Abstract:
There is provided a power supply apparatus supplying a power to an electronic device, especially, a light emitting diode, capable of stably supplying power to different loads with simple circuit configuration, and maintaining the balance of the current supplied to light emitting diodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 10-2013-0073608 filed on Jun. 26, 2013, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a power supply apparatus supplying power to at least one light emitting diode. 
         [0004]    2. Description of the Related Art 
         [0005]    In general, a power supply apparatus is frequently used in information devices such as personal computers and home appliances such as air conditioners, audio systems and audio-visual devices. 
         [0006]    Such a power supply apparatus may provide a plurality of output powers depending on the electronic devices to which such power supply apparatuses are applied. 
         [0007]    A typical multiple-power supply apparatus converts an input direct current (DC) power into alternating current (AC) power using a single transformer, and rectifies the AC power so as to output a plurality of direct current power currents. In a typical multiple-power supply apparatus, however, if the voltage level of one of the plurality of direct current power is changed, it influences other direct current power so that cross-regulation is not maintained. To overcome the problem, a step-down chopper circuit is commonly used at the output stage. However, after primary power conversion, power conversion is made by the step-down chopper circuit once again, and thus the efficiency of power conversion is lowered. Further, adding the step-down chopper circuit increases the number of the components and thereby increases manufacturing costs. 
         [0008]    Especially when a power supply apparatus is used for driving light emitting diodes, a step-up or step-down circuit is employed for compensating for voltage deviations between the light emitting diodes after the primary power conversion as disclosed in Patent Document 1. Therefore, the efficiency of power conversion is lowered, and adding such a step-up or step-down chopper circuit increases the number of required components, thereby increasing manufacturing costs. 
       RELATED ART DOCUMENT  
       [0000]    
       
         (Patent Document 1) Korean Patent Laid-open Publication No. 10-2007-0068804 
       
     
       SUMMARY OF THE INVENTION 
       [0010]    An aspect of the present invention provides a power supply apparatus supplying power to an electronic device, especially a light emitting diode, capable of stably supplying power to different loads with a simple circuit configuration, and maintaining the balance of the current supplied to light emitting diodes. 
         [0011]    According to an aspect of the present invention, there is provided a power supply apparatus including: a power supplying unit switching an input power so as to supply at least two powers; a first control unit controlling primary side switching of the power supplying unit in a predetermined first manner according to the state of one of the at least two powers from the power supplying unit; a current balancing unit receiving another of the least two powers from the power supplying unit and maintaining the balance of currents between at least two light emitting diodes so as to transfer the power; and a second control unit controlling secondary side switching of the power supplying unit in a predetermined second manner according to the state of the power transferred to the least one light emitting diode of the at least two light emitting diodes. 
         [0012]    The first control unit may control the primary side switching frequency of the power supplying unit. 
         [0013]    The second control unit may control the secondary side switching duty of the power supplying unit. 
         [0014]    The power supplying unit may include: a power switching unit switching the input power under the control of the first control unit; a transformer having a primary winding receiving the switched powers from the switching units, and a plurality of secondary windings magnetically coupled to the primary winding to have a predetermined turns ratio; and a switching unit connected to one of the plurality of secondary windings and switching the transferred power under the control of the second control unit to supply the switched power to the current balancing unit. 
         [0015]    The plurality of secondary windings of the transformer may be made up of: a first secondary winding group having some of the plurality of secondary windings and supplying power to the light emitting diodes; and a second secondary winding group having the other of the plurality of secondary windings. 
         [0016]    The current balancing unit may include: a diode group having a plurality of diodes each connected between both ends of a secondary winding of the first secondary winding group so as to provide a power transfer path; and capacitors respectively located between one diode of the diode group and a corresponding secondary winding so as to maintain the balance of currents flowing in one direction and the other direction of the corresponding secondary winding. 
         [0017]    Each of the at least two light emitting diodes may be connected to one end of each of the first secondary winding group via at least one diode of the diode group, or to the other terminal of each of the capacitors with one terminal connected to the other end of each of the secondary windings. 
         [0018]    The first control unit may include: a power control unit controlling the switching of the power supplying unit according to the output state of at least one of the powers from the power supplying unit; a frequency control unit controlling the switching frequency of the power supplying unit according to the switching control of the power control unit; and a gate driver driving the switching of the power switching unit according to the switching frequency of the frequency control unit. 
         [0019]    The first control unit may further include a transfer unit which has one side to receive a signal and the other side to transmit a signal, electrically isolated from each other, so that the control signal from the power control unit input to the one side is transmitted to the frequency control unit connected to the other side. 
         [0020]    The second control unit may include: a PI control unit comparing a current value flowing in one of the at least two light emitting diodes with a command current value; and a switching control unit comparing the comparison result from the PI control unit with a predetermined reference signal so as to control the switching duty of the switching unit. 
         [0021]    According to another aspect of the present invention, there is provided a power supply apparatus including: a power supplying unit switching an input power to supply a predetermined power; and a current balancing unit alternately supplying the power from the power supplying unit to at least two light emitting diodes according to the switching of the power supplying unit so as to maintain the balance of currents between the at least two light emitting diodes. 
         [0022]    The first secondary winding group may include N secondary windings, wherein N is a natural number equal to or greater than 1, and the other end of each of the N secondary windings is connected to one end of and adjacent secondary winding via at least one diode of the diode group so as to supply power to (N+1) light emitting diodes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    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: 
           [0024]      FIG. 1  is a schematic circuit diagram of a power supply apparatus according to an embodiment of the present invention; 
           [0025]      FIG. 2  is a circuit diagram schematically showing the current balancing unit employed in the power supply apparatus according to the embodiment of the present invention; 
           [0026]      FIGS. 3 and 4  are circuit diagrams illustrating the operation of the current balancing unit employed in the power supply apparatus according to the embodiment of the present invention; 
           [0027]      FIGS. 5A to 5C  are circuit diagrams illustrating various examples of the current balancing unit employed in the power supply apparatus according to the embodiment of the present invention; 
           [0028]      FIG. 6  is a circuit diagram illustrating the power supply apparatus according to the embodiment of the present invention in more detail; 
           [0029]      FIG. 7  is a graph showing operation waveforms of the power supply apparatus; and 
           [0030]      FIG. 8  to  FIG. 10  are graphs showing voltage or current waveforms of main components of the power supply apparatus according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Hereinafter, embodiments of the present invention will be described in detail. 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. Throughout the drawings, the same or like reference numerals will be used to designate the same or like elements. 
         [0032]      FIG. 1  is a schematic circuit diagram of a power supply apparatus according to an embodiment of the present invention. 
         [0033]    Referring to  FIG. 1 , the power supply apparatus  100  according to the embodiment may include a power supplying unit  110 , a first control unit  120 , a second control unit  130 , and a current balancing unit  140 . 
         [0034]    The power supplying unit  110  may include a power switching unit  111  switching an input power and a transformer T. 
         [0035]    The power switching unit  111  may include power switches M 1  and M 2  switching an input power, and the power switches M 1  and M 2  may perform power-conversion by alternately switching the input power according to control. 
         [0036]    The transformer T may include a primary winding Np and at least two secondary windings Ns 1  to Nsn and Nsm. 
         [0037]    The primary winding Np receives the power switched by the power switching unit. The at least two secondary windings Ns 1  to Nsn and Nsm are electrically isolated from and magnetically coupled to the primary winding Np to have predetermined turns ratios, such that power input to the primary winding Np is converted according to the turns ratios so as to be output. 
         [0038]    The at least two secondary windings Ns 1  to Nsn and Nsm may be divided into a first secondary winding group Ns 1  to Nsn and a second secondary winding group Nsm. 
         [0039]    The first secondary winding group Ns 1  to Nsn may include at least one secondary winding, and may include a plurality of secondary windings if a plurality of light emitting diodes were employed. Each secondary winding of the first secondary winding group Ns 1  to Nsn may supply power to a light emitting diode in a LED unit, and the second secondary winding group Nsm including at least one secondary winding may supply power necessary for operating an electronic device including alight emitting diode. The power supplied to electronic devices may be output from the second secondary winding group Nsm, stabilized by a diode and capacitor, and then supplied. 
         [0040]    The first control unit  120  may control the power switches M 1  and M 2  of the power switching unit in a predetermined manner according to the power condition of the power supplied to the electronic device from the second secondary winding group Nsm. Specifically, the first control unit  120  may control the switching frequencies of the switches according to the power condition. 
         [0041]    To this end, the first control unit  120  may include a power control unit  121 , a transfer unit  122 , a frequency control unit  123 , and a gate driver  124 . 
         [0042]    The power control unit  121  may provide a control signal controlling the switching of the power switches M 1  and M 2  according to the power condition. The transfer unit  122  is configured as an isolated element such as a photo coupler or an isolated transformer since the primary winding Np and the secondary windings Ns 1  to Nsn and Nsm are electrically isolated as mentioned above, so that the control signal from the power control unit  121  on the secondary side may be transferred to the frequency control unit  123  on the primary side. 
         [0043]    The frequency control unit  123  may provide a control signal for controlling the switching frequencies of the power switches M 1  and M 2  based on the control signal from the transfer unit  122 . The gate driver  124  may provide a driving signal for driving the power switches M 1  and M 2  based on the control signal from the frequency control unit  123 . 
         [0044]    The second control unit  130  may control the supply of power by the first secondary winding group Ns 1  to Nsn in a predetermined control manner. Specifically, the second control unit  130  may control the switching duty of the power transferred via the first secondary winding group Ns 1  to Nsn. To this end, the power supplying unit  110  may further include a switching unit switching the power transferred via the first secondary winding group Ns 1  to Nsn. The switching unit may include a switch M AUX  that is connected to one of the first secondary winding group Ns 1  to Nsn and switches the power according to the switching control by the second control unit  130 . 
         [0045]    The current balancing unit  140  may maintain balance of current in the power transferred to a light emitting diode via the first secondary winding group Ns 1  to Nsn. 
         [0046]      FIG. 2  is a circuit diagram schematically showing the current balancing unit employed in the power supply apparatus according to the embodiment of the present invention. 
         [0047]    Referring to  FIG. 2 , the current balancing unit  140  employed in the power supply apparatus according to the embodiment may include a diode group having a plurality of diodes and a capacitor connected to the secondary winding. 
         [0048]    For example, when first to fourth light emitting diodes LED 1 , LED 2 , LED 3  and LED 4  are powered to be driven, the first second winding group may include the first to third windings Ns 1 , Ns 2 , and Ns 3 , and a current may flow while alternating in the current directions from one end to the other end of the first to third secondary windings Ns 1  to Ns 3 , and vice versa, according to the switching of the power supplying unit  110 . Here, the power supplying unit  110  may include a LLC (inductor-inductor-capacitor) resonant converter. 
         [0049]    Each of the first to third secondary windings Ns 1  to Ns 3  may share a light emitting diode, to which they supply power, with the adjacent secondary winding, and thus may drive four light emitting diodes with three secondary windings. However, the number of the secondary windings and that of the light emitting diodes are not limited to the above numbers. Two light emitting diodes may be driven with one secondary winding, and N+1 light emitting diodes may be powered with N secondary windings to be driven, where N is a natural number equal to or greater than “1.” 
         [0050]    The diode group having a plurality of diodes may provide a transfer path of the power transferred from the first to third winding Ns 1  to Ns 3  to the light emitting diodes LED 1  to LED 4 . A capacitor may be connected between one of the secondary windings and a corresponding diode to maintain the balance of current according to the charge balance law. 
         [0051]      FIGS. 3 and 4  are circuit diagrams illustrating the operation of the current balancing unit employed in the power supply apparatus according to the embodiment of the present invention. 
         [0052]    Referring to  FIG. 3  in conjunction with  FIG. 2 , currents isec 1 _P, isec 2 _P and isec 3 _P may flow in the first to third secondary windings Ns 1  to Ns 3 , respectively, in the direction from the other end to the one end thereof, and a corresponding diode DoP may be turned on so that the currents isec 1 _P, isec 2 _P and isec 3 _P flowing the direction may be transmitted to the corresponding light emitting diodes LED 1  and LED 3 . 
         [0053]    Referring to  FIG. 4  in conjunction with  FIG. 2 , currents isec 1 _N, isec 2 _N and isec 3 _N may flow in the first to third secondary windings Ns 1  to Ns 3 , respectively, in the direction from the one end to the other end thereof, and a corresponding diode DoN may be turned on so that the currents isec 1 _N, isec 2 _N and isec 3 _N flowing the direction may be transmitted to the corresponding light emitting diodes LED 2  and LED 4 . 
         [0054]    The above-described operation of supplying power will be described with respect to current balance along with the switching of the power supplying unit  110 . When the second power switch M 2  is turned on, a current conduction path as shown in  FIG. 3  is made, so that Equation 1 maybe established as follows: 
         [0000]      (Equation 1) 
         [0000]      I sec     —     P     —     1 =I sec     —     P     —     2 =I LED1 , I sec     —     P     —     3 =I LED3    (1).
 
         [0055]    Next, when the first power switch M 1  is turned on, a current conduction path as shown in  FIG. 4  is made, so that Equation 2 may be established as follows: 
         [0000]      (Equation 2) 
         [0000]      I sec     —     N     —     1 −I LED2 , I sec     —     N     —     2 −I sec     —     N     —     3 −I LED4    (2).
 
         [0056]    Here, due to charge balance law of capacitors connected the other end of the first to third secondary windings Ns 1  to Ns 3 , the average value of the DC offset in currents is removed, and thereby Equation 3 may be established as follows: 
         [0000]      (Equation 3) 
         [0000]      I sec     —     P     —     1 =I sec     —     N     —     1 , I sec     —     P     —     2 =I sec     —     N     —     2 , I sec     —     P     —     3 =I sec     —     N     —     3    (3).
 
         [0057]    Finally, as shown in Equation 4, the current values transferred to the first to fourth light emitting diodes LED 1  to LED 4  may be controlled so that they are the same, which may be equally applied to N light emitting diodes. 
         [0000]      &lt;I sec     —     P     —     1 &gt;=&lt;I sec     —     N     —     1 &gt;= . . . =&lt;I sec     —     P     —     n &gt;=&lt;I sec     —     N     —     n &gt;=I LED1 =I LED2 = . . . =I LEDn    (4).
 
         [0058]      FIGS. 5A to 5C  are circuit diagrams illustrating various examples of the current balancing unit employed in the power supply apparatus according to the embodiment of the present invention. 
         [0059]    Referring to  FIGS. 5A to 5C , as shown in  FIG. 5A , two light emitting diodes LED 1  and LED 2  may be powered to be driven with one secondary winding Ns 1 , and current balance is maintained between the two light emitting diodes LED 1  and LED 2 . 
         [0060]    Likewise, as shown in  FIG. 5B , three light emitting diodes LED 1 , LED 2  and LED 3  may be powered to be driven with two secondary windings Ns 1  and Ns 2 , or as shown in  FIG. 5C , N+1 light emitting diodes LED 1 , LED 2 , LED 3 , . . . , LEDn+1 may be powered to be driven with N secondary windings Ns 1 , Ns 2 , . . . , Nsn, where N is a natural number equal to or greater than 1, such that current balance may be made maintained among three light emitting diodes LED 1 , LED 2  and LED 3  or N+1 light emitting diodes LED 1 , LED 2 , LED 3 , . . . , LEDn+1. 
         [0061]      FIG. 6  is a circuit diagram illustrating the power supply apparatus according to the embodiment of the present invention in more detail, and  FIG. 7  is a graph showing operation waveforms of the power supply apparatus. For the sake of easy explanation of the overall system operation,  FIGS. 6 and 7  illustrate the configurations and the operation waveforms of the simplest example, a single power stage power converter for a two-channel LED backlight. 
         [0062]    Referring to  FIG. 6 , the power supply apparatus  100  according to the embodiment may control the switching frequency of a primary side power switching based on the output of the second secondary winding group, and may control the switching duty of a secondary side power switching based on the output of the first secondary winding group. 
         [0063]    To this end, the second control unit  130  may include a PI control unit  131  having a comparator OP 1  and a switching control unit  132  having a comparator OP 2 . 
         [0064]    The PI control unit  131  may compare a current value flowing in a light emitting diode with a command current value indicating a current value intended to flow in the light emitting diode, to provide the result Vero. 
         [0065]    The switching control unit  132  may compare the result Vero from the PI control unit  131  with a reference signal Vsaw of a predetermined triangular wave, to provide a control signal Vgs for controlling the switching duty of the switch M AUX  of the power supplying unit  110 . 
         [0066]    Referring to  FIG. 7 , the output V AUD  from the second secondary winding group may alternately turn on and off the power switches M 1  and M 2  by the first control unit  110 . Here, the power switches M 1  and M 2  are controlled by the pulse frequency modulation (PFM) operation in which the operation frequency is varied while the duty ratio is fixed at 50%. Further, the current in the first light emitting diode LED 1  is controlled by pulse width modulation (PWM) operation in which the duty ratio of the switch M AUX  is varied by the PI control unit  131  and the switching control unit  132 . That is, the current in the first light emitting diode LED 1 , one of the light emitting diode channels, is detected using a resistance sensor or current sensor, and then is input to the PI control unit  131 , such that the result voltage Vero is adjusted so that the current in the input first light emitting diode LED 1  is equalized with the command current Icom. The result voltage Vero output from the PI control unit  131  is input to the switching control unit  132 , and is compared to a reference signal having a lamp-waveform Vsaw generated by an external signal Vsync, so that the duty ratio of the generated control signal Vgs is varied, to thereby control the current flowing in the first light emitting diode LED 1 . 
         [0067]    The operating principle of each of the modes according to switching states shown in  FIG. 7  will be described. 
         [0068]    Mode 1: the first power switch M 1  is turned on and a positive (+) voltage is applied to a non-dot of the transformer, and thus the voltage Vsec 1  at the first secondary winding has a negative (−) value, such that diodes Do 1 , D 1  and D 2  of the diode group are all turned off, and both of the output from the second secondary winding group i AUD  and the current i LED2  flowing in the second light emitting diode become zero. In addition, although a conduction path is made toward the first light emitting diode LED 1  via the diodes D 3  and D 4 , the current i LED1  in first light emitting diode does not flow either since the switch M AUX  is in the off state. 
         [0069]    Mode 2: like Mode 1, the first power switch M 1  is turned on, and the positive (+) voltage is applied to the Non-dot of the transformer, such that the voltage at the first secondary winding Vsec 1  has the negative (−) value. When a control signal Vgs is applied to the switch M AUX , a current flows in the first light emitting diode LED 1  via a path of the switch M AUX  and the diodes D 3  and D 4 , such that the current iLED 1  in the first light emitting diode is controlled so that it becomes the command current Icom. At the same time, the voltage Vsec 2  at the second secondary winding group also has the negative value, such that the diode Do 1  is blocked, and thus no current flow toward the output i AUD  of the second secondary winding group. 
         [0070]    Mode 3: when the first power switch M 1  is turned off and the second power switch M 2  is turned on, the positive (+) voltage is applied to Dot of the transformer. Accordingly, the voltage at the second secondary winding group Vsec 2  has a positive value and thus the diode Do 1  is turned on, such that the output current i AUD  from the second secondary winding group flows toward the output, to output an output voltage VAUD as shown in  FIG. 7 . At the same time, toward the light emitting diodes, the first secondary winding voltage Vsec 1  has a positive value and the diodes D 1  and D 2  are turned on, such that current i LED2  flows in the second light emitting diodes LED 2 . At this time, by the capacitor C B , the average current having the same amplitude as the current I LED1  flowing toward the first light emitting diode LED 1  in Mode 2 flows toward the second light emitting diode LED 2  as the current i LED2  such that the same average currents flow in the first light emitting diode LED 1  and the second light emitting diode LED 2  and thereby current balancing is achieved. 
         [0071]      FIG. 8  to  FIG. 10  are graphs showing voltage or current waveforms of main components of the power supply apparatus according to the embodiment of the present invention. The input/output specifications used in the experiment are: input voltage Vin=400 Vdc, VAUD=13V/2.5 A, output LED=4 channels/RLED (LED equivalent resistance)=428 ohm, 375 ohm, 333 ohm, 300 ohm, and PWM dimming frequency=200 Hz. Here, in order to exhibit current balancing performance of light emitting diodes, the equivalent resistance of each of the light emitting diodes are differently set as shown in  FIG. 8 . 
         [0072]    Even though the equivalent resistance of each of the light emitting diodes are different such that different voltages are applied as shown in  FIG. 8 , the current balances in each of the light emitting diodes are constantly maintained as shown in  FIGS. 9 and 10 . 
         [0073]    As set forth above, according to embodiments of the present invention, power can be stably supplied to different loads and the area of the circuit and manufacturing cost can be reduced by a single power stage of the transformer and by the primary side switching between different loads and the switching of one of the plurality of secondary windings. Further, the balance of the current supplied to light emitting diodes can be maintained and the area of the circuit and manufacturing cost can be further reduced by the connection of the plurality of secondary windings and capacitors. 
         [0074]    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.