vf 4  are calculated as follows.
 
 Vfn=Vo−Vkn , where  n= 1 to 4
 
     The control unit  24  compares the calculated forward voltages Vf 1  to Vf 4  with each other, and determines the maximum forward voltage Vfmax (step S 6 ). In this embodiment, the forward voltages Vf 1  to Vf 4  have relationships of Vf 1 &lt;Vf 2 &lt;Vf 3 &lt;Vf 4  and thus the control unit  24  selects Vf 4  as the maximum forward voltage Vfmax. 
     The control unit  24  determines the supply voltage Vo from the power supply unit  22  based on the maximum forward voltage Vfmax (step S 8 ). The drivers  32  and the current dividers  34  include components such as the switching components Q and the resistors R, respectively. Therefore, the minimum driver voltage Vkmin is required for each driver  32  for normal operation of these components. The control unit  24  calculates the supply voltage Vo from the maximum forward voltage Vfmax and the minimum driver voltage Vkmin. Therefore, voltages applied to the light emitting unit  30 , the drivers  32 , and the current dividers  34  are less likely to become insufficient. Furthermore, excessive voltages are less likely to be applied to the drivers  32  and the current dividers  34 . The following is an equation for calculating the supply voltage Vo.
 
 Vo=Vf max+ Vk min
 
     The control unit  24  controls the current divider  36  of the parallel circuit H 4 , the forward voltage Vf 4  of which is the maximum forward voltage Vfmax, to restrict the flow of the divided current Id 4  in the current divider  34  of the parallel circuit H 4  (step S 10 ). The control unit  24  calculates a power P 4  consumed by the driver  32  of the parallel circuit H 4  (step S 12 ). Namely, the driver current Ik 4 , the divided current Id 4 , and the power P 4  are expressed as follows.
 
 Ik 4= Io, Id 4=0 , P 4= Vk 4× Ik 4=( Vo−Vf 4)× Io  
 
     The control unit  24  determines the driver currents Ik 1  to Ik 3  so that the powers P 1  to P 3  of the drivers  32  of the parallel circuits H 1  to H 3  are equal to or lower than the power P 4  of the driver  32  of the parallel circuit H 4  (step S 14 ). The powers P 1  to P 3  of the drivers  32  of the parallel circuits H 1  to H 3  are expressed as follows.
 
 Pn=Vkn×Ikn =( Vo−Vfn )× Ikn , where  n= 1 to 3
 
     As described earlier, the forward voltages Vf 1  to Vf 4  have the relationships of Vf 1 &lt;Vf 2 &lt;Vf 3 &lt;Vf 4 . Therefore, the control unit  24  is required to adjust the driver currents Ik 1  to Ik 3  to have relationships of Ik 1 &lt;Ik 2 &lt;Ik 3 &lt;Io. Furthermore, the control unit  24  adjusts the divided currents Id 1  to Id 3  to have relationships of Id 1 &gt;Id 2 &gt;Id 3 &gt;0. Namely, the control unit  24  controls the parallel circuits H 1  to H 4  in which the forward voltages Vf of the light emitting units  30  are relatively small so that relatively large amount of the divided currents Id flow in the current dividers  34 . 
     3. Waveforms of Driver and Current Divider 
     Waveforms of the driver current Ik 1 , the divided current Id 1 , and the current that flows through the light emitting unit  30  (Ik 1 +Id 1 ) in the parallel circuit H 1  are illustrated in  FIG. 4 . The letter “H” indicates a high state in which the current is large and the letter “L” indicates a low state in which the current is small. Root mean square (RMS) control is performed on the currents that flow in the light emitting units  30 , the drivers  32 , and the current dividers  34  in the parallel circuits H 1  to H 4 . As illustrate in  FIG. 4 , the switching component Q 1  is controlled such that a RMS value of the driver current Ik 1  that flows in the driver  32  per reference time remains constant (as indicated by a broken line in  FIG. 4 ). Furthermore, the switching component Q 2  is controlled such that a RMS of the divided current that flows in the current divider  34  per reference time remains constant (as indicated by a broken line in  FIG. 4 ). With the control, a RMS value of the current that flows in the light emitting unit  30  per reference time is adjusted to the constant value Io. 
     In this embodiment, as illustrated in  FIG. 4 , the divided current Id 1  is stopped for feeding the driver current Ik 1 , and the divided current Id 1  is fed for stopping the driver current Ik 1 . A period in which the power is consumed by the driver  32  can be separated from a period in which the power is regenerated by the regenerator  40 . The power is regenerated by the regenerator  40  in a non-display period of a liquid crystal device that includes the LED backlight  10  by synchronizing the on/off timing of the driver  32  with the on/off timing of the liquid crystal display. 
     4. Features of this Embodiment 
     (1) In the LED backlight  10  of this embodiment, the current dividers  34  are connected in parallel to the drivers  32  of the series circuit T 1  to T 4 , respectively, in the parallel circuits H 1  to H 4 . Therefore, some of the current that flows through each light emitting unit  30  can be fed to the corresponding current divider  34 . The driver current Ik that flows in the driver  32  can be adjusted to a lower amount than the current Io that flows through the light emitting unit  30 . According to the LED backlight  10  of this embodiment, even if excessive voltages higher than the minimum driver voltage Vimin are applied to the drivers  32 , the driver currents Ik that flow in the drivers  32  are adjusted to small amounts. With this configuration, losses of the power P are reduced and the temperature increases can be reduced in the driver  32 . Therefore, heatsinks for reducing the temperature increases in the drivers  32  are not required or a size of the heatsinks can be reduced even in a case that the heatsinks are required. The configuration of the LED backlight  10  can be simplified and the manufacturing cost can be reduced. 
     (2) In the parallel circuit H 1  to H 4  of the LED backlight  10  of this embodiment, the forward voltages Vf of the light emitting units  30  are set relatively low in the parallel circuits H 1  to H 4 . Furthermore, the parallel circuits H 1  to H 4  are configured such that the relatively large divided current Id flows in the current divider  34  connected to the series circuit T in which the relatively high driver voltages Vk are applied to the drivers  32 . Namely, the driver currents Ik that flow in the drivers  32  are set relatively small in the series circuits T in which the relatively high driver voltages Vk are applied to the drivers  32 . According to the LED backlight  10  of the present invention, the losses of power P by the drivers  32  can be reduced and the temperature increases in the drivers  32  can be reduced. 
     (3) The LED backlight  10  of this embodiment includes the regenerator  40  in the parallel circuit H 1 . The power P that may be consumed by the drivers  32  according to known technologies is stored by the regenerator  40  and thus the losses of power P in the LED backlight can be reduced. 
     (4) In the LED backlight  10  of the present invention, the control unit  24  controls the current divider driver  36  to restrict the flows of the divided current Id in the current divider  34  in the parallel circuit H 4  in which the forward voltage Vf of the light emitting units  30  is the maximum. In the parallel circuit H 4 , the minimum driver voltage Vkmin is applied to the driver  32 . The loss of power P 4  is not large in the driver  32  and the temperature increase in the driver  32  is small. Therefore, a heatsink or any other measure is not required. In the LED backlight  10  of the present invention, the current divider driver  36  of the parallel circuit H 4  is control as described above, and an open or closed status thereof is not altered according to time. Therefore, the control by the control unit  24  can be simplified. 
     Other Embodiments 
     The present invention is not limited to the embodiment illustrated in the above description and the drawings. For example, the following embodiments may be included in the technical scope of the present invention. 
     (1) In the above embodiment, the driver current Ik 1  and the divided current Id 1  are adjusted according to time. However, the scope of the present invention is not limited to such a configuration. As illustrated in  FIG. 5 , the driver current Ik 1  and the divided current Id 1  may be maintained constant. As illustrated in  FIG. 6 , one of the driver current Ik 1  and the divided current Id 1  may be adjusted according to time and the other one of them may be maintained constant. 
     (2) In the above embodiment, the current dividers  34  and the current divider drivers  36  are provided and controlled by the control unit  24 . However, the scope of the present invention is not limited to such a configuration. As illustrated in  FIG. 7 , each current divider  34  may include a coil L 1  and a resistor R 4 , and the current divider  34  may be connected to a part of the current divider  34 , that is, the ground and the midpoint between the switching component Q 1  and the resistor R 1 . With this configuration, a current that flows through the light emitting unit  30  can be adjusted by the driver  32 . Furthermore, the resistors R 1  and R 4  and the coil L 1  may be configured based on the forward voltages Vf 1  to Vf 4  of the light emitting units  30  of the parallel circuits H 1  to H 4 . By do so, a ratio of each driver current Ik to the corresponding divided current Id can be adjusted and thus the control by the control unit  24  can be simplified. 
     (3) In the above embodiment, the regenerator  40  includes the capacitor C 1 . However, the scope of the present invention is not limited to such a configuration. For example, the regenerator  40  may include a storage cell or any other type of component configured to store energy. 
     (4) In the above embodiment, the LEDs  42  are provided as light emitting components. However, the scope of the present invention is not limited to such a configuration. For example, laser diodes or light emitting components, currents of which are adjustable, may be provided. 
     The technical elements described in this specification and the drawings may be used independently or in combination to achieve the technical benefits. The combinations and the category are not limited to those in original claims. For example, the following methods may provide the technical benefits. 
     A method illustrated in this specification and the drawings is to drive a lighting device having the following configuration. The lighting device includes light emitting units, drivers, a current divider, and a power supply unit. Each light emitting unit includes a plurality of light emitting components connected in series. The drivers are configured to control driving of the light emitting units and connected in series with the light emitting components. The current divider is connected in parallel to the drivers in the series circuits including the light emitting units and the drivers. The power supply unit is configured to apply a voltage to the series circuits. The series circuits to which the current divider is connected are connected in parallel to each other. The power supply unit applies the same voltage to the series circuits. Currents that flow through the light emitting units in the series circuits are adjusted to a common constant amount. The method includes adjusting the first divided current Id 1  in the first current divider connected to the first series circuit is larger than the second divided current Id 2  in the second current divider connected to the second series circuit to satisfy the following condition. The first forward voltage Vf 1  of the first light emitting unit of the first series circuit is lower than second forward voltage Vf 2  of the second light emitting unit of the second series circuit. 
     According to the method of driving the light emitting device illustrated in this specification and the drawings, a relatively small current flows in the driver of the series circuit to which a relatively high excessive voltage is applied to the driver. Therefore, the power loss in the driver can be reduced and effects for reducing the temperature increase in the driver can be achieved. 
     With the technologies described in this specification and the drawings, multiple objects may be accomplished at the same time. However, the technical benefits can be achieved by accomplishing even only one of the objects. 
     EXPLANATION OF SYMBOLS 
       10 : LED backlight,  20 : Circuit,  22 : Power supply unit,  24 : Control unit,  30 : Light emitting unit,  32 : Driver,  34 : Current divider,  36 : Current divider driver,  40 : Regenerator,  42 : LED, H: Parallel circuit, T: Series circuit, Vf: Forward voltage, Vk: Driver voltage, Ik: Driver current, Id: Divided current