DC-DC Converter and Control Method Thereof

Abstract

Limit the maximum magnetic flux density in the transformer within a required range. 【Solution means】The control unit (control device 40) of the DC-DC converter (1) sets a predetermined value derived from the limit value of the maximum magnetic flux density generated in the core of the transformer (Tr) as an upper limit, and based on the ratio of the first voltage (V small ) to the second voltage (V large ) and the phase difference between the bridges, determines the phase difference between the first legs on the first voltage side, and further multiplies the phase difference between the first legs by the ratio to determine the phase difference between the second legs on the second voltage side.

Description

Technical Field

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

Background Art

[0002] In a dual active bridge (DAB) type DC-DC converter capable of bidirectional DC power transmission, the applicant has proposed a technique that can reduce the loss of the entire DC-DC converter by suppressing the switching loss in the required leg (Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The switching control method of the DC-DC converter of Patent Document 1 does not consider the maximum magnetic flux density in the transformer used in the DAB type DC-DC converter. When a transformer with a limited allowable maximum magnetic flux density is used in a DC-DC converter due to constraints in terms of size or cost, the switching control method cannot be applied as it is. One aspect of the present disclosure aims to realize a DC-DC converter capable of restricting the maximum magnetic flux density in a transformer within a required range and reducing the switching loss.

Means for Solving the Problems

[0005] To solve the above problems, one aspect of the present disclosure provides a DC-DC converter that includes a primary bridge circuit having a plurality of switching elements and having a first leg and a second leg, a secondary bridge circuit having 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. The DC-DC converter transports power between a first terminal pair on the primary side and a second terminal pair on the secondary side. The control unit sets the smaller of the inter-terminal voltages at the first terminal pair and the second terminal pair, represented as the converted voltage of the transformer, as a first voltage V small and sets the other inter-terminal voltage as a second voltage V large . Also, when the converted voltage is represented as the converted voltage to the first voltage V small side, according to the power or current transmitted between the first terminal pair and the second terminal pair, a bridge-to-bridge phase difference Φ B between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side is determined. Based on a ratio of the first voltage to the second voltage and the bridge-to-bridge phase difference Φ B , with a predetermined value Φ large3 derived from the limit value Bo’ of the maximum magnetic flux density generated in the core of the transformer as an upper limit, a first leg-to-leg phase difference between the two legs in the bridge circuit on the first voltage side is determined. Further, the ratio is multiplied by the first leg-to-leg phase difference to determine a second leg-to-leg phase difference between the two legs in the bridge circuit on the second voltage side. With the duty of each switching element being constant, the switching of each switching element is controlled according to the determined bridge-to-bridge phase difference, the first leg-to-leg phase difference, and the second leg-to-leg phase difference.

[0006] To solve the above problems, another aspect of the present disclosure is a control method for a DC-DC converter that includes a primary-side bridge circuit including a plurality of switching elements and having a first leg and a second leg, a secondary-side 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-side bridge circuit and the secondary-side bridge circuit, and a control unit that controls the switching elements, and transports power between a first terminal pair on the primary side and a second terminal pair on the secondary side. Among the inter-terminal voltages at the first terminal pair and the second terminal pair, expressed as the converted voltage of the transformer, the smaller inter-terminal voltage is defined as the first voltage V small and the other inter-terminal voltage is defined as the second voltage V large . Also, when the converted voltage is expressed as the converted voltage to the first voltage V small side, according to the power or current transmitted between the first terminal pair and the second terminal pair, the bridge-to-bridge phase difference Φ B between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side is determined, and based on the ratio of the first voltage to the second voltage and the bridge-to-bridge phase difference Φ large3 with the upper limit being a predetermined value Φ B derived from the limit value Bo' of the maximum magnetic flux density generated in the core of the transformer, the first leg-to-leg phase difference between the two legs in the bridge circuit on the first voltage side is determined. Further, the ratio is multiplied by the first leg-to-leg phase difference to determine the second leg-to-leg phase difference between the two legs in the bridge circuit on the second voltage side. With the duty of each switching element being constant, the switching of each switching element is controlled according to the determined bridge-to-bridge phase difference, the first leg-to-leg phase difference, and the second leg-to-leg phase difference.

[0007] The control unit (control device) according to each aspect of the present invention may be implemented by a computer. In this case, a DC-DC converter control program for causing the computer to operate as each unit (software element) included in the control unit to implement the control unit by the computer, and a computer-readable recording medium on which the program is recorded also fall within the scope of the present invention.

Effects of the Invention

[0008] According to one aspect of the present disclosure, a DC-DC converter capable of restricting the maximum magnetic flux density in a transformer within a required range and reducing switching losses can be realized.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0010] 〔Embodiment 1〕 Hereinafter, embodiments 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 an electric circuit, and does not necessarily mean that only an external connection terminal such as a connector is provided.

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

[0012] Hereinafter, 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, but this does not necessarily mean that power is transported from the primary side to the secondary side, and it is a name for convenience. During 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 the first terminal pair (p1, q1) of the DC-DC converter 1, and a second DC voltage source is connected to the second terminal pair (p2, q2) of the DC-DC converter 1. In this case, the DC-DC converter 1 according to the embodiment can perform power transmission corresponding to any of a boosting operation, a rated voltage ratio operation, and a bucking operation from the first DC voltage source to the second DC voltage source. Further, the DC-DC converter 1 can also perform power transmission corresponding to any of a boosting operation, a rated voltage ratio operation, and a bucking operation from the second DC voltage source to the first DC voltage source.

[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 connected in series in this order to the first terminal pair. The secondary-side smoothing circuit 15 and the secondary-side bridge circuit 14 are connected in series in this order to the second terminal pair. 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 between the first terminal pair, and is configured to include a capacitor Cs1 provided between the terminal p1 and the terminal 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 between the second terminal pair, and is configured to include a capacitor Cs2 provided between the terminal p2 and the terminal 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 with each other. The first leg 21 is formed by connecting a switching element S1 and a switching element S2 in series, and the connection point thereof is one terminal x1 of the AC-side terminal pair of the primary-side bridge circuit 12. The second leg 22 is formed by connecting a switching element S3 and a switching element S4 in series, and the connection point thereof 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 with each other. The third leg 23 is formed by connecting a switching element S5 and a switching element S6 in series, and the connection point thereof is one terminal x2 of the AC-side terminal pair of the secondary-side bridge circuit 14. The fourth leg 24 is formed by connecting a switching element S7 and a switching element S8 in series, and the connection point thereof 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] The switching elements S1, S3, S5, and S7 are the high-potential-side switching elements in the first leg 21, the second leg 22, the third leg 23, and the fourth leg 24, respectively, and are referred to as upper arms. The switching elements S2, S4, S6, and S8 are the low-potential-side switching elements in the first leg 21, the second leg 22, the third leg 23, and the fourth leg 24, respectively, and are referred to as lower arms.

[0019] Each of the switching elements S1 to S4 included in the primary-side bridge circuit 12 is a primary-side switching element. Each of the switching elements S5 to S8 included in the secondary-side bridge circuit 14 is a secondary-side switching element. Each of these switching elements S1 to S8 may be provided with a freewheeling diode as shown in the circuit diagram of FIG. 1.

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

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

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

[0023] <Base control method> First, a base control method regarding the switching control of each of the switching elements S1 to S8 in the DC-DC converter 1 according to the embodiment will be described. Since this control method is a known technique disclosed in Patent Document 1, detailed description thereof will be omitted.

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

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

[0026] The control device 40 monitors the power transported between the first terminal pair (p1, q1) and the second terminal pair (p2, q2), and determines the inter-bridge phase difference Φ by feedback control so that the transported power becomes the target value. That is, when the power to be transported is smaller than the target value, the control device 40 increases the magnitude of the inter-bridge phase difference Φ B , and when it is larger than the target value, adjusts it to decrease the magnitude of the inter-bridge phase difference Φ B . B

[0027] Of the input / output voltage V1 between the first terminal pair and the input / output voltage n·V2 between the second terminal pair, which are the converted voltages for the transformer Tr with the turns ratio n, the lower voltage is defined as the first voltage V small , and the higher voltage is defined as the second voltage V large . However, when these voltages are the same, the first voltage V small and the second voltage Vlarge shall be the same value. Note that as the converted voltage, the converted voltage to the first voltage V of the transformer Tr small side shall be adopted. Therefore, here, the case where the converted voltage to the primary side is adopted is taken as an example for explanation.

[0028] In addition, when power is transported 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 transported from the first voltage V small side to the second voltage V large side, it is a step-up operation, and the reverse case 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 minimum 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 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 small side from the relational expression Φ small = Φ large ×V small / V large from.

[0030] Figure 2 is a waveform diagram showing an operation example of the DC-DC converter 1 when power is transported from the first terminal pair (p1, q1) to the second terminal pair (p2, q2). In Figure 2, the primary-side AC voltage V ac1The waveform of the voltage V across the AC side terminals of the secondary bridge circuit 14, which is the secondary AC voltage ac2 is shown.

[0031] In FIG. 2, the waveforms of the AC voltages V ac1 , V ac2 are shown together in one graph. Among them, the dark-colored waveform represents the AC voltage V ac1 , and the light-colored waveform represents the AC voltage V ac2 . The same applies to FIG. 3 described later. In FIG. 2, the waveform of the primary AC current I ac1 flowing through the AC side terminals x1 and y1 of the primary bridge circuit 12 is also shown.

[0032] Furthermore, in FIG. 2, the waveform of the magnetic flux density B in the core of the transformer Tr and the waveform of the power P transmitted from the first terminal pair (p1, q1) to the second terminal pair (p2, q2) are also shown. Since the power transmitted by the DC-DC converter 1 is DC power, the waveform of the power P shows a constant value.

[0033] In the example of FIG. 2, the phase difference between the legs of the primary bridge circuit 12 is determined as the above phase difference Φ large , and the phase difference between the legs of the secondary bridge circuit 14 is determined as the above phase difference Φ small . That is, when comparing 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 converted voltage, the latter is larger, and it is a step-up operation. Note that the bridge-to-bridge phase difference Φ B is the phase difference in switching between the first leg 21 and the third leg 23.

[0034] In the first leg 21, the turn-on operation or turn-off operation of the switching elements S1 and S2 is executed at the phases t3 and t6 shown in FIG. 2 during the switching period. In the second leg 22, the turn-on operation or turn-off operation of the switching elements S3 and S4 is executed at the phases t1 and t4.

[0035] In the third leg 23, the turn-on operation or turn-off operation of the switching elements S5 and S6 is executed at phases t3 and t6 during the switching period. In the fourth leg 24, the turn-on operation or turn-off operation of the switching elements S7 and S8 is executed at phases t2 and t5. In the example of FIG. 2, the phase difference Φ large = π, which is the maximum, and the timings of phase t3 and phase t4, and phase t6 and phase t1 are the same, respectively.

[0036] AC current I ac1 As is clear from comparison with the waveform of, zero-current switching (ZCS: Zero Current Switching) is realized in the switching at the timings of phases t1, t3, t4, and t6. Therefore, zero-current switching is realized in the first leg 21, the second leg 22, and the third leg 23. Therefore, according to the base control method, reduction of switching loss is realized for the entire DC-DC converter 1.

[0037] Note that Φ large ≦ Φ large1 : Relational expression 1, Φ large ≦ Φ large2 : Relational expression 2, being simultaneously satisfied is the condition for realizing zero-current switching (ZCS) in at least one leg in each of the primary-side bridge circuit 12 and the secondary-side bridge circuit 14.

[0038] Therefore, in the base control method, the value of the inter-leg phase difference Φ small in the bridge circuit on the first voltage V large side may be set to the first limit value, which is the minimum value of the phase difference Φ large1 and the phase difference Φ large2 . Alternatively, the value of the inter-leg phase difference Φ large may be set to a value equal to or less than the first limit value.

[0039] <Principle of the control method of the DC-DC converter according to this embodiment> When controlling the switching of each switching element in the DC-DC converter 1 by the above-described base control method, the theoretically maximum value B of the magnetic flux density of the core of the transformer Tr max will be considered. Here, the theoretically maximum value means that it is intended to be in a situation not restricted by the configuration of the transformer Tr.

[0040] The theoretically maximum value B of the magnetic flux density by the base control method max is when the value of the first voltage V small and the inter-leg phase difference Φ large take the following values. For the first voltage V small , it is the case where the value of V small becomes the upper limit value Vo of the range defined as the operating range of the DC-DC converter 1, i.e., V small = Vo. For the inter-leg phase difference Φ large , it is the case where Φ large = π. Since B max = V small ·Φ large / (2N·S·2π·f), the following equation holds: B max = V small ·Φ large / (2N·S·2π·f) = Vo / (4·f·N·S) Here, N is the number of turns of the winding on the side of the first voltage V small of the transformer Tr, S is the core cross-sectional area, and f is the switching frequency. The switching frequency f is also the AC frequency of the AC power transported between the AC side terminal pair (x1, y1) of the primary side bridge circuit 12 and the AC side terminal pair (x2, x2) of the secondary side bridge circuit 12 via the conversion unit 13.

[0041] From the middle side and the right side of the above equation, Φ large = Vo / (2·f·V small ) is expressed, which is the condition under which the magnetic flux density can take the above maximum value B max . Φ largeBy reducing the value of [[ID=]] to a value represented by this formula, the magnetic flux density of the core of the transformer Tr is proportionally reduced.

[0042] Therefore, in the DC-DC converter 1 of Embodiment 1, an expression representing such a proportional coefficient as x / 100 Φ large3 =Vo / (2·f·V small )×x / 100 is used to limit the range of Φ large . By doing so, a control method can be realized in which the magnetic flux density that can occur in the core of the transformer Tr becomes a value reduced according to the proportional coefficient x / 100 from the ideal maximum value B max of the magnetic flux density by the base control method.

[0043] By the above method, when the phase difference Φ small between legs in the bridge circuit on the first voltage V large side is adopted as the value Φ large3 , the maximum magnetic flux density Bo' generated in the core of the transformer Tr by the control method of the DC-DC converter of this embodiment is Bo’=B max ·x / 100 as represented. Therefore, such a maximum magnetic flux density Bo' is also the limit value of the maximum magnetic flux density generated in the core of the transformer Tr, which is limited by the control method of the DC-DC converter of this embodiment. By using such a limit value Bo' of the maximum magnetic flux density, the above Φ large3 can be Φ large3 =2·N·S·Bo’ / V small described as.

[0044] <Control Method of DC-DC Converter of this Embodiment> To summarize, in the control method of the DC-DC converter 1 of this embodiment, the method for determining the phase difference Φ large is changed as follows from the base control method. Φ large1 , Φ large2 , Φ large3Let the minimum value among them be the phase difference Φ large be defined as such.

[0045] Φ large1 =(π - |Φ B |)×V large / V small (1) Φ large2 =|Φ B |×V large / (V large -V small ) (2) Φ large3 =Vo / (2·f·V small )×x / 100 (3) Here, x / 100 is the ratio of the magnitude of the maximum magnetic flux density allowed by the transformer Tr applied to the DC-DC converter 1, that is, the limiting value Bo’, to the value calculated as B max =Vo·π / (2NS·2πf).

[0046] Figure 3 is a waveform diagram showing the operation of the DC-DC converter 1 when the control method of the DC-DC converter of the present embodiment is applied. Figure 3 shows the waveforms of the above-described physical quantities shown in Figure 2 when the same power is transmitted as in the operation of the DC-DC converter 1 by the base control method shown in Figure 2.

[0047] In the case shown in Figure 3, according to the above formula (3), the magnitude of the phase difference Φ large is limited. That is, switching control is performed so that the magnetic flux density B does not reach the ideal maximum value B max of the magnetic flux density by the base control method. Therefore, the value of Φ large in Figure 3 is smaller than that in the case of the example shown in Figure 2. Also, the magnetic flux density B of the core of the transformer Tr has a waveform that levels off (is clipped) at its magnitude Bo’. Note that Bo’ is also shown for reference in the graph of the waveform of the magnetic flux B in Figure 2.

[0048] When compared with the case of the base control method shown in FIG. 2, in the control method of the DC-DC converter of the present embodiment shown in FIG. 3, although the current value Iac1 is smaller than the peak value at the timings where the phases are t3 and t6, zero-current switching is not achieved.

[0049] Therefore, in the control method of the DC-DC converter of the present embodiment, zero-current switching is not achieved in the first leg 21 when compared with the case of the base control method. However, even in the control method of the DC-DC converter of the present embodiment, zero-current switching is achieved in the second leg 22 and the third leg 23, and the switching loss is reduced for the entire DC-DC converter 1.

[0050] The following shows a comparison of the outlines of the ranges of parameters (physical quantities) in each part of the DC-DC converter between the case of the base control method in FIGS. 4 to 6 and the case of the control method of the DC-DC converter of the present embodiment in FIGS. 7 to 9.

[0051] FIG. 4 is a graph showing the maximum value P of the output power that can be achieved by the base control method when the input-output voltage V2 between the second terminal pair is fixed and the input-output voltage V1 between the first terminal pair changes. MAX According to the base control method, when the input-output voltages on the primary side and the secondary side are balanced (in the case of the rated voltage ratio operation), the maximum value P of the transmitted power MAX can be maximized, and the situation of decreasing as it deviates from the balanced state is shown.

[0052] In FIG. 4, the voltage Voo is the minimum value of the input-output voltage V1 defined for the operation range of the DC-DC converter, and the voltage Vo is the maximum value. Also, the power Po is the maximum value of the power defined in the same way and may be referred to as the rated power. In FIG. 4, the hatched range Rb is the operation range defined for the DC-DC converter. Thus, usually, the actual operation range of the DC-DC converter is defined within the range defined as the rating.

[0053] FIG. 5 is a graph showing the relationship between the magnitude B (T) of the maximum magnetic flux density in the core of transformer Tr when a certain input / output voltage V1 is selected in the graph of FIG. 4 and the output power P is changed there. Similar to FIG. 4, the operating range Rb1 is shown in the figure. FIG. 6 is a graph showing the relationship with the power transmission efficiency η when a certain input / output voltage V1 is selected in the graph of FIG. 4 and the output power P is changed there. Similar to FIG. 4, the operating range Rb2 is shown in the figure.

[0054] As shown in FIG. 7, in the control method of the DC-DC converter of the present embodiment, by providing a limit in the range of the phase difference, the maximum value P of the output power possible by the switching control method MAX ’ is lower than the maximum value P in FIG. 4. The dashed line indicates the maximum value P of the transmitted power in the case of FIG. 4 for comparison. However, the rated power Po of the DC-DC converter itself does not need to be changed from the case of the base control method, and the actual operating range defined for the DC-DC converter 1 is not restricted from the case of the base control method. MAX For comparison with FIG. 5, FIG. 7 shows a situation where the magnitude B (T) of the maximum magnetic flux density in the core of transformer Tr is clipped at the limit value Bo' in the control method of the DC-DC converter 1 of the present embodiment. For comparison with FIG. 6, FIG. 8 shows a situation where the power transmission efficiency η is lower than that of the base control method in a situation where the maximum magnetic flux density in the core of transformer Tr is restricted. MAX However, as described above, according to the control method of the DC-DC converter 1 of the present embodiment, an operation is possible to make the magnitude of the maximum magnetic flux density in the core of transformer Tr within the range of the limit value Bo'. Therefore, it becomes possible to select the transformer Tr applied to the DC-DC converter 1 while considering the size or cost aspect.

[0055]

[0056]

[0057] [Embodiment 2] This application proposes a technology that uses a control method for a DC-DC converter that can achieve zero voltage switching (ZVS) or zero current switching in a specific leg as the basic operation, and switches the control of the leg, upper arm, and lower arm from the switching control based on the basic operation every period (Patent Publication No. 7648971).

[0058] According to this technology, the occurrence of switching loss in each switching element can be averaged between the legs. Also, the magnitude of the maximum magnetic flux density in the core of the transformer Tr is the same as when each switching element is controlled in the basic operation. The control method of the DC-DC converter 1 according to the first embodiment can be combined with this technology. Specifically, the control of each switching element is executed as follows.

[0059] The control device 40 of the DC-DC converter according to the second embodiment performs the switching control for the first leg 21 to the fourth leg 24 in the first embodiment on the first virtual leg to the fourth virtual leg, respectively, as a basic operation for transporting power. Then, the first leg 21 of the DC-DC converter circuit in Fig. 1 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, and a first operation is performed in each leg to control the switching elements by the basic operation.

[0060] 1 as the first virtual leg, the first leg 21 as the second virtual leg, the fourth leg 24 as the third virtual leg, and the third leg 23 as the fourth virtual leg, and in each leg, a second operation is performed in which the on / off control of the upper arm and the on / off control of the lower arm in the basic operation are interchanged to control each switching element. The control device 40 controls each switching element by alternately executing such first and second operations for each switching period.

[0061] For example, in the operation example shown in FIG. 3, zero-current switching was achieved only on the second leg 22 side in the primary-side bridge circuit 12 and only on the third leg 23 side in the secondary-side bridge circuit 14. By applying the control of Embodiment 2, in each of the primary-side bridge circuit 12 and the secondary-side bridge circuit 14, the leg in which zero-current switching is achieved changes every switching period. Therefore, according to Embodiment 2, the switching losses generated in each switching element can be averaged among the legs.

[0062] 〔Example of Realization by Software〕 The functions of the control device 40 (hereinafter referred to as the "device") can be realized by a program for causing 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. By executing the program with this control device and storage device, the respective functions described in the above embodiments are realized.

[0063] The above program may be recorded on one or more computer-readable recording media, not temporarily. This recording medium may or may not be provided in the device. In the latter case, the above program may be supplied to the device via any wired or wireless transmission medium. Also, part or all of the functions of the control device 40 can be realized by a logic circuit. For example, an integrated circuit in which a logic circuit that functions as part or all of the functions of the control device 40 is formed is also included in the scope of the present invention.

[0064] 〔Summary〕 Aspect 1 of the present disclosure is a DC-DC converter that includes a primary-side bridge circuit having a plurality of switching elements and having a first leg and a second leg, a secondary-side bridge circuit transformer having a plurality of switching elements and having a third leg and a fourth leg, a conversion unit connected between the primary-side bridge circuit and the secondary-side bridge circuit, and a control unit that controls the switching elements. The DC-DC converter transports power between a first terminal pair on the primary side and a second terminal pair on the secondary side. The control unit uses the smaller of the inter-terminal voltages at the first terminal pair and the second terminal pair, expressed as the converted voltage of the transformer, as the first voltage V small and the other inter-terminal voltage as the second voltage V large . Also, when the converted voltage is expressed as the converted voltage to the first voltage V small side, according to the power or current transmitted between the first terminal pair and the second terminal pair, the bridge-to-bridge phase difference Φ B between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side is determined. Based on the ratio of the first voltage to the second voltage and the bridge-to-bridge phase difference Φ large3 with a predetermined value Φ B derived from the limit value Bo' of the maximum magnetic flux density generated in the core of the transformer as the upper limit, the first leg-to-leg phase difference between the two legs in the bridge circuit on the first voltage side is determined. Further, the ratio is multiplied by the first leg-to-leg phase difference to determine the second leg-to-leg phase difference between the two legs in the bridge circuit on the second voltage side. With the duty of each switching element kept constant, the switching of each switching element is controlled according to the determined bridge-to-bridge phase difference, the first leg-to-leg phase difference, and the second leg-to-leg phase difference.

[0065] The DC-DC converter according to Aspect 2 of the present disclosure is, in the above Aspect 1, where the predetermined value Φ large3 is from the limit value Bo', the number of turns N of the winding on the first voltage V small side of the transformer, and the core cross-sectional area S of the transformer, Φ large3 = 2·N·S·Bo’ / Vsmall It is calculated as

[0066] The DC-DC converter according to Embodiment 3 of the present disclosure, in the above Embodiment 1, the maximum value Vo of the first voltage V small , the switching frequency f, the number of turns N of the winding on the first voltage V small side of the transformer, and the core cross-sectional area S of the transformer, B max =Vo / (4·f·N·S) The magnetic flux density B calculated by max is used to set the limit value Bo’ as Bo’=B max ·x / 100 When expressed as large3 , the predetermined value Φ Φ large3 =Vo / (2·f·V small )×x / 100 It is calculated as

[0067] The DC-DC converter according to Embodiment 4 of the present disclosure, in any one of the above Embodiments 1 to 3, the first leg phase difference is Φ large1 =(π-|Φ B |)×V large / V small The phase difference Φ obtained by large1 , and Φ large2 =|Φ B |×V large / (V large -V small ) The phase difference Φ obtained by large2 , and the minimum value among the predetermined value Φ large3 is

[0068] The DC-DC converter according to Embodiment 5 of the present disclosure, in any one of the above Embodiments 1 to 4, the control unit, the bridge phase difference Φ BIt is characterized by being determined by feedback control with reference to the power or current transported between the first terminal pair and the second terminal pair.

[0069] Aspect 6 of the present disclosure includes a primary-side bridge circuit that is a bridge circuit including a plurality of switching elements and having a first leg and a second leg, a secondary-side bridge circuit that is a 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-side bridge circuit and the secondary-side bridge circuit, and a control unit that controls the switching elements, and is a control method for a DC-DC converter that transports power between a first terminal pair on the primary side and a second terminal pair on the secondary side. The lower of the inter-terminal voltages at the first terminal pair and the second terminal pair, expressed as the converted voltage of the transformer, is defined as the first voltage V small and the other inter-terminal voltage is defined as the second voltage V large Also, when the converted voltage is expressed as the converted voltage to the first voltage V small side, according to the power or current transmitted between the first terminal pair and the second terminal pair, the bridge-to-bridge phase difference Φ B between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side is determined, and a predetermined value Φ large3 derived from the limit value Bo’ of the maximum magnetic flux density generated in the core of the transformer is used as an upper limit, and based on the ratio of the first voltage to the second voltage and the bridge-to-bridge phase difference Φ B , the first leg-to-leg phase difference between the two legs in the bridge circuit on the first voltage side is determined, the ratio is multiplied by the first leg-to-leg phase difference to determine the second leg-to-leg phase difference between the two legs in the bridge circuit on the second voltage side, and with the duty in each switching element being constant, the switching of each switching element is controlled according to the determined bridge-to-bridge phase difference, the first leg-to-leg phase difference, and the second leg-to-leg phase difference.

[0070] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims. Embodiments obtained by appropriately combining the disclosed technical means are also included in the technical scope of the present invention.

Explanation of Reference Numerals

[0071] 1 DC-DC converter (p1, q1) First terminal pair (p2, q2) Second terminal pair 11 Primary-side smoothing circuit 12 Primary-side bridge circuit 21 First leg 22 Second leg (x1, y1) AC-side terminal pair 13 Conversion unit Tr Transformer 14 Secondary-side 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 side bridge circuit including a plurality of switching elements and having a first leg and a second leg; a secondary-side 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, and a DC-DC converter that transfers power between a first terminal pair on a primary side and a second terminal pair on a secondary side, The control unit is The lesser of the terminal voltages at the first and second terminal pairs, expressed as a reduced voltage of the transformer, is defined as a first voltage V ｓｍａｌｌ The voltage between the other terminals is the second voltage V ｌａｒｇｅ The converted voltage is the first voltage V ｓｍａｌｌ When expressed as a converted voltage to the A bridge phase difference Φ between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side in response to power or current transmitted between the first terminal pair and the second terminal pair. Ｂ Determine, A predetermined value Φ derived from a limit value Bo' of the maximum magnetic flux density generated in the core of the transformer ｌａｒｇｅ３ The ratio of the first voltage to the second voltage and the bridge-to-bridge phase difference Φ Ｂ determining a first inter-leg phase difference between the two legs in the bridge circuit on the first voltage side based on the multiplying the first inter-leg phase difference by the ratio to determine a second inter-leg phase difference between the two legs in the bridge circuit on the second voltage side; a DC-DC converter that controls switching of each of the switching elements in accordance with the determined inter-bridge phase difference, the first leg phase difference, and the second leg phase difference, with a duty of each of the switching elements kept constant.

2. The predetermined value Φ ｌａｒｇｅ３ is the limit value Bo', the first voltage V of the transformer ｓｍａｌｌ From the number of turns N of the winding on the side of the transformer and the core cross-sectional area S of the transformer, Φ ｌａｒｇｅ３ =2・N・S・Bo' / V ｓｍａｌｌ The DC-DC converter according to claim 1 , wherein the DC-DC converter is calculated as follows:

3. The first voltage V ｓｍａｌｌ the maximum value Vo of the first voltage V of the transformer, the switching frequency f, ｓｍａｌｌ From the number of turns N of the winding on the side of the transformer and the core cross-sectional area S of the transformer, B ｍａｘ =Vo / (4・f・N・S) The magnetic flux density B is calculated as ｍａｘ The limit value Bo' is calculated by: B / '=B ｍａｘ ・x / 1000 When expressed as above, the predetermined value Φ ｌａｒｇｅ３ but, Φ ｌａｒｇｅ３ =Vo / (2・f・V ｓｍａｌｌ )×+ / 100 The DC-DC converter according to claim 1 , wherein the DC-DC converter is calculated as follows:

4. The first inter-leg phase difference is F ｌａｒｇｅ１ =(π-|Φ Ｂ |)×V ｌａｒｇｅ / V ｓｍａｌｌ The phase difference Φ is calculated by ｌａｒｇｅ１ and, Φ ｌａｒｇｅ２ =|Φ Ｂ |×V ｌａｒｇｅ / (V ｌａｒｇｅ -V ｓｍａｌｌ ) The phase difference Φ is calculated by ｌａｒｇｅ２ and, The predetermined value Φ ｌａｒｇｅ３ 4. The DC-DC converter according to claim 1, wherein the minimum value of

5. The control unit determines the inter-bridge phase difference Φ Ｂ 4. The DC-DC converter according to claim 1, wherein the first terminal pair and the second terminal pair are connected to each other via a feedback control.

6. a primary side bridge circuit including a plurality of switching elements and having a first leg and a second leg; a secondary-side 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 elements, and a DC-DC converter that transfers power between a first terminal pair on a primary side and a second terminal pair on a secondary side, comprising: The lesser of the terminal voltages at the first and second terminal pairs, expressed as a reduced voltage of the transformer, is defined as a first voltage V ｓｍａｌｌ The voltage between the other terminals is the second voltage V ｌａｒｇｅ The converted voltage is the first voltage V ｓｍａｌｌ When expressed as a converted voltage to the A bridge phase difference Φ between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side in response to power or current transmitted between the first terminal pair and the second terminal pair. Ｂ Determine, A predetermined value Φ derived from a limit value Bo' of the maximum magnetic flux density generated in the core of the transformer ｌａｒｇｅ３ The ratio of the first voltage to the second voltage and the bridge-to-bridge phase difference Φ Ｂ determining a first inter-leg phase difference between the two legs in the bridge circuit on the first voltage side based on the multiplying the first inter-leg phase difference by the ratio to determine a second inter-leg phase difference between the two legs in the bridge circuit on the second voltage side; A control method for a DC-DC converter, comprising: controlling switching of each of the switching elements in accordance with the determined inter-bridge phase difference, the determined first leg phase difference, and the determined second leg phase difference, with a duty of each of the switching elements kept constant.

Images