Abstract:
Disclosed is a DC-DC converter for suitably converting the voltage of a solar panel into a desired output voltage to be supplied to various observation equipment installed in an artificial satellite. The DC-DC converter  10  comprises an input coil L 1,  a first intermediate capacitor C 1  and a first intermediate coil Lm 1  connected in series between positive and negative terminals of an input voltage source E, a switch S and a diode D having their one ends connected to a node a of the input coil L 1  and the first intermediate capacitor C 1,  a second intermediate coil Lm 2  connected between the other end (node d) of the switch S and the negative terminal of the input voltage source E, a second intermediate capacitor C 2  connected between the other end (node c of the diode D and the node d and a load R connected to the node c through an output coil L 2.

Description:
INCORPORATION BY REFERENCE 
       [0001]    This invention is based upon and claims the benefit of priority from Japanese patent application no. 2007-259402, filed Oct. 3, 2007, the disclosure of which is incorporated herein in its entirety by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a DC-DC converter, more specifically to a switching type DC-DC converter particularly suited for supplying operation power for various observation equipment installed on an artificial satellite by stepping-up the output voltage of a solar panel to a desired voltage required by those equipment as a load. 
       BACKGROUND OF THE INVENTION 
       [0003]    Artificial satellites, space crafts, particularly those for exploring planets that are in the orbit largely changing their distances from the sun generally use solar panels that utilize solar power as the power supply for driving various observation and control equipment and machines that are installed in such satellites. The use of solar panels enables to relatively constantly supply electrical power for an extended time in the space. 
         [0004]    The output voltage that is acquired from such solar panel is insufficient or impossible to stably provide a desired voltage required for properly operating various observation equipment that are mentioned hereinabove. Particularly, in case of planet exploration space crafts having large changes in distance from the sun, it is normal to use a switching type DC-DC converter including one or more switching device for converting the output voltage from the solar panel into a desired voltage. Moreover, in case of planet exploration space crafts for observing planets&#39; electric and/or magnetic field, DC-DC converters to be used in such satellites are absolutely required not only to output a stable voltage but also to be a low noise in which the generated noise level is quite low. 
         [0005]      FIG. 9  shows a general example of a conventional DC-DC converter (Boost converter). As shown in  FIG. 9(   a ), the DC-DC converter  90  comprises a coil (inductor) L and a switching device including, for example, a transistor or the like (simply referred to as a switch below) S connected in series between both ends of an input voltage source Vi and a parallel circuit of a load resistor R and a smoothing capacitor C connected across both ends of the switch S through a diode (rectifying device) D. 
         [0006]    In  FIG. 9 , (b) is a transfer function of the DC-DC converter  90  as shown in (a). (c) shows ripple currents flowing through the inductor L and the diode D. (d) is a ripple voltage (or ripple potential) on the node a of the coil L and the diode D. And (e) is a voltage on the coil L. 
         [0007]    In the conventional DC-DC converter  90  as described hereinabove, ripple currents as shown in  FIG. 9(   c ) flow in response to ON/OFF operation of the switch S. That is, the ripple current flowing through the coil L is generally triangular. The amplitude of the triangular ripple current is proportional to the ON time of the switch S. And the energy stored in the coil L during the ON time of the switch S is supplied to the load R through the diode D during the OFF time of the switch S. In other words, it is possible to supply a desired voltage to load R by controlling the ON time of the switch S in response to the voltage Vi of the input voltage source. 
         [0008]    Unfortunately, although such general DC-DC converter  90  is able to stably provides a desired output voltage from a fluctuating input voltage source, the ripple current in the coil L unavoidably accompanies with large noise at the switching frequency of the switch S as well as harmonic frequencies in the multiple times of the switching frequency. Such triangular ripple current provides a relatively low noise level as compared with, for example, a rectangular pulses but is not acceptable as a power supply for planet exploring space crafts in which highly sensitive observation equipment for observing very weak electric and/or magnetic field are installed. 
         [0009]    Conventional DC-DC converters such as those described hereinabove have the following problems or drawbacks. That is, in the conventional steep-up type DC-DC converter (Boost converter)  90  as shown in  FIG. 9 , the input current is a triangle wave, while the output current is a pulse wave, thereby exhibiting a large output noise. The fact that the output current is a pulse wave means a large current change in time at the switching frequency, thereby making it impossible or very difficult to apply such converter to a power supply for aforementioned sensitive observation equipment. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention was made in consideration of the above circumstances and it is a primary object of the present invention to provide a DC-DC converter that is simple in circuit construction and capable of supplying a desired step-up output voltage that is non-inverted or has the same polarity as the input voltage. 
         [0011]    In order to achieve the above objectives, the DC-DC converter according to the present invention employs the following unique construction. That is, the DC-DC converter includes a series connection of an input inductance and a switching device connected between positive and negative terminals of an input voltage source and a load connected to the node of the input inductor and the switching device through a rectifying device for converting and outputting to the load a desired voltage of the same polarity as the input voltage source, further comprising: 
         [0012]    a first intermediate inductor connected between the output side of the input inductor and the negative terminal of the input voltage source; and 
         [0013]    a second intermediate inductor connected between the output side of the switching device and the negative terminal of the input voltage source; 
         [0014]    wherein the input inductor and the output inductor are magnetically coupled to the first intermediate inductor and the second intermediate inductor. 
         [0015]    Also, the DC-DC converter according to the present invention employs the following unique construction. That is, the DC-DC converter includes a series circuit of an input inductor and a switching device connected between positive and negative terminals of an input voltage source and a load connected to the node of the switching device and the input inductor through a rectifying device for converting and outputting a desired step-up voltage to the load a desired step-up voltage of the same polarity as the input voltage source, further comprising: 
         [0016]    a series connection of a first intermediate inductor and a first intermediate capacitor connected between the node of the input inductor and the rectifying device and the negative terminal of the input voltage source; 
         [0017]    an output inductor connected between the rectifying device and the load; 
         [0018]    a second intermediate inductor connected between the output end of the switching device and the negative terminal of the input voltage source; and 
         [0019]    a second capacitor connected between the node of the second intermediate inductor and the switching device and the node of the rectifying device and the output inductor. 
         [0020]    The DC-DC converter according to the present invention exhibits the following unique advantages. That is, the construction is simple because mutually magnetically coupled input and output inductors as well as the intermediate inductors are only added to the conventional DC-DC converter. And it exhibits low noise by significantly reducing or essentially eliminating ripple currents in the input and output inductors. Moreover, since the input, output and intermediate inductors are magnetically coupled to equalize terminal voltages thereacross, a single transformer may be used to configure these inductors by winding coils on a common magnetic core. As a result, it finds particularly preferable applications as a power supply for planet exploration space crafts that absolutely require a stabilized low noise output voltage from a fluctuating input voltage source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The above and other objects and advantages of the present invention will be best understood by reading the following descriptions made with reference to the accompanying drawings, wherein: 
           [0022]      FIG. 1  shows a preferred embodiment of the present invention, wherein (a) is a circuit schematic, (b) is the transfer function, (c 1 ˜c 4 ) show ripple currents in different circuit portions under different conditions, (d) shows ripple voltages and (e) shows voltages across coils; 
           [0023]      FIG. 2  shows circuit schematics for describing operations of the DC-DC converter as shown in  FIG. 1(   a ), wherein (a) is the entire circuit schematic, (b) shows potentials and currents in and on various circuit portions when the switch S is ON and (c) shows potentials and currents in and on various circuit portions when the switch S is OFF; 
           [0024]      FIG. 3  is illustrations for describing how to reduce ripple currents in the DC-DC converter according to the present invention; 
           [0025]      FIG. 4  is an exemplified circuit schematic of the DC-DC converter apparatus using the DC-DC converter according to the present invention; 
           [0026]      FIG. 5  illustrates voltage waveforms (a)˜(d) and current waveforms (e)˜(h) in various coils of the DC-DC converter according to the present invention when coils are not magnetic coupled; 
           [0027]      FIG. 6  illustrates voltage waveforms (a)˜(d) and current waveforms (e)˜(h) in various coils of the DC-DC converter according to the present invention when the coils are magnetically coupled; 
           [0028]      FIG. 7  is a circuit schematic of another embodiment of the DC-DC converter apparatus using the DC-Dc converter according to the present invention; 
           [0029]      FIG. 8  is a circuit schematic of still another embodiment of the DC-DC converter apparatus using the DC-DC converter according to the present invention; and 
           [0030]      FIG. 9  is a circuit schematic of a conventional step-up type DC-DC converter. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0031]    Now, the construction and operation of a preferred embodiment of the DC-DC converter according to the present invention will be described in greater detail with reference to the accompanying drawings. 
         [0032]    Firstly, a reference will be made on  FIG. 1  for describing the preferred embodiment of the DC-DC converter according to the present invention.  FIG. 1(   a ) is a circuit schematic to illustrate the construction of the DC-DC converter.  FIG. 1(   b ) is the transfer function.  FIG. 1(   c   1 )˜( c   4 ) shows current waveforms through a plurality of coils (inductors) constituting the DC-DC converter.  FIG. 1(   d ) shows ripple potential waveforms on various portions of the circuit as shown in  FIG. 1(   a ). Finally,  FIG. 1(   e ) shows voltages across various coils. 
         [0033]    Now, the construction of the preferred embodiment of the DC-DC converter  10  according to the present invention will be described with reference to  FIG. 1(   a ). The DC-DC converter  10  comprises an input voltage source E such as, for example, a solar panels, an input coil (inductor) L 1 , a first intermediate capacitor C 1  and a first intermediate coil Lm 1  connected in series between positive and negative terminals of the input voltage source E, a switch (switching device) S and a second intermediate coil Lm 2  connected in series between a node a of the input coil L 1  and the first intermediate capacitor C 1  and the negative terminal (−) of the input voltage source E, a second intermediate capacitor C 2 , an output coil L 2  and a load (parallel connection of a resistor r and a smoothing capacitor C) connected in series across the second intermediate coil Lm 2 , and a diode (rectifying device) D connected between the node a and a node c of the second intermediate capacitor C 2  and the output coil L 2 . In addition to the aforementioned nodes a and c, there are a node b of the first intermediate capacitor C 1  and the first intermediate coil Lm 1  and a node d of the second intermediate capacitor C 2  and the second intermediate coil Lm 2 . 
         [0034]      FIG. 1(   c   1 )˜( c   4 ) illustrate approximate waveforms of the ripple currents through the input coil L 1 , the first intermediate coil Lm 1 , the second intermediate coil Lm 2  and the output coil L 2  under different coupling conditions between these coils. That is,  FIG. 1(   c   1 ) illustrates ripple currents through the input coil L 1 , the first intermediate coil Lm 1 , the output coil L 2  and the second intermediate coil Lm 2  in case of no coupling between these coils from left to right in the drawing, respectively. They indicate in this case that all of the ripple currents through these coils L 1 , Lm 1 , L 2  and Lm 2  are large and triangular.  FIG. 1(   c   2 ) illustrates ripple currents through the respective coils in case of coupling between the input coil L 1  and the first intermediate coil Lm 1  with coupling factor k 11 =n 11 . In this case, although only the ripple current through the input coil L 1  is suppressed, all other ripple currents (through the remaining coils Lm 1 , L 2  and Lm 2 ) remain unchanged from those in  FIG. 1(   c   1 ). 
         [0035]    On the other hand,  FIG. 1(   c   3 ) illustrates ripple currents through the respective coils in case of coupling between the output coil L 2  and the second intermediate coil Lm 2  with coupling factor k 22 =n 22 . In this case, only the ripple current through the output coil L 2  is suppressed, while the other ripple currents (through the remaining coils L 1 , Lm 1  and Lm 2 ) remain unchanged from those in  FIG. 1(   c   1 ). Finally,  FIG. 1(   c   4 ) illustrates ripple currents through the respective coils in case of coupling between the input coils L 1  and the first intermediate coil Lm 1  with coupling factor k 11 =n 11  and also coupling between the output coil L 2  and the second intermediate coupling Lm 2  with coupling factor k 22 =n 22 . In this case, both ripple currents through the input coil L 1  and the output coil L 2  are suppressed, thereby achieving low noise. 
         [0036]    Now, a reference is made to  FIG. 2  for describing the operation of the DC-DC converter  10  according to the present invention as shown in  FIG. 1  more in detail.  FIG. 2(   a ) is the same circuit schematic as shown in  FIG. 1(   a ) showing the construction of the DC-DC converter  10  according to the present invention.  FIG. 2(   b ) shows currents flowing through the respective coils L 1 , L 2 , Lm 1 , Lm 2 , the capacitors C 1 , C 2  and the load R as well as potentials on the nodes a˜d when the switch S is ON. On the other hand, shown in  FIG. 2(   c ) are currents through the respective coils L 1 , L 2 , Lm 1 , Lm 2 , the capacitors C 1 , C 2  and the load R as well as potentials on the nodes a˜d when the switch S is OFF. 
         [0037]    Firstly, a description will be made with reference to  FIG. 2(   b ). When the switch S is ON, exciting currents flow through all of the coils L 1 , L 2 , Lm 1  and Lm 2  as indicated by dotted lines in the drawing, thereby providing an output current through the load R from the input voltage source E. The current through the first intermediate capacitor C 1  is in the discharging direction during the former half and in the charging direction during the latter half. On the other hand, the current through the second intermediate capacitor C 2  is in the discharging direction. 
         [0038]    Now, a reference is made to  FIG. 2(   c ) for describing the operation when the switch S is OFF. In this condition, a releasing current flows from the input voltage source E through all of the coils L 1 , L 2 , Lm 1  and Lm 2  to provide an output current into the load R by way of the diode D. Opposite to the aforementioned direction when the switch S is ON, the direction of the current through the first intermediate capacitor C 1  is in the charging direction during the former half, while in the discharging direction during the charging direction. On the other hand, the current through the second intermediate capacitor C 2  is in the charging direction. 
         [0039]    Now, potentials on the respective nodes a˜d as shown in  FIG. 2(   b ) and  FIG. 2(   c ) are in the case of L 1 =L 2  (i. e., the inductance of the input coil L 1  is equal to that of the output coil L 2 ) and Lm 1 =Lm 2  (i. e., the inductance of the first intermediate coil Lm 1  is equal to that of the second intermediate coil Lm 2 ) that will be described hereinafter. It is to be noted that currents flow through the input coil L 1  and the output coil L 2  during the time when the switch S is ON and OFF. That is, the current is increasing (positive going) when the switch S is ON, while decreasing (negative going) when the switch S is OFF, thereby developing a triangular wave (note that the current is not a pulse wave, i. e., a rectangular wave). 
         [0040]    Now, the operation of the DC-DC converter  10  according to the present invention will be analyzed hereunder. In this operational analysis, it is assumed that the switch S is an ideal switch, the diode D is also an ideal diode and time durations when the switch S is ON and OFF are referred to as ton toff, respectively. Moreover, it is assumed that each of the first intermediate capacitor C 1  and the second intermediate capacitor C 2  has sufficiently low impedance at the switching frequency of the switch S (i. e., these capacitors C 1  and C 2  have sufficiently large capacitance) and the first and second intermediate capacitors C 1  and C 2  can be considered as power sources having voltages equal to the voltage Vi of the input voltage source E and the voltage Vo of the output voltage, respectively. 
         [0041]    (a) When the switch S is ON 
         [0042]    Potentials (Va˜Vd) on the nodes a˜d and ripple currents (ΔIL 1 ˜ΔILm 2 ) of the respective coils L 1 , L 2 , Lm 1 , Lm 2  have the following relationships: 
         [0000]    
       
      
       Vb=Va−Vi  
      
     
         [0000]    
       
      
       Vc=Va−Vo  
      
     
         [0000]      Vd=Va 
         [0000]        ΔIL 1+Δ ILm 1=Δ ILm 2+Δ IL 2 
         [0043]    Amplitudes of the ripple currents (ΔIL 1 ˜ΔILm 2 ) through the respective coils L 1 ˜Lm 2  are given by the following mathematical expressions. (It is to be noted herein that           means “equals to” in case of L 1 =L 2  and Lm 1 =Lm 2 .) 
         [0000]      Δ IL 1=(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi×t on/ L 1         ( Vi×t on/ L 1/2) 
         [0000]      Δ ILm 1=(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi×t on/ Lm 1)         ( Vi×t on/ Lm 1/2) 
         [0000]      Δ IL 2=(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi×t on/ L 2         ( Vi×t on/ L 2/2) 
         [0000]      Δ ILm 2=(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi×t on/ Lm 2         ( Vi×t on/ Lm 2/2) 
         [0044]    Potentials (Va˜Vd) on the respective nodes a˜d are given by the following mathematical expressions. (Again, it is to be noted herein that           means “equals to” in case of L 1 =L 2  and Lm 1 =Lm 2 .) 
         [0000]        Va =(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi           ( Vi/ 2) 
         [0000]        Vb =−(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi           −( Vi/ 2) 
         [0000]        Vc =(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi+Vo           ( Vi/ 2)+ Vo    
         [0000]        Vd =(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi           ( Vi/ 2) 
         [0045]    Voltages across the respective coils L 1 ˜Lm 2  are given by the following mathematical expressions. (It is to be noted herein that           means “equal to” in case of L 1 =L 2  and Lm 1 =Lm 2 .) 
         [0000]        VL 1=(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi           ( Vi/ 2) 
         [0000]        VLm 1=(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi           ( Vi/ 2) 
         [0000]        VL 2=(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi           ( Vi/ 2) 
         [0000]        VLm 2=(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)× Vi           ( Vi/ 2) 
         [0046]    (b) When the switch S is OFF 
         [0047]    Potentials on the respective nodes a˜d and ripple currents (ΔIL 1 ˜ΔILm 2 ) through the respective coils L 1 ˜Lm 2  are given by the following mathematical expressions: 
         [0000]    
       
      
       Vb=Va−Vi  
      
     
         [0000]      Vc=Va 
         [0000]    
       
      
       Vd=Va−Vo  
      
     
         [0000]      Δ IL 1+Δ ILm 1=Δ ILm 2+Δ IL 2 
         [0048]    The ripple currents (ΔIL 1 ˜ΔILm 2 ) through the respective coils L 1 ˜Lm 2  are given by the following mathematical expressions. (It is to be noted herein that           means “equal to” in case of L 1 =L 2  and Lm 1 =Lm 2 .) 
         [0000]      Δ IL 1=(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         (( Vo−Vi )/2) 
         [0000]      Δ ILm 1=(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         (( Vo−Vi )/2) 
         [0000]      Δ IL 2=(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         (( Vo−Vi )/2) 
         [0000]      Δ ILm 2=(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         (( Vo−Vi )/2) 
         [0049]    Potentials (Va˜Vd) on the respective nodes a˜d are given by the following mathematical expressions. (It is to be noted herein that           means “equals to” in case of L 1 =L 2  and Lm 1 =Lm 2 .) 
         [0000]        Va ={(1/ L 1+1/ Lm 1)× Vi +(1/ L 2+1/ Lm 2)× Vo }/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)         ( Vi+Vo )/2 
         [0000]        Vb =(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         ( Vo−Vi )/2 
         [0000]        Vc ={(1/ L 1+1/ Lm 1)× Vi +(1/ L 2+1/ Lm 2)× Vo }/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)         ( Vi+Vo )/2 
         [0000]        Vd =−(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         −( Vo−Vi )/2 
         [0050]    Voltages (VL 1 ˜VLm 2 ) across the respective coils L 1 ˜Lm 2  are given by the following mathematical expressions. (It is to be noted herein that           means “equals to” in case of L 1 =L 2  and Lm 1 =Lm 2 .) 
         [0000]        VL 1=−(1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         −( Vo−Vi )/2 
         [0000]        VLm 1=−((1/ L 2+1/ Lm 2)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         −( Vo−Vi )/2 
         [0000]        VL 2=−(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         −( Vo−Vi )/2 
         [0000]        VLm 2=−(1/ L 1+1/ Lm 1)/(1/ L 1+1/ Lm 1+1/ L 2+1/ Lm 2)×( Vo−Vi )         −( Vo−Vi )/2 
         [0051]    Conditions that the DC-DC  10  converter operates normally include: 
         [0000]      Δ Ix (ON)=Δ Ix (OFF) 
         [0000]        Vx (ON)× t on=− Vx (OFF)× t off 
         [0052]    It is to be noted herein that x indicates either one of the coils L 1 , Lm 1 , L 2  and Lm 2 . Solution of the above equations leads to conclusions as follows: 
         [0000]        Vo=Vi ×( t on+ t off)/ t off= Vi /(1− D ) 
         [0000]      Where,  D=t on/( t on+ t off) 
         [0000]    This suggests that the DC-DC converter  10  is capable of operating as a voltage step-up converter. 
         [0053]    As apparent from the above description, the DC-DC converter  10  according to the present invention is capable of operating as a voltage step-up DC-DC converter in which the ripple currents through the input coil L 1  and the output coil L 2  are triangular. 
         [0054]    (c) Reduction or zero ripple currents through input and output coils 
         [0055]    Now, reduction or zero ripple currents through the input coil L 1  and the output coil L 2  in the DC-DC converter  10  will be described with reference to illustrations in  FIG. 3 .  FIG. 3(   a ) illustrates the two coils (namely input coil L 1  and output coil L 2 ) in the DC-DC converter circuit, ripple currents through these coils and voltages thereacross. If there are two coils L 1  and L 2  in the circuit that develop equal voltage across these coils as illustrated in  FIG. 3(   a ) and the coils L 1  and L 2  are coupled in the same polarity as shown in  FIG. 3(   b ), these two coils L 1  and L 2  can be represented as the equivalent circuit as illustrated in  FIG. 3(   c ). Now, it is assumed that the coupling factor and the winding ratio of these two coils L 1  and L 2  satisfy the relationship as illustrated in  FIG. 3(   d )( 1 ), the ripple currents through these coils can be reduced to one half as compared to those before coupling. If the coupling factor and the winding ratio are of the relationship as illustrated in  FIG. 3(   e )( 2 ), the ripple current through the coil L 1  remain unchanged from that before coupling but the ripple current through the coil L 2  becomes zero (i. e., zero ripple). On the other hand, if the coupling factor and the winding ratio of the coils L 1  and L 2  are of the relationship as illustrated in  FIG. 3(   f )( 3 ), the ripple current through the coil L 1  becomes zero, while the ripple current through the coil L 2  remains unchanged from that before coupling. 
         [0056]    In the DC-DC converter  10  according to the present invention, the relationships VL=VLm, or namely VL 1 =VLm 1  and VL 2 =VLm 2  always hold true as shown in  FIG. 1  and as apparent from the above mathematical expressions. Particularly, when L 1 =L 2  and Lm 1 =Lm 2 , the relationship VL 1 =VLm 1 =VL 2 =VLm 2  holds true, thereby equalizing voltage waveforms across all of the coils L 1 , L 2 , Lm 1  and Lm 2 . This means that the ripple current or currents through the input coil L 1  and/or the output coil L 2  can be reduced by properly coupling these coils L 1 ˜Lm 2 . 
         [0057]    In the DC-DC converter  10  as shown in  FIG. 1 , illustrated are examples of ripple current waveforms for suppressing the ripple current through only the input coil L 1  by coupling only the input coil L 1  and the first intermediate coil Lm 1 , the ripple current through only the output coil L 2  by coupling only the output coil L 2  and the second intermediate coil Lm 2  and ripple currents through both of the input coil L 1  and the output coil L 2  by coupling the input coil L 1  and the first intermediate coil Lm 1  as well as the output coil L 2  and the second intermediate coil Lm 2 . It is to be noted herein that the coils to be coupled may be interchanged to have the similar result, i. e., by coupling the input coil L 1  and the second intermediate coil Lm 2  and also the output coil L 2  and the first intermediate coil Lm 1 . 
         [0058]    Now, applications or practical examples using the DC-DC converter according to the present invention will be described hereinafter.  FIG. 4  shows a circuit schematic of a practical example of a DC-DC converter apparatus according to the present invention. The DC-DC converter apparatus  40  is designed to supply a stabilized step-up voltage to a load  46  such as an electrical/electronic circuit, another DC-DC converter, a battery or the like by the DC-DC converter  42  according to the present invention to which an unstable DC voltage from an input DC voltage source  44  is applied from a battery, a solar panel or the like. ON time of the switch S of the DC-DC converter  42  is controlled by a feedback control circuit  48  for providing a feedback so that a manner that the output voltage to the load  46  remains within a specified voltage range. In the DC-DC converter apparatus  40  as shown in  FIG. 4 , the input coil L 1  and the first intermediate coil Lm 1  as well as the output coil L 2  and the second intermediate coil Lm 2  are properly magnetically coupled for significantly reducing the ripple currents through the input coil L 1  and the output coil L 2  or making such ripple currents substantially zero. 
         [0059]    In other words, the DC-DC converter apparatus  40  as shown in  FIG. 4  comprises the DC-DC converter  42  including the input DC voltage source  42 , the input coil L 1 , the output coil L 2 , intermediate coils Lm 1 , Lm 2  and intermediate capacitors C 1 , C 2 , the load  46  including the load resistor R and the output (or smoothing) capacitor C and the feedback control circuit  48  for controlling the ON time of the switch by feeding back the output voltage across the load  46 . 
         [0060]    Now, a reference is made to operation waveforms in  FIG. 5  and  FIG. 6  for describing the operation of the DC-DC converter apparatus  40  as shown in  FIG. 4 .  FIG. 5  and  FIG. 6  show operation waveforms that are simulation results of the ripple currents through the input coil L 1  and the output coil L 2  of the DC-DC converter apparatus  40  under the following zero ripple current conditions:
   Vi=50V, Vo=120V   L 1 =L 2 =118 μH, Lm 1 =Lm 2 =50 μH   C 1 =C 2 =5 μF, C=100 μF   S=ideal switch, D=ideal diode   Switching frequency=100 kHz, ton=4.17 μS   
 
         [0066]      FIG. 5(   a )˜( h ) illustrate operational waveforms in case of no coupling between the coils L 1 ˜Lm 2  of the DC-DC converter apparatus  40  as shown in  FIG. 4 .  FIG. 5(   a )˜( d ) are voltage waveforms across the coils L 1 , L 2 , Lm 1  and Lm 2 , while  FIG. 5(   e )˜( h ) are current waveforms through these coils, respectively. It is understood that the voltages across all of these coils L 1 ˜Lm 2  are equal and are Vi/2≈60V when the switch S is ON and −(Vo−Vi)/2≈−35V when the switch S is OFF. The ripple currents through the respective coils L 1 ˜Lm 2  are ΔIL 1 =ΔIL 2 =Vi/2/L×ton≈1.2A and ΔILm 1 =ΔILm 2 =Vi/2/L×ton≈2.9A. 
         [0067]    Now,  FIG. 6(   a )˜( h ) illustrate operation waveforms in case of coupling between the input coil L 1  and the first intermediate coil Lm 1  as well as between the output coil L 2  and the second intermediate coil Lm 2  of the DC-DC converter apparatus  40  as shown in  FIG. 4  with the following coupling conditions. Similarly to the case in  FIG. 5 , it is to be noted herein that  FIG. 6(   a )˜( h ) are voltage and current waveforms across and through the coils L 1 , L 2 , Lm 1  and Lm 2 , respectively.
   Winding ratio:   Between L 1  and Lm 1 : n 11 =√(Lm 1 /L 1 )=0.65   Between L 2  and Lm 2 : n 22 =√(Lm 2 /L 2 )=0.65   Coupling factor:   Between L 1  and Lm 1 : k 11 =n 11 =0.65   Between L 2  and Lm 2 : k 22 =n 22 =0.65   
 
         [0074]    It is understood that the voltages across all of the coils L 1 ˜Lm 2  are equal and are Vi/2≈60V when the switch S is ON and −(Vo−Vi)/2≈−35V when the switch S is OFF. The ripple currents through the respective coils L 1 ˜Lm 2  are ΔIL 1 =ΔIL 2 ≈0A (zero ripple) and ΔILm 1 =ΔILm 2 =Vi/2/L×ton≈2.9A. This means that the ripple currents through the input coil L 1  and the output coil L 2  are significantly reduced or substantially zero. In other words, the use of the DC-DC converter according to the present invention enables to reduce the ripple currents through the input coil L 1  and the output coil L 2  essentially zero, thereby significantly reducing noise as illustrated in  FIGS. 6(   e ) and ( f ). Moreover, high electromagnetic adaptive performance helps to reduce the size of the filter to be added, thereby making the DC-DC converter compact. Additionally, since the voltages across the two or four coils are equal, it is possible to couple all of the coils in a single transformer, thereby enhancing compact and less expensive design of the DC-DC converter. 
         [0075]    Now, other embodiments of the DC-Dc converter according to the present invention will be made with reference to  FIGS. 7 and 8 .  FIG. 7  shows a circuit schematic of another example of the DC-DC converter apparatus according to the present invention. The DC-Dc converter apparatus  70  comprises a DC-DC converter  72 , a DC input voltage source  74 , a load  76  and a feedback control circuit  78 . The DC-DC converter apparatus  70  differs from the DC-DC converter apparatus  40  as shown in  FIG. 4  in that the switch S of the DC-DC converter  72  comprises a bipolar transistor T and all other circuit configurations are essentially the same. 
         [0076]      FIG. 8  shows a circuit schematic of still another example of the DC-DC converter apparatus according to the present invention. This DC-DC converter apparatus  80  comprises a DC-DC converter  82 , a DC input voltage source  84 , a load  86  and a feedback control circuit  88 . Although the DC-DC converter apparatus  80  is similar to the DC-DC converter apparatus  40 ,  70  as shown respectively in FIGS.  4  and  7 , it differs in the use of power MOSFETs (abbreviated to MT) in place of the switch S and the diode D. A diode connected in parallel with each power MOSFET represents a parasitic diode. The output voltage is fed back by the feedback control circuit  88  by controlling ON time of the switch MT so that the output voltage is within a predetermined voltage range. Power loss of the power MOSFET can be reduced by turning ON the power MOSFET replacing the diode D in the OFF time of the switch MT (synchronous rectifying). 
         [0077]    Now, the DC-DC converter and the DC-DC converter apparatus according to the present invention have been described hereinabove with reference to preferred embodiments and examples. However, it is to be understood that such embodiments and examples are simply for the purpose of describing the present invention rather than for restricting the present invention. A person having an ordinary skill in the art may be able to easily make various modifications and alternations without departing from the scope and spirit of the present invention. 
         [0078]    The DC-DC converter according to the present invention having the particular construction and exhibiting unique advantages as described hereinabove finds wide applications. It can be applied to a power supply system and apparatus in which low noise is essential, such as a power supply system and apparatus that receives an input power from a solar panels, a power supply system and apparatus that receives an input power from a battery, a battery charging/discharging system and apparatus, or the like.