Patent ID: 12206328

DESCRIPTION OF EMBODIMENT

FIG.1is a circuit block diagram of a DC-DC converter11according to an exemplary embodiment of the present invention. DC-DC converter11includes series assembly14of switching elements12and13connected in series to each other, high-voltage positive end15, high-voltage negative end16, inductance element17, low-voltage positive end18, low-voltage negative end19, start-up signal receiver20, and controller21.

High-voltage positive end15is connected to one end14A of series assembly14. High-voltage negative end16is connected to another end14B of series assembly14. One end of inductance element17is connected to node J1at which switching element12is connected to switching element13. Another end of inductance element17is connected to low-voltage positive end18. Low-voltage negative end19is connected to high-voltage negative end16or another end14B of series assembly14.

Switching element12includes one end12A and another end12B. Switching element13includes one end13A and another13B. One end13A of switching element13is connected to one end12A of switching element12at node J1. High-voltage positive end15is connected to another end12B of switching element12. High-voltage negative end16is connected to another end13B of switching element13. Low-voltage negative end19is connected to another end13B of switching element13. Inductance element17includes one end17A and another end17B. One end17A of inductance element17is connected to node J1. Low-voltage positive end18is connected to another end17B of inductance element17.

Start-up signal receiver20is configured to receive start-up signal SG. Controller21is configured to detect start-up signal SG via start-up signal receiver20. Controller21is configured to detect high-side voltage VH between high-voltage positive end15and high-voltage negative end16. Controller21is configured to detect low-side voltage VL between low-voltage positive end18and low-voltage negative end19. Controller21is configured to determine a length of an ON-period of switching elements12and13in each of control cycles T repeating subsequently at predetermined time intervals. Controller21is configured to determine a duty (the ratio of the ON-period to a length of control cycle T) of switching elements12and13, thereby controlling an operation of switching elements12and13.

Upon detecting start-up signal SG, controller21detects high-side voltage VH to obtain high voltage value VH1, and detects low-side voltage VL to obtain low voltage value VL1. Controller21then determines length LT1of ON-period Ton1in which switching element12is turned on and length LT2of ON-period Ton2in which switching element13is turned on based on high voltage value VH1and low voltage value VL1. Controller21decreases a length of a preceding period of ON-periods Ton1and Ton2of first control cycle Tf among control cycles T that follows the detection of start-up signal SG.

Controller21turns on and off switching elements12and13complementarily. That is, controller21turns on switching element12and turns off switching element13for ON-period Ton1. Controller21turns off switching element12and turns on switching element13on for ON-period Ton2.

The above configuration and operation suppress a steady current flowing averagely in one of switching elements12and13that has been turned on firstly at the start-up of DC-DC converter11. Since the steady current flowing averagely in the switching element that has been turned on first at the start-up of DC-DC converter11is suppressed within a short period of time through the simple control, DC-DC converter11is controlled substantially in a balanced state in which no current flows in any direction. Thus, undesired power is no supplied to high-voltage positive end15, high-voltage negative end16, low-voltage positive end18, and low-voltage negative end19. This suppresses adverse effects, such as degradation of characteristics high-voltage battery22connected to high-voltage positive end15and high-voltage negative end16and degradation of characteristics of low-voltage battery23connected low-voltage positive end18and low-voltage negative end19.

In conventional power supply device1illustrated inFIG.5, a reference time ratio of the ON time for switching element4to the ON time for switching element5is determined bases on a ratio of the voltage of battery2to the voltage of battery3, and then, the ON and OFF operation of switching elements4and5is started and continued. However, a biased current flows in one of switching elements4and5that has been turned on first at the start-up of power supply device1, flowing as an initial steady current through reactor6.

Accordingly, in order to reduce the initial steady-state current to zero, the respective ON times of switching elements4and5are temporarily controlled with a time ratio value different from the determined reference time ratio, which is based on the ratio between the voltages of batteries2and3. 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 device1and may promote deterioration of connected batteries2and3of power supply device1.

A configuration and operation of DC-DC converter11will be detailed below.FIG.2is a circuit block diagram of vehicle26having DC-DC converter11mounted thereto.FIG.3is a timing chart illustrating an operation of DC-DC converter11.

The timing chart ofFIG.3also illustrates an operation of DC-DC converter500as a comparative example. At start-up of DC-DC converter500of the comparative example, ON-period Ton1and ON-period Ton2are 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 Ton2from timing t0to timing t2, and reaches a minimum value of −dI at timing t2in firstly-appearing control cycle T. After that, output current Iout increases by amplitude dI in ON-period Ton1from timing t2to timing t3, and reaches a maximum value of zero within first control cycle T at timing t3.

Since the maximum value is zero mentioned above in DC-DC converter500of 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 converter11according to the embodiment suppresses the output current Iout, which appears in the start-up of DC-DC converter500of the comparative example to a smaller value.

Configurations of DC-DC converter11and vehicle26having DC-DC converter11mounted thereto will be described below. DC-DC converter11includes series assembly14of switching elements12and13connected in series to each other, high-voltage positive end15, high-voltage negative end16, inductance element17, low-voltage positive end18, low-voltage negative end19, start-up signal receiver20, controller21, input capacitor24, and output capacitor25.

High-voltage battery22has a positive electrode connected to high-voltage positive end15, and has a negative electrode connected to high-voltage negative end16. Low-voltage battery23has a positive electrode connected to low-voltage positive end18, and has a negative electrode connected to low-voltage negative end19. High-voltage battery22has rated voltage VHC while low-voltage battery23has rated voltage VLC. Rated voltage VHC is higher than rated voltage VLC.

DC-DC converter11, high-voltage battery22, and low-voltage battery23are mounted to body27of vehicle26. In accordance with the embodiment, low-voltage battery23is, e.g. a storage battery configure to power auxiliary device28installed in vehicle26. High-voltage battery22is a storage battery configured to power, e.g. driving load29installed in vehicle26. DC-DC converter11performs a bidirectional operation. In other words, DC-DC converter11charges low-voltage battery23with power from high-voltage battery22in a stepping-down operation. DC-DC converter11charges high-voltage battery22with power from low-voltage battery23in a boosting-up operation.

Controller21includes 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 controller21. Alternatively, the control circuit, the memory, the drive circuit, and the detection circuit may be distributed or integrated.

High-voltage positive end15is connected to one end14A of series assembly14, and high-voltage negative end16is connected to another end14B of series assembly14. Input capacitor24has one end connected to one end14A of series assembly14, and has another end connected to another end14B of the series assembly14. Alternatively, the one end of input capacitor24is connected to high-voltage positive end15, and another end of input capacitor24is connected to high-voltage negative end16. One end17A of inductance element17is connected to node J1at which switching elements12and13are connected to each other. Another end17B of inductance element17is connected to low-voltage positive end18. Low-voltage negative end19is connected to high-voltage negative end16or another end14B of series assembly14. Output capacitor25has one end connected to low-voltage positive end18, and has another end connected to low-voltage negative end19.

In the device shown inFIGS.1and2, switching element12is illustrated as a high-potential-side arm, and switching element13is illustrated as a low-potential-side arm. Switching elements12and13are 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 inFIGS.1and2, diodes12D and13D are connected parallel to switching elements12and13, respectively. A cathode of diode12D is connected to a positive electrode side of switching element12, and an anode of diode12D is connected to a negative electrode side of switching element12. A cathode of diode13D is connected to a positive electrode side of switching element13, and an anode of diode13D is connected to a negative electrode side of switching element13. In the case that switching elements12and13include diodes12D and13D as parasitic diodes, respectively, external diodes12D and13D may not necessarily be connected.

Operations of DC-DC converter11and vehicle26having DC-DC converter11mounted thereto or operations of vehicle26having power supply device32, including DC-DC converter11, high-voltage battery22, and low-voltage battery23, mounted thereto will be described below.

When a driver in vehicle26activates start switch30to start up vehicle26, start switch30transmits start-up signal SG to start-up signal receiver20. At timing t0, controller21detects start-up signal SG via start-up signal receiver20. Before timing t0, switching elements12and13are both turned off as an initial state. In other words, DC-DC converter11is not in operation in before timing t0.

Upon detecting the start-up signal SG at timing t0, controller21detects high-side voltage VH between high-voltage positive end15and high-voltage negative end16. In other words, controller21detects high-side voltage VH which is a voltage of high-voltage battery22at timing t0. Controller21detects low-side voltage VL between low-voltage positive end18and low-voltage negative end19. In other words, controller21detects low-side voltage VL which is a voltage of low-voltage battery23. Controller21may detect high-side voltage VH and low-side voltage VL at the same time or at different timings. Considering a processing load of controller21, controller21preferably 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 t0, controller21generates a timer signal for timing of calculation and timing of control in controller21. 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 controller21. Length LT of control cycle T is determined based on, e.g. a circuit value of inductance element17or circuit values of inductance element17and output capacitor25.

Based on of voltage VH of high-voltage battery22and voltage VL of low-voltage battery23detected by controller21at timing t0, controller21determines, by calculation, length LT1of ON-period Ton1in which switching element12is turned on and length LT2of ON-period Ton2in which switching element13is turned on. Length LT1of ON-period Ton1and length LT2of ON-period Ton2are determined to be values that prevent DC-DC converter11from charging and discharging both high-voltage battery22and low-voltage battery23. In other words, bases on voltage VH and voltage VL, controller21determines lengths LT1and LT2of ON-periods Ton1and Ton2, namely, the duty which is the ratio of length LT1of ON-period Ton1to length LT of control cycle T, or the duty which is the ratio of length LT2of ON-period Ton2to length LT of control cycle T, so that an average value of a current is zero to prevent a current from flowing through inductance element17.

If detected voltages VH and VL are 50 V and 20 V, respectively, the ratio of length LT1of ON-period Ton1to length LT2of ON-period Ton2is about ⅔. In other words, the ratio of length LT1of ON-period Ton1to length LT of control cycle T is about ⅖. Length LT of control cycle T is the sum of length LT1of ON-period Ton1, length LT2of ON-period Ton2, and a length of a dead time. During the dead time, both switching elements12and13are 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 end15and high-voltage positive end16and damage to switching elements12and13. In actual control, the dead time and circuit losses in DC-DC converter11cause the ratio of length LT1of ON-period Ton1to length LT2of ON-period Ton2not to be ⅔ exactly. In other words, the ratio of length LT1of ON-period Ton1to length LT of control cycle T is not set to be ⅖ exactly. In the operation illustrated in the timing chart ofFIG.3, for simplified illustration, detected voltages VH and VL are, for example, 24 V and 12V, respectively, and the ratio of length LT1of ON-period Ton1to length LT2of ON-period Ton2is set to be 1. Since the dead time is extremely shorter than ON-periods Ton1and Ton2, the operation is described with the dead time omitted for convenience sake. Controller21turns on switching elements12and13complementarily. That is, controller21turns off switching element13while turning on switching element12. Controller21turns off switching element12while turning on switching element13. Controller21turns off both switching elements12and13during the extremely short dead time. Controller21does not turns on switching elements12and13simultaneously.

Upon determining lengths LT1and LT2of ON-periods Ton1and Ton2, controller21starts the switching operation of switching elements12and13. Lengths LT1and LT2of ON-periods Ton1and Ton2for control cycle T secondly provided and control cycles T subsequent to the secondly provided control cycle T in order to prevent DC-DC converter11from charging and discharging both high-voltage battery22and low-voltage battery23.

Control cycle Tf among subsequently-repeating control cycles T which firstly starts from timing t0includes ON-period Ton2in which switching element13as the low-potential-side arm is turned on first. ON-period Ton2includes initial ON-duration Ton2fhaving length LT2fshorter than length LT2of ON-period Ton2that has been determined by controller21by calculation. Initial ON-duration Ton2fof firstly-provided control cycle Tf starts from timing t1after timing t0. Since the set ratio of length LT1of ON-period Ton1to length LT2of ON-period Ton2is 1 in the operation illustrated inFIG.3, initial ON-duration Ton2for ON-period Ton2of firstly-provided control cycle Tf ends at timing t2, that is, at a half of control cycle Tf or control cycle T. Then, for ON-period Ton1from timing t2to timing t3, controller21turns on switching element12and turns off switching element13.

As described above, initial ON-duration Ton2fstarts from timing t1that is after timing to. Therefore, while switching element13is turned on and switching element12is turned off, output current Iout flows through inductance element17in direction Dr shown inFIG.3. However, since initial ON-duration Ton2fof control cycle Tf is shorter than ON-period Ton2, output current Iout reaches a minimum value −dI/2 at timing t2in firstly-provided control cycle Tf. The output current Iout with a negative value shown in the timing chart ofFIG.3flows in direction Dr through inductance element17shown inFIG.1or2.

While switching element12is turned on and switching element13is turned off in ON-period Ton1from timing t2to timing t3, output current Iout flows in direction Df. The output current Iout increases by amplitude dI in ON-period Ton1and reaches a maximum value dI/2 at timing t3in firstly-provided control cycle Tf.

For secondly-provided control cycle T from timing t3to timing t5and each of subsequent control cycles T, controller21provides ON-periods Ton1and Ton2with predetermined lengths LT1and LT2, respectively. The ratio of length LT1of ON-period Ton1to length LT2of ON-period Ton2is set to be 1 as mentioned above. Therefore, in ON-period Ton2from timing t3to timing t4, switching element13is turned on, and switching element12is turned off. In ON-period Ton1from timing t4to timing t5, switching element12is turned on, and switching element13is turned off.

In ON-period Ton2from timing t3to timing t4, the output current Iout decreases by amplitude dI and reaches the minimum value −dI/2 at timing t4in secondly-provided control cycle T. In ON-period Ton1from timing t4to timing t5, output current Iout increases by amplitude dI and reaches the maximum value dI/2 at timing t5in secondly-provided control cycle T.

Since controller21sets length LT2fof initial ON-duration Ton2fin firstly-provided control cycle Tf shorter than length LT2of ON-period Ton2that has been determined based on high voltage value VH1and low voltage value VL1by 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 elements12and13that is turned on first in the start-up of DC-DC converter11is 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 battery22and the characteristic degradation of low-voltage battery23.

Length LT2fof initial ON-duration Ton2fin firstly-provided control cycle Tf is preferably short, about a half of length LT2of ON-period Ton2that controller21has determined based on high voltage value VH1and low voltage value VL1. 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 LT2fof initial ON-duration Ton2fin firstly-provided control cycle Tf is a half of length LT2of ON-period Ton2, so that output current Iout flows through inductance element17in direction Dr at timing t2and takes the minimum value −dI/2 in first control cycle Tf, thus providing a desired steady output current. However, length LT2fof initial ON-duration Ton2fin firstly-provided control cycle Tf is not limited to the half of length LT2of ON-period Ton2. Length LT2fshorter than length LT2also reduces the steady output current.

In the above description, since initial ON-duration Ton2fof firstly-provided control cycle Tf is shorter than ON-period Ton2with length LT2that controller21has determined by the calculation based on high voltage value VH1and low voltage value VL1, 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 Ton1, initial ON-duration Ton2f, and idle duration Ts that is from timing t0to timing t1. Controller21turns off switching elements12and13in idle duration Ts. Although controller21turns on switching elements12and13on complementarily, controller21turns off switching elements12and13in 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 t0to timing t1or no idle duration Ts. Firstly-provided control cycle Tf may include only ON-period Ton1and initial ON-duration Ton2f.

In the above description of the operation of DC-DC converter11, an operation of switching elements12and13particularly in firstly-provided control cycle Tf and an operation of switching elements12and13in secondly and subsequently-provided control cycles T. In the above description, the determination of lengths LT1and LT2of ON-periods Ton1and Ton2of DC-DC converter11is based on voltage VH of high-voltage battery22and voltage VL of low-voltage battery23that have been detected at timing t0. An operation of DC-DC converter11in secondly and subsequently-provided control cycles T in that case that reference voltage VHR for high-voltage battery22and reference voltage VLR for low-voltage battery23are different from at least one of detected voltage VH of high-voltage battery22and detected voltage VL of low-voltage battery23at timing t0. 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, controller21detects voltage VH of high-voltage battery22and voltage VL of low-voltage battery23at timing t0. Controller21compares with detected voltage VH of high-voltage battery22. Reference voltage VHR is preset or previously stored in controller21for high-voltage battery22. Controller21also compares reference voltage VLR with detected voltage VL of low-voltage battery23. Reference voltage VLR is preset or previously stored in controller21for low-voltage battery23. The comparison of these voltages is performed at timing t0. Alternatively, the comparison of these voltages may be performed at any timing in control cycle Tf from timing t0to timing t3.

If voltage VH of high-voltage battery22is different from reference voltage VHR for high-voltage battery22or deviates from reference voltage VHR for high-voltage battery22by a difference larger than a predetermined value, DC-DC converter11charges or discharges high-voltage battery22in secondly and subsequently-provided control cycles T to adjust voltage VH of high-voltage battery22and voltage VL of low-voltage battery23to reference voltages VHR and VLR, respectively.

When, for example, voltage VH is lower than reference voltage VHR, DC-DC converter11performs a boost-up operation of charging high-voltage battery22with power from low-voltage battery23. When voltage VH is higher than reference voltage VHR, DC-DC converter11performs a stepping-down operation of discharging power from high-voltage battery22to charge low-voltage battery23. In an alternative stepping down operation that DC-DC converter11may perform when voltage VH is higher than reference voltage VHR, power of high-voltage battery22is discharged to auxiliary device28.

On the other hand, if voltage VL of low-voltage battery23is different from reference voltage VLR for low-voltage battery23or deviates from reference voltage VLR for low-voltage battery23by a difference larger than a predetermined value, DC-DC converter11charges or discharges low-voltage battery23in secondly and subsequently-provided control cycles T. Voltage VH of high-voltage battery22and voltage VL of low-voltage battery23are thus adjusted to reference voltages VHR and VLR, respectively.

When, for example, voltage VL is lower than reference voltage VLR, DC-DC converter11performs a stepping-down operation of charging low-voltage battery23with power from high-voltage battery22. When voltage VL is higher than reference voltage VLR, DC-DC converter11performs a boost-up operation of discharging power from low-voltage battery23to charge high-voltage battery22. In an alternative boosting-up operation that DC-DC converter11may perform when voltage VL is higher than reference voltage VLR, power of low-voltage battery23is discharged to driving load29.

The above operations are just examples. When voltage VH of high-voltage battery22is lower than reference voltage VHR for high-voltage battery22, and voltage VL of low-voltage battery23is lower than reference voltage VLR for low-voltage battery23, controller21may cause power generation circuit31connected to low-voltage battery23to charge low-voltage battery23in secondly and subsequently-provided control cycles T so as to increase voltage VL of low-voltage battery23up to reference voltage VLR; meanwhile, DC-DC converter11may perform a boost-up operation of charging high-voltage battery22with power of power generation circuit31so as to increase voltage VH of high-voltage battery22up to reference voltage VHR.

The power supply from power generation circuit31to low-voltage battery23here is controlled by a vehicle controller. Alternatively, the power supply from power generation circuit31to low-voltage battery23may be controlled by controller21. Power generation circuit31may include a power generator.

Since high-voltage battery22and low-voltage battery23that are mounted to vehicle26have 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 battery22and low-voltage battery23.

As mentioned earlier, inFIG.1andFIG.2, switching element12and switching element13are 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 element13, first in ON-period Ton2. In the operation that starts with switching element13, switching element13starts operating with a negative potential applied between a drain and a source of the switching element, especially in the case that switching elements12and13are implemented by FETs. This configuration allows the switching operation of switching element13and subsequent switching operation of switching element12to be controlled respectively by low-voltage-value control signals.

The switching operation may start with turning on switching element12, which is the high-potential-side arm, if a bootstrap circuit is provided to supply a high-voltage control signal to switching element12.FIG.4is a timing chart illustrating an operation of DC-DC converter11in this case. InFIG.4, items identical to those ofFIG.3are denoted by the same reference numerals. In the operation illustrated inFIG.4, switching element12is turned on, and switching element13is turned off for ON-period Ton2of control cycle T. Switching element12is turned off, and switching element13is turned on for ON-period Ton1. Therefore, for the cycle firstly provided, namely, for firstly-provided control cycle Tf, switching element12is turned on, and switching element13is turned off in initial ON-duration Ton2f. Then, switching element12is turned off, and switching element13is turned on in ON-period Ton1. Output current Iout has polarity reversed to output current Iout illustrated inFIG.3. The operation illustrated inFIG.4allows average value Iav of output current Iout to become extremely small in value, that is, about zero similarly to the operation illustrated inFIG.3.

High-voltage battery22may be a lead-acid battery, a lithium-ion battery, an electrical double-layer capacitor, or a lithium-ion capacitor. Similarly to high-voltage battery22, low-voltage battery23may be a lead-acid battery, a lithium-ion battery, an electrical double-layer capacitor, or a lithium-ion capacitor.

As described above, controller21is configured to provide control cycles T subsequently repeated in response to start-up signal SG received by start-up signal receiver20. Each of control cycles T includes ON-period Ton2and ON-period Ton1subsequent to ON-period Ton2. For ON-period Ton2, controller21turns on one of switching elements12and13and turns off another of switching elements12and13. For ON-period Ton1, controller21turns off the one of switching elements12and13and turns on another of switching elements12and13. Controller21is configured to determine length LT2of ON-period Ton2and length LT1of ON-period Ton1based on high-side voltage VH between high-voltage positive end15and high-voltage negative end16and low-side voltage VL between low-voltage positive end18and low-voltage negative end19. Controller21turns on the one of switching elements12and13in initial ON-duration Ton2fincluded in ON-period Ton2of firstly-provided control cycle Tf among control cycles T. Initial ON-duration Ton2fis shorter than determined length LT2of ON-period Ton2. Controller21turns off switching element12and switching element13in a duration (idle duration Ts) in ON-period Ton2of firstly-provided control cycle Tf other than the initial ON-duration. Controller21turns on the one of switching elements12and13and turns off another of switching elements12and13throughout determined length LT2of ON-period Ton2of one or more control cycles T other than firstly-provided control cycle Tf. Controller21turns off the one of switching elements12and13and turns on another of switching elements12and13throughout determined length LT1of ON-period Ton1of each of control cycles T.

Length LT2fof initial ON-duration Ton2fmay be a half of determined length LT2of ON-period Ton2.

In the first control cycle Tf, ON-period Ton1may be subsequent to initial ON-duration Ton2f.

The one of switching elements12and13is switching element12, and another of switching elements12and13is switching element13.

Alternatively, the one of switching elements12and13is switching element13, and another of switching elements12and13is switching element12.

ON-period Ton2of first control cycle Tf has a length shorter than determined length LT2of ON-period Ton2.

REFERENCE MARKS IN THE DRAWINGS

11DC-DC converter12switching element (first switching element)13switching element (second switching element)14series assembly15high-voltage positive end16high-voltage negative end17inductance element18low-voltage positive end19low-voltage negative end20start-up signal receiver21controller22high-voltage battery23low-voltage battery24input capacitor25output capacitor26vehicle27body28auxiliary device29power load to be driven30start switch31power generation circuit32power supply deviceT control cycleTon1ON-period (second ON-period)Ton2ON-period (first ON-period)Ton2finitial ON-duration