DC-DC converter and control method therefor

Abstract

To reduce the possibility of occurrence of excessive current.SOLUTION: A control unit (control device 40) of a DC-DC converter (1) defines an inter-leg phase difference to be equal to or less than a smaller one of a first limit value that is a maximum value at which ZCS can be realized with at least one leg (21 to 24) in a bridge circuit (12, 14) and a second limit value that is a maximum value at which the magnetic flux density of a core of a transformer (Tr) is at a prescribed magnitude or less, and further performs switching control to alternately switch legs.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] The present 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) DC-DC converter that can transfer DC power in both directions. The present 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. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7315886 [Patent Document 2] Japanese Patent Application Publication No. 2023-166914 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 as described above. In view of this, an object of one aspect 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 excessive current generation due to magnetic saturation. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, one aspect of the present disclosure provides a DC-DC converter including: a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg; a secondary bridge circuit including a plurality of secondary 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 primary switching elements and the secondary switching elements. the control unit alternately executes a first operation of regarding the first leg as a first virtual leg, the second leg as a second virtual leg, the third leg as a third virtual leg, and the fourth leg as a fourth virtual leg, and a second operation of regarding the second leg as the first virtual leg, the first leg as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg, and performs the first operation and the second operation to determine the first virtual leg, the second virtual leg, the third virtual leg, and the fourth leg as a fourth virtual leg. a first inter-leg phase difference is provided between the first virtual leg and the second virtual leg, and a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg; a ratio of the first inter-leg phase difference to the second inter-leg phase difference is set to a value corresponding to a ratio of an input / output voltage on a primary side to an input / output voltage on a secondary side of the DC-DC converter; and further, the greater of the first inter-leg phase difference and the second inter-leg phase difference is set to be equal to or less than the smaller of a first limit value that is a maximum value at which zero current switching can be achieved in at least one leg in each of the primary side bridge circuit and the secondary side bridge circuit, and a second limit value that is a maximum value at which magnetic flux density in a core of the transformer is equal to or less than a predetermined magnitude, thereby controlling switching of the primary side switching elements and each of the secondary side switching elements. [Effects of the Invention]

[0006] 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 excessive current occurring due to magnetic saturation. [Brief explanation of the drawings]

[0007] [Figure 1] This is a diagram showing the circuit configuration of a DC-DC converter according to an embodiment of the present disclosure. [Figure 2] This is a waveform diagram showing the voltage and current of each part for explaining the basic control method of the switching control performed in the DC-DC converter according to an embodiment of the present disclosure. [Figure 3] This is a waveform diagram showing the transformer magnetic flux density when the basic control method is applied to the circuit configuration of the above DC-DC converter. [Figure 4] This is a waveform diagram showing the transformer magnetic flux density when a control method incorporating a technique for averaging the losses of each leg is applied to the circuit configuration of the above DC-DC converter in addition to the basic control method. [Figure 5] This is a waveform diagram showing the operation of the DC-DC converter according to an embodiment of the present disclosure and the DC-DC converter of the comparative example at low output. [Figure 6] This is a waveform diagram showing the operation of the DC-DC converter according to an embodiment of the present disclosure at high output. [Figure 7] This is a waveform diagram showing the operation of the DC-DC converter of the comparative example at high output.

Mode for Carrying Out the Invention

[0008] 〔Embodiment〕 Hereinafter, embodiments according to one aspect of the present disclosure will be described based on the drawings. In the following description, the term "terminal" is also used to mean a connection point of a specific circuit element inside an electric circuit, and does not necessarily mean that only an external connection terminal such as a connector is provided.

[0009] <Outline of the Configuration of DC-DC Converter 1> 1 is a circuit diagram of a DC-DC converter 1 according to an 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 a first terminal pair and a second terminal pair.

[0010] The first terminal pair consists of a high-potential terminal p1 and a low-potential terminal q1. The second terminal pair consists of a high-potential terminal p2 and a low-potential terminal q2. In the following, the first terminal pair side may be referred to as the primary side and the second terminal pair side as the secondary side, but this does not necessarily mean that power is transferred from the primary side to the secondary side; it is just a convenient designation.

[0011] For example, a first DC voltage source is connected to a first terminal pair of the DC-DC converter 1, and a second DC voltage source is connected to a second terminal pair of the DC-DC converter 1. In this case, the DC-DC converter 1 according to the embodiment can transfer power from the first DC voltage source to the second DC voltage source by any of step-up operation, rated operation, and step-down operation. The DC-DC converter 1 can also transfer power from the second DC voltage source to the first DC voltage source by any of step-up operation, rated operation, and step-down operation.

[0012] 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 in this order to a first terminal pair. The secondary-side smoothing circuit 15 and the secondary-side bridge circuit 14 are cascade-connected in this order to a second terminal pair. The conversion unit 13 is provided between the primary-side bridge circuit 12 and the secondary-side bridge circuit 14.

[0013] 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 C1 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 C2 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.

[0014] 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 their connection point 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 their connection point is the other terminal y1 of the AC-side terminal pair. The AC-side terminal pair of the primary-side bridge circuit 12 inputs and outputs AC power to and from the primary side of the conversion unit 13.

[0015] 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 their connection point 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 their connection point is the other terminal y2 of the AC-side terminal pair. The AC-side terminal pair of the secondary-side bridge circuit 14 inputs and outputs AC power to and from the secondary side of the conversion unit 13.

[0016] The switching elements S1 to S4 included in the primary-side bridge circuit 12 are primary-side switching elements. The switching elements S5 to S8 included in the secondary-side bridge circuit 14 are secondary-side switching elements. Each of these switching elements S1 to S8 may include a freewheeling diode, as shown in the circuit diagram of FIG. 2.

[0017] The on / off of each of the switching elements S1 to S8 is controlled by a control device 40 (controller). An IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor field-effect transistor) can be used for each of the switching elements S1 to S8.

[0018] The conversion unit 13 has an isolated transformer Tr, the primary winding of which is connected to a pair of AC terminals of the primary bridge circuit 12, and the secondary winding of which is connected to a pair of AC terminals of the secondary bridge circuit 14. In the circuit diagram of Fig. 2, the conversion unit 13 is represented by a simplified equivalent circuit made up of the transformer Tr as an ideal transformer and an inductance component L arranged between terminal x1 of the primary bridge circuit 12 and the primary winding.

[0019] 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 takes on 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.

[0020] <Base control method> First, a description will be given of a basic control method relating to switching control of each of the switching elements S1 to S8 in the DC-DC converter 1 according to the embodiment. This control method is a publicly known technique disclosed in Patent Document 1, and therefore a detailed description thereof will be omitted.

[0021] In each of the first leg 21 to the fourth leg 24, the control device 40 controls the on / off of each of the switching elements S1 to S8 with an on-duty of 50%, with the on / off states of the switching elements connected in series reversed so that they are not simultaneously conductive. Here, the on-duty of 50% does not exclude the provision of a so-called dead time in the switching, but rather expresses the intention that each switching element will repeat on / off approximately every half cycle.

[0022] Then, the control device 40 controls the on / off of each switching element at a timing determined by the following three parameters: (1) an inter-bridge phase difference Φ, which is the phase difference of switching between the primary bridge circuit 12 and the secondary bridge circuit 14; B (2) Inter-leg phase difference, which is the phase difference of switching between the legs in the primary bridge circuit 12. (3) Inter-leg phase difference, which is the phase difference of switching between the legs in the secondary bridge circuit 14.

[0023] The control device 40 monitors the power transferred between the first terminal pair and the second terminal pair, and performs feedback control so that the transferred power reaches a target value, thereby controlling the inter-bridge phase difference Φ B That is, when the transferred power is smaller than the target value, the control device 40 determines the inter-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

[0024] The lower of the voltage V1 between the first terminal pair and the voltage n·V2 between the second terminal pair as the primary-side converted voltage for 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 small and the second voltage V large shall have the same value.

[0025] When power is transferred from the first terminal pair to the second terminal pair, if V1 < n·V2, it is called a step-up operation; if V1 = n·V2, it is called a rated operation; if V1 > n·V2, it is called a step-down operation. That is, when power is transferred 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.

[0026] The control device 40 calculates 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 ), and determines the smaller value as the inter-leg phase difference Φ small in the bridge circuit on the first voltage V large side. Further, the control device 40 uses the inter-leg phase difference Φ large to determine the inter-leg phase difference Φ large in the bridge circuit on the second voltage V[[ID=X]] small from the relational expression ΦX small = Φ large ×V small / V large .

[0027] Figure 2 is a waveform diagram showing the operation of the DC-DC converter 1 when the above-described base control method is applied. In Figure 2, the primary AC voltage (Vac1) represents the voltage waveform between the AC-side terminal pairs of the primary-side bridge circuit 12, and the secondary AC voltage (Vac2) represents the voltage waveform between the AC-side terminal pairs of the secondary-side bridge circuit 14. In Figure 2, the primary AC current (Iac1) represents the terminal current waveform of the AC-side terminal pairs of the primary-side bridge circuit 12.

[0028] Also shown in Figure 2 are the voltage waveforms applied to each of the switching elements S1, S3, S5, S7 and the current waveforms flowing through them. When the voltage applied to each switching element is approximately 0, the switching element is on; otherwise, it is off. In the example of Figure 2, the inter-block phase difference Φ Bis the phase difference of switching between the second leg 22 including the switching element S3 of the primary bridge circuit 12 and the fourth leg 24 including the switching element S7 corresponding to the second leg in the secondary bridge circuit 14.

[0029] 2 also shows a case where relatively low power is transferred from the first terminal pair to the second terminal pair in a step-up operation. In other words, when comparing the voltage V1 between the first terminal pair and the voltage n·V2 between the second terminal pair as the primary-side converted voltage, the latter is larger. Therefore, the phase difference between the legs of the primary-side bridge circuit 12 is equal to the phase difference Φ large and the phase difference between the legs of the secondary bridge circuit 14 is determined as the phase difference Φ small It has been determined as.

[0030] As shown in the current waveforms of switching elements S1, S3, and S5 in Figure 2, in the first leg 21, second leg 22, and third leg 23 that include these switching elements, switching is achieved when the current is zero, i.e., zero current switching (ZCS).

[0031] 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.

[0032] 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 the phase difference Φ large1 and phase difference Φ large2 Alternatively, the inter-leg phase difference Φ large The value may be set to a value equal to or less than the first limit value.

[0033] <Control to average the losses of each leg> A method for averaging losses in each leg is also applied to the switching control of each switching element S1 to S8 in the DC-DC converter 1 according to this embodiment. This method is defined as follows. Note that the "period" here refers to the switching period in the above-described base control method, and is also the period of the primary AC voltage Vac1 and the secondary AC voltage Vac2.

[0034] Here, in the above-described basic control method, 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 is assumed to be performed virtually for the first virtual leg, the second virtual leg, the third virtual leg, and the fourth virtual leg, respectively.

[0035] That is, the phase difference between the first virtual leg and the second virtual leg is the first inter-leg phase difference, and the phase difference between the third virtual leg and the fourth virtual leg is the second inter-leg phase difference. As described above, the first inter-leg phase difference and the second inter-leg phase difference are calculated by comparing the magnitude of the converted voltage V1 and the voltage n·V2, and are then calculated as the phase difference Φ large or phase difference Φ small Either one of the following will be assigned.

[0036] Thus, the first operation and the second operation are executed alternately every period. In the first operation, the first leg 21 is assigned to the first virtual leg, the second leg 22 is assigned to the second virtual leg, the third leg 23 is assigned to the third virtual leg, and the fourth leg 24 is assigned to the fourth virtual leg. In the second operation, the second leg 22 is assigned to the first virtual leg, the first leg 21 is assigned to the second virtual leg, the fourth leg 24 is assigned to the third virtual leg, and the third leg 23 is assigned to the fourth virtual leg.

[0037] That is, in the primary-side bridge circuit 12, the roles of the first leg 21 and the second leg 22 are switched every cycle, and in sync with this, the roles of the third leg 23 and the fourth leg 24 are switched in the secondary-side bridge circuit 12. For example, in the example of Fig. 2 using the base control method described above, ZCS was achieved in the third leg 23, but not in the fourth leg 24. However, when this method is further applied here, ZCS is achieved alternately in the third leg 23 and the fourth leg 24, and losses are averaged out in these legs.

[0038] Figure 3 is a graph showing the waveform of transformer magnetic flux density B, which is the magnetic flux density in the core of the transformer Tr, alongside the waveforms of the primary AC voltage Vac1 and secondary AC voltage Vac2 when the example of Figure 2 is implemented using the base control method described above. Figure 4 is a graph of each waveform when a method for averaging losses in each leg is further applied to the base control method. The vertical scale of each waveform in Figure 4 is the same as in Figure 3.

[0039] By applying this method, the roles of each leg are swapped every cycle, resulting in the primary AC voltage Vac1 taking on a double-pulse waveform, with two consecutive positive pulses followed by two consecutive negative pulses, as shown in Figure 4. Since the transformer magnetic flux density B is the integral of the input voltage to the transformer Tr, in the examples of Figures 3 and 4, the integral waveform of the input primary AC voltage Vac1 becomes the waveform of the transformer magnetic flux density B. As a result, it was found that the maximum value of the transformer magnetic flux density B becomes twice as large when this method, which swaps the roles of each leg every cycle, is applied.

[0040] <Control Method of This Embodiment> In this way, when this technique of switching the leg roles every cycle is applied to the base control method described above, the maximum value of the transformer magnetic flux density B doubles, making magnetic saturation more likely to occur in the transformer Tr.

[0041] As is known, when magnetic saturation occurs in the transformer Tr, the excitation inductance becomes apparently close to zero, and an excessive current called magnetic saturation current flows through the transformer Tr. Such a magnetic saturation current may cause a malfunction in the circuit of the DC-DC converter 1. Therefore, in the control method for the DC-DC converter 1 of this embodiment, the above-mentioned base control method is partially modified, and the above technique of exchanging the roles of the legs every cycle is applied.

[0042] The rated voltage Vo of the transformer Tr that gives the maximum rated magnetic flux density Bo is expressed as Vo=4·f·N·S·Bo, where f is the frequency, N is the number of windings, and S is the core cross-sectional area. According to the control method that is the basis of the above, the first voltage V that gives the maximum rated magnetic flux density Bo is small Phase difference Φ between legs of the bridge circuit on the large’ , Bo=V small Φ large’ / (2·N·S).

[0043] Furthermore, considering that the maximum value of the transformer magnetic flux density B doubles, when the above method of exchanging the leg roles every cycle is applied, Bo = V small Φ large3 / (N·S). The first voltage V small Phase difference Φ between legs of the bridge circuit on the large is the second limiting value of such a phase difference Φ large3 If it is below 、 The maximum value of the transformer magnetic flux density B is equal to or less than the maximum rated magnetic flux density Bo.

[0044] From the above equations, the first voltage V small Phase difference Φ between legs of the bridge circuit on the large , Φ large3 =Vo / (4 f V small ) or less, magnetic saturation will not occur even if this method of switching the roles of the legs every cycle is applied to the above-mentioned base control method. Thus, in the control method of this embodiment, in addition to Relational Expression 1 and Relational Expression 2, Φ large ≦Φ large3 :Relationship 3 The first voltage V small The phase difference between the legs of the bridge circuit on the side is determined.

[0045] <Example of operation according to the control method of this embodiment> Next, an example of operation of the DC-DC converter 1 according to this embodiment will be shown in comparison with a comparative example. Figures 5 and 6 are diagrams showing examples of operation of the DC-DC converter 1 according to this embodiment at low output and high output, respectively. Figure 7 is a diagram showing an example of operation of the comparative DC-DC converter at the same output as in Figure 6. For each waveform, the vertical axis scale is the same in Figures 5 to 7.

[0046] In the examples of Figures 5 to 7, the winding ratio n is 2 / 3. The voltage V1 between the first terminal pair and the voltage V2 between the second terminal pair are both 500 V. Therefore, the examples of Figures 5 to 7 are voltage step-down operations when power is transferred from the first terminal pair to the second terminal pair. Therefore, the phase difference Φ large The first voltage V small The bridge circuit on the primary side is a secondary bridge circuit 14.

[0047] As described above, in the DC-DC converter 1 according to this embodiment, the first voltage V small Phase difference Φ between legs of the bridge circuit on the large is determined to satisfy the relations 1 to 3. On the other hand, in the DC-DC converter of the comparative example, the maximum rated magnetic flux density Bo is not taken into consideration, and only the relations 1 and 2 are taken into consideration, and the first voltage V small Phase difference Φ between legs of the bridge circuit on the large is determined.

[0048] In the operation of the DC-DC converter 1 according to this embodiment at low output shown in FIG. large1 = 46.9 μs, Φ large2 = 31.2 μs, Φ large3 = 31.2 μs. Note that the phase difference here is expressed in units of time, with the switching frequency f being 12 kHz. Therefore, Φ according to relation 2 large2 = 31.2μs is the phase difference between legs Φlarge In this way, in the case of Figure 5, the relational expression 3 is large Since the voltage difference between the DC-DC converter and the DC-DC converter of the comparative example does not affect the determination of the voltage difference between the DC-DC converter and the DC-DC converter of the comparative example, the DC-DC converter of the comparative example also operates in the same manner.

[0049] In the high output operation of the DC-DC converter 1 according to this embodiment shown in FIG. large1 = 41.5 μs, Φ large2 = 42.0 μs, Φ large3 = 31.2μs. Therefore, Φ large3 = 31.2μs is the phase difference between legs Φ large In other words, the phase difference between the legs Φ large The maximum rated magnetic flux density Bo of the transformer Tr was reflected in the determination of

[0050] On the other hand, in the operation of the DC-DC converter according to the comparative example shown in FIG. 7 at the same output as in FIG. 6, Φ large1 = 42.5 μs, Φ large2 = 39.9μs. Therefore, Φ large2 =39.9μs is the phase difference between legs Φ large In this case, too, Φ large3 The calculated value is 31.2 μs, and the phase difference between the legs Φ large exceeded this value, which meant that there was a risk of excessive current flowing in the circuit.

[0051] Note that the waveforms in Figure 7 are drawn assuming that magnetic saturation does not occur. If magnetic saturation actually occurs, the transformer magnetic flux density B will have a waveform clamped at the saturation magnetic flux density, and while it is clamped, the primary AC current Iac1 will have a waveform in which an excessive magnetic saturation current occurs.

[0052] In the DC-DC converter 1 according to this embodiment, the transformer magnetic flux density B was able to be kept below the maximum rated magnetic flux density Bo up to an output of 112 kW. However, in the DC-DC converter 1 of the comparative example, the transformer magnetic flux density B was able to be kept below the maximum rated magnetic flux density Bo only up to an output of 45 kW. Therefore, in the DC-DC converter 1 according to this embodiment, switching control was achieved that can increase output while avoiding the generation of excessive current due to magnetic saturation.

[0053] [Software implementation example] The functions of the control device 40 (hereinafter referred to as "device") can be realized by a program that causes a computer to 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 storage device.

[0054] The program may be non-transitory and may be recorded on one or more computer-readable recording media. The recording media may or may not be included in the device. In the latter case, the program may be supplied to the device via any wired or wireless transmission medium. Also, some or all of the functions of each of the control blocks may be realized by logic circuits. For example, an integrated circuit in which a logic circuit functioning as each of the control blocks is formed is also included in the scope of the present invention.

[0055] 〔summary〕 A first aspect of the present disclosure is a DC-DC converter including: a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg; a secondary bridge circuit including a plurality of secondary 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 primary switching elements and the secondary switching elements, The control unit alternately performs a first operation of regarding the first leg as a first virtual leg, the second leg as a second virtual leg, the third leg as a third virtual leg, and the fourth leg as a fourth virtual leg, and a second operation of regarding the second leg as the first virtual leg, the first leg as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg, and performs the first operation and the second operation of regarding the first virtual leg, the second virtual leg, the third virtual leg, and the fourth virtual leg. a first inter-leg phase difference is provided between the first virtual leg and the second virtual leg and a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg; a ratio of the first inter-leg phase difference to the second inter-leg phase difference is set to a value corresponding to a ratio of an input / output voltage on a primary side to an input / output voltage on a secondary side of the DC-DC converter; and a configuration in which the non-smaller of the first inter-leg phase difference and the second inter-leg phase difference is set to be equal to or less than the smaller of a first limit value that is a maximum value at which zero current switching can be achieved in at least one leg in each of the primary side bridge circuit and the secondary side bridge circuit, and a second limit value that is a maximum value at which magnetic flux density in the core of the transformer is equal to or less than a predetermined magnitude, thereby controlling switching of the primary side switching elements and each of the secondary side switching elements.

[0056] The DC-DC converter of the second aspect of the present disclosure is the DC-DC converter of the first aspect, wherein the smaller of the input / output voltages of the primary side and the input / output voltages of the secondary side, expressed as converted voltages for the transformer, is converted into a first voltage V small , and the other voltage is the second voltage V largeWhen the phase difference between the first leg and the second leg is set as above, the input / output voltage is set to the first voltage V small Phase difference Φ between legs on the side large and the input / output voltage is the second voltage V large Phase difference Φ between legs on the side small The ratio of Φ small / Φ large =V small / V large The configuration is given by:

[0057] A DC-DC converter according to a third aspect of the present disclosure is the DC-DC converter according to the first or second aspect, wherein the control unit switches between the first operation and the second operation every cycle.

[0058] A fourth aspect of the present disclosure provides a DC-DC converter according to the third aspect, wherein the second limit value is calculated from a rated voltage V0 of the transformer expressed as the converted voltage, where V is a switching frequency, and Φ large3 =Vo / (4 f V small ) The present invention has a configuration given as follows:

[0059] A DC-DC converter according to a fifth aspect of the present disclosure is the DC-DC converter according to any one of the second to fourth aspects, wherein the first limit value is a bridge-to-bridge phase difference Φ B from, Φ large1 =(π-|Φ B |)×V large / V small , Φ large2 =|Φ B |×V large / (V large -V small ) The configuration is given as the smaller value of

[0060] A sixth aspect of the present disclosure is a control method for a DC-DC converter including a primary-side bridge circuit including a plurality of primary-side switching elements and having a first leg and a second leg, a secondary-side bridge circuit including a plurality of secondary-side switching elements and having a third leg and a fourth leg, and a conversion unit having a transformer and connected between the primary-side bridge circuit and the secondary-side bridge circuit, the method comprising: alternately performing a first operation in which the first leg is regarded as a first virtual leg, the second leg as a second virtual leg, the third leg as a third virtual leg, and the fourth leg as a fourth virtual leg, and a second operation in which the second leg is regarded as the first virtual leg, the first leg as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as a fourth virtual leg; and performing, through the first operation and the second operation, a control method for controlling a DC-DC converter including a primary-side bridge circuit including a primary-side bridge circuit including a first bridge circuit including a first leg, a second virtual leg, a second virtual leg, a third virtual leg, and a fourth virtual leg, a first inter-leg phase difference is provided between the first virtual leg and the second virtual leg, and a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg; a ratio of the first inter-leg phase difference to the second inter-leg phase difference is set to a value corresponding to a ratio of an input / output voltage on a primary side to an input / output voltage on a secondary side of the DC-DC converter; and a configuration in which the greater of the first inter-leg phase difference and the second inter-leg phase difference is set to be equal to or less than the smaller of a first limit value that is the maximum value at which zero current switching can be achieved in at least one leg in each of the primary side bridge circuit and the secondary side bridge circuit, and a second limit value that is the maximum value at which magnetic flux density in the core of the transformer is equal to or less than a predetermined magnitude, thereby controlling switching of each of the primary side switching elements and each of the secondary side switching elements.

[0061] 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. [Explanation of symbols]

[0062] 1 DC-DC converter (p1, q1) First terminal pair (p2, q2) Second terminal pair 11 Primary side smoothing circuit 12 Primary bridge circuit 21 First Leg 22 Second Leg (x1, y1) AC side terminal pair 13 Conversion unit Tr transformer 14 Secondary bridge circuit 23 Third Leg 24 Fourth 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 primary switching elements and having a first leg and a second leg; a secondary bridge circuit including a plurality of secondary 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 primary side switching element and the secondary side switching element, The control unit a first operation of regarding the first leg as a first virtual leg, the second leg as a second virtual leg, the third leg as a third virtual leg, and the fourth leg as a fourth virtual leg; a second operation of regarding the second leg as a first virtual leg, the first leg as a second virtual leg, the fourth leg as a third virtual leg, and the third leg as a fourth virtual leg, and through the first operation and the second operation, each switching element in the first virtual leg, the second virtual leg, the third virtual leg, and the fourth virtual leg is switched every half cycle; a first inter-leg phase difference is provided between the first virtual leg and the second virtual leg; a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg; a ratio of the phase difference between the first and second legs to a value corresponding to a ratio of an input / output voltage on a primary side to an input / output voltage on a secondary side of the DC-DC converter; Furthermore, the non-smaller value of the first inter-leg phase difference and the second inter-leg phase difference is set to be equal to or less than the smaller value of a first limit value, which is the maximum value at which zero current switching can be achieved in at least one leg in each of the primary side bridge circuit and the secondary side bridge circuit, and a second limit value, which is the maximum value at which magnetic flux density in the core of the transformer is equal to or less than a predetermined magnitude; A DC-DC converter controls switching of each of the primary-side switching elements and each of the secondary-side switching elements.

2. The lesser of the primary input / output voltage and the secondary input / output voltage expressed as a reduced voltage for the transformer is defined as a first voltage V 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 set to the first voltage V ｓｍａｌｌ Phase difference Φ between legs on the side ｌａｒｇｅ and the input / output voltage is the second voltage V ｌａｒｇｅ Phase difference Φ between legs on the side ｓｍａｌｌ The ratio of Φ ｓｍａｌｌ / Φ ｌａｒｇｅ =V ｓｍａｌｌ / V ｌａｒｇｅ 2. The DC-DC converter of claim 1, wherein:

3. The control unit 3. The DC-DC converter according to claim 2, wherein the first operation and the second operation are switched every cycle.

4. The second limit value is the rated voltage V of the transformer expressed as the converted voltage. ０ From this, the switching frequency is f, Φ ｌａｒｇｅ３ =Vo / (4・f・V ｓｍａｌｌ ) 4. The DC-DC converter of claim 3, wherein:

5. The first limit value is a bridge-to-bridge phase difference Φ Ｂ from, F ｌａｒｇｅ１ =(π-|Φ Ｂ |)×V ｌａｒｇｅ / V ｓｍａｌｌ Φ ｌａｒｇｅ２ =|Φ Ｂ |×V ｌａｒｇｅ / (V ｌａｒｇｅ -V ｓｍａｌｌ ) 5. The DC-DC converter according to claim 2, wherein the smaller value of

6. a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg; a secondary bridge circuit including a plurality of secondary 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: a first operation of regarding the first leg as a first virtual leg, the second leg as a second virtual leg, the third leg as a third virtual leg, and the fourth leg as a fourth virtual leg; a second operation of regarding the second leg as a first virtual leg, the first leg as a second virtual leg, the fourth leg as a third virtual leg, and the third leg as a fourth virtual leg, and through the first operation and the second operation, each switching element in the first virtual leg, the second virtual leg, the third virtual leg, and the fourth virtual leg is switched every half cycle; a first inter-leg phase difference is provided between the first virtual leg and the second virtual leg; a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg; a ratio of the phase difference between the first and second legs to a value corresponding to a ratio of an input / output voltage on a primary side to an input / output voltage on a secondary side of the DC-DC converter; Furthermore, the non-smaller value of the first inter-leg phase difference and the second inter-leg phase difference is set to be equal to or less than the smaller value of a first limit value, which is the maximum value at which zero current switching can be achieved in at least one leg in each of the primary side bridge circuit and the secondary side bridge circuit, and a second limit value, which is the maximum value at which magnetic flux density in the core of the transformer is equal to or less than a predetermined magnitude; A control method for a DC-DC converter, which controls switching of each of the primary-side switching elements and each of the secondary-side switching elements.

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