JP2017063582A - Bidirectional DC-DC converter - Google Patents

Abstract

To provide a bidirectional DC-DC converter capable of suppressing recovery loss without increasing the size of the apparatus.
A bidirectional DC-DC converter includes a high voltage power supply unit, a low voltage power supply unit, a transformer, a high voltage side switching circuit unit, and a low voltage side switching circuit unit. The high voltage side switching elements Q1, Q2, Q3, Q4 and the low voltage side switching elements Q5, Q6 are appropriately turned on / off to perform the step-down operation and the step-up operation. The on / off control of the low voltage side switching elements Q5, Q6 during the boosting operation is performed so that the pair of low voltage side switching elements Q5, Q6 are alternately turned on so that the on periods of the low voltage side switching elements Q5, Q6 partially overlap. Done. In the step-up operation, at least one of the high-voltage side switching elements Q1, Q2, Q3, and Q4 is turned on in synchronization with the timing of switching the low-voltage side switching elements Q5 and Q6 from off to on.
[Selection] Figure 2

Description

  The present invention relates to a bidirectional DC-DC converter that can perform bidirectional power conversion between step-down and step-up between a high-voltage power supply unit and a low-voltage power supply unit.

  For example, a DC-DC converter mounted on a hybrid vehicle, an electric vehicle, or the like steps down the voltage of a high-voltage power supply unit for driving a motor, supplies power to the low-voltage power supply unit, and supplies it to auxiliary devices. As such a DC-DC converter, a full bridge type push-pull converter is generally used. The full-bridge type push-pull converter includes a high-voltage side switching circuit unit connected between the high-voltage power supply unit and the transformer, and a low-voltage side switching circuit unit connected between the transformer and the low-voltage power supply unit. Each of the high-voltage side switching circuit unit and the low-voltage side switching circuit unit includes a plurality of switching elements. Then, the step-down operation is performed by appropriately turning on and off the high-voltage side switching element.

  On the other hand, the DC-DC converter may be required to perform a boosting operation in order to supply power from the low-voltage power supply unit to the high-voltage power supply unit. As described above, various bidirectional DC-DC converters having both a step-down function for converting a voltage from a high voltage to a low voltage and a step-up function for converting a voltage from a low voltage to a high voltage have been proposed (for example, see Patent Document 1). ).

  Here, the on / off control of the pair of low voltage side switching elements during the boosting operation is performed so that the pair of low voltage side switching elements are alternately turned on. And in order to perform continuous electric power transmission smoothly, on-off control is carried out so that the ON period of a pair of low voltage | pressure side switching element may overlap partially. As a result, the bidirectional DC-DC converter has a problem that it is difficult to increase the output power during the boosting operation.

  That is, during the step-up operation, the switching element of the high-voltage side switching circuit generates heat due to recovery loss, so that it is difficult to increase the output power. That is, a recovery loss occurs due to a recovery current flowing through a parasitic diode parasitic on the switching element. This recovery loss occurs when the parasitic diode is turned on and off due to voltage oscillation between the transformer windings that occurs during the period when both of the two switching elements are on in the low voltage side switching circuit. Therefore, in order to increase the output power during the boost operation, it is necessary to reduce this recovery loss.

  Here, the timing at which recovery loss occurs is due to the switching timing of the low-voltage side switching element. The invention of Patent Document 1 proposes lowering the driving frequency of the low-voltage side switching element by paying attention to this. More specifically, during the boosting operation, the driving frequency of the low-voltage side switching element is lowered as the output current value on the high-voltage side increases.

JP 2014-27750 A

  However, with the above control, the drive frequency of the low-voltage side switching element decreases as the output current value increases. Therefore, to obtain a desired output voltage at the time of boosting, the transformer must be enlarged. If it does so, it will lead to the enlargement of a bidirectional | two-way DC-DC converter.

  The present invention has been made in view of such a background, and an object of the present invention is to provide a bidirectional DC-DC converter capable of suppressing recovery loss without increasing the size of the apparatus.

One aspect of the present invention includes a high-voltage power supply unit (11),
A low-voltage power supply unit (12) having a lower voltage than the high-voltage power supply unit;
A transformer (2) comprising a high voltage side coil (21) and a low voltage side coil (22);
A high-voltage side switching circuit unit (3) connected between the high-voltage power supply unit and the high-voltage side coil and performing power conversion between DC power and AC power;
A low-voltage side switching circuit unit (4) connected between the low-voltage side coil and the low-voltage power source unit and performing power conversion between DC power and AC power;
The high-voltage side switching circuit unit includes a plurality of high-voltage side switching elements (Q1, Q2, Q3, Q4),
The low-voltage side switching circuit unit includes at least a pair of low-voltage side switching elements (Q5, Q6),
Step-down operation for stepping down the high voltage of the high-voltage power supply unit to the low voltage of the low-voltage power supply unit by appropriately turning on and off the high-voltage side switching element and the low-voltage side switching element, and low voltage of the low-voltage power supply unit Boosting operation to boost the voltage to the high voltage of the high-voltage power supply unit,
The on / off control of the pair of low voltage side switching elements during the boosting operation is performed so that the pair of low voltage side switching elements are alternately turned on so that the on periods of the pair of low voltage side switching elements partially overlap. I,
In the step-up operation, the low-voltage side switching element is configured to turn on at least one of the high-voltage side switching elements in synchronization with the timing of switching the low-voltage side switching element from off to on.
Bidirectional DC-DC converter (1).

  The bidirectional DC-DC converter is configured to turn on at least one of the high-voltage side switching elements in synchronization with the timing of switching the low-voltage side switching element from OFF to ON during the boosting operation. Thereby, the recovery current flowing through the parasitic diode that is parasitic on the high-voltage side switching element can be effectively suppressed.

  That is, the timing at which the low-voltage side switching elements are switched from OFF to ON is the timing at which the pair of low-voltage side switching elements are simultaneously turned on. At this timing, a recovery current tends to flow through the parasitic diode of the high voltage side switching element. Therefore, at this timing, at least one of the high-voltage side switching elements is turned on. Thereby, the transformer side and the high voltage power supply unit side are brought into conduction through the high voltage side switching element. Thereby, the recovery current generated in the parasitic diode can be suppressed, and the recovery loss can be suppressed.

  In the above control, it is not necessary to lower the drive frequency of the switching element. Therefore, the transformer is not increased in size.

As mentioned above, according to the said aspect, the bidirectional | two-way DC-DC converter which can suppress a recovery loss can be provided, without causing the enlargement of an apparatus.
In addition, the code | symbol in the parenthesis described in the means to solve a claim and a subject shows the correspondence with the specific means as described in embodiment mentioned later, and limits the technical scope of this invention. It is not a thing.

FIG. 2 is a circuit diagram of a bidirectional DC-DC converter in the first embodiment. FIG. 3 is a timing chart in the step-up operation of the bidirectional DC-DC converter in Embodiment 1. FIG. In Embodiment 1, (A) Explanatory drawing of the state of the 1st period, (B) Explanatory drawing of the state of the 2nd period. (A) Explanatory drawing of the state of the 3rd period in Embodiment 1, (B) Explanatory drawing of the state of the 4th period. Explanatory drawing of the current which flows into a parasitic diode when not controlling a high voltage | pressure side switching element in particular. 9 is a timing chart at the time of step-up operation of the bidirectional DC-DC converter in the second embodiment. 9 is a timing chart at the time of step-up operation of the bidirectional DC-DC converter in the third embodiment.

(Embodiment 1)
An embodiment of a bidirectional DC-DC converter will be described with reference to FIGS.
As shown in FIG. 1, the bidirectional DC-DC converter 1 of the present embodiment includes a high voltage power supply unit 11, a low voltage power supply unit 12 having a voltage lower than that of the high voltage power supply unit 11, a transformer 2, and a high voltage side switching circuit unit 3. And a low-voltage side switching circuit unit 4.

The transformer 2 includes a high voltage side coil 21 and a low voltage side coil 22.
The high-voltage side switching circuit unit 3 is connected between the high-voltage power source unit 11 and the high-voltage side coil 21 and performs power conversion between DC power and AC power.
The low voltage side switching circuit unit 4 is connected between the low voltage side coil 22 and the low voltage power source unit 12 and performs power conversion between DC power and AC power.

The high-voltage side switching circuit unit 3 includes a plurality of high-voltage side switching elements Q1, Q2, Q3, and Q4.
The low-voltage side switching circuit unit 4 includes a pair of low-voltage side switching elements Q5 and Q6.
The high-voltage side switching element 3 and the low-voltage side switching element 4 are appropriately turned on / off. Thereby, the step-down operation for stepping down the high voltage of the high-voltage power supply unit 11 to the low voltage of the low-voltage power supply unit 12 and the step-up operation for boosting the low voltage of the low-voltage power supply unit 12 to the high voltage of the high-voltage power supply unit 11 are performed. It is configured.

  As shown in FIG. 2, the on / off control of the pair of low voltage side switching elements Q5 and Q6 during the boosting operation is performed so that the on periods of the pair of low voltage side switching elements Q5 and Q6 partially overlap. Q5 and Q6 are performed alternately.

  In the step-up operation, at least one of the high-voltage side switching elements Q1, Q2, Q3, and Q4 is turned on in synchronization with the timing of switching the low-voltage side switching elements Q5 and Q6 from off to on.

  The bidirectional DC-DC converter 1 of this embodiment is mounted on a hybrid vehicle, an electric vehicle, or the like. The bidirectional DC-DC converter 1 steps down the voltage of the high-voltage power supply unit 11 for driving the motor, supplies power to the low-voltage power supply unit 12, and supplies it to auxiliary devices. The bidirectional DC-DC converter 1 is a full-bridge type push-pull converter whose circuit diagram is shown in FIG.

  The high-voltage side switching circuit unit 3 includes a first switching element Q1, a second switching element Q2, a third switching element Q3, and a fourth switching element Q4 as high-voltage side switching elements. The first switching element Q1 and the second switching element Q2 are connected to the positive electrode side of the high-voltage power supply unit 11. The third switching element Q3 and the fourth switching element Q4 are connected to the negative electrode side of the high-voltage power supply unit 11.

  The first switching element Q1 and the third switching element Q3 are connected in series to each other to form a first switching leg 321. The second switching element Q <b> 2 and the fourth switching element Q <b> 4 are connected in series to form a second switching leg 322. The first switching leg 321 and the second switching leg 322 are connected in parallel to each other. The connection point between the first switching element Q1 and the third switching element Q3 in the first switching leg 321 and the connection point between the second switching element Q2 and the fourth switching element Q4 in the second switching leg 322 are the high voltage side coil. 21 are connected to both terminals.

The low voltage side coil 22 has a first coil portion 221 and a second coil portion 222 configured to allow current to flow individually.
The low-voltage side switching circuit unit 4 includes a fifth switching element Q5 and a sixth switching element Q6 as low-voltage side switching elements. The fifth switching element Q5 turns on and off the supply of current from the low-voltage power supply unit 12 to the first coil unit 221. The sixth switching element Q6 turns on and off the supply of current from the low voltage power supply unit 12 to the second coil unit 222.

  In the low-voltage side switching circuit unit 4, the fifth switching element Q 5 and the sixth switching element Q 6 are connected to the positive electrode of the low-voltage power supply unit 12 via the choke coil 41. A capacitor 42 is connected between the positive electrode and the negative electrode of the low-voltage power supply unit 12.

  In the step-up operation, as shown in FIG. 2, the fifth switching element Q5 and the sixth switching element Q6 are switched so as to be alternately turned on and off while providing a partial overlap of the on periods. As a result, as shown in FIGS. 3 and 4, a period in which current flows only in the first coil part 221 of the low-voltage side coil 22 and a period in which current flows in both the first coil part 221 and the second coil part 222. And the period during which current flows only through the second coil portion 222 of the low-voltage side coil 22 is switched in an extremely short time unit of, for example, 10 microseconds. Accordingly, a voltage Vt is induced in the high voltage side coil 21 in the transformer 2. A current flows through the high-voltage side switching circuit unit 3. At this time, the current flows in a forward direction through a parasitic diode that is parasitic on the plurality of high voltage side switching elements Q1 to Q4 in the high voltage side switching circuit unit 3. That is, the high-voltage side switching elements Q1 to Q4 are composed of, for example, MOS field effect transistors (hereinafter referred to as MOSFETs) or the like, and parasitic diodes are parasitic on the MOSFETs. A return current flows through the parasitic diode.

  Specifically, during the period in which the fifth switching element Q5 is on and the sixth switching element Q6 is off, as shown in FIG. 3A, the high voltage side coil 21, the first switching element Q1, and the fourth switching element Q4. A current It flows through a current path including That is, in the first period T1 shown in FIG. 2, the current It flows through the diodes parasitic on the first switching element Q1 and the fourth switching element Q4.

  Further, during the period when the fifth switching element Q5 is off and the sixth switching element Q6 is on, as shown in FIG. 4A, the high-voltage side coil 21, the second switching element Q2, and the third switching element Q3 are included. A current It flows in the current path. That is, in the third period T3 shown in FIG. 2, the current It flows through the diodes parasitic on the second switching element Q2 and the third switching element Q3.

Note that, during the period in which both the fifth switching element Q5 and the sixth switching element Q6 are on, no voltage is induced in the high-voltage coil 21 of the transformer 2, and ideally no current flows.
As shown in FIG. 2, as the switching pattern of the low-voltage side switching elements Q5 and Q6, the following first period T1, second period T2, third period T3, and fourth period T4 are sequentially switched. Then, the first period T1 to the fourth period T4 are repeated. In FIG. 2, reference signs T <b> 1 to T <b> 4 are given only in the first period to the fourth period for one cycle. Here, the first period T1 is a period in which the fifth switching element Q5 is turned on and the sixth switching element Q6 is turned off. The second period T2 is a period in which both the fifth switching element Q5 and the sixth switching element Q6 are turned on following the first period T1. The third period T3 is a period in which the fifth switching element Q5 is turned off and the sixth switching element Q6 is turned on following the second period T2. The fourth period T4 is a period in which both the fifth switching element Q5 and the sixth switching element Q6 are turned on following the third period T3.

  The first switching element Q1, the second switching element Q2, the third switching element Q3, the fourth switching element Q5 at the timing of switching the fifth switching element Q5 from OFF to ON and the timing of switching the sixth switching element Q6 from OFF to ON. At least one of the switching elements Q4 is turned on. That is, at the timing of switching from the first period T1 to the second period T2 and the timing of switching from the third period T3 to the fourth period T4, at least one of the four high-voltage side switching elements Q1 to Q4 is turned on.

That is, as described above, ideally, no current flows through the high-voltage side switching circuit unit 3 in the second period T2 and the fourth period T4. However, when switching from the first period T1 to the second period T2, as shown in FIG. 3B and FIG. 5, the current flowing in the forward direction in the parasitic diodes of the high-voltage side switching elements Q1 to Q4 suddenly increases. After the decrease, the recovery current Ir tends to flow slightly in the reverse direction. For example, when the first switching element Q1, the high-voltage coil 21 and the fourth switching element Q4 are switched to the second period T2 from the first period T1 in which the current It flows in the current path, the first switching element Q1 A recovery current Ir is generated in the parasitic diode of Q1. Then, LC resonance occurs in the current path sandwiched between the first switching element Q1 and the fourth switching element Q4. As a result, ringing occurs and loss increases. Therefore, at least one of the high-voltage side switching elements Q1 to Q4 is turned on to contain this ringing. That is, at the timing when the recovery current is generated, at least one of the high-voltage side switching elements Q1 to Q4 is turned on. This stabilizes the potential of the current path between the first switching element Q1 and the fourth switching element Q4. In this way, it is possible to suppress the generation of recovery current and to suppress the loss accompanying this.
Such a phenomenon similarly occurs at the timing of switching from the third period T3 to the fourth period T4 as shown in FIGS.

  In the present embodiment, as shown in FIG. 2, in particular, the first switching element is synchronized with the timing at which the fifth switching element Q5 is switched from OFF to ON and the timing at which the sixth switching element Q6 is switched from OFF to ON. Q1 and the second switching element Q2 are configured to be turned on. That is, the first switching element Q1 and the second switching element Q2 are turned on in synchronization with the timing of switching from the first period T1 to the second period T2 and the timing of switching from the third period T3 to the fourth period T4.

  In the present embodiment, the timing of switching the first switching element Q1 and the second switching element Q2 from on to off is synchronized with the turn-off timing of the fifth switching element Q5 or the sixth switching element Q6. That is, the turn-on and turn-off of the first switching element Q1 and the second switching element Q2 are synchronized with the turn-on and turn-off of the fifth switching element Q5 or the sixth switching element Q6. Thereby, the signal of switching control can be simplified.

  In addition, instead of the first switching element Q1 and the second switching element Q2, the timing for switching the third switching element Q3 and the fourth switching element Q4 from the first period T1 to the second period T2, and from the third period T3 to the second switching element Q2 You may make it turn on synchronizing with the timing switched to 4 period T4.

  Further, the duty and timing of on / off control of each switching element can be controlled by peak current mode control. That is, the switching duty and timing can be controlled by comparing the inductor current detected by an external circuit, a microcomputer, or the like with a current command value obtained by calculation.

  In the timing chart of FIG. 2, Vt indicates a change in voltage generated in the high voltage side coil 21 of the transformer 2. IL indicates a change in the current flowing through the low-voltage side switching circuit unit 4.

  The bidirectional DC-DC converter 1 is configured to turn on the high voltage side switching elements Q1, Q2 in synchronization with the timing of switching the low voltage side switching elements Q5, Q6 from OFF to ON during the boosting operation. . Thereby, the recovery current flowing through the parasitic diode parasitic on the high voltage side switching elements Q1 and Q2 can be effectively suppressed.

  That is, the timing at which the low-voltage side switching elements Q5, Q6 are switched from OFF to ON is the timing at which the pair of low-voltage side switching elements Q5, Q6 are simultaneously turned ON. At this timing, a recovery current tends to flow through the parasitic diodes of the high voltage side switching elements Q1 and Q2. Therefore, at this timing, the high voltage side switching elements Q1 and Q2 are turned on. Thereby, the transformer 2 side and the high-voltage power supply unit 11 side are brought into conduction through the high-voltage side switching elements Q1 and Q2. Thereby, the recovery current generated in the parasitic diode can be suppressed, and the recovery loss can be suppressed.

  In the above control, it is not necessary to lower the drive frequency of the switching element. Therefore, the transformer 2 is not increased in size, and the bidirectional DC-DC converter 1 is not increased in size.

  As described above, according to the present example, it is possible to provide a bidirectional DC-DC converter that can suppress recovery loss without increasing the size of the apparatus.

(Embodiment 2)
In the present embodiment, during the step-up operation, as shown in FIG. 6, the on / off operation of the first switching element Q1 and the third switching element Q3 is performed alternately.
That is, in synchronization with the timing of switching the fifth switching element Q5 from OFF to ON, the first switching element Q1 is turned ON and in synchronization with the timing of switching the sixth switching element Q6 from OFF to ON, the third switching element Q3 Turn on.
Other configurations are the same as those of the first embodiment.

In the present embodiment, the on / off operation of the first switching element Q1 and the third switching element Q3 is performed alternately. Therefore, during the continuous operation of the bidirectional DC-DC converter 1, the on / off switching frequency of the first switching element Q1 and the third switching element Q3 can be equalized. As a result, heat concentration in the switching element can be reduced.
In addition, the same effects as those of the first embodiment are obtained.

  In the above embodiment, on / off of the first switching element Q1 may be replaced with on / off of the second switching element Q2, and on / off of the third switching element Q3 may be replaced with on / off of the fourth switching element Q4. That is, the second switching element Q2 and the fourth switching element Q4 may be alternately turned on / off. Alternatively, the first switching element Q1 and the second switching element Q2 may be simultaneously turned on / off, and the third switching element Q3 and the fourth switching element Q4 may be simultaneously turned on / off. In this case, the first switching element Q1 and the second switching element Q2, and the third switching element Q3 and the fourth switching element Q4 are alternately turned on and off.

(Embodiment 3)
In the present embodiment, as shown in FIG. 7, in the boost operation, the second switching element Q2 is turned on and the sixth switching element Q6 is turned on in synchronization with the timing of switching the fifth switching element Q5 from OFF to ON. The first switching element Q1 is turned on in synchronization with the timing of switching from off to on.

That is, during the step-up operation, during the period T1 in which only the fifth switching element Q5 among the low-voltage side switching elements is on, the current path including the first switching element Q1 and the fourth switching element Q4 is changed in the high-voltage side switching circuit unit 3. The high-voltage power supply unit 11 is charged via this. From this period T1, when the fifth switching element Q5 is turned on and becomes the period T2, the second switching element Q2 is turned on. Thereby, the recovery current which tends to flow through the first switching element Q1 and the fourth switching element Q4 can be suppressed smoothly.
Other configurations are the same as those of the first embodiment, and the same effects are obtained.

  In the present embodiment, the third switching element Q3 may be turned on / off instead of turning on / off the second switching element Q2. Further, the fourth switching element Q4 may be turned on / off instead of turning on / off the first switching element Q1. That is, in synchronization with the timing of switching the fifth switching element Q5 from OFF to ON, the third switching element Q3 is turned ON, and in synchronization with the timing of switching the sixth switching element Q6 from OFF to ON, the fourth switching element Q4 It may be configured to turn on.

  Alternatively, in synchronization with the timing of switching the fifth switching element Q5 from OFF to ON, the second switching element Q2 and the third switching element Q3 are turned ON, and in synchronization with the timing of switching the sixth switching element Q6 from OFF to ON. The first switching element Q1 and the fourth switching element Q4 may be turned on. In this case, the recovery current can be more effectively suppressed.

  In addition, this invention is not limited to the said embodiment, A various form can be employ | adopted. That is, it is only necessary to control such that at least one of the high-voltage side switching elements Q1 to Q4 is turned on at the timing when both of the low-voltage side switching elements Q5 and Q6 are turned on during the boosting operation. However, it is desirable to provide a dead time for the on / off control of the high voltage side switching elements Q1 to Q4. That is, with respect to the on / off operation of the switching elements included in the same switching leg, it is possible to more reliably prevent problems such as a short circuit by providing a time zone in which the switching elements are simultaneously turned off.

DESCRIPTION OF SYMBOLS 1 Bidirectional DC-DC converter 11 High voltage side power supply part 12 Low voltage side power supply part 2 Transformer 21 High voltage side coil 22 Low voltage side coil 3 High voltage side switching circuit part 4 Low voltage side switching circuit part Q1, Q2, Q3, Q4 High voltage side switching element Q5, Q6 Low voltage side switching element

Claims (7)

A high-voltage power supply (11);
A low-voltage power supply unit (12) having a lower voltage than the high-voltage power supply unit;
A transformer (2) comprising a high voltage side coil (21) and a low voltage side coil (22);
A high-voltage side switching circuit unit (3) connected between the high-voltage power supply unit and the high-voltage side coil and performing power conversion between DC power and AC power;
A low-voltage side switching circuit unit (4) connected between the low-voltage side coil and the low-voltage power source unit and performing power conversion between DC power and AC power;
The high-voltage side switching circuit unit includes a plurality of high-voltage side switching elements (Q1, Q2, Q3, Q4),
The low-voltage side switching circuit unit includes at least a pair of low-voltage side switching elements (Q5, Q6),
Step-down operation for stepping down the high voltage of the high-voltage power supply unit to the low voltage of the low-voltage power supply unit by appropriately turning on and off the high-voltage side switching element and the low-voltage side switching element, and low voltage of the low-voltage power supply unit Boosting operation to boost the voltage to the high voltage of the high-voltage power supply unit,
The on / off control of the pair of low voltage side switching elements during the boosting operation is performed so that the pair of low voltage side switching elements are alternately turned on so that the on periods of the pair of low voltage side switching elements partially overlap. I,
In the step-up operation, the low-voltage side switching element is configured to turn on at least one of the high-voltage side switching elements in synchronization with the timing of switching the low-voltage side switching element from off to on.
Bidirectional DC-DC converter (1). The high-voltage side switching circuit unit includes, as the high-voltage side switching element, a first switching element (Q1) and a second switching element (Q2) connected to a positive electrode side of the high-voltage power supply unit, and a negative electrode side of the high-voltage power supply unit A first switching leg (321) comprising a third switching element (Q3) and a fourth switching element (Q4) connected to each other, wherein the first switching element and the third switching element are connected in series with each other; A second switching leg (322) formed by connecting the second switching element and the fourth switching element in series with each other is connected in parallel to each other, and the first switching element in the first switching leg and the above-mentioned The connection point with the third switching element and the second switch in the second switching leg. A connection point between the ring element and the fourth switching element is connected to both terminals of the high voltage side coil,
The low voltage side coil has a first coil part (221) and a second coil part (222) configured to allow current to flow individually,
The low-voltage side switching circuit unit includes, as the low-voltage side switching element, a fifth switching element (Q5) for turning on and off the supply of current from the low-voltage power source unit to the first coil unit, and the second low-voltage power source unit to the second A sixth switching element (Q6) for turning on and off the supply of current to the coil section;
During the step-up operation, a current path including the high-voltage coil, the first switching element, and the fourth switching element is passed during a period in which the fifth switching element is on and the sixth switching element is off. In a period in which the current flows, and the fifth switching element is off and the sixth switching element is on, a current path including the high-voltage coil, the second switching element, and the third switching element Configured to carry current,
The first switching element, the second switching element, the third switching element, and the fourth switching at the timing of switching the fifth switching element from off to on and the timing of switching the sixth switching element from off to on. The bidirectional DC-DC converter of claim 1, configured to turn on at least one of the elements.   In the step-up operation, the first switching element and the second switching are synchronized with the timing at which the fifth switching element is switched from OFF to ON and the timing at which the sixth switching element is switched from OFF to ON. The bidirectional DC-DC converter according to claim 2, wherein the bidirectional DC-DC converter is configured to turn on the element.   In the step-up operation, the third switching element and the fourth switching are synchronized with the timing of switching the fifth switching element from OFF to ON and the timing of switching the sixth switching element from OFF to ON. The bidirectional DC-DC converter according to claim 2, wherein the bidirectional DC-DC converter is configured to turn on the element.   In the step-up operation, the first switching element or the second switching element is turned on and the sixth switching element is turned from off to on in synchronization with the timing of switching the fifth switching element from off to on. The bidirectional DC-DC converter according to claim 2, wherein the bidirectional DC-DC converter is configured to turn on the third switching element or the fourth switching element in synchronization with a switching timing.   In the step-up operation, the second switching element or the third switching element is turned on and the sixth switching element is turned off to on in synchronization with the timing of switching the fifth switching element from off to on. The bidirectional DC-DC converter according to claim 2, wherein the bidirectional DC-DC converter is configured to turn on the first switching element or the fourth switching element in synchronization with a switching timing.   In the step-up operation, the first switching element and the fourth switching element are turned on and the sixth switching element is turned from off to on in synchronization with the timing of switching the fifth switching element from off to on. The bidirectional DC-DC converter according to claim 2, wherein the bidirectional DC-DC converter is configured to turn on the second switching element and the third switching element in synchronization with a switching timing.

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