CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of Korean Patent Application No. 10-2012-0155771 filed on Dec. 28, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for directly driving alight emitting diode (LED) with alternating current (AC) power. 
     2. Description of the Related Art 
     A Light Emitting Diode (LED) is a semiconductor device having a p-n junction structure and emitting light through the recombination of electrons and holes, and has recently been applied to various technological fields in accordance with the development of semiconductor technologies. In particular, LEDs have high efficiency, relatively long service lives, and environmentally friendly characteristics, as compared to existing incandescent light bulbs, and therefore applications thereof have been continuously broadened. 
     In general, LEDs may be driven by applying several volts of direct current (DC) power thereto, due to structures thereof, and therefore a separate unit is required in order to drive an LED with commercially available alternating current (AC) power which is generally used in domestically, commercially, industrially, and the like. In order to drive an LED with commercially available AC power, an apparatus for driving an LED includes a rectifier circuit, an AC-DC converter, and the like. 
     However, a general AC-DC converter has a large volume and high power consumption, and therefore, when the general AC-DC converter is applied to the apparatus for driving an LED, LED advantages such as high efficiency, a small packaging size, a long service life, and the like may be offset thereby. 
     Therefore, in recent years, research into an apparatus for directly driving an LED with AC power without any AC-DC converter has been conducted. 
     In a case in which an LED is directly driven by AC power without an AC-DC converter, a method of enabling a current to flow at a constant rate in such a manner that a plurality of LEDs may be respectively connected to a plurality of switches, and ON/OFF switching of the plurality of switches may be controlled in accordance with levels of the AC power may be generally used. 
     In the Related Art Document below, Patent Document 1 discloses an apparatus for driving an LED. Here, the apparatus may emit light directly from an LED with AC power by controlling operations of switches connected to middle and final nodes of an LED array. In Patent Document 2, there is disclosed an apparatus for driving an LED. Here, the apparatus may control ON/OFF switching of switches in an order in which an LED array is connected to the switches. 
     However, in both Patent Documents 1 and 2, technology for controlling the operation of the switch in accordance with levels of AC power has been described, but there is a problem in that electro-magnetic interference (EMI) may occur due to the switching operation. As a result, in order to solve this problem, a separate EMI filter may be adopted, resulting in increases in manufacturing costs and in a circuit area. 
     Related Art Document 
     
         
         (Patent Document 1) Korean Patent Registration Publication No. 10-0997050 
         (Patent Document 2) Korean Patent Registration Publication No. 10-0995793 
       
    
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides an apparatus for driving an LED, enabling a pair of LED groups to be switch-operated by a phase difference of 180 degrees therebetween. 
     According to an aspect of the present invention, there is provided an apparatus for driving a light-emitting diode (LED), including: a switching unit that alternately switches a first LED unit and a second LED unit of at least one pair of LED units emitting light by receiving rectified power, in accordance with a predetermined phase difference; a current limiting unit that limits a current flowing in the at least one pair of LED units through the switching of the switching unit; and a driving control unit that controls switching driving of the switching unit in accordance with a voltage level of the rectified power. 
     Each of the first and second LED units may include a plurality of LEDs connected to each other in series. 
     The switching unit may include a switch group having a plurality of switches respectively connected between a connection point between respective LEDs of the plurality of LEDs of each of the first and second LEDs units and the current limiting unit. 
     The switching unit may include a first switch group connected to the first LED unit of the at least one pair of LED units and a second switch group connected to the second LED unit thereof. 
     The driving control unit may control the alternate switching of the switching unit by a phase difference of 180 degrees. 
     The driving control unit may include a comparison unit that compares the rectified power with a predetermined reference voltage, a driving unit that provides a switching driving signal for controlling the switching driving of the first LED unit of the at least one pair of LED units in accordance with a comparison result of the comparison unit, and a phase shift unit that shifts a phase of the switching driving signal from the driving unit so as to control switching driving of the second LED unit. 
     The comparison unit may include a plurality of comparators corresponding to the number of LEDs contained within the first or second LED unit. 
     The driving unit may provide a plurality of switching driving signals corresponding to the number of LEDs contained within the first LED unit. 
     Each of the first and second switch groups may include a plurality of switches corresponding to the number of the plurality of LEDs contained within the first or second LED unit. 
     The apparatus for driving a light-emitting diode may further include a rectifying unit that rectifies alternating current (AC) power and provides the rectified AC power to the at least one pair of LED units. 
     According to another aspect of the present invention, there is provided an apparatus for driving a light-emitting diode (LED), including: a rectifying unit that rectifies AC power and provides the rectified AC power; a switching unit that includes a first switch group for driving a first LED unit of at least one pair of LED units emitting light by receiving the rectified power from the rectifying unit and a second switch group for driving a second LED unit of the at least one pair of LED units, and alternately switches the first switch group and the second switch group in accordance with a predetermined phase difference; a current limiting unit that limits a current flowing in each of the first and second LED units through the switching of the switching unit; and a driving control unit that controls switching driving of the switching unit in accordance with a voltage level of the rectified power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic circuit diagram showing an apparatus for driving an LED according to an embodiment of the present invention; 
         FIG. 2  is a schematic configuration diagram showing a driving control unit adapted in an apparatus for driving an LED according to an embodiment of the present invention; and 
         FIGS. 3 and 4  are graphs showing electrical characteristics of an apparatus for driving an LED according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. 
     Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
       FIG. 1  is a schematic circuit diagram showing an apparatus for driving an LED according to an embodiment of the present invention. 
     Referring to  FIG. 1 , an apparatus  100  for driving an LED according to an embodiment of the present invention may include a rectifying unit  110  and LED driving circuits A, B, and C. 
     The rectifying unit  110  may receive AC power to half-wave or full-wave rectify the received power through a bridge diode, and provide the rectified power rec to an LED so as to allow the LED to emit light. 
     Meanwhile, a plurality of LED driving circuits A, B, and C may be provided. The plurality of LED driving circuits A, B, and C may have the same configuration, and a single LED driving circuit A among theses will be described in detail. 
     The LED driving circuit A may include a pair of LED units LEDG 1  and LEDG 2 , a switching unit  120 , a driving control unit  130 , and a current limiting unit  140 . 
     The pair of LED units LEDG 1  and LEDG 2  may include a first LED unit LEDG 1  and a second LED unit LEDG 2  which are connected in parallel. 
     Each of the first LED unit LEDG 1  and the second LED unit LEDG 2  may include a plurality of LEDs LED 11  to LED 1 N and LED 21  to LED 2 N connected to each other in series. 
     The switching unit  120  may include a first switch group  121  and a second switch group  122 . 
     The first and second switch groups  121  and  122  respectively have a plurality of switches, the first switch group  121  enables light-emitting operations of the plurality of LEDs LED 11  to LED 1 N of the first LED unit LEDG 1  to be performed or stopped, and the second switch group  122  enables light-emitting operations of the plurality of LEDs LED 21  to LED 2 N of the second LED unit LEDG 2  to be performed or stopped. 
     More specifically, the plurality of switches Q 11  to Q 1 N of the first switch group  121  may respectively correspond to the plurality of LEDs LED 11  to LED 1 N of the first LED unit LEDG 1 , and likewise, the plurality of switches Q 21  to Q 2 N of the second switch group  122  may respectively correspond to the plurality of LEDs LED 21  to LED 2 N of the second LED unit LEDG 2 . 
     The plurality of respective switches Q 11  to Q 1 N and Q 21  to Q 2 N are connected between a connection point connecting adjacent LEDs of the plurality of LEDs LED 11  to LED 1 N and LED 21  to LED 2 N and the current limiting unit  140  as shown in  FIG. 1 , and form a current path of corresponding LEDs by performing ON/OFF switching in accordance with switching driving signals, thereby controlling drive of the corresponding LEDs. 
     The driving control unit  130  compares the rectified power red from the rectifying unit  110  and a predetermined reference voltage, thereby controlling drive of a corresponding switch. 
       FIG. 2  is a schematic configuration diagram showing a driving control unit adapted in the apparatus for driving the LED according to an embodiment of the present invention. 
     Referring to  FIGS. 1 and 2 , the driving control unit  130  may include a comparison unit  131 , a driving unit  132 , and a phase shift unit  133 . 
     The comparison unit  131  may include a plurality of comparators  131 - 1  to  131 -N, and the plurality of respective comparators  131 - 1  to  131 -N may compare a plurality of reference voltages Vref 1  to VrefN which are set in advance and the rectified power rec of the rectifying unit  110 . 
     A comparison result of each of the plurality of comparators  131 - 1  to  131 -N of the comparison unit  131  may be transmitted to the driving unit  132 . 
     The driving unit  132  may provide switching driving signals SQ 11  to SQ 1 N for driving ON/OFF switching of the corresponding switch based on the comparison result of each of the plurality of comparators  131 - 1  to  131 -N. 
     The phase shift unit  133  may shift a phase of the switching driving signals SQ 11  to SQ 1 N of the driving unit  132  to thereby output different switching driving signals SQ 21  to SQ 2 N. 
     More specifically, the number of the plurality of comparators  131 - 1  to  131 -N may correspond to the number of the plurality of switches Q 11  to Q 1 N or Q 21  to Q 2 N of the first switch group  121  or the second switch group  122 . 
     Therefore, the comparison result of each of the plurality of comparators  131 - 1  to  131 -N may be based on signals for driving the plurality of switches Q 11  to Q 1 N or Q 21  to Q 2 N of the first switch group  121  or the second switch group  122 . 
     The switching driving signals SQ 11  to SQ 1 N of the driving unit  132  may be Pulse Width Modulation (PWM) signals, and repeatedly perform ON/OFF switching of the corresponding switch while the corresponding LED is driven. The duration of driving of the corresponding LED may be determined based on the comparison result of each of the plurality of comparators  131 - 1  to  131 -N. 
     The switching driving signals SQ 11  to SQ 1 N of the driving unit  132  may PWM-drive the plurality of switches Q 11  to Q 1 N or Q 21  to Q 2 N of the first switch group  121  or second switch group  122 . Therefore, the phase shift unit  133  may shift the phase of the switching driving signals SQ 11  to SQ 1 N, and specifically shift the phase of the switching driving signals SQ 11  to SQ 1 N by 180 degrees so that switches Q 21  to  2 N or Q 11  to Q 1 N different from the switches Q 11  to Q 1 N or Q 21  to Q 2 N in charge of the switching driving signals SQ 11  to SQ 1 N of the driving unit  132  may be PWM-driven. 
     For example, when the switching driving signals SQ 11  to SQ 1 N of the driving unit  132  PWM-drive the plurality of switches Q 11  to Q 1 N of the first switch group  121 , the phase shift unit  133  may PWM-drive the plurality of switches Q 21  to Q 2 N of the second switch group  122  by a phase difference of 180 degrees. The reverse may also be adopted in the same manner. 
     The current limiting unit  140  may be connected to a distal end of each of the switches Q 11  to Q 1 N and Q 21  to Q 2 N of the first and second switch groups  121  and  122  of the switching unit  120  so as to limit a current flowing in the corresponding LED by PWM-driving each of the switches Q 11  to Q 1 N and Q 21  to Q 2 N of the first and second switch groups  121  and  122 . 
       FIGS. 3 and 4  are graphs showing electrical characteristics of the apparatus for driving the LED according to an embodiment of the present invention. 
     Referring to  FIGS. 3 and 4  together with  FIGS. 1 and 2 , for example, when the pair of LED units LEDG 1  and LEDG 2  adopted in the LED driving circuit A of the apparatus for driving the LED according to an embodiment of the present invention each include three LEDs connected to each other in series, a corresponding switch is turned on in accordance with respective voltage levels of the rectified power rec as shown in  FIG. 3  so that 3-STEP power flows. In addition, as shown in  FIG. 4 , the first and second LED units LEDG 1  and LEDG 2  are PWM-driven by a phase difference of 180 degrees. 
     As set forth above, according to the embodiments of the present invention, a pair of LED groups may be switching-operated by a phase difference of 180 degrees therebetween so as to suppress electromagnetic interference, and therefore a separate EMI filter may not be adopted, thereby reducing manufacturing costs and circuit area, and improving a power factor and Total Harmonic Distortion (THD) characteristics. 
     While the present invention has been shown and described in connection with the embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.
There is provided an apparatus for driving a light emitting diode (LED), performing switching-operation with respect to a pair of LED groups by a phase difference of 180 degrees. The apparatus includes a switching unit that alternately switches a first LED unit and a second LED unit of at least one pair of LED units emitting light by receiving rectified power, in accordance with a predetermined phase difference, a current limiting unit that limits a current flowing in the at least one pair of LED units through the switching of the switching unit, and a driving control unit that controls switching driving of the switching unit in accordance with a voltage level of the rectified power.