JP2023034862A - DC-DC converter - Google Patents

Abstract

To make an output power change to an output side a monotonous change.SOLUTION: A DC-DC converter includes a primary-side bridge circuit (10), a secondary-side bridge circuit (20), and a conversion unit (30). The primary-side bridge circuit (10) includes multiple primary-side switching elements (S1-4). The secondary-side bridge circuit (20) includes multiple secondary-side switching elements (S5-8), a return diode, and a capacitor. All secondary-side switching elements are turned off and a rectifier mode for controlling a phase difference between legs in the primary-side bridge circuit and a phase shift mode for controlling the on/off of secondary-side switching elements are switched.SELECTED DRAWING: Figure 1

Description

The present invention relates to DC-DC converters.

Generally, in a dual active bridge type DC-DC converter, a transformer is provided between the primary side and the secondary side in order to insulate them. Depending on the winding ratio between the primary side and the secondary side of the transformer, the voltage on the primary side can be stepped up or stepped down and output to the secondary side.

Patent Literature 1 discloses a method of switching between first control and second control according to the output power in order to adjust the output power when power is output from the input side to the output side. In the first control, the duty is increased as the output power increases. After the duty reaches 50%, as the second control, the inter-bridge phase difference between the input-side bridge circuit and the output-side bridge circuit is increased as the output power increases.

JP 2020-10594 A

When the control method is switched, the output power to the output side does not monotonically change, and the output power may change from increasing to decreasing along with the switching of the control method. That is, plotting the change in output power on a graph results in a curve with extremes such as maxima or minima. Therefore, a parameter cannot be determined uniquely from the output power, and control is not easy.

Accordingly, it is an object of one aspect of the present invention to realize a DC-DC converter that can monotonically change the output power to the output side even when the control method is switched, thereby facilitating control. .

In order to solve the above problems, a DC-DC converter according to an aspect of the present invention includes a plurality of primary side switching elements, a plurality of free wheel diodes connected in parallel to each of the primary side switching elements, and a primary bridge circuit having a first leg and a second leg; a plurality of secondary switching elements; and a plurality of secondary switching elements connected in parallel to each of the secondary switching elements. a secondary bridge circuit having a third leg and a fourth leg, including a freewheeling diode and a capacitor; and a transformer, between the primary bridge circuit and the secondary bridge circuit. and a control unit configured to control switching of the primary side switching elements and the secondary side switching elements, wherein the control unit turns off all of the secondary side switching elements and switches off the first switching element. A first control that adjusts the inter-leg phase difference between the leg and the second leg in the range of π to 0 according to the output, and a third leg that sets the inter-leg phase difference between the first leg and the second leg to 0. A second control for setting the inter-leg phase difference with the fourth leg to 0 and adjusting the inter-bridge phase difference between the primary side bridge circuit and the secondary side bridge circuit according to the output, according to the first control The first control and the second control are switched and executed in a state where the output and the output by the second control are equal.

According to one aspect of the present invention, the change in output power that accompanies parameter changes is monotonous, and control is easy.

1 is a circuit diagram showing a DC-DC converter according to Embodiment 1; FIG. 4 is a diagram showing the relationship between parameters and output power of the DC-DC converter according to Embodiment 1. FIG. 4 is a timing chart when the phase difference between legs is 0 in the rectifier mode according to Embodiment 1; 4 is a timing chart in phase shift mode according to Embodiment 1. FIG. 7 is a timing chart while increasing the duty at the time of mode transition according to the first embodiment; 4 is a diagram showing changes in output power when the DC-DC converter according to Embodiment 1 is shifted from the rectifier mode to the phase shift mode; FIG. 4 is a block diagram of a control section in a rectifier mode in the DC-DC converter according to Embodiment 1. FIG. 4 is a block diagram of a control section in phase shift mode in the DC-DC converter according to Embodiment 1. FIG. FIG. 4 is a diagram showing the relationship between parameters and output power of a DC-DC converter according to a reference operation example; FIG. 4 is a diagram showing changes in output power when the DC-DC converter according to the reference operation example shifts from the first control to the second control;

[Embodiment 1]
The present invention will be described in detail below with reference to FIGS. 1 to 10. FIG.

(Configuration of DC-DC converter 1)
FIG. 1 is a circuit diagram showing a DC-DC converter 1 according to Embodiment 1. FIG. The DC-DC converter 1 includes a primary side bridge circuit 10, a secondary side bridge circuit 20, a conversion section 30, and a control section 40.

The primary side bridge circuit 10 is connected to the DC power supply E1 at the input terminal. The secondary bridge circuit 20 is connected to the DC power source E2 at its output terminal. The voltage between the input terminals of the primary side bridge circuit 10 is the primary side voltage V1, and the current flowing through the input terminals of the primary side bridge circuit 10 is the primary side current I1. The voltage across the output terminals of the secondary bridge circuit 20 is the secondary voltage V2, and the current flowing through the output terminals of the secondary bridge circuit 20 is the secondary current I2. Here, the primary side voltage V1, the primary side current I1, the secondary side voltage V2, and the secondary side current I2 are time-average values obtained by the control unit 40, and are used for control described later.

Here, "input" and "output" are expressions assuming that power is transmitted from the DC power supply E1 side to the DC power supply E2 side, that is, from the primary side to the secondary side. However, this is an expression for convenience, and the same applies to the following. The DC-DC converter 1 of Embodiment 1 is a bidirectional dual active bridge type DC-DC converter, and is capable of transmitting power from the secondary side to the primary side.

In the primary side bridge circuit 10, a capacitor element C1 is connected in parallel to a full bridge circuit provided with four primary side switching elements S1 to S4. The primary side bridge circuit 10 is composed of a first leg 11, a second leg 12, and a capacitor element C1. In the first leg 11, the primary side switching element S1 and the primary side switching element S2 are connected in series. The second leg 12 has a primary switching element S3 and a primary switching element S4 connected in series.

In the secondary bridge circuit 20, a capacitor element C2 is connected in parallel to a full bridge circuit provided with four secondary side switching elements S5 to S8. The secondary bridge circuit 20 is composed of a third leg 21, a fourth leg 22, and a capacitor element C2. The third leg 21 has a secondary switching element S5 and a secondary switching element S6 connected in series. The fourth leg 22 has a secondary switching element S7 and a secondary switching element S8 connected in series.

The primary side switching elements S1 to S4 and the secondary side switching elements S5 to S8 (hereinafter collectively referred to as switching elements S1 to S8) are MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or other FETs (Field Effect Transistors). Transistor). Alternatively, the switching elements S1 to S8 may be composed of IGBTs (Insulated Gate Bipolar Transistors) or other transistors.

Freewheeling diodes D1 to D8 are connected in parallel to the switching elements S1 to S8, respectively. Snubber capacitors Csnub1 to Csnub8 are connected in parallel to the switching elements S1 to S8, respectively.

The conversion unit 30 includes at least a transformer Tr with a winding ratio of n. In the circuit diagram of FIG. 1, the inductance component of the conversion unit 30 is equivalently expressed as a reactor L1 and a reactor L2 provided on the primary side. In the equivalent circuit of FIG. 1, the reactor L1 is connected to the connection point between the primary switching element S1 and the primary switching element S2 and to the primary terminal of the primary winding of the transformer Tr. The reactor L2 is connected to a connection point between the primary switching element S3 and the primary switching element S4 and to a secondary terminal of the primary winding of the transformer Tr.

The inductance component represented by reactor L1 and reactor L2 includes leakage inductance of transformer Tr. A secondary winding of the transformer Tr is connected to a connection point between the secondary switching element S5 and the secondary switching element S6 and a connection point between the secondary switching element S7 and the secondary switching element S8. When the conversion unit 30 is provided with a reactor element as an actual element, the reactor element may be arranged on the primary side of the transformer Tr, on the secondary side, or on both sides. good.

The control unit 40 controls switching of the switching elements S1 to S8 with reference to the primary voltage V1, the primary current I1, the secondary voltage V2, and the secondary current I2.

（参考動作例：従来技術による制御方法の変更とその出力電力）
参考動作例として、ＤＣ－ＤＣコンバータ１での特許文献１に基づく制御を実行した場合の動作が示される。 (Simulation conditions)
After that, the output power from the primary side bridge circuit 10 to the secondary side bridge circuit 20 in each control method is calculated by simulation. At this time, calculation was performed under the following conditions.

(Reference operation example: change of control method by conventional technology and its output power)
As a reference operation example, an operation when control based on Patent Document 1 is executed in the DC-DC converter 1 is shown.

FIG. 9 is a diagram showing the relationship between parameters and output power of the DC-DC converter 1 according to the reference operation example. In FIG. 9, the horizontal axis represents the ratio of output power to the reference output power (hereinafter referred to as PU), the first vertical axis represents duty, and the second vertical axis represents phase difference between bridges. The reference output power will be described later.

In the first control, the output power is increased by increasing the duty of each switching element S1 to S8 from 0% to 50%. At this time, the inter-bridge phase difference between the primary bridge circuit 10 and the secondary bridge circuit 20 is zero.

Next, in the reference operation example, when the duty reaches 50%, the control shifts to the second control. In the second control, the inter-bridge phase difference is increased from 0 to π/2 while the duty is kept constant at 50%. This results in a monotonically increasing trend in the output power.

FIG. 10 is a diagram showing changes in output power when the DC-DC converter 1 according to the reference operation example shifts from the first control to the second control. In FIG. 10, the horizontal axis represents the number of plots representing the passage of time in the simulation, and the vertical axis represents the ratio of the output power to the reference output power.

In FIG. 10, between plot number 1 and plot number 10 is the section where the first control is performed, duty is between 0.05 and 0.48 in increments of 0.05 (0.05 from plot number 9 to plot number 10). 48 increments). Also, the interval between 11 and 20 plots is a section in which the second control is performed, and the inter-bridge phase difference φ is increased from 0° to 90° in 9° increments.

As shown in FIG. 10, in the first control, at a duty of less than 50%, there is an inflection point at which a monotonous increase turns to a monotonous decrease as the duty increases. Also, it can be seen that in the second control, it monotonically increases according to the increase in the inter-bridge phase difference. That is, the inflection point at which monotonous decrease in the first control transitions to monotonous increase in the second control is also present at the point of transition between the first control and the second control.

In the first control and the second control, when the primary side voltage V1 and the secondary side voltage V2 fluctuate and the ratio of the primary side voltage V1 and the secondary side voltage V2 deviates from the winding ratio n, the first control A drop occurs in the output power at the stage of shifting to the second control. Interlocking control of the first control and the second control may alleviate the drop, but there are a huge number of voltage fluctuation patterns, and it is very difficult to set the control conditions based on the duty and the phase difference between the bridges. .

(rectifier mode)
Next, the operation of the DC-DC converter 1 according to Embodiment 1 will be described. In Embodiment 1, the output power is increased from the state where the output power is 0 by the rectifier mode (first control mode).

In the rectifier mode, the secondary side switching elements S5 to S8 of the secondary side bridge circuit 20 are always turned off. That is, the secondary side bridge circuit 20 forms a full diode bridge circuit by the free wheel diodes D5 to D8.

In the rectifier mode, the primary side switching elements S1 to S4 are switched every half cycle, and the inter-leg phase difference φL between the first leg 11 and the second leg 12 is adjusted in the range from π to 0, so that the output Power can be adjusted.

FIG. 2 is a diagram showing the relationship between parameters and output power of the DC-DC converter 1 according to the first embodiment. In FIG. 2, the horizontal axis is the ratio of the output power to the reference output power, the first vertical axis is the duty, and the second vertical axis is the inter-bridge phase difference. As shown in FIG. 2, the closer the leg-to-leg phase difference φL is to π, the more P.L. U is small, and the closer the inter-leg phase difference φL is to 0, the more P.U is. U tends to be large. Also, the output power changes monotonically.

FIG. 3 is a timing chart when the inter-leg phase difference φL is 0 in the rectifier mode according to the first embodiment. As shown in FIG. 3, when the inter-leg phase difference φL is 0, a phase difference occurs between the primary side converter voltage Vtr1 and the secondary side converter voltage Vtr2. This phase difference is caused by the relationship between the freewheeling of the primary side bridge circuit and the conducting sections of the freewheeling diodes D5-D8 of the secondary side bridge circuit that occur in the rectifier mode. In other words, the waveform includes a phase difference due to the difference in timing between the off period of the primary switching elements S1 to S4 and the conduction period of the freewheeling diodes D5 to D8 on the secondary side. Further, this phase difference is calculated by the control unit 40 so as to be equal to the bridge-to-bridge phase difference φB, which will be described later.

(Phase shift mode)
After the inter-leg phase difference φL becomes 0 in the rectifier mode, control is performed in the phase shift mode (second control mode) in order to further increase the output power.

In the phase shift mode, the primary side switching elements S1 to S4 and the secondary side switching elements S5 to S8 are switched every half cycle (50% duty). In the phase shift mode, an inter-bridge phase difference φB is provided between the primary side bridge circuit 10 and the secondary side bridge circuit 20 according to the output power. At this time, the inter-leg phase difference φL between the first leg 11 and the second leg 12 is zero, and the inter-leg phase difference between the third leg 21 and the fourth leg 22 is also zero.

FIG. 4 is a timing chart in phase shift mode according to the first embodiment. In particular, FIG. 4 is also the state immediately after completing the mode transition to the phase shift mode.

As shown in FIG. 2, the closer the inter-bridge phase difference .phi.B is to .pi. U is large, and the closer the bridge-to-bridge phase difference φB is to 0, the more P.U. U tends to be small. Also, the output power changes monotonically.

Here, in FIGS. 9 and 10, P.P., which is the ratio of the output power to the reference output power, is described above. U becomes 1 when the inter-bridge phase difference φB is π/2. That is, the ratio to the output power at this time is P. is U.

(mode transition)
After the inter-leg phase difference φL becomes 0 in the rectifier mode, the control unit 40 shifts the mode from the rectifier mode to the phase shift mode in order to further increase the output power.

FIG. 5 is a timing chart while increasing the duty at the time of mode transition according to the first embodiment.

ここで、Ｐｏｕｔは２次側ブリッジ回路２０での出力電力を、ｆｓｗはスイッチング周波数を表す。すなわち、ブリッジ間位相差φＢは、２次側ブリッジ回路２０での出力電力Ｐｏｕｔに応じて定まる値である。数１により、整流器モードでの最大出力と等しい出力を、位相シフトモードで得るためのブリッジ間位相差φＢを求める。 First, an inter-bridge phase difference φB is provided between the primary bridge circuit 10 and the secondary bridge circuit 20 . φB is derived from the relational expression shown below.

Here, Pout represents the output power of the secondary bridge circuit 20, and fsw represents the switching frequency. That is, the inter-bridge phase difference φB is a value determined according to the output power Pout of the secondary bridge circuit 20 . From Equation 1, the phase difference φB between the bridges for obtaining an output equal to the maximum output in the rectifier mode in the phase shift mode is obtained.

Next, as shown in FIG. 4, in the mode transition, the duty of the secondary side switching elements S5 to S8 is increased from 0% to 50%. During this time, the primary-side switching elements S1 to S4 are switched in a half cycle (50% duty), and the inter-leg phase difference φL between the first leg 11 and the second leg 12 is 0, and the third leg 21 and The inter-leg phase difference with the fourth leg 22 is also zero. The mode transition from the rectifier mode to the phase shift mode is completed by switching the secondary side switching elements S5 to S8 in a half cycle (with a duty of 50%).

As shown in FIGS. 3 to 5, during mode transition from the rectifier mode to the phase shift mode, the secondary conversion voltage Vtr2 and the inductor current iL do not change, so the duty changes (from 0% to 50%), but the output power does not increase.

This phenomenon is caused by the fact that when the mode is shifted to the phase shift mode when the phase difference φL between the legs is 0, no current flows through the secondary switching elements S5 to S8, and only the freewheeling diodes D5 to D8. It's becoming Therefore, in the mode transition, even if the duty is increased (changed), the output power does not increase. When the mode transition is completed, the mode is switched to the phase shift mode, and the inter-bridge phase difference φB is increased, current flows through the secondary side switching elements S5 to S8 for the first time, and the output power begins to increase.

(Relationship between rectifier mode and phase shift mode)
FIG. 6 is a diagram showing the results of changing the output power by shifting from the rectifier mode to the phase shift mode in the DC-DC converter 1 according to the first embodiment. In FIG. 6, the horizontal axis represents the number of plots representing the passage of time in the simulation, and the vertical axis represents the ratio of the output power to the reference output power.

In FIG. 6, plot numbers 1 to 10 are sections in which the rectifier mode is performed, and the inter-leg phase difference φL is increased from 0° to 90° in increments of 9°. Further, the period between 11 and 20 plots is a section in which the phase shift mode is performed, and the inter-bridge phase difference φ is increased from 0° to 90° in increments of 9°.

As shown in FIG. 6, by shifting from the rectifier mode to the phase difference shift mode and manipulating the leg-to-leg phase difference φL or the bridge-to-bridge phase difference φB, the output power monotonically increases. Therefore, the desired P.I. Since the combination of the phase difference φL between legs and the phase difference φB between bridges is uniquely determined for U, control is easy. Further, in the rectifier mode, phase shift mode, and mode transition, the output power can be controlled by controlling the phase difference φL between legs or the phase difference φB between bridges by feedback control.

(Brief Summary)
Therefore, in the process of shifting from the rectifier mode to the phase shift mode in order to increase the output power, the output power characteristics do not drop and the output power increases monotonously. Therefore, by controlling the output power with respect to the target power, the leg-to-leg phase difference φL or the bridge-to-bridge phase difference φB, which is a parameter, is uniquely determined, which facilitates control.

Also in the rectifier mode, the inter-bridge phase difference φB corresponding to the output power is calculated also in the rectifier mode in order to continuously change the rectifier mode and the phase shift mode. Therefore, even in the rectifier mode, it is sufficient to calculate the inter-bridge phase difference φB in addition to the leg-to-leg phase difference φL, so there is an advantage that the design is easier than the reference example.

(Control block diagram in rectifier mode)
FIG. 7 is a block diagram of the controller 40 in the rectifier mode in the DC-DC converter 1 according to the first embodiment. As shown in FIG. 7, the output power Pout is calculated from the secondary voltage V2 and the secondary current I2. A power deviation ΔPout is derived from the output power Pout and the target power Pout*.

Power deviation ΔPout, primary side voltage V1, secondary side voltage V2, winding ratio n, switching frequency fsw, and inductor L are converted into deviation φe in the phase difference region by the phase deviation calculator. do. The inter-leg phase difference φL is generated from the difference between the deviation φe amplified by the PI calculation and π.

If the inter-leg phase difference φL is in the range of 0<φL<π, 0 is set to the flag a. In rectifier mode, this condition is satisfied.

By outputting a PWM (Pulse Width Modulation) signal to the primary side switching elements S1 to S4, the primary side switching elements S1 to S4 are switched. The primary side switching elements S1 and S2 of the first leg 11 are in opposite phases to each other. Further, the phases of the primary side switching elements S3 and S4 of the second leg 12 are delayed with respect to the first leg by the inter-leg phase difference φL. Further, the secondary side switching elements S5 to S8 are always off.

In addition to the output power Pout, the bridge-to-bridge phase difference φB is derived from the primary side voltage V1, the secondary side voltage V2, the winding ratio n, the switching frequency fsw, and the inductor L.

(Control block diagram in phase shift mode)
FIG. 8 is a block diagram of the control section 40 in the phase shift mode in the DC-DC converter 1 according to Embodiment 1. As shown in FIG. As shown in FIG. 8, the inter-leg phase difference φL is derived by the same procedure as in the rectifier mode. Here, in the phase shift mode, the inter-leg phase difference φL is a negative value. Therefore, after setting the flag a to 1, the inter-leg phase difference is set to 0.

A PWM signal is output to the primary side switching elements S1 to S4 to switch the primary side switching elements S1 to S4. Here, the primary side switching elements S2 and S3 are in opposite phase to the primary side switching elements S1 and S4.

When flag a is assigned to the ramp function and flag a is set to 1, the ramp function begins to rise. A signal obtained by comparing the ramp function and the triangular wave determines the duty. A signal whose phase is delayed by the bridge-to-bridge phase difference φB is generated with respect to the signal, and is output to the secondary side switching elements S5 to S8 to switch the secondary side switching elements S5 to S8.

Further, when the inter-leg phase difference φL becomes a positive value during the phase shift mode control, the flag a is set to 0, and the ramp function starts falling. As a result, the duty is reduced and the phase shift mode is switched to the rectifier mode.

[Modification]
Although the inter-leg phase difference is controlled in the rectifier mode in Embodiment 1, the present invention is not limited to this. Any control may be used as long as the duty of the primary side switching elements S1 to S4 becomes 50% at the maximum output in the rectifier mode. For example, the duty or the inter-leg phase difference φL may be controlled according to the output power.

[Example of realization by software]
The function of the DC-DC converter 1 (hereinafter referred to as "device") is a program for causing a computer to function as the device, and the computer as each control block of the device (especially each part included in the control unit 40) It can be realized by a program for functioning.

In this case, the apparatus comprises a computer having at least one control device (eg processor) and at least one storage device (eg memory) as hardware for executing the program. Each function described in each of the above embodiments is realized by executing the above program using the control device and the storage device.

The program may be recorded on one or more computer-readable recording media, not temporary. The recording medium may or may not be included in the device. In the latter case, the program may be supplied to the device via any transmission medium, wired or wireless.

Also, part or all of the functions of the above control blocks can be realized by logic circuits. For example, integrated circuits in which logic circuits functioning as the control blocks described above are formed are also included in the scope of the present invention. In addition, it is also possible to implement the functions of the control blocks described above by, for example, a quantum computer.

〔summary〕
In order to solve the above problems, a DC-DC converter according to an aspect of the present invention includes a plurality of primary side switching elements, a plurality of free wheel diodes connected in parallel to each of the primary side switching elements, and a primary bridge circuit having a first leg and a second leg; a plurality of secondary switching elements; and a plurality of secondary switching elements connected in parallel to each of the secondary switching elements. a secondary bridge circuit having a third leg and a fourth leg, including a freewheeling diode and a capacitor; and a transformer, between the primary bridge circuit and the secondary bridge circuit. and a control unit configured to control switching of the primary side switching elements and the secondary side switching elements, wherein the control unit turns off all of the secondary side switching elements and switches off the first switching element. A first control mode in which the inter-leg phase difference between the leg and the second leg is adjusted in the range from π to 0 according to the output, and a inter-leg phase difference between the first leg and the second leg is set to 0 and the third leg and a second control mode in which the inter-leg phase difference between the and the fourth leg is set to 0, and the inter-bridge phase difference between the primary side bridge circuit and the secondary side bridge circuit is adjusted according to the output of the first The output by the control mode and the output by the second control mode are switched and executed in the same state.

With the above configuration, the output power can be monotonously changed from the first control mode to the second control mode. Therefore, it is easy to control the output power.

The control unit changes the duty of the secondary side switching element from 0 to values may be gradually changed.

According to the above configuration, it is possible to switch from the first control mode to the second control mode without changing the output power. Therefore, the output power can be monotonously changed between the first control mode and the second control mode.

In the first control mode, the control section may control the primary side switching elements so that they perform switching every half cycle.

According to the above configuration, it is easy to control the primary side switching element in the first control mode.

In the second control mode, the control section may control the primary side switching element and the secondary side switching element so that they each perform switching every half cycle.

According to the above configuration, it is easy to control the primary side switching element and the secondary side switching element in the second control mode.

[Additional notes]
The present invention is not limited to the above-described embodiments, but can be modified in various ways within the scope of the claims, and can be obtained by appropriately combining technical means disclosed in different embodiments. is also included in the technical scope of the present invention.

1, 100 DC-DC converter 10 primary side bridge circuit 11 first leg 12 second leg 20 secondary side bridge circuit 21 third leg 22 fourth leg 30 converter 40 controller Csnub1 to Csnub8 snubber capacitors (capacitors)
D1~D8 Freewheeling diode S1~S4 Primary side switching element S5~S8 Secondary side switching element Tr Transformer

Claims (4)

A primary side bridge having a first leg and a second leg, including a plurality of primary side switching elements, and a plurality of freewheeling diodes and capacitors connected in parallel to each of the primary side switching elements. a circuit;
A secondary side bridge including a plurality of secondary side switching elements, and a plurality of freewheeling diodes and capacitors connected in parallel to each of the secondary side switching elements, and having a third leg and a fourth leg. a circuit;
a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit;
a control unit that controls switching of the primary side switching element and the secondary side switching element,
The control unit turns off all the secondary side switching elements, and adjusts an inter-leg phase difference between the first leg and the second leg in a range from π to 0 according to the output; The inter-leg phase difference between the leg and the second leg is set to 0, the inter-leg phase difference between the third leg and the fourth leg is set to 0, and the inter-bridge position between the primary side bridge circuit and the secondary side bridge circuit is set to 0. A DC-DC converter characterized in that a second control mode for adjusting a phase difference according to an output is switched and executed in a state in which the output in the first control mode and the output in the second control mode are equal. The control unit gradually changes the duty of the secondary switching element from 0 to the value in the second control mode when switching from the first control mode to the second control mode, The DC-DC converter according to claim 1. 3. The DC-DC converter according to claim 1, wherein, in the first control mode, the control unit controls the primary side switching elements so that they perform switching every half cycle. . 4. The control unit controls the primary side switching element and the secondary side switching element so that they are switched every half cycle in the second control mode. The DC-DC converter according to any one of 1.

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