JP7547855B2 - DC/DC converter and method for controlling DC/DC converter - Google Patents

Description

The present invention relates to a DC/DC converter and a method for controlling a DC/DC converter.

As a DC/DC converter, a DAB (Dual Active Bridge) type DC/DC converter as seen in Patent Document 1 is known. A DAB type DC/DC converter has two bridge circuits connected to the primary and secondary windings of a transformer via reactors, and primary and secondary smoothing capacitors connected to the bridge circuits, respectively. In a DAB type DC/DC converter, the reactor installed between the transformer and the bridge circuits does not need to have a very large inductance, so the leakage inductance of the transformer is sometimes used as the reactor.

JP 2014-87134 A

In DAB DC/DC converters like the one described above, the inductance of the reactor is small, so the current rises more quickly immediately after startup than in DC/DC converters of other types, and a large current may flow in the circuit immediately after startup. For this reason, it is necessary to use expensive components with a high current resistance that can withstand the large current immediately after startup as components of the DC/DC converter, which leads to increased manufacturing costs.

The present invention was made in consideration of these circumstances, and the problem it aims to solve is to make it difficult for excessive current to flow through a DC/DC converter immediately after startup.

The DC/DC converter that solves the above problem includes a transformer having two windings, a primary side and a secondary side, a bridge circuit connected to the primary side winding via a reactor, the primary side bridge circuit having a plurality of switching elements, a secondary side bridge circuit connected to the secondary side winding via a reactor, a primary side smoothing capacitor connected to the primary side bridge circuit without a reactor, a secondary side smoothing capacitor connected to the secondary side bridge circuit without a reactor, and a drive circuit that switches each switching element of the primary side bridge circuit and each switching element of the secondary side bridge circuit. Furthermore, the drive circuit in the DC/DC converter performs the switching at a predetermined steady switching frequency after a predetermined time has elapsed since the start of the DC/DC converter, and performs the switching at a startup switching frequency that is a frequency higher than the steady switching frequency before the predetermined time has elapsed.

The method for controlling a DC/DC converter that solves the above problem is a method for controlling a DC/DC converter that includes a transformer having two windings, a primary side and a secondary side, a bridge circuit connected to the primary side winding via an inductor, the primary side bridge circuit having a plurality of switching elements, a secondary side bridge circuit connected to the secondary side winding via an inductor, the secondary side bridge circuit having a plurality of switching elements, a primary side smoothing capacitor connected to the primary side bridge circuit without a reactor, and a secondary side smoothing capacitor connected to the secondary side bridge circuit without a reactor, in which after a predetermined time has elapsed since the start-up of the DC/DC converter, the switching of each switching element is performed at a predetermined steady switching frequency, and before the predetermined time has elapsed, the switching is performed at a startup switching frequency that is a frequency higher than the steady switching frequency.

In the above DC/DC converter and control method for the DC/DC converter, during the period from start-up of the DC/DC converter until a predetermined time has elapsed, switching of each switching element is performed at a start-up switching frequency that is higher than the steady-state switching frequency, so the current flowing through the transformer, etc. immediately after start-up is suppressed. Therefore, it is difficult for an excessive current to flow immediately after start-up of the DC/DC converter.

Here, the switching pattern of each of the switching elements immediately before the predetermined time has elapsed since the start of the DC/DC converter is set as a first switching pattern. At this time, it is preferable that the drive circuit in the DC/DC converter alternately switches the switching patterns in the order of the second switching pattern and the first switching pattern after the predetermined time has elapsed. Furthermore, it is preferable that the drive circuit alternately switches the switching pattern immediately after the start of the DC/DC converter as the first switching pattern. Furthermore, in the control method for the DC/DC converter, it is preferable that the switching patterns alternately switch in the order of the second switching pattern and the first switching pattern after the predetermined time has elapsed. Furthermore, in the control method, it is preferable that the switching pattern immediately after the start of the DC/DC converter is set as the first switching pattern.

According to the present invention, it is difficult for an excessive current to flow immediately after starting up the DC/DC converter.

FIG. 2 is an electrical circuit diagram of one embodiment of a DC/DC converter. 4 is a time chart showing, during operation of the DC/DC converter, (a) to (d) the transition of the drive signal of each switching element, and (e) and (f) the transition of the voltage between the connection points of each bridge circuit. 13A to 13D are time charts showing the transitions of the drive signals of the switching elements, and FIG. 13E is a time chart showing the transitions of the transformer current when the DC/DC converter is in steady-state operation with the phase difference φ set to 0°. 1A to 1D are time charts showing the transitions of the drive signals of the switching elements and FIG. 1E is a time chart showing the transitions of the transformer current during a period before and after startup in a comparative example of a DC/DC converter having the same electric circuit configuration as the above embodiment, in which steady-state operation is started immediately after startup. 5A to 5D are time charts showing transitions of drive signals for each switching element, and FIG. 5E is a time chart showing transitions of a transformer current during a period before and after the start-up of the DC/DC converter of the embodiment. 11 is a time chart showing, in a period before and after the start of the DC/DC converter according to the modified example of the embodiment, (a) to (d) a transition of the drive signal of each switching element, and (e) a transition of the transformer current. 13 is a time chart showing, in a period before and after the start of another modified example of the DC/DC converter of the embodiment, (a) to (d) a transition of the drive signal of each switching element, and (e) a transition of the transformer current.

Below, an embodiment of a DC/DC converter and a control method thereof will be described in detail with reference to Figs. 1 to 7. The DC/DC converter of this embodiment is configured as a DAB type bidirectional DC/DC converter.

As shown in FIG. 1, the DC/DC converter 10 of this embodiment is configured to perform bidirectional power conversion between two DC power sources E1 and E2. The DC/DC converter 10 has a transformer TR having a primary winding N1 and a secondary winding N2, primary and secondary bridge circuits B1 and B2, and primary and secondary smoothing capacitors C1 and C2.

The primary bridge circuit B1 has a first leg 11 and a second leg 12 connected in parallel. The first leg 11 has a first and a second switching element Q1, Q2 connected in series. A connection point P1 between the first switching element Q1 and the second switching element Q2 is connected to one end of the primary winding N1 of the transformer TR via an inductor L1. The second leg 12 has a third and a fourth switching element Q3, Q4 connected in series. A connection point P2 between the third switching element Q3 and the fourth switching element Q4 is connected to the other end of the primary winding N1 of the transformer TR. In addition, the primary smoothing capacitor C1 is connected in parallel to the first leg 11 and the second leg 12 in the primary bridge circuit B1.

The secondary bridge circuit B2 has a third leg 21 and a fourth leg 22 connected in parallel. The third leg 21 has fifth and sixth switching elements Q5 and Q6 connected in series. A connection point P3 between the fifth switching element Q5 and the sixth switching element Q6 is connected to one end of the secondary winding N2 of the transformer TR via an inductor L2. The fourth leg 22 has seventh and eighth switching elements Q7 and Q8 connected in series. A connection point P4 between the seventh switching element Q7 and the eighth switching element Q8 is connected to the other end of the secondary winding N2 of the transformer TR. In addition, the secondary smoothing capacitor C2 is connected in parallel to the third leg 21 and the fourth leg 22 in the secondary bridge circuit B2.

Each switching element Q1 to Q8 in the first and second bridge circuits B1, B2 is composed of an IGBT or MOSFET with a diode connected in inverse parallel. In this DAB DC/DC converter 10, the primary and secondary smoothing capacitors C1, C2 are connected to the primary and secondary bridge circuits B1, B2, respectively, without a reactor. In this DAB DC/DC converter 10, the inductors L1, L2 between the windings N1, N2 and the bridge circuits B1, B2 do not need to have a large inductance. Therefore, it is possible to use the leakage inductance of the transformer TR as the inductors L1, L2.

The DC/DC converter 10 further includes a drive circuit 30 that drives each of the switching elements Q1 to Q8 in the primary and secondary bridge circuits B1 and B2. The drive circuit 30 drives each of the switching elements Q1 to Q8 by outputting a pulse-shaped drive signal to each of the switching elements Q1 to Q8.

Next, referring to FIG. 2, the power conversion operation of the DC/DC converter 10 of this embodiment will be described. FIGS. 2(a) to 2(d) show the transition of the drive signals of the switching elements Q1 to Q8 when power is flowed from the DC power source E1 to the DC power source E2. FIG. 2(e) shows the transition of the primary side AC voltage VA, which is the voltage between the connection points P1 and P2 of the primary side bridge circuit B1, and FIG. 2(f) shows the transition of the secondary side AC voltage VB, which is the voltage between the connection points P3 and P4 of the secondary side bridge circuit B2, in the above case. Note that the waveforms of the drive signals of the switching elements in FIG. 2 and in FIGS. 3 to 7 described later are shown in a form that omits the dead time set when the switching elements in each bridge circuit B1 and B2 are switched on and off.

At this time, the drive circuit 30 drives the switching elements Q1 to Q8 of the primary and secondary bridge circuits B1 and B2 in the manner shown in Figures 2(a) to (d). In other words, the drive circuit 30 switches the switching elements Q1 to Q4 of the primary bridge circuit B1 at the same cycle and in the same phase.

The drive circuit 30 also switches the switching elements Q5 to Q8 of the secondary bridge circuit B2 at the same cycle as the switching elements Q1 to Q4 of the primary bridge circuit B1. However, the switching elements Q6 and Q7 are driven so that they are off when the switching elements Q5 and Q8 are on, and on when the switching elements Q5 and Q8 are off.

Furthermore, the drive circuit 30 drives each of the switching elements Q5 to Q8 of the secondary bridge circuit B2 with a delay of the phase difference φ with respect to each of the switching elements Q1 to Q4 of the primary bridge circuit B1. As a result, the secondary AC voltage VB is delayed by the phase difference φ with respect to the primary AC voltage VA, and power flows from the DC power source E1 to the DC power source E2. The power flowing from the DC power source E1 to the DC power source E2 is 0 when the phase difference φ is 0°, and is maximum when the phase difference φ is 90°. Note that, conversely to the case of FIG. 2, if the phase of the switching period of each of the switching elements Q1 to Q4 of the primary bridge circuit B1 is delayed from the phase of the switching period of each of the switching elements Q5 to Q8 of the secondary bridge circuit B2, power flows from the DC power source E2 to the DC power source E1. In this way, in the DC/DC converter 10, the magnitude and direction of the power flowing between the DC power sources E1 and E2 change depending on the phase difference φ of the switching period between the primary and secondary bridge circuits B1 and B2. In this DC/DC converter 10, the drive circuit 30 controls the magnitude and direction of the power flowing between the DC power sources E1 and E2 by changing the phase difference φ in response to an external command.

After a predetermined time has elapsed since the start-up of the DC/DC converter 10, the drive circuit 30 switches each of the switching elements Q1 to Q8 at a constant frequency as described above. In the following explanation, the switching frequency of the switching elements Q1 to Q8 at this time is referred to as the steady-state switching frequency Fa. In the following explanation, the switching of each of the switching elements Q1 to Q8 at this steady-state switching frequency Fa is also referred to as steady-state drive.

Figures 3(a) to (d) show the transitions of the drive signals of the switching elements Q1 to Q8 when the DC/DC converter 10 is in steady-state operation with a phase difference φ of 0°, and Figure 3(e) shows the transitions of the transformer current iT. In the following description, a switching pattern in which the switching elements Q1, Q4, Q5, and Q8 are turned on while the switching elements Q2, Q3, Q6, and Q7 are turned off is referred to as a first switching pattern A. A switching pattern in which the switching elements Q1, Q4, Q5, and Q8 are turned off while the switching elements Q2, Q3, Q6, and Q7 are turned on is referred to as a second switching pattern B. During steady-state operation of the DC/DC converter 10 with a phase difference φ of 0°, the first switching pattern A and the second switching pattern B are alternately switched.

As shown in FIG. 3(e), the transformer current iT changes in the positive direction over time during the drive period with the first switching pattern A, and gradually changes in the negative direction over time during the drive period with the second switching pattern B. As a result, the transformer current iT during steady-state drive with the phase difference φ at 0° repeatedly increases and decreases at a constant amplitude Ia centered on 0 [A]. In other words, the transformer current iT at this time repeatedly increases and decreases within a range with a lower limit of "-Ia/2" and an upper limit of "Ia/2". Therefore, the maximum absolute value of the transformer current iT at this time is "Ia/2".

When the DC/DC converter 10 is started, the drive circuit 30 starts driving each of the switching elements Q1 to Q8 with the phase difference φ set to 0° and the power flowing between the primary and secondary circuits set to 0. However, as described below, if steady-state driving is started immediately after startup, there is a risk that an excessive current will flow through the transformer TR, etc.

Figures 4 (a) to (d) show the transitions of the drive signals of the switching elements Q1 to Q8 when steady-state operation with a phase difference φ of 0° is started immediately after the start of the DC/DC converter 10, and (e) shows the transition of the transformer current iT at that time. As shown in Figure 4, in this case, from time t0 when the DC/DC converter 10 is started, switching of the switching elements Q1 to Q8 is started with the switching frequency set to the steady-state switching frequency Fa. Even at this time, the transformer current iT repeatedly increases and decreases at a constant amplitude Ia after the start of switching of the switching elements Q1 to Q8.

However, the transformer current iT before time t0 when the DC/DC converter 10 is stopped is 0 [A]. Therefore, as shown in FIG. 4(e), during the driving period of each switching element Q1 to Q8 in the first switching pattern A after startup, the transformer current iT starts to change in the positive direction from 0 A. Therefore, the transformer current iT immediately after the startup of the DC/DC converter 10 repeatedly increases and decreases within a range with a lower limit of 0 and an upper limit of Ia. Incidentally, if the driving of each switching element Q1 to Q8 continues in the above manner, the amplitude center of the transformer current iT gradually approaches 0 [A] and eventually transitions to the state shown in FIG. 3(e). However, immediately after the startup of the DC/DC converter 10, the maximum absolute value of the transformer current iT becomes "Ia", which is twice as large as in the case of FIG. 3.

In contrast, in the DC/DC converter 10 of this embodiment, the switching elements Q1 to Q8 are driven in the following manner at startup to prevent an excessive current from flowing through the circuit immediately after startup.

Figure 5 shows the driving state before and after the start of the DC/DC converter 10 of this embodiment. Note that Figures 5(a) to (d) show the transition of the drive signal of each switching element Q1 to Q8 before and after the start, and Figure 5(e) shows the transition of the transformer current iT before and after the start. In the case of this embodiment as well, immediately after the start of the DC/DC converter 10 at time t0, each switching element Q1 to Q8 is driven with the first switching pattern A. However, the drive circuit 30 drives each switching element Q1 to Q8 with the first first switching pattern A from time t0 at a startup switching frequency Fb, which is a frequency higher than the steady switching frequency Fa. In the following description, the drive of each switching element Q1 to Q8 at a frequency higher than the steady switching frequency Fa immediately after the start of the DC/DC converter 10 is described as startup drive. In this embodiment, in the startup drive, switching of each switching element Q1 to Q8 with the first switching pattern A is performed only once. In other words, in this embodiment, startup drive is performed for a period equivalent to half the switching period of each switching element Q1 to Q8.

When the startup drive ends at time t1, the drive circuit 30 changes the switching frequency to the steady-state switching frequency Fa, and then alternates between the second switching pattern B and the first switching pattern A. In other words, steady-state drive begins at time t1.

In this embodiment, the startup switching frequency Fb is set so that steady-state operation begins when the transformer current iT reaches "Ia/2" after switching of the switching elements Q1 to Q8 according to the first switching pattern A from time t0. The value of the startup switching frequency Fb set in this manner is slightly greater than twice the steady-state switching frequency Fa.

In this embodiment, the switching of each switching element Q1 to Q8 immediately after the start of the DC/DC converter 10 is performed at a startup switching frequency Fb that is higher than the steady switching frequency Fa. Therefore, the amount of change in the transformer current iT in the positive direction during the period when the switching of each switching element Q1 to Q8 is performed in the first switching pattern A immediately after the start of the DC/DC converter 10 is smaller than "Ia". Furthermore, in this embodiment, the startup switching frequency Fb is set so that the amount of change in the transformer current iT in the positive direction during the above period is "Ia/2". Therefore, immediately after the start of the DC/DC converter 10, the transformer current iT repeatedly increases and decreases within a range with a lower limit of "-Ia/2" and an upper limit of "Ia/2".

According to the DC/DC converter 10 and the control method thereof of the present embodiment described above, the following effects can be achieved.
(1) After a predetermined time has elapsed since the start-up of the DC/DC converter 10, the switching elements Q1 to Q8 are switched at the steady switching frequency Fa, and before the predetermined time has elapsed, the switching elements Q1 to Q8 are switched at a startup switching frequency Fb that is a frequency higher than the steady switching frequency Fa. As a result, an increase in the transformer current iT immediately after the start-up of the DC/DC converter 10 is suppressed, and an excessive current is less likely to flow immediately after the start-up of the DC/DC converter 10.

(2) Because the current flowing through the circuit immediately after startup is suppressed, there is no need to use expensive components with high current resistance as components of the DC/DC converter 10. This reduces the manufacturing costs of the DC/DC converter 10.

(3) The value of the startup switching frequency Fb is set so that the amplitude of the transformer current iT when each switching element Q1 to Q8 is switched at the startup switching frequency Fb is half the amplitude of the transformer current iT when each switching element Q1 to Q8 is switched at the steady-state switching frequency Fa. Therefore, immediately after the start of steady-state operation, the transformer current iT begins to repeatedly increase and decrease around 0 [A]. Therefore, the range of change of the transformer current iT after the start of steady-state operation is less likely to deviate from the expected range.

This embodiment can be modified as follows: This embodiment and the following modifications can be combined with each other to the extent that there is no technical contradiction.
The states of the switching elements Q1 to Q8 in the first switching pattern A and the second switching pattern B may be opposite to those in the above embodiment. That is, the first switching pattern is a switching pattern in which the switching elements Q1, Q4, Q5, and Q8 are turned off while the switching elements Q2, Q3, Q6, and Q7 are turned on. The second switching pattern B is a switching pattern in which the switching elements Q1, Q4, Q5, and Q8 are turned on while the switching elements Q2, Q3, Q6, and Q7 are turned off.

In the above embodiment, the startup switching frequency Fb is set so that the amplitude of the transformer current iT when each switching element Q1 to Q8 is switched at the startup switching frequency Fb is half the amplitude of the transformer current iT when each switching element Q1 to Q8 is switched at the steady switching frequency Fa. Not limited to this, if the startup switching frequency Fb is set to a frequency higher than the steady switching frequency Fa, the increase in the transformer current iT immediately after the startup of the DC/DC converter 10 can be suppressed.

〇 In the above embodiment, the switching of each switching element Q1 to Q8 was performed for half a cycle at the startup switching frequency Fb immediately after the startup of the DC/DC converter 10. For example, as shown in FIG. 6, the switching of each switching element Q1 to Q8 at the startup switching frequency Fb immediately after the startup of the DC/DC converter 10 may be performed for a longer period. FIGS. 6(a) to (d) show the transition of the drive signal of each switching element Q1 to Q8 in such a case, and FIG. 6(e) shows the transition of the transformer current iT in such a case. In the example of FIG. 6, immediately after the startup of the DC/DC converter 10, the switching of each switching element Q1 to Q8 is performed for one and a half cycles at the startup switching frequency Fb, and then the switching of each switching element Q1 to Q8 is started at the steady switching frequency Fa. In this case, the period from time t0 when the DC/DC converter 10 is started to time t2 when the switching of each switching element Q1 to Q8 at the startup switching frequency Fb is performed for one and a half cycles is the period during which startup drive is performed. As shown in Figure 6 (e), even in such a case, the increase in transformer current iT immediately after startup is suppressed.

For example, as shown in FIG. 7, the switching frequency may be changed in stages during startup drive. FIGS. 7(a)-(d) show the transitions of the drive signals of the switching elements Q1-Q8 before and after startup in such a case, and FIG. 7(e) shows the transitions of the transformer current iT before and after startup in such a case. In the example of FIG. 7, during the period from time t0 when the DC/DC converter 10 is started to time t3 when two and a half cycles of switching are completed, the switching frequency of each of the switching elements Q1-Q8 is lowered in stages within a range that is higher than the steady switching frequency Fa.

○ The period during which the startup drive is performed may be changed as appropriate. In order to prevent an excessive current from flowing through the DC/DC converter 10 immediately after startup, it is desirable to set the period during which the startup drive is performed so that the center of the amplitude of the transformer current iT immediately after the transition from startup drive to steady drive is close to 0 [A]. To achieve this, it is desirable to set the period during which the startup drive is performed so that the switching of each of the switching elements Q1 to Q8 is performed for N and a half periods during the startup drive. Here, "N" indicates any natural number. Here, consider a case in which the switching of each of the switching elements Q1 to Q8 in the DC/DC converter 10 is performed by alternately switching between two patterns, a first switching pattern A and a second switching pattern B. Suppose that switching is performed for N and a half periods during the startup drive, and the switching pattern immediately before the transition from startup drive to steady drive is the first switching pattern A. In this case, if the switching pattern immediately after the startup of the DC/DC converter 10 is the first switching pattern A, switching is performed for N and a half periods during the startup drive.

The DC/DC converter 10 is configured to operate by switching each of the switching elements Q1 to Q8 to alternate between two switching patterns, a first switching pattern A and a second switching pattern B. Even in a DC/DC converter configured to operate by periodically switching between three or more switching patterns, if the switching is performed at a higher frequency during the period from startup until a predetermined time has elapsed than after the predetermined time has elapsed, excessive current is less likely to flow immediately after startup.

The technical ideas and effects obtained from the above-described embodiment and its modified examples will be described below.
(i) A DC/DC converter as claimed in claim 1 or 2, wherein the drive circuit performs the startup drive in a state where a phase difference between a switching period of each switching element of the primary bridge circuit and a switching period of each switching element of the secondary bridge circuit is set to 0, and starts the steady-state drive in a state where the phase difference is set to 0.

(b) The method for controlling a DC/DC converter according to claim 3 or 4, wherein the startup drive is performed in a state where the phase difference between the switching period of each switching element of the secondary bridge circuit and the switching period of each switching element of the primary bridge circuit is set to 0, and the steady drive is started in a state where the phase difference is set to 0.

According to the DC/DC converter of (A) above and the control method of the DC/DC converter of (B) above, the power flowing between the bridge circuits on the primary and secondary sides is zero during the period from when the DC/DC converter is started to when steady-state operation begins. This makes it difficult for unintended power to flow through the DC/DC converter during the period when operation is unstable immediately after startup.

10...DC/DC converter, 30...drive circuit, B1...primary bridge circuit, B2...secondary bridge circuit, C1...primary smoothing capacitor, C2...secondary smoothing capacitor, L1, L2...inductor, N1...primary winding, N2...secondary winding, Q1 to Q8...switching elements, TR...transformer.

Claims (4)

A transformer having two windings, a primary winding and a secondary winding;
a primary-side bridge circuit in which a first leg having a plurality of switching elements connected in series and a second leg having a plurality of switching elements connected in series are connected in parallel to each other, wherein a connection point between an upper arm of the first leg and a lower arm of the first leg is connected to one end of the primary-side winding via a first reactor, and a connection point between the upper arm of the second leg and the lower arm of the second leg is connected to the other end of the primary-side winding;
a secondary-side bridge circuit in which a third leg having a plurality of switching elements connected in series and a fourth leg having a plurality of switching elements connected in series are connected in parallel to each other, wherein a connection point between an upper arm of the third leg and a lower arm of the third leg is connected to one end of the secondary-side winding via a second reactor, and a connection point between the upper arm of the fourth leg and the lower arm of the fourth leg is connected to the other end of the secondary-side winding;
a primary-side smoothing capacitor connected to the primary-side bridge circuit;
a secondary-side smoothing capacitor connected to the secondary-side bridge circuit;
a drive circuit that alternately switches the switching patterns of each switching element of the primary bridge circuit and each switching element of the secondary bridge circuit between a first switching pattern and a second switching pattern in which the on/off states of each switching element are opposite to those of the first switching pattern;
A DAB DC/DC converter comprising:
the drive circuit performs the switching at a predetermined steady switching frequency after a predetermined time has elapsed since the start-up of the DC/DC converter, and performs the switching at a startup switching frequency that is higher than the steady switching frequency before the predetermined time has elapsed;
when the switching pattern of each of the switching elements immediately before the predetermined time has elapsed since the start-up of the DC/DC converter is the first switching pattern, after the predetermined time has elapsed, the switching pattern is switched to the second switching pattern, and then the switching patterns are alternately switched in the order of the second switching pattern and the first switching pattern;
a switching pattern immediately after the start-up of the DC/DC converter is set as the first switching pattern;
A DC/DC converter comprising: A transformer having two windings, a primary winding and a secondary winding;
a primary-side bridge circuit in which a first leg having a plurality of switching elements connected in series and a second leg having a plurality of switching elements connected in series are connected in parallel to each other, wherein a connection point between an upper arm of the first leg and a lower arm of the first leg is connected to one end of the primary-side winding via a first reactor, and a connection point between the upper arm of the second leg and the lower arm of the second leg is connected to the other end of the primary-side winding;
a secondary-side bridge circuit in which a third leg having a plurality of switching elements connected in series and a fourth leg having a plurality of switching elements connected in series are connected in parallel to each other, wherein a connection point between an upper arm of the third leg and a lower arm of the third leg is connected to one end of the secondary-side winding via a second reactor, and a connection point between the upper arm of the fourth leg and the lower arm of the fourth leg is connected to the other end of the secondary-side winding;
a primary-side smoothing capacitor connected to the primary-side bridge circuit;
a secondary-side smoothing capacitor connected to the secondary-side bridge circuit;
a drive circuit that alternately switches the switching patterns of each switching element of the primary bridge circuit and each switching element of the secondary bridge circuit between a first switching pattern and a second switching pattern in which the on/off states of each switching element are opposite to those of the first switching pattern;
A DAB DC/DC converter comprising:
the drive circuit performs the switching at a predetermined steady switching frequency after a predetermined time has elapsed since the start-up of the DC/DC converter, and performs the switching at a startup switching frequency that is higher than the steady switching frequency , for only a half cycle or one or more cycles and a half cycle, before the predetermined time has elapsed;
when the switching pattern of each of the switching elements immediately before the predetermined time has elapsed since the start-up of the DC/DC converter is the first switching pattern, after the predetermined time has elapsed, the switching pattern is switched to the second switching pattern, and then the switching patterns are alternately switched in the order of the second switching pattern and the first switching pattern;
a switching pattern immediately after the start-up of the DC/DC converter is set as the first switching pattern;
A DC/DC converter comprising: a bridge circuit in which a transformer having two windings, a primary side and a secondary side, a first leg having a plurality of switching elements connected in series, and a second leg having a plurality of switching elements connected in series are connected in parallel to each other, wherein a connection point between an upper arm of the first leg and a lower arm of the first leg is connected to one end of the primary side winding via a first reactor, and a connection point between the upper arm of the second leg and the lower arm of the second leg is connected to the other end of the primary side winding; a secondary bridge circuit connected to one end of the secondary winding via a second reactor, and a connection point between the upper arm of the fourth leg and the lower arm of the fourth leg being connected to the other end of the secondary winding, a primary smoothing capacitor connected to the primary bridge circuit, and a secondary smoothing capacitor connected to the secondary bridge circuit, wherein a switching pattern of each switching element of the primary bridge circuit and each switching element of the secondary bridge circuit is alternately switched between a first switching pattern and a second switching pattern in which an on/off state of each switching element is opposite to that of the first switching pattern,
After a predetermined time has elapsed since the start-up of the DC/DC converter, the switching is performed at a predetermined steady switching frequency, and before the predetermined time has elapsed, the switching is performed at a startup switching frequency that is a frequency higher than the steady switching frequency;
when the switching pattern of each of the switching elements immediately before the predetermined time has elapsed since the start-up of the DC/DC converter is the first switching pattern, after the predetermined time has elapsed, the switching pattern is switched to the second switching pattern, and then the switching patterns are alternately switched in the order of the second switching pattern and the first switching pattern;
a switching pattern immediately after the start-up of the DC/DC converter is set as the first switching pattern;
2. A method for controlling a DC/DC converter comprising: a bridge circuit in which a transformer having two windings, a primary side and a secondary side, a first leg having a plurality of switching elements connected in series, and a second leg having a plurality of switching elements connected in series are connected in parallel to each other, wherein a connection point between an upper arm of the first leg and a lower arm of the first leg is connected to one end of the primary side winding via a first reactor, and a connection point between the upper arm of the second leg and the lower arm of the second leg is connected to the other end of the primary side winding; a secondary bridge circuit connected to one end of the secondary winding via a second reactor, and a connection point between the upper arm of the fourth leg and the lower arm of the fourth leg being connected to the other end of the secondary winding, a primary smoothing capacitor connected to the primary bridge circuit, and a secondary smoothing capacitor connected to the secondary bridge circuit, wherein a switching pattern of each switching element of the primary bridge circuit and each switching element of the secondary bridge circuit is alternately switched between a first switching pattern and a second switching pattern in which an on/off state of each switching element is opposite to that of the first switching pattern,
After a predetermined time has elapsed since the start-up of the DC/DC converter, the switching is performed at a predetermined steady-state switching frequency, and before the predetermined time has elapsed, the switching is performed for only a half cycle, or one or more cycles and a half cycle , at a startup switching frequency that is a frequency higher than the steady-state switching frequency;
when the switching pattern of each of the switching elements immediately before the predetermined time has elapsed since the start-up of the DC/DC converter is the first switching pattern, after the predetermined time has elapsed, the switching pattern is switched to the second switching pattern, and then the switching patterns are alternately switched in the order of the second switching pattern and the first switching pattern;
a switching pattern immediately after the start-up of the DC/DC converter is set as the first switching pattern;
2. A method for controlling a DC/DC converter comprising: