Patent Publication Number: US-2023132923-A1

Title: Dc-dc conversion device having coupling inductor

Description:
TECHNICAL FIELD 
     The present disclosure relates to a DC-DC conversion device having a coupling inductor, which may reduce a ripple of an output current compared to a conventional DC-DC converter. 
     BACKGROUND AND SUMMARY 
       FIG.  1    is a diagram schematically illustrating a bus power system in a satellite. 
     As illustrated in  FIG.  1   , the bus power system in the satellite includes a solar panel, a battery charge-discharge regulator (BCDR), a battery, and a load. The bus power system in the satellite typically uses the solar panel as a voltage source to generate power, some of the power generated by the solar panel is charged to the battery, and the remaining power after charging the battery or the power which is already charged in the battery is supplied to an output capacitor and an output resistor through the BCDR. 
     The output resistor corresponds to the load consuming the power generated by the solar panel, and the output capacitor serves to store the power required by the output resistor, and then to supply the power stored in the output capacitor to the output resistor when the power is required by the output resistor. The output capacitor is charged by an effective output current (RMS current), and in this case, if the effective output current is too large, the output capacitor is deteriorated and a lifespan of the output capacitor is shortened. 
     Here, the shortening of the lifespan of the output capacitor means that the lifespan of the satellite including the output capacitor is also shortened. Therefore, in order to ensure the lifespan of the satellite, the output capacitor should be able to be charged by a stable output current, and to this end, it is necessary to reduce a ripple of an output current flowing through an output terminal of a DC-DC conversion device. 
       FIG.  2    is a diagram schematically illustrating a conventional Weinberg DC-DC conversion device, and the Weinberg DC-DC conversion illustrated in  FIG.  2    is a device included in each BCDR illustrated in  FIG.  1   . 
     Such a Weinberg DC-DC conversion device is connected to a voltage source  1  and an output capacitor  2  in a bus power system, and serves to charge a DC-DC converted power to the output capacitor  2 . However, the Weinberg DC-DC conversion device has a relatively large ripple of the output current, as will be described later. Accordingly, since the bus power system including the Weinberg DC-DC conversion may charge the output capacitor  2  by the output current having the relatively large ripple, there is a risk of shortening the lifespan of the output capacitor  2  and furthermore the lifespan of the satellite. 
     In order to solve such a problem, in Patent Document 1, the ripple of the output current flowing through the output terminal of the DC-DC conversion is reduced by adding a third switch Q 3  and a fourth switch Q 4  in addition to a first switch Q 1  and a second switch Q 2  included in the Weinberg DC-DC conversion device. 
     However, the third switch Q 3  and the fourth switch Q 4  added to Patent Document 1 correspond to active elements like the first switch Q 1  and the second switch Q 2 , the control complexity of a control unit should be increased to control a plurality of active elements as described above, and drivers corresponding to the number of added active devices (e.g., gate drivers when the active elements are FETs) should be added. Consequently, there is a problem in that the volume, weight, and price of the DC-DC conversion device are increased. 
     RELATED ART DOCUMENT 
     Patent Document 
     
         
         Korean Patent No. 2005881 
       
    
     Non-Patent Document 
     
         
         A. H. Weinberg et al. “A HIGH POWER, HIGH FREQUENCY, DC TO DC CONVERTER FOR SPACE APPLICATIONS”. IEEE. 1992. 
       
    
     DISCLOSURE 
     Technical Problem 
     An object of the present disclosure is to provide a DC-DC conversion device capable of reducing a ripple of an output current. 
     Further, an object of the present disclosure is to provide a DC-DC conversion device capable of reducing complexity, reducing volume and weight, and reducing cost that are caused by controlling a plurality of active elements. 
     Technical Solution 
     In one general aspect, a DC-DC conversion device includes: a first transformer connected between a ground and a first node between an input terminal to which an input voltage is applied and an output terminal to which an output voltage is applied, and including a first inductor and a second inductor that are magnetically coupled to each other; a first switch connected in series with the first inductor between the first node and the ground; a second switch connected in series with the second inductor between the first node and the ground; a first reverse current preventing element connected to a second node between the first inductor and the first switch and a fourth node between the input terminal and the output terminal; a second reverse current preventing element connected to a third node between the second inductor and the second switch, and the fourth node; a second transformer positioned between the input terminal and the output terminal and including a third inductor and a fourth inductor that are magnetically coupled to each other; a third reverse current preventing element connected to the fourth inductor and the fourth node; a link capacitor connected to the fourth node, and the first switch and the second switch; and an output inductor connected to the fourth node and the output terminal, in which the output inductor may be magnetically coupled to the third inductor and the fourth inductor. 
     One terminal of the third inductor may be connected to the input terminal, and the other terminal of the third inductor may be connected to the first node, and one terminal of the fourth inductor may be connected to the first node, and the other terminal of the fourth inductor may be connected to the third reverse current preventing element. 
     One terminal of the third inductor may be connected to the first switch, the second switch, and the link capacitor, and the other terminal of the third inductor may be connected to the ground, and one terminal of the fourth inductor may be connected to the first node, and the other terminal of the fourth inductor may be connected to the third reverse current preventing element. 
     The DC-DC conversion device may have sections in which the second switch is in a turned-off state when the first switch is turned on, the first switch is in the turned-off state when the second switch is turned on, and both the first switch and the second switch are in the turned-off state. 
     The first switch and the second switch may be alternately turned on at the same duty ratio, and the duty ratio may be less than 50%. 
     In the section in which both the first switch and the second switch are in the turned-off state, an output current may flow only through the third reverse current preventing element. 
     The first switch may be a first field effect transistor (FET) whose turn-on and turn-off operations are controlled by a first control signal input to a gate electrode, and the second switch may be a second FET whose turn-on and turn-off operations are controlled by a second control signal input to a gate electrode. 
     The DC-DC conversion device may further include a control unit for outputting the first control signal to the gate electrode of the first FET and outputting the second control signal to the gate electrode of the second FET. 
     Advantageous Effects 
     The DC-DC conversion device having a coupling inductor according to the present disclosure prevents a phenomenon in which the output current flowing to the output terminal causes a sudden change through passive elements such as a link capacitor and an output inductor, and according to the present disclosure, it is possible to significantly reduce the ripple generated in the output current. 
     Further, according to the present disclosure, since the number of active elements is small compared to the DC-DC conversion device of Patent Document 1, the complexity caused by controlling a plurality of active elements may be reduced, and the number of drivers for driving the plurality of active elements may also be reduced, and as a result, the volume and weight of the DC-DC conversion device may be reduced, and the price thereof may also be reduced. 
     Further, according to the present disclosure, since the output inductor is not formed of a separate inductor element, but is formed by being magnetically coupled to the third inductor and the fourth inductor constituting the second transformer, the volume and weight of the DC-DC conversion device may be reduced, and the price thereof may also be reduced compared to the case in which the output inductor is formed of the separate inductor element independently from the third and fourth inductors. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG.  1    is a diagram schematically illustrating a bus power system in a satellite. 
         FIG.  2    is a diagram illustrating a conventional Weinberg DC-DC conversion device. 
         FIG.  3 A  is a diagram illustrating a waveform of a voltage applied to a first switch in the Weinberg DC-DC conversion device of  FIG.  2   . 
         FIG.  3 B  is a diagram illustrating a waveform of a voltage applied to a second switch in the Weinberg DC-DC conversion device of  FIG.  2   . 
         FIG.  3 C  is a diagram illustrating a waveform of an output current in the Weinberg DC-DC conversion device of  FIG.  2   . 
         FIG.  4    is a diagram illustrating a DC-DC conversion device having a coupling inductor according to a first embodiment of the present disclosure. 
         FIG.  5 A  is a diagram illustrating a waveform of a voltage applied to a first switch in the DC-DC conversion device of  FIG.  4   . 
         FIG.  5 B  is a diagram illustrating a waveform of a voltage applied to a second switch in the DC-DC conversion device of  FIG.  4   . 
         FIG.  5 C  is a diagram illustrating a waveform of an output current in the DC-DC conversion device of  FIG.  4   . 
         FIG.  6    is a diagram illustrating a DC-DC conversion device having a coupling inductor according to a second embodiment of the present disclosure. 
     
    
    
     BEST MODE 
     Hereinafter, a DC-DC conversion device having a coupling inductor according to the present device will be described in detail with reference to the accompanying drawings. The accompanying drawings are provided by way of example in order to sufficiently transfer the spirit of the present disclosure to those skilled in the art, and the present disclosure is not limited to the accompanying drawings provided below, but may be implemented in other forms. 
     Before describing the DC-DC conversion device according to the present disclosure, a Weinberg DC-DC conversion device, which is one of the prior art, will be first described with reference to  FIGS.  2  and  3   . 
       FIG.  2    is a diagram schematically illustrating a conventional Weinberg DC-DC conversion device. 
     The Weinberg DC-DC conversion device illustrated in  FIG.  2    may be included in each BCDR illustrated in  FIG.  1   , and includes a first transformer  10 , a first switch  20 , a second switch  30 , a first reverse current preventing element  40 , a second reverse current preventing element  50 , a second transformer  60 , and a third reverse current preventing element  70 . 
     The first transformer  10  is connected between a ground and a first node n 1  between an input terminal n in  and an output terminal n out , and includes a first inductor  11  and a second inductor  12  that are magnetically coupled to each other. Here, the first inductor  11  and the second inductor  12  are magnetically coupled to each other means that the first inductor  11  and the second inductor  12  share the same core and are inductively coupled to each other by the number of coils wound around the same core. 
     An input voltage V in  is applied to the input terminal n in  by a voltage source  1 , and the input voltage V in  causes an input current I in  to flow. In addition, an output voltage V o  is applied to the output terminal n out , and the magnitude of the output voltage V o  depends on a root mean square (RMS) value of an output current I out . 
     The first switch  20  is connected in series with the first inductor  11  between the first node n 1  and the ground. In addition, the second switch  30  is connected in series with the second inductor  12  between the first node n 1  and the ground. 
     The first reverse current preventing element  40  is connected to a second node n 2  between the first inductor  11  and the first switch  20  and the output terminal n out . In addition, the second reverse current preventing element  50  is connected to a third node n 3  between the second inductor  12  and the second switch  30  and the output terminal n out . 
     The second transformer  60  is connected between the input terminal n in  and the output terminal n out , and includes a third inductor  61  and a fourth inductor  62  that are magnetically coupled to each other. Here, the third inductor  61  and the fourth inductor  62  are magnetically coupled to each other means that the third inductor  61  and the fourth inductor  62  share the same core and are inductively coupled to each other by the number of coils wound around the same core. The third inductor  61  is connected to the input terminal n in  and the first node n 1 , and the fourth inductor  62  is connected to the first node n 1  and the third reverse current preventing element  70 . In addition, the third reverse current preventing element  70  is connected to the fourth inductor  62  and the output terminal n out . 
     The output capacitor  2  may be connected to the output terminal n out  and the ground of the Weinberg DC-DC conversion device, and an output resistor  3  may be connected in parallel to the output capacitor  2 . Here, the output resistor  3  corresponds to a load consuming the power generated by the voltage source  1 , and the output capacitor  2  serves to store the power required by the output resistor  3 , and then to supply the power stored in the output capacitor  2  to the output resistor  3  when the power is required by the output resistor  3 . The output capacitor  2  is charged by an RMS value of the output current I out . 
       FIG.  3 A  is a diagram illustrating a waveform of a voltage applied to a first switch in the Weinberg DC-DC conversion device of  FIG.  2   ,  FIG.  3 B  is a diagram illustrating a waveform of a voltage applied to a second switch in the Weinberg DC-DC conversion device of  FIG.  2   , and  FIG.  3 C  is a diagram illustrating a waveform of an output current in the Weinberg DC-DC conversion device of  FIG.  2   . A ripple of the output current of the Weinberg DC-DC conversion device illustrated in  FIG.  2    has a relatively large value as illustrated in  FIG.  3 C . 
       FIG.  3    will be described in detail. First,  FIG.  3 A  illustrates a control signal applied to the first switch  20  of the Weinberg DC-DC conversion device of  FIG.  2   , and  FIG.  3 B  illustrates a control signal applied to the second switch  30  of the Weinberg DC-DC conversion device of  FIG.  2   , where the Weinberg DC-DC conversion device also has sections in which the switch  30  is in the turned-off state when the first switch  20  is turned on, the first switch  20  is in the turned-off state when the second switch  30  is turned on, and both the first switch  20  and the second switch  30  are in the turned-off state. 
     First, when the first switch  20  is turned on in the state in which the second switch  30  is turned off, the input current I in  by the voltage source  1  mainly flows through a current path including the third inductor  61 , the first inductor  11 , and the first switch  20 , and a small amount of input current I in  flows through a current path including the third inductor  61 , the fourth inductor  62 , and the third reverse current preventing element  70 . In this case, an induced current is generated in the second inductor  12  by the current flowing through the first inductor  11 , and the induced current generated in the second inductor  12  flows through the third node n 3  to the output terminal n out  through the second reverse current preventing element  50 . 
     Next, when the first switch  20  is switched from the turned-on state to the turned-off state in the state in which the second switch  30  is turned off, the input current I in  by the voltage source  1  flows through the third inductor  61 . In this case, an induced current is generated in the fourth inductor  62  by the current flowing through the third inductor  61 , and the induced current generated in the fourth inductor  62  flows to the output terminal n out  through the third reverse current preventing element  70 . 
     Next, when the second switch  10  is turned on in the state in which the first switch  20  is continuously turned off, the input current I in  by the voltage source  1  mainly flows through a current path including the third inductor  61 , the second inductor  12 , and the second switch  30 , and a small amount of input current I in  flows through a current path including the third inductor  61 , the fourth inductor  62 , and the third reverse current preventing element  70 . In this case, an induced current is generated in the first inductor  11  by the current flowing through the second inductor  12 , and the induced current generated in the first inductor  11  flows through the second node n 2  to the output terminal n out  through the first reverse current preventing element  40 . 
     As described above, when the first switch  20  is turned on in the state in which the second switch  30  is turned off, the induced current generated in the second inductor  12  flows through the third node n 3  to the output terminal n out  through the second reverse current preventing element  50 . Thereafter, when the first switch  20  is switched from the turned-on state to the turned-off state in the state in which the second switch  30  is continuously turned off, only the induced current generated in the fourth inductor  62  needs to flow to the output terminal n out  through the third reverse current preventing element  70 . 
     If there is no leakage inductance L k  or magnetizing inductance L m  in the transformers  10  and  60 , no ripple occurs in the output current I out  of the Weinberg DC-DC conversion device. However, since a leakage inductance L k  or a magnetizing inductance L m  exists in the transformers  10  and  60 , the ripple as illustrated in  FIG.  3 C  occurs in the output current I out  of the Weinberg DC-DC conversion device. 
       FIG.  3 C  is a result obtained by simulating under the conditions as illustrated in Table 1 below. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Weinberg DC-DC Conversion 
               
               
                 Simulation Conditions 
                 Device illustrated in FIG. 2 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Input Voltage V in [V] 
                 36 
               
               
                 Output Voltage V o [V] 
                 50 
               
               
                 Output Power P o  [W] 
                 750 
               
               
                 Turn Ratio of First Transformer 
                 1:1 
               
               
                 Turn Ratio of Second Transformer 
                 1:1 
               
               
                 Magnetizing Inductance L m  [μH] 
                 1.86 
               
               
                 Leakage Inductance L k  [μH] 
                 0.48 
               
               
                 Output Capacitance C o  [μF] 
                 13.2 
               
               
                   
               
            
           
         
       
     
     In Table 1 above, the first transformer  10  includes the first inductor  11  and the second inductor  12 . In addition, a turn ratio of the first transformer  10  refers to a ratio with respect to the number of coils wound on the same core, where the first inductor  11  and the second inductor  12  constituting the first transformer  10  share one same core, and the number of coils wound around the core is 1:1. In Table 1 above, the second transformer  60  includes the third inductor  61  and the fourth inductor  62 . In addition, a turn ratio of the second transformer  60  refers to a ratio with respect to the number of coils wound on the same core, where the third inductor  61  and the fourth inductor  62  constituting the second transformer  60  share another same core, and the number of coils wound around the core is 1:1. 
     When the first switch  20  is switched from the turned-on state to the turned-off state in the state in which the second switch  30  is turned off, the induced current generated in the fourth inductor  62  not only flows to the output terminal n out  through the third reverse current preventing element  70 , but also a current flow to the output terminal n out  instantaneously occurs even through the first reverse current preventing element  40  and the second reverse current preventing element  50  due to the leakage inductance L k  or magnetizing inductance L m  existing in the transformers  10  and  60 . In addition, due to such an instantaneous current flow, a ripple (about 22.7 A) as illustrated in  FIG.  3 C  is generated in the output current I out  of the Weinberg DC-DC conversion device. 
     In the same way, when the second switch  30  is switched from the turned-on state to the turned-off state in the state in which the first switch  20  is turned off, the induced current generated in the fourth inductor  62  not only flows to the output terminal n out  through the third reverse current preventing element  70 , but also a current flow to the output terminal n out  instantaneously occurs even through the first reverse current preventing element  40  and the second reverse current preventing element  50  due to the leakage inductance L k  or magnetizing inductance L m  existing in the transformers  10  and  60 . In addition, due to such an instantaneous current flow, a ripple (about 22.7 A) as illustrated in  FIG.  3 C  is generated in the output current I out  of the Weinberg DC-DC conversion device. 
     As such, when a ripple having a relatively large value occurs in the output current I out , the RMS value of the output current I out  also increases, thereby deteriorating the output capacitor  2  connected to the output terminal n out , and shortening the lifespan of the output capacitor  2 , and when the Weinberg DC-DC conversion device illustrated in  FIG.  2    is used in a battery charge/discharge regulator of a satellite, the lifespan of the satellite is also shortened. Accordingly, it is necessary to provide a DC-DC conversion device capable of reducing the ripple of the output current. 
       FIG.  4    is a diagram illustrating a DC-DC conversion device having a coupling inductor according to a first embodiment of the present disclosure. 
     Referring to  FIG.  4   , a DC-DC conversion device according to a first embodiment of the present disclosure includes a first transformer  100 , a first switch  200 , a second switch  300 , a first reverse current preventing element  400 , a second reverse current preventing element  500 , a second transformer  600 , a third reverse current preventing element  700 , a link capacitor  800 , and an output inductor  900 . 
     The first transformer  100  is connected between a ground and a first node n 1  between an input terminal n in  and an output terminal n out , and includes a first inductor  110  and a second inductor  120  that are magnetically coupled to each other. Here, the first inductor  110  and the second inductor  120  are magnetically coupled to each other means that the first inductor  110  and the second inductor  120  share the same core and are inductively coupled to each other by the number of coils wound around the same core. 
     An input voltage V in  is applied to the input terminal n in  by a voltage source  1000 , and the input voltage V in  causes an input current I in  to flow. In addition, an output voltage V o  is applied to the output terminal n out , and the magnitude of the output voltage V o  depends on an RMS value of an output current I Lo . 
     The first switch  200  is connected in series with the first inductor  110  between the first node n 1  and the ground. The first switch  200  may be a FET (i.e., a first FET) controlled by a first control signal output from a control unit (not illustrated). When the first switch  200  is the first FET, the first control signal may be input to a gate electrode of the first FET, and turn-on and turn-off operations of the first FET may be controlled by the first control signal. In this case, a drain electrode of the first FET may be connected to a second node n 2 , and a source electrode of the first FET may be connected to the ground. 
     The second switch  300  is connected in series with the second inductor  120  between the first node n 1  and the ground. The second switch  300  may be a FET (i.e., a second FET) controlled by a second control signal output from the control unit. When the second switch  300  is the second FET, the second control signal may be input to a gate electrode of the second FET, and turn-on and turn-off operations of the second FET may be controlled by the second control signal. In this case, a drain electrode of the second FET may be connected to a third node n 3 , and a source electrode of the second FET may be connected to the ground. 
     However, in the present disclosure, the first switch  200  or the second switch  300  is not necessarily limited to only the FET, and any element whose on-off operation may be controlled by the control signal of the control unit, such as a bipolar junction transistor (BJT) or a relay, may correspond to the first switch  200  or the second switch  300  according to the present disclosure. 
     The first reverse current preventing element  400  is connected to the second node n 2  between the first inductor  110  and the first switch  200 , and a fourth node n 4  between the input terminal n in  and the output terminal n out . More specifically, an anode electrode of the first reverse current preventing element  400  is connected to the second node n 2 , and a cathode electrode of the first reverse current preventing element  400  is connected to the fourth node n 4 , so that a forward current may flow from the second node n 2  to the fourth node n 4 . 
     The second reverse current preventing element  500  is connected to the third node n 3  between the second inductor  120  and the second switch  300 , and the fourth node n 4  between the input terminal n in  and the output terminal n out . More specifically, an anode electrode of the second reverse current preventing element  500  is connected to the third node n 3 , and a cathode electrode of the second reverse current preventing element  500  is connected to the fourth node n 4 , so that a forward current may flow from the third node n 3  to the fourth node n 4 . 
     The second transformer  600  is positioned between the input terminal n in  and the output terminal n out , and includes a third inductor  610  and a fourth inductor  620  that are magnetically coupled to each other. Here, the third inductor  610  and the fourth inductor  620  are magnetically coupled to each other means that the third inductor  610  and the fourth inductor  620  share the same core and are inductively coupled to each other by the number of coils wound around the same core. 
     In the DC-DC conversion device illustrated in  FIG.  4   , one terminal of the third inductor  610  is connected to the input terminal n in , and the other terminal of the third inductor  610  is connected to the first node n 1 . In addition, one terminal of the fourth inductor  620  is connected to the first node n 1 , and the other terminal of the fourth inductor  620  is connected to the anode of the third reverse current preventing element  700 . 
     The third reverse current preventing element  700  is connected to the fourth inductor  620  and the fourth node n 4 . More specifically, the anode of the third reverse current preventing element  700  is connected to the fourth inductor  620 , and the cathode of the third reverse current preventing element  700  is connected to the fourth node n 4 . 
     The link capacitor  800  is connected to the fourth node n 4 , and the first switch  200  and the second switch  300 . That is, one terminal of the link capacitor  800  is connected to the fourth node n 4 , and the other terminal of the link capacitor  800  is connected to the first switch  200  and the second switch  300 . The link capacitor  800  serves to receive a DC voltage converted by the first transformer  100 , the first switch  200 , the second switch  300 , the first reverse current preventing element  400 , the second reverse current preventing element  500 , and the second transformer, and store the DC voltage. In addition, the link capacitor  800  also serves to remove an AC component from the converted DC voltage. 
     The output inductor  900  is connected to the fourth node n 4  and the output terminal n out . That is, one terminal of the output inductor  900  is connected to the fourth node n 4 , and the other terminal of the output inductor  900  is connected to the output terminal n out . The output inductor  900  serves to prevent an abrupt change in the output current I Lo  when the output current I Lo  flows from the fourth node n 4  to the output terminal n out . 
     In the present disclosure, the output inductor  900  is not formed of a separate inductor element, but is formed by being magnetically coupled to the third inductor  610  and the fourth inductor  620  constituting the second transformer  600 . That is, in the present disclosure, the second transformer  600  may include the third inductor  610 , the fourth inductor  620 , and the output inductor  900 . Here, the output inductor  900  is magnetically coupled to the third inductor  610  and the fourth inductor  620  means that the output inductor  900  shares the same core with the third inductor  610  and the fourth inductor  620  and is inductively coupled to each other by the number of coils wound around the same core. That is, in the present disclosure, the output inductor  900  constitutes a coupling inductor together with the third inductor  610  and the fourth inductor  620 . 
     As described above, in the present disclosure, as the output inductor  900  is magnetically coupled to each other with the third inductor  610  and the fourth inductor  620 , the volume and weight of the DC-DC conversion device may be reduced, and the price thereof may also be reduced, compared to the case in which the output inductor  900  is formed of the separate inductor element independently from the third inductor  610  and fourth inductor  620 . 
     An output capacitor  2000  may be connected to the output terminal n out  and the ground of the Weinberg DC-DC conversion device illustrated in  FIG.  4   , and an output resistor  3000  may be connected in parallel to the output capacitor  2000 . Here, the output resistor  3000  corresponds to a load consuming the power generated by the voltage source  1000 , and the output capacitor  2000  serves to store the power required by the output resistor  3000 , and then to supply the power stored in the output capacitor  2  to the output resistor  3000  when the power is required by the output resistor  3000 . The output capacitor  2000  is charged by an RMS value of the output current I Lo . 
       FIG.  5 A  is a diagram illustrating a waveform of a voltage applied to a first switch in the DC-DC conversion device of  FIG.  4   ,  FIG.  5 B  is a diagram illustrating a waveform of a voltage applied to a second switch in the DC-DC conversion device of  FIG.  4   , and  FIG.  5 C  is a diagram illustrating a waveform of an output current in the DC-DC conversion device of  FIG.  4   . 
       FIG.  5    will be described in detail. First,  FIG.  5 A  illustrates a control signal applied to the first switch  200  of the DC-DC conversion device according to the first embodiment of the present disclosure, and  FIG.  5 B  illustrates a control signal applied to the second switch  300  of the DC-DC conversion device according to the first embodiment of the present disclosure, where the DC-DC conversion device also has sections in which the switch  300  is in the turned-off state when the first switch  200  is turned on, the first switch  200  is in the turned-off state when the second switch  300  is turned on, and both the first switch  200  and the second switch  300  are in the turned-off state. 
     The first switch  200  and the second switch  300  may each have a duty ratio of less than 50%. The first switch  200  and the second switch  300  may be alternately turned on at the same duty ratio (e.g., 30%) by a control signal output from the control unit. In this case, a 40% section in which both the first switch  200  and the second switch  300  are in the turn-off state corresponds to a section in which the control signal from the control unit is not input to the first switch  200  and the second switch  300 . 
     First, when the first switch  200  is turned on in the state in which the second switch  300  is turned off, the input current I in  by the voltage source  1000  mainly flows through a current path including the third inductor  610 , the first inductor  110 , and the first switch  200 , and a small amount of input current I in  flows through a current path including the third inductor  610 , the fourth inductor  620 , and the third reverse current preventing element  700 . In this case, an induced current is generated in the second inductor  120  by the current flowing through the first inductor  110 , and the induced current generated in the second inductor  120  flows through the third node n 3  to the fourth node n 4  through the second reverse current preventing element  500 . 
     Next, when the first switch  200  is switched from the turned-on state to the turned-off state in the state in which the second switch  300  is turned off, the input current I in  by the voltage source  1000  flows through the third inductor  610 . In this case, an induced current is generated in the fourth inductor  620  by the current flowing through the third inductor  610 , and the induced current generated in the fourth inductor  620  flows to the fourth node n 4  through the third reverse current preventing element  700 . 
     Next, when the second switch  300  is turned on in the state in which the first switch  200  is continuously turned off, the input current I in  by the voltage source  1000  mainly flows through a current path including the third inductor  610 , the second inductor  120 , and the second switch  300 , and a small amount of input current I in  flows through a current path including the third inductor  610 , the fourth inductor  620 , and the third reverse current preventing element  700 . In this case, an induced current is generated in the first inductor  110  by the current flowing through the second inductor  120 , and the induced current generated in the first inductor  110  flows through the second node n 2  to the fourth node n 4  through the first reverse current preventing element  400 . 
     As described above, when the first switch  200  is turned on in the state in which the second switch  300  is turned off, the induced current generated in the second inductor  120  flows through the third node n 3  to the fourth node n 4  through the second reverse current preventing element  500 . Thereafter, when the first switch  200  is switched from the turned-on state to the turned-off state in the state in which the second switch  300  is continuously turned off, only the induced current generated in the fourth inductor  620  needs to flow to the fourth node n 4  through the third reverse current preventing element  700 . 
     If there is no leakage inductance L k  or magnetizing inductance L m  in the transformers  100  and  600 , no ripple occurs in the output current I Lo  of the DC-DC conversion device. However, since a leakage inductance L k  or a magnetizing inductance L m  exists in the transformers  100  and  600 , the ripple as illustrated in  FIG.  5 C  occurs in the output current I Lo  of the DC-DC conversion device. 
       FIG.  5 C  is a result obtained by simulating under the conditions as illustrated in Table 2 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 DC-DC Conversion Device 
               
               
                   
                 Simulation Conditions 
                 illustrated in FIG. 4 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Input Voltage V in [V] 
                 36 
               
               
                   
                 Output Voltage V o [V] 
                 50 
               
               
                   
                 Output Power P o  [W] 
                 750 
               
               
                   
                 Turn Ratio of First Transformer 
                 1:1 
               
               
                   
                 Turn Ratio of Second Transformer 
                 1:1:1.3 
               
               
                   
                 Magnetizing Inductance L m  [μH] 
                 1.86 
               
               
                   
                   
                 L k1  = 0.48 
               
               
                   
                 Leakage Inductance L k  [μH] 
                 L k2  = 0.48 
               
               
                   
                   
                 L k3  = 1.28 
               
               
                   
                 Output Capacitance C o  [μF] 
                 11 
               
               
                   
                 Link Capacitance C link  [μF] 
                 2.2 
               
               
                   
                   
               
            
           
         
       
     
     In Table 2, a turn ratio of the first transformer  100  refers to a ratio with respect to the number of coils wound on the same core, where the first inductor  110  and the second inductor  120  constituting the first transformer  100  share one same core, and the number of coils wound around the core is 1:1. In Table 2, the second transformer  600  also includes the output inductor  900  in addition to the third inductor  610  and the fourth inductor  620 . In addition, a turn ratio of the second transformer  600  refers to a ratio with respect to the number of coils wound on the same core, where the third inductor  610 , the fourth inductor  620 , and the output inductor  900  constituting the second transformer  600  share another same core, and the number of coils wound around the core is 1:1:1.3. 
     When the first switch  200  is switched from the turned-on state to the turned-off state in the state in which the second switch  300  is turned off, or when the second switch  300  is switched from the turned-on state to the turned-off state in the state in which the first switch  200  is turned off, the induced current generated in the fourth inductor  620  not only flows to the fourth node n 4  through the third reverse current preventing element  700 , but also a current flow to the fourth node n 4  instantaneously occurs even through the first reverse current preventing element  400  and the second reverse current preventing element  500  due to the leakage inductance L k  or the magnetizing inductance L m  existing in the transformers  100  and  600 . In addition, due to such an instantaneous current flow, a ripple as illustrated in  FIG.  5 C  is generated in the output current I Lo  of the DC-DC conversion device according to the first embodiment of the present disclosure. 
     However, it may be seen that only a ripple of about 5.9 A is generated in the output current I Lo  of the DC-DC conversion device according to the first embodiment of the present disclosure as illustrated in  FIG.  5 C  as compared to the ripple of 22.7 A being generated in the output current I out  of the Weinberg DC-DC conversion according to  FIG.  2    as illustrated in  FIG.  3 C . 
     This is because the output inductor  900  prevents an abrupt change in the output current I Lo  flowing from the fourth node n 4  to the output terminal n out  even though the current flows through all the reverse current preventing elements  400 ,  500 , and  700  like the Weinberg DC-DC conversion device according to  FIG.  2   , when the second switch  300  is continuously turned off and the first switch  200  is switched from the turned-on state to the turned-off, or when the first switch  200  is continuously turned off and the second switch  300  is switched from the turned-on state to the turned-off state, in the DC-DC conversion device according to the first embodiment of the present disclosure. 
       FIG.  6    is a diagram illustrating a DC-DC conversion device having a coupling inductor according to a second embodiment of the present disclosure. Compared to the DC-DC conversion device illustrated in  FIG.  4   , the DC-DC conversion device illustrated in  FIG.  6    differs only in the arrangement of the third inductor  610 , and the arrangement and functions of other components are the same as those described above with respect to  FIG.  4   . Therefore, the DC-DC conversion device according to the second embodiment of the present disclosure will be mainly described with respect to the difference compared to the DC-DC converter according to the first embodiment. 
     In the DC-DC conversion device illustrated in  FIG.  6   , one terminal of the third inductor  610  is connected to the first switch  200 , the second switch  300 , and the link capacitor  800 , and the other terminal of the third inductor  620  is connected to the ground. In addition, one terminal of the fourth inductor  620  is connected to the first node n 1 , and the other terminal of the fourth inductor  620  is connected to the anode of the third reverse current preventing element  700 . 
     The third reverse current preventing element  700  is connected to the fourth inductor  620  and the fourth node n 4 . More specifically, the anode of the third reverse current preventing element  700  is connected to the fourth inductor  620 , and the cathode of the third reverse current preventing element  700  is connected to the fourth node n 4 . 
     The DC-DC conversion device illustrated in  FIG.  6    operates in the same manner as the DC-DC conversion device illustrated in  FIG.  4   . That is, the DC-DC conversion device also has sections in which the switch  300  is in the turned-off state when the first switch  200  is turned on, the first switch  200  is in the turned-off state when the second switch  300  is turned on, and both the first switch  200  and the second switch  300  are in the turned-off state. 
     The first switch  200  and the second switch  300  may each have a duty ratio of less than 50%. The first switch  200  and the second switch  300  may be alternately turned on at the same duty ratio (e.g., 30%) by a control signal output from the control unit. In this case, a 40% section in which both the first switch  200  and the second switch  300  are in the turn-off state corresponds to a section in which the control signal from the control unit is not input to the first switch  200  and the second switch  300 . 
     First, when the first switch  200  is turned on in the state in which the second switch  300  is turned off, the input current I in  by the voltage source  1000  mainly flows through a current path including the first inductor  110 , the first switch  200 , and the third inductor  610 , and a small amount of input current I in  flows through a current path including the fourth inductor  620  and the third reverse current preventing element  700 . In this case, an induced current is generated in the second inductor  120  by the current flowing through the first inductor  110 , and the induced current generated in the second inductor  120  flows through the third node n 3  to the fourth node n 4  through the second reverse current preventing element  500 . 
     Next, when the first switch  200  is switched from the turned-on state to the turned-off state in the state in which the second switch  300  is turned off, the input current I in  by the voltage source  1000  flows through the fourth inductor  620 , and the current flowing through the fourth inductor  620  flows to the fourth node n 4  through the third reverse current preventing element  700 . 
     Next, when the second switch  300  is turned on in the state in which the first switch  200  is continuously turned off, the input current I in  by the voltage source  1000  mainly flows through a current path including the second inductor  120 , the second switch  300 , and the third inductor  610 , and a small amount of input current I in  flows through a current path including the fourth inductor  620  and the third reverse current preventing element  700 . In this case, an induced current is generated in the first inductor  110  by the current flowing through the second inductor  120 , and the induced current generated in the first inductor  110  flows through the second node n 2  to the fourth node n 4  through the first reverse current preventing element  400 . 
     As described above, when the first switch  200  is turned on in the state in which the second switch  300  is turned off, the induced current generated in the second inductor  120  flows through the third node n 3  to the fourth node n 4  through the second reverse current preventing element  500 . Thereafter, when the first switch  200  is switched from the turned-on state to the turned-off state in the state in which the second switch  300  is continuously turned off, the current needs to flow to the fourth node n 4  only through the path including the fourth inductor  620  and the third reverse current preventing element  700 . 
     If there is no leakage inductance L k  or magnetizing inductance L m  in the transformers  100  and  600 , no ripple occurs in the output current I Lo  of the DC-DC conversion device. However, since a leakage inductance L k  or a magnetizing inductance L m  exists in the transformers  100  and  600 , the ripple as illustrated in  FIG.  5 C  occurs in the output current I Lo  of the DC-DC conversion device. 
     That is, the DC-DC conversion device illustrated in  FIG.  6    exhibits the same waveform of the output current under the same conditions as the DC-DC conversion device illustrated in  FIG.  4   . This is because the output inductor  900  prevents an abrupt change in the output current I Lo  flowing from the fourth node n 4  to the output terminal n out  even though the current flows through all the reverse current preventing elements  400 ,  500 , and  700  like the Weinberg DC-DC conversion device according to  FIG.  2   , when the second switch  300  is continuously turned off and the first switch  200  is switched from the turned-on state to the turned-off, or when the first switch  200  is continuously turned off and the second switch  300  is switched from the turned-on state to the turned-off state, also in the DC-DC conversion device according to the first embodiment of the present disclosure. 
     In the present disclosure, the reverse current preventing element can be a diode, but is not necessarily limited thereto, and a FET, a BJT, a relay, and the like may also be used as the reverse current preventing element of the present disclosure, as long as it is a device capable of preventing a reverse current. 
     As described above, although the present disclosure has been described with reference to the limited embodiments and drawings, the present disclosure is not limited to the above embodiments, and various modifications and variations are possible from these descriptions by those of ordinary skill in the art to which the present disclosure pertains. Therefore, the technical spirit of the present disclosure should be understood only by the claims, and all equivalents or equivalent modifications thereof are intended to fall within the scope of the technical spirit of the present disclosure. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               100 : first transformer 
               110 : first inductor 
               120 : second inductor 
               200 : first switch 
               300 : second switch 
               400 : first reverse current preventing element 
               500 : second reverse current preventing element 
               600 : second transformer 
               610 : third inductor 
               620 : fourth inductor 
               700 : third reverse current preventing element 
               800 : link capacitor 
               900 : output inductor 
               1000 : voltage source 
               2000 : output capacitor 
               3000 : output resistor