Abstract:
A DC-DC converter includes a transformer, a switching circuit provided on the primary side of the transformer, and a rectifier circuit provided on the secondary side of the transformer. The rectifier circuit includes a first rectifier part that is serially connected body of a first transistor and a second transistor having a first electrode connected to a second electrode of the first transistor. The first and second transistors each include a parasitic diode connected forward between the second and first electrode, and the withstanding voltage between the first and second electrodes of the first transistor is higher than the withstanding voltage between the first and second electrodes of the second transistor.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a DC-DC converter using a transformer, a solar power controller and a mobile body that use the DC-DC converter. 
       BACKGROUND ART 
       [0002]    A DC-DC converter using a transformer is provided with a rectification portion on a secondary side. 
         [0003]    In a two-way DC-DC converter, which is disclosed in a non-patent document 1 and uses a transformer, each of four rectification portions disposed on a secondary side is composed of a single MOS transistor. 
       CITATION LIST 
     Non-Patent Literature 
       [0004]    Non-patent document 1: Florian Krismer, Johann W. Kolar, “Accurate Power Loss Model Derivation of a High-Current Dual Active Bridge Converter for an Automotive Application” , IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 3, MARCH 2010 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    However, in the two-way DC-DC converter disclosed in the non-patent document 1, each of the four rectification portions disposed on the secondary side is composed of a single MOS transistor; accordingly, there is a problem that a large recovery current (reverse recovery current) flows in a parasitic diode of the MOS transistor; a large loss occurs; and electricity transmission efficiency dramatically declines during a low power transmission period. 
         [0006]    In the meantime, to solve the recovery current problem, it is conceivable to compose each rectification portion on the secondary side by using a fast recovery diode. However, in this case, synchronous rectification is impossible; accordingly, there is a problem that the efficiency declines compared with the case where each rectification portion on the secondary side is composed of a MOS transistor. 
         [0007]    In light of the above situation, it is an object of the present invention to provide a highly efficient DC-DC converter, a solar power controller and a mobile body that use the DC-DC converter. 
       Solution to Problem 
       [0008]    A DC-DC converter according to the present invention has a structure (first structure) that comprises: a transformer, a switching circuit disposed on a primary side of the transformer, and a rectification circuit disposed on a secondary side of the transformer, wherein the rectification circuit includes a first rectification portion that is a series-connected body of a first transistor and a second transistor whose first electrode is connected to a second electrode of the first transistor, each of the first and second transistors has a parasitic diode that is connected in a forward direction between the second and first electrodes, and a withstand voltage between the first and second electrodes of the first transistor is higher than a withstand voltage between the first and second electrodes of the second transistor. 
         [0009]    Besides, in the DC-DC converter having the above first structure, a structure (second structure) may be employed, in which the rectification circuit is a rectification bride circuit that includes: a second rectification portion that is a series-connected body of a third transistor and a fourth transistor whose first electrode is connected to a second electrode of the third transistor, a third rectification portion that is a series-connected body of a fifth transistor and a sixth transistor whose first electrode is connected to a second electrode of the fifth transistor, a fourth rectification portion that is a series-connected body of a seventh transistor and an eighth transistor whose first electrode is connected to a second electrode of the seventh transistor, wherein the first electrode of the first transistor is connected to the first electrode of the fifth transistor, the second electrode of the second transistor is connected to the first electrode of the third transistor, the second electrode of the sixth transistor is connected to the first electrode of the seventh transistor, the second electrode of the fourth transistor is connected to the first electrode of the eighth transistor, each of the third, fourth, fifth, sixth, seventh, and eighth transistors has a parasitic diode that is connected in a forward direction between the second and first electrodes, a withstand voltage between the first and second electrodes of the third transistor is higher than a withstand voltage between the first and second electrodes of the fourth transistor, a withstand voltage between the first and second electrodes of the fifth transistor is higher than a withstand voltage between the first and second electrodes of the sixth transistor, and a withstand voltage between the first and second electrodes of the seventh transistor is higher than a withstand voltage between the first and second electrodes of the eighth transistor. 
         [0010]    Besides, in the DC-DC converter having the above first structure, a structure (third structure) may be employed, in which the rectification circuit includes a second rectification portion that is a series-connected body of a third transistor and a fourth transistor whose first electrode is connected to a second electrode of the third transistor, the first electrode of the first transistor is connected to the first electrode of the third transistor, each of the third and fourth transistors has a parasitic diode that is connected in a forward direction between the second and first electrodes, and a withstand voltage between the first and second electrodes of the third transistor is higher than a withstand voltage between the first and second electrodes of the fourth transistor. 
         [0011]    Besides, in the DC-DC converter having any one of the above first to third structures, a structure (fourth structure) may be employed, in which all (2k−1)th (k is a natural number) transistors included in the rectification circuit are transistors of depletion type, and all (2k)th (k is a natural number) transistors included in the rectification circuit are transistors of enhancement type. 
         [0012]    Besides, in the DC-DC converter having any one of the above first to third structures, a structure (fifth structure) may be employed, in which all (2k−1)th (k is a natural number) transistors and all (2k)th (k is a natural number) transistors included in the rectification circuit are transistors of enhancement type. 
         [0013]    Besides, in the DC-DC converter having any one of the above first to fifth structures, a structure (sixth structure) may be employed, in which an output voltage from the DC-DC converter is set in a range of 100 V to 1000 V. 
         [0014]    Besides, a solar power controller according to the present invention has a structure which comprises the DC-DC converter having any one of the above first to sixth structures. 
         [0015]    Besides, a mobile body according to the present invention has a structure which comprises the DC-DC converter having any one of the above first to sixth structures. 
       Advantageous Effects of Invention 
       [0016]    According to the present invention, it is possible to achieve high efficiency of a DC-DC converter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a circuit block diagram showing a structure of a DC-DC converter according to a first embodiment of the present invention. 
           [0018]      FIG. 2  is a time chart showing a measurement result of an electric current output from a secondary winding of a transformer during a low power transmission period of the DC-DC converter according to the first embodiment of the present invention. 
           [0019]      FIG. 3  is a circuit block diagram showing a structure of a DC-DC converter as a comparative example. 
           [0020]      FIG. 4  is a time chart showing a measurement result of an electric current output from a secondary winding of a transformer during a low power transmission period of a DC-DC converter as a comparative example. 
           [0021]      FIG. 5  is a circuit block diagram showing a modification example of the DC-DC converter according to the first embodiment of the present invention. 
           [0022]      FIG. 6  is a circuit block diagram showing a structure of a DC-DC converter according to a second embodiment of the present invention. 
           [0023]      FIG. 7  is a circuit block diagram showing a structure of a DC-DC converter according to a third embodiment of the present invention. 
           [0024]      FIG. 8  is a view showing a schematic structure of a mobile body according to a fourth embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0025]    A DC-DC converter according to a first embodiment of the present invention includes, as shown in  FIG. 1 , a primary side transformer drive circuit  2 , a transformer T 1 , a smoothing reactor L 3 , N-channel MOS transistors Q 1  to Q 8 , capacitors C 1  and C 2 , diodes D 1  and D 2 , a gate power source  3 , and a driver  4 , applies DC-DC conversion to a d.c. voltage output from a d.c. power source  1 , and supplies a d.c. voltage (output voltage) after the DC-DC conversion to a load  5 . 
         [0026]    The output voltage from the DC-DC converter according to the first embodiment of the present invention is not especially limited, but in a case where the output voltage is large, an effect by high efficiency becomes remarkable compared with the two-way DC-DC converter disclosed in the non-patent document 1; accordingly, it is desirable that the output voltage from the DC-DC converter according to the first embodiment of the present invention is set in a range of 100 V to 1000 V, for example. 
         [0027]    The primary side transformer drive circuit  2  is a switching circuit which has a switching element, converts the d.c. power output from the d.c. power source  1  into a transformer drive voltage by switching of the switching element, and supplies the transformer drive voltage to a primary winding L 1  of the transformer T 1 . In the primary side transformer drive circuit  2 , it is possible to use a circuit of, for example, full bridge type, half bridge type, push-pull type, forward type, fly back type or the like. 
         [0028]    An electric current occurring in a secondary winding L 2  of the transformer T 1  is smoothed by the smoothing reactor L 3 , rectified by a rectification bridge circuit that includes the transistors Q 1  to Q 8 , thereafter, supplied to the load  5 . 
         [0029]    Each of the transistors Q 1  to Q 8  incorporates a parasitic diode. An anode of the parasitic diode is connected to a source of one of the transistors Q 1  to Q 8  corresponding to the parasitic diode, and a cathode of the parasitic diode is connected to a drain of the one of the transistors Q 1  to Q 8  corresponding to the parasitic diode. 
         [0030]    Each of the transistors Q 1 , Q 3 , Q 5 , and Q 7  is a high withstand voltage transistor whose on-resistance is, for example, 0.099 Ω, and whose source-drain withstand voltage is 600 V, for example. Each of the transistors Q 2 , Q 4 , Q 6 , and Q 8  is a low withstand voltage transistor whose on-resistance is, for example, 0.079 Ω, and whose source-drain withstand voltage is 30 V, for example. 
         [0031]    Generally, a recovery current of a low withstand voltage transistor is smaller than a recovery current of a high withstand voltage transistor. Because of this, in the present embodiment, the recovery currents of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8  are set to be smaller than the recovery currents of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7 . 
         [0032]    Both drains of the transistors Q 1 , Q 5  are connected to one end of the load  5 . Drains of the transistors Q 2 , Q 6  are connected respectively to sources of the transistors Q 1 , Q 5 ; a source of the transistor Q 2  is connected to a drain of the transistor Q 3  and one end of the secondary winding L 2  of the transformer T 1 ; and a source of the transistor Q 6  is connected to a drain of the transistor Q 7  and the other end of the secondary winding L 2  of the transformer T 1  via the smoothing reactor L 3 . Drains of the transistors Q 4 , Q 8  are connected respectively to sources of the transistors Q 3 , Q 7 ; and both sources of the transistors Q 4 , Q 8  are connected to a line of a ground voltage GND. 
         [0033]    Cathodes of the diodes D 1 , D 2  are connected respectively to gates of the transistors Q 1 , Q 5 . A first output terminal  31  of the gate power source  3  is connected to anodes of the diodes D 1 , D 2  and the driver  4 . A second output terminal  32  of the gate power source  3  is connected to gates of the transistors Q 3 , Q 7 . The gate power source  3  outputs, from a first output node, a d.c. voltage (e.g., 0.2 to 50 V) that is higher than a sum voltage of a threshold value voltage VTH of each of the high withstand voltage transistors Q 1 , Q 5  (e.g., transistors of enhancement type whose threshold value voltage VTH=0.1 to 7 V) and a forward drop voltage of the diodes D 1 , D 2 , and outputs, from a second output node, a d.c. voltage (e.g., 0.2 to 50 V) that is higher than a threshold value voltage VTH of each of the high withstand voltage transistors Q 3 , Q 7  (e.g., transistors of enhancement type whose threshold value voltage VTH=0.1 to 7 V). 
         [0034]    The capacitor C 1  is connected between the gate of the transistor Q 1  and the one end of the secondary winding L 2  of the transformer T 1 . The capacitor C 2  is connected between the gate of the transistor Q 5  and the smoothing reactor L 3 . In the meantime, the capacitor C 1  is disposed to apply, to the gate of the transistor Q 1 , a voltage which is a sum of a voltage from the one end of the secondary winding L 2  of the transformer T 1  and a voltage from the first output terminal  31  of the gate power source  3 . The capacitor C 2  is disposed to apply, to the gate of the transistor Q 5 , a voltage which is a sum of a voltage from an end portion of the smoothing reactor L 3  not-connected to the secondary winding L 2  and the voltage from the first output terminal  31  of the gate power source  3 . For example, when the one end of the secondary winding L 2  of the transformer T 1  is at the ground potential, the output potential from the gate power source  3  is applied to the gate of the high withstand voltage transistor Q 1  via the diode D 1 . Thereafter, even if the potential of the one end of the secondary winding L 2  of the transformer T 1  rises, a potential difference between the one end of the secondary winding L 2  of the transformer T 1  and the gate of the high withstand voltage transistor Q 1  is kept because of the capacitance coupling of the capacitors C 1 , C 2 . Likewise, also a potential difference between the end portion of the smoothing reactor L 3  not-connected to the secondary winding L 2  and the gate of the high withstand voltage transistor Q 5  is kept at the output voltage from the gate power source  3 . 
         [0035]    The gates of the transistors Q 1 , Q 2 , Q 4 , Q 5 , Q 6 , and Q 8 , the one end of the secondary winding L 2  of the transformer T 1 , and the end portion of the smoothing reactor L 3  not-connected to the secondary winding L 2  are connected to the driver  4 . The driver  4  controls gate voltages of the transistors Q 2 , Q 4 , Q 6 , and Q 8  to perform on/off control of the transistors Q 2 , Q 4 , Q 6 , and Q 8 . 
         [0036]    Next, operation of the DC-DC converter according to the first embodiment of the present invention is described. In the case where the DC-DC converter according to the first embodiment of the present invention supplies d.c. power to the load  5 , first the gate power source  3  goes to an on-state; a d.c. voltage, which is higher than the sum voltage of the threshold value voltage VTH of each of the high withstand voltage transistors Q 1 , Q 5  and the forward drop voltage of the diodes D 1 ,  2 D, is applied to the gate of each of the high withstand voltage transistors Q 1 , Q 5 ; and a d.c. voltage, which is higher than the threshold value voltage VTH of each of the high withstand voltage transistors Q 3 , Q 7 , is applied to the gate of each of the high withstand voltage transistors Q 3 , Q 7 . 
         [0037]    In this state, in a case where an electric current is output, in an arrow direction of  FIG. 1 , from the secondary winding L 2  of the transformer T 1  because of the transformer drive voltage from the primary side transformer drive circuit  2 , first the low withstand voltage transistors Q 2 , Q 8  are turned on by the driver  4 . In this way, also the high withstand voltage transistors Q 1 , Q 7  are turned on and the electric current is supplied to the load  5 . 
         [0038]    Next, the low withstand voltage transistors Q 2 , Q 8  are turned off by the driver  4  at a timing (timing when the absolute value of the electric current in the arrow direction of  FIG. 1  flowing from the secondary winding L 2  of the transformer T 1  decreases to reach a predetermined value near zero) when a predetermined time elapses after the electric current in the arrow direction of  FIG. 1  begins to flow from the secondary winding L 2  of the transformer T 1  because of the transformer drive voltage from the primary side transformer drive circuit  2 . At this time, if a reflux current flows in the DC-DC converter according to the first embodiment of the present invention, the electric current flows back in an electric-current route that includes: channel portions of the high withstand voltage transistors Q 1 , Q 7  kept at the on-state; and parasitic diodes of the low withstand voltage transistors Q 2 , Q 8  in the off-state. In this way, the recovery currents flow in the low withstand voltage transistors Q 2 , Q 8 . However, the recovery currents of the low withstand voltage transistors Q 2 , Q 8  are smaller than the recovery currents of the high withstand voltage transistors Q 1 , Q 7 ; accordingly, recovery characteristics of the low withstand voltage transistors Q 2 , Q 8  are good. Besides, the reflux current flows in the channel portions of the high withstand voltage transistors Q 1 , Q 7  kept at the on-state; accordingly, also recovery characteristics of the high withstand voltage transistors Q 1 , Q 7  are good. Therefore, recovery characteristics of each of: a rectification portion including the high withstand voltage transistor Q 1  and the low withstand voltage transistor Q 2 ; and a rectification portion including the high withstand voltage transistor Q 7  and the low withstand voltage transistor Q 8  become good. 
         [0039]    Next, the low withstand voltage transistors Q 4 , Q 6  are turned on at a timing when the reflux current disappears. In this way, if the high withstand voltage transistors Q 3 , Q 5  also are turned on and an electric current in a direction opposite to the arrow direction of  FIG. 1  begins to be output from the secondary winding L 2  of the transformer T 1  because of the transformer drive voltage from the primary side transformer drive circuit  2 , an electric current is supplied to the load  5 . 
         [0040]    Next, the low withstand voltage transistors Q 4 , Q 6  are turned off by the driver  4  at a timing (timing when the absolute value of the electric current in the direction opposite to the arrow direction of  FIG. 1  flowing from the secondary winding L 2  of the transformer T 1  decreases to reach a predetermined value near zero) when a predetermined time elapses after the electric current in the direction opposite to the arrow direction of  FIG. 1  begins to flow from the secondary winding L 2  of the transformer T 1  because of the transformer drive voltage from the primary side transformer drive circuit  2 . At this time, if a reflux current flows in the DC-DC converter according to the first embodiment of the present invention, the electric current flows back in an electric-current route that includes: channel portions of the high withstand voltage transistors Q 3 , Q 5  kept at the on-state; and parasitic diodes of the low withstand voltage transistors Q 4 , Q 6  in the off-state. In this way, the recovery currents flow in the low withstand voltage transistors Q 4 , Q 6 . However, the recovery currents of the low withstand voltage transistors Q 4 , Q 6  are smaller than the recovery currents of the high withstand voltage transistors Q 3 , Q 5 ; accordingly, recovery characteristics of the low withstand voltage transistors Q 4 , Q 6  are good. Besides, the reflux current flows in the channel portions of the high withstand voltage transistors Q 3 , Q 5  kept at the on-state; accordingly, also recovery characteristics of the high withstand voltage transistors Q 3 , Q 5  are good. Therefore, recovery characteristics of each of: a rectification portion including the high withstand voltage transistor Q 3  and the low withstand voltage transistor Q 4 ; and a rectification portion including the high withstand voltage transistor Q 5  and the low withstand voltage transistor Q 6  become good. 
         [0041]    Hereinafter, likewise, the d.c. power is supplied to the load  5 . 
         [0042]    In a case where the DC-DC converter according to the first embodiment of the present invention stops the supply of the d.c. power to the load  5 , the gate power source  3  goes to an off-state, the gates of the transistors Q 1 , Q 3 , Q 5 , and Q 7  are brought to a “L” level, and the transistors Q 1 , Q 3 , Q 5 , and Q 7  are fixed at an off-state. Besides, also the transistors Q 2 , Q 4 , Q 6 , and Q 8  are fixed at the off-state by the driver  4 . In the meantime, one capacitor may be connected between the gates of the transistors Q 3 , Q 4 , and another capacitor may be connected between the gates of the transistors Q 7 , Q 8 . Besides, a cathode and an anode of one diode may be connected to the gates of the transistors Q 3 , Q 4 , respectively, and also a cathode and an anode of another diode may be connected to the gates of the transistors Q 7 , Q 8 , respectively. 
         [0043]      FIG. 2  shows a measurement result of the electric current output from the secondary winding L 2  of the transformer T 1  during a low power transmission period when the DC-DC converter according to the first embodiment of the present invention supplies low power (output voltage 250 V, output current 0.1 A) to the load  5 . Besides, as a comparative example,  FIG. 4  shows a measurement result of the electric current output from the secondary winding L 2  of the transformer T 1  during a low power transmission period when a DC-DC converter, in which each of four rectification portions is disposed on the secondary side, composed of a single high withstand voltage transistor whose on-resistance is, for example, 0.099 Ω and whose source-drain withstand voltage is, for example, 600 V and shown in  FIG. 3 , supplies low power (output voltage 250 V, output current 0.1 A) to the load  5 . 
         [0044]    As understood from comparison of  FIG. 2  and  FIG. 4 , in the comparative example, the recovery current is large; accordingly, a large current flows in the opposite direction and the power loss becomes large, while almost no recovery current flows in the DC-DC converter according to the first embodiment of the present invention; accordingly, there is almost no backward current and the power loss is small. As a result of this, in the low power transmission of the output voltage 250 V and output current 0.1 A, the power transmission efficiency is 52% in the comparative example, while the power transmission efficiency increases to 89% in the DC-DC converter according to the first embodiment of the present invention. 
         [0045]    In the meantime, it is desirable that source-drain withstand voltages of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8  are in a range of 3 to 200 V. If the source-drain withstand voltages of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8  exceed 200 V, the recovery currents in the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8  increase. Besides, in a case where the source-drain withstand voltages of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8  are under 3 V, durability of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8  to noise in the power source circuit declines. 
         [0046]    Besides, it is desirable that source-drain withstand voltages of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7  are in a range of 3 times or higher to 100 times or less than the source-drain withstand voltages of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8 . In a case where the source-drain withstand voltages of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7  are smaller than 3 times the source-drain withstand voltages of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8 , difference between the recovery current of the high withstand voltage transistor and the recovery current of the low withstand voltage transistor becomes small, and the effect of the present embodiment becomes small. Besides, in a case where the source-drain withstand voltages of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7  are larger than 100 times the source-drain withstand voltages of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8 , the durability of the low withstand voltage transistor to switching noise declines. 
         [0047]    In the present embodiment, voltages, which are higher than sum voltages obtained by adding the threshold value voltages of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7  to the source potentials of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8 , are applied to the gates of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7 , respectively. Because of this, it is possible to nearly nullify voltage difference between the source and drain of each of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7 . Accordingly, it is possible to fully show the effect of reducing the recovery currents of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7 . 
         [0048]    Besides, in the present embodiment, the capacitor C 1  is connected between the gate of the high withstand voltage transistor Q 1  and the one end of the secondary winding L 2  of the transformer T 1 , and the capacitor C 2  is connected between the gate of the high withstand voltage transistor Q 5  and the end portion of the smoothing reactor L 3  not-connected to the secondary winding L 2 . Further, the cathodes of the diodes D 1 , D 2  are connected to the gates of the high withstand voltage transistors Q 1 , Q 5 , respectively. A structure is employed, in which voltages, which are higher than the threshold value voltages of the high withstand voltage transistors Q 1 , Q 5 , are applied to the anodes of the diodes D 1 , D 2 , respectively. In this way, even if a voltage occurring in the secondary winding L 2  of the transformer T 1  changes during the period of supplying the d.c. power to the load  5 , it is possible to continue to give a voltage enough to turn on the high withstand voltage transistors Q 1 , Q 5  to the gates of the high withstand voltage transistors Q 1 , Q 5  because of the capacitance coupling of the capacitors C 1 , C 2 ; accordingly, it is possible to surely reduce the recovery currents of the high withstand voltage transistors Q 1 , Q 5 . 
         [0049]    Besides, in a case where the power supply to the load  5  is not performed, the gate potentials of the high withstand voltage transistors Q 1 , Q 5  stabilize at potentials that are respectively near a potential of the one end of the secondary winding L 2  of the transformer T 1  and a potential of the end portion of the smoothing reactor L 3  not-connected to the secondary winding L 2  because of the capacitance coupling of the capacitors C 1 , C 2 . Because of this, it is possible to prevent the high withstand voltage transistors Q 1 , Q 5  from being turned on unsuitably by surge or the like; accordingly, it is possible to raise safety. 
         [0050]    In the present embodiment , it is possible to switch the on/off of each element of the rectification bridge circuit; accordingly, it is also possible to use the DC-DC converter as a two-way DC-DC converter. 
         [0051]    In the present embodiment, the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7  are transistors of enhancement type. However, the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7  may be changed to transistors of depletion type. In the case where the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7  are changed to transistors of depletion type, as shown in  FIG. 5 , the gates of the high withstand voltage transistors Q 1 , Q 3 , Q 5 , and Q 7  are connected to the sources of the low withstand voltage transistors Q 2 , Q 4 , Q 6 , and Q 8 , respectively. 
         [0052]    Besides, in the present embodiment, the electric current is flowed in the parasitic diodes of the low withstand voltage transistors Q 4 , Q 6  (or Q 2 , Q 8 ) by normally keeping the low withstand voltage transistors Q 4 , Q 6  (or Q 2 , Q 8 ), which are prohibited from being switched, at the off-state, which is however not limiting, but synchronous rectification may be performed. In the synchronous rectification, when an electric current begins to flow in the parasitic diodes of the low withstand voltage transistors Q 4 , Q 6  (or Q 2 , Q 8 ), the low withstand voltage transistors Q 4 , Q 6  (or Q 2 , Q 8 ) are turned on, and the low withstand voltage transistors Q 4 , Q 6  (or Q 2 , Q 8 ) are turned off immediately before the low withstand voltage transistors Q 2 , Q 8  (or Q 4 , Q 6 ) under switching are turned on, namely, immediately before the electric current stops flowing in the low withstand voltage transistors Q 4 , Q 6  (or Q 2 , Q 8 ). In this way, it is possible to further reduce the power loss. 
       Second Embodiment 
       [0053]      FIG. 6  shows a structure of a DC-DC converter according to a second embodiment of the present invention. In the meantime, in  FIG. 6 , the same components as  FIG. 1  are indicated by the same reference numbers and detailed description of them is skipped. 
         [0054]    The DC-DC converter according to the second embodiment of the present invention is different from the DC-DC converter according to the first embodiment of the present invention in that the rectification circuit disposed on the secondary side of the transformer T 1  is composed of only the high withstand voltage transistors Q 1 , Q 3  and the low withstand voltage transistors Q 2 , Q 4  unlike the rectification bride circuit of full bridge type of the DC-DC converter according to the first embodiment of the present invention. 
         [0055]    Also in the DC-DC converter according to the second embodiment of the present invention, like in the DC-DC converter according to the first embodiment of the present invention, almost no recovery current flows; accordingly, there is almost no backward current and the power loss is small. As a result of this, it is possible to raise the power transmission efficiency also in the low power transmission. 
       Third Embodiment 
       [0056]      FIG. 7  shows a structure of a DC-DC converter according to a third embodiment of the present invention. In the meantime, in  FIG. 7 , the same components as  FIG. 1  are indicated by the same reference numbers and detailed description of them is skipped. 
         [0057]    The DC-DC converter according to the third embodiment of the present invention is different from the DC-DC converter according to the first embodiment of the present invention in that the rectification circuit disposed in the secondary side of the transformer T 1  is composed of only the high withstand voltage transistors Q 1  and the low withstand voltage transistors Q 2  unlike the rectification bride circuit of full bridge type of the DC-DC converter according to the first embodiment of the present invention. 
         [0058]    Also in the DC-DC converter according to the third embodiment of the present invention, like in the DC-DC converter according to the first embodiment of the present invention, almost no recovery current flows; accordingly, there is almost no backward current and the power loss is small. As a result of this, it is possible to raise the power transmission efficiency also in the low power transmission. 
       Fourth Embodiment 
       [0059]      FIG. 8  is a view showing a schematic structure of a mobile body according to a fourth embodiment of the present embodiment. In the meantime, in  FIG. 8 , a connection line connected to a ground potential is not shown. The mobile body shown in  FIG. 8  is, for example, an electric car, an electric bike or the like, and comprises: a solar power controller that includes a solar panel  11 , an MPPT (Maximum Power Point Tracking)  12 , a battery management portion  13 , a sub-battery  14 , a two-way DC-DC converter  15 , a control circuit  16 , a battery management portion  17 , and a main battery  18 ; an inverter  19 ; and a motor  20 . 
         [0060]    The solar panel  11  arranges therein a plurality of solar battery cells in a panel shape and is disposed on a roof of an electric car, for example. 
         [0061]    The MPPT  12  is a DC-DC converter that controls an operation point of the solar panel  11  to maximize power generation of the solar panel  11 . An output end of the solar panel  11  is connected to an input end of the MPPT  12 , and an output end of the MPPT  12  is connected to the sub-battery  14 . 
         [0062]    The battery management portion  13  manages the sub-battery  14  to control charge and discharge of the sub-battery  14 . 
         [0063]    The battery management portion  17  manages the main battery  18  to control charge and discharge of the main battery  18 . 
         [0064]    In the present embodiment, a voltage of the main battery  18  is larger than a voltage of the sub-battery  14 . For example, by setting a voltage range of the main battery  18  at 100 to 600 V and setting a voltage range of the sub-battery at 10 to 48 V, the voltage range of the main battery  18  becomes a range suitable for driving the motor  20 , and the voltage range of the sub-battery  14  becomes a range suitable for charging the power generated at the solar panel  11 . 
         [0065]    The two-way DC-DC converter  15  is, for example, the DC-DC converter according to any one of the above first to third embodiments, and transmits power between the sub-battery  14  and the main battery  18 . A first input/output terminal  21  of the two-way DC-DC converter  15  is connected to the sub-battery  14  via the battery management portion  13 , and a second input/output terminal  22  of the two-way DC-DC converter  15  is connected to the main battery  18  via the battery management portion  17 . 
         [0066]    The control portion  16  controls power transmission (output voltage or output current) of the two-way DC-DC converter  15 . 
         [0067]    The inverter  19  converts a d.c. voltage output from the main battery  18  into a motor drive a.c. voltage. The motor  20  is driven to rotate by the motor drive a.c. voltage output from the inverter  19 . A drive wheel of the mobile body rotates because of the rotation of the motor  20 . Regenerative energy occurring in the motor  20  during a damping period of the mobile body is recovered by the battery management portion  17  and stored into the main battery  18 . Besides, a d.c. voltage output from the sub-battery  14  is used as a power source for electrical components such as a headlight and the like. 
       Others 
       [0068]    It should be considered that the embodiments disclosed this time are examples in all respects and are not limiting. It is intended that the scope of the present invention is not indicated by the above description but by the claims, and all modifications within the scope of the claims and the meaning equivalent to the claims are covered. 
       REFERENCE SIGNS LIST 
       [0069]      1  d.c. power source 
         [0070]      2  primary side transformer drive circuit 
         [0071]      3  gate power source 
         [0072]      4  driver 
         [0073]      5  load 
         [0074]      11  solar panel 
         [0075]      12  MPPT 
         [0076]      13  battery management portion 
         [0077]      14  sun-battery 
         [0078]      15  two-way DC-DC converter 
         [0079]      16  control circuit 
         [0080]      17  battery management portion 
         [0081]      18  main battery 
         [0082]      19  inverter 
         [0083]      20  motor 
         [0084]      21  first input/output terminal 
         [0085]      22  second input/output terminal 
         [0086]      31  first output terminal 
         [0087]      32  second output terminal 
         [0088]    C 1 , C 2  capacitors 
         [0089]    D 1 , D 2  diodes 
         [0090]    L 1  primary winding 
         [0091]    L 2  secondary winding 
         [0092]    L 3  smoothing reactor 
         [0093]    Q 1 , Q 3 , Q 5 , Q 7  high withstand voltage transistors (N-channel MOS transistors) 
         [0094]    Q 2 , Q 4 , Q 6 , Q 8  low withstand voltage transistors (N-channel MOS transistors) 
         [0095]    T 1  transformer