Patent Publication Number: US-2022216790-A1

Title: Dc-dc converter and power supply device

Description:
TECHNICAL FIELD 
     The present invention relates to a DC-DC converter and a power supply device to be used in various electronic devices. 
     BACKGROUND ART 
       FIG. 5  is a circuit block diagram of a conventional power supply device. Power supply device  1  includes batteries  2  and  3 , switching elements  4  and  5  disposed between batteries  2  and  3 , and reactor  6 . 
     In start-up of power supply device  1 , the ratio of an ON time for switching element  4  to an ON time for switching element  5  is determined based of voltages of batteries  2  and  3 , and switching elements  4  and  5  are started to turn on and off. 
     A conventional power supply device similar to power supply device  1  is disclosed in, for example, PTL 1. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent Laid-Open Publication No. 2002-112534 
       
    
     SUMMARY 
     A DC-DC converter includes a first switching element, a second switching element having one end connected to one end of the first switching element at a node, a high-voltage positive end connected to another end of the first switching element, a high-voltage negative end connected to another end of the second switching element, a low-voltage negative end connected to the another end of the second switching element, an inductance element having one end connected to the node, a low-voltage positive end connected to another end of the inductance element, a start-up signal receiver configured to receive a start-up signal, and a controller configured to turn on the first switching element and the second switching element complementarily. The controller is configured to provide a plurality of control cycles subsequently in response to the start-up signal received by the start-up signal receiver, each of the plurality of control cycles consisting of a first ON-period and a second ON-period subsequent to the first ON-period. The controller is configured to turn on one of the first switching element and the second switching element and turn off another of the first switching element and the second switching element for the first ON-period. The controller is configured to turn off the one of the first switching element and the second switching element and turn on the another of the first switching element and the second switching element for the second ON-period. The controller is configured to determine a length of the first ON-period and a length of the second ON-period based on a high-side voltage between the high-voltage positive end and the high-voltage negative end and a low-side voltage between the low-voltage positive end and the low-voltage negative end. The controller is configured to turn on the one of the first switching element and the second switching element in an initial ON-duration in the first ON-period of a first control cycle among the plurality of control cycles which is firstly provided among the plurality of control cycles, the initial ON-duration having a length shorter than the determined length of the first ON-period, and turn off the first switching element and the second switching element in a duration of the first ON-period of the first control cycle other than the initial ON-duration. The controller is configured to turn on the one of the first switching element and the second switching element and turn off the another of the first switching element and the second switching element for the determined length of the first ON-period of each of one or more control cycles among the plurality of control cycles other than the first control cycle. The controller is configured to turn off the one of the first switching element and the second switching element and turn on the another of the first switching element and the second switching element for the determined length of the second ON-period of each of the plurality of control cycles. 
     This DC-DC converter suppresses unnecessary current and unnecessary power output during at its start-up. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit block diagram of a DC-DC converter according to an exemplary embodiment. 
         FIG. 2  is a circuit block diagram of a vehicle having the DC-DC converter mounted thereto according to the embodiment. 
         FIG. 3  illustrates an operation of the DC-DC converter according to the embodiment. 
         FIG. 4  is a timing chart illustrating another operation of the DC-DC converter according to the embodiment. 
         FIG. 5  is a circuit block diagram of a conventional power supply device. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
       FIG. 1  is a circuit block diagram of a DC-DC converter according to an exemplary embodiment of the present invention. DC-DC converter  11  includes series assembly  14  of switching elements  12  and  13  connected in series to each other, high-voltage positive end  15 , high-voltage negative end  16 , inductance element  17 , low-voltage positive end  18 , low-voltage negative end  19 , start-up signal receiver  20 , and controller  21 . 
     High-voltage positive end  15  is connected to one end  14 A of series assembly  14 . High-voltage negative end  16  is connected to another end  14 B of series assembly  14 . One end of inductance element  17  is connected to node J 1  at which switching element  12  is connected to switching element  13 . Another end of inductance element  17  is connected to low-voltage positive end  18 . Low-voltage negative end  19  is connected to high-voltage negative end  16  or another end  14 B of series assembly  14 . 
     Switching element  12  includes one end  12 A and another end  12 B. Switching element  13  includes one end  13 A and another  13 B. One end  13 A of switching element  13  is connected to one end  12 A of switching element  12  at node J 1 . High-voltage positive end  15  is connected to another end  12 B of switching element  12 . High-voltage negative end  16  is connected to another end  13 B of switching element  13 . Low-voltage negative end  19  is connected to another end  13 B of switching element  13 . Inductance element  17  includes one end  17 A and another end  17 B. One end  17 A of inductance element  17  is connected to node J 1 . Low-voltage positive end  18  is connected to another end  17 B of inductance element  17 . 
     Start-up signal receiver  20  is configured to receive start-up signal SG. Controller  21  is configured to detect start-up signal SG via start-up signal receiver  20 . Controller  21  is configured to detect high-side voltage VH between high-voltage positive end  15  and high-voltage negative end  16 . Controller  21  is configured to detect low-side voltage VL between low-voltage positive end  18  and low-voltage negative end  19 . Controller  21  is configured to determine a length of an ON-period of switching elements  12  and  13  in each of control cycles T repeating subsequently at predetermined time intervals. Controller  21  is configured to determine a duty (the ratio of the ON-period to a length of control cycle T) of switching elements  12  and  13 , thereby controlling an operation of switching elements  12  and  13 . 
     Upon detecting start-up signal SG, controller  21  detects high-side voltage VH to obtain high voltage value VH 1 , and detects low-side voltage VL to obtain low voltage value VL 1 . Controller  21  then determines length LT 1  of ON-period Ton 1  in which switching element  12  is turned on and length LT 2  of ON-period Ton 2  in which switching element  13  is turned on based on high voltage value VH 1  and low voltage value VL 1 . Controller  21  decreases a length of a preceding period of ON-periods Ton 1  and Ton 2  of first control cycle Tf among control cycles T that follows the detection of start-up signal SG. 
     Controller  21  turns on and off switching elements  12  and  13  complementarily. That is, controller  21  turns on switching element  12  and turns off switching element  13  for ON-period Ton 1 . Controller  21  turns off switching element  12  and turns on switching element  13  on for ON-period Ton 2 . 
     The above configuration and operation suppress a steady current flowing averagely in one of switching elements  12  and  13  that has been turned on firstly at the start-up of DC-DC converter  11 . Since the steady current flowing averagely in the switching element that has been turned on first at the start-up of DC-DC converter  11  is suppressed within a short period of time through the simple control, DC-DC converter  11  is controlled substantially in a balanced state in which no current flows in any direction. Thus, undesired power is no supplied to high-voltage positive end  15 , high-voltage negative end  16 , low-voltage positive end  18 , and low-voltage negative end  19 . This suppresses adverse effects, such as degradation of characteristics high-voltage battery  22  connected to high-voltage positive end  15  and high-voltage negative end  16  and degradation of characteristics of low-voltage battery  23  connected low-voltage positive end  18  and low-voltage negative end  19 . 
     In conventional power supply device  1  illustrated in  FIG. 5 , a reference time ratio of the ON time for switching element  4  to the ON time for switching element  5  is determined bases on a ratio of the voltage of battery  2  to the voltage of battery  3 , and then, the ON and OFF operation of switching elements  4  and  5  is started and continued. However, a biased current flows in one of switching elements  4  and  5  that has been turned on first at the start-up of power supply device  1 , flowing as an initial steady current through reactor  6 . 
     Accordingly, in order to reduce the initial steady-state current to zero, the respective ON times of switching elements  4  and  5  are temporarily controlled with a time ratio value different from the determined reference time ratio, which is based on the ratio between the voltages of batteries  2  and  3 . Then, when the average current becomes zero, controlled time ratio switching is needed for using the reference time ratio again. Moreover, unwanted charging and discharging with the average current is involved in every start-up of power supply device  1  and may promote deterioration of connected batteries  2  and  3  of power supply device  1 . 
     A configuration and operation of DC-DC converter  11  will be detailed below.  FIG. 2  is a circuit block diagram of vehicle  26  having DC-DC converter  11  mounted thereto.  FIG. 3  is a timing chart illustrating an operation of DC-DC converter  11 . 
     The timing chart of  FIG. 3  also illustrates an operation of DC-DC converter  500  as a comparative example. At start-up of DC-DC converter  500  of the comparative example, ON-period Ton 1  and ON-period Ton 2  are provided in this order in a firstly-appearing one of control cycles T. In this case, output current Iout decreases by amplitude dI in ON-period Ton 2  from timing t 0  to timing t 2 , and reaches a minimum value of −dI at timing t 2  in firstly-appearing control cycle T. After that, output current Iout increases by amplitude dI in ON-period Ton 1  from timing t 2  to timing t 3 , and reaches a maximum value of zero within first control cycle T at timing t 3 . 
     Since the maximum value is zero mentioned above in DC-DC converter  500  of the comparative example, a steady output current that is an average value of output current Iout over control cycles T continuously has a negative value. DC-DC converter  11  according to the embodiment suppresses the output current, which appears in the start-up of DC-DC converter  500  of the comparative example to a smaller value. 
     Configurations of DC-DC converter  11  and vehicle  26  having DC-DC converter  11  mounted thereto will be described below. DC-DC converter  11  includes series assembly  14  of switching elements  12  connected in series to each other, high-voltage positive end  15 , high-voltage negative end  16 , inductance element  17 , low-voltage positive end  18 , low-voltage negative end  19 , start-up signal receiver  20 , controller  21 , input capacitor  24 , and output capacitor  25 . 
     High-voltage battery  22  has a positive electrode connected to high-voltage positive end  15 , and has a negative electrode connected to high-voltage negative end  16 . Low-voltage battery  23  has a positive electrode connected to low-voltage positive end  18 , and has a negative electrode connected to low-voltage negative end  19 . High-voltage battery  22  has rated voltage VHC while low-voltage battery  23  has rated voltage VLC. Rated voltage VHC is higher than rated voltage VLC. 
     DC-DC converter  11 , high-voltage battery  22 , and low-voltage battery  23  are mounted to body  27  of vehicle  26 . In accordance with the embodiment, low-voltage battery  23  is, e.g. a storage battery configure to power auxiliary device  28  installed in vehicle  26 . High-voltage battery  22  is a storage battery configured to power, e.g. driving load  29  installed in vehicle  26 . DC-DC converter  11  performs a bidirectional operation. In other words, DC-DC converter  11  charges low-voltage battery  23  with power from high-voltage battery  22  in a stepping-down operation. DC-DC converter  11  charges high-voltage battery  22  with power from low-voltage battery  23  in a boosting-up operation. 
     Controller  21  includes a control circuit, a memory, a drive circuit, and a detection circuit. The control circuit, the memory, the drive circuit, and the detection circuit may be unitized to be provided in controller  21 . Alternatively, the control circuit, the memory, the drive circuit, and the detection circuit may be distributed or integrated. 
     High-voltage positive end  15  is connected to one end  14 A of series assembly  14 , and high-voltage negative end  16  is connected to another end  14 B of series assembly  14 . Input capacitor  24  has one end connected to one end  14 A of series assembly  14 , and has another end connected to one end  14 A of the series assembly  14 . Alternatively, the one end of input capacitor  24  is connected to high-voltage positive end  15 , and another end of input capacitor  24  is connected to high-voltage negative end  16 . One end  17 A of inductance element  17  is connected to node J 1  at which switching elements  12  are connected to each other. Another end  17 B of inductance element  17  is connected to low-voltage positive end  18 . Low-voltage negative end  19  is connected to high-voltage negative end  16  or another end  14 B of series assembly  14 . Output capacitor  25  has one end connected to low-voltage positive end  18 , and has another end connected to low-voltage negative end  19 . 
     In the device shown in  FIGS. 1 and 2 , switching element  12  is illustrated as a high-potential-side arm, and switching element  13  is illustrated as a low-potential-side arm. Switching elements  12  and  13  are implemented by field-effect transistors (FETs), and may be implemented by other semiconductor switches, such as insulated-gate bipolar transistors (IGBTs). In the device shown in  FIGS. 1 and 2 , diodes  12 D and  13 D are connected parallel to switching elements  12  and  13 , respectively. A cathode of diode  12 D is connected to a positive electrode side of switching element  12 , and an anode of diode  12 D is connected to a negative electrode side of switching element  12 . A cathode of diode  13 D is connected to a positive electrode side of switching element  13 , and an anode of diode  13 D is connected to a negative electrode side of switching element  13 . In the case that switching elements  12  and  13  include diodes  12 D and  13 D as parasitic diodes, respectively, external diodes  12 D and  13 D may not necessarily be connected. 
     Operations of DC-DC converter  11  and vehicle  26  having DC-DC converter  11  mounted thereto or operations of vehicle  26  having power supply device  32 , including DC-DC converter  11 , high-voltage battery  22 , and low-voltage battery  23 , mounted thereto will be described below. 
     When a driver in vehicle  26  activates start switch  30  to start up vehicle  26 , start switch  30  transmits start-up signal SG to start-up signal receiver  20 . At timing t 0 , controller  21  detects start-up signal SG via start-up signal receiver  20 . Before timing t 0 , switching elements  12  and  13  are both turned off as an initial state. In other words, DC-DC converter  11  is not in operation in before timing t 0 . 
     Upon detecting the start-up signal at timing t 0 , controller  21  detects high-side voltage VH between high-voltage positive end  15  and high-voltage negative end  16 . In other words, controller  21  detects high-side voltage VH which is a voltage of high-voltage battery  22  at timing t 0 . Controller  21  detects low-side voltage VL between low-voltage positive end  18  and low-voltage negative end  19 . In other words, controller  21  detects low-side voltage VL which is a voltage of low-voltage battery  23 . Controller  21  may detect high-side voltage VH and low-side voltage VL at the same time or at different timings. Considering a processing load of controller  21 , controller  21  preferably detect high-side voltage VH and low-side voltage VL at different timings. However, even in that case, a time difference between these timings is generally as short as milliseconds, that is to say, less than or equal to a second. Therefore, there is no problem even if high-side voltage VH and low-side voltage VL are detected at different timings. 
     At timing t 0 , controller  21  generates a timer signal for timing of calculation and timing of control in controller  21 . The timer signal is generated for each control cycle T having predetermined length LT. Length LT of control cycle T is predetermined or preliminary stored in controller  21 . Length LT of control cycle T is determined based on, e.g. a circuit value of inductance element  17  or circuit values of inductance element  17  and output capacitor  25 . 
     Based on of voltage VH of high-voltage battery  22  and voltage VL of low-voltage battery  23  detected by controller  21  at timing t 0 , controller  21  determines, by calculation, length LT 1  of ON-period Ton 1  in which switching element  12  is turned on and length LT 2  of ON-period Ton 2  in which switching element  13  is turned on. Length LT 1  of ON-period Ton 1  and length LT 2  of ON-period Ton 2  are determined to be values that prevent DC-DC converter  11  from charging and discharging both high-voltage battery  22  and low-voltage battery  23 . In other words, bases on voltage VH and voltage VL, controller  21  determines lengths LT 1  and LT 2  of ON-periods Ton 1  and Ton 2 , namely, the duty which is the ratio of length LT 1  of ON-period Ton 1  to length T of control cycle T, or the duty which is the ratio of length LT 2  of ON-period Ton 2  to length T of control cycle T, so that an average value of a current is zero to prevent a current from flowing through inductance element  17 . 
     If detected voltages VH and VL are 50 V and 20 V, respectively, the ratio of length LT 1  of ON-period Ton 1  to length LT 2  of ON-period Ton 2  is about ⅔. In other words, the ratio of length LT 1  of ON-period Ton 1  to length LT of control cycle T is about ⅖. Length LT of control cycle T is the sum of length LT 1  of ON-period Ton 1 , length LT 2  of ON-period Ton 2 , and a length of a dead time. During the dead time, both switching elements  12  and  13  are turned off in order to prevent the current from flowing simultaneously due to a delay in operation or a rise time in reality. This configuration prevents the flow of overcurrent to high-voltage negative end  15  and high-voltage positive end  16  and damage to switching elements  12  and  13 . In actual control, the dead time and circuit losses in DC-DC converter  11  cause the ratio of length LT 1  of ON-period Ton 1  to length LT 2  of ON-period Ton 2  not to be ⅔ exactly. In other words, the ratio of length LT 1  of ON-period Ton 1  to length LT of control cycle T is not set to be ⅖ exactly. In the operation illustrated in the timing chart of  FIG. 3 , for simplified illustration, detected voltages VH and VL are, for example, 24 V and 12V, respectively, and the ratio of length LT 1  of ON-period Ton 1  to length LT 2  of ON-period Ton 2  is set to be 1. Since the dead time is extremely shorter than ON-periods Ton 1  and Ton 2 , the operation is described with the dead time omitted for convenience sake. Controller  21  turns on switching elements  12  and  13  complementarily. That is, controller  21  turns off switching element  13  while turning on switching element  12 . Controller  21  turns off switching element  12  while turning on switching element  13 . Controller  21  turns off both switching elements  12  and  13  during the extremely short dead time. Controller  21  does not turns on switching elements  12  and  13  simultaneously. 
     Upon determining lengths LT 1  and LT 2  of ON-periods Ton 1  and Ton 2 , controller  21  starts the switching operation of switching elements  12  and  13 . Lengths LT 1  and LT 2  of ON-periods Ton 1  and Ton 2  for control cycle T secondly provided and control cycles T subsequent to the secondly provided control cycle T in order to prevent DC-DC converter  11  from charging and discharging both high-voltage battery  22  and low-voltage battery  23 . 
     Control cycle Tf among subsequently-repeating control cycles T which firstly starts from timing t 0  includes ON-period Ton 2  in which switching element  13  as the low-potential-side arm is turned on first. ON-period Ton 2  includes initial ON-duration Ton 2   f  having length LT 2   f  shorter than length LT 2  of ON-period Ton 2  that has been determined by controller  21  by calculation. Initial ON-duration Ton 2   f  of firstly-provided control cycle Tf starts from timing t 1  after timing t 0 . Since the set ratio of length LT 1  of ON-period Ton 1  to length LT 2  of ON-period Ton 2  is 1 in the operation illustrated in  FIG. 3 , initial ON-duration Ton 2   f  or ON-period Ton 2  of firstly-provided control cycle Tf ends at timing t 2 , that is, at a half of control cycle Tf or control cycle T. Then, for ON-period Ton 1  from timing t 3  to timing t 3 , controller  21  turns on switching element  12  and turns off switching element  13 . 
     As described above, initial ON-duration Ton 2   f  starts from timing t 1  that is after timing t 0 . Therefore, while switching element  13  is turned on and switching element  12  is turned off, output current Iout flows through inductance element  17  in direction Dr shown in  FIG. 3 . However, since initial ON-duration Ton 2   f  of control cycle Tf is shorter than ON-period Ton 2 , output current Iout reaches a minimum value −dI/2 at timing t 2  in firstly-provided control cycle Tf. The current with a negative value shown in the timing chart of  FIG. 3  flows in direction Dr through inductance element  17  shown in  FIG. 1 or 2 . 
     While switching element  12  is turned on and switching element  13  is turned off in ON-period Ton 1  from timing t 2  to timing t 3 , output current Iout flows in direction Df. The output current increases by amplitude dI in ON-period Ton 1  and reaches a maximum value dI/2 at timing t 3  in firstly-provided control cycle Tf. 
     For secondly-provided control cycle T from timing t 3  to timing t 5  and each of subsequent control cycles T, controller  21  provides ON-periods Ton 1  and Ton 2  with predetermined lengths LT 1  and LT 2 , respectively. The ratio of length LT 1  of ON-period Ton 1  to length LT 2  of ON-period Ton 2  is set to be 1 as mentioned above. Therefore, in ON-period Ton 2  from timing t 3  to timing t 4 , switching element  13  is turned on, and switching element  12  is turned off. In ON-period Ton 1  from timing t 4  to timing t 5 , switching element  12  is turned on, and switching element  13  is turned off. 
     In ON-period Ton 2  from timing t 3  to timing t 4 , the output current decreases by amplitude dI and reaches the minimum value −dI/2 at timing t 4  in secondly-provided control cycle T. In ON-period Ton 1  from timing t 4  to timing t 5 , output current Tout increases by amplitude dI and reaches the maximum value dI/2 at timing t 5  in secondly-provided control cycle T. 
     Since controller  21  sets length LT 2   f  of initial ON-duration Ton 2   f  in firstly-provided control cycle Tf shorter than length LT 2  of ON-period Ton 2  that has been determined based on high voltage value VH 1  and low voltage value VL 1  by calculation, output current Tout reverses its polarity in every half of control cycle T with the maximum value dI/2 and the minimum value −dI/2 which have the same absolute value in control cycle T. This configuration drastically decreases a steady output current which is average value lay of output current Tout to about zero over single control cycle T or plural control cycles T. Moreover, a time for the output current to become the extremely-small steady current of about zero is about control cycle T. Thus, the steady output current becomes extremely small within an extremely short time by such a simple control. 
     The steady output current flowing toward one of switching elements  12  and  13  that is turned on first in the start-up of DC-DC converter  11  is suppressed by the simple control, and output current Tout is controlled nearly in the equilibrium state of flowing on average in neither of directions Dr and Df. As a result, the power supply associated with unwanted charging and discharging is minimized. This configuration suppresses the adverse effects, such as the characteristic degradation of high-voltage battery  22  and the characteristic degradation of low-voltage battery  23 . 
     Length LT 2   f  of initial ON-duration Ton 2   f  in firstly-provided control cycle Tf is preferably short, about a half of length LT 2  of ON-period Ton 2  that controller  21  has determined based on high voltage value VH 1  and low voltage value VL 1 . The steady output current which is average value lay of output current Tout becomes extremely small, about zero, over single control cycle T or plural control cycles T. 
     In the above-described example, length LT 2   f  of initial ON-duration Ton 2   f  in firstly-provided control cycle Tf is a half of length LT 2  of ON-period Ton 2 , so that output current Iout flows through inductance element  17  in direction Dr at timing t 2  and takes the minimum value −dI/2 in first control cycle Tf, thus providing a desired steady output current. However, length LT 2   f  of initial ON-duration Ton 2   f  in firstly-provided control cycle Tf is not limited to the half of length LT 2  of ON-period Ton 2 . Length LT 2   f  shorter than length LT 2  also reduces the steady output current. 
     In the above description, since initial ON-duration Ton 2   f  of firstly-provided control cycle Tf is shorter than ON-period Ton 2  with length LT 2  that controller  21  has determined by the calculation based on high voltage value VH 1  and low voltage value VL 1 , output current Iout reverses its polarity with the maximum value and the minimum value having different polarities in control cycle Tf, thus suppressing the steady output current. In other words, the length of firstly-provided control cycle Tf is equal to lengths of secondly and subsequently provided control cycles T. Firstly-provided control cycle Tf includes ON-period Ton 1 , initial ON-duration Ton 2   f , and idle duration Ts that is from timing t 0  to timing t 1 . Controller  21  turns off switching elements  12  and  13  in idle duration Ts. Although controller  21  turns on switching elements  12  and  13  on complementarily, controller  21  turns off switching elements  12  and  13  in idle period Ts in addition to the dead time. 
     Firstly-provided control cycle Tf may be shorter than secondly and subsequently-provided control cycles T, for example, by having a shorter length of idle duration Ts from timing t 0  to timing t 1  or no idle duration Ts. Firstly-provided control cycle Tf may include only ON-period Ton 1  and initial ON-duration Ton 2   f.    
     In the above description of the operation of DC-DC converter  11 , an operation of switching elements  12  and  13  particularly in firstly-provided control cycle Tf and an operation of switching elements  12  and  13  in secondly and subsequently-provided control cycles T. In the above description, the determination of lengths LT 1  and LT 2  of ON-periods Ton 1  and Ton 2  of DC-DC converter  11  is based on voltage VH of high-voltage battery  22  and voltage VL of low-voltage battery  23  that have been detected at timing t 0 . An operation of DC-DC converter  11  in secondly and subsequently-provided control cycles T in that case that reference voltage VHR for high-voltage battery  22  and reference voltage VLR for low-voltage battery  23  are different from at least one of detected voltage VH of high-voltage battery  22  and detected voltage VL of low-voltage battery  23  at timing t 0 . As long as the operation to be described here is performed in secondly and subsequently-provided control cycles T, the operation may be performed in thirdly-provided control cycle T and later control cycles T. 
     As described above, controller  21  detects voltage VH of high-voltage battery  22  and voltage VL of low-voltage battery  23  at timing t 0 . Controller  21  compares with detected voltage VH of high-voltage battery  22 . Reference voltage VHR is preset or previously stored in controller  21  for high-voltage battery  22 . Controller  21  also compares reference voltage VLR with detected voltage VL of low-voltage battery  23 . Reference voltage VLR is preset or previously stored in controller  21  for low-voltage battery  23 . The comparison of these voltages is performed at timing t 0 . Alternatively, the comparison of these voltages may be performed at any timing in control cycle Tf from timing t 0  to timing t 3 . 
     If voltage VH of high-voltage battery  22  is different from reference voltage VHR for high-voltage battery  22  or deviates from reference voltage VHR for high-voltage battery  22  by a difference larger than a predetermined value, DC-DC converter  11  charges or discharges high-voltage battery  22  in secondly and subsequently-provided control cycles T to adjust voltage VH of high-voltage battery  22  and voltage VL of low-voltage battery  23  to reference voltages VHR and VLR, respectively. 
     When, for example, voltage VH is lower than reference voltage VHR, DC-DC converter  11  performs a boost-up operation of charging high-voltage battery  22  with power from low-voltage battery  23 . When voltage VH is higher than reference voltage VHR, DC-DC converter  11  performs a stepping-down operation of discharging power from high-voltage battery  22  to charge low-voltage battery  23 . In an alternative stepping down operation that DC-DC converter  11  may perform when voltage VH is higher than reference voltage VHR, power of high-voltage battery  22  is discharged to auxiliary device  28 . 
     On the other hand, if voltage VL of low-voltage battery  23  is different from reference voltage VLR for low-voltage battery  23  or deviates from reference voltage VLR for low-voltage battery  23  by a difference larger than a predetermined value, DC-DC converter  11  charges or discharges low-voltage battery  23  in secondly and subsequently-provided control cycles T. Voltage VH of high-voltage battery  22  and voltage VL of low-voltage battery  23  are thus adjusted to reference voltages VHR and VLR, respectively. 
     When, for example, voltage VL is lower than reference voltage VLR, DC-DC converter  11  performs a stepping-down operation of charging low-voltage battery  23  with power from high-voltage battery  22 . When voltage VL is higher than reference voltage VLR, DC-DC converter  11  performs a boost-up operation of discharging power from low-voltage battery  23  to charge high-voltage battery  22 . In an alternative boosting-up operation that DC-DC converter  11  may perform when voltage VL is higher than reference voltage VLR, power of low-voltage battery  23  is discharged to driving load  29 . 
     The above operations are just examples. When voltage VH of high-voltage battery  22  is lower than reference voltage VHR for high-voltage battery  22 , and voltage VL of low-voltage battery  23  is lower than reference voltage VLR for low-voltage battery  23 , controller  21  may cause power generation circuit  31  connected to low-voltage battery  23  to charge low-voltage battery  23  in secondly and subsequently-provided control cycles T so as to increase voltage VL of low-voltage battery  23  up to reference voltage VLR; meanwhile, DC-DC converter  11  may perform a boost-up operation of charging high-voltage battery  22  with power of power generation circuit  31  so as to increase voltage VH of high-voltage battery  22  up to reference voltage VHR. 
     The power supply from power generation circuit  31  to low-voltage battery  23  here is controlled by a vehicle controller. Alternatively, the power supply from power generation circuit  31  to low-voltage battery  23  may be controlled by controller  21 . Power generation circuit  31  may include a power generator. 
     Since high-voltage battery  22  and low-voltage battery  23  that are mounted to vehicle  26  have large capacities, the steady output current may often have a large value suddenly changing. Accordingly, the steady output current is suppressed only in firstly-provided control cycle T or a limited number of control cycles T, and then, the control of the voltage to adjust it to an appropriate value is performed in subsequently-provided control cycles T. This configuration prevents sudden current inflow and significantly suppresses the adverse effects, such as degradation of characteristics of high-voltage battery  22  and low-voltage battery  23 . 
     As mentioned earlier, in  FIG. 1  and  FIG. 2 , switching element  12  and switching element  13  are illustrated as the high-potential-side arm and the low-potential-side arm, respectively. As described, the switching operation starts with turning on the low-potential-side arm, which is switching element  13 , first in ON-period Ton 2 . In the operation that starts with switching element  13 , switching element  13  starts operating with a negative potential applied between a drain and a source of the switching element, especially in the case that switching elements  12  and  13  are implemented by FETs. This configuration allows the switching operation of switching element  13  and subsequent switching operation of switching element  12  to be controlled respectively by low-voltage-value control signals. 
     The switching operation may start with turning on switching element  12 , which is the high-potential-side arm, if a bootstrap circuit is provided to supply a high-voltage control signal to switching element  12 .  FIG. 4  is a timing chart illustrating an operation of DC-DC converter  11  in this case. In  FIG. 4 , items identical to those of  FIG. 3  are denoted by the same reference numerals. In the operation illustrated in  FIG. 4 , switching element  12  is turned on, and switching element  13  is turned off for ON-period Ton 2  of control cycle T. Switching element  12  is turned off, and switching element  13  is turned on for ON-period Ton 1 . Therefore, for the cycle firstly provided, namely, for firstly-provided control cycle Tf, switching element  12  is turned on, and switching element  13  is turned off in initial ON-duration Ton 2   f . Then, switching element  12  is turned off, and switching element  13  is turned on in ON-period Ton 1 . Output current Iout has polarity reversed to output current Iout illustrated in  FIG. 3 . The operation illustrated in  FIG. 4  allows average value Iav of output current Iout to become extremely small in value, that is, about zero similarly to the operation illustrated in  FIG. 3 . 
     High-voltage battery  22  may be a lead-acid battery, a lithium-ion battery, an electrical double-layer capacitor, or a lithium-ion capacitor. Similarly to high-voltage battery  22 , low-voltage battery  23  may be a lead-acid battery, a lithium-ion battery, an electrical double-layer capacitor, or a lithium-ion capacitor. 
     As described above, controller  21  is configured to provide control cycles T subsequently repeated in response to start-up signal SG received by start-up signal receiver  20 . Each of control cycles T includes ON-period Ton 2  and ON-period Ton 1  subsequent to ON-period Ton 2 . For ON-period Ton 2 , controller  21  turns on one of switching elements  12  and  13  and turns off another of switching elements  12  and  13 . For ON-period Ton 1 , controller  21  turns off the one of switching elements  12  and  13  and turns on another of switching elements  12  and  13 . Controller  21  is configured to determine length LT 2  of ON-period Ton 2  and length LT 1  of ON-period Ton 1  based on high-side voltage VH between high-voltage positive end  15  and high-voltage negative end  16  and low-side voltage VL between low-voltage positive end  18  and low-voltage negative end  19 . Controller  21  turns on the one of switching elements  12  and  13  in initial ON-duration Ton 2   f  included in ON-period Ton 2  of firstly-provided control cycle Tf among control cycles T. Initial ON-duration Ton 2   f  is shorter than determined length LT 2  of ON-period Ton 2 . Controller  21  turns off switching element  12  and switching element  13  in a duration (idle duration Ts) in ON-period Ton 2  of firstly-provided control cycle Tf other than the initial ON-duration. Controller  21  turns on the one of switching elements  12  and  13  and turns off another of switching elements  12  and  13  throughout determined length LT 2  of ON-period Ton 2  of one or more control cycles T other than firstly-provided control cycle Tf. Controller  21  turns off the one of switching elements  12  and  13  and turns on another of switching elements  12  and  13  throughout determined length LT 1  of ON-period Ton 1  of each of control cycles T. 
     Length Ton 2   f  of initial ON-duration Ton 2   f  may be a half of determined length LT 2  of ON-period Ton 2 . 
     In the first control cycle Tf, ON-period Ton 1  may be subsequent to initial ON-duration Ton 2   f.    
     The one of switching elements  12  and  13  is switching element  12 , and another of switching elements  12  and  13  is switching element  13 . 
     Alternatively, the one of switching elements  12  and  13  is switching element  13 , and another of switching elements  12  and  13  is switching element  12 . 
     ON-period Ton 2  of first control cycle Tf has a length shorter than determined length LT 2  of ON-period Ton 2 . 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           11  DC-DC converter 
           12  switching element (first switching element) 
           13  switching element (second switching element) 
           14  series assembly 
           15  high-voltage positive end 
           16  high-voltage negative end 
           17  inductance element 
           18  low-voltage positive end 
           19  low-voltage negative end 
           20  start-up signal receiver 
           21  controller 
           22  high-voltage battery 
           23  low-voltage battery 
           24  input capacitor 
           25  output capacitor 
           26  vehicle 
           27  body 
           28  auxiliary device 
           29  power load to be driven 
           30  start switch 
           31  power generation circuit 
           32  power supply device 
         T control cycle 
         Ton 1  ON-period (second ON-period) 
         Ton 2  ON-period (first ON-period) 
         Ton 2   f  initial ON-duration