DC-DC converter and control method thereof

Abstract

It evens out the heating between the legs while reducing the possibility of excessive current draw. [Solution] A control unit (control device 40) of a DC-DC converter (1) alternately executes a first operation of assigning the first leg (21) to the first virtual leg, the second leg (22) to the second virtual leg, the third leg (23) to the third virtual leg, and the fourth leg (24) to the fourth virtual leg every N periods (N is a natural number) of a switching cycle, and a second operation of assigning the second leg to the first virtual leg, the first leg to the second virtual leg, the fourth leg to the third virtual leg, and the third leg to the fourth virtual leg, and swapping on / off control of the upper arm and on / off control of the lower arm.

Description

[Technical field]

[0001] The present invention relates to a DC-DC converter and a control method thereof. [Background technology]

[0002] The applicant has proposed a technology that can reduce the overall loss of a DC-DC converter by suppressing switching loss in a required leg in a dual active bridge (DAB) type DC-DC converter that can transmit DC power in both directions (Patent Document 1).The applicant has also proposed a technology that can average the loss of each leg by switching the leg in which switching loss is suppressed every switching period (Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7315886 [Patent Document 2] JP 2023-166914 A Summary of the Invention [Problem to be solved by the invention]

[0004] It is desirable to realize a DC-DC converter with further advantages in the technology capable of averaging the losses in each leg, such as that in Patent Document 2. In view of this, an object of one embodiment of the present disclosure is to realize a DC-DC converter that can average the losses in each leg and can reduce the possibility of an excessive current occurring due to magnetic saturation. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, one aspect of the present disclosure is a DC-DC converter including a primary bridge circuit including a plurality of switching elements and having a first leg and a second leg, a secondary bridge circuit including a plurality of switching elements and having a third leg and a fourth leg, a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit, and a control unit that controls the switching elements, wherein assuming a first virtual leg and a second virtual leg that are assigned one-to-one to the first leg and the second leg, and a third virtual leg and a fourth virtual leg that are assigned one-to-one to the third leg and the fourth leg, the control unit performs on / off control of an upper arm in each of the virtual legs as a basic operation when the DC-DC converter transports power. a phase of on / off control for the upper arm is shifted by half a switching period, and further, the first leg is assigned to a first virtual leg, the second leg to a second virtual leg, the third leg to a third virtual leg, and the fourth leg to a fourth virtual leg, and in each of the legs, a first operation of controlling the switching elements by the basic operation is performed, and a second operation of assigning the second leg to the first virtual leg, the first leg to the second virtual leg, the fourth leg to the third virtual leg, and the third leg to the fourth virtual leg, and in each of the legs, switching between the on / off control for the upper arm and the on / off control for the lower arm in the basic operation and controlling each of the switching elements is performed alternately every N periods (N is a natural number) of the switching period.

[0006] In order to solve the above problem, another aspect of the present disclosure is a control method for a DC-DC converter including a primary bridge circuit including a plurality of switching elements and having a first leg and a second leg, a secondary bridge circuit including a plurality of switching elements and having a third leg and a fourth leg, and a conversion unit including a transformer and connected between the primary bridge circuit and the secondary bridge circuit, in which, assuming a first virtual leg and a second virtual leg assigned one-to-one to the first leg and the second leg, and a third virtual leg and a fourth virtual leg assigned one-to-one to the third leg and the fourth leg, the control method includes the steps of: The phase of the off control is shifted by half a switching period, and further, the first leg is assigned to a first virtual leg, the second leg to a second virtual leg, the third leg to a third virtual leg, and the fourth leg to a fourth virtual leg, and in each of the legs, a first operation of controlling the switching elements by the basic operation is performed, and a second operation of assigning the second leg to the first virtual leg, the first leg to the second virtual leg, the fourth leg to the third virtual leg, and the third leg to the fourth virtual leg, and in each of the legs, switching between the on / off control for the upper arm and the on / off control for the lower arm in the basic operation and controlling each of the switching elements is performed alternately every N periods (N is a natural number) of the switching period.

[0007] The control unit according to each aspect of the present invention may be realized by a computer. In this case, a control program for a DC-DC converter that causes a computer to operate as each unit (software element) of the control unit to realize the control unit, and a computer-readable recording medium on which the control program is recorded, also fall within the scope of the present invention. Effect of the Invention

[0008] According to one aspect of the present disclosure, it is possible to realize a DC-DC converter that can average the losses in each leg and reduce the possibility of an excessive current occurring due to magnetic saturation. [Brief description of the drawings]

[0009] [Figure 1] 1 is a diagram showing a circuit configuration of a DC-DC converter according to an embodiment of the present disclosure. [Diagram 2] 1 is a waveform diagram showing voltages and currents at various parts of a DC-DC converter, which shows a comparison between a base control method, a control method for averaging losses in each leg, and a control method according to an embodiment of the present disclosure. [Diagram 3] FIG. 2 is a diagram illustrating, as a logic circuit, an example of calculation for implementing switching control performed in a DC-DC converter according to an embodiment of the present disclosure. [Figure 4] 4 is a truth table for the logic circuit shown in FIG. 3. [Diagram 5] 4 is a waveform diagram showing a step-down operation at low output of the DC-DC converter according to the embodiment of the present disclosure. FIG. [Figure 6] FIG. 4 is a waveform diagram showing a step-down operation at low output of the DC-DC converter according to the embodiment of the present disclosure, showing the element voltage and element current of each switching element. [Figure 7] 4 is a waveform diagram showing a boost operation during low output of the DC-DC converter according to the embodiment of the present disclosure. FIG. [Figure 8] FIG. 4 is a waveform diagram showing a boost operation during low output of the DC-DC converter according to the embodiment of the present disclosure, showing the element voltage and element current of each switching element. [Figure 9] 5A to 5C are waveform diagrams showing operation at high output or rated voltage ratio of the DC-DC converter according to the embodiment of the present disclosure. [Figure 10] 1 is a waveform diagram showing the operation of the DC-DC converter according to the embodiment of the present disclosure at high output or at the rated voltage ratio, showing the element voltage and element current of each switching element. FIG. [Figure 11]It is a waveform diagram of the AC voltage, AC current of the DC-DC converter, and the magnetic flux density in the core of the transformer. It shows a comparison of the basic control method, the control method for averaging the losses of each leg, and the control method according to the embodiment of the present disclosure. [Figure 12] It is a waveform diagram showing the voltage and current of each part of the DC-DC converter. It shows a comparison of the basic control method, the control method for averaging the losses of each leg, and the control method according to another case of the embodiment of the present disclosure.

Mode for Carrying Out the Invention

[0010] 〔Embodiment〕 Hereinafter, an embodiment according to one aspect of the present disclosure will be described with reference to the drawings. In the following description, the term "terminal" is also used to mean a connection point of a specific circuit element inside the electric circuit, and does not necessarily mean that only an external connection terminal such as a connector is provided.

[0011] <Configuration Outline of DC-DC Converter 1> FIG. 1 is a circuit diagram of the DC-DC converter 1 according to the embodiment. The DC-DC converter 1 is an isolated DC-DC converter of the dual active bridge (DAB) type that enables bidirectional power transfer between the first terminal pair and the second terminal pair. The first terminal pair is composed of a high-potential terminal p1 and a low-potential terminal q1. The second terminal pair is composed of a high-potential terminal p2 and a low-potential terminal q2.

[0012] In the following, the first terminal pair side may be referred to as the primary side, and the second terminal pair side may be referred to as the secondary side. However, this does not necessarily mean that power is transported from the primary side to the secondary side, and it is a convenient name. During the operation of the DC-DC converter 1, the voltage between the first terminal pair is the primary side input / output voltage V1 (primary side input / output voltage), and the voltage between the second terminal pair is the secondary side input / output voltage V2 (secondary side input / output voltage).

[0013] For example, a first DC voltage source is connected to a first terminal pair (p1, q1) of the DC-DC converter 1, and a second DC voltage source is connected to a second terminal pair (p2, q2) of the DC-DC converter 1. In this case, the DC-DC converter 1 according to the embodiment can transmit power in response to any of a step-up operation from the first DC voltage source to the second DC voltage source, a rated voltage ratio operation, and a step-down operation. The DC-DC converter 1 can also transmit power in response to any of a step-up operation from the second DC voltage source to the first DC voltage source, a rated voltage ratio operation, and a step-down operation.

[0014] The DC-DC converter 1 includes a primary side smoothing circuit 11, a primary side bridge circuit 12, a conversion unit 13, a secondary side bridge circuit 14, a secondary side smoothing circuit 15, and a control device 40. The primary side smoothing circuit 11 and the primary side bridge circuit 12 are cascade connected to a first terminal pair in this order. The secondary side smoothing circuit 15 and the secondary side bridge circuit 14 are cascade connected to a second terminal pair in this order. The conversion unit 13 is provided between the primary side bridge circuit 12 and the secondary side bridge circuit 14.

[0015] The primary side smoothing circuit 11 may be any circuit that smoothes the voltage across the first terminal pair, and is configured to include a capacitor Cs1 provided between terminals p1 and q1. The DC side of the primary side bridge circuit 12 is connected to the primary side smoothing circuit 11. Similarly, the secondary side smoothing circuit 15 may be any circuit that smoothes the voltage across the second terminal pair, and is configured to include a capacitor Cs2 provided between terminals p2 and q2. The DC side of the secondary side bridge circuit 14 is connected to the secondary side smoothing circuit 15.

[0016] The primary-side bridge circuit 12 is composed of a first leg 21 and a second leg 22 connected in parallel to each other. The first leg 21 is composed of a switching element S1 and a switching element S2 connected in series, and the connection point between them is one terminal x1 of the AC-side terminal pair of the primary-side bridge circuit 12. The second leg 22 is composed of a switching element S3 and a switching element S4 connected in series, and the connection point between them is the other terminal y1 of the AC-side terminal pair. The AC-side terminal pair (x1, y1) of the primary-side bridge circuit 12 inputs and outputs AC power to and from the primary side of the conversion unit 13.

[0017] The secondary-side bridge circuit 14 is composed of a third leg 23 and a fourth leg 24 connected in parallel to each other. The third leg 23 is composed of a switching element S5 and a switching element S6 connected in series, and the connection point between these is one terminal x2 of the AC-side terminal pair of the secondary-side bridge circuit 14. The fourth leg 24 is composed of a switching element S7 and a switching element S8 connected in series, and the connection point between these is the other terminal y2 of the AC-side terminal pair. The AC-side terminal pair (x2, y2) of the secondary-side bridge circuit 14 inputs and outputs AC power to and from the secondary side of the conversion unit 13.

[0018] Switching elements S1, S3, S5, and S7 are high-potential side switching elements in the first leg 21, second leg 22, third leg 23, and fourth leg 24, respectively, and are referred to as upper arms. Switching elements S2, S4, S6, and S8 are low-potential side switching elements in the first leg 21, second leg 22, third leg 23, and fourth leg 24, respectively, and are referred to as lower arms.

[0019] The switching elements S1 to S4 included in the primary bridge circuit 12 are primary side switching elements. The switching elements S5 to S8 included in the secondary bridge circuit 14 are secondary side switching elements. Each of these switching elements S1 to S8 may include a free wheel diode as shown in the circuit diagram of FIG.

[0020] Each of the switching elements S1 to S8 is controlled to be turned on / off by a control device 40 (controller). An IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor field-effect transistor) can be used as each of the switching elements S1 to S8.

[0021] The conversion unit 13 has an insulating transformer Tr, the primary winding of which is connected to the AC terminal pair (x1, y1) of the primary bridge circuit 12, and the secondary winding of which is connected to the AC terminal pair (x2, y2) of the secondary bridge circuit 14. In the circuit diagram of Fig. 1, the conversion unit 13 is represented by a simplified equivalent circuit composed of the transformer Tr as an ideal transformer, and an inductance component L arranged between the terminal x1 of the primary bridge circuit 12 and the primary winding.

[0022] Such inductance component L is an equivalent circuit representation of the leakage inductance of the transformer Tr and the inductance component of a reactor element provided as a real element in the conversion unit 13. Therefore, in the actual conversion unit 13, a reactor as a real element that bears part of the inductance component L of the conversion unit 13 may be appropriately inserted between each winding and each terminal x1, y1, x2, y2.

[0023] <Base control method> First, a basic control method for controlling the switching of each of the switching elements S1 to S8 in the DC-DC converter 1 according to the embodiment will be described. The basic control method is an example of the "basic operation" in the claims. This control method is a publicly known technique disclosed in Patent Document 1, so a detailed description will be omitted.

[0024] The control device 40 controls the on / off of each of the switching elements S1 to S8 with an on-duty of 0.5 in a state where the on / off of the switching elements connected in series in each of the first leg 21 to the fourth leg 24 is inverted so that they are not simultaneously conductive. Here, the on / off control for the upper arm is a phase shift of the on / off control for the lower arm by half a switching period. The on-duty of 0.5 does not exclude the provision of a so-called dead time in switching, but is an expression intended to indicate that each switching element repeats on / off approximately every half of the switching period.

[0025] In addition, the control device 40 controls the on / off of each switching element at a timing determined by the following three parameters: (a) An inter-bridge phase difference Φ, which is a phase difference of switching between the primary bridge circuit 12 and the secondary bridge circuit 14. B (b) Inter-leg phase difference which is a phase difference in switching between the legs in the primary bridge circuit 12. (c) Inter-leg phase difference which is a phase difference in switching between the legs in the secondary bridge circuit 14.

[0026] The control device 40 monitors the power transmitted between the first terminal pair (p1, q1) and the second terminal pair (p2, q2) and performs feedback control so that the transmitted power reaches a target value, thereby controlling the inter-bridge phase difference Φ B That is, when the transmitted power is smaller than the target value, the control device 40 determines the bridge-to-bridge phase difference Φ B If the magnitude of is larger than the target value, the phase difference between the bridges Φ B Adjust to reduce the magnitude of

[0027] The lower of the input / output voltage V1 between the first terminal pair and the input / output voltage n·V2 between the second terminal pair as the primary side conversion voltage of a transformer Tr with a winding ratio n is called the first voltage V. small , the higher voltage is the second voltage V large However, if these voltages are the same, the first voltage V smalland the second voltage V large shall have the same value.

[0028] When power is transmitted from the first terminal pair (p1, q1) to the second terminal pair (p2, q2), if V1 < n·V2, it is a step-up operation; if V1 = n·V2, it is a rated voltage ratio operation; if V1 > n·V2, it is a step-down operation. That is, when the first voltage V small and the second voltage V large are different, when power is transmitted from the first voltage V small side to the second voltage V large side, it is a step-up operation, and vice versa is a step-down operation. Also, when the first voltage V small and the second voltage V large are equal, it is a rated voltage ratio operation.

[0029] The control device 40 determines the smaller value of the phase difference Φ large1 =(π - |Φ B |)×V large / V small and the phase difference Φ large2 =|Φ B |×V large / (V large - V small ) as the leg-to-leg phase difference Φ small on the first voltage V large side of the bridge circuit. Further, the control device 40 determines the leg-to-leg phase difference Φ large on the second voltage V large side of the bridge circuit from the relational expression Φ small = Φ small ×V large ×V small / V large .

[0030] Figure 2 is a waveform diagram showing the operation of the DC-DC converter 1 in each control method. In Figure 2, the waveform of the primary-side AC voltage V ac1 between the AC-side terminal pairs of the primary-side bridge circuit 12 and the waveform of the secondary-side AC voltage V ac2 between the AC-side terminal pairs of the secondary-side bridge circuit 14 are shown.

[0031] In Figure 2, AC voltage V ac1 , V ac2 The waveforms of are shown together on one graph, and the black (dark) waveform is the AC voltage V ac1 The gray (light color) waveform is the AC voltage V ac2 The same applies to each of the following figures. Also, FIG. 2 shows the primary side AC current I ac1 The waveform of is also shown.

[0032] The upper part of Figure 2 shows the operation of the DC-DC converter 1 using the base control method. The upper part of Figure 2 shows an example of low-power power transfer using step-down operation from the first terminal pair (p1, q1) to the second terminal pair (p2, q2). In other words, when comparing the input / output voltage V1 between the first terminal pair and the input / output voltage n·V2 between the second terminal pair, which are primary-side converted voltages, the former is greater.

[0033] Therefore, in the example of FIG. 2, the phase difference between the legs of the primary bridge circuit 12 is the above-mentioned phase difference Φ small and the phase difference between the legs of the secondary bridge circuit 14 is determined as the phase difference Φ large The phase difference between the bridges is determined as Φ B is the phase difference of switching between the first leg 21 and the third leg 23.

[0034] In the first leg 21, the switching elements S1 and S2 are turned on or off at phases t2 and t5 in the switching period as shown in Fig. 2. In the second leg 22, the switching elements S3 and S4 are turned on or off at phases t1 and t4. In the third leg 23, the switching elements S5 and S6 are turned on or off at phases t3 and t6 in the switching period. In the fourth leg 24, the switching elements S7 and S8 are turned on or off at phases t1 and t4.

[0035] AC current I ac1As is clear from a comparison with the waveforms in Fig. 1, zero current switching (ZCS) is achieved in the switching at the timings of phases t1, t3, t4, and t6. Therefore, zero current switching is achieved in the second leg 22, the third leg 23, and the fourth leg 24. Therefore, according to the base control method, a reduction in switching loss is achieved in the DC-DC converter 1 as a whole.

[0036] On the other hand, at the timing of the on-state or off-state operation in the first leg 21 (phases t2 and t5), the AC current I ac1 is not 0. Therefore, a current flows through the switching element that is on in the first leg 21 immediately before phases t2 and t5, and zero current switching is not achieved in that switching element.

[0037] In this case, the first leg 21 experiences so-called hard switching, which results in large switching losses and causes the switching elements S1 and S2 to generate more heat than the switching elements in the other legs in which zero current switching is realized. In Fig. 2, the symbols of the switching elements in which hard switching occurs are shown along with their timing.

[0038] In addition, Φ large ≦Φ large1 :Relationship 1, Φ large ≦Φ large2 :Relationship 2, are simultaneously satisfied, this is the condition for realizing zero current switching (ZCS) in at least one leg in each of the primary bridge circuit 12 and the secondary bridge circuit 14.

[0039] Therefore, in the basic control method, the first voltage V small Phase difference Φ between legs in the bridge circuit on the large The value of phase difference Φ large1 and phase difference Φ large2Alternatively, the inter-leg phase difference Φ large The value may be set to a value equal to or less than the first limit value.

[0040] <Control method to average the losses of each leg> As described above, in the base control method, the heat generation of the switching elements in a specific leg becomes large, so the present applicant further proposed a switching control method that adds control for averaging the losses in each leg. This control method as a control method for averaging the losses in each leg is a publicly known technique disclosed in Patent Document 2, so a detailed description will be omitted. Note that the "period" hereinafter refers to the period of switching control in the base control method described above.

[0041] Here, it is assumed that the switching control for each switching element of the first leg 21, the second leg 22, the third leg 23, and the fourth leg 24 in the above-mentioned base control method is virtually performed for a first virtual leg, a second virtual leg, a third virtual leg, and a fourth virtual leg, respectively. The first virtual leg and the second virtual leg are virtual legs assigned to the first leg 21 and the second leg 22 in a one-to-one relationship. The third virtual leg and the fourth virtual leg are virtual legs assigned to the third leg 23 and the fourth leg 24 in a one-to-one relationship.

[0042] That is, the phase difference between the first virtual leg and the second virtual leg is the first leg phase difference, and the phase difference between the third virtual leg and the fourth virtual leg is the second leg phase difference. As described above, the first leg phase difference and the second leg phase difference are each calculated by comparing the magnitude of the input / output voltage V1 as a converted voltage with the input / output voltage n·V2. large or phase difference Φ small Either one of the following will be assigned.

[0043] Thus, the first operation and the second operation are alternately executed every N switching periods (N is a natural number). In the first operation, the first leg 21 is assigned to the first virtual leg, the second leg 22 to the second virtual leg, the third leg 23 to the third virtual leg, and the fourth leg 24 to the fourth virtual leg. In the second operation, the second leg 22 is assigned to the first virtual leg, the first leg 21 to the second virtual leg, the fourth leg 24 to the third virtual leg, and the third leg 23 to the fourth virtual leg.

[0044] Thus, in the primary bridge circuit 12, the roles of the first leg 21 and the second leg 22 are switched every N cycles, and in synchronization therewith, in the secondary bridge circuit 14, the roles of the third leg 23 and the fourth leg 24 are switched. The middle part of Fig. 2 is a waveform diagram showing the operation of the DC-DC converter 1 when a method for averaging the losses in each leg is further applied. The middle part of Fig. 2 shows an example in which the first operation and the second operation are repeated every cycle.

[0045] In the figure, period C2 corresponds to a period during which the legs are switched from the base control method in each of the primary bridge circuit 12 and the secondary bridge circuit 14. Moreover, period C1 is a period during which the control of each switching element by the base control method is applied as is.

[0046] From the viewpoint of the converter 13, this switching of the legs in the primary bridge circuit 12 and the secondary bridge circuit 14 corresponds to a switching of roles between the AC terminals x1 and y1 of the primary bridge circuit 12 and between the AC terminals x2 and y2 of the secondary bridge circuit 14. Therefore, during the period C2 in which the legs are switched, the AC voltage V ac1、 AC voltage V ac2 The waveforms of are inverted in sign from period C1 onwards.

[0047] However, since the legs of the primary bridge circuit 12 and the secondary bridge circuit 14 are switched synchronously, even during the period C2 in which the legs are switched, power is transported between the first terminal pair (p1, q1) and the second terminal pair (p2, q2) in the same manner as in the period C1.

[0048] 2, using the above-mentioned base control method, in the primary-side bridge circuit 12, zero current switching (ZCS) is achieved in the second leg 22, but zero current switching is not achieved in the first leg 21. When this method of averaging the losses in each leg is further applied to this, zero current switching is achieved alternately in the first leg 21 and the second leg 22, and the losses are averaged between these legs.

[0049] In other words, according to the control method for averaging the loss in each leg, in each of the primary bridge circuit 12 and the secondary bridge circuit 14, the switching loss of the switching elements in each leg can be equalized.

[0050] By applying this method, the roles of each leg are switched every N cycles, so for example, when N=1, the primary side AC voltage Vac1 has a double-pulse waveform in which two consecutive positive pulses are followed by two consecutive negative pulses, and this is repeated. This type of double-pulse waveform also occurs when switching the roles of each leg when N>1. As a result, as will be described in detail later, it was found that this method creates an issue in which the maximum value of the transformer magnetic flux density B, which is the magnetic flux density in the core of the transformer Tr, becomes larger than the base control method.

[0051] <Outline of the Control Method of the DC-DC Converter of the Present Embodiment> In this way, when this technique of switching the roles of the legs every cycle is applied to the base control method described above, the maximum value of the transformer magnetic flux density B becomes larger, making magnetic saturation more likely to occur in the transformer Tr.

[0052] As is known, when magnetic saturation occurs in the transformer Tr, the excitation inductance becomes apparently close to zero, and an excessive current called a magnetic saturation current flows through the transformer Tr. Such a magnetic saturation current may cause a breakdown in the circuit of the DC-DC converter 1. Therefore, in the control method of the DC-DC converter 1 according to this embodiment, a further modification is made to the control method for averaging the losses in each leg.

[0053] The lower part of Fig. 2 shows the control method according to this embodiment. In the control method according to this embodiment, in period C2, in all legs (first leg 21 to fourth leg 24), the control of the upper arm and the lower arm are switched from the virtual leg assigned by the control method that averages the losses of each leg. Then, switching of each switching element in each leg is executed.

[0054] Here, for each virtual leg, the on / off control for the upper arm is a phase shift of the on / off control for the lower arm by half a switching period. Therefore, swapping the control for the upper arm and the lower arm is equivalent to shifting the control phase for each of the upper arm and the lower arm by half a switching period in the basic control. The dotted arrow between the middle and lower parts of Figure 2 indicates this.

[0055] By performing such control, the primary side AC voltage V ac1 and the secondary AC voltage V ac2 The waveforms of the primary side AC voltage V ac1 and the secondary AC voltage V ac2 The waveform of is the same as that of the base control method, or equivalently, with the positive and negative signs reversed, throughout periods C1 and C2. This eliminates the double pulse that occurs in the control method that averages the losses of each leg.

[0056] (1) In the above explanation, the control method of this embodiment executes a first operation (period C1) that assigns the first leg 21 to the first virtual leg, the second leg 22 to the second virtual leg, the third leg 23 to the third virtual leg, and the fourth leg 24 to the fourth virtual leg, based on the base control method for each virtual leg.

[0057] Then, in the second operation (period C2), the second leg 22 is assigned to the first virtual leg, the first leg 21 to the second virtual leg, the fourth leg 24 to the third virtual leg, and the third leg 23 to the fourth virtual leg. Also, in the second operation (period C2), the virtual control over the virtual legs is switched between on / off control over the upper arm and on / off control over the lower arm, and actual control of each switching element in each leg is performed.

[0058] (2) However, the control method of this embodiment can be expressed with substantially the same meaning as follows: The control method of this embodiment executes a first operation (period C1) of assigning the first leg 21 to the first virtual leg, the second leg 22 to the second virtual leg, the third leg 23 to the third virtual leg, and the fourth leg 24 to the fourth virtual leg, based on a base control method for each virtual leg.

[0059] In the first operation (period C1), the virtual control for the virtual legs is replaced by the on / off control for the upper arm and the on / off control for the lower arm to control the actual switching elements in each leg. Then, based on the base control method for each virtual leg, the second operation (period C2) is executed to assign the second leg 22 to the first virtual leg, the first leg 21 to the second virtual leg, the fourth leg 24 to the third virtual leg, and the third leg 23 to the fourth virtual leg. Figure 12 shows an example of the operation (2) in comparison with the base control method and the control method for averaging the losses of each leg.

[0060] The above expressions (1) and (2) are essentially synonymous for the following reason: For the base control method for each virtual leg, the operation in which the on-off control for the upper arm and the on-off control for the lower arm are interchanged is essentially synonymous with the original operation, except that the control operation is shifted by a half cycle.

[0061] <Example of a circuit for implementing the control method for the DC-DC converter according to this embodiment> A circuit example for implementing the control method of the DC-DC converter 1 of this embodiment will be described below. Fig. 3 is a diagram showing, as a logic circuit, an example of calculation for implementing switching control performed in the DC-DC converter 1 according to this embodiment.

[0062] 3 shows a logic circuit that calculates on / off control signals S1-S4 for each of the switching elements S1-S4 in the switching control performed in the DC-DC converter 1 according to this embodiment from on / off control signals S1'-S4' for each of the switching elements S1-S4 in the base control method. Note that here, for example, the on / off control signal for the real switching element S1 is represented by the same symbol S1, and the same applies to the other real switching elements S2-S8.

[0063] In Fig. 3, signal SEL is a signal for instructing switching between the first operation and the second operation. When the first operation and the second operation are alternately executed every N periods, the signal SEL alternates between a logical value 0 and a logical value 1 every N periods. In other words, the signal period of signal SEL is 2N periods. In each logic circuit in Fig. 3, when signal SEL has a logical value of 0, the output of the partial circuit enclosed in a frame as mode A is finally output as control signals S1 to S4, and when signal SEL has a logical value of 1, the output of the partial circuit enclosed in a frame as mode B is finally output as control signals S1 to S4.

[0064] The control signals S1 and S3 are calculated from the control signals S1' and S3' in the base control strategy for the virtual leg. The control signals S2 and S4 are calculated from the control signals S2' and S4' in the base control strategy for the virtual leg. Figure 4 is a truth table based on the logic circuit shown in Figure 3.

[0065] 4, by replacing S1' with S5', S2' with S6', S3' with S7', S4' with S8', S1 with S5, S2 with S6, S3 with S7, and S4 with S8, the control signals for each switching element of the secondary bridge circuit 14 can be obtained. Therefore, the control signals S5 to S8 for each switching element of the secondary bridge circuit 14 correspond to the control signals S1 to S4 for each switching element of the primary bridge circuit 12 shown in FIG. 3, respectively, and can be calculated based on a similar logic circuit.

[0066] <Operation example 1: Step-down operation at low output> The operation of the DC-DC converter 1 when the control method of the DC-DC converter of this embodiment is applied will be described below. Fig. 5 is a waveform diagram showing an operation example 1, which is a step-down operation at low output when power is transferred from the primary side, i.e., the first terminal pair (p1, q1) side to the secondary side, i.e., the second terminal pair (p2, q2) side. In the operation example 1, the phase difference between the legs Φ large As the phase difference Φ large2 This corresponds to the case where the above is adopted.

[0067] 5 shows the control signals S1', S3', S5', and S7' for each virtual leg, the signal SEL, and the control signals S1, S3, S5, and S7 for each switching element. ac1 , secondary AC voltage V ac2、 Primary AC current I ac1 is also shown.

[0068] In mode A where the value of the signal SEL is 0, the phase difference Φ between the legs of the primary bridge circuit 12 in the base control method is smallappears as an ON section of the switching element S1 or an ON section of the switching element S3. This ON section corresponds to the primary side AC voltage V ac1 This appears as the application time.

[0069] In addition, the phase difference Φ between the legs of the secondary bridge circuit 14 in the base control method large appears as an ON section of the switching element S5 or an ON section of the switching element S7. This ON section corresponds to the secondary side AC voltage V ac2 The phase difference between the bridges Φ B appears as a phase difference from the turn-off of switching element S1 to the turn-off of switching element S5, and further appears as a phase difference from the turn-off of switching element S3 to the turn-off of switching element S7.

[0070] In mode B where the value of the signal SEL is 1, the phase difference Φ between the legs of the primary bridge circuit 12 in the base control method is small appears as an off-period of the switching element S1 or the switching element S3. This off-period occurs when the primary side AC voltage V ac1 This appears as the application time.

[0071] In addition, the phase difference Φ between the legs of the secondary bridge circuit 14 in the base control method large appears as an off-period of the switching element S5 or an off-period of the switching element S7. This off-period occurs when the AC voltage V ac2 The phase difference between the bridges Φ B appears as a phase difference from when switching element S1 is turned on to when switching element S5 is turned on, and further appears as a phase difference from when switching element S3 is turned on to when switching element S7 is turned on.

[0072] Switching between Mode A and Mode B by the signal SEL controls the AC voltage V ac1 , V ac2If this occurs within the zero section of the gate signal, an unnecessary small pulse may be generated. Therefore, the switching between Mode A and Mode B by the signal SEL is performed by the AC voltage V ac1 , V ac2 The timing (phase) of switching between Mode A and Mode B by the signal SEL is set so that it occurs outside the zero interval of the control signal S3'. sel Let us assume that.

[0073] This setting is possible by switching between Mode A and Mode B by the signal SEL during a period in which the control signals S1' and S3' are inverted and the control signals S5' and S7' are inverted. As is clear from the truth table in Fig. 4, even if the signal SEL is inverted during such a period, the switching elements S1 to S8 do not switch on and off due to the inversion of the signal SEL.

[0074] The symbol ΔΦ in FIG. 5 is a phase difference between bridges that is not used for feedback control, and ΔΦ=0 during step-down / step-up operation and low output. ΔΦ will be described later. In the case of operation example 1, the phase difference Φ sel is (ΔΦ) or more (Φ small ) or less, (ΔΦ+π) or more (Φ small +π) or less, (ΔΦ-π) or more (Φ small -π) or less, (ΔΦ-2π) or more (Φ small -2π) or less. Note that the positive side here is the delay direction.

[0075] In this way, it is demonstrated by Fig. 5 that the control signals for each virtual leg are converted into control signals for each switching element by following the logic circuit shown in Fig. 3 and the truth table shown in Fig. 4. Also, the primary side AC voltage V ac1 , secondary AC voltage V ac2、 Primary AC current I ac1 is the same as that in the control method of the embodiment shown in the lower part of FIG.

[0076] Fig. 5 also shows periods C1 and C2, as in Fig. 2. It can be seen from Fig. 5 that in period C1, the on / off control signals S1'-S8' of the virtual legs are output as they are as control signals S1-S8 of the switching elements. Also, in period C2, it can be seen that the on / off control signals S1'-S8' of the virtual legs are output as control signals S1-S8 of the switching elements after swapping the legs in each bridge, and then the control signals for the upper arm and the control signals for the lower arm are swapped in all legs.

[0077] Fig. 6 is a waveform diagram showing the element voltages and element currents of the switching elements S1 to S8 in the operation example 1. When the element voltage is 0, the switching element is on, and when a voltage is applied to the element, the switching element is off. In Fig. 6, the timing and location where hard switching occurs are indicated by dotted lines.

[0078] As shown in the figure, according to the control method of this embodiment, in the step-down operation at low output when transmitting power from the primary side to the secondary side, it can be seen that hard switching is alternately performed in the four switching elements S1 to S4 of the primary side bridge circuit 12. In mode A, hard switching is performed in the switching elements S1 and S3, and zero current switching is performed in the other switching elements. In mode B, hard switching is performed in the switching elements S2 and S4, and zero current switching is performed in the other switching elements.

[0079] In this way, when the base control method is applied to the primary side bridge circuit 12, heat generation is concentrated in one leg. However, according to the control method of this embodiment, such heat generation is distributed to each leg.

[0080] <Operation example 2: Boost operation at low output> 7 is a waveform diagram showing an operation example 2, which is a boost operation at low output when transmitting power from the primary side, i.e., the first terminal pair (p1, q1) side to the secondary side, i.e., the second terminal pair (p2, q2) side. Operation example 2 also has a phase difference between the legs Φ large As the phase difference Φ large2 7 and 8 are waveform diagrams for this example, corresponding to FIGS. 5 and 6, respectively.

[0081] In mode A where the value of the signal SEL is 0, the phase difference Φ between the legs of the primary bridge circuit 12 in the base control method is large appears as an ON section of the switching element S1 or an ON section of the switching element S3. This ON section corresponds to the primary side AC voltage V ac1 This appears as the application time.

[0082] In addition, the phase difference Φ between the legs of the secondary bridge circuit 14 in the base control method small appears as an ON section of the switching element S5 or an ON section of the switching element S7. This ON section corresponds to the secondary side AC voltage V ac2 The phase difference between the bridges Φ B appears as a phase difference from when switching element S1 is turned on to when switching element S5 is turned on, and also appears as a phase difference from when switching element S3 is turned on to when switching element S7 is turned on.

[0083] In mode B where the value of the signal SEL is 1, the phase difference Φ between the legs of the primary bridge circuit 12 in the base control method is large appears as an off-period of the switching element S1 or the switching element S3. This off-period occurs when the primary side AC voltage V ac1 This appears as the application time.

[0084] In addition, the phase difference Φ between the legs of the secondary bridge circuit 14 in the base control method small appears as an off-period of the switching element S5 or an off-period of the switching element S7. This off-period occurs when the AC voltage Vac2 The phase difference between the bridges Φ B appears as a phase difference from the turn-off of switching element S1 to the turn-off of switching element S5, and also appears as a phase difference from the turn-off of switching element S3 to the turn-off of switching element S7.

[0085] In the case of operation example 2, the phase difference Φ sel is (Φ B ) or more (Φ large ) below, (Φ B +π) or more (Φ large +π) or less, (-π+Φ B ) or more (-π+Φ large ) or less, (-2π+Φ B ) or more (-2π+Φ large ) or any value below.

[0086] As shown in Fig. 8, according to the control method of this embodiment, in the boost operation at low output when power is transferred from the primary side to the secondary side, it can be seen that hard switching is alternately performed in the four switching elements S5 to S8 of the secondary side bridge circuit 14. In mode A, hard switching is performed in switching elements S6 and S8, and zero current switching is performed in other switching elements. In mode B, hard switching is performed in switching elements S5 and S7, and zero current switching is performed in other switching elements.

[0087] In this way, when the base control method is applied to the secondary-side bridge circuit 14, heat generation is concentrated in one leg. However, according to the control method of this embodiment, such heat generation is distributed to each leg.

[0088] <Operation example 3: When operating at rated voltage ratio, at high output> FIG. 9 is a waveform diagram showing the rated voltage ratio operation (V1=n·V2) when power is transferred from the primary side, i.e., the first terminal pair (p1, q1) side to the secondary side, i.e., the second terminal pair (p2, q2) side. Similar waveforms are also obtained when the DC-DC converter 1 operates at high output. Operation example 3 shows the phase difference between the legs Φ large As the phase difference Φ large1 9 and 10 are waveform diagrams for this example, corresponding to FIGS. 5 and 6, respectively.

[0089] As shown in FIG. 9, when the DC-DC converter 1 operates at the rated voltage ratio or at high output, the phase difference ΔΦ between the bridges not used for feedback control is not 0. In particular, when the DC-DC converter 1 operates at the rated voltage ratio, Φ small =Φ large and ΔΦ=Φ B In the operation example 3, the phase difference Φ sel may be set in the same manner as in Operation Example 1 during step-down operation, and may be set in the same manner as in Operation Example 2 during step-up operation.

[0090] 10, according to the control method of this embodiment, in the rated voltage ratio operation or high output operation, hard switching is alternately performed in the four switching elements of each of the primary bridge circuit 12 and the secondary bridge circuit 14. In mode A, hard switching is performed in the switching elements S1, S3, S6, and S8, and zero current switching is performed in the other switching elements. In mode B, hard switching is performed in the switching elements S2, S4, S5, and S7, and zero current switching is performed in the other switching elements.

[0091] In this way, when the base control method is applied to each of the primary side bridge circuit 12 and the secondary side bridge circuit 14, heat generation is concentrated in one leg. However, according to the control method of this embodiment, such heat generation is distributed to each leg.

[0092] <Actions and Effects> As described above, when the base control method is applied, zero current switching is realized in at least one leg in each of the primary bridge circuit 12 and the secondary bridge circuit 14, reducing switching loss throughout the DC-DC converter 1. However, in this case, heat is generated in a concentrated manner in one leg in the primary bridge circuit 12 and / or the secondary bridge circuit 14.

[0093] However, according to the control method of this embodiment, heat generated by hard switching is distributed to both legs of the primary bridge circuit 12 and the secondary bridge circuit 14. Therefore, according to the control method of this embodiment, heat dissipation from the switching elements becomes easy, and the cost of the DC-DC converter 1 can be reduced.

[0094] FIG. 11 shows the AC voltage V ac1 , V ac2 and AC current I ac1 In addition, the waveform of the magnetic flux density B in the core of the transformer Tr is shown. When the switching control of the DC-DC converter 1 is performed by applying a method for averaging the losses in each leg, the AC voltage V ac1 , V ac2 In each of the above, the voltage waveform in the cycle immediately after switching from period C1 to period C2 is the positive / negative inversion of the voltage waveform in the cycle immediately before the switching.

[0095] Therefore, typically, the AC voltage V ac1 , V ac2 A double pulse waveform with successive positive or negative pulses appears in each of the above. As a result, the maximum value of the magnetic flux density B in the core of the transformer Tr becomes twice as high as in the case of the base control method when magnetic flux saturation is not taken into account.

[0096] On the other hand, according to the control method of this embodiment, the AC voltage V ac1 , V ac2are similar to the base control method, there is no increase in the maximum value of magnetic flux density B from the base control method. Therefore, according to the control method of this embodiment, magnetic saturation in the transformer Tr is not likely to occur, unlike the control method that averages the losses in each leg.

[0097] 〔others〕 The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the disclosed technical means are also included in the technical scope of the present invention.

[0098] For example, the base control method in the above embodiment as a basic operation for a virtual leg in the present invention is not limited to an on-duty of each switching element being 0.5, and may be a fixed control with an on-duty of less than 0.5. Alternatively, the on-duty of each switching element may be controlled to vary according to a predetermined regulation within a range of 0.5 or less.

[0099] Furthermore, the basic operation of the virtual leg in the present invention may be based on the control method in the above embodiment as long as it satisfies the following conditions: As the basic operation when the DC-DC converter transports power, the on / off control of the upper arm in each leg may be shifted in phase from the on / off control of the lower arm by half a switching period.

[0100] Even if a switching control method that meets such conditions is used instead of the above-mentioned base control method, the AC voltage V can be controlled by switching the on / off control of the upper arm and the on / off control of the lower arm in the present invention. ac1 , V ac2 It is clear that each of the above can be inverted positive and negative. Therefore, as the basic operation for the virtual leg in the present invention, any switching control method for a DC-DC converter that satisfies the above conditions can be applied.

[0101] Even if the basic operation for such virtual legs is a control method in which zero current switching is performed only on some of the first virtual leg to the fourth virtual leg, the DC-DC converter of the present invention can equalize the difference in heat generation between the legs in each bridge without making magnetic saturation likely to occur.Alternatively, the same applies even if the basic operation for such virtual legs is a control method in which zero voltage switching (ZVS) is performed only on some of the first virtual leg to the fourth virtual leg.

[0102] [Software implementation example] The functions of the control device 40 (hereinafter referred to as the "device") can be realized by a program for making a computer function as the device. In this case, the device includes a computer having at least one control device (e.g., a processor) and at least one storage device (e.g., a memory) as hardware for executing the program. The functions described in each of the above embodiments are realized by executing the program using the control device and the storage device.

[0103] The program may be recorded on one or more computer-readable recording media, not temporarily. The device may or may not have the recording media. In the latter case, the program may be supplied to the device via any wired or wireless transmission medium. In addition, some or all of the functions of the control device 40 may be realized by a logic circuit. For example, an integrated circuit in which a logic circuit that functions as some or all of the functions of the control device 40 is formed is also included in the scope of the present invention.

[0104] 〔summary〕 A first aspect of the present disclosure is a DC-DC converter including a primary bridge circuit including a plurality of switching elements and having a first leg and a second leg, a secondary bridge circuit including a plurality of switching elements and having a third leg and a fourth leg, a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit, and a control unit that controls the switching elements, wherein assuming a first virtual leg and a second virtual leg that are assigned one-to-one to the first leg and the second leg, and a third virtual leg and a fourth virtual leg that are assigned one-to-one to the third leg and the fourth leg, the control unit performs on / off control of an upper arm and on / off control of a lower arm in each of the virtual legs as a basic operation when the DC-DC converter transports power. the phase of the on / off control performed by the first leg is shifted by half a switching period, and further, the first leg is assigned to a first virtual leg, the second leg to a second virtual leg, the third leg to a third virtual leg, and the fourth leg to a fourth virtual leg, and in each of the legs, a first operation of controlling the switching elements by the basic operation is performed alternately every N periods (N is a natural number) of the switching period. The second leg is assigned to the first virtual leg, the first leg to the second virtual leg, the fourth leg to the third virtual leg, and the third leg to the fourth virtual leg, and in each of the legs, the on / off control for the upper arm and the on / off control for the lower arm in the basic operation are interchanged to control each of the switching elements.

[0105] The DC-DC converter of aspect 2 of the present disclosure is similar to aspect 1 in that the basic operation is to make all on-duties of on-off control of the switching elements in the first virtual leg and the second virtual leg the same, provide a first-leg phase difference between the virtual first leg and the virtual second leg, make all on-duties of on-off control of the switching elements in the third virtual leg and the fourth virtual leg the same, provide a second-leg phase difference between the virtual third leg and the virtual fourth leg, and perform switching control by setting a ratio between the first-leg phase difference and the second-leg phase difference to a value corresponding to a ratio between a primary side input / output voltage and a secondary side input / output voltage of the DC-DC converter.

[0106] The DC-DC converter of the third aspect of the present disclosure is the DC-DC converter of the second aspect, further comprising: small , the other voltage is the second voltage V large When the phase difference between the first leg and the second leg is set as above, the input / output voltage is the first voltage V small Phase difference between legs on the side Φ large The input / output voltage is the second voltage V large Phase difference between legs on the side Φ small The ratio of Φ small / Φ large =V small / V large It is characterized in that it is given by:

[0107] A DC-DC converter according to a fourth aspect of the present disclosure is the DC-DC converter according to the second or third aspect, further comprising: an on-duty of on-off control of the switching elements in each of the virtual legs is 0.5; and the inter-leg phase difference Φ large is the bridge phase difference Φ between the primary bridge circuit and the secondary bridge circuit. B from, Φ large1 =(π-|Φ B |)×V large / V small Φlarge2 =|Φ B |×V large / (V large -V small ) It is characterized in that it is given as the smaller value of

[0108] A DC-DC converter of aspect 5 of the present disclosure is configured in any one of aspects 1 to 4, wherein the basic operation is such that zero current switching is performed only on some of the first virtual leg to the fourth virtual leg, or zero voltage switching is performed only on some of the virtual legs.

[0109] A sixth aspect of the present disclosure is a control method for a DC-DC converter including a primary bridge circuit including a plurality of switching elements and having a first leg and a second leg, a secondary bridge circuit including a plurality of switching elements and having a third leg and a fourth leg, and a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit, wherein, assuming a first virtual leg and a second virtual leg assigned one-to-one to the first leg and the second leg, and a third virtual leg and a fourth virtual leg assigned one-to-one to the third leg and the fourth leg, the control method includes the steps of: in each of the virtual legs, an on / off control for an upper arm is phase-shifted with respect to an on / off control for a lower arm; the first leg is shifted by half a switching period, and further, the first leg is assigned to a first virtual leg, the second leg to a second virtual leg, the third leg to a third virtual leg, and the fourth leg to a fourth virtual leg, and in each of the legs, a first operation of controlling the switching elements by the basic operation is performed alternately every N periods (N is a natural number) of the switching period. the second leg is assigned to the first virtual leg, the first leg to the second virtual leg, the fourth leg to the third virtual leg, and the third leg to the fourth virtual leg, and in each of the legs, an on / off control of the upper arm and an on / off control of the lower arm in the basic operation are interchanged to control each of the switching elements. [Explanation of symbols]

[0110] 1 DC-DC converter (p1, q1) First terminal pair (p2, q2) Second terminal pair 11 Primary side smoothing circuit 12 Primary bridge circuit 21 1st Leg 22 2nd Leg (x1, y1) AC side terminal pair 13 Conversion section Tr Transformer 14 Secondary bridge circuit 23 Third Leg 24 4th Leg (x2, y2) AC side terminal pair 15 Secondary side smoothing circuit 40 Control device (control unit)

Claims

1. a primary bridge circuit including a plurality of switching elements and having a first leg and a second leg; a secondary bridge circuit including a plurality of switching elements and having a third leg and a fourth leg; a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit; A control unit that controls the switching element, Assuming that a first virtual leg and a second virtual leg are assigned to the first leg and the second leg in a one-to-one relationship, and a third virtual leg and a fourth virtual leg are assigned to the third leg and the fourth leg in a one-to-one relationship, The control unit is As a basic operation when the DC-DC converter transports power, in each of the virtual legs, an on / off control for an upper arm is shifted in phase from an on / off control for a lower arm by a half cycle of a switching period; a first operation of assigning the first leg to a first virtual leg, the second leg to a second virtual leg, the third leg to a third virtual leg, and the fourth leg to a fourth virtual leg, and controlling the switching elements in each of the legs by the basic operation; a second operation of controlling each of the switching elements by interchanging the on / off control of the upper arm and the on / off control of the lower arm in the basic operation in each of the legs, and a second operation of controlling each of the switching elements by interchanging the on / off control of the upper arm and the on / off control of the lower arm in the basic operation in each of the legs, the second operation being alternately executed every N periods (N is a natural number) of the switching period.

2. The basic operation is as follows: on-duties of on-off control of the switching elements in the first virtual leg and the second virtual leg are all set to be the same, and a first inter-leg phase difference is provided between the first virtual leg and the second virtual leg; on-duties of on-off control of the switching elements in the third virtual leg and the fourth virtual leg are all set to be the same, and a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg; 2. The DC-DC converter according to claim 1, wherein a ratio of the phase difference between the first legs and the phase difference between the second legs is determined to a value corresponding to a ratio between a primary side input / output voltage and a secondary side input / output voltage of the DC-DC converter, and switching control is performed.

3. The lesser of the primary input / output voltage and the secondary input / output voltage expressed as a reduced voltage for the transformer is called a first voltage V ｓｍａｌｌ , the other voltage is the second voltage V ｌａｒｇｅ When the input / output voltage of the first leg phase difference and the second leg phase difference is the first voltage V ｓｍａｌｌ Phase difference between legs on the side Φ ｌａｒｇｅ The input / output voltage is the second voltage V ｌａｒｇｅ Phase difference between legs on the side Φ ｓｍａｌｌ The ratio of Φ ｓｍａｌｌ / Φ ｌａｒｇｅ =V ｓｍａｌｌ / V ｌａｒｇｅ 3. The DC-DC converter of claim 2, wherein:

4. an on-duty of on-off control of the switching elements in each of the virtual legs is 0.5; The phase difference between the legs Φ ｌａｒｇｅ is a bridge-to-bridge phase difference Φ, which is a phase difference between the primary bridge circuit and the secondary bridge circuit. Ｂ from, F ｌａｒｇｅ１ =(π-|Φ Ｂ |)×V ｌａｒｇｅ / V ｓｍａｌｌ Φ ｌａｒｇｅ２ =|Φ Ｂ |×V ｌａｒｇｅ / (V ｌａｒｇｅ -V ｓｍａｌｌ ) 4. The DC-DC converter of claim 3, wherein the input current is given as the smaller value of

5. The basic operation is as follows:

2. The DC-DC converter according to claim 1, wherein zero current switching is performed on only some of the first virtual leg to the fourth virtual leg, or zero voltage switching is performed on only some of the virtual legs.

6. a primary bridge circuit including a plurality of switching elements and having a first leg and a second leg; a secondary bridge circuit including a plurality of switching elements and having a third leg and a fourth leg; a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit; A control method for a DC-DC converter comprising: Assuming that a first virtual leg and a second virtual leg are assigned to the first leg and the second leg in a one-to-one relationship, and a third virtual leg and a fourth virtual leg are assigned to the third leg and the fourth leg in a one-to-one relationship, As a basic operation when the DC-DC converter transports power, in each of the virtual legs, an on / off control for an upper arm is shifted in phase from an on / off control for a lower arm by a half cycle of a switching period; a first operation of assigning the first leg to a first virtual leg, the second leg to a second virtual leg, the third leg to a third virtual leg, and the fourth leg to a fourth virtual leg, and controlling the switching elements in each of the legs by the basic operation; a second operation of controlling each of the switching elements by switching between the on / off control of the upper arm and the on / off control of the lower arm in the basic operation, in each of the legs, and alternately executing these operations every N periods (N is a natural number) of the switching period.

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