DC-DC converter and control method

Abstract

To provide a DC-DC converter with minimized loss.SOLUTION: A DC-DC converter (1) includes a transformer (Tr) between a primary side bridge circuit (10) connected to a first terminal pair (13) and a secondary side bridge circuit (20) connected to a second terminal pair (23). Lesser of terminal tube voltage at the first and second terminal pairs, which is expressed as conversion voltage for the transformer, is called as first voltage, and the one which is not smaller is called as second voltage. The DC-DC converter determines a phase difference between bridges, a phase difference between first legs and a phase difference between second legs and controls each switching element based on current to be transmitted, first voltage and second voltage.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Dual active bridge type (hereinafter abbreviated as DAB) DC-DC converters are widely used for transmitting DC power. The DC-DC converter disclosed in Patent Document 1 includes a transformer between a bridge circuit connected to a first terminal pair and a bridge circuit connected to a second terminal pair. In the DC-DC converter, the greater of the terminal voltages at the first terminal pair and the second terminal pair expressed as converted voltages for the transformer is called a first voltage, and the lesser of the terminal voltages is called a second voltage. Furthermore, in the DC-DC converter, the bridge phase difference, the first leg phase difference, and the second leg phase difference are determined based on the current to be transmitted, the first voltage, and the second voltage, and each switching element is controlled. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7315886 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the DC-DC converter of Patent Document 1, the conduction loss and switching loss are minimized. However, Patent Document 1 does not take into consideration the iron loss. Therefore, in the DC-DC converter of Patent Document 1, the overall loss is not necessarily minimized.

[0005] An object of one embodiment of the present disclosure is to provide a DC-DC converter or the like in which losses are minimized. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, a DC-DC converter according to one embodiment of the present disclosure is a DC-DC converter including: a bridge circuit including a plurality of switching elements and having two legs between a first terminal pair; a bridge circuit including a plurality of switching elements and having two legs between a second terminal pair; a conversion unit including a transformer and connected between the two bridge circuits; and a control unit that controls switching of each of the switching elements of the two bridge circuits, wherein when the smaller of the inter-terminal voltages at the first terminal pair and the second terminal pair, expressed as a converted voltage for the transformer, is referred to as a first voltage Vsmall and the smaller inter-terminal voltage is referred to as a second voltage Vlarge, the control unit calculates a switching voltage according to power or current transmitted from the first terminal pair to the second terminal pair. determining a bridge phase difference φB between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side; determining a first leg phase difference between the two legs in the bridge circuit on the first voltage side based on the ratio of the first voltage to the second voltage and the bridge phase difference; multiplying the first leg phase difference by the ratio to determine a second leg phase difference between the two legs in the bridge circuit on the second voltage side; fixing a duty of each of the switching elements, and controlling switching of each of the switching elements in accordance with the determined bridge phase difference, the first leg phase difference, and the second leg phase difference; and setting the first leg phase difference to a value of φlarge that minimizes third relational expression (3) in a range that satisfies first relational expression (1) and second relational expression (2). φlarge≦(Vlarge / Vsmall)×(π-φB) (1) φlarge≦(Vlarge / (Vlarge-Vsmall))×φB (2) Wloss(φlarge)=Wfe+Wsw+Wcon (3) Wfe: Iron loss Wsw: Switching loss Wcon: Conduction loss Further, a control method according to an embodiment of the present disclosure is a control method for a DC-DC converter including a bridge circuit including a plurality of switching elements and having two legs between a first terminal pair, a bridge circuit including a plurality of switching elements and having two legs between a second terminal pair, and a conversion unit including a transformer and connected between the two bridge circuits, the control method comprising: controlling switching of each of the switching elements of the two bridge circuits, the control method comprising: controlling a switching voltage of the bridge circuit on the first voltage side and a switching voltage of the bridge circuit on the second terminal pair side according to power or current transmitted from the first terminal pair side to the second terminal pair side, the control method comprising: controlling a switching voltage of each of the switching elements of the two bridge circuits; determining a first-leg phase difference between the two legs in the bridge circuit on the first voltage side based on a ratio of the first voltage to the second voltage and the inter-bridge phase difference; and determining a second-leg phase difference between the two legs in the bridge circuit on the second voltage side by multiplying the first-leg phase difference by the ratio, wherein a duty of each of the switching elements is fixed, and switching of each of the switching elements is controlled in accordance with the determined inter-bridge phase difference, the first-leg phase difference, and the second-leg phase difference, and the first-leg phase difference is set to a value of φlarge that minimizes the third relation (3) within a range that satisfies the first relation (1) and the second relation (2). φlarge≦(Vlarge / Vsmall)×(π-φB) (1) φlarge≦(Vlarge / (Vlarge-Vsmall))×φB (2) Wloss(φlarge)=Wfe+Wsw+Wcon (3) Wfe: Iron loss Wsw: Switching loss Wcon: Conduction loss Effect of the Invention

[0007] According to one aspect of the present disclosure, it is possible to realize a DC-DC converter or the like in which losses are minimized. [Brief description of the drawings]

[0008] [Figure 1] 1 is a circuit diagram of a DC-DC converter according to an embodiment of the present invention. [Diagram 2] FIG. 4 is a block diagram showing the operation of a control unit. [Diagram 3] 13 is a graph showing the operation of the DC-DC converter when φlarge does not satisfy the second relational expression. [Figure 4] 4 is a graph showing an example of the relationship between the first inter-leg phase difference and loss in the DC-DC converter according to the present embodiment. [Diagram 5] 1 is a graph showing the relationship between AC voltage, AC power, magnetic flux density in a transformer, and output power of a DC-DC converter. [Figure 6] 4 is a graph showing current and voltage waveforms in each of the switching elements. [Figure 7] FIG. 13 is a diagram for explaining switching loss. [Figure 8] FIG. 13 is a diagram for explaining a current calculation formula when the DC-DC converter is operating in a square mode. [Figure 9] FIG. 13 is a diagram for explaining a current calculation formula when the DC-DC converter is operating in a triangular mode. [Figure 10] 1 is a graph comparing the efficiency of an embodiment and a conventional example. [Figure 11] 11 is a graph comparing the inter-bridge phase difference between an embodiment and a conventional example. [Figure 12] 11 is a graph comparing the phase difference between the first legs of an embodiment and a conventional example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] An embodiment of the present disclosure will be described in detail below. Fig. 1 is a circuit diagram of a DC-DC converter 1 according to this embodiment. The DC-DC converter 1 includes a primary bridge circuit 10, a secondary bridge circuit 20, a conversion unit 30, and a control unit 40.

[0010] (Configuration of DC-DC converter 1) The primary bridge circuit 10 is connected to a first terminal pair 13. The voltage of the first terminal pair 13, that is, the voltage from the terminal 13a to the terminal 13b, is a voltage V1. A power source or a load may be connected to the first terminal pair 13. The current flowing into the terminal 13b is a current I1.

[0011] The secondary bridge circuit 20 is connected to the second terminal pair 23. The voltage of the second terminal pair 23, that is, the voltage from the terminal 23a to the terminal 23b, is a voltage V2. Note that a power source or a load may be connected to the second terminal pair 23. Also, the current flowing into the terminal 23b is a current I2.

[0012] The voltage V1, the voltage V2, the current I1, and the current I2 are time average values ​​obtained by the control unit 40 and are used for the control described below.

[0013] In this specification, the transmission of power from the primary bridge circuit 10 to the secondary bridge circuit 20 is referred to as "power running", and the transmission of power from the secondary bridge circuit 20 to the primary bridge circuit 10 is referred to as "regeneration". The primary bridge circuit 10 side is also referred to as the "primary side", and the secondary bridge circuit 20 side is also referred to as the "secondary side". In "power running", the output power Pout>0. In "regeneration", the output power Pout<0.

[0014] The primary bridge circuit 10 is a full bridge circuit provided with four switching elements S1 to S4, to which a capacitor element C1 is connected in parallel. The primary bridge circuit 10 is composed of a first leg 11, a second leg 12, and a capacitor element C1. The first leg 11 is composed of a switching element S1 and a switching element S2 connected in series. The second leg 12 is composed of a switching element S3 and a switching element S4 connected in series.

[0015] The secondary bridge circuit 20 is a full bridge circuit provided with four switching elements S5 to S8, and a capacitor element C2 is connected in parallel to the full bridge circuit. The secondary bridge circuit 20 is composed of a third leg 21, a fourth leg 22, and a capacitor element C2. The third leg 21 is composed of a switching element S5 and a switching element S6 connected in series. The fourth leg 22 is composed of a switching element S7 and a switching element S8 connected in series.

[0016] The switching elements S1 to S8 may be configured with MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or other FETs (Field Effect Transistors), or may be configured with IGBTs (Insulated Gate Bipolar Transistors) or other transistors.

[0017] The conversion unit 30 includes a transformer Tr having a 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 Fig. 1, the inductance component of the conversion unit 30 is equivalently represented as a reactor L provided on the primary side.

[0018] Here, the reactor L is shown connected to the connection point between the switching element S1 and the switching element S2 and one end of the primary winding of the transformer Tr. The other end of the primary winding of the transformer Tr is shown connected to the connection point between the switching element S3 and the switching element S4.

[0019] Here, the reactor L is described as being connected to the primary winding of the transformer Tr, but is not limited to this. Also, the reactor L is expressed including an inductance component not included in the transformer Tr, but an actual reactor element does not have to exist on the circuit. When a reactor element as an actual element is provided in the conversion unit 30, the reactor element may be disposed on the primary side of the transformer Tr, the secondary side, or both.

[0020] The reactor L may include a leakage inductance of the transformer Tr. In the circuit diagram of Fig. 1, the secondary winding of the transformer Tr is shown as being connected to a connection point between the switching element S5 and the switching element S6 and a connection point between the switching element S7 and the switching element S8.

[0021] The voltage on the primary side of the conversion unit 30, i.e., the voltage from the connection point between the switching element S3 and the switching element S4 to the connection point between the switching element S1 and the switching element S2, is defined as the primary side AC voltage Vac1. The current on the primary side of the conversion unit 30, i.e., the current flowing between the conversion unit 30 and the primary side bridge circuit 10, is defined as the primary side AC current Iac1.

[0022] The voltage on the secondary side of the conversion unit 30, i.e., the voltage from the connection point between the switching element S7 and the switching element S8 to the connection point between the switching element S5 and the switching element S6, is defined as the secondary side AC voltage Vac2. The current on the secondary side of the conversion unit 30, i.e., the current flowing between the conversion unit 30 and the secondary side bridge circuit 20, is defined as the secondary side AC current Iac2.

[0023] (Block diagram) 2 is a block diagram showing the operation of the control unit 40. The control unit 40 controls each of the switching elements S1 to S8 according to the block diagram. In the block diagram, the phase difference between each of the switching elements is determined.

[0024] (Phase difference between bridges φB) In reference numeral 41, the control unit 40 compares the target current Iref with the current output current Iout. The target current Iref and the output current Iout are the target value and the current value of the current I2 of the secondary bridge circuit 20, and the power transmission from the primary side to the secondary side is positive. That is, in reference numeral 41, the control unit 40 obtains the deviation of the output current Iout from the target current Iref.

[0025] In the reference numeral 42, the control unit 40 performs PI control on the deviation. The result of performing the PI control is the bridge phase difference φB. Here, the bridge phase difference φB is the phase difference between the primary bridge circuit 10 and the secondary bridge circuit 20. Focusing on the primary AC voltage Vac1 and the secondary AC voltage Vac2, the bridge phase difference φB is positive when the primary AC voltage Vac1 is a leading phase with respect to the secondary AC voltage Vac2. That is, the control unit 40 determines the bridge phase difference φB by feedback control with reference to the power or current transmitted from the first terminal pair 13 side to the second terminal pair 23 side. Note that the bridge phase difference φB may take a negative value, and a positive value indicates power running, and a negative value indicates regeneration. The bridge phase difference φB is a value in the range of -π / 2 to π / 2.

[0026] (Turns ratio n, first voltage Vsmall and second voltage Vlarge) The turns ratio n of the transformer Tr can be expressed by the following equation (4) using the number of turns n1 of the primary winding and the number of turns n2 of the secondary winding. n=n1 / n2 (4) Therefore, the voltage of the first terminal pair 13, expressed as the converted voltage on the primary side of the transformer Tr, is V1, and the voltage of the second terminal pair 23 is nV2. At reference numeral 43, as shown in Equation 2, the control unit 40 compares the magnitudes of these voltages, designates the one that is not larger as Vsmall, and the one that is not smaller as Vlarge. When V1 ≧ n × V2, Vsmall = n × V2, Vlarge = V1 When V1 < n × V2, Vsmall = V1, Vlarge = n × V2 (Phase difference φlarge between the first legs) At reference numeral 44, the control unit 40 derives the following first relational expression (5) using Vsmall and Vlarge. φlarge ≦ (Vlarge / Vsmall) × (π - |φB|) (5) At reference numeral 45, the control unit 40 derives the following second relational expression (6) using Vsmall and Vlarge. φlarge ≦ (Vlarge / (Vlarge - Vsmall)) × |φB| (6) At reference numeral 46, the control unit 40 derives the following third relational expression (7-1). The specific content of each term in the third relational expression (7-1) will be described later. Wloss(φlarge) = Wfe + Wsw + Wcon (7-1) Wfe: Iron loss Wsw: Switching loss Wcon: Conduction loss At reference numeral 47, the control unit 40 calculates φlarge that minimizes the third relational expression (7-1) within the range that satisfies the first relational expression (5) and the second relational expression (6), and sets it as the phase difference φlarge between the first legs.

[0027] Also, when the third relational expression (7-1) has an extreme point within the range that satisfies the first relational expression (5) and the second relational expression (6), the loss gradient becomes 0 at the extreme point. Therefore, the control unit 40 may calculate φlarge that satisfies the following equation (7-2). dWloss / dφlarge = 0 (7-2) However, the third relational expression (7-1) may not have a minimum point within the range in which the first relational expression (5) and the second relational expression (6) are satisfied. In other words, the loss may monotonically decrease within the range in which the first relational expression (5) and the second relational expression (6) are satisfied. In this case, the control unit 40 may calculate φlarge using the above third relational expression (7-1).

[0028] The phase difference between the first legs φlarge is the phase difference between two legs in a bridge circuit on the side of the first voltage Vsmall. If the first voltage Vsmall is a bridge circuit on the primary side, the phase difference between the first legs φlarge is positive when the second leg 12 leads the first leg 11. If the first voltage Vsmall is a bridge circuit on the secondary side, the phase difference between the first legs φlarge is positive when the fourth leg 22 leads the third leg 21. The phase difference between the first legs φlarge is always a positive value. The phase difference between the first legs φlarge is a value in the range of 0 to π.

[0029] The above first relational expression (5) and second relational expression (6) show the conditions for determining the first inter-leg phase difference φlarge so as to provide a zero current section in the AC current flowing through the transformer Tr and to increase efficiency. Specifically, if φlarge does not satisfy the first relational expression (5), there is no zero current section. Also, if φlarge does not satisfy the second relational expression (6), the proportion of reactive current increases and efficiency decreases.

[0030] Fig. 3 is a graph showing the operation of the DC-DC converter 1 when φlarge does not satisfy the second relational expression (6). In Fig. 3, reference numeral 301 indicates a primary side AC voltage Vac1 and a secondary side AC voltage Vac2, and reference numeral 302 indicates a primary side AC current Iac1 and a secondary side AC current Iac2. Also, in Fig. 3, reference numerals 303 to 306 are graphs showing the waveforms of current and voltage in each of the switching elements S1, S3, S5, and S7. The on / off of the switching elements S2, S4, S6, and S8 is in the opposite phase to the on / off of the switching elements S1, S3, S5, and S7, respectively.

[0031] When φlarge satisfies the second relation (6), switching loss occurs only in the switching element S5. On the other hand, when φlarge does not satisfy the second relation (6), as shown in FIG. 3, in the switching elements S5 and S7, the current does not become zero when the voltage rises. Therefore, compared to when φlarge satisfies the second relation (6), the number of elements in which switching loss occurs increases, and the efficiency decreases. Also, in this case, a period in which the current becomes negative occurs in an interval in which the voltage is positive, and therefore the reactive current increases. For these reasons, when φlarge does not satisfy the second relation (6), the efficiency of the DC-DC converter 1 decreases compared to when φlarge satisfies the second relation (6).

[0032] In the conventional technology, the value of φlarge is set to the maximum value that satisfies the first relational expression (5) and the second relational expression (6). In the present application, the control unit 40 determines φlarge so as to further minimize the loss Wloss under the above conditions.

[0033] (Phase difference between the second legs φsmall) At 48, the control unit 40 multiplies the first inter-leg phase difference φlarge by the ratio (Vlarge / Vsmall) of the second voltage Vlarge to the first voltage Vsmall to obtain the second inter-leg phase difference φsmall.

[0034] The phase difference φsmall between the second legs is the phase difference between the two legs in the bridge circuit on the side of the second voltage Vlarge. If the second voltage Vlarge is a bridge circuit on the primary side, the phase difference φsmall between the second legs is positive when the second leg 12 leads the first leg 11. If the second voltage Vlarge is a bridge circuit on the secondary side, the phase difference φsmall between the second legs is positive when the fourth leg 22 leads the third leg 21. The phase difference φsmall between the second legs is always a positive value. The phase difference φsmall between the second legs is a value in the range of 0 to π.

[0035] (Triangle wave comparison) At 49, the control unit 40 controls the switching of each of the switching elements S1 to S8 according to the determined inter-bridge phase difference φB, first leg phase difference φlarge, and second leg phase difference φsmall. At this time, the duty of each of the switching elements S1 to S8 is fixed at, for example, 0.5. Note that the control unit 40 may use a so-called triangular wave comparison method in which the magnitude of a triangular wave and a value corresponding to the phase difference are compared to determine the switching timing in order to generate a required switching phase difference for each of the switching elements S1 to S8.

[0036] The means for generating the control signal is not limited, and the control unit 40 may use any means that takes into consideration the inter-bridge phase difference φB, the first leg phase difference φlarge, and the second leg phase difference φsmall.

[0037] (DC-DC converter 1 operation) When control is performed based on the block diagram shown in Fig. 2, the operation of the DC-DC converter 1 can be classified from two viewpoints: the power transmission direction and the voltage difference between the primary side and the secondary side. As described above, in terms of the power transmission direction, power transmission from the primary side to the secondary side is called "power running", and power transmission from the secondary side to the primary side is called "regeneration". When the secondary side AC voltage Vac2 is in a lagging phase with respect to the primary side AC voltage Vac1, power running is performed. Conversely, when the primary side AC voltage Vac1 is in a lagging phase with respect to the secondary side AC voltage Vac2, regeneration is performed.

[0038] Furthermore, there are three main patterns for the voltage difference between the primary and secondary sides: rated voltage operation, step-up operation, and step-down operation. Rated voltage operation is a state in which the converted voltages of the primary and secondary sides are roughly balanced. Step-up operation is a state in which the voltage on the secondary side is higher than the primary side. Step-down operation is a state in which the voltage on the secondary side is lower than the primary side. In the block diagram above, there are a total of six operating patterns that combine the power transmission direction and the voltage difference between the primary and secondary sides, and all of them are executable.

[0039] 4 is a graph showing an example of the relationship between the first inter-leg phase difference φlarge and loss in the DC-DC converter 1. In this specification, the loss refers to the amount of reduction in the output power relative to the power input to the DC-DC converter 1.

[0040] Before describing the operation of the DC-DC converter 1, three examples of the operation of the DC-DC converter 1 will be described. In Fig. 4, the horizontal axis is the phase difference φlarge (°) between the first legs, and the vertical axis is the loss (W). In Fig. 4, graph 401 shows the sum of the conduction loss Wcon and the switching loss Wsw, and graph 402 shows the iron loss Wfe. Also in Fig. 4, graph 403 shows the sum of the conduction loss Wcon, the switching loss Wsw, and the iron loss Wfe. All of these graphs are for the case where the output power of the DC-DC converter 1 is 20% of the rated value.

[0041] The losses in the DC-DC converter 1 include conduction loss Wcon, switching loss Wsw, and iron loss Wfe. The conduction loss Wcon is a loss due to resistance in the switching elements S1 to S8 and the transformer Tr, and increases when a large current flows through the switching elements S1 to S8 and the transformer Tr in a short time. The switching loss Wsw is a loss that occurs when the switching elements S1 to S8 are switched, and increases when a large current flows through the switching elements S1 to S8 when they are switched. The iron loss Wfe is a loss that occurs when the iron core of the transformer Tr is magnetized, and increases when current is applied to the transformer Tr for a long time. For this reason, the conduction loss Wcon and switching loss Wsw decrease as the phase difference φlarge between the first legs increases, but the iron loss Wfe increases as the phase difference φlarge between the first legs increases.

[0042] In the DC-DC converter of Patent Document 1, the phase difference φlarge between the first legs is determined so that the conduction loss Wcon is minimized. In the example shown in FIG. 4, in the DC-DC converter of Patent Document 1, the phase difference φlarge between the first legs is about 160°. In this case, the switching loss Wsw is also minimized. However, in this case, the iron loss Wfe is large, so the loss Wloss(φlarge) of the entire DC-DC converter, which is the sum of the conduction loss Wcon, the switching loss Wsw, and the iron loss Wfe, is not minimized. In other words, in the DC-DC converter of Patent Document 1, the phase difference φlarge between the first legs is determined with the main focus on suppressing the loss and heat generation in the switching elements.

[0043] As described above, in the DC-DC converter 1 of the present disclosure, the first inter-leg phase difference φlarge is determined so as to minimize the loss Wloss(φlarge) in the entire DC-DC converter 1. In the example shown in Fig. 4, in the DC-DC converter 1, the first inter-leg phase difference φlarge is about 85°.

[0044] 4, if the phase difference φlarge between the first legs is determined so as to minimize the iron loss Wfe, the phase difference φlarge between the first legs will be about 40°. However, in this case, the conduction loss Wcon and the switching loss Wsw will be large, and therefore the loss in the entire DC-DC converter 1 will not be minimized.

[0045] In the following description, an example of the DC-DC converter 1 in which φlarge is 85° is referred to as an embodiment, an example in which φlarge is 160° is referred to as a conventional example, and an example in which φlarge is 40° is referred to as an operating example.

[0046] Fig. 5 is a graph showing the relationship between AC voltage, AC power, magnetic flux density in the transformer Tr, and output power of the DC-DC converter 1. In Fig. 5, reference numeral 501 denotes a group of graphs relating to an operation example, reference numeral 502 denotes a group of graphs relating to an embodiment, and reference numeral 503 denotes a group of graphs relating to a conventional example. The graphs included in each of the groups of graphs 501 to 503 are, from top to bottom, Primary AC voltage Vac1 and secondary AC voltage Vac2 Primary AC current Iac1 and secondary AC current Iac2 Magnetic flux density in transformer Tr Output power of DC-DC converter 1 Shows.

[0047] 6 is a graph showing the waveforms of current and voltage in each of the switching elements S1, S3, S5, and S7. In FIG. 6, reference numeral 601 denotes a group of graphs relating to an operation example, reference numeral 602 denotes a group of graphs relating to an embodiment, and reference numeral 603 denotes a group of graphs relating to a conventional example. The graphs in reference numerals 601 to 603 are, from top to bottom, Voltage applied to and current flowing through switching element S1 Voltage applied to and current flowing through switching element S3 Voltage applied to and current flowing through switching element S5 Voltage applied to and current flowing through switching element S7 5. The on / off of switching elements S2, S4, S6, and S8 is in the opposite phase to the on / off of switching elements S1, S3, S5, and S7, respectively. In practice, dead time is provided in the operation of each of the switching elements S1 to S8 to prevent short circuits, but for simplicity, the dead time is omitted here. In addition, in FIG. 5, current Isw indicates an example of the current during switching.

[0048] As described above, φlarge is determined so as to satisfy the first relational expression (5) and the second relational expression (6). In this case, zero current switching (ZCS) is possible in four or more of the switching elements S1 to S8, resulting in low loss.

[0049] In the DC-DC converter 1, the larger the fluctuation range of the magnetic flux density in the transformer Tr, the larger the iron loss Wfe. As shown in Fig. 4, the conventional example (φlarge = 160°) has a larger fluctuation range of the magnetic flux density than the embodiment (φlarge = 85°), so it can be said that the iron loss Wfe is larger.

[0050] Furthermore, in the DC-DC converter 1, the larger the current flowing through the switching elements S1 to S8 and the transformer Tr, the larger the conduction loss Wcon becomes. As shown in Fig. 4, in the operation example (φlarge = 40°), the AC current is larger than in the embodiment, so that the conduction loss Wcon is larger.

[0051] Furthermore, in the DC-DC converter 1, the larger the current during switching, the larger the switching loss Wsw that occurs when cutting it off. As shown in Fig. 5, in the operation example, the current during switching is larger than in the embodiment and the conventional example, and therefore it can be said that the switching loss Wsw is larger.

[0052] (iron loss) In general, iron loss Wfe is expressed by the following equation (8). Wfe = Kh × f × B β +Ke×B 2 ×f 2 (8) Kh: Hysteresis loss coefficient β: Steinmetz constant Ke: Eddy current loss coefficient f: frequency B: Magnetic flux density In equation (8), the first term corresponds to the hysteresis loss, and the second term corresponds to the eddy current loss. Here, the maximum magnetic flux density B in equation (8) is calculated by the following equation (9).

number

[0053] (switching loss) The switching loss Wsw occurs when each of the switching elements S1 to S8 starts and ends conduction. The switching loss Wsw is derived by combining the switching losses in each of the switching elements S1 to S8, which are calculated based on the switching loss-interruption current characteristics per switching in each of the switching elements S1 to S8 and the current conditions at the start and end of conduction in each of the switching elements S1 to S8. Specifically, the switching loss Wsw can be derived by referring to the data sheets of the switching elements S1 to S8, based on the current conditions at the start and end of conduction in the switching elements S1 to S8, i.e., turn-on and turn-off, which are calculated using a current calculation formula described later.

[0054] Specifically, the data sheets for the switching elements S1 to S8 describe the switching loss [J] vs. cutoff current [A] characteristics per switching operation. The switching loss per switching operation can be calculated from the characteristics and the current conditions calculated using the current calculation formula described below, and then multiplied by the drive frequency to calculate the switching loss per unit time [W].

[0055] During ZCS operation, the switching loss Wsw is 0. For this reason, the switching loss Wsw needs to be calculated only for those switching elements S1 to S8 that are not in ZCS operation.

[0056] Fig. 7 is a diagram for explaining switching loss Wsw. In Fig. 7, the horizontal axis is time, and the vertical axis is current or voltage. In Fig. 7, reference numeral 701 denotes a current flowing through switching elements S1 to S8 at the time of switching in the operation example, and reference numeral 702 denotes a current flowing through switching elements S1 to S8 at the time of switching in the embodiment. Reference numeral 703 denotes a voltage applied to switching elements S1 to S8 at the time of switching in both the operation example and the embodiment.

[0057] In Fig. 7, the area where the current and voltage overlap indicates the switching loss Wsw. As shown in Fig. 7, the area where the graph of current in the example overlaps with the graph of voltage is smaller than the graph of current in the operational example. Therefore, it can be said that the switching loss Wsw is smaller in the example than in the operational example.

[0058] (Conduction loss) The conduction loss Wcon is derived from the total ohmic loss of each switching element when it is conducting and the copper loss of the transformer. Specifically, the conduction loss Wcon can be calculated by the following formula (10) using the effective current value Irms of the primary AC current Iac calculated from a current calculation formula described later, and the total value R of the resistance values ​​of the elements on the path through which the current flows among the switching elements S1 to S8 and the winding resistance value of the transformer Tr. Wcon = R × Irms 2 (10) In the example shown in FIG. 1, a current flows alternately through the set of switching elements S1, S4, S6, and S7 and the set of switching elements S2, S3, S5, and S8.

[0059] As will be described later, the current value is divided into I1 to I3 or I1 and I2 for each section within one period depending on the operation mode. Irms in formula (10) can be calculated by the following formula (11). When the current value is divided into I1 and I2, I3 in formula (11) is 0.

[0060]

number

[0061] In the square mode, the area of ​​the shape formed by the waveform and the straight line with the current value of zero tends to be large, making it possible to transmit electricity at a high output. On the other hand, in the triangular mode, the area of ​​the shape formed by the waveform and the straight line with the current value of zero tends to be small, making it possible to transmit electricity at a low output.

[0062] The current calculation formula differs depending on whether the DC-DC converter 1 is operating in square mode or triangular mode. The current calculation formula for each mode is explained below.

[0063] (Current calculation formula in square mode) Fig. 8 is a diagram for explaining a current calculation formula when the DC-DC converter 1 is operating in the square mode. In Fig. 8, reference numeral 801 is a graph with the horizontal axis representing time and the vertical axis representing the primary AC voltage Vac1 and the secondary AC voltage Vac2. Also, in Fig. 8, reference numeral 802 is a graph with the horizontal axis representing time and the primary AC current Iac1 and the secondary AC current Iac2. Also, in Fig. 8, φL1 indicates the period from the rise to the fall of the primary AC voltage Vac1, and φL2 indicates the period from the rise to the fall of the secondary AC voltage Vac2.

[0064] 8, a section from the rising edge of the primary AC voltage Vac1 to the rising edge of the secondary AC voltage Vac2 is defined as a first section sq1. At this time, a current I1 in the first section sq1 is expressed by the following equation (12-1). I1 = (V1 / L) × t (12-1) In equation (10-1), t is the elapsed time expressed in phase from the start time of the first interval sq1 being t=0.

[0065] 8, a section in which both the primary side AC voltage Vac1 and the secondary side AC voltage Vac2 are in a rising state is defined as a second section sq2. At this time, a current I2 in the second section sq2 is expressed by the following equation (12-2). I2=(V1 / L)×φB+((V1-V2) / L)×t (12-2) 8, a section from the falling edge of the primary side AC voltage Vac1 to the falling edge of the secondary side AC voltage Vac2 is defined as a third section sq3. At this time, a current I3 in the third section sq3 is expressed by the following equation (12-3). I3=(V1 / L)×φB+((V1-V2) / L)×(φL1-φB)-(V2 / L)×t (12-3) Moreover, the relationship between the phase difference φlarge between the first legs and the phase difference φsmall between the second legs is expressed by the following formula (13). φlarge=(Vlarge / Vsmall)×φsmall (13) V1 and V2 are compared, and the larger one is designated as Vlarge and the smaller one is designated as Vsmall. In addition, the pulse width of Vlarge is designated as φsmall, and the pulse width of Vsmall is designated as φlarge. In this case, the output power Psq is expressed by the following equation (14-1), where the time of one period is T.

[0066]

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[0069]

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[0070] (Current calculation formula in triangular mode) Fig. 9 is a diagram for explaining a current calculation formula when the DC-DC converter 1 is operating in the triangular mode. In Fig. 9, reference numeral 901 is a graph with the horizontal axis representing time and the vertical axis representing the primary AC voltage Vac1 and the secondary AC voltage Vac2. Also, in Fig. 9, reference numeral 902 is a graph with the horizontal axis representing time and the vertical axis representing the primary AC current Iac1 and the secondary AC current Iac2. Also, φL1 and φL2 in Fig. 9 are the same as φL1 and φL2 in Fig. 8.

[0071] 9, a section from the rising edge of the primary AC voltage Vac1 to the rising edge of the secondary AC voltage Vac2 is defined as a first section tri1. At this time, a current I1 in the first section tri1 is expressed by the following equation (15-1). I1 = (V1 / L) × t (15-1) 9, a section from the rising edge of the secondary AC voltage Vac2 to the falling edges of the primary AC voltage Vac1 and the secondary AC voltage Vac2 is defined as a second section tri2. In this case, a current I2 in the second section tri2 is expressed by the following equation (15-2). I2=(V1 / L)×φB+((V1-V2) / L)×t (15-2) The relationship between the phase difference φlarge between the first legs and the phase difference φsmall between the second legs is expressed by the following equation (16) in addition to the above equation (13). φB=((Vlarge-Vsmall) / Vlarge)×φlarge (14) When the DC-DC converter 1 is operating in triangular mode, φB can be rewritten with φlarge as shown in equation (16). This allows the current calculation equation when the DC-DC converter 1 is operating in triangular mode, as well as the switching loss Wsw and conduction loss Wcon, to be expressed as equations for φlarge.

[0072] When the DC-DC converter 1 is operating in the triangular mode, the power calculation formula is the following formula (17).

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[0074] Fig. 11 is a graph comparing the inter-bridge phase difference φB of the embodiment and the conventional example. In Fig. 11, the horizontal axis represents the ratio to the rated value of the output power, and the vertical axis represents the inter-bridge phase difference φB. In Fig. 11, graph 1101 represents the inter-bridge phase difference φB of the embodiment, and graph 1102 represents the inter-bridge phase difference φB of the comparative example.

[0075] Fig. 12 is a graph comparing the phase difference φlarge between the first legs of the embodiment and the conventional example. In Fig. 12, the horizontal axis represents the ratio to the rated value of the output power, and the vertical axis represents the phase difference φlarge between the first legs. In Fig. 12, graph 1101 represents the phase difference φlarge between the first legs of the embodiment, and graph 1102 represents the phase difference φlarge between the first legs of the comparative example.

[0076] 10 to 12, when the output power was set to less than 60% of the rated value, the efficiency of the example was improved compared to the efficiency of the comparative example. Furthermore, the inter-bridge phase difference φB in the example was larger than the inter-bridge phase difference φB in the comparative example, and the first inter-leg phase difference φlarge in the example was smaller than the first inter-leg phase difference φlarge in the comparative example.

[0077] From the above, it can be said that the DC-DC converter 1 of the present disclosure has improved performance compared to the DC-DC converter of Patent Document 1, particularly when operated in a state where the output power is low.

[0078] 〔summary〕 A DC-DC converter according to a first aspect of the present disclosure is a DC-DC converter including a bridge circuit including a plurality of switching elements and having two legs between a first terminal pair, a bridge circuit including a plurality of switching elements and having two legs between a second terminal pair, a conversion unit including a transformer and connected between the two bridge circuits, and a control unit that controls switching of each of the switching elements of the two bridge circuits, wherein, when the inter-terminal voltage between the first terminal pair and the second terminal pair, which is expressed as a converted voltage for the transformer, is referred to as a first voltage Vsmall and the inter-terminal voltage between the first terminal pair and the second terminal pair, which is not larger, is referred to as a second voltage Vlarge, the control unit controls the first voltage Vsmall in accordance with power or current transmitted from the first terminal pair to the second terminal pair. determining a bridge phase difference φB between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side, determining a first leg phase difference between the two legs in the bridge circuit on the first voltage side based on the ratio of the first voltage to the second voltage and the bridge phase difference, multiplying the first leg phase difference by the ratio to determine a second leg phase difference between the two legs in the bridge circuit on the second voltage side, controlling switching of the switching elements in accordance with the determined bridge phase difference, the first leg phase difference, and the second leg phase difference while fixing a duty of each of the switching elements, and setting the first leg phase difference to a value of φlarge that minimizes third relational expression (3) within a range that satisfies first relational expression (1) and second relational expression (2). φlarge≦(Vlarge / Vsmall)×(π-φB) (1) φlarge≦(Vlarge / (Vlarge-Vsmall))×φB (2) Wloss(φlarge)=Wfe+Wsw+Wcon (3) Wfe: Iron loss Wsw: Switching loss Wcon: Conduction loss In a DC-DC converter according to a second aspect of the present disclosure, in the first aspect, the iron loss Wfe in the third relational expression (3) is expressed by the following expression (4). Wfe = Kh × f × B β +Ke×B 2 ×f 2 (4) Kh: Hysteresis loss coefficient β: Steinmetz constant Ke: Eddy current loss coefficient f: frequency B: Magnetic flux density In a DC-DC converter according to aspect 3 of the present disclosure, in aspect 1 or 2, the switching loss Wsw in the third relational expression (3) is derived by combining the switching losses in each of the switching elements calculated based on the switching loss-interruption current characteristics per switching in each of the switching elements and the current conditions at the start or end of conduction in each of the switching elements.

[0079] A DC-DC converter according to a fourth aspect of the present disclosure is a DC-DC converter as described in claim 1, in which, in any one of aspects 1 to 3, the conduction loss Wcon is derived from a combined loss of ohmic losses when each of the switching elements is conductive and a copper loss of the transformer.

[0080] In a DC-DC converter according to aspect 5 of the present disclosure, in any one of aspects 1 to 4, the control unit determines the bridge-to-bridge phase difference φB by feedback control with reference to the power or current transmitted from the first terminal pair side to the second terminal pair side.

[0081] A control method according to a sixth aspect of the present disclosure is a control method for a DC-DC converter including a bridge circuit including a plurality of switching elements and having two legs between a first terminal pair, a bridge circuit including a plurality of switching elements and having two legs between a second terminal pair, and a conversion unit including a transformer and connected between the two bridge circuits, the control method controlling switching of each of the switching elements of the two bridge circuits, the control method including: when the inter-terminal voltage at the first terminal pair and the second terminal pair, expressed as a converted voltage for the transformer, is referred to as a first voltage Vsmall, and the inter-terminal voltage at the second terminal pair, which is not larger, is referred to as a second voltage Vlarge, the control method controlling the switching of the bridge circuit on the first voltage side according to power or current transmitted from the first terminal pair side to the second terminal pair side, determining a first-leg phase difference between the two legs in the bridge circuit on the first voltage side based on a ratio of the first voltage to the second voltage and the inter-bridge phase difference; and determining a second-leg phase difference between the two legs in the bridge circuit on the second voltage side by multiplying the first-leg phase difference by the ratio, wherein a duty of each of the switching elements is fixed, and switching of each of the switching elements is controlled in accordance with the determined inter-bridge phase difference, the first-leg phase difference, and the second-leg phase difference, and the first-leg phase difference is set to a value of φlarge that minimizes third relational expression (3) within a range that satisfies first relational expression (1) and second relational expression (2). φlarge≦(Vlarge / Vsmall)×(π-φB) (1) φlarge≦(Vlarge / (Vlarge-Vsmall))×φB (2) Wloss(φlarge)=Wfe+Wsw+Wcon (3) Wfe: Iron loss Wsw: Switching loss Wcon: Conduction loss The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. [Explanation of symbols]

[0082] 1 DC-DC converter 10 Primary bridge circuit 11 First leg 12 2nd Leg 13 First terminal pair 20 Secondary bridge circuit 21 Third Leg 22 Fourth Leg 23 Second terminal pair 30 Conversion unit 40 Control section S1~S8 Switching elements Tr Transformer

Claims

1. a bridge circuit including a plurality of switching elements and having two legs between a first terminal pair; a bridge circuit including a plurality of switching elements and having two legs between the second terminal pair; A conversion unit having a transformer and connected between the two bridge circuits; A control unit that controls switching of each switching element of the two bridge circuits, The control unit is When the lesser of the inter-terminal voltages at the first terminal pair and the second terminal pair, expressed as a reduced voltage for the transformer, is referred to as a first voltage Vsmall and the lesser inter-terminal voltage is referred to as a second voltage Vlarge, determining an inter-bridge phase difference φB between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side in accordance with power or current transmitted from the first terminal pair side to the second terminal pair side; determining a first inter-leg phase difference between the two legs in the bridge circuit on the first voltage side based on the ratio of the first voltage to the second voltage and the inter-bridge phase difference; 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; fixing a duty of each of the switching elements, and controlling switching of each of the switching elements in accordance with the determined inter-bridge phase difference, the first inter-leg phase difference, and the second inter-leg phase difference; the first inter-leg phase difference is set to a value of φlarge that minimizes the third relational expression (3) within a range that satisfies the first relational expression (1) and the second relational expression (2). φlarge≦(Vlarge / Vsmall)×(π−φB) (1) φlarge≦(Vlarge / (Vlarge−Vsmall))×φB (2) Wloss(φlarge)=Wfe+Wsw+Wcon (3) Wfe: Iron loss Wsw: Switching loss Wcon: Conduction loss

2. 2. The DC-DC converter according to claim 1, wherein the iron loss Wfe in the third relational expression (3) is expressed by the following expression (4): Wfe=Kh×f×B β +Ke×B ２ ×f ２ (4) Kh: Hysteresis loss coefficient β: Steinmetz constant Ke: Eddy current loss coefficient f: frequency B: magnetic flux density

3. 2. The DC-DC converter according to claim 1, wherein the switching loss Wsw in the third relational expression (3) is derived by combining the switching losses in each of the switching elements calculated based on the switching loss-interruption current characteristics per switching in each of the switching elements and the current conditions at the start or end of conduction in each of the switching elements.

4. 2. The DC-DC converter according to claim 1, wherein the conduction loss Wcon in the third relational expression (3) is derived from a total loss of ohmic losses when the switching elements are conductive and a copper loss of the transformer.

5. 2. The DC-DC converter according to claim 1, wherein the control unit determines the inter-bridge phase difference φB by feedback control with reference to power or current transmitted from the first terminal pair side to the second terminal pair side.

6. a bridge circuit including a plurality of switching elements and having two legs between a first terminal pair; a bridge circuit including a plurality of switching elements and having two legs between the second terminal pair; A control method for controlling switching of each switching element of the two bridge circuits in a DC-DC converter including a conversion unit having a transformer and connected between the two bridge circuits, comprising: When the lesser of the inter-terminal voltages at the first terminal pair and the second terminal pair, expressed as a reduced voltage for the transformer, is referred to as a first voltage Vsmall and the lesser inter-terminal voltage is referred to as a second voltage Vlarge, determining a bridge phase difference φB between the bridge circuit on the first voltage side and the bridge circuit on the second voltage side according to power or current transmitted from the first terminal pair side to the second terminal pair side; determining a first inter-leg phase difference between the two legs in the bridge circuit on the first voltage side based on a ratio of the first voltage to the second voltage and the inter-bridge phase difference; 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; fixing a duty of each of the switching elements, and controlling switching of each of the switching elements in accordance with the determined inter-bridge phase difference, the first inter-leg phase difference, and the second inter-leg phase difference; the first inter-leg phase difference is set to a value of φlarge that minimizes the third relational expression (3) within a range that satisfies the first relational expression (1) and the second relational expression (2). φlarge≦(Vlarge / Vsmall)×(π−φB) (1) φlarge≦(Vlarge / (Vlarge−Vsmall))×φB (2) Wloss(φlarge)=Wfe+Wsw+Wcon (3) Wfe: Iron loss Wsw: Switching loss Wcon: Conduction loss

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