Patent Publication Number: US-8981649-B2

Title: Light emitting diode driving apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of Korean Patent Application No. 10-2012-0157056 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 a light emitting diode (LED) driving apparatus for directly driving an LED using alternating current (AC) power. 
     2. Description of the Related Art 
     A light emitting diode (LED) is a semiconductor element having a p-n junction structure and emitting light through electron-hole recombination, and has recently been used in various fields, according to the development of semiconductor technology. 
     Particularly, since an LED has high efficiency, a relatively long lifespan, and environmentally friendly characteristics, as compared to a light emitting device according to the related art, a range of applications thereof is continuously being widened. 
     Generally, an LED may be driven by applying several volts of direct current (DC) voltage thereto, due to a structure thereof. Therefore, in order to drive the LED using commercial alternating current (AC) power, commonly used domestically, commercially, or the like, a separate unit is required. 
     In order to drive the LED using commercial AC power, an LED driving apparatus typically includes a rectifying circuit, an alternating current-direct current converter (an AC-DC converter), and the like. 
     However, a general AC-DC converter has large volume and consumes a significant amount of power. Therefore, in a case in which a general AC-DC converter is applied to the LED driving apparatus, advantages of the LED such as high efficiency, a small packaging size, a long lifespan, and the like are largely canceled. 
     Therefore, research into an apparatus capable of directly driving an LED with AC power without using an AC-DC converter has been recently conducted. 
     In the case of directly driving the LED with AC power without using the AC-DC converter, a method in which a plurality of switches are respectively connected to a plurality of LEDs and switching on and switching off of the plurality of switches are controlled according to a level of the AC power to allow current to uniformly flow may be generally applied. 
     Patent Document 1 relates to an LED driving apparatus, in which an LED is directly driven to emit light using AC power by controlling operations of switches connected to a medium node and the last node of an LED array. 
     Patent Document 2 also relates to an LED driving apparatus, and discloses a configuration controlling switching on and switching off of a switch in an order in which LED arrays are connected. 
     However, while both Patent Documents 1 and 2 control the operation of the switch according to a level of the AC power, a waveform of input current due to switch driving is formed as a step-type waveform, thereby deteriorating a power factor and total harmonic distortion characteristics. 
     RELATED ART DOCUMENT 
     
         
         (Patent Document 1) Korean Patent No. 10-0997050-0000 
         (Patent Document 2) Korean Patent No. 10-0995793-0000 
       
    
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides a light emitting diode (LED) driving apparatus configured such that a waveform of current input to an LED follows a sine wave. 
     According to an aspect of the present invention, there is provided a light emitting diode (LED) driving apparatus, including: a switching unit switching an LED unit having a plurality of LEDs receiving rectified power and emitting light; a driving control unit controlling switching driving of the switching unit according to a voltage level of the rectified power; a current limiting unit limiting current flowing in the LED unit; and an adjusting unit adjusting current limitation of the current limiting unit according to the voltage level of the rectified power. 
     The adjusting unit may adjust the current of the current limiting unit according to a voltage waveform of the rectified power. 
     The adjusting unit may include: a voltage dividing unit dividing the voltage level of the rectified power; and an automatic gain adjusting unit adjusting a current amount of the current limiting unit according to the divided voltage level of the voltage dividing unit. 
     The automatic gain adjusting unit may include: a comparator comparing the divided voltage level of the voltage dividing unit with a level of a preset operational reference voltage; a dividing resistor group dividing the divided voltage level of the voltage dividing unit according to a preset resistance ratio; a limiting unit limiting the voltage level divided by the dividing resistor group to a preset level; a converting unit converting a peak value of the limited voltage level from the limiting unit into a digital signal; and a peak controlling unit controlling the peak value of the voltage level from the converting unit to maintain a constant peak voltage regardless of the divided voltage level of the voltage dividing unit. 
     The switching unit may include a plurality of switches, respectively connected between connection points between the plurality of respective LEDs of the LED unit and the current limiting unit. 
     The driving control unit may include: a comparing unit comparing the rectified power with a preset reference voltage; and a driving unit providing a switching driving signal controlling switching driving of the LED unit according to a comparison result of the comparing unit. 
     The comparing unit may include a plurality of comparators having a number corresponding to that of the LEDs of the LED unit. 
     The current limiting unit may include: a current control unit having a plurality of switches controlling current flowing in the switching unit according to a driving control of the driving control unit; and a current source unit having a plurality of current sources adjusting a current amount according to a current adjustment of the adjusting unit. 
     The LED driving apparatus may further include a rectifying unit rectifying alternating current (AC) power to supply the rectified power to at least one pair of the LEDs. 
     According to another aspect of the present invention, there is provided a light emitting diode (LED) driving apparatus, including: a rectifying unit rectifying and supplying alternating current power; a switching unit switching an LED unit having a plurality of LEDs receiving rectified power and emitting light; a driving control unit controlling switching driving of the switching unit according to a voltage level of the rectified power; a current limiting unit limiting current flowing in the LED unit; and an adjusting unit adjusting current limitation of the current limiting unit according to a voltage waveform 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 view schematically showing a configuration of a light emitting diode (LED) driving apparatus according to an embodiment of the present invention; 
         FIG. 2  is a view schematically showing a configuration of a driving control unit used in the LED driving apparatus according to the embodiment of the present invention; 
         FIG. 3  is a circuit diagram schematically showing an example of the LED driving apparatus according to the embodiment of the present invention; 
         FIG. 4  is a configuration view schematically showing an adjusting unit used in the LED driving apparatus according to the embodiment of the present invention; and 
         FIGS. 5A and 5B  are graphs showing electrical characteristics of the LED driving apparatus according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     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. In the drawings, the shapes and dimensions of elements may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like elements. 
       FIG. 1  is a view schematically showing a configuration of a light emitting diode (LED) driving apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 1 , an LED driving apparatus  100  according to the embodiment of the present invention may include a rectifying unit  110 , a switching unit  120 , a driving control unit  130 , a current limiting unit  140 , and an adjusting unit  150 . 
     The rectifying unit  110  may receive alternating current (AC) power to half-wave rectify or full-wave rectify the AC power through a bridge diode, and may supply rectified power rec to LEDs to enable the LEDs to perform a light emitting operation. 
     An LED unit may include a plurality of LEDs LED 1  through LEDn connected to one another in series. 
     The switching unit  120  includes a plurality of switches Q 1  through Qn respectively correspond to the plurality of LEDs LED 1  through LEDn of the LED unit. 
     Each of the plurality of switches Q 1  through Qn is connected between a connection point between the LEDs adjacent to each other and the current limiting unit  140  as shown in  FIG. 1  and is switched on or switched off according to a switching driving signal, such that each switch may form a current path for each LED corresponding thereto to control the driving of the corresponding LED. 
     For example, when the first switch Q 1  is switched on, the first diode LED 1  may be driven, and when the second switch Q 2  is switched on, the first and second diodes LED 1  and LED 2  may be driven. Similarly, when the n-th switch Qn is switched on, the first through N-th diodes LED 1  through LEDn may be driven. 
     The driving control unit  130  may compare the rectified power rec from the rectifying unit  110  with a preset reference voltage to control the driving of the corresponding switch. 
       FIG. 2  is a view schematically showing a configuration of a driving control unit used in the LED driving apparatus according to the embodiment of the present invention. 
     Referring to  FIGS. 1 and 2 , the driving control unit  130  may include a comparing unit  131  and a driving unit  132 . 
     The comparing unit  131  may include a plurality of comparators  131   a - 1  through  131   a - n , and the plurality of respective comparators  131   a - 1  through  131   a - n  may compare preset plurality reference voltages Vref 1  through VrefN with the rectified power rec from the rectifying unit  110 . 
     Compared results of the plurality of respective comparators  131   a - 1  through  131   a - n  of the comparing unit  131  may be transferred to the driving unit  132 . 
     The driving unit  132  may supply switching driving signals SQ 1  through SQn respectively driving switching on and switching off of the corresponding switch based on the comparison result of each of the plurality of comparators  131   a - 1  through  131   a - n.    
     More specifically, the number of the plurality of comparators  131   a - 1  through  131   a - n  of the comparator unit  131  may correspond to the number of the plurality of switches Q 1  through Qn. 
     Therefore, the comparison result of each of the plurality of comparators  131   a - 1  through  131   a - n  may be a basis of a signal for driving a plurality of switches Q 11  through Q 1 N or Q 21  through Q 2 N of a first switch group  121  or a second switch group  122 . 
     The switching driving signals SQ 1  through SQn of the driving unit  132  may be pulse width modulation (PWM) signals and each switching signal may repeatedly switch on and switch off the corresponding switch during the driving of the corresponding LED. A period in which the corresponding LED is driven may be determined depending on the comparison result of each of the plurality of comparators  131   a - 1  through  131   a - n . The switching driving signals SQ 1  through SQn of the driving unit  132  may PWM drive the plurality of switches Q 1  through Qn. 
     The current limiting unit  140  may be connected to respective ends the plurality of switches Q 1  through Qn to limit the current flowing in the corresponding LED by the PWM driving of each of the plurality of switches Q 1  through Qn. 
     The adjusting unit  150  may adjust a current limit amount of the current limiting unit  140  according to a voltage level of the rectified power rec. 
       FIG. 3  is a circuit diagram schematically showing an example of the LED driving apparatus according to the embodiment of the present invention. 
     Referring to  FIGS. 1 and 3 , for example, in the case in which the LED driving apparatus  100  according to the embodiment of the present invention drives the first through third LEDs LED 1 , LED 2 , and LED 3 , the switching unit  120  may include the first through third switch Q 1 , Q 2 , and Q 3 , such that the first LED LED 1 , the first and second LEDs LED 1  and LED 2 , or the first through third LEDs LED 1 , LED 2 , and LED 3  may be driven. To this end, the driving control unit  130  compares the voltage level of the rectified power rec with each of the preset plurality reference voltage Vref 1 , Vref 2 , and Vref 3 , and the driving unit  132  may provide the switching driving signals driving the first through third switches Q 1 , Q 2 , and Q 3  through logical operations using an AND element and an inverted AND element according to the comparison results of the respective comparators of the comparing unit  131 . 
     The current limiting unit  140  may include a current control unit  141  and a current source unit  142 . In this case, the current control unit  141  inverts the switching driving signals to perform switching on and switching off operations complementarily in conjunction with the switching on and switching off operations of the first through third switches Q 1 , Q 2 , and Q 3 , such that the current control unit  141  may be switched on when the corresponding switch is switched off to connect the current path to a ground and may control the current. 
     The current source unit  142  may include current sources respectively connected to the ends of the first through third switches Q 1 , Q 2 , and Q 3 , and the respective current sources may limit the current flowing in the respective first through third LEDs LED 1 , LED 2 , and LED 3  according to the control of the adjusting unit  150 . 
     Here, although the embodiment illustrates the case in which the current control unit  141  and the current source unit  142  include logic elements, switches, and current sources corresponding to the first through third switches Q 1 , Q 2 , and Q 3 , the number of the logical elements, the switches, and the current sources may also increase in the case in which the number of the switches increases. 
     The adjusting unit  150  may include a voltage dividing unit  151  and an automatic gain adjusting unit  152 . 
     The voltage dividing unit  151  may divide the voltage level of the rectified power rec into voltage levels that may be processed, according to a preset resistance ratio and thus, may include a plurality of resistors R 1  and R 2  for the division. 
     The automatic gain adjusting unit  152  adjusts a current amount of the current limiting unit  140  according to the divided voltage level of the voltage dividing unit to enable a current waveform of the power input to the LED unit to follow a voltage waveform of the rectified power rec. 
       FIG. 4  is a configuration view schematically showing an adjusting unit used in the LED driving apparatus according to the embodiment of the present invention. 
     Referring to  FIG. 4 , the automatic gain adjusting unit  152  may include a comparator  152   a , a dividing resistor group  152   b , a limiting unit  152   c , a converting unit  152   d , and a peak controlling unit  152   e.    
     The comparator  152   a  may compare the divided voltage level of the voltage dividing unit  151  with a level of a preset operational reference voltage Verf to provide a clock signal CLK according to the comparison result, the dividing resistor group  152   b  may divide the divided voltage level of the voltage dividing unit  151  according to the preset resistance ratio, and the limiting unit  152   c  limits the divided voltage level divided by the dividing resistor group  152   b  to a preset level. 
     The converting unit  152   d  may convert a peak value of the limited voltage level from the limiting unit  152   c  into a digital signal according to the clock signal CLK from the comparator  152   a , and the peak controlling unit  152   e  may control the peak value of the voltage level from the converting unit  152   d  to maintain a constant peak voltage regardless of the divided voltage level of the voltage dividing unit  151 . 
       FIGS. 5A and 5B  are graphs showing electrical characteristics of the LED driving apparatus according to the embodiment of the present invention. 
     Referring to  FIG. 5A , in the LED driving apparatus, when the current is controlled in an one-step scheme, total harmonic distortion characteristics (THD) of 31.7% and a power factor of 0.953 are exhibited, while when the current is controlled in a two-step scheme, THD characteristics may be decreased to 15.6% and the power factor may be increased to 0.994. That is, since the voltage waveform of the rectified power has a sine wave form, in the case in which the current input to the LED approximately follows the voltage waveform of the rectified power, the THD and the power factor characteristics may be improved. 
       FIG. 5B  shows a current waveform (an upper side graph) of the current input to the LED of the LED driving apparatus according to the embodiment of the present invention and a current waveform (a lower side graph) according to switch driving in the case of using three switches as shown in  FIG. 4 . 
     As shown in  FIG. 5B , since the current waveform of the current input to the LED follows the sine wave except for dead time due to the switch, the THD and power factor characteristics may be improved in the LED driving apparatus according to the embodiment of the present invention. 
     As set forth above, according to the embodiment of the present invention, the current waveform of the current input to the LED follows the sine wave, whereby the power factor and THD characteristics can be improved. 
     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.