DC-DC converter

Abstract

To reduce switching loss in a DC-DC converter.SOLUTION: A DC-DC converter (1) comprises: a primary bridge circuit (10) having a first leg (11) and a second leg (12); and a secondary bridge circuit (20) having a third leg (21) and a fourth leg (22). The DC-DC converter includes a first virtual leg (Q1) and a second virtual leg (Q2), and a third virtual leg (Q3) and a fourth virtual leg (Q4) which have a phase difference from each other. In the DC-DC converter, each of the virtual legs for controlling each of the legs are switched. The DC-DC converter determines a phase difference between the primary bridge circuit and the secondary bridge circuit in accordance with a voltage difference between a voltage applied to the primary bridge circuit and a primary conversion voltage of a voltage applied to the secondary bridge circuit. The DC-DC converter determines a phase difference between the first virtual leg and the second virtual leg, and a phase difference between the third virtual leg and the fourth virtual leg in accordance with electric power to be carried.SELECTED DRAWING: Figure 15

Description

Technical Field

[0001] The present invention relates to a DC-DC converter. Background Art

[0002] Patent Document 1 discloses, in a dual active bridge type DC-DC converter, a method of bidirectionally transmitting power and performing step-up / step-down by controlling a phase difference between bridges, a phase difference between legs, and a duty ratio. Prior Art Documents Patent Documents

[0003] Patent Document 1 Japanese Unexamined Patent Application Publication No. 2021-048702 Summary of the Invention Problem to be Solved by the Invention

[0004] However, in the above-described conventional technology, heat loss occurs in specific legs. This results in heat loss being concentrated only on specific legs, making it necessary to intensively dissipate heat from the relevant legs, and thermal design is not easy.

[0005] One aspect of the present invention aims to level out heat loss generated in a DC-DC converter within a bridge, and facilitate heat dissipation of the DC-DC converter. Means for Solving the Problem

[0006] To solve the above problems, a DC-DC converter according to one aspect of the present invention comprises: 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 switching of the primary switching elements and the secondary switching elements, wherein the control unit performs a first operation in which the first leg is treated 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 the second leg as the first virtual leg The system alternately performs a second operation in which the first leg is treated as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg. The system also determines the inter-bridge phase difference between the primary bridge circuit and the secondary bridge circuit according to the voltage difference between a first voltage, which is the voltage applied to the primary bridge circuit from outside the DC-DC converter, and a second voltage, which is the primary-side converted voltage of the voltage applied to the secondary bridge circuit from outside the DC-DC converter. The system controls the inter-leg phase difference between the first virtual leg and the second virtual leg, and the inter-leg phase difference between the third virtual leg and the fourth virtual leg, according to the power transported from the primary side to the secondary side or from the secondary side to the primary side.

[0007] To solve the above problems, a DC-DC converter according to one aspect of the present invention comprises: 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 switching of the primary switching elements and the secondary switching elements, wherein the control unit controls the first leg as a first virtual leg, the second leg as a second virtual leg, and the third leg as a third virtual leg. The switching of each primary-side switching element and each secondary-side switching element is controlled by alternately performing a first operation in which a virtual leg and the fourth leg are treated as the fourth virtual leg, and a second operation in which the second leg is treated 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, while also providing a first-leg phase difference between the first virtual leg and the second virtual leg, a second-leg phase difference between the third virtual leg and the fourth virtual leg, and a bridge phase difference between the primary-side bridge circuit and the secondary-side bridge circuit. [Effects of the Invention]

[0008] According to one aspect of the present invention, heat loss in a DC-DC converter can be leveled within the bridge. [Brief explanation of the drawing]

[0009] [Figure 1] These are the circuit diagram and block diagram of DC-DC converter 1 related to a reference operating example. [Figure 2] This timing chart shows an example of operation in the low-power region at rated voltage (motoring) when variable a = 1. [Figure 3] This timing chart shows an example of operation (powering) in the low-power region at rated voltage when variable a > a > 0. [Figure 4]This timing chart shows an example of operation in the high-power region at rated voltage (motoring) when variable a = 1. [Figure 5] This timing chart shows an example of operation (motoring) in the high-power region at rated voltage when variable a > a > 0. [Figure 6] This graph shows the relationship between the phase difference φL1 between the first legs and the power Pout. [Figure 7] This timing chart shows an example of operation in the low-power region at rated voltage (regeneration) when a=1 for variable a. [Figure 8] This timing chart shows an example of operation in the high-power region under rated voltage (regeneration) when a=1 for variable a. [Figure 9] This is a timing chart showing an example of operation during voltage boosting. [Figure 10] This is a timing chart showing an example of operation during step-down voltage conversion. [Figure 11] This is another timing chart showing an example of operation in the low-power region at rated voltage (motoring) for variable a in the reference operating example, where 1>a>0. [Figure 12] This timing chart shows an example of operation (motoring) in the low-power region at rated voltage for variable a in Embodiment 1, where 1>a>0. [Figure 13] This is another timing chart showing an example of operation in the high-power region under rated voltage (motoring) for variable a in the reference operating example, where 1>a>0. [Figure 14] This timing chart shows an example of operation in the high-power region under rated voltage (motoring) for variable a in Embodiment 1, where 1>a>0. [Figure 15] These are the circuit diagram and block diagram of the DC-DC converter 1 according to Embodiment 1. [Figure 16] This is an example of a timing chart that controls the first and second virtual legs using the first virtual leg, the second virtual leg, and a switching signal. [Modes for carrying out the invention]

[0010] [Reference Operation Example] Prior to the description of the first embodiment, a reference operation example of the present invention will first be described in detail with reference to FIGS. 1 to 10. FIG. 1 is a circuit diagram and a block diagram of a DC-DC converter 1 according to the reference operation example. The DC-DC converter 1 includes a primary-side bridge circuit 10, a secondary-side bridge circuit 20, a conversion unit 30, and a control unit 40.

[0011] (Configuration of DC-DC Converter 1) The primary-side bridge circuit 10 is connected to a DC power supply via input terminals. The secondary-side bridge circuit 20 is connected to a DC load via output terminals. The voltage between the input terminals of the primary-side bridge circuit 10, that is, the voltage applied to the primary-side bridge circuit 10 from outside the DC-DC converter 1, is a primary-side voltage E1, and the current flowing through the input terminals of the primary-side bridge circuit 10 is a primary-side current I1. The voltage between the output terminals of the secondary-side bridge circuit 20, that is, the voltage output from the secondary-side bridge circuit 20 to the outside of the DC-DC converter 1, is a secondary-side voltage E2, and the current flowing through the output terminals of the secondary-side bridge circuit 20 is a secondary-side current I2. Here, each of the primary-side voltage E1, the primary-side current I1, the secondary-side voltage E2, and the secondary-side current I2 is a time-average value acquired by the control unit 40, and is used for control described later.

[0012] Here, the terms "input" and "output" are expressions based on the assumption that power is transmitted from the primary-side bridge circuit 10 to the secondary-side bridge circuit 20. However, this is an expression for convenience, and the same applies hereinafter. The DC-DC converter 1 according to the first embodiment is a bidirectional dual active bridge type DC-DC converter, and power transmission from the secondary side to the primary side is also possible. In addition, in this specification, transmission of power Pout from the primary side to the secondary side is referred to as powering (Pout>0), and transmission of power Pout from the secondary side to the primary side is referred to as regeneration (Pout<0).

[0013] The primary-side bridge circuit 10 is a circuit in which a capacitor element C1 is connected in parallel to a full-bridge circuit provided with four primary-side switching elements S1 to S4. The primary-side bridge circuit 10 is configured by a first leg 11, a second leg 12, and the capacitor element C1. The first leg 11 is configured by connecting the primary-side switching element S1 and the primary-side switching element S2 in series. The second leg 12 is configured by connecting the primary-side switching element S3 and the primary-side switching element S4 in series.

[0014] The secondary-side bridge circuit 20 is a circuit in which a capacitor element C2 is connected in parallel to a full-bridge circuit provided with four secondary-side switching elements S5 to S8. The secondary-side bridge circuit 20 is configured by a third leg 21, a fourth leg 22, and the capacitor element C2. The third leg 21 is configured by connecting the secondary-side switching element S5 and the secondary-side switching element S6 in series. The fourth leg 22 is configured by connecting the secondary-side switching element S7 and the secondary-side switching element S8 in series.

[0015] The primary-side switching elements S1 to S4 and the secondary-side switching elements S5 to S8 (hereinafter collectively referred to as switching elements S1 to S8) can each be configured of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or another FET (Field Effect Transistor). Alternatively, the switching elements S1 to S8 may be configured of an IGBT (Insulated Gate Bipolar Transistor) or another transistor.

[0016] The conversion unit 30 comprises a transformer Tr with winding ratio n and a reactor L, and is connected between the primary bridge circuit 10 and the secondary bridge circuit 20. In the circuit diagram of Figure 1, the inductance component of the conversion unit 30 is equivalently represented as the reactor L provided on the primary side. Here, the reactor L is shown as being connected to the connection point between the primary switching element S1 and the primary switching element S2 and to one end of the primary winding of the transformer Tr. The other end of the primary winding of the transformer Tr is shown as being connected to the connection point between the primary switching element S3 and the primary switching element S4.

[0017] Here, the reactor L is described as being connected to the primary winding of the transformer Tr, but this is not the only option. Also, the reactor L is described to include inductance not included in the transformer L, meaning that the actual reactor element does not necessarily have to be present in the circuit. If the conversion unit 30 is provided with an actual reactor element, the reactor element may be placed on the primary side of the transformer Tr, on the secondary side, or on both sides.

[0018] The reactor L may include the leakage inductance of the transformer Tr. In the circuit diagram of Figure 1, the secondary winding of the transformer Tr is shown to be connected to the connection point between secondary switching element S5 and secondary switching element S6 and the connection point between secondary switching element S7 and secondary switching element S8.

[0019] The primary voltage of the conversion unit 30, that is, the voltage from the connection point between primary switching element S3 and primary switching element S4 to the connection point between primary switching element S1 and primary switching element S2, is defined as the primary AC voltage Vac1. The primary current of the conversion unit 30, that is, the current flowing between the conversion unit 30 and the primary bridge circuit 10, is defined as the primary AC current Iac1.

[0020] The secondary voltage of the conversion unit 30, that is, the voltage from the connection point between the secondary switching element S7 and the secondary switching element S8 to the connection point between the secondary switching element S5 and the secondary switching element S6, is defined as the secondary AC voltage Vac2. The secondary current of the conversion unit 30, that is, the current flowing between the conversion unit 30 and the secondary bridge circuit 20, is defined as the secondary AC current Iac2.

[0021] Here, the primary voltage E1 is also referred to as the first voltage (=E1), and the voltage obtained by multiplying the secondary voltage E2 by the winding ratio n is also referred to as the second voltage (=nE2). In other words, the second voltage is the primary-side equivalent voltage of the secondary voltage E2.

[0022] (Comparative example: DC-DC converter using conventional technology) In the invention described in Patent Document 1, power transmission and voltage boosting / bucking are performed by simultaneously controlling each of the above-mentioned control parameters. Furthermore, each switching element reduces losses by setting its voltage or current to zero during switching. However, since the switching element whose losses are reduced in this way changes depending on the control mode, it is necessary to implement the same level of thermal countermeasures for all switching elements.

[0023] Furthermore, when the primary and secondary voltages are in voltage balance, or when the secondary voltage is greater than the primary voltage (boost operation: Figures 12 and 16 of Patent Document 1), a large output can be obtained, but it also has the disadvantage of high losses. In the case of voltage balance, Patent Document 1 achieves a reduction in switching losses by ZCS in one leg. Also, when the secondary voltage is smaller than the primary voltage (buck operation: Figures 6 and 15 of Patent Document 1), it has the disadvantage that the output cannot be increased if the voltage difference is small. The buck operation in Patent Document 1 utilizes the fact that energy is stored when the potential difference between the power supplies on both sides is applied to the inductor.

[0024] (Setting of winding ratio n) The winding ratio n of a transformer Tr can be expressed as follows, using the number of turns n1 of the primary winding and the number of turns n2 of the secondary winding.

number

[0025] The control unit 40 controls the switching of the switching elements S1 to S8 by appropriately referring to the secondary voltage E2 and secondary current I2.

[0026] The winding ratio n of the transformer Tr is set such that the following relationship is satisfied by the maximum value E1max within the range of primary voltage E1 and the minimum value E2min within the range of secondary voltage E2 to which the DC-DC converter 1 in the reference operating example applies.

number

[0027] However, the DC-DC converter 1 in the reference operating example is applicable only when the winding ratio n is greater than 1.

[0028] As mentioned above, DC-DC converter 1 is a bidirectional dual active bridge type DC-DC converter, so the terms primary and secondary are merely for convenience. If the relationship between the maximum value E2max of the secondary voltage E2 and the minimum value E1min of the primary voltage E1 satisfies E2max > E1min, then the secondary bridge circuit can be reinterpreted as the primary bridge circuit, and the primary bridge circuit as the secondary bridge circuit, and DC-DC converter 1 in the reference operating example can be applied.

[0029] (Block diagram) The control unit 40 controls each switching element S1 to S8 according to the block diagram shown in Figure 1. The block diagram determines the phase difference between each switching element. The block diagram shown in Figure 1 is broadly divided into block diagram 41, block diagram 42, and block diagram 43. The duty cycle of each switching element S1 to S8 is fixed, for example, to 0.5. Therefore, since there is no need to control the duty cycle, control is easy.

[0030] (Determination of the phase difference φL1 between the first leg and the phase difference φL2 between the second leg) Next, we will explain the details of each block diagram. First, we will explain block diagrams 41 and 42.

[0031] Block diagram 41 shows that the control unit 40 determines the phase difference between legs (first leg phase difference φL1) in the primary bridge circuit 10 according to the power Pout of the DC-DC converter 1. The first leg phase difference φL1 is the phase difference between the first leg 11 and the second leg 12, and the sign of this value changes depending on the control mode.

[0032] The control unit 40 determines the power Pout of the DC-DC converter 1 from the secondary voltage E2 and secondary current I2, and feeds back the deviation between the power Pout and the target power Pref_n using PI control. The control unit 40 determines the phase difference φL1 between the first legs through a limiter that restricts the absolute value of the controlled quantity determined by PI control to an appropriate value that does not saturate.

[0033] Block diagram 42 shows that the control unit 40 determines the phase difference between legs in the secondary bridge circuit 20 (second leg phase difference φL2) according to the first leg phase difference φL1 and the primary voltage E1 and secondary voltage E2. The second leg phase difference φL2 is the phase difference between the third leg 21 and the fourth leg 22, and the sign of this value changes depending on the control mode.

[0034] The control unit 40 multiplies the phase difference φL1 between the first legs by the ratio of the second voltage to the first voltage (=E1 / nE2) to obtain the phase difference φL2 between the second legs.

[0035] Therefore, depending on the power being transported from the primary bridge circuit 10 to the secondary bridge circuit 20, or from the secondary bridge circuit 20 to the primary bridge circuit 10, the phase difference between the first legs, φL1 (the phase difference between the first leg 11 and the second leg 12), is determined, and the phase difference between the second legs, φL2 (the phase difference between the third leg 21 and the fourth leg 22), is determined and controlled.

[0036] In the example operation, the phase difference between the first and second legs, φL1 and φL2, were determined by feedback control, but this is not the only method. In other words, the control unit 40 can determine the phase difference between the second and third legs, φL2, which is the phase difference between the third and fourth legs, by multiplying the phase difference between the first and second legs, φL1 (the phase difference between the first and second legs, 11), by the ratio of the second voltage to the first voltage.

[0037] (Phase difference between bridges φB) Block diagram 43 shows that the control unit 40 determines the phase difference (inter-bridge phase difference φB) between the primary bridge circuit 10 and the secondary bridge circuit 20 according to the primary voltage E1 and the secondary voltage E2. The inter-bridge phase difference φB is positive when the primary bridge circuit 10 leads the secondary bridge circuit 20.

[0038] The control unit 40 determines the variable a by the following equation.

number

[0039] Subsequently, the control unit 40 checks whether variable a is less than or equal to 0, and if it is, it performs a limit process to set variable a to 0. In other words, if variable a is less than or equal to 0, the inter-bridge phase difference is set to 0. Also, if variable a is greater than 1, it performs a limit process to set variable a to 1. Furthermore, the control unit 40 determines the inter-bridge phase difference φB by multiplying variable a by the maximum value of the inter-bridge phase difference φB_max, which is a constant specific to the DC-DC converter 1.

[0040] In other words, the inter-bridge phase difference φB is the phase difference between the primary bridge circuit 10 and the secondary bridge circuit 20, determined according to the voltage difference between the first voltage and the second voltage. The inter-bridge phase difference φB does not depend on the output power. Note that when variable a is 1, the first voltage and the second voltage are equal.

[0041] (Control mode) The DC-DC converter 1 in the reference operating example operates in three control modes. These three control modes are rated voltage operation, boost operation, and buck operation. Of these, boost operation and buck operation are operating modes when the relationship between the primary voltage E1 and the secondary voltage E2 deviates significantly from the balanced state determined by the winding ratio n of the transformer Tr. In this case, the operation of the DC-DC converter 1 in the reference operating example is restricted so that the power in boost operation becomes powering and the power in buck operation becomes regenerative.

[0042] In contrast, rated voltage operation is an operating mode in which the relationship between the primary voltage E1 and the secondary voltage E2 is matched to the winding ratio n of the transformer Tr, and the power can be used for both traction and regeneration. Rated voltage operation is a state in which the primary voltage E1 and the secondary voltage E2 are in voltage balance.

[0043] (Operates at rated voltage) Rated voltage operation occurs when variable a is positive. That is, when the voltage difference between the first voltage (=E1) and the second voltage (=nE2) is less than the reference voltage Diff. In rated voltage operation, the phase difference φL1 between the first legs is positive when the second leg 12 leads the first leg 11, and the phase difference φL2 between the second legs is positive when the fourth leg 22 leads the third leg 21. Furthermore, rated voltage operation is broadly divided into low-power region and high-power region.

[0044] Figure 2 is a timing chart showing an example of operation in the low-power region at rated voltage (motoring) when variable a = 1 (when the first voltage (= E1) and the second voltage (= nE2) are equal). Figure 3 is a timing chart showing an example of operation in the low-power region at rated voltage (motoring) when variable a > a > 0. As shown in Figures 2 and 3, in the low-power region at rated voltage operation, the currents in the second leg 12 and the third leg 21 become 0 when the second leg 12 and the third leg 21 switch. Therefore, there is no power loss associated with the switching of the second leg 12 and the third leg 21, and ZCS (Zero Current Switching) is possible.

[0045] The low-output region occurs when the voltage difference between the first voltage (E1) and the second voltage (nE2) is less than the reference voltage Diff (variable a > 0), and when the phase difference between bridges φB > phase difference between the first leg φL1 or the phase difference between bridges φB > phase difference between the second leg φL2 is true.

[0046] The output power in the low-power region can be expressed by the following equation.

number

[0047] Figure 4 is a timing chart showing an example of operation in the high-power region at rated voltage (motoring) when a=1 for variable a. Figure 5 is a timing chart showing an example of operation in the high-power region at rated voltage (motoring) when 1>a>0 for variable a. As shown in Figures 4 and 5, in the high-power region at rated voltage, the second leg 12 and the third leg 21 are ZCS (Zero-Critical Speed ​​Control).

[0048] Note that in Figures 4 and 5, only the odd-numbered switching elements (upper arms) are shown as representatives from each leg. However, the state of the even-numbered switching elements (lower arms) is 180° out of phase with the state of the odd-numbered switching elements within the same leg. This relationship is the same in other drawings as well.

[0049] The high-output region occurs when the voltage difference between the first voltage (=E1) and the second voltage (=nE2) is less than the reference voltage Diff (variable a>0), and when the phase difference between bridges φB < phase difference between the first leg φL1 or the phase difference between bridges φB < phase difference between the second leg φL2 is satisfied.

[0050] The output power in the high-power range can be expressed by the following equation.

number

[0051] Figure 6 is a graph showing the relationship between the phase difference φL1 between the first legs and the power Pout. As shown in Figure 6, in the case of rated voltage operation, the low-power region and the high-power region are continuous, and seamless mode switching is possible.

[0052] In both the low-power and high-power regions, the above procedure determines the phase difference between the bridges φB, the phase difference between the first legs φL1, and the phase difference between the second legs φL2. Therefore, the changes in the phase difference between the bridges φB, the phase difference between the first legs φL1, and the phase difference between the second legs φL2 are continuous, and their values ​​do not fluctuate discontinuously. As a result, the change in power is also continuous.

[0053] Furthermore, the above-mentioned examples of rated voltage operation relate to powering (Pout>0), but similar trends can be obtained even in regeneration (Pout<0). Figure 7 is a timing chart showing an example of operation in the low-power region rated voltage operation (regeneration) with variable a = 1. Figure 8 is a timing chart showing an example of operation in the high-power region rated voltage operation (regeneration) with variable a = 1. Note that in regeneration, the phase difference between the first leg φL1 and the phase difference between the second leg φL2 are negative numbers.

[0054] As shown in Figures 7 and 8, ZCS is possible in the second leg 12 and the third leg 21 not only during powering but also during regeneration. In other words, during rated voltage operation (voltage balance), switching loss reduction is achieved by ZCS in both legs, resulting in a higher switching loss reduction effect than the method described in Patent Document 1.

[0055] (Boost operation) Boost operation occurs when variable a is negative; that is, when the voltage difference between the first voltage (E1) and the second voltage (nE2) is greater than or equal to the reference voltage Diff. In boost operation, the phase difference φL1 between the first legs is positive when the second leg 12 leads the first leg 11, and the phase difference φL2 between the second legs is positive when the fourth leg 22 leads the third leg 21.

[0056] Figure 9 is a timing chart showing an example of operation during boost operation. In boost operation, variable a becomes 0 due to limit processing, so the phase difference φB between bridges also becomes 0. Therefore, in boost operation, power, voltage, and current are controlled by the phase difference φL1 between the first legs and the phase difference φL2 between the second legs.

[0057] In the boost operation, the process involves first determining the phase difference φL1 between the first legs, and then using that value to determine the phase difference φL2 between the second legs. This ensures that the time product of the AC voltages applied to the transformer Tr of the conversion unit 30 is equal. By making the time product of the AC voltages on the primary and secondary sides equal, the effect of preventing reactive current from flowing in the circuit is achieved.

[0058] As shown in Figure 9, in boost operation, the current generated when the switching elements in the first leg 11, second leg 12, and third leg 21 switch is zero, making ZCS (Zero Switching System) possible. In other words, in boost operation, switching losses occur only in the fourth leg 22. Therefore, power can be driven with low loss, and the cooling mechanism can be reduced.

[0059] The output power Pout in this case can be expressed by the following equation.

number

[0060] (Step-down operation) Step-down operation occurs when variable a is negative. That is, when the voltage difference between the first voltage (=E1) and the second voltage (=nE2) is greater than or equal to the reference voltage Diff. In step-down operation, the phase difference φL1 between the first legs is positive when the first leg 11 leads the second leg 12, and the phase difference φL2 between the second legs is positive when the third leg 21 leads the fourth leg 22.

[0061] Figure 10 is a timing chart showing an example of operation in buck-down mode. In buck-down mode, variable a becomes 0 due to limit processing, so the phase difference φB between bridges also becomes 0. Therefore, in buck-down mode, power, voltage, and current are controlled by the phase difference φL1 between the first legs and the phase difference φL2 between the second legs.

[0062] Furthermore, just like in boost operation, measures are taken to prevent reactive current from flowing through the circuit during step-down operation.

[0063] As shown in Figure 10, in step-down operation, the current generated when the switching elements in the first leg 11, second leg 12, and third leg 21 switch is zero, making zero-switching (ZCS) possible. In other words, in step-down operation, switching losses occur only in the fourth leg 22. Therefore, power can be regenerated with low loss, and the cooling mechanism can be reduced.

[0064] Furthermore, the output power Pout in this case is the same as that in equation (6). Therefore, when transitioning between rated voltage operation and step-down operation, there is no need to discontinuously change each phase difference, and seamless control can be achieved.

[0065] (summary) The control modes for rated voltage operation, boost operation, and buck operation can be seamlessly switched. In this case, significant heat loss occurs because ZCS cannot be performed for some of the switching elements S1 to S8, so only the cooling of those switching elements needs to be enhanced. Therefore, sufficient heat dissipation is only required for some of the switching elements S1 to S8, which simplifies thermal design and reduces costs. In addition, because each control parameter is continuous, the transformer does not become magnetized.

[0066] Furthermore, regardless of the type of operation (boost or buck), a large output can be obtained even when the voltage difference between the first voltage and the second voltage is small. In addition, losses can be reduced compared to the control method described in Patent Document 1.

[0067] [Embodiment 1] Next, the operation of the DC-DC converter 1 according to Embodiment 1, with its heat loss leveled, will be described. The DC-DC converter 1 according to Embodiment 1 has a circuit similar to that of the DC-DC converter 1 according to the reference operation example.

[0068] First, an overview of Embodiment 1 will be described. In the reference operating example, ZCS was possible in the second leg 12 and the third leg 21 during rated voltage operation. That is, heat loss occurs in the first leg 11 and the fourth leg 22 during rated voltage operation.

[0069] In contrast, Embodiment 1 operates in a manner that equalizes (levels out) the legs that generate heat loss. To achieve this, the operation of each leg in the reference operation example is swapped between the first leg 11 and the second leg 12 and between the third leg 21 and the fourth leg 22 at each switching cycle, thereby leveling the heat generation of each leg and equalizing the heat loss. By leveling the heat loss, the DC-DC converter 1 only needs to dissipate heat uniformly, thus simplifying the thermal design.

[0070] Specifically, in Embodiment 1, a first virtual leg Q1, a second virtual leg Q2, a third virtual leg Q3, and a fourth virtual leg Q4 are defined, and the control unit controls the first virtual leg Q1, the second virtual leg Q2, the third virtual leg Q3, and the fourth virtual leg Q4 in the same way as the first leg 11, the second leg 12, the third leg 21, and the fourth leg 22 in the reference operation example. Furthermore, each switching element is controlled to alternately perform, with each switching cycle, a first operation in which the first leg 11 is treated as the first virtual leg Q1, the second leg 12 as the second virtual leg Q2, the third leg 21 as the third virtual leg Q3, and the fourth leg 22 as the fourth virtual leg Q4, and a second operation in which the second leg 12 is treated as the first virtual leg Q1, the first leg 11 as the second virtual leg Q2, the fourth leg 22 as the third virtual leg Q3, and the third leg 21 as the fourth virtual leg Q4.

[0071] (Changes in the timing chart during rated voltage operation) Figure 11 is another timing chart showing an example of operation for variable a in the reference operation example at rated voltage in the low-power region (motoring) when 1>a>0. Figure 12 is a timing chart showing an example of operation for variable a in Embodiment 1 at rated voltage in the low-power region (motoring) when 1>a>0. Figure 13 is another timing chart showing an example of operation for variable a in the reference operation example at rated voltage in the high-power region (motoring) when 1>a>0. Figure 14 is a timing chart showing an example of operation for variable a in Embodiment 1 at rated voltage in the high-power region (motoring) when 1>a>0. Figure 11 corresponds to Figure 3, and Figure 13 corresponds to Figure 5, with each timing chart showing an expanded range.

[0072] In Figure 11, it can be seen that the primary switching element S3 is always leading the primary switching element S1. Also, it can be seen that the secondary switching element S7 is always leading the secondary switching element S5. In contrast, in Figure 12, it can be seen that the primary switching element S3 alternates between leading and lagging the primary switching element S1 with each switching cycle. Also, it can be seen that the secondary switching element S7 alternates between leading and lagging the secondary switching element S5 with each switching cycle.

[0073] Furthermore, Figure 13 shows that the primary switching element S3 is always leading the primary switching element S1. Also, it can be seen that the secondary switching element S7 is always leading the secondary switching element S5. In contrast, Figure 14 shows that the primary switching element S3 alternates between leading and lagging the primary switching element S1 with each switching cycle. Also, it can be seen that the secondary switching element S7 alternates between leading and lagging the secondary switching element S5 with each switching cycle.

[0074] In other words, in Embodiment 1, the operation of the first leg 11 in the reference operation example may be performed by the first leg 11 or by the second leg 12 in this embodiment. Similarly, in Embodiment 1, the operation of the third leg 21 in the reference operation example may be performed by the third leg 21 or by the fourth leg 22 in this embodiment. In Embodiment 1, the operations of the first leg 11 to the fourth leg 22 in the reference operation example are designated as the first virtual leg Q1 to the fourth virtual leg Q4, respectively, and the legs controlled by the first virtual leg Q1 to the fourth virtual leg Q4 are controlled by appropriately switching between the first leg 11 to the fourth leg 22.

[0075] As a result, in rated voltage operation, ZCS is possible in the second virtual leg Q2 and the third virtual leg Q3. In other words, in the first operation, ZCS is possible in the second leg 12 and the third leg 21, and in the second operation, ZCS is possible in the first leg 11 and the fourth leg 22. As the legs on which ZCS is possible change with each switching cycle, heat loss is leveled out. Therefore, the heat generated when ZCS cannot be achieved in each switching element is also leveled out, and the DC-DC converter 1 only needs to dissipate heat uniformly.

[0076] Comparing Figures 11 and 12, and Figures 13 and 14, in the reference operating examples (Figures 11 and 13), the primary AC voltage Vac1, secondary AC voltage Vac2, primary AC current Iac1, and secondary AC current Iac2 each exhibit an oscillating waveform in which the positive and negative signs reverse every half period.

[0077] In contrast, in Embodiment 1 (Figures 12 and 14), the primary AC voltage Vac1, secondary AC voltage Vac2, primary AC current Iac1, and secondary AC current Iac2 each have an oscillating waveform in which the positive and negative signs reverse every period. However, this oscillating waveform oscillates within a range where the positive and negative signs do not reverse every half period, and as a result, it oscillates twice within a range where the positive and negative signs do not reverse in one period.

[0078] Furthermore, even when switching between the first and second operations, the output power Pout does not change. In other words, the secondary voltage E2 and secondary current I2 remain unchanged between Embodiment 1 and the reference operation example.

[0079] (Block diagram) Figure 15 shows the circuit diagram and block diagram of the DC-DC converter 1 according to Embodiment 1. Unlike Figure 1, which shows a reference operation example, Figure 15 according to Embodiment 1 includes the additional processing shown in block diagrams 44, 45, and 46.

[0080] Block diagram 44 shows the logic for determining the first virtual leg Q1 to the fourth virtual leg Q4 with a duty cycle of 0.5. In block diagram 44, the first virtual leg Q1 to the fourth virtual leg Q4 are generated by the phase difference φL1 between the first legs, the phase difference φL2 between the second legs, and the phase difference φB between the bridges.

[0081] (SEL switching signal) Block diagram 45 shows the process of creating the switching signal SEL. The switching signal SEL performs the process of switching between the first operation and the second operation. The switching signal SEL is a signal that switches every cycle.

[0082] In block diagram 46, the switching signal SEL and the first virtual leg Q1 to the fourth virtual leg Q4 are used to actually control the first leg 11 to the fourth leg 22, and the logic circuit determines the control signals for each switching element S1 to S8.

[0083] Figure 16 is an example of a timing chart that controls the first leg 11 and the second leg 12 using the first virtual leg Q1, the second virtual leg Q2, and the switching signal SEL.

[0084] Furthermore, the timing chart for controlling the third and fourth legs 21 and 22 by the third virtual leg Q3, the fourth virtual leg Q4, and the switching signal SEL is shown in Figure 16, but with the first virtual leg Q1 replaced by the third virtual leg Q3, the second virtual leg Q2 replaced by the fourth virtual leg Q4, control signal S1 replaced by control signal S5, and control signal S3 replaced by control signal S7. Control signals S2, S4, S6, and S8 are the inverse logic of control signals S1, S3, S5, and S7, respectively.

[0085] (summary) Therefore, when transmitting power in the DC-DC converter 1, the switching elements performing ZCS are replaced periodically to level out the heat, and the large amount of heat generated in the switching elements S1 to S8 that do not perform ZCS can be leveled out while maintaining the output of the DC-DC converter 1. As a result, the heat dissipation efficiency of the switching elements S1 to S8 that constitute the DC-DC converter 1 is improved.

[0086] [Variation] (Boost operation / Buck operation) Embodiment 1 describes only the operation at the rated voltage, but is not limited thereto. That is, the boost and buck operations shown in the reference operation examples may also be implemented using the control method shown in Embodiment 1.

[0087] In other words, in both boost and buck operations, the heat generated due to ZCS not being achieved solely in the fourth leg 22 can be equalized across the third leg 21 and the fourth leg 22, thereby reducing the heat generated in each of them. As a result, the thermal design of the DC-DC converter 1 as a whole becomes easier.

[0088] (Regarding the switching signal SEL) In Embodiment 1, the switching signal SEL is switched every period, but it is not limited to this. For example, it may be switched every integer period. Alternatively, the temperature of each switching element may be measured, and the switching signal SEL may be controlled so that heat is generated in the switching element with the lowest temperature.

[0089] 〔summary〕 To solve the above problems, a DC-DC converter according to embodiment 1 of the present invention comprises: 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 switching of the primary switching elements and the secondary switching elements, wherein the control unit performs a first operation in which the first leg is treated 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 the second leg as the first virtual leg The system alternately performs a second operation in which the first leg is treated as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg. The system also determines the inter-bridge phase difference between the primary bridge circuit and the secondary bridge circuit according to the voltage difference between a first voltage, which is the voltage applied to the primary bridge circuit from outside the DC-DC converter, and a second voltage, which is the primary-side converted voltage of the voltage applied to the secondary bridge circuit from outside the DC-DC converter. The system controls the inter-leg phase difference between the first virtual leg and the second virtual leg, and the inter-leg phase difference between the third virtual leg and the fourth virtual leg, according to the power transported from the primary side to the secondary side or from the secondary side to the primary side.

[0090] With the above configuration, it is not possible to achieve ZCS, and the legs in which switching losses occur can be leveled within each bridge circuit. As a result, the heat generated in the switching elements can be distributed throughout the entire DC-DC converter, making thermal design easier.

[0091] In the DC-DC converter according to embodiment 2 of the present invention, in embodiment 1, the control unit may determine a variable obtained by subtracting the voltage difference from the reference voltage and dividing the value by the reference voltage, and determine the first inter-leg phase difference and the second inter-leg phase difference according to the variable.

[0092] With the above configuration, it is possible to appropriately select and control rated voltage operation, boost operation, and buck operation, respectively, during rated voltage operation.

[0093] In the DC-DC converter according to embodiment 3 of the present invention, in embodiment 2, the control unit may set the bridge phase difference to 0 when the variable is 0 or less.

[0094] With the above configuration, when transitioning from rated voltage operation to boost or buck operation, or vice versa, seamless and continuous control of each control parameter, namely the phase difference, can be achieved. Therefore, because each control parameter is continuous, the transformer does not become magnetized.

[0095] In the DC-DC converter according to aspect 4 of the present invention, in aspect 2 above, the control unit may determine the inter-bridge phase difference by multiplying the variable by a predetermined maximum phase difference when the variable is positive.

[0096] With the above configuration, the phase difference between bridges can be kept within a range controllable by the DC-DC converter, enabling stable control.

[0097] In the DC-DC converter according to embodiment 5 of the present invention, in any of embodiments 1 to 4 above, the control unit may determine the second inter-leg phase difference by multiplying the first inter-leg phase difference by the ratio of the second voltage to the first voltage.

[0098] With the above configuration, seamless and continuous control of the phase difference, which is a control parameter, can be achieved with respect to the output power during rated voltage operation, boost operation, and buck operation. Therefore, because each control parameter is continuous, the transformer does not become magnetized.

[0099] To solve the above problems, a DC-DC converter according to embodiment 6 of the present invention comprises: 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 switching of the primary switching elements and the secondary switching elements, wherein the control unit controls the first leg as a first virtual leg, the second leg as a second virtual leg, and the third leg as a third virtual leg. The switching of each primary-side switching element and each secondary-side switching element is controlled by alternately performing a first operation in which a virtual leg and the fourth leg are treated as the fourth virtual leg, and a second operation in which the second leg is treated 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, while also providing a first-leg phase difference between the first virtual leg and the second virtual leg, a second-leg phase difference between the third virtual leg and the fourth virtual leg, and a bridge phase difference between the primary-side bridge circuit and the secondary-side bridge circuit.

[0100] With the above configuration, the role of each leg can be leveled within the bridge. Therefore, heat loss can be leveled.

[0101] In the DC-DC converter according to embodiment 7 of the present invention, in any of embodiments 1 to 5 above, the winding ratio of the transformer may be the ratio of the maximum value in the voltage range of the primary bridge circuit to the minimum value in the voltage range of the secondary bridge circuit.

[0102] In the DC-DC converter according to embodiment 8 of the present invention, in any of embodiments 1 to 5 above, the maximum value of the first voltage range and the minimum value of the second voltage range may be equal.

[0103] With the above configuration, even when the first voltage and the second voltage are different, seamless and continuous control of the phase difference between the bridges can be achieved.

[0104] In the DC-DC converter according to embodiment 9 of the present invention, the duty cycle of the first virtual leg, the second virtual leg, the third virtual leg, and the fourth virtual leg may be 0.5 in any of embodiments 1 to 8 above.

[0105] With the above configuration, there is no need to control the duty cycle, making it easy to control the DC-DC converter.

[0106] In the DC-DC converter according to embodiment 10 of the present invention, the first operation and the second operation may be switched every one cycle in any of embodiments 1 to 9 above.

[0107] With the above configuration, the leg that undergoes ZCS can be switched every cycle.

[0108] In the DC-DC converter according to embodiment 11 of the present invention, the first operation and the second operation may be switched every N periods (where N is a natural number of 2 or more) in any of embodiments 1 to 9.

[0109] With the above configuration, the ZCS leg can be switched at intervals that are N times the period.

[0110] [Additional Notes] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of Symbols]

[0111] 1 DC-DC converter 10 Primary bridge circuit 11 Leg 1 12 Leg 2 20 Secondary bridge circuit 21 Leg 3 22 Leg 4 30 Conversion section 40 Control Unit S1~4 Primary switching elements S5~8 Secondary switching elements

Claims

1. A primary side bridge circuit comprising multiple primary side switching elements and having a first leg and a second leg, A secondary bridge circuit comprising multiple 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 DC-DC converter comprising a control unit for controlling the switching of the primary switching element and the secondary switching element, The control unit, A first operation in which the first leg is considered the first virtual leg, the second leg the second virtual leg, the third leg the third virtual leg, and the fourth leg the fourth virtual leg, The second operation, in which the second leg is treated 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, is performed alternately. A DC-DC converter that determines the inter-bridge phase difference between the primary-side bridge circuit and the secondary-side bridge circuit according to the voltage difference between a first voltage, which is a voltage applied to the primary-side bridge circuit from outside the DC-DC converter, and a second voltage, which is the primary-side converted voltage of a voltage applied to the secondary-side bridge circuit from outside the DC-DC converter, and controls the inter-leg phase difference between the first virtual leg and the second virtual leg, and the inter-leg phase difference between the third virtual leg and the fourth virtual leg, according to the power transported from the primary side to the secondary side or from the secondary side to the primary side.

2. The control unit obtains a variable by dividing the value obtained by subtracting the voltage difference from the reference voltage by the reference voltage, The DC-DC converter according to claim 1, wherein the phase difference between the first and second legs is determined according to the aforementioned variable.

3. The DC-DC converter according to claim 2, wherein the control unit sets the bridge phase difference to 0 when the variable is 0 or less.

4. The DC-DC converter according to claim 2, wherein the control unit determines the inter-bridge phase difference by multiplying the variable by a predetermined maximum phase difference when the variable is positive.

5. The DC-DC converter according to claim 1, wherein the control unit determines the second inter-leg phase difference by multiplying the first inter-leg phase difference by the ratio of the second voltage to the first voltage.

6. A primary side bridge circuit comprising multiple primary side switching elements and having a first leg and a second leg, A secondary bridge circuit comprising multiple 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 DC-DC converter comprising a control unit for controlling the switching of the primary switching element and the secondary switching element, The control unit, A first operation in which the first leg is considered the first virtual leg, the second leg the second virtual leg, the third leg the third virtual leg, and the fourth leg the fourth virtual leg, The second operation, in which the second leg is treated 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, is performed alternately. A phase difference between the first and second virtual legs is provided between the first and second virtual legs. A second-leg phase difference is provided between the third virtual leg and the fourth virtual leg. A DC-DC converter characterized by controlling the switching of each primary-side switching element and each secondary-side switching element by providing an inter-bridge phase difference between the primary-side bridge circuit and the secondary-side bridge circuit.

7. The DC-DC converter according to any one of claims 1 to 5, wherein the winding ratio of the transformer is the ratio of the maximum value in the voltage range of the primary bridge circuit to the minimum value in the voltage range of the secondary bridge circuit.

8. The DC-DC converter according to any one of claims 1 to 5, wherein the maximum value in the first voltage range is equal to the minimum value in the second voltage range.

9. The DC-DC converter according to any one of claims 1 to 6, wherein the duty cycle of the first virtual leg, the second virtual leg, the third virtual leg, and the fourth virtual leg is 0.

5.

10. A DC-DC converter according to any one of claims 1 to 6, characterized in that the first operation and the second operation are switched every one cycle.

11. A DC-DC converter according to any one of claims 1 to 6, characterized in that the first operation and the second operation are switched every N periods (where N is a natural number of 2 or more).

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