Abstract:
A current balancing apparatus includes a first transformer having a first primary winding and a first secondary winding electromagnetically coupled with the first primary winding, the first primary winding having a first end connected to a first load that passes a first current; a second transformer having a second primary winding and a second secondary winding electromagnetically coupled with the second primary winding, the second primary winding having a first end connected to a second load that passes a second current having an AC component substantially having a 180-degree phase difference with respect to the first current; and a series circuit including the first secondary winding, the second secondary winding, and a current smoother, to balance the first current and second current with each other.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a current balancing apparatus, a current balancing method, and a power supply apparatus, for balancing currents passing through a plurality of loads connected in parallel. 
         [0003]    2. Description of the Related Art 
         [0004]    An example of an apparatus for supplying power to a plurality of loads is an apparatus for lighting a plurality of LEDs (Light Emitting Diodes) disclosed in Japanese Unexamined Patent Application Publication No. 2003-332624 (Document 1). 
         [0005]      FIG. 1  illustrates the LED driving apparatus disclosed in Document 1. The apparatus has a DC power source Vdd, a step-up circuit  27 , LEDs  21  to  26 , sink drivers  12  to  14 , bypass units  15  to  17 , and a selector  18 . The sink drivers  12  to  14  turn on/off in response to time division signals S 1  to S 3 . Each of ends of the sink drivers  12  to  14  is connected to related one of terminals P 23  to P 25  each connected to the LEDs  21  to  26 . The bypass units  15  to  17  are connected in parallel with the sink drivers  12  to  14  and pass currents when the sink drivers  12  to  14  are OFF, the currents being small not to make the LEDs  21  to  26  emit light. 
         [0006]    The selector  18  detects a drain-source voltage of one of the sink drivers  12  to  14  and a current passing through one of the three lines of the LEDs  21  to  26  and controls an output voltage from the step-up circuit (converter)  27 . 
         [0007]    According to this related art, the sink drivers  12  to  14  pass necessary currents through the LEDs  21  to  26  during a period of lighting the LEDs  21  to  26 . During a period of not lighting the LEDs  21  to  26 , the sink drivers  12  to  14  stop the currents and the bypass units  15  to  17  bypass small currents, to prevent an output voltage from the converter  27  from jumping up. 
         [0008]    Other related arts are disclosed in, for example, Japanese Unexamined Patent Application Publications No. H11-67471 and No. 2002-8409. 
       SUMMARY OF THE INVENTION 
       [0009]    According to the related art illustrated in  FIG. 1 , a step-up reactor L 27  and a high-frequency switch Q 27  are used to generate a stepped-up, high-frequency voltage, which is rectified and smoothed with a diode D 27  and an electrolytic capacitor C 27 , to apply a stepped-up DC voltage to the LEDs  21  to  26 . 
         [0010]    Generally, LEDs have variations in forward voltages Vf. Accordingly, currents passing through the LEDs  21  to  26  connected in parallel are not equal to one another. For this, the related art employs the sink drivers  12  to  14  that are constant current circuits (current mirror circuits), to apply different voltages according to the different Vf values, to balance the currents passing through the LEDs  21  to  26  with one another. The sink drivers  12  to  14  cause losses depending on applied voltages and thereby deteriorate efficiency. 
         [0011]    The present invention provides a current balancing apparatus, a current balancing method, and a power supply apparatus, capably of minimizing losses that occur when balancing currents passing through loads and improving efficiency. 
         [0012]    According to a first aspect of the present invention, the current balancing apparatus includes a first transformer having a first primary winding and a first secondary winding electromagnetically coupled with the first primary winding, the first primary winding having a first end connected to a first load that passes a first current; a second transformer having a second primary winding and a second secondary winding electromagnetically coupled with the second primary winding, the second primary winding having a first end connected to a second load that passes a second current; and a series circuit including the first secondary winding, the second secondary winding, and a current smoother, wherein the first current and the second current load are balanced with each other. 
         [0013]    According to a second aspect of the present invention, the power supply apparatus includes a series resonant circuit including a transformer; a plurality of switching elements to pass a current to the series resonant circuit; a first transformer connected to an output of the series resonant circuit and having a first primary winding and a first secondary winding electromagnetically coupled with the first primary winding, the first primary winding having a first end connected to a first load; a second transformer connected to an output of the series resonant circuit and having a second primary winding and a second secondary winding electromagnetically coupled with the second primary winding, the second primary winding having a first end connected to a second load; a series circuit including the first secondary winding, the second secondary winding, and a current smoother; a current detector to detect a current passing through the series circuit; and a controller to turn on/off the plurality of switching elements according to an output from the current detector. 
         [0014]    According to a third aspect of the present invention, the current balancing method includes connecting a primary winding of a first transformer to a first load that passes a first current; connecting a primary winding of a second transformer to a second load that passes a second current; and connecting a secondary winding of the first transformer electromagnetically coupled with the primary winding of the first transformer, a secondary winding of the second transformer electromagnetically coupled with the primary winding of the second transformer, and a current smoother in series, thereby passing a current to balance the first current and second current with each other. 
         [0015]    According to these aspects of the present invention, the first secondary winding, second secondary winding, and current smoother that form the series circuit pass a current to balance the first current and second current with each other, thereby reducing losses that may occur when balancing the currents passing through the loads and improving efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a block diagram illustrating an LED lighting apparatus according to a related art; 
           [0017]      FIG. 2  is a block diagram illustrating a power supply apparatus having a current balancing apparatus according to an embodiment of the present invention; and 
           [0018]      FIG. 3  is a timing chart illustrating operation of the power supply apparatus of  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    A current balancing apparatus, a current balancing method, and a power supply apparatus according to embodiments of the present invention will be explained in detail with reference to the drawings. 
         [0020]      FIG. 2  is a block diagram illustrating a power supply apparatus having a current balancing apparatus according to an embodiment of the present invention. In this embodiment, the power supply apparatus having the current balancing apparatus is used as an LED lighting apparatus. 
         [0021]    In  FIG. 2 , both ends of a DC power source Vin are connected to a series circuit including switching elements Q 1  and Q 2  made of MOSFETs. A connection point between the switching elements Q 1  and Q 2  is connected to a series resonant circuit including a primary winding Np of a transformer T and a current resonant capacitor Cri. The transformer T has a leakage inductance. 
         [0022]    The transformer T has a secondary winding Ns whose first end is connected to LEDs  21   a  to  21   e  connected in series, LEDs  22   a  to  22   e  connected in series, and a flywheel diode D 10 . 
         [0023]    A second end of the secondary winding Ns of the transformer T is connected to LEDs  23   a  to  23   e  connected in series, LEDs  24   a  to  24   e  connected in series, and a flywheel diode D 11 . 
         [0024]    A cathode of the LED  21   e  is connected to a first end of a primary winding P 1  of a transformer T 1  (corresponding to the “first transformer” stipulated in the claims). A second end of the primary winding P 1  is grounded. A cathode of the LED  22   e  is connected to a first end of a primary winding P 2  of a transformer T 2  (corresponding to the “first transformer” stipulated in the claims). A second end of the primary winding P 2  is grounded. 
         [0025]    A cathode of the LED  23   e  is connected to a first end of a primary winding P 3  of a transformer T 3  (corresponding to the “second transformer” stipulated in the claims). A second end of the primary winding P 3  is grounded. A cathode of the LED  24   e  is connected to a first end of a primary winding P 4  of a transformer T 4  (corresponding to the “second transformer” stipulated in the claims). A second end of the primary winding P 4  is grounded. 
         [0026]    A secondary winding S 1  of the transformer T 1 , a secondary winding S 2  of the transformer T 2 , a secondary winding S 3  of the transformer T 3 , a secondary winding S 4  of the transformer T 4 , a resistor Rs, and a reactor L 1  are connected in series to form a closed-loop constant current circuit. The constant current circuit operates as a balancing circuit due to its function. The reactor L 1  corresponds to the “current smoother” stipulated in the claims and smoothes a current passing through the constant current circuit. In the smoothed current, an AC component is left to achieve a current balancing action (explained later). 
         [0027]    A connection point between the resistor Rs and the secondary winding S 4  is grounded. The resistor Rs serves as a current detector. A connection point between the resistor Rs and the reactor L 1  is connected to a series circuit including a resistor R 3  and a capacitor C 3 . The series circuit converts a voltage containing an AC component into a DC voltage. 
         [0028]    A PFM circuit  1  compares the voltage of the capacitor C 3  with a reference voltage Vref and generates a pulse signal. At this time, the PFM circuit  1  changes the frequency of the pulse signal according to the voltage of the capacitor C 3 . 
         [0029]    An inverter  2  inverts the pulse signal from the PFM circuit  1  and supplies the inverted pulse signal to a high-side driver  4 . A low-side driver  3  receives the pulse signal from the PFM circuit  1 , and according to the pulse signal, turns on/off the switching element Q 1 . The high-side driver  4  turns on/off the switching element Q 2  according to the inverted pulse signal from the inverter  2 . 
         [0030]    Alternately turning on/off the switching elements Q 1  and Q 2  and the frequency of the pulse signal control input voltages to the LEDs  21   a  to  21   e , LEDs  22   a  to  22   e , LEDs  23   a  to  23   e , and LEDs  24   a  to  24   e.    
         [0031]    Operation of the LED lighting apparatus of the above-mentioned configuration will be explained with reference to  FIG. 3 . 
         [0032]    In  FIG. 3 , a waveform Q 1   v  illustrates a drain-source voltage of the switching element Q 1 , a waveform Q 1   i  a drain current of the switching element Q 1 , a waveform Q 2   v  a drain-source voltage of the switching element Q 2 , a waveform Q 2   i  a drain current of the switching element Q 2 , a waveform D 10   i  a current to the flywheel diode D 10 , and a waveform D 11   i  a current to the flywheel diode D 11  in the same manner. 
         [0033]    At time t 0 , the switching element Q 1  is OFF and the switching element Q 2  turns on to pass the current Q 2   i  in a minus (counterclockwise) direction through a path extending along Vin (positive terminal), Q 2 , Np, Cri, and Vin (negative terminal). As time passes, the current increases into a plus (clockwise) direction to charge the current resonant capacitor Cri. 
         [0034]    At this time, the secondary winding Ns of the transformer T generates a voltage to pass a transformer current Nsi, LED currents, and the current D 11   i  through a path extending along the first end of Ns, LEDs  21   a  to  21   e  (LEDs  22   a  to  22   e ), P 1  (P 2 ), D 11 , and the second end of Ns. 
         [0035]    At time t 1 , the switching element Q 2  turns off and the switching element Q 1  turns on. The primary winding Np of the transformer T generates a voltage in a reverse direction so that the current Q 1   i  passes in a minus (clockwise) direction through a path extending along Cri, Np, Q 1 , and Cri. As time passes, the current increases into a plus (counterclockwise) direction to discharge the current resonant capacitor Cri. 
         [0036]    At this time, the secondary winding Ns of the transformer T generates a voltage in response to the voltage of the reverse direction generated by the primary winding Np. This results in passing the transformer current Nsi, LED currents, and current D 10   i  through a path extending along the second end of Ns, LEDs  23   a  to  23   e  (LEDs  24   a  to  24   e ), P 3  (P 4 ), D 10 , and the first end of Ns. 
         [0037]    Namely, a current passing through the LEDs  23   a  to  23   e  and P 3  (LEDs  24   a  to  24   e  and P 4 ) has an AC component that substantially has a 180-degree phase difference with respect to a current passing through the LEDs  21   a  to  21   e  and P 1  (LEDs  22   a  to  22   e  and P 2 ). Operation after time t 2  is the same as that in the period from t 0  to t 2 , and therefore, the explanation thereof is omitted. 
         [0038]    A current balancing method according to an embodiment of the present invention will be explained. 
         [0039]    As explained above, at time t 0 , the LEDs  21   a  to  21   e  and the primary winding P 1  of the transformer T 1  pass an equal LED current. This LED current causes the primary winding P 1  to generate magnetic flux. This magnetic flux causes the secondary winding S 1  of the transformer T 1  to generate magnetic flux. This magnetic flux causes the secondary winding S 1  to generate a current passing through the closed-loop constant current circuit. 
         [0040]    Also at time t 0 , the LEDs  22   a  to  22   e  and the primary winding P 2  of the transformer T 2  pass an equal LED current. This LED current causes the primary winding P 2  to generate magnetic flux. This magnetic flux causes the secondary winding S 2  of the transformer T 2  to generate magnetic flux. This magnetic flux causes the secondary winding S 2  to generate a current passing through the closed-loop constant current circuit. 
         [0041]    At time t 1 , the LEDs  23   a  to  23   e  and the primary winding P 3  of the transformer T 3  pass an equal LED current. This LED current causes the primary winding P 3  to generate magnetic flux. This magnetic flux causes the secondary winding S 3  of the transformer T 3  to generate magnetic flux. This magnetic flux causes the secondary winding S 3  to generate a current passing through the closed-loop constant current circuit. 
         [0042]    Also at time t 1 , the LEDs  24   a  to  24   e  and the primary winding P 4  of the transformer T 4  passes an equal LED current. This LED current causes the primary winding P 4  to generate magnetic flux. This magnetic flux causes the secondary winding S 4  of the transformer T 4  to generate magnetic flux. This magnetic flux causes the secondary winding S 4  to generate a current passing through the closed-loop constant current circuit. 
         [0043]    The currents based on the magnetic flux generated by the secondary windings S 1  to S 4  all pass through the closed-loop constant current circuit, and therefore, are balanced (equalized) to a constant value even if the currents inherently differ from one another. This results in balancing (equalizing) the magnetic flux generated by the secondary windings S 1  to S 4 , thereby balancing (equalizing) the magnetic flux generated by the primary windings P 1  to P 4 . As results, the LED current passing through the LEDs  21   a  to  21   e  and primary winding P 1 , the LED current passing through the LEDs  22   a  to  22   e  and primary winding P 2 , the LED current passing through the LEDs  23   a  to  23   e  and primary winding P 3 , and the LED current passing through the LEDs  24   a  to  24   e  and primary winding P 4  are balanced (equalized) with one another. 
         [0044]    In this way, the LED lighting apparatus, i.e., the power supply apparatus having the current balancing apparatus according to the embodiment balances (equalizes) currents passing through the primary windings P 1  to P 4 . The reactor L 1  smoothes the LED currents. As results, the LEDs  21   a  to  21   e , LEDs  22   a  to  22   e , LEDs  23   a  to  23   e , and LEDs  24   a  to  24   e  uniformly emit light. 
         [0045]    The embodiment does not employ the sink drivers  12  to  14  of the related art made of constant current drivers, and therefore, the embodiment reduces losses in the balancing circuit and improves efficiency. 
         [0046]    According to the embodiment, the PFM circuit  1  compares a voltage representative of a current detected by the current detector with the reference voltage Vref, to alternately turn on/off the switching elements Q 1  and Q 2  and control voltages supplied to the LEDs  21   a  to  21   e , LEDs  22   a  to  22   e , LEDs  23   a  to  23   e , and LEDs  24   a  to  24   e . Namely, the embodiment does not require the electrolytic capacitor C 27  of the related art having a short service life. The LED lighting apparatus, i.e., the power supply apparatus having the current balancing apparatus according to the embodiment is manufacturable at low cost, is small, and has a long service life. 
         [0047]    The present invention is not limited to the LED lighting apparatus mentioned above. According to the above-mentioned embodiment, the first end of the secondary winding Ns of the transformer T is connected to two groups of series-connected LEDs and the second end of the secondary winding Ns is connected to two groups of series-connected LEDs. The number of groups of series-connected LEDs is optional, for example, one, three, or more, provided that each of the first and second ends of the secondary winding Ns is connected to the same number of groups of series-connected LEDs. 
         [0048]    The present invention is applicable to an LED lighting apparatus to light LEDs serving as, for example, backlights of a liquid crystal display. 
         [0049]    This application claims benefit of priority under 35USC §119 to Japanese Patent Application No. 2009-022415, filed on Feb. 3, 2009, the entire contents of which are incorporated by reference herein. Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the teachings. The scope of the invention is defined with reference to the following claims.