Patent Publication Number: US-2011069514-A1

Title: Dc conversion apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a DC conversion apparatus having a plurality of current resonant converters that are controlled to have a phase difference. 
     2. Description of the Related Art 
     DC conversion apparatuses with current resonant converters have been used as high-efficiency, low-noise power source apparatuses. Generally, the DC conversion apparatus switches power from a DC power source into a switching output through a switching circuit, supplies the switching output to a resonant circuit, provides a resonant output from windings of a transformer in the resonant circuit, and converts the resonant output into a DC output. When large output power is needed, the DC conversion apparatus must be designed in consideration of an increase in the temperature of the transformer and other parts. This results in enlarging the size of the apparatus and increasing the cost thereof. 
     To provide large output power without enlarging the size or increasing the cost, there is a known technique of connecting a plurality of current resonant converters in parallel.  FIG. 1  is a circuit diagram illustrating a DC conversion apparatus according to such a related art. In  FIG. 1 , the DC conversion apparatus employs two current resonant converters connected in parallel. The first current resonant converter has switching elements Q 11  and Q 12 , a resonant reactor L 1 , a transformer T 1 , a resonant capacitor C 1 , diodes D 11  and D 12 , and a smoothing capacitor C. The second current resonant converter has switching elements Q 21  and Q 22 , a resonant reactor L 2 , a transformer T 2 , a resonant capacitor C 2 , diodes D 21  and D 22 , and the smoothing capacitor C. 
     Both ends of a DC power source Vin are connected to a series circuit including the switching elements Q 11  and Q 12  that are MOSFETs. In parallel with this series circuit, a series circuit of the switching elements Q 21  and Q 22  that are MOSFETs is connected. Drains of the switching elements Q 11  and Q 21  are connected to a positive electrode of the DC power source Vin. Sources of the switching elements Q 12  and Q 22  are connected to a negative electrode of the DC power source Vin. 
     The resonant reactor L 1 , a primary winding P 1  of the transformer T 1 , and the resonant capacitor C 1  form a series resonant circuit that is connected between the drain and source of the switching element Q 12 . The resonant reactor L 2 , a primary winding P 2  of the transformer T 2 , and the resonant capacitor C 2  form a series resonant circuit that is connected between the drain and source of the switching element Q 22 . 
     The transformer T 1  has the primary winding P 1  and secondary windings S 11  and S 12 . A first end of the secondary winding S 11  is connected to an anode of the diode D 11 . A second end of the secondary winding S 11  and a first end of the secondary winding S 12  are connected to a first end of the smoothing capacitor C. A second end of the secondary winding S 12  is connected to an anode of the diode D 12 . Cathodes of the diodes D 11  and D 12  are connected to a second end of the smoothing capacitor C. 
     The transformer T 2  has the primary winding P 2  and secondary windings S 21  and S 22 . A first end of the secondary winding S 21  is connected to an anode of the diode D 21 . A second end of the secondary winding S 21  and a first end of the secondary winding S 22  are connected to the first end of the smoothing capacitor C. A second end of the secondary winding S 22  is connected to an anode of the diode D 22 . Cathodes of the diodes D 21  and D 22  are connected to the second end of the smoothing capacitor C. 
     A controller  1  uses an output voltage Vout from the smoothing capacitor C, to output control signals to gates of the switching elements Q 11 , Q 12 , Q 21 , and Q 22  and control the output voltage Vout of the smoothing capacitor C to a constant value. 
     Under the control of the controller  1 , the series-connected switching elements Q 11  and Q 12  alternately turn on to switch power from the DC power source Vin into a switching output and supply the switching output to the series resonant circuit including the primary winding P 1  of the transformer T 1 . 
     The series resonant circuit including the resonant reactor L 1 , primary winding P 1 , and resonant capacitor C 1  passes a sinusoidal current corresponding to a switching frequency of the switching elements Q 11  and Q 12 . At this time, the secondary windings S 11  and S 12  magnetically coupled with the primary winding P 1  induce voltages, which are converted into a direct current by the diodes D 11  and D 12  connected to the secondary windings S 11  and S 12  and the smoothing capacitor C. 
     Under the control of the control circuit  1 , the series-connected switching elements Q 21  and Q 22  alternately turn on to switch power from the DC power source Vin into a switching output and supply the switching output to the series resonant circuit including the primary winding P 2  of the transformer T 2 . 
     A sinusoidal current corresponding to a switching frequency of the switching elements Q 21  and Q 22  passes through the series resonant circuit including the resonant reactor L 2 , primary winding P 2 , and resonant capacitor C 2 . At this time, the secondary windings S 21  and S 22  magnetically coupled with the primary winding P 2  induce voltages, which are converted into a direct current by the diodes D 21  and D 22  connected to the secondary windings S 21  and S 22  and the smoothing capacitor C. 
     Operation of the DC conversion apparatus of  FIG. 1  will briefly be explained. The controller  1  turns on the switching element Q 11 , to pass a current clockwise on the primary side of the transformer T 1  through a path extending along Vin, Q 11 , L 1 , P 1 , C 1 , and Vin, thereby charging the resonant capacitor C 1 . At this time, a voltage induced on the secondary side of the transformer T 1  is provided from the secondary winding S 11 , is rectified and smoothed with the diode D 11  and smoothing capacitor C, and is provided as the output Vout to a load. 
     The controller  1  turns off the switching element Q 11  and on the switching element Q 12 , to discharge the resonant capacitor C 1  through the primary winding P 1 . At this time, a current passes through the primary winding P 1  in an opposite direction to that of charging the resonant capacitor C 1  and induces a voltage on the secondary side of the transformer T 1 . The voltage induced on the secondary side of the transformer T 1  is provided from the secondary winding S 12 , is rectified and smoothed with the diode D 12  and smoothing capacitor C, and is provided as the output Vout to the load. 
     The controller  1  changes an ON period of the switching elements Q 11  and Q 12 , i.e., a charge/discharge period of the resonant capacitor C 1 , thereby controlling an amount of power induced on the secondary side of the transformer T 1 . 
     In this way, the controller  1  controls the switching elements Q 11  and Q 12  and operates the first current resonant converter. The controller  1  also controls the switching elements Q 21  and Q 22 , to operate the second current resonant converter having a phase difference of 90 degrees with respect to the first current resonant converter. The DC conversion apparatus of  FIG. 1  operates the current resonant converters in parallel, to provide large output power. 
     Japanese Unexamined Patent Application Publication No. 2005-33956 (Patent Document 1) discloses a power source apparatus that operates two or more current resonant switching converters in parallel and balances currents to equalize output power borne by the converters. This power source apparatus includes first and second field effect transistors (FETs) connected to a drive circuit arranged on a primary winding side of a transformer, a source of the first FET being connected to a drain of the second FET, a capacitor arranged between a source of the second FET and a first end of the primary winding of the transformer, and a smoothing circuit arranged on a secondary winding side of the transformer, to form a resonant converter block. At least two resonant converter blocks are arranged in parallel and choke coils that form the smoothing circuits are magnetically coupled with each other. 
     This power source apparatus has a simple circuit configuration to operate the two or more current resonant switching converters in parallel and provide large output power without increasing the size and cost of the apparatus. 
     This power source apparatus magnetically couples the choke coils arranged on the secondary side of the resonant converter transformers, to correct an inductance and optimize resonant conditions of the parallel operation of the two or more current resonant switching converters. Consequently, the power source apparatus is capable of adjusting the phase and amplitude of currents, balancing the currents, equalizing the temperatures of parts used in the apparatus, averaging output power from the current resonant switching converters operated in parallel, and equalizing the service life of the parts. 
     SUMMARY OF THE INVENTION 
     The DC conversion apparatus with a plurality of current resonant converters according to the related art stabilizes the output of each current resonant converter by controlling a switching frequency and changing the impedance of the series resonant circuit. Generally, circuit constant values of the resonant capacitor and resonant reactor that form each series resonant circuit slightly differ among the current resonant converters. It is very difficult, therefore, to equalize currents borne by the current resonant converters. If the currents are imbalanced, i.e., if the current resonant converters provide different current values, the efficiency of the apparatus will deteriorate and the transformers and switching elements will generate heat and break. 
     For example, if the resonant reactors L 1  and L 2  in the DC conversion apparatus of the related art illustrated in  FIG. 1  have the same inductance and if the resonant capacitors C 1  and C 2  have the same capacitance, the series resonant circuits in the current resonant converters will have the same impedance. Then, it will relatively be easy to equalize currents borne by the current resonant converters. Equalizing the impedances of the series resonant circuits with each other based on the characteristics of parts, however, needs the measurement and selection of the resonant reactors and resonant capacitors in connection with their circuit constants. This involves a large cost. 
       FIG. 2  is a waveform diagram illustrating currents passing to the resonant capacitors C 1  and C 2  in the DC conversion apparatus of the related art of  FIG. 1  with the series resonant circuits of the current resonant converters involving a constant difference between them. In  FIG. 2 , the constant difference such as L 1 ≠L 2 , or C 1 ≠C 2  causes a difference in resonant conditions between the series resonant circuits of the current resonant converters. As a result, a current peak value of the resonant capacitor C 1  greatly differs from that of the resonant capacitor C 2 . 
     The power source apparatus of the Patent Document 1 magnetically couples the choke coils arranged on the secondary winding side of each resonant converter transformer, to correct an inductance and balance currents. This power source apparatus, however, needs the magnetic circuits whose number is dependent on the number of units to be operated in parallel. Namely, this related art has a problem of increasing cost and circuit size as the number of units to be operated in parallel increases. 
     To solve the problems of the related arts mentioned above, the present invention provides a DC conversion apparatus that is manufacturable at low cost, has a simple configuration, and is capable of properly balancing currents passing through a plurality of current resonant converters that are operated at a predetermined phase difference. 
     According to an aspect of the present invention, the DC conversion apparatus includes (i) a plurality of current resonant converters each including two switching elements connected in series, a transformer having a primary winding and a secondary winding, a series resonant circuit including a resonant reactor, the primary winding of the transformer, and a resonant capacitor, and a rectifying circuit configured to rectify a voltage generated by the secondary winding of the transformer, (ii) a smoothing circuit including a reactor and a smoothing capacitor and arranged after connection points to which output terminals of the rectifying circuits of the plurality of current resonant converters are commonly connected, and (iii) a controller configured to control, according to an output voltage from the smoothing circuit, ON/OFF of the two switching elements of each of the plurality of current resonant converters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a DC conversion apparatus according to a related art; 
         FIG. 2  is a waveform diagram illustrating currents passing through resonant capacitors in current resonant converters of the DC conversion apparatus of  FIG. 1  with the current resonant converters involving a deviation among their circuit constants; 
         FIG. 3  is a circuit diagram illustrating a DC conversion apparatus according to Embodiment 1 of the present invention; 
         FIG. 4  is a diagram illustrating waveforms sent from a controller to gates of switching elements in the DC conversion apparatus of Embodiment 1; 
         FIG. 5  is a waveform diagram illustrating currents passing through resonant capacitors and a ripple current passing through a reactor on the secondary side in the DC conversion apparatus of Embodiment 1; and 
         FIG. 6  is a circuit diagram illustrating a DC conversion apparatus according to a modification of Embodiment 1. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A DC conversion apparatus according to an embodiment of the present invention will be explained in detail with reference to the drawings. 
     Embodiment 1 
       FIG. 3  is a circuit diagram illustrating a DC conversion apparatus according to Embodiment 1 of the present invention. The DC conversion apparatus has two current resonant converters, a smoothing circuit, and a controller  1   a . Although the number of the current resonant converters arranged in the DC conversion apparatus of Embodiment 1 is two, this does not limit the present invention. The DC conversion apparatus according to the present invention may employ any number of current resonant converters. 
     The two current resonant converters in the DC conversion apparatus illustrated in  FIG. 3  have the same circuit configuration. Namely, each of the current resonant converters has two switching elements connected in series, a transformer having primary and secondary windings, a series resonant circuit including a resonant reactor, the primary winding of the transformer, and a resonant capacitor, and a rectifying circuit to rectify a voltage generated by the secondary winding of the transformer. 
     In  FIG. 3 , the first current resonant converter has the two switching elements Q 11  and Q 12  connected in series, the transformer T 1  having the primary winding P 1  and secondary windings S 11  and S 12 , the series resonant circuit including the resonant reactor L 1 , primary winding P 1 , and resonant capacitor C 1 , and the rectifying circuit including diodes D 11  and D 12  to rectify voltages generated by the secondary windings S 11  and S 12 . 
     The secondary windings S 11  and S 12  of the transformer T 1  are connected in series in phase. Voltages generated by the secondary windings S 11  and S 12  are rectified with the diodes D 11  and D 12  and are smoothed with a reactor L 3  and a smoothing capacitor C, to provide an output voltage Vout. 
     The second current resonant converter has the two switching elements Q 21  and Q 22  connected in series, the transformer T 2  having the primary winding P 2  and secondary windings S 21  and S 22 , the series resonant circuit including the resonant reactor L 2 , primary winding P 2 , and resonant capacitor C 2 , and the rectifying circuit including diodes D 21  and D 22  to rectify voltages generated by the secondary windings S 21  and S 22 . 
     The secondary windings S 21  and S 22  of the transformer T 2  are connected in series in phase. Voltages generated by the secondary windings S 21  and S 22  are rectified with the diodes D 21  and D 22  and are smoothed with the reactor L 3  and smoothing capacitor C, to provide the output voltage Vout. 
     In each of the transformers T 1  and T 2 , a voltage on the output side is lower than a voltage on the input side. Namely, in the transformer T 1 , the number of turns of the secondary winding S 11  (S 12 ) is smaller than that of the primary winding P 1 , to carry out a step-down operation. In the transformer T 2 , the number of turns of the secondary winding S 21  (S 22 ) is smaller than that of the primary winding P 2 , to carry out a step-down operation. The transformers T 1  and T 2  have the same turn ratio. 
     The series circuit of the switching elements Q 11  and Q 12  is connected to both ends of a DC power source Vin. The series circuit of the switching elements Q 21  and Q 22  is also connected to the ends of the DC power source Vin. The switching elements Q 11 , Q 12 , Q 21 , Q 22  are, for example, MOSFETs. Drains of the switching elements Q 11  and Q 21  are connected to a positive electrode of the DC power source Vin. Sources of the switching elements Q 12  and Q 22  are connected to a negative electrode of the DC power source Vin. 
     The series resonant circuit of the resonant reactor L 1 , primary winding P 1 , and resonant capacitor C 1  is connected between the drain and source of the switching element Q 12 . The series resonant circuit of the resonant reactor L 2 , primary winding P 2 , and resonant capacitor C 2  is connected between the drain and source of the switching element Q 22 . 
     The smoothing circuit of the DC conversion apparatus according to the present invention is arranged after connection points (a, b) to which output ends of the rectifying circuits of the current resonant converters are commonly connected. The smoothing circuit includes the reactor and smoothing capacitor. Namely, the smoothing circuit is arranged after the connection points as depicted by “a” and “b” to which the output ends of the rectifying circuits of the two current resonant converters are commonly connected. The smoothing circuit includes the reactor L 3  and smoothing capacitor C. The output voltage Vout of the smoothing circuit is provided from ends depicted by “c” and “d” of the smoothing capacitor C. 
     The controller  1   a  controls, based on the voltage Vout from the smoothing circuit, ON/OFF of the two switching elements of each current resonant converter, i.e., the switching elements Q 11  and Q 12  of the first current resonant converter and the switching elements Q 21  and Q 22  of the second current resonant converter. 
     Compared with the DC conversion apparatus of the related art illustrated in  FIG. 1 , Embodiment 1 of  FIG. 3  is characterized in that Embodiment 1 additionally has the reactor L 3 . 
     Operation of Embodiment 1 will be explained.  FIG. 4  is a diagram illustrating waveforms applied from the controller  1   a  to gates of the switching elements Q 11 , Q 12 , Q 21 , and Q 22  in the DC conversion apparatus of Embodiment 1. As illustrated in  FIG. 4 , the controller  1   a  carries out multiphase control to establish a phase difference of 90 degrees between the two current resonant converters. 
     If the number of the current resonant converters is increased, the phase difference is adjusted according to the number of the current resonant converters connected in parallel, i.e., the number of phases. Namely, the controller  1   a  controls ON/OFF of the two switching elements contained in each current resonant converter so that a phase difference of π/n (n is the number of the current resonant converters) is given among the phases of sinusoidal currents passing through the primary windings of the transformers in the current resonant converters. In the case of two phases like Embodiment 1, the controller  1   a  provides a phase difference of 90 degrees between the two current resonant converters. In the case of three phases, the controller  1   a  provides a phase difference of 60 degrees among three current resonant converters, and in the case of four phases, a phase difference of 45 degrees among four current resonant converters. 
     Operation of the controller  1   a  according to Embodiment 1 will be explained. The controller  1   a  controls the switching elements Q 11  and Q 12  so that they alternately turn on/off with the same ON width as illustrated in  FIG. 4 , to pass a sinusoidal current corresponding to a switching frequency through the series resonant circuit of the resonant reactor L 1 , primary winding P 1 , and resonant capacitor C 1 . 
     Also, the controller  1   a  controls the switching elements Q 21  and Q 22  so that they alternately turn on/off with the same ON period at a phase difference of 90 degrees with respect to the first current resonant converter, to pass a sinusoidal current corresponding to a switching frequency through the series resonant circuit of the resonant reactor L 2 , primary winding P 2 , and resonant capacitor C 2 . 
     A resonant time constant of the series resonant circuit of the resonant reactor L 1 , primary winding P 1 , and resonant capacitor C 1  is equalized with that of the series resonant circuit of the resonant reactor L 2 , primary winding P 2 , and resonant capacitor C 2 , so that the sinusoidal current passing through the series resonant circuit including the resonant capacitor C 2  has a phase difference of 90 degrees with respect to the sinusoidal current passing through the series resonant circuit including the resonant capacitor C 1 . 
     The reactor L 3  arranged on the secondary side contributes to adjust resonant conditions between the two series resonant circuits. According to Embodiment 1, the transformers T 1  and T 2  have the same turn ratio and the number of turns on the primary side is larger than that on the secondary side. Accordingly, an apparent impedance observed from the primary side becomes larger than an actual impedance of the reactor L 3  on the secondary side and is equal to a square of the turn ratio (the ratio of the number of turns on the primary side to the number of turns on the secondary side). 
     Even if circuit constant values related to the resonant capacitors and resonant reactors slightly differ between the two current resonant converters, the inductance of the reactor L 3  largely influences on the primary side, to adjust resonant conditions of the series resonant circuits so that the circuit constant difference causes no imbalance in currents. 
     As mentioned above, the reactor L 3  is arranged after the connection points “a” and “b” to which the output ends of the rectifying circuits of the current resonant converters are commonly connected. Accordingly, the reactor L 3  evenly acts on each current resonant converter, to adjust, on the secondary side, resonant conditions. 
     An output voltage of the transformer becomes lower as the turn ratio of the transformer becomes larger, and as the turn ratio becomes larger, a resonant inductance that is usually set on the primary side makes an output impedance on the secondary side smaller. This means that the inductance of the reactor L 3  used to adjust resonant conditions on the secondary side is allowed to be small, such as 1 μH or lower. 
       FIG. 5  is a waveform diagram illustrating currents passing through the resonant capacitors C 1  and C 2  and a ripple current passing through the reactor L 3  on the secondary side in the DC conversion apparatus of Embodiment 1. The current waveforms (2 A/div) of  FIG. 5  are obtained when an input voltage from the DC power source Vin is 400 V and output power is 12V and 40 A. The reactor L 3  adjusts resonant conditions of the series resonant circuits in the current resonant converters and properly balances currents so that the currents passing through the resonant capacitors C 1  and C 2  have a phase difference of 90 degrees and substantially the same peak value as illustrated in  FIG. 5 . 
     The sinusoidal current passing through the primary winding P 1  of the transformer T 1  produces voltages on the secondary windings S 11  and S 12  of the transformer T 1 . These voltages are rectified with the rectifying circuit (diodes D 11  and D 12 ). Similarly, the sinusoidal current through the primary winding P 2  of the transformer T 2  produces voltages on the secondary windings S 21  and S 22  of the transformer T 2 . These voltages are rectified with the rectifying circuit (diodes D 21  and D 22 ). 
     The rectified currents pass through the reactor L 3 . Namely, the currents from the two current resonant converters have a phase difference of 90 degrees and are full-wave-rectified so that, when they join together at the reactor L 3 , they complement each other to reduce a ripple current. The ripple current illustrated in  FIG. 5  passing through the reactor L 3  is 980 mArms with respect to the output of 40 A and is very small. The effect of the interleave operation to reduce the ripple current is demonstrated even when the number of the current resonant converters is increased such as three phases at a phase difference of 60 degrees, four phases at a phase difference of 45 degrees, and the like. 
     The smoothing capacitor C is usually an electrolytic capacitor having a specified allowable ripple current. To satisfy the specified value, several pieces of electrolytic capacitors are usually connected in parallel to form the smoothing capacitor C. The DC conversion apparatus of Embodiment 1 reduces the ripple current, and therefore, is capable of reducing the number of electrolytic capacitors that form the smoothing capacitor C, thereby reducing the cost and size of the apparatus. In addition, the apparatus of Embodiment 1 is capable of elongating the service life of the electrolytic capacitors. 
     The frequency of a current passing through the reactor L 3  is twice as large as the frequency of a current from each current resonant converter. The reactor L 3  of Embodiment 1 is arranged after the connection points to which the output ends of the rectifying circuits on the secondary side of the current resonant converters are commonly connected, and the current resonant converters have a phase difference. Due to these two points, the reactor L 3  passes a current of high frequency. For example, in a case where the number of phases is two, the reactor L 3  passes a current whose frequency is twice as large as that of a current passing through a reactor of a single phase or a reactor arranged in each phase. In a case where the number of phases is three, the reactor L 3  passes a current whose frequency is triple as large as that of a current passing through a reactor of a single phase or a reactor arranged in each phase. Consequently, the apparatus of Embodiment 1 can be miniaturized and can have a low inductance value. 
     As mentioned above, the DC conversion apparatus according to Embodiment 1 has a simple configuration and is manufacturable at low cost. When operating the current resonant converters in parallel at a predetermined phase difference, the DC conversion apparatus of Embodiment 1 is capable of properly balancing currents passing through the current resonant converters. 
     Compared with the DC conversion apparatus of the related art illustrated in  FIG. 1 , the DC conversion apparatus of Embodiment 1 is characterized in that the reactor L 3  is arranged after the connection points to which the output ends of the rectifying circuits arranged in the two current resonant converters are commonly connected, as illustrated in  FIG. 3 . This configuration is simple and manufacturable at low cost. The apparatus of Embodiment 1 is capable of adjusting resonant conditions of the series resonant circuits arranged on the primary side of the current resonant converters, to properly balance currents. 
     The DC conversion apparatus of Embodiment 1 needs no measurement nor selection of resonant reactors and resonant capacitors to adjust resonant conditions of the series resonant circuits, thereby further reducing the cost of the apparatus. The apparatus of Embodiment 1 properly balances currents passed through the current resonant converters, to prevent the transformers and switching elements from generating heat or being broken. 
     Unlike the power source apparatus of the Patent Document 1 that needs the same number of magnetic circuits as the number of units operated in parallel, to increase the cost and size of the apparatus as the number of units operated in parallel increases, the DC conversion apparatus of Embodiment 1 needs only one additional reactor without regard to the number of units operated in parallel (the number of parallel connections), thereby providing a remarkable effect in terms of cost and package size. 
     Embodiment 1 carries out an interleave operation on the current resonant converters at a phase difference of π/n where n is the number of the current resonant converters. In addition, Embodiment 1 arranges the reactor L 3  after the common connection points of the output ends of the current resonant converters, to increase the frequency of a current passing through the reactor L 3 . This means that the inductance value of the reactor L 3  is allowed to be low. 
     The interleave operation carried out by the DC conversion apparatus of Embodiment 1 has an effect of reducing a ripple current in the output of the apparatus. Due to this, the number of electrolytic capacitors used as the smoothing capacitor C can be reduced to decrease the cost and size of the apparatus and elongate the service life of the electrolytic capacitors. 
     The transformer in each current resonant converter of the DC conversion apparatus according to Embodiment 1 is of a step-down type with the number of turns of the secondary winding of the transformer being smaller than that of the primary winding thereof. 
     Accordingly, an apparent impedance observed from the primary side becomes larger than an actual impedance of the reactor L 3  on the secondary side and is equal to a square of the turn ratio (the ratio of the number of turns on the primary side to the number of turns on the secondary side). Even with the small inductance value, the reactor L 3  gives a large influence on the primary side, to adjust resonant conditions of the series resonant circuits and properly balance currents. 
     Modification of Embodiment 1 
       FIG. 6  is a circuit diagram illustrating a DC conversion apparatus according to a modification of Embodiment 1. The modification of  FIG. 6  differs from Embodiment 1 of  FIG. 3  in that the modification additionally has a voltage dividing circuit including voltage dividing capacitors C 10  and C 20 . 
     The voltage dividing circuit has the same number of capacitors as the number of current resonant converters employed by the DC conversion apparatus. The two voltage dividing capacitors C 10  and C 20  are employed to the modification. 
     These capacitors are connected in series, to divide a source voltage from a DC power source Vin and supply DC power to each of the current resonant converters. 
     The voltage dividing capacitor C 10  is connected in parallel with a series circuit of switching elements Q 11  and Q 12 . The voltage dividing capacitor C 20  is connected in parallel with a series circuit of switching elements Q 21  and Q 22 . 
     The DC conversion apparatus of  FIG. 6  differs from the DC conversion apparatus of  FIG. 3  in the supply source of an input voltage to each current resonant converter and achieves the same operation and effect as the apparatus of  FIG. 3 . If parts such as the switching elements of the current resonant converters have a withstand voltage of 400 V and if the DC power source Vin provides an input voltage of 800 V, the modification has an advantage that the voltage dividing capacitors divide the input voltage for use by the parts. Namely, even if the input voltage is higher than the rated capacity of each part of the DC conversion apparatus, the modification of  FIG. 6  can divide the input voltage for use by the parts. The DC conversion apparatus according to the modification is applicable without regard to the magnitude of an input voltage. 
     As mentioned above, the DC conversion apparatus according to the present invention has a simple configuration, is realizable at low cost, and is capable of balancing currents passing through current resonant converters that are operated in parallel at a given phase difference. 
     The DC conversion apparatus according to the present invention is usable as a DC conversion apparatus of a power source circuit in which a plurality of current resonant converters are connected in parallel. 
     This application claims benefit of priority under 35 USC §119 to Japanese Patent Application No. 2009-219079, filed on Sep. 24, 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.