PATENT ABSTRACT
The inventive bi-directional DC-DC converter addresses a problem of an insufficient step-up ratio during the step-up operation that is caused when a turns ratio of the transformer is determined to, for example, match the step-up operation and also address a contrary problem of an insufficient step-down ratio during the step-down operation that is caused when a turns ratio is determined to match the step-up operation. In the inventive bi-directional DC-DC converter that uses a transformer for both step-down and step-up operations, a switching frequency for operating a switching device is set separately for the step-down and step-up operations. For example, when the switching frequency during the step-up operation is lower than the switching frequency during the step-down operation, the range in which the duty ratio in PWM control can be controlled is widened, compensating for step-up ratio insufficiency. Conversely, step-down ratio insufficiency is compensated for by making the switching frequency during the step-down operation lower than the switching frequency during the step-up operation.

PATENT DESCRIPTION
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese application serial no. 2005-367862, filed on Dec. 21, 2005, the content of which is hereby incorporated by reference into this application.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a DC-DC converter that is provided between a first voltage power supply and a second voltage power supply and performs forward power conversion from a first voltage to a second voltage and backward power conversion from the second voltage to the first voltage.  
       BACKGROUND OF THE INVENTION  
       [0003]     With a background of social problems such as global warming and an increase in crude oil prices, there is a rapid spread of hybrid electric vehicles (HEVs) and other vehicles targeted at a high mileage. In general, an HEV includes a main high-voltage battery for driving an engine assisting motor and an auxiliary low-voltage battery for supplying electric power to electronic devices mounted on the vehicle. The main high-voltage battery is charged when the engine rotates the motor and produces (regenerates) electric power. The generated electric power is converted by a DC-DC converter to electric power for the auxiliary low-voltage battery and supplied to the vehicle-mounted electronic devices. Thus, the main purpose of the DC-DC converter disposed between the main high-voltage battery and the auxiliary low-voltage battery is to cause a step-down operation from the main high-voltage battery to the auxiliary low-voltage battery. However, there is also a need to cause a step-up operation from the auxiliary low-voltage battery to the main high-voltage battery. For example, the engine may not be capable of being started due to a low voltage of the main high-voltage battery. In this case, if electric power can be supplied from the auxiliary low-voltage battery to the main high-voltage battery, the auxiliary low-voltage battery can compensate for the power insufficiency to start the engine through the main high-voltage battery alone. Accordingly, a bi-directional DC-DC converter having both a step-down function that serves from the high-voltage side to the low-voltage side and a step-up function that serves from the low-voltage side to the high-voltage side is demanded.  
         [0004]     Examples of the prior art related to this type of bi-directional DC-DC converter are disclosed in, for example, Patent Documents 1 to 3.  
         [0005]     Patent Document 1: Japanese Patent Laid-open No. 2003-111413  
         [0006]     Patent Document 2: Japanese Patent Laid-open No. 2002-165448  
         [0007]     Patent Document 3: Japanese Patent Laid-open No. 11(1999)-8910  
       SUMMARY OF THE INVENTION  
       [0008]     Suppose that a step-down ratio and a step-up ratio are determined by a ratio between the number of turns on the primary side of a transformer and the number of turns on the secondary side. If a ratio of the number of turns on the transformer that is optimum for a step-down operation is set, a problem of the inability to meet a step-up ratio arises. Conversely, if the step-up ratio is focused in setting a ratio of the number of turns on the transformer, another problem of a too low voltage during a step-down operation occurs. Even if a bi-directional DC-DC converter is structured without a transformer, when a difference between the step-down ratio and the step-up ratio is relatively large, desired bi-directional voltage ratios cannot be obtained easily.  
         [0009]     An object of the present invention is to provide a DC-DC converter, for bi-directionally converting electric power between two different voltages, from which a voltage is obtained across two terminals in a desired range even when a difference between its step-down ratio and step-up ratio is needed.  
         [0000]     [Means of Solving the Problems] 
         [0010]     With a usual switching power supply, the step-down ratio and step-up ratio can be adjusted by adjusting the duty ratio of a pulse width modulation (PWM) signal (a pulse frequency modulation (PFM) signal may be used instead, which is also true for the description that follows) that controls the switching device. When a transformer is used, the step-down ratio and step-up ratio can be determined by the ratio between the number of turns on the primary side of the transformer and the number of turns on the secondary side. However there may be a large difference between a demanded step-down ratio (N 1 ) and step-up ratio (N 2 ). In this case, the above-mentioned PWM control and transformer turns ratio alone may be insufficient.  
         [0011]     In a preferred mode of the present invention, there is a difference in duty ratio range in PWM control between the step-down operation and the step-up operation.  
         [0012]     As well known, the duty ratio in PWM control cannot be adjusted over a range from 0% to 100% due to restrictions on the minimum turned-on and turned-off times of a switching device. An allowable range of the duty ratio is, for example, 5% to 95%. Since the minimum turned-on and turned-off times of the switching device are unchangeable, when the switching frequency is lowered to prolong the cycle, the allowable duty ratio range can be widened accordingly. It is possible to obtain an allowable duty ratio range of, for example, 3% to 97%. Therefore, the easiest method of adjusting the duty ratio range is to adjusting the switching frequency.  
         [0013]     In the preferred mode of the present invention, a means for setting a duty ratio range for the step-down operation and a duty ratio range in the step-up operation separately is provided.  
         [0014]     In another preferred mode of the present invention, a DC-DC converter, which includes a transformer that connects a step-down conversion circuit and a step-up conversion circuit and converts electric power between two voltages, has a turns ratio switching means for switching the turns ratios of the transformer between the step-down operation and the step-up operation.  
         [0015]     According to the preferred mode of the present invention, the duty ratio range in PWM control can be adjusted independently for the step-down operation and the step-up operation by making a switching frequency during the step-down operation different from, for example, a switching frequency during the step-up operation. Accordingly, when the frequency for the step-down ratio or step-up ratio, whichever is insufficient, is set to a value lower than the frequency for the other (the cycle, that is, the length of time of one cycle, is prolonged) to expand the duty ratio range in PWM control, the adjustable range of the step-down ratio or the step-up ratio can be expanded. Of course, it is also possible to use a duty ratio range adjusting means other than to adjust the switching frequency.  
         [0016]     According to the other preferred mode of the present invention, since there is provided a means for using a different transformer turns ratio between the primary side and the secondary side depending on whether the voltage is dropped or boosted when a single transformer is used to drop and boost the voltage, transformer turns ratios optimum for the step-down ratio and step-up ratio can be set. As a result, the adjustable range of the step-down ratio or step-up ratio can be expanded.  
         [0017]     These two types,of techniques can be used separately or together, enabling the range of the step-down ratio or step-up ratio to be expanded.  
         [0018]     Other purposes and features of the present invention will be clarified in the description of embodiments that follow. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  shows the entire structure of a bi-directional DC-DC converter according to a first embodiment of the present invention.  
         [0020]      FIG. 2  illustrates the relation between the step-down ratio and the step-up ratio of the bidirectional DC-DC converter.  
         [0021]      FIG. 3A  shows first example of the structure of the switching frequency setting and adjusting means in the first embodiment of the present invention.  
         [0022]      FIG. 3B  shows second example of the structure of the switching frequency setting and adjusting means in the first embodiment of the present invention.  
         [0023]      FIG. 3C  shows third example of the structure of the switching frequency setting and adjusting means in the first embodiment of the present invention.  
         [0024]      FIG. 4  shows a specific example of the structure of the step-down control circuit in the first embodiment.  
         [0025]      FIG. 5A  illustrates first relation between the frequencies set by the switching frequency setting means.  
         [0026]      FIG. 5B  illustrates second relation between the frequencies set by the switching frequency setting means.  
         [0027]      FIG. 6  shows the entire structure of a bi-directional DC-DC converter according to a second embodiment of the present invention.  
         [0028]      FIG. 7  shows the entire structure of a bi-directional DC-DC converter according to a third embodiment of the present invention.  
         [0029]      FIG. 8  shows the entire structure of a bi-directional DC-DC converter according to a fourth embodiment of the present invention.  
         [0030]      FIG. 9  shows the entire structure of a bi-directional DC-DC converter according to a fifth embodiment of the present invention.  
         [0031]      FIG. 10  shows the entire structure of a bi-directional DC-DC converter according to a sixth embodiment of the present invention.  
         [0032]      FIG. 11  shows the entire structure of a bi-directional DC-DC converter according to a seventh embodiment of the present invention.  
         [0033]      FIG. 12  shows the entire structure of a bi-directional DC-DC converter according to an eighth embodiment of the present invention.  
         [0034]      FIG. 13  shows an example of timing charts when a step-down operation is performed in  FIG. 12 .  
         [0035]      FIG. 14  shows an example of timing charts when a step-down operation is performed in  FIG. 12 .  
         [0036]      FIG. 15  shows the entire structure of a bi-directional DC-DC converter according to a ninth embodiment of the present invention.  
         [0037]      FIG. 16  shows the entire structure of a bi-directional DC-DC converter according to a tenth embodiment of the present invention.  
         [0038]      FIG. 17  shows the entire structure of a bi-directional DC-DC converter according to an eleventh embodiment of the present invention.  
         [0039]      FIG. 18  shows the entire structure of a bi-directional DC-DC converter according to a twelfth embodiment of the present invention.  
         [0040]      FIG. 19  shows the entire structure of a bi-directional DC-DC converter according to a thirteenth embodiment of the present invention.  
         [0041]      FIG. 20  shows examples of timing charts during a step-down operation and step-up operation in the thirteenth embodiment in  FIG. 19 .  
         [0042]      FIG. 21  shows, as a fourteenth embodiment of the present invention, a system structure in which a bi-directional DC-DC converter is applied to a vehicle-mounted hybrid system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]     In, for example, a DC-DC converter that has two batteries with two different voltages and bi-directionally converts electric power between the two voltages, the voltage range of the main high-voltage battery is determined according to the secondary battery mounted, required system specifications, and other factors. The voltage range of the auxiliary low-voltage battery is also determined similarly.  
         [0044]      FIG. 2  shows a step-down ratio and step-up ratio when electric power conversion is performed between two different voltages, high voltage and low voltage. The largest difference between N 1  of the step-down ratio 1/N 1  and the step-up ratio N 2  may be present in  FIG. 2 . The step-down ratio 1/N 1  during the step-down operation from the high-voltage side to the low-voltage side is defined as 1/N 1 =1/(HV 1 /LV 2 ), and the step-up ratio N 2  during the step-up operation from the low-voltage side to the high-voltage side is defined as HV 2 /LV 1 . If N 1  of the step-down ratio 1/N 1  is relatively close to the step-up ratio N 2 , a bi-directional DC-DC converter can be designed with ease. However, HV 1 , HV 2 , LV 1 , and LV 2  vary according to the charted states of the two batteries, battery deterioration states, and other conditions, so there may be often a large difference between N 1  and N 2 , making the design difficult. Preferred embodiments of the present invention that addresses this problem will be described below in detail with reference to the drawings.  
       First Embodiment  
       [0045]      FIG. 1  shows the entire structure of a bi-directional DC-DC converter according to a first embodiment of the present invention. The main circuits in  FIG. 1  are a high-voltage DC power supply HV, a low-voltage DC power supply LV, a main high-voltage circuit  1  having a switching means, and a main low-voltage circuit  2  having a switching means, and a transformer  3 .  
         [0046]     Provided as control circuits are a step-down control circuit  4  for dropping the voltage from the HV side to the LV side, a step-up control circuit  5  for boosting the voltage, a switching frequency setting means  6  for a switching signal generated by the step-down control circuit  4 , and a frequency setting means  7  for the step-up control circuit  5 . Selectors  8  and  9  are also included; the selector  8  selectively selects a control signal sent from the step-down control circuit  4  and a control signal sent from the step-up control circuit  5  and sends the selected signal to the main high-voltage circuit  1 ; the selector  9  selectively selects a control signal sent from the step-down control circuit  4  and a control signal sent from the step-up control circuit  5  and sends the selected signal to the main low-voltage circuit  2 .  
         [0047]     The above components excluding the power supplies HV and LV constitute the bi-directional DC-DC converter  10 .  
         [0048]     The bi-directional DC-DC converter  10  is structured so that a step-down/step-up control switching signal  12  is received from a high-end controller  11  such as engine controller.  
         [0049]     Next, operation in  FIG. 1  will be described. In the step-down operation from the high-voltage DC power supply HV to the low-voltage DC power supply LV, a DC voltage of the HV is converted to an AC voltage in the main high-voltage circuit  1 , the AC voltage is transferred to the LV by the transformer  3 , and the transferred AC voltage is rectified in the main low-voltage circuit  2 . At this time, the switching means in the main high-voltage circuit  1  and main low-voltage circuit  2  are controlled by control signals generated by the step-down control circuit  4  and selected by the selectors  8  and  9 . The step-down/step-up control switching signal  12  sent from the high-end controller  11  and input to the selectors  8  and  9  commands a step-down operation. The step-down control circuit  4  generates control signals to be supplied to the switching means according to the switching frequency set by the switching frequency setting means  6 .  
         [0050]     During the step-down operation, the step-up control circuit  5  and switching frequency setting means  7  may or may not operate because they do not affect the step-down operation. To reduce the power consumption, however, the step-up control circuit  5  and switching frequency setting means  7  are preferably stopped. As such, the step-down operation from the high-voltage DC power supply HV to the low-voltage DC power supply LV is performed.  
         [0051]     In the step-up operation from the low-voltage DC power supply LV to the high-voltage DC power supply HV, the DC voltage of the LV is converted into an AC voltage in the main low-voltage circuit  2 . The converted AC voltage is transferred by the transformer  3  to the HV and then rectified in the main high-voltage circuit  1 . At this time, the switching means in the main low-voltage circuit  2  and main high-voltage circuit  1  are controlled by control signals generated in the step-up control circuit  5  and selected by the selectors  8  and  9 . The step-down/step-up control switching signal  12  input from the selectors  8  and  9  from the high-end controller  11  commands a step-up operation. The step-up control circuit  5  generates controls signals to be supplied to the switching means, according to the switching frequency set by the switching frequency setting means  7 .  
         [0052]     During the step-up operation, the step-down control circuit  4  and switching frequency setting means  6  may or may not operate because they do not affect the step-up operation. To reduce the power consumption, however, the step-down control circuit  4  and switching frequency setting means  6  are preferably stopped. As such, the step-up operation from the low-voltage DC power supply LV to the high-voltage DC power supply HV is performed.  
         [0053]     The main high-voltage circuit  1  operates as an inverter that converts a DC voltage into an AC voltage during the step-down operation and as a rectifier that converts an AC voltage into a DC voltage during the step-up operation. The main low-voltage circuit  2  operates as a rectifier that converts an AC voltage into a DC voltage during the step-down operation and as an inverter that converts a DC voltage into an AC voltage during the step-up operation.  
         [0054]     The switching means included in the main high-voltage circuit  1  and main low-voltage circuit  2  may be operated by diodes alone that are connected in parallel according to the operation, without having them perform a switching operation. This is because, during the rectification operation, for example, rectification by the diodes can basically achieve the purpose. When the switching means is turned on actively during the rectification operation, its purpose is usually to perform synchronous rectification with a switching device with less loss than the diode.  
         [0055]     Next, the relation among the step-down ratio, the step-up ratio, the turns ratios of the transformer, and switching frequencies fsw 1  and fsw 2  will be described with reference again to  FIG. 2 .  
         [0056]      FIG. 2  illustrates the relation between the voltage range (HV 1  to HV 2 ) of the high-voltage DC power supply HV and the voltage range (LV 1  to LV 2 ) of the low-voltage power supply LV. In the step-down operation, the step-down ratio (indicated by N 1 ) is minimized when the HV is at the lowest voltage (HV 1 ) and the LV is at the highest voltage (LV 2 ). In the step-up operation, the step-up ratio (indicated by N 2 ) is maximized when the LV is at the lowest voltage (LV 1 ) and the HV is at the highest voltage (HV 2 ).  
         [0057]     When there is a large difference between the step-down ratio and the step-up ratio, as described above, a significant design parameter in  FIG. 1  is the turns ratio of the transformer. When both the step-down operation from the HV to LV and the step-up operation from the LV to the HV are performed, the step-down ratio and step-up ratio are largely affected by the turns ratio of the transformer because the transformer is shared by the main high-voltage circuit  1  and main low-voltage circuit  2 . If the turns ratio, for example, is determined with the step-down operation prioritized, a sufficient step-up ratio may not be obtained. Conversely, if the turns ratio is determined with the step-up operation prioritized, a sufficient step-down ratio cannot be obtained, resulting in a too low LV voltage.  
         [0058]     In this embodiment, the above-mentioned switching frequencies during the step-down and step-up operations are set independently, so the step-down and step-up ratios can be set in a wide range. The switching frequencies fsw 1  and fsw 2  respectively set in the switching frequency setting means  6  and  7  are factory-set to unique values; they may be left unchanged after the product is shipped or may be changed during an operation after the shipping, according to the voltages of the HV and LV, the value of the load current (large or small), or another factor.  
         [0059]      FIGS. 3A  to  3 C show an example of the structure of the switching frequency setting means in the first embodiment.  
         [0060]     In  FIG. 3A , switches  311  to  313 , used as the switching means of the switching frequency setting means  6  and  7 , selectively select resistors  321  to  323 , respectively, to change the frequency fsw of an oscillator  310 . In  FIG. 3B , a plurality of oscillators  331  to  333  with different frequencies are provided; to change the output frequency fsw, one oscillator is selected with a switch  341 ,  342 , or  343 . In  FIG. 3C , the signal frequency of a discrete component, such as a carrier oscillator, in a PWM modulator  350  is adjusted by selecting the constant of an external component, such as the capacitance of a capacitor  361 ,  362 , or  363 , with a switch  371 ,  372 , or  373 .  
         [0061]      FIG. 4  shows a specific example of the structure of the step-down control circuit according to the first embodiment.  FIG. 4  is the same as  FIG. 1  except that the structure of the control system of the step-down control circuit  4  is depicted in detail. An error amplifier  111  amplifies the difference between the voltage of the low-voltage DC power supply LV and a reference voltage  112  and sends the amplified error to a PWM modulator (or PFM modulator)  110 . The PWM modulator  110  performs PWM modulation (or PFM modulation) on the amplified result received from the error amplifier  111  and sends the resulting signal to the switching means in the main high-voltage circuit  1  and main low-voltage circuit  2 . Although the step-up control circuit  5  in  FIG. 1  is omitted in  FIG. 4 , it has the same structure as the step-down control circuit  4  except that the step-up control circuit  5  receives a voltage from the high-voltage DC power supply HV and outputs it to terminals, on the selectors  8  and  9 , not used by the step-down control circuit  4 .  
         [0062]      FIG. 5A  illustrates first relation between the turns ratio of the transformer  3  and the frequencies fsw 1  and fsw 2  set by the switching frequency setting means  6  and  7 . In  FIG. 5A , the transformer turns ratio (N 1 ) required for dropping the voltage and the transformer turns ratio (N 2 ) required for boosting the voltage are indicated on the horizontal axis. There is no problem if transformer turns ratios that satisfy the conditions for both the step-down and step-up operations are selected. When losses in the transformer, the switching device, and other circuits are considered, it is difficult to satisfy both conditions. In this case, either the step-down or step-up operation must be prioritized when transformer turns ratios are determined. When the switching frequency fsw 1  during the step-down operation and the switching frequency fsw 2  during the step-up operation are set as shown in  FIG. 5A , the step-up and step-down ratios, which are difficult to satisfy simultaneously with only the transformer turns ratio, can be satisfied.  
         [0063]      FIG. 5B  illustrates second relation between the turns ratio of the transformer  3  and the frequencies fsw 1  and fsw 2  set by the switching frequency setting means  6  and  7 . Especially,  FIG. 5B  shows an example in which the transformer turns ratios obtained from calculations of the step-down and step-up ratios cannot be originally satisfied simultaneously. In this case as well, either the step-down or step-up operation must be prioritized when transformer turns ratios are determined. When the switching frequencies fsw 1  and fsw 2  are set as shown in  FIG. 5B , the step-down and step-up ratios can be set in as wide a range as possible.  
         [0064]     Now, the relation between the step-up ratio and the transformer turns ratio required for the step-up operation will be described. During the step-up operation, the main low-voltage circuit is operated as the step-up circuit. The product (N 2 _ 1 ×N 2 _ 2 ) of the step-up ratio N 2 _ 1  of the step-up circuit and the transformer turns ratio N 2 _ 2  is used to satisfy the step-up ratio. In this type of example, the transformer turns ratio N 2 _ 2  actually required for the step-up operation is N 2 _ 2  N 2 /N 2 _ 1 . The transformer turns ratio N 2 _ 2  required for the step-up operation that has been described refers to the step-up ratio required for the transformer itself (in this case, the step-up ratio is N 2 _ 2 ).  
         [0065]     As described above, if the switching frequency is reduced and the length of one cycle is prolonged, the duty ratio width in PWM control can be expanded, widening the step-down or step-up ratio range.  
         [0066]     According to this embodiment, in a bi-directional DC-DC converter that cannot satisfy both step-down and step-up ratios simultaneously, a switching frequency selected during a step-down operation and a switching frequency selected during a step-up operation are set independently to different values. A resulting effect is that the step-down and step-up ratios can be set in a wide range. Another effect is that since one more design parameter is used in a design of a bi-directional DC-DC converter, the design can be completed more quickly.  
       Second Embodiment  
       [0067]      FIG. 6  shows the entire structure of a bi-directional DC-DC converter according to a second embodiment of the present invention. The functional parts in  FIG. 6  that are identical to the corresponding ones in  FIG. 1  are assigned the same reference numerals to eliminate duplicate description.  FIG. 6  differs from  FIG. 1  in that a switching circuit  13  that switches between the switching frequencies fsw 1  and fsw 2  is provided. A switching frequency setting means  14  sets the switching frequency fsw 1  according to an fsw 1  switching signal  16  from the switching circuit  13 . A switching frequency setting means  15  sets the switching frequency fsw 2  according to an fsw 2  switching signal  17  from the switching circuit  13 . The switching circuit  13  is structured so that it receives the step-down/step-up control switching signal  12  sent from the high-end controller  11 , a voltage signal  18  from the high-voltage DC power supply HV, and a voltage signal  19  from the low-voltage DC power supply LV. This completes the description of the structure of the bi-directional DC-DC converter  20 .  
         [0068]     The basic operation in the second embodiment is similar to the one in the first embodiment in  FIG. 1 . Operations different from  FIG. 1  will be described below. In  FIG. 1 , the switching frequencies fsw 1  and fsw 2  cannot be changed during operation; in  FIG. 6 , however, they can be changed. Specifically, the switching frequency fsw 1 /fsw 2  switching circuit  13  calculates a step-down or step-up ratio at that time from the voltage  18  of the high-voltage DC power supply HV and the voltage  19  of the low-voltage DC power supply LV. The switching circuit  13  can generate switching signals  16  and  17  for setting the required switching frequency fsw 1  and fsw 2  and send them to the switching frequency setting means  14  and  15 .  
         [0069]     The switching frequency fsw 1 /fsw 2  switching circuit  13  receives the step-down/step-up control switching signal  12  supplied from the high-end controller  11 . The switching circuit  13  can thus switch between calculation for generating fsw 1  and another calculation for generating fsw 2 .  
         [0070]     If the switching frequency fsw 1 /fsw 2  switching circuit  13  includes an independent calculation circuit for generating fsw 1  and fsw 2 , the absence of the step-down/step-up control switching signal  12  causes no operational problem. If the step-down/step-up control switching signal  12  is input externally, there is no need to provide an independent calculation circuit for generating fsw 1  and fsw 2  in the switching circuit  13 , providing an effect of structuring the switching circuit  13  with less hardware.  
         [0071]     According to the second embodiment, the switching frequencies fsw 1  and fsw 2  can be changed during a DC-DC converter operation according to the voltages of the high-voltage DC power supply HV and low-voltage DC power supply LV, thereby enabling a bi-directional DC-DC converter that widens the step-down and step-up ratio ranges to be obtained.  
       Third Embodiment  
       [0072]      FIG. 7  shows the entire structure of a bi-directional DC-DC converter according to a third embodiment of the present invention. The functional parts in  FIG. 7  that are identical to the corresponding ones in  FIG. 1  are assigned the same reference numerals to eliminate duplicate description.  FIG. 7  differs from  FIG. 1  in that the structure in  FIG. 6  is further modified; an operation switching circuit  22  is provided, which receives a control signal  21  from the high-end controller  11  and switches the operation of the DC-DC converter  23 .  
         [0073]     The control signal  21  from the high-end controller  11  includes a command for indicating a step-down or step-up operation and frequency setting information about the switching frequency fsw 1  during the step-down operation and the switching frequency fsw 2  during the step-up operation. The operation switching circuit  22  generates a step-down/step-up control switching signal  12  according to the control signal  21  from the high-end controller  11 , and also generates switching signals  16  and  17  to be respectively sent to the switching frequency setting means  14  and  15 .  
         [0074]     According to the third embodiment, a bi-directional DC-DC converter can be operated according to a command from a high-end controller  11 . The high-end controller  11  monitors the states of a high-voltage DC power supply HV and low-voltage DC power supply LV and controls an entire system in which the DC-DC converter  23  is mounted, so the high-end controller  11  can command the DC-DC converter to perform an optimum operation according to the state.  
       Fourth Embodiment  
       [0075]      FIG. 8  shows the entire structure of a bi-directional DC-DC converter according to a fourth embodiment of the present invention. The functional parts in  FIG. 8  that are identical to the corresponding ones in  FIG. 1  are assigned the same reference numerals to eliminate duplicate description.  FIG. 8  differs from  FIG. 1  in that the structure in  FIG. 7  is further modified; an operation switching circuit  24  is structured so that it can make a switchover for the DC-DC converter  25  at its discretion, without receiving an external command. Specifically, the operation switching circuit  24  respectively receives voltages  18  and  19  from the high-voltage DC power supply HV and low-voltage DC power supply LV, selects an operation mode in which the DC-DC converter  25  should operate according to the voltage values, and outputs a step-down/step-up control switching signal  12 . The operation switching circuit  24  also generates switching signals  16  and  17  to be respectively sent to the switching frequency setting means  14  and  15 . When, for example, the voltage of the high-voltage DC power supply HV rises to or above a prescribed voltage and the voltage of the low-voltage DC power supply LV falls to or below a prescribed voltage, the operation switching circuit  24  sends a step-down control signal as the step-down/step-up control switching signal  12 , and sends a switching frequency fsw 1  switching signal suitable for the HV and LV voltages. When the HV voltage is equal to or below the prescribed voltage and the LV voltage is equal to or above the prescribed voltage, the operation switching circuit  24  sends a step-up signal as the step-down/step-up control switching signal  12 , and sends a switching frequency fsw 2  switching signal suitable for the HV and LV voltages.  
         [0076]     According to the fourth embodiment, the DC-DC converter  25  can perform control by itself according to the values of the voltages of the high-voltage DC power supply HV and low-voltage DC power supply LV, even when there is no signal from a high-end system.  
       Fifth Embodiment  
       [0077]      FIG. 9  shows the entire structure of a bi-directional DC-DC converter according to a fifth embodiment of the present invention. The functional parts in  FIG. 9  that are identical to the corresponding ones in  FIG. 1  are assigned the same reference numerals to eliminate duplicate description.  FIG. 9  differs from  FIG. 1  in that the structure in  FIG. 6  is further modified; the bi-directional DC-DC converter further comprises a battery controller  26  for monitoring and controlling the state of the battery in the high-voltage DC power supply HV and a battery controller  27  for monitoring and controlling the state of the battery in the low-voltage DC power supply LV. A signal line  29 , which includes information about the HV voltage and current and the like, connects the high-voltage DC power supply HV to the battery controller  26 . An operation selecting circuit  28  receives a state signal  31  concerning the HV from the battery controller  26 . Similarly, a signal line  30  connects the LV to the battery controller  27 , and the battery controller  27  inputs a state signal  32  concerning the LV into the operation selecting circuit  28 . The operation selecting circuit  28  thus switches between step-down control and step-up control of the bi-directional DC-DC converter  33 , according to the states of the batteries of the high-voltage DC power supply HV and low-voltage DC power supply LV respectively sent from the battery controllers  26  and  27 . That is, the operation selecting circuit  28  receives the HV state signal  31  from the battery controller  26  and the LV state signal  32  from the batter controller  27 , and outputs the step-down/step-up control signal  12 , fsw 1  switching signal  16 , and fsw 2  switching signal  17 .  
         [0078]     According to the fifth embodiment, the battery controllers  26  and  27 , which monitor the states of the HV and LV batteries, enables precise switching between step-down control and step-up control and precise setting of the switching frequencies fsw 1  and fsw 2 . Since signals can be received from battery controllers specific to battery state monitoring, processing for battery state confirmation does not need to be performed in the operation selecting circuit  28 , providing an effect of reducing the size of the operation selecting circuit  28 .  
       Sixth Embodiment  
       [0079]      FIG. 10  shows the entire structure of a bi-directional DC-DC converter according to a sixth embodiment of the present invention. The functional parts in  FIG. 10  that are identical to the corresponding ones in  FIG. 1  are assigned the same reference numerals to eliminate duplicate description.  FIG. 10  differs from  FIG. 9  in that a switching frequency switching means  34  is provided as a modified part. Other parts not shown are structured as shown in  FIG. 9 . The switching means  34  outputs a clock frequency switching signal  35  used to set frequencies for control signals generated by the step-down control circuit  4  and step-up control circuit  5 . A clock frequency switching signal  36  is used to set the frequency of the clock signal  35 . Reference numeral  37  indicates a bi-directional DC-DC converter. The step-down/step-up control switching signal  12  and clock frequency switching signal  36  are generated as illustrated in FIGS.  6  to  9 .  
         [0080]     During the step-down operation, the step-down/step-up control switching signal  12  commands a voltage drop, so the frequency switching means  34  outputs a clock signal  35  for the step-down operation. The step-down control circuit  4  receives the clock signal  35  and outputs a control signal for the step-down operation. The control signal is supplied to the main high-voltage circuit  1  and main low-voltage circuit  2  through the selectors  8  and  9 . In this case, the selectors  8  and  9  select a signal from the step-down control circuit  4  according to the step-down/step-up control switching signal  12 , and output it. The clock signal  35  for step-down control is also supplied to the step-up control circuit  5 , so the step-up control circuit  5  also outputs to the selectors a signal at the same frequency as the signal in the step-down control circuit  4 . However, the selectors  8  and  9  have selected the signals from the step-down control circuit  4 , causing no problem. It is also possible to use the step-down/step-up control switching signal  12  or the like to control the step-up control circuit  5  so that it does not operate.  
         [0081]     During the step-up operation, the step-down/step-up control switching signal  12  commands voltage boosting, so the frequency switching means  34  outputs a clock signal  35  for the step-up operation. The step-up control circuit  5  receives the clock signal  35  and outputs a control signal for the step-up operation. The control signal is supplied to the main high-voltage circuit  1  and main low-voltage circuit  2  through the selectors  8  and  9 . In this case, the selectors  8  and  9  select a signal from the step-up control circuit  5  according to the step-down/step-up control switching signal  12  and output it. The clock signal  35  for step-up control is also supplied to the step-down control circuit  4 , but no problem occurs as in the step-down operation. In the step-up operation as well, it is also possible to use the step-down/step-up control switching signal  12  or the like to control the step-down control circuit  4  so that it does not operate.  
         [0082]     The frequency switching means  34  shown in  FIG. 10  can function during both the step-down operation and step-up operation in a single circuit block, according to the step-down/step-up control switching signal  12  and clock signal  36 . This idea can also be applied to the embodiments in FIGS.  6  to  9 .  
         [0083]     According to the sixth embodiment, there is no need to provide the frequency switching means  34  for each of the step-down and step-up operations, so a switching frequency for step-down control and a switching frequency for step-up control can be set separately with less circuit devices.  
       Seventh Embodiment  
       [0084]      FIG. 11  shows the entire structure of a bi-directional DC-DC converter according to a seventh embodiment of the present invention. The functional parts in  FIG. 11  that are identical to the corresponding ones in  FIG. 10  are assigned the same reference numerals to eliminate duplicate description. Only differences from  FIG. 10  will be described. In  FIG. 11 , reference numerals  38  and  39  each indicate an OR circuit; reference numeral  40  indicates a step-down control circuit with an enable terminal; reference numeral  41  indicates a step-up control circuit with an enable terminal; reference numeral  42  indicates a bi-directional DC-DC converter.  
         [0085]     In the seventh embodiment as well, the step-down/step-up control switching signal  12  and clock frequency switching signals  16  and  17  are generated as illustrated in FIGS.  6  to  9 , so they are not shown.  
         [0086]     During a step-down operation, the step-down/step-up control switching signal  12  commands a voltage drop, so the step-down control circuit  40  operates and the step-up control circuit  41  does not operate. The step-up control circuit  41  is controlled so that when it is not operational, its output signal is low. The OR circuits  38  and  39  each OR the outputs of the step-down control circuit  40  and step-up control circuit  41  and send the resulting signal. Since the output of the step-up control circuit  41  is low, the output of the step-down control circuit  40  is sent to the main high-voltage circuit  1  and main low-voltage circuit  2 . At this time, the switching frequency setting means  14  and  15  respectively supply a clock signal to the step-down control circuit  40  and step-up control circuit  41 , according to the switching signals  16  and  17 .  
         [0087]     During a step-up operation, the step-down/step-up control switching signal  12  commands voltage boosting, so the step-down control circuit  40  does not operate and the step-up control circuit  41  operates. The step-down control circuit  40  is controlled so that when it is not operational, its output signal is low. The OR circuits  38  and  39  each OR the outputs of the step-down control circuit  40  and step-up control circuit  41  and send the resulting signal. Since the output of the step-down control circuit  40  is low, the output of the step-up control circuit  41  is sent to the main high-voltage circuit  1  and main low-voltage circuit  2 . At this time, the switching frequency setting means  14  and  15  respectively supply a clock signal to the step-down control circuit  40  and step-up control circuit  41 , according to the switching signals  16  and  17 .  
         [0088]     According to the seventh embodiment, Enable signals are input to the step-down control circuit  40  and step-up control circuit  41  so that they do not operate actively when they do not need to operate, providing an effect of reducing the power consumption of the control circuits. Of course, it is also possible to reduce the power consumption of the switching frequency setting means  14  and  15  by supplying Enable signals to them so that they stop when they do not need to operate. Furthermore, in the above structure, a circuit for selecting a signal from the step-down control circuit  40  and a signal from the step-up control circuit  41  can be implemented as a simple OR circuit.  
       Eighth Embodiment  
       [0089]      FIG. 12  shows the entire structure of a bi-directional DC-DC converter according to an eighth embodiment of the present invention. The functional parts in  FIG. 12  that are identical to the corresponding ones in  FIG. 1  are assigned the same reference numerals to eliminate duplicate description.  FIG. 12  shows examples of the internal structures of the main high-voltage circuit  1  and main low-voltage circuit  2  in  FIG. 1 .  
         [0090]     First, the structure of the main high-voltage circuit  1  will be described. Connected to the high-voltage DC power supply HV are a smoothing capacitor  43 , a pair of switching devices  44  and  45  connected in series, and another pair of switching devices  46  and  47  connected in series. Freewheel diodes  48  to  51  are respectively connected to the switching devices  44  to  47  in parallel. When the switching devices  44  to  47  are metal-oxide semiconductor field effect transistors (MOSFETs), body diodes can be used.  
         [0091]     During the step-down operation, when the switching devices  44  to  47  are operated, a DC voltage is converted into an AC voltage and the AC voltage is generated on the primary winding  53  of the transformer  3  through an auxiliary reactor  52 . When the polarity of the current flowing in the primary winding  53  of the transformer  3  is inverted, the auxiliary reactor  52  adjusts the current gradient. The auxiliary reactor  52  may be replaced with a leak inductance of the transformer  3 ; in this case, the auxiliary reactor  52  can be eliminated.  
         [0092]     During the step-up operation, the AC voltage generated on the primary winding  53  of the transformer  3  is rectified and converted by diodes  48  to  51  into a DC voltage. The switching devices  44  to  47  may be kept turned on while forward current flows from the anode to the cathode in each of the diodes  48  to  51 , that is, so-called synchronous rectification may be performed.  
         [0093]     Next, the structure of the main low-voltage circuit  2  will be described. In the example in  FIG. 12 , a current-doubler synchronous rectifier is used as the main low-voltage circuit. The current-doubler synchronous rectifier is well-known, as disclosed in, for example, Japanese Patent Laid-open No. 2003-199339. Connected in parallel to the low-voltage DC power supply LV are a smoothing capacitor  61 , a pair of a reactor  59  and switching device  56  connected in series, and another pair of a reactor  60  and switching device  55  connected in series; the smoothing capacitor  61  and the reactor  60  and switching device  55  pairs are connected in parallel. Freewheel diodes  58  and  57  are respectively connected to the switching devices  56  and  55  in parallel. When the switching devices  56  and  55  are MOSFETs, body diodes can be used.  
         [0094]     During the step-down operation, the main low-voltage circuit  2  configured as the current-doubler circuit rectifies the AC voltage generated on the transformer  3  by using the diodes  57  and  58 . The reactors  59  and  60  and the capacitor  61  smooth the rectified voltage to obtain a DC voltage LV. The switching devices  55  and  56  may be kept turned on while forward current flows from the anode to the cathode in each of the diodes  57  and  58 , that is, so-called synchronous rectification may be performed.  
         [0095]     During the step-up operation, the switching devices  55  and  56  are turned on alternately to convert the DC voltage LV to an AC voltage and generate the AC voltage on the secondary winding  54  of the transformer  3 . The generated AC voltage is converted according to the turns ratio of the transformer  3 , and then rectified into a DC voltage by the main high-voltage circuit  1 , resulting in a high DC voltage.  
         [0096]     In the example in the eighth embodiment, MOSFETs are used as the switching devices, but switching devices such as insulated gate bipolar transistors (IGBTs) may be used without problems.  
         [0097]      FIG. 13  shows an example of timing charts when the step-down operation is performed in  FIG. 12 . The gate signals of the switching devices  44  to  47 ,  55 , and  56  are indicated by A to F.  
         [0098]     The gate signals A and B have a period during which they are kept low concurrently so that both switching devices  44  and  45  are not turned on concurrently. Similarly, the gate signals C and D have a period during which they are kept low concurrently so that both switching devices  46  and  47  are not turned on concurrently. In this case, A and C are controlled in such a way that they are shifted from each other. While both A and D are on and both B and C are on, a voltage is generated on the primary winding of the transformer  3  and electric power is supplied to the low-voltage side through the transformer  3 . The switching devices  55  and  56  on the low-voltage side perform synchronous rectification according to the control signals E and F shown in  FIG. 13  so that the AC voltage generated on the secondary winding of the transformer  3  is rectified. The switching frequency at that time is 1/T 1 . The switching frequency setting means  6  enables a switching frequency suitable for the step-down operation to be set without being affected by the step-up operation.  
         [0099]      FIG. 14  shows examples of timing charts when the step-up operation is performed in  FIG. 12 . In this example, the AC voltage generated on the primary winding of the transformer  3  is rectified by the diodes  48  to  51  with A to D turned off. The control signals E and F used to control the switching devices  55  and  56  on the low-voltage side are switched alternately as shown in  FIG. 14  so as to generate an AC voltage on the secondary winding  54  of the transformer  3  and supply electric power to the high-voltage side. The switching frequency at that time is 1/T 2 . The switching frequency setting means  7  enables a switching frequency suitable for the step-up operation to be set without being affected by the step-down operation.  
       Ninth Embodiment  
       [0100]      FIG. 15  shows the entire structure of a bi-directional DC-DC converter according to a ninth embodiment of the present invention. The functional parts in  FIG. 15  that are identical to the corresponding ones in  FIG. 12  are assigned the same reference numerals to eliminate duplicate description.  FIG. 15  differs from  FIG. 12  in that the secondary winding of the transformer  62  has a center tap, at which the winding is divided into segments  63  and  64 . Accordingly, the main low-voltage circuit is changed to a structure indicated by reference numeral  70 . The main low-voltage circuit  70  comprises a reactor  65 , switching devices  66  and  67 , and diodes  68  and  69  connected in parallel to these switching devices. When the switching devices  66  and  67  are metal-oxide MOSFETs, body diodes can be used as the diodes  68  and  69 .  
         [0101]     The operation of the main circuit  70  having a center tap is well known through, for example, documents, so its detailed description will be omitted. Timing charts for controlling the embodiment in  FIG. 15  indicate operations similar to those in  FIGS. 13 and 14 .  
         [0102]     Although exemplary circuits that practice embodiments  8  and  9  of the present invention were shown in  FIGS. 12 and 15  in detail, it would be appreciated that the main high-voltage circuit and main low-voltage circuit are not limited to the circuits shown in these drawings, but any circuits that can operate as both an inverter and a rectifier can be used.  
       Tenth Embodiment  
       [0103]      FIG. 16  shows the entire structure of a bi-directional DC-DC converter according to a tenth embodiment of the present invention. The functional parts in  FIG. 16  that are identical to the corresponding ones in  FIG. 12  are assigned the same reference numerals to eliminate duplicate description. The bi-directional DC-DC converter  78  in the tenth embodiment is structured so that the transformer turns ratios are changed by switches  76  and  77  between the step-down operation and the step-up operation. The primary winding of the transformer  72  is divided into segments  73  and  74 . The secondary winding is indicated by reference numeral  75 .  
         [0104]     During the step-down operation, the switch  76  is turned on and the switch  77  is turned off so that only the segment  73  of the primary winding is used to reduce the turns ratio (N 1 ) of the transformer  72 . During the step-up operation, the switch  76  is turned off and the switch  77  is turned on so that the segments  73  and  74  of the primary winding are connected in series to increase the turns ratio (N 2 ) of the transformer  72 . Since the turns ratio of the transformer  72  is changed between the step-down operation and the step-up operation as described above, the step-down ratio and step-up ratio can be set to values optimal to the respective operations. In the tenth embodiment, the step-down control circuit  4  and step-up control circuit  5  are operated according to signals generated by the switching frequency setting means  6 , so the switching frequencies during the step-down operation and the step-up operation are the same. Therefore, the transformer  72  is used to make a switchover between the step-down ratio and the step-up ratio. The operations in the tenth embodiment are the same as in the embodiment shown in  FIG. 1  except that the turns ratios of the primary transformer are changed.  
         [0105]     According to the tenth embodiment, the step-down ratio and step-up ratio can be changed to desired value by operating switches such as relays. When the converter is mounted on a vehicle, relays and other switches may cause incorrect contacts due to vibration, bi-directional DC-DC converters as described so far are considered to be more preferable.  
         [0106]     It would be understood that with a switching frequency setting means for the step-down control circuit  4  and another switching frequency setting means for the step-up control circuit  5  provided independently as shown in  FIG. 1 , a means for setting switching frequencies optimal for the step-down operation and step-up operation can be provided together.  
       Eleventh Embodiment  
       [0107]      FIG. 17  shows the entire structure of a bi-directional DC-DC converter according to an eleventh embodiment of the present invention. The functional parts in  FIG. 17  that are identical to the corresponding ones in  FIG. 16  are assigned the same reference numerals to eliminate duplicate description. The bi-directional DC-DC converter  85  in the eleventh embodiment is also structured so that the transformer turns ratios are switched between the step-down operation and the step-up operation.  FIG. 17  differs from  FIG. 16  in that the turns ratios are switched by switches  83  and  84  between the step-down operation and the step-up operation on the secondary winding side of the transformer  79 . The primary winding  80  of the transformer  79  is divided into segments  81  and  82 . Reference numeral  83  and  84  indicates switches, and reference numeral  85  indicates a bi-directional DC-DC converter.  
         [0108]     During the step-down operation, the switch  83  is turned off and the switch  84  is turned on so that the segments  81  and  82  of the secondary winding are connected in series to decrease the turns ratio (N 1 ). During the step-up operation, the switch  83  is turned on and the switch  84  is turned off so that only the segment  81  of the secondary winding is used to increase the turns ratio (N 2 ) of the transformer  79 . This type of operation provides an effect similar to that in the tenth embodiment shown in  FIG. 16 .  
       Twelfth Embodiment  
       [0109]      FIG. 18  shows the entire structure of a bi-directional DC-DC converter according to a twelfth embodiment of the present invention. The functional parts in  FIG. 18  that are identical to the corresponding ones in  FIG. 12  are assigned the same reference numerals to eliminate duplicate description. In the twelfth embodiment, the structure of the main circuit in  FIG. 12  is used as the base, and the taps of the transformer are selectively used to switch reactor values and transformer turns ratios between the step-down operation and the step-up operation. During the step-down operation, the switch  136  is turned off and the switch  137  is turned on so that the auxiliary reactor  135  and primary winding  132  are operated effectively. During the step-up operation, the switch  136  is turned on and the switch  137  is turned off so that the auxiliary reactor  134  and the primary windings  131  and  132  are operated effectively. Accordingly, the auxiliary reactor value during the step-up operation is made small and the transformer turns ratios are made large, relative to the step-down operation. The reason why a small auxiliary reactor value is set during the step-up operation is that due to a voltage drop caused by the auxiliary reactor, the voltages generated on the primary windings  131  and  132  are not supplied effectively to the high-voltage DC power supply HV.  
       Thirteenth Embodiment  
       [0110]      FIG. 19  shows the entire structure of a bi-directional DC-DC converter according to a thirteenth embodiment of the present invention. The functional parts in  FIG. 19  that are identical to the corresponding ones in the previous drawings are assigned the same reference numerals to eliminate duplicate description. The bi-directional DC-DC converter in the thirteenth embodiment is an example of a non-insulated bi-directional DC-DC converter that does not use a transformer for electric power conversion. Reference numeral  86  indicates a smoothing capacitor on the high-voltage side, reference numerals  87  and  88  indicate switching devices, and reference numerals  89  and  90  indicate diodes. When the switching devices  87  and  88  are MOSFETs, body diodes can be used as the diodes  89  and  90 . Reference numeral  91  indicates a reactor, and reference numeral  92  indicates a smoothing capacitor on the low-voltage side.  
         [0111]     When the switching device  87  is operated during the step-down operation, electric power is sent from the HV side to the LV side. Specifically, when the switching device  87  is turned off, the current flowing in the reactor  91  causes the diode  90  to supply a forward current. At that time, the switch  88  can be turned on to perform synchronous rectification.  
         [0112]     When the switching device  88  is operated during the step-up operation, electric power is sent from the LV side to the HV side. Specifically, when the switching device  88  is turned off, the current flowing in the reactor  91  causes the diode  89  to supply a forward current. At that time, the switch  87  can be turned on to perform synchronous rectification. The bi-directional DC-DC converter is indicated by reference numerals  93 .  
         [0113]      FIG. 20  shows examples of timing charts when the bi-directional DC-DC converter according to the thirteenth embodiment of the present invention in  FIG. 19  performs the step-down operation and step-up operation, assuming that synchronous rectification is performed. The switching frequency cycle during the step-down operation, given as T 1 , and the switching frequency cycle during the step-up operation, given as T 2 , can be controlled independently.  
         [0114]     According to the thirteenth embodiment, if the switching frequency during the step-down operation and the switching frequency during the step-up operation are controlled independently, it is possible in the non-insulated converter as well to set the step-down ratio and step-up ratio in a wide range.  
       Fourteenth Embodiment  
       [0115]      FIG. 21  shows, as a fourteenth embodiment of the present invention, a system structure in which a bi-directional DC-DC converter is applied to a vehicle-mounted hybrid system. Reference numeral  100  indicates an engine; reference numeral  101  indicates a motor/generator for powering and regeneration, which operates as the inverter during powering and operates as the generator during regeneration; reference numerals  102  indicates an inverter/converter, which operates as the inverter during powering and rotates a motor by using electric power of the high-voltage DC power supply HV, and operates as the converter during regeneration and converts the AC voltage generated by the generator and charges the high-voltage DC power supply HV.  
         [0116]     The bi-directional DC-DC converter  103  is disposed between the HV and the LV and performs bi-directional power conversion. An electronic unit  104  is mounted on the vehicle. Battery controllers  105  and  106  respectively control the power of the HV and LV. An electronic control unit ECU  106  functions as a high-end unit that controls the bi-directional DC-DC converter  103 . Specifically, the ECU  106  switches the bi-directional DC-DC converter  103  between the step-down operation and the step-up operation, sends setting information about the switching frequency to the DC-DC converter  103 , and receives the operation state and other information from the DC-DC converter  103 . The battery controllers  105  and  106  and electronic control unit ECU  106  mutually communicate through a network  108  to transmit and receive information.  
         [0117]     The DC-DC converter  103  in the fourteenth embodiment communicates directly with the electronic control unit ECU  106 . However, the DC-DC converter  103  may also use the network  108  to communicate with the electronic control unit ECU  106  and battery controllers  105  and  106 .  
         [0118]     In the fourteenth embodiment, it is assumed that, during the step-down operation, the DC-DC converter  103  functions to supply electric power to the vehicle-mounted electronic unit connected to the LV power supply and that, during the step-up operation, it functions as an emergency unit to start the engine when the voltage of the HV is lowered. However, the present invention is not limited to these applications but can be used to convert electric power between DC voltages. The high-voltage DC power supply and low-voltage power DC power supply described above are assumed to comprise a secondary battery, a capacitor, and other parts.  
         [0119]     The above embodiments of the present invention are effective in bi-directionally converting electric power between a high-voltage DC power supply and a low-voltage DC power supply in a vehicle-mounted system when there is a large difference in voltage between the power supplies and their voltages largely vary during an operation.  
       INDUSTRIAL APPLICABILITY  
       [0120]     The above embodiments have been mainly described about vehicle-mounted applications, but the present invention can also be applied to other applications in which, for example, DC-DC power conversion is necessary in a battery charging/discharging system.