Patent Publication Number: US-10334681-B2

Title: Device for driving light emitting element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the National Phase of PCT International Application No. PCT/KR2015/011819, filed on Nov. 5, 2015, which claims priority under 35 U.S.C. 119(a) to Patent Application No. 10-2014-0185732, filed in the Republic of Korea on Dec. 22, 2014, all of which are hereby expressly incorporated by reference into the present application. 
     TECHNICAL FIELD 
     Embodiments relate to a device for driving a light emitting element. 
     BACKGROUND ART 
     Recently, an LED light having high luminance, comparable to that of a lighting device such as an incandescent lamp, while being driven with low power has attracted increasing attention. Particularly, light driving devices for driving the LED light by controlling uniform current to flow through the LED light are actively researched and developed. 
     Such a light driving device has various lighting functions and, particularly, can enable lighting in various forms by changing dimming levels of LED elements arranged in serial/parallel connection. 
     In general, a light driving device can include a rectification circuit for rectifying full waves output from an AC power supply, a transformation circuit for transforming the voltage output from the rectification circuit and outputting the transformed voltage, a power factor correction circuit for correcting a power factor of power output from the AC power supply by controlling the output voltage of the transformation circuit, a smoothing circuit for smoothing the voltage output from the transformation circuit to output a stable DC voltage and supplying the output voltage to an LED module, a constant current driving circuit for controlling LED current such that uniform driving current flows through the LED module, and a dimming control circuit for controlling current flow in the LED module by controlling the constant current driving circuit according to PWM (Pulse Width Modulation), thereby controlling dimming. 
     DISCLOSURE 
     Technical Problem 
     Embodiments provide a device for driving a light emitting element which can improve power efficiency and prevent flickering. 
     Technical Solution 
     A device for driving a light-emitting element according to an embodiment includes: a voltage generator for providing a DC signal for driving a light-emitting unit; a sensing resistor; and a dimming unit connected between the light-emitting unit and the sensing resistor and controlling current flowing through the sensing resistor and the light-emitting unit, wherein the dimming unit adjusts a level of the DC signal on the basis of a first sensing voltage according to a result obtained by sensing a voltage of a first node at which the light-emitting unit and a switch are connected and a second sensing voltage according to a result obtained by sensing a voltage of a second node at which the switch and the sensing resistor are connected. 
     The dimming unit may adjust the level of the DC signal such that a difference between the first sensing voltage and the second sensing voltage becomes equal to or lower than a first reference voltage. 
     The dimming unit may block current flow between the light-emitting unit and the sensing resistor when the difference between the first sensing voltage and the second sensing voltage exceeds a second reference voltage. 
     The dimming unit may include: a switch connected between the light-emitting unit and the sensing resistor; an amplifier including a first input terminal receiving a constant-current control signal, a second input terminal connected to the second node, and an output terminal; a voltage sensing unit outputting the first sensing voltage and the second sensing voltage; and a controller for generating a dimming signal on the basis of the first and second sensing voltages, wherein the switch is switched in response to output of the amplifier and the voltage generator adjusts the level of the DC signal on the basis of the dimming signal. 
     The constant-current control signal may be an analog signal. 
     The dimming unit may smooth a pulse width modulation signal and provide a signal according to a smoothing result as the constant-current control signal. 
     The controller may adjust the level of the DC signal such that the difference between the first sensing voltage and the second sensing voltage becomes equal to or lower than the first reference voltage. 
     The switch may be implemented as a transistor and the first reference voltage may be a drain-source on state voltage of the switch. 
     The controller may decrease the level of the DC signal when the difference between the first sensing voltage and the second sensing voltage exceeds the first reference voltage and is equal to or lower than the second reference voltage. 
     The controller may change a level of the constant-current control signal to zero when the difference between the first sensing voltage and the second sensing voltage exceeds the second reference voltage. 
     The device for driving a light-emitting element may further include: a rectifier for rectifying an AC signal and providing a rectified signal according to the rectification result; and a power factor correction unit for correcting a power factor of the rectified signal and outputting the power-factor-corrected rectified signal to the voltage generator. 
     The controller may calculate sensing current flowing through the sensing resistor on the basis of the second sensing voltage and turn on or off the power factor correction unit on the basis of the calculated sensing current. 
     The controller may turn off the power factor correction unit when the sensing current is lower than a reference current value. 
     A device for driving a light-emitting element according to another embodiment includes: a voltage generator for providing a DC signal for driving a light-emitting unit on the basis of a dimming signal; an amplifier including a first input terminal receiving a constant-current control signal, a second input terminal and an output terminal; a sensing resistor, one terminal of which is connected to the second input terminal; a switch connected between the light-emitting unit and the sensing resistor and switched in response to an output of the amplifier; a voltage sensing unit outputting a first sensing voltage according to a result obtained by sensing a voltage of a first node at which the light-emitting unit and the switch are connected and a second sensing voltage according to a result obtained by sensing a voltage of a second node at which the switch and one terminal of the sensing resistor are connected; and a controller for providing the dimming signal for adjusting the level of the DC signal on the basis of a difference between the first sensing voltage and the second sensing voltage to the voltage generator. 
     The device for driving a light-emitting element may further include a smoothing circuit for smoothing a pulse width modulation signal and providing a signal according to the smoothing result as the constant-current control signal. 
     The controller may provide the pulse width modulation signal. 
     The device for driving a light-emitting element may further include: a rectifier for rectifying an AC signal and providing a rectified signal according to the rectification result; and a power factor correction unit for correcting a power factor of the rectified signal and outputting the power-factor-corrected rectified signal to the voltage generator. 
     The voltage generator may change the level of the power-factor-corrected rectified signal on the basis of the dimming signal and generate the DC signal according to the level change result. 
     The controller may calculate sensing current flowing through the sensing resistor on the basis of the second sensing voltage and turn on or off the power factor correction unit on the basis of the calculated sensing current. 
     A device for driving a light-emitting element according to another embodiment includes: a voltage generator for providing a DC signal for driving a plurality of light-emitting units; a plurality of sensing resistors; a plurality of dimming units for controlling current flowing through the plurality of light-emitting units; and a controller for providing a constant-current control signal to each of the plurality of dimming units and adjusting the level of the DC signal, wherein each of the plurality of dimming units includes: an amplifier including a first input terminal receiving the constant-current control signal, a second input terminal connected to a corresponding one of the plurality of sensing resistors, and an output terminal; a switch connected between a corresponding one of the plurality of light-emitting units and one terminal of a corresponding one of the plurality of sensing resistors and switched in response to an output of the amplifier; and a voltage sensing unit outputting first sensing voltages according to results obtained by sensing a voltage of a first node at which a corresponding one of the plurality of light-emitting units and the switch are connected and second sensing voltages according to results obtained by sensing a voltage of a second node at which the switch and one terminal of a corresponding one of the plurality of sensing resistors are connected, wherein the controller adjusts the level of the DC signal on the basis of differences between the first sensing voltages and the second sensing voltages. 
     Advantageous Effects 
     Embodiments can improve power efficiency and prevent flickering. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a configuration of a lighting apparatus according to an embodiment. 
         FIG. 2 a    illustrates an embodiment of a first sensing unit shown in  FIG. 1 . 
         FIG. 2 b    illustrates another embodiment of the first sensing unit shown in  FIG. 1 . 
         FIG. 3  illustrates a configuration of a lighting apparatus according to another embodiment. 
         FIG. 4  illustrates a configuration of a lighting apparatus according to another embodiment. 
         FIG. 5  is a flowchart illustrating an operation of a controller to control the level of a DC voltage supplied from a voltage generator to a light-emitting unit shown in  FIGS. 1 and 3 . 
         FIG. 6  is a flowchart illustrating an operation of the controller to control a power factor correction unit of  FIG. 4 . 
         FIG. 7 a    illustrates light emission of a light-emitting unit when constant current control is performed using a duty ratio of a PWM signal. 
         FIG. 7 b    illustrates light emission of a light-emitting unit according to an embodiment. 
         FIG. 8  illustrates a configuration of a lighting apparatus according to another embodiment. 
     
    
    
     BEST MODE 
     Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. In description of embodiments, it will be understood that when a layer (film), region, pattern or structure is referred to as being “above”/“on” or “below”/“under” another layer (film), region, pattern or structure, it can be directly “above”/“on” the other layer (film), region, pattern or structure or an intervening element may be present therebetween. Furthermore, relative terms, such as “lower”/“bottom” and “upper”/“top” may be used herein to describe one element&#39;s relationship to another elements as illustrated in the Figures. 
     In the drawings, dimensions of layers are exaggerated, omitted or schematically illustrated for clarity and convenience of description. In addition, dimensions of constituent elements do not entirely reflect actual dimensions thereof. The same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1  illustrates a configuration of a lighting apparatus  100  according to an embodiment. 
     Referring to  FIG. 1 , the lighting apparatus  100  includes a light-emitting unit  101  and a light-emitting element driving device  102  for driving the light-emitting unit  101 . 
     The light-emitting unit  101  includes a plurality of light-emitting element arrays D 1  to Dn (n being a natural number greater than 1) connected in series. 
     Each of the light-emitting element arrays D 1  to Dn (n being a natural number greater than 1) may include one or more light-emitting elements, for example, light-emitting diodes. 
     When a plurality of light-emitting elements is included in a light-emitting element array, the light-emitting elements may be connected in series, in parallel or in series and parallel. 
     The light-emitting element driving device  102  includes an AC power supply  110 , an EMI filter  115 , a rectifier  120 , a power factor correction unit  125 , a power generator  130 , a dimming unit  140  and a sensing resistor Rsen. 
     The AC power supply unit  110  provides an AC signal. 
     For example, the AC signal AC may be an AC voltage and/or AC current. 
     The EMI (Electromagnetic Interference) filter  115  filters external electromagnetic noise and removes noise included in the AC signal AC supplied from the AC power supply  110 , for example, conductive noise. The EMI filter  115  may be implemented to include at least one of a capacitor, a transformer and an inductor. 
     The rectifier  120  rectifies the AC signal AC from which the electromagnetic noise has been removed by the EMI filter  115  and provides a rectified signal (ripple current) VR according to the rectification result. 
     For example, the rectifier  120  may full-wave rectify the AC signal AC and output the rectified signal VR according to the full-wave rectification result. That is, the rectified signal VR may be a signal obtained by full-wave rectifying the AC signal AC. 
     While the rectifier  120  may be implemented as a full-wave diode bridge circuit including four bridge-connected diodes, the rectifier  120  is not limited thereto. 
     The power factor correction unit  125  adjusts phase differences of the voltage and current of the rectified signal VR to correct the power factor of the rectified signal VR and outputs a power-factor-corrected rectified signal VR 1 . 
     The voltage generator  130  changes the level of the rectified signal VR 1  having the power factor corrected by the power factor correction unit  125  on the basis of a dimming signal DS provided by the dimming unit  140  and outputs a level-changed DC signal VR 2 . For example, the DC signal VR 2  may be a DC voltage. 
     Here, the level of the DC signal VR 2  output from the voltage generator  130  may be set or changed on the basis of the dimming signal DS provided by the dimming unit  140 . 
     The DC signal VR 2  output from the voltage generator  130  is provided to the light-emitting unit  101 . For example, the DC signal VR 2  output from the voltage generator  130  can be provided to an input terminal  105  of the light-emitting unit  101 . Here, the input terminal  105  of the light-emitting unit  101  may be a positive terminal of the first light-emitting element array D 1  of the serially connected light-emitting element arrays D 1  to Dn. 
     The voltage generator  130  may be implemented as a converter that can change the DC level of the rectified signal VR 1 . For example, the voltage generator  130  may be implemented to include at least one of a DC-DC converter, a resonant LLC half bridge converter, a fly back converter, and a buck converter. 
     The dimming unit  140  connects the light-emitting unit  101  and the sensing resistor Rsen and adjusts the luminance of the light-emitting unit  101  by controlling current flowing through the light-emitting unit  101 . 
     Further, the dimming unit  140  changes the level of the DC signal VR 2  supplied from the voltage generator  130  such that a voltage VN between an output terminal  106  of the light-emitting unit  101  and one terminal  107  of the sensing resistor Rsen is maintained at a predetermined reference voltage. 
     Here, the output terminal  106  of the light-emitting unit  101  may be a negative terminal of the last light-emitting element array Dn of the serially connected light-emitting element arrays D 1  to Dn. The predetermined reference voltage will be described with reference to  FIG. 5 . 
     The dimming unit  140  may adjust the level of the DC signal VR 2  on the basis of a first sensing voltage Vsen 1  obtained by sensing a voltage of a first node N 1  at which the light-emitting unit  101  and a switch  142  are connected and a second sensing voltage Vsen 2  obtained by sensing a voltage of a second node N 2  at which the switch  142  and the sensing resistor Rsen are connected. 
     For example, the dimming unit  140  can generate the dimming signal DS on the basis of the first sensing voltage Vsen 1  obtained by sensing the voltage of the first node N 1  at which the light-emitting unit  101  and the switch  142  are connected and the second sensing voltage Vsen 2  obtained by sensing the voltage of the second node N 2  at which the switch  142  and the sensing resistor Rsen are connected. 
     The dimming unit  140  may adjust the level of the DC signal VR 2  such that the difference Vsen 1 -Vsen 2  between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  is equal to or lower than a first reference voltage. 
     Further, the dimming unit  140  may block current flow between the light-emitting unit  101  and the sensing resistor Rsen when the difference Vsen 1 -Vsen 2  between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  exceeds a second reference voltage. For example, the dimming unit  140  can decrease the level of the DC signal VR 2  to a level that is insufficient to turn on the light-emitting unit  101  or control the voltage generator  130  to change the level of the DC signal VR 2  to zero when the difference Vsen 1 -Vsen 2  between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  exceeds the second reference voltage. 
     The dimming unit  140  may include the switch  142 , a voltage sensing unit  144 , an amplifier  146  and a controller  148 . 
     The switch  142  is connected between the output terminal  106  of the light-emitting unit  101  and one terminal  107  of the sensing resistor Rsen and is switched on the basis of a constant-current control signal Vset supplied from the controller  148 . 
     For example, the switch  142  can be implemented as a transistor, for example, an FET or a BJT. 
     For example, the switch  142  can be implemented as an NMOS transistor including a drain connected to the output terminal  106  of the light-emitting unit  101 , a source connected to one terminal  107  of the sensing resistor Rsen and a gate to which the output of the amplifier  146  is input. However, the switch  142  is not limited thereto and may be implemented as a PMOS transistor in other embodiments. 
     The switch  142  may be implemented in various forms that electrically connect the output terminal  106  of the light-emitting unit  101  and one terminal  107  of the sensing resistor Rsen in response to the output CS of the amplifier  146 . 
     The voltage VN between the output terminal  106  of the light-emitting unit  101  and one terminal  107  of the sensing resistor Rsen may be a voltage between the source and drain of the switch  142  implemented as a transistor. 
     The voltage sensing unit  144  may sense the voltage of the first node N 1  at which the output terminal  106  of the light-emitting unit  101  and the switch  142  are connected and the voltage of the second node N 2  at which one terminal  107  of the sensing resistor Rsen and the switch  142  are connected. 
     For example, the voltage sensing unit  144  can sense the voltage of the first node N 1  and provide the first sensing voltage Vsen 1  to the controller  148  according to the sensing result. 
     In addition, the voltage sensing unit  144  can sense the voltage of the second node N 2  and provide the second sensing voltage Vsen 2  to the controller  148  according to the sensing result. 
     The voltage sensing unit  144  may include a first sensing unit  144 - 1  for sensing the voltage of the first node N 1  and providing the first sensing voltage Vsen 1  and a second sensing unit  144 - 2  for sensing the voltage of the second node N 2  and providing the second sensing voltage Vsen 2 . 
       FIG. 2 a    illustrates an embodiment  144   a  of the first sensing unit  144 - 1  shown in  FIG. 1 . 
     Referring to  FIG. 2 a   , the first sensing unit  144   a  may include a plurality of resistors (e.g., R 1  and R 2 ) serially connected between the first node N 1  and a ground power supply GND and may provide a voltage applied to at least one of the plurality of resistors (e.g., R 1  and R 2 ) to the controller  148  as the first sensing voltage Vsen 1 . 
       FIG. 2 b    illustrates another embodiment  144   b  of the first sensing unit  144 - 1  shown in  FIG. 1 . 
     Referring to  FIG. 2 b   , the first sensing unit  144   b  may include a plurality of resistors (e.g., R 1  and R 2 ) serially connected between the first node N 1  and the ground power supply GND and a Zener diode  201  connected in parallel with at least one (e.g., R 2 ) of the plurality of resistors (e.g., R 1  and R 2 ) and provide a voltage applied across the Zener diode  201  to the controller  148  as the first sensing voltage Vsen 1 . 
     For example, the first sensing unit  144   b  can include first and second resistors R 1  and R 2  serially connected between the first node N 1  and the ground power supply GND and the Zener diode  201  connected between a connecting node of the first and second resistors R 1  and R 2  and the ground power supply GND and provide the voltage applied across the Zener diode  201  to the controller  148  as the first sensing voltage Vsen 1 . 
     The second sensing unit  144 - 2  may provide the voltage applied to the second node N 2  to the controller  148  as the second sensing voltage Vsen 2 . 
     For example, the second sensing unit  144 - 2  can sense the voltage applied to the sensing resistor Rsen and provide the voltage applied to the sensing resistor Rsen to the controller  148 . 
     The embodiments illustrated in  FIGS. 2 a  and 2 b    may be applied to the second sensing unit  144 - 2  in other embodiments. However, values of resistors included in the second sensing unit  144 - 2  may differ from those of the first sensing unit  144 - 1 . 
     The amplifier  146  amplifies the constant-current control signal Vset supplied from the controller  148  and the voltage of the second node N 2  and outputs an amplified signal CS according to the amplification result. For example, the constant-current control signal Vset supplied from the controller  148  shown in  FIG. 1  may be an analog signal such as a DC voltage instead of a pulse signal such as a PWM signal. 
     The amplifier  146  may include a first input terminal  146   a  to which the constant-current control signal Vset is input, a second input terminal  146   b  connected to the second node N 2  and an output terminal  146   c  through which the amplified signal CS is output. While the amplifier  146  may be implemented as an operational amplifier or a differential amplifier, the amplifier  146  is not limited thereto. For example, the first input terminal  146   a  may be a positive input terminal (+) of an operational amplifier and the second input terminal  146   b  may be a negative input terminal (−) of the operational amplifier. 
     Current flowing through the sensing resistor Rsen may be determined by the constant-current control signal Vset provided by the controller  148 , and thus current flowing through the light-emitting unit  101  can be controlled in the present embodiment. According to characteristics of the operational amplifier, the voltage of the second node N 2  is the constant-current control signal Vset input to the first input terminal  146   a  and thus sensing current Isen flowing through the sensing resistor Rsen may be obtained by dividing the constant-current control signal Vset by the value of the sensing resistor Rsen. 
     Since the constant-current control signal Vset is not a pulse signal but is an analog signal, the current flowing through the light-emitting unit  101  can be linear unless the level of the constant-current control signal Vset is changed by the light-emitting unit  101  and thus flickering of the light-emitting unit  101  can be reduced or eliminated. 
     The controller  148  may control the voltage generator  130  to change the level of the DC signal VR 2  output from the voltage generator  130  on the basis of the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  supplied from the voltage sensing unit  144 . 
     For example, the controller  148  can generate the dimming signal DS for controlling the voltage generator  130  on the basis of the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2 , and the voltage generator  130  can change the level of the rectified signal VR 1  on the basis of the dimming signal DS and output the level-changed DC signal VR 2 . That is, the level of the DC signal VR 2  supplied from the voltage generator  130  to the light-emitting unit  101  can be determined on the basis of the dimming signal DS. 
     The controller  148  may adjust the level of the DC signal VR 2  of the voltage generator  130  such that the difference Vsen 1 -Vsen 2  between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  becomes equal to or lower than a predetermined reference voltage. 
     For example, the controller  148  can adjust the level of the DC signal VR 2  of the voltage generator  130  such that the difference Vsen 1 -Vsen 2  between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  becomes equal to a predetermined first reference voltage. 
     For example, the predetermined first reference voltage can be a drain-source on state voltage of the switch  142  implemented as a transistor. However, the predetermined first reference voltage is not limited thereto. For example, while the predetermined reference voltage can be 0.4 V, the predetermined reference voltage is not limited thereto. 
     To drive the serially connected light-emitting element arrays, a first voltage corresponding to the sum of rated operating voltages of the light-emitting element arrays may be applied across both terminals of the light-emitting element arrays. 
     When a function temperature of the light-emitting element arrays increases, operating voltages of the light-emitting element arrays may decrease. Such operating voltage decreases in the light-emitting element arrays cause a difference between the first voltage and an operating voltage actually applied across both terminals of the light-emitting element arrays. This voltage difference can result in generation of heat by other elements of the light-emitting element driving device, resulting in power efficiency reduction in the lighting apparatus. 
     It is possible to prevent power consumption wasted as heat in the switch  142  by decreasing the level of the DC signal VR 2  provided to the light-emitting unit  101  on the basis of a result obtained by sensing the voltage across the switch  142  according to the present embodiment. 
     Since the dimming unit  140  senses the difference between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  and adjusts the level of the DC signal VR 2  provided to the light-emitting unit  101  such that the difference Vsen 1 -Vsen 2  between the first and second sensing voltages is maintained as a predetermined voltage according to the sensing result, power consumed by the switch  142  can remain uniform even when the operating voltage of the light-emitting unit  101  is changed and power efficiency reduction in the lighting apparatus  100  can be prevented. 
     If the dimming controller  140  does not perform the aforementioned control operation, the difference between the voltage supplied from the voltage generator  130  and the voltage actually applied to the light-emitting unit  101  can be consumed as heat in the switch  142  due to operating voltage reduction in the light-emitting unit  101 , and thus power efficiency of the lighting apparatus  100  can be reduced. 
     The controller  148  may turn off the light-emitting unit  101  by preventing the voltage generator  130  from providing the DC signal VR 2  to the light-emitting unit  101  when the difference Vsen 1 -Vsen 2  between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  exceeds the second reference voltage. 
     Alternatively, the controller  148  may set or change the level of the constant-current control signal Vset to zero when the difference Vsen 1 -Vsen 2  between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  exceeds the second reference voltage. 
     When the difference Vsen 1 -Vsen 2  between the first sensing voltage Vsen 1  and the second sensing voltage Vsen 2  exceeds the second reference voltage, the controller  148  needs to prevent current from flowing through the light-emitting unit for protecting the light-emitting unit  101  upon determining that short-circuit is generated in the light-emitting unit  101 . To this end, the controller  148  may block provision of the DC signal VR 2  or change the level of the constant-current control signal Vset to 0. 
       FIG. 3  illustrates a configuration of the lighting apparatus  100  according to another embodiment. The same reference numbers will be used in  FIGS. 1 and 3  to refer to the same or like parts, and a repeated description thereof will be simplified or omitted. 
     Referring to  FIG. 3 , the lighting apparatus  200  includes the light-emitting unit  101  and a light-emitting element driving device  102   a  for driving the light-emitting unit  101 . 
     The light-emitting element driving unit  102   a  includes the AC power supply  110 , the EMI filter  115 , the rectifier  120 , the power factor correction unit  125 , the power generator  130 , a dimming unit  140   a  and the sensing resistor Rsen. 
     The dimming unit  140   a  may include the switch  142 , the voltage sensing unit  144 , the amplifier  146 , a smoothing circuit  310  and the controller  148 . 
     The dimming unit  140   a  illustrated in  FIG. 3  may further include the smoothing circuit  310  in addition to the dimming unit  140  shown in  FIG. 1 . 
     The smoothing circuit  310  smooths a signal Pw supplied form the controller  148  and outputs a constant-current control signal Vset 1  according to the smoothing result. 
     The signal Pw supplied from the controller  148  may be a pulse width modulation (PWM) signal. When constant current control for the light-emitting unit  101  is performed on the basis of the duty ratio of such a PWM signal, current flowing through the light-emitting unit  101  has a ripple component and thus flickering may occur in the light-emitting unit  101  due to the ripple component. 
     The smoothing circuit  310  smooths the PWM signal supplied from the controller  148  in order to remove such flickering and generates the constant-current control signal Vset 1  that is a DC analog signal from which a ripple current component has been removed according to the smoothing result. 
     The ripple component of the current flowing through the light-emitting unit  101  can be reduced by the constant-current control signal Vset 1  generated by the smoothing circuit  310 . The present embodiment can perform constant current control with respect to the light-emitting unit  101  using the level of the constant-current control signal Vset 1  corresponding to an analog signal instead of the duty ratio of a PWM signal to thereby reduce or remove flickering of the light-emitting unit  101 . 
     While the smoothing circuit  310  may be implemented as an RC smoothing circuit including a resistor R 3  connected between the controller  148  and a first input terminal  146   a  of the amplifier  146  and a capacitor C 1  connected between the first input terminal  146   a  of the amplifier  146  and the ground power supply GND, the smoothing circuit  310  is not limited thereto and may be implemented in various forms including a resistor, a capacitor or an inductor. 
       FIG. 7 a    illustrates light emission of a light-emitting unit when dimming control is performed using the duty ratio of a PWM signal and  FIG. 7 b    illustrates light emission of the light-emitting unit  101  according to an embodiment. 
     Flickering is generated due to a contrast difference in light emission of the light-emitting unit illustrated in  FIG. 7 a   . Conversely, there is little contrast difference and flickering in light emission of the light-emitting unit illustrated in  FIG. 7   b.    
     The present embodiment can adjust a dimming range up to 1% of maximum current that can flow through the light-emitting unit  101  because flickering is not generated even at low illumination, thereby reducing energy consumption. 
     According to the present embodiment, accurate current control can be performed because the current flowing through the light-emitting unit  101  or the luminance of the light-emitting unit  101  is controlled by adjusting the DC level of the constant-current control signal Vset 1 . 
       FIG. 5  is a flowchart illustrating the operation of the controller  148  to control the level of the DC voltage VR 2  supplied from the voltage generator  130  to the light-emitting unit  101  shown in  FIG. 3 . 
     Referring to  FIG. 5 , the controller  148  sets the constant-current control signal Vset 1  supplied to the first input terminal  146   a  of the amplifier  146  using an external signal S 1  (refer to  FIG. 3 ) received through a communication interface (S 510 ). For example, the level of the analog signal may be a target to be set with respect to Vset of  FIG. 1  and the duty ratio of the PWM signal may be a target to be set with respect to Vset 1  of  FIG. 3 . The constant-current control signal Vset or Vset 1  that determines the luminance of the light-emitting unit  101  may be set according to user selection. For example, a dimming degree may be determined in S 510 . 
     For example, the controller  148  can output a pulse width modulation signal Pw corresponding to the signal S 1  received from the outside and the signal Pw provided by the controller  148  can be converted into the constant-current control signal Vset 1  corresponding to an analog signal, as shown in  FIG. 3 . The level of the constant-current control signal Vset 1  can be determined by the duty ratio of the signal Pw supplied from the controller  148 . For example, the level of the constant-current control signal Vset 1  can be proportional to the duty ratio of the signal Pw supplied from the controller  148 . 
     Then, the controller  148  receives the first and second sensing voltages Vsen 1  and Vsen 2  supplied from the voltage sensing unit  144  (S 520 ). 
     Subsequently, the controller  148  compares the set constant-current control signal Vset or Vset 1  with the second sensing voltage Vsen 2  in order to determine whether the voltage Vsen 2  actually applied to the sensing resistor Rsen due to the current which flows through the light-emitting unit  101  according to the DC signal VR 2  supplied from the voltage generator  130  is identical to the set constant-current control signal Vset or Vset 1  (S 530 ). 
     When the second sensing voltage Vsen 2  is not identical to the set constant-current control signal Vset or Vset 1 , the controller  148  changes the level of the DC signal VR 2  supplied from the voltage generator  130  to the light-emitting unit  101  (S 540 ). The controller  148  may repeatedly perform steps S 520  to S 540  until the second sensing voltage Vsen 2  becomes identical to the set constant-current control signal Vset or Vset 1 . 
     For example, the second sensing voltage Vsen 2  may be lower than the set constant-current control signal Vset or Vset 1 . In this case, the controller  148  can change the level of the DC signal VR 2  until the set constant-current signal Vset or Vset 1  becomes the second sensing voltage Vsen 2 . 
     On the contrary, when the second sensing voltage Vsen 2  is identical to the set constant-current control signal Vset or Vset 1 , the controller  148  determines whether the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  is equal to or lower than the predetermined first reference voltage Vref 1  (S 550 ). 
     For example, the predetermined first reference voltage Vref 1  may be a drain-source on state voltage of the switch  142  implemented as a transistor. For example, the predetermined first reference voltage Vref 1  can be 0.4 V. However, the first reference voltage Vref 1  is not limited thereto. 
     When the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  is equal to or lower than the predetermined first reference voltage Vref 1 , the controller  148  does not change the level of the DC signal VR 2  and maintains the set constant-current control signal Vset or Vset 1  (S 560 ). 
     The fact that the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  is equal to or lower than the predetermined first reference voltage Vref 1  means that there is no or little power wasted as heat in the switch  142 , and thus the controller  148  does not change the level of the DC signal VR 2 . The opposite case means that lots of power is wasted as heat in the switch  142 , and thus the controller  148  reduces the level of the DC signal VR 2 . 
     When the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  exceeds the predetermined first reference voltage Vref 1 , the controller  148  determines whether the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  exceeds the second reference voltage Vref 2  (S 570 ). The second reference voltage Vref 2  is higher than the first reference voltage Vref 1  (Vref 2 &gt;Vref 1 ). 
     The second reference signal Vref 2  may be a voltage by which the light-emitting unit  101  is determined to short-circuit. For example, the second reference voltage Vref 2  can be 3.5 V. However, the second reference voltage Vref 2  is not limited thereto. 
     When the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  exceeds the predetermined first reference voltage Vref 1  and is equal to or lower than the second reference voltage Vref 2  (Vref 1 &lt;Vsen 1 -Vsen 2 ≤Vref 2 ), the controller  148  changes the level of the DC signal VR 2  supplied from the voltage generator  130  to the light-emitting unit  101  (S 550 →S 570 →S 540 ). 
     The controller  149  repeatedly performs steps S 520 , S 530 , S 550 , S 570  and S 540  until the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  becomes equal to or lower than the first reference voltage Vref 1 . For example, the controller  148  can control the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  to be equal to or lower than the first reference voltage Vref 1  by decreasing the level of the DC signal VR 2  supplied from the voltage generator  130  to the light-emitting unit  101 . 
     For example, when the junction temperature of the light-emitting unit  101  increases and thus the driving voltage of the light-emitting unit  101  decreases, the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  increases. When the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  increases to be equal to or lower than the second reference voltage Vref 2  while exceeding the first reference voltage Vref 1 , the controller  148  can decrease the level of the DC signal VR 2  to improve power efficiency. 
     When the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  exceeds the second reference signal Vref 2  (Vsen 1 -Vsen 2 &gt;Vref 2 ), the controller  148  can change the level of the set constant-current control signal Vset or Vset 1  to zero. 
     When the difference Vsen 1 -Vsen 2  between the received first sensing voltage Vsen 1  and second voltage Vsen 2  exceeds the second reference signal Vref 2 , the controller  148  can change the level of the constant-current control signal Vset or Vset 1  to 0 such that current does not flow through the light-emitting unit  101  in order to protect the light-emitting unit  101  and the light-emitting element driving device  102  upon determining that short-circuit is generated in the light-emitting unit  101 . 
       FIG. 4  illustrates a configuration of a lighting apparatus  300  according to another embodiment. The same reference numbers will be used in  FIGS. 1 and 4  to refer to the same or like parts, and a repeated description thereof will be simplified or omitted. 
     Referring to  FIG. 4 , the lighting apparatus  300  includes a light-emitting unit  101  and a light-emitting element driving device  102   b  for driving the light-emitting unit  101 . 
     The light-emitting element driving device  102   b  includes the AC power supply  110 , the EMI filter  115 , the rectifier  120 , the power factor correction unit  125 , the power generator  130 , a dimming unit  140   b  and the sensing resistor Rsen. 
     The dimming unit  140   b  may include the switch  142 , the voltage sensing unit  144 , the amplifier  146 , the smoothing circuit  310  and a controller  148 - 1 . 
     The controller  148 - 1  outputs the dimming signal DS for controlling the voltage generator  130  and a PFC control signal TS for controlling the power factor correction unit  125 . 
     Description of the dimming signal DS is identical to description with reference to  FIG. 1  and thus is omitted to avoid redundant description. 
     The controller  148 - 1  calculates sensing current Isen flowing through the sensing resistor Rsen on the basis of the second sensing voltage Vsen 2  supplied from the second sensing unit  144 - 2  and turns on or off the power factor correction unit  125  on the basis of the calculated sensing current Isen. 
       FIG. 6  is a flowchart illustrating an operation of the controller  148 - 1  to control the power factor correction unit  125  of  FIG. 4 . 
     Referring to  FIG. 6 , the controller  148 - 1  detects the sensing current Isen flowing through the sensing resistor Rsen on the basis of the second sensing voltage Vsen 2  supplied from the second sensing unit  144 - 2  (S 610 ). 
     The controller  148 - 1  can store the value of the sensing resistor Rsen and calculate the sensing current Isen by dividing the second sensing voltage Vsen 2  received from the second sensing unit  144 - 2  by the stored value of the sensing resistor Rsen. 
     Then, the controller  148 - 1  determines whether the value of the detected sensing current Isen is equal to or greater than a predetermined reference current value Iref (S 620 ). For example, the current flowing through the light-emitting unit  101  can be controlled by the constant-current control signal Vset or Vset 1  supplied from the controller  148 , and the predetermined reference current value Iref may be 20% to 50% of maximum current that can flow through the light-emitting unit  101  in response to the constant-current control signal Vset or Vset 1 . 
     For example, the predetermined reference current value Iref can be 20% of the maximum current that can flow through the light-emitting unit  101  in response to the maximum constant-current control signal Vset or Vset 1 . 
     Then, when the value of the detected sensing current Isen is lower than the predetermined reference current value Iref, the controller  148 - 1  turns off the power factor correction unit  125  such that the power factor correction unit  125  does not operate. That is, when the value of the detected sensing current Isen is lower than the predetermined reference current value Iref, the controller  148 - 1  turns off the power factor correction unit  125  such that power is not consumed by the power factor correction unit  125 . 
     On the other hand, when the value of the detected sensing current Isen is equal to or greater than the predetermined reference current value Iref, the controller  148 - 1  turns on the power factor correction unit  125  such that the power factor correction unit  125  performs an operation. For example, the controller  148 - 1  can turn off or on the power factor correction unit  125  by blocking power provided to the power factor correction unit  125  or supplying power to the power factor correction unit  125  using the PFC control signal TS. However, embodiments are not limited thereto. 
     In a period in which the current flowing through the light-emitting unit  101  is lower than the reference current value Iref, power factor improvement is insufficient even if power factor correction is performed. Accordingly, the present embodiment can prevent the power factor correction unit  125  from consuming power by turning off the power factor correction unit  125  in the period in which the current flowing through the light-emitting unit  101  is lower than the reference current value Iref, thereby improving power efficiency. 
     Furthermore, the present embodiment can secure an EMI (Electromagnetic Interference) margin by suspending the operation of the power factor correction unit  125  in a period in which power factor correction is not needed in order to reduce EMI. 
       FIG. 8  illustrates a configuration of a lighting apparatus  400  according to another embodiment. The same reference numbers will be used in  FIGS. 1, 3 and 8  to refer to the same or like parts, and a repeated description thereof will be simplified or omitted. 
     Referring to  FIG. 8 , the lighting apparatus  400  includes a plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) and a light-emitting element driving device  102   c  for driving the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1). 
     Each of the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than n) may be implemented to be identical to the light-emitting unit  101  described with reference to  FIG. 1  and description thereof is omitted to avoid redundant description. 
     The light-emitting element driving device  102   c  includes the AC power supply  110 , the EMI filter  115 , the rectifier  120 , the power factor correction unit  125 , the power generator  130 , a plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1), a plurality of sensing resistors Rsen_ 1  to Rsen_n (n being a natural number greater than 1) and a controller  148   a.    
     The AC power supply  110 , the EMI filter  115 , the rectifier  120 , the power factor correction unit  125  and the power generator  130  of the light-emitting element driving device  102   c  may be identical to those described with reference to  FIGS. 1 and 3 . The DC signal VR 2  output from the voltage generator  130  is simultaneously provided to the plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1). 
     Each of the plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1) may include: an amplifier  146  having a first input terminal to which a corresponding one of constant-current control signals Vset 1  to Vset_n (n being a natural number greater than 1) is input, a second input terminal connected to a corresponding one of the plurality of sensing resistors Rsen_ 1  to Rsen_n (n being a natural number greater than 1), and an output terminal; a switch  142  connected between a corresponding one of the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) and one terminal of a corresponding one of the plurality of sensing resistors Rsen_ 1  to Rsen_n (n being a natural number greater than 1) and switched in response to the output of the amplifier  146 ; and a voltage sensing unit  144  outputting first sensing voltages Vsen 1 _ 1  to Vsen 1 _ n  (n being a natural number greater than 1) according to results obtained by sensing a voltage of the first node N 1  at which a corresponding one of the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) and the switch  142  are connected and second sensing voltages Vsen 2 _ 1  to Vsen 2 _ n  (n being a natural number greater than 1) according to results obtained by sensing a voltage of the second node N 2  at which the switch  142  and one terminal of a corresponding one of the plurality of sensing resistors Rsen_ 1  to Rsen_n (n being a natural number greater than 1) are connected. 
     The controller  148   a  may adjust the level of the DC signal VR 2  on the basis of the differences Vsen 1 _ 1 -Vsen 2 _ 1  to Vsen 1 _ n -Vsen 2 _ n  between the first sensing voltages and the second sensing voltages. 
     The plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1) connects corresponding light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) to corresponding sensing resistors Rsen_ 1  to Rsen_n (n being a natural number greater than 1) and controls luminance of the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) by adjusting current flowing through the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1). 
     Each of the dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1) may include the switch  142 , the voltage sensing unit  144  and the amplifier  146 . Description of the switch  142 , the voltage sensing unit  144  and the amplifier  146  of  FIG. 1  may be equally applied to the plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1). 
     In another embodiment, each of the plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1) may further include the smoothing circuit  310  shown in  FIG. 3 . 
     The switch  142  of each of the plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1) may be connected between the output terminal  106  of a corresponding one of the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) and a corresponding one of the plurality of sensing resistors Rsen_ 1  to Rsen_n (n being a natural number greater than 1) and may be switched on the basis of a corresponding one of the constant-current control signals Vset_ 1  to Vset_n (n being a natural number greater than 1) supplied from the controller  148   a.    
     The plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1) can output the first sensing voltages Vsen 1 _ 1  to Vsen 1 _ n  (n being a natural number greater than 1) according to a result obtained by sensing the voltage of the first node N 1  and the second sensing voltages Vsen 2 _ 1  to Vsen 2 _ n  (n being a natural number greater than 1) according to a result obtained by sensing the voltage of the second node N 1 . 
     The controller  158   a  provides the constant-current control signals Vset_ 1  to Vset_n (n being a natural number greater than 1) for dimming to the plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1). 
     The controller  148   a  may control the voltage generator  130  to change the level of the DC signal VR 2  output from the voltage generator  130  on the basis of the differences Vsen 1 _ 1 -Vsen 2 _ 1  to Vsen 1 _ n -Vsen 2 _ n  between the first sensing voltages Vsen 1 _ 1  to Vsen 1 _ n  (n being a natural number greater than 1) and the second sensing voltages Vsen 2 _ 1  to Vsen 2 _ n  (n being a natural number greater than 1). 
     For example, the controller  148   a  can calculate the differences Vsen 1 _ 1 -Vsen 2 _ 1  to Vsen 1 _ n -Vsen 2 _ n  between the first sensing voltages Vsen 1 _ 1  to Vsen 1 _ n  (n being a natural number greater than 1) and the second sensing voltages Vsen 2 _ 1  to Vsen 2 _ n  (n being a natural number greater than 1) supplied from the plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1) and set a first reference value and a second reference value on the basis of the calculated differences Vsen 1 _ 1 -Vsen 2 _ 1  to Vsen 1 _ n -Vsen 2 _ n  between the first sensing voltages and the second sensing voltages. 
     The controller  148   a  may decrease the level of the DC signal VR 2  supplied from the voltage generator  130  by the first reference value. 
     The first reference value may be a value obtained by subtracting a predetermined first reference voltage from the largest value among the calculated differences Vsen 1 _ 1 -Vsen 2 _ 1  to Vsen 1 _ n -Vsen 2 _ n  between the first sensing voltages and the second sensing voltages. Here, the predetermined first reference voltage may be a drain-source on state voltage of the switch  142  implemented as a transistor. 
     Operating voltages of the light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) may decrease due to a junction temperature increase in the light-emitting element arrays and dimming according to variations in the constant-current control signals Vset_ 1  to Vset_n. Here, operating voltage reductions in the light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) may be different and power losses as heat in the plurality of dimming units  140 - 1  to  140 - n  (n being a natural number greater than 1) in response to the operating voltage reductions may be different. 
     When the operating voltages of the light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) decrease, the present embodiment reduces the level of the DC signal VR 2  supplied from the voltage generator  130  by the first reference value to meet desired luminance levels (e.g., luminance levels of 100% or 50%) of all light-emitting elements  101 - 1  to  101 - n  (n being a natural number greater than 1) and to improve power efficiency. 
     Furthermore, the controller  148   a  may reduce the level of the DC signal VR 2  supplied from the voltage generator  130  by the sum of the first reference value and the second reference value, for example. 
     The second reference value may be less than the difference between the largest value and the smallest value from among the differences Vsen 1 _ 1 -Vsen 2 _ 1  to Vsen 1 _ n -Vsen 2 _ n  of the calculated first and second voltages. 
     For example, the second reference value may be half the difference between the largest value and the smallest value from among the differences Vsen 1 _ 1 -Vsen 2 _ 1  to Vsen 1 _ n -Vsen 2 _ n  of the calculated first and second voltages. However, the second reference value is not limited thereto. 
     Here, the second reference value is subtracted from the level of the DC voltage VR 2  in order to further improve power efficiency even if some of the light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) cannot satisfy desired luminance levels (e.g., luminance level of 100% or 50%). 
     As described above, the present embodiment can improve power efficiency by reducing the level of the DC signal VR 2  commonly provided to the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1) in response to operating voltage variations in the plurality of light-emitting units  101 - 1  to  101 - n  (n being a natural number greater than 1). 
     The detailed description of the preferred embodiments of the present invention has been given to enable those skilled in the art to implement and practice the invention. Although the invention has been described with reference to the preferred embodiments, those skilled in the art will appreciate that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention described in the appended claims. Accordingly, the invention should not be limited to the specific embodiments described herein, but should be accorded the broadest scope consistent with the principles and novel features disclosed herein. 
     INDUSTRIAL APPLICABILITY 
     The present invention is used for a light-emitting element driving device capable of improving power efficiency and preventing flickering.