JP2024063642A - DC-DC Converter - Google Patents

Abstract

The present invention provides a method for reducing switching losses in a DC-DC converter.
[Solution] A DC-DC converter (1) comprising a primary side bridge circuit (10) having a first leg (11) and a second leg (12), and a secondary side bridge circuit (20) having a third leg (21) and a fourth leg (22), and including a first virtual leg (Q1) and a second virtual leg (Q2), as well as a third virtual leg (Q3) and a fourth virtual leg (Q4) having a phase difference from each other, and switching between each virtual leg for controlling each leg, determines the phase difference between the primary side bridge circuit and the secondary side bridge circuit in accordance with the voltage difference between a voltage applied to the primary side bridge circuit and a primary side converted voltage of a voltage applied to the secondary side bridge circuit, and determines the phase difference between the first virtual leg and the second virtual leg and the phase difference between the third virtual leg and the fourth virtual leg in accordance with the power to be transported.
[Selection] Figure 15

Description

The present invention relates to a DC-DC converter.

Patent document 1 discloses a method for bidirectional power transmission and step-up/step-down in a dual active bridge DC-DC converter by controlling the phase difference between the bridges, the phase difference between the legs, and the duty ratio.

JP 2021-048702 A

However, in the conventional technology described above, heat loss occurs in a specific leg. This means that heat loss is concentrated in only that specific leg, making it necessary to dissipate heat in a concentrated manner in that leg, which makes thermal design difficult.

One aspect of the present invention aims to equalize the heat loss generated in a DC-DC converter within the bridge, making it easier to dissipate heat from the DC-DC converter.

In order to solve the above problem, a DC-DC converter according to one aspect of the present invention is a DC-DC converter including a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg, a secondary bridge circuit including a plurality of secondary switching elements and having a third leg and a fourth leg, a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit, and a control unit that controls 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 regarded as a first virtual leg, the second leg is regarded as a second virtual leg, the third leg is regarded as a third virtual leg, and the fourth leg is regarded as a fourth virtual leg, and a second operation in which the second leg is regarded as a first virtual leg. , and a second operation in which the first leg is regarded as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg are alternately executed, and a bridge phase difference between the primary bridge circuit and the secondary bridge circuit is determined according to a voltage difference between a first voltage, which is a voltage applied to the primary bridge circuit from outside the DC-DC converter, and a second voltage, which is a primary-converted voltage of a voltage applied to the secondary bridge circuit from outside the DC-DC converter, and a first leg phase difference between the first virtual leg and the second virtual leg and a second leg phase difference between the third virtual leg and the fourth virtual leg are controlled according to power transported from the primary side to the secondary side or from the secondary side to the primary side.

In order to solve the above problem, a DC-DC converter according to one aspect of the present invention is a DC-DC converter including a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg, a secondary bridge circuit including a plurality of secondary switching elements and having a third leg and a fourth leg, a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit, and a control unit that controls switching of the primary switching elements and the secondary switching elements, wherein the control unit refers to 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 present invention is characterized in that a first operation in which the second leg is regarded as the first virtual leg, the fourth leg as the fourth virtual leg, and a second operation in which the second leg is regarded as the first virtual leg, the first leg as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg are alternately performed, and a first inter-leg phase difference is provided between the first virtual leg and the second virtual leg, a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg, and a bridge phase difference is provided between the primary bridge circuit and the secondary bridge circuit, thereby controlling switching of each of the primary side switching elements and each of the secondary side switching elements.

According to one aspect of the present invention, heat loss in a DC-DC converter can be averaged within the bridge.

1A and 1B are a circuit diagram and a block diagram of a DC-DC converter 1 according to a reference operation example. 11 is a timing chart showing an example of operation during rated voltage operation (power running) in the low output region when a=1 for a variable a. 11 is a timing chart showing an example of operation in rated voltage operation (power running) in a low output region where 1>a>0 for a variable a. 11 is a timing chart showing an example of operation during rated voltage operation (power running) in the high output region when a=1 for a variable a. 11 is a timing chart showing an example of operation in rated voltage operation (power running) in the high output region where 1>a>0 for the variable a. 11 is a graph showing the relationship between a first inter-leg phase difference φL1 and power Pout. 11 is a timing chart showing an example of operation in rated voltage operation (regeneration) in the low output region when a=1 for the variable a. 11 is a timing chart showing an example of operation in rated voltage operation (regeneration) in the high output region when a=1 for the variable a. 11 is a timing chart showing an example of a boost operation. 4 is a timing chart showing an example of a step-down operation. 13 is another timing chart showing an example of operation in rated voltage operation (power running) in the low output region where 1>a>0 for the variable a in the reference operation example. 10 is a timing chart showing an example of operation during rated voltage operation (power running) in a low output region where 1>a>0 for a variable a in the first embodiment. 13 is another timing chart showing an example of operation during rated voltage operation (power running) in the high output region where 1>a>0 for the variable a in the reference operation example. 10 is a timing chart showing an example of operation during rated voltage operation (power running) in a high output region where 1>a>0 for a variable a in the first embodiment. 1A and 1B are a circuit diagram and a block diagram of a DC-DC converter 1 according to a first embodiment. 10 is an example of a timing chart showing a first virtual leg, a second virtual leg, and control of the first leg and the second leg by a switching signal.

[Reference operation example]
Prior to describing the first embodiment, a reference operation example of the present invention will be described in detail with reference to Figures 1 to 10. Figure 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 bridge circuit 10, a secondary bridge circuit 20, a conversion unit 30, and a control unit 40.

(Configuration of DC-DC Converter 1)
The primary bridge circuit 10 is connected to a DC power source at its input terminals. The secondary bridge circuit 20 is connected to a DC power source at its output terminals. The voltage between the input terminals of the primary bridge circuit 10, i.e., the voltage applied to the primary bridge circuit 10 from outside the DC-DC converter 1, is a primary voltage E1, and the current flowing through the input terminals of the primary bridge circuit 10 is a primary current I1. The voltage between the output terminals of the secondary bridge circuit 20, i.e., the voltage applied to the secondary bridge circuit 20 from outside the DC-DC converter 1, is a secondary voltage E2, and the current flowing through the output terminals of the secondary bridge circuit 20 is a secondary current I2. Here, the primary voltage E1, the primary current I1, the secondary voltage E2, and the secondary current I2 are time average values acquired by the control unit 40 and are used for the control described below.

The terms "input" and "output" are used here assuming that power is transmitted from the primary bridge circuit 10 to the secondary bridge circuit 20. However, this is merely a convenient expression, and the same applies below. The DC-DC converter 1 of the first embodiment is a bidirectional dual active bridge type DC-DC converter, and is also capable of transmitting power from the secondary side to the primary side. In this specification, the transmission of power Pout from the primary side to the secondary side is referred to as power running (Pout>0), and the transmission of power Pout from the secondary side to the primary side is referred to as regeneration (Pout<0).

The primary side bridge circuit 10 is a full bridge circuit having four primary side switching elements S1 to S4, to which a capacitor element C1 is connected in parallel. The primary side 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 primary side switching element S1 and a primary side switching element S2 connected in series. The second leg 12 is composed of a primary side switching element S3 and a primary side switching element S4 connected in series.

The secondary bridge circuit 20 is a full bridge circuit having four secondary switching elements S5 to S8, to which a capacitor element C2 is connected in parallel. 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 secondary switching element S5 and a secondary switching element S6 connected in series. The fourth leg 22 is composed of a secondary switching element S7 and a secondary switching element S8 connected in series.

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 with a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or other FETs (Field Effect Transistors). Alternatively, the switching elements S1 to S8 may be configured with an IGBT (Insulated Gate Bipolar Transistor) or other transistors.

The conversion unit 30 includes a transformer Tr with 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. Here, the reactor L is represented as being connected to the connection point between the primary switching element S1 and the primary 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 represented as being connected to the connection point between the primary switching element S3 and the primary switching element S4.

Here, the reactor L is described as being connected to the primary winding of the transformer Tr, but this is not limiting. The reactor L is described to include inductance that is not included in the transformer L, meaning that 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 placed on the primary side of the transformer Tr, on the secondary side, or on both sides.

The reactor L may include the 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 the connection point between the secondary switching element S5 and the secondary switching element S6 and the connection point between the secondary switching element S7 and the secondary switching element S8.

The voltage on the primary side of the conversion unit 30, i.e., the voltage from the connection point between the primary side switching element S3 and the primary side switching element S4 to the connection point between the primary side switching element S1 and the primary side switching element S2, is defined as the primary side AC voltage Vac1. Also, 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.

The voltage on the secondary side of the conversion unit 30, i.e., the voltage from the connection point between the secondary side switching element S7 and the secondary side switching element S8 to the connection point between the secondary side switching element S5 and the secondary side 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.

Here, the primary voltage E1 is also called the first voltage (= E1), and the voltage obtained by multiplying the secondary voltage E2 by the winding ratio n is also called the second voltage (= nE2). In other words, the second voltage is the primary conversion voltage of the secondary voltage E2.

(Comparative Example: DC-DC Converter According to Prior Art)
In the invention of Patent Document 1, the above-mentioned control parameters are simultaneously controlled to transmit power and step up/down. In addition, the voltage or current of each switching element is set to zero during switching to reduce loss. However, the switching elements in which loss is reduced in this way change depending on the control mode, so it is necessary to take the same level of heat countermeasures for all switching elements.

Furthermore, when the primary and secondary voltages are in voltage balance or when the secondary voltage is greater than the primary voltage (boost operation: Figs. 12 and 16 of Patent Document 1), a large output can be obtained, but there is a drawback in that losses are also large. When the voltage is in balance, Patent Document 1 achieves a reduction in switching loss by ZCS in one leg. Also, when the secondary voltage is smaller than the primary voltage (step-down operation: Figs. 6 and 15 of Patent Document 1), there is a drawback in that the output cannot be increased if the voltage difference is small. The step-down operation in Patent Document 1 utilizes the fact that the potential difference between the power supplies on both sides is applied to the inductor and energy is stored.

…（１）。 (Setting the turns ratio n)
The turns ratio n of the 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:

…(1).

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

…（２）。 The turns ratio n of the transformer Tr is set so as to satisfy the following relationship based on the maximum value E1max of the range of the primary voltage E1 and the minimum value E2min of the range of the secondary voltage E2 to which the DC-DC converter 1 of the reference operation example is applied.

…(2).

However, when the turns ratio n is greater than 1, the DC-DC converter 1 in the reference operation example is applied.

As mentioned above, the DC-DC converter 1 is a bidirectional dual active bridge DC-DC converter, so the designations primary side and secondary side are for convenience. If the relationship between the maximum value E2max of the secondary side voltage E2 and the minimum value E1min of the primary side voltage E1 satisfies E2max > E1min, the secondary side bridge circuit can be read as a primary side bridge circuit, and the primary side bridge circuit as a secondary side bridge circuit, and the DC-DC converter 1 of the reference operation example can be applied.

(Block Diagram)
The control unit 40 controls each of the switching elements S1 to S8 in accordance with the block diagram shown in Fig. 1. In the block diagram, the phase difference between each of the switching elements is determined. The block diagram shown in Fig. 1 is roughly divided into block diagram 41, block diagram 42, and block diagram 43. Note that the duty ratio of each of the switching elements S1 to S8 is fixed at, for example, 0.5. Therefore, there is no need to control the duty ratio, making control easy.

(Determination of Phase Difference φL1 Between First Legs and Phase Difference φL2 Between Second Legs)
Next, each block diagram will be described in detail. First, block diagram 41 and block diagram 42 will be described.

Block diagram 41 shows that the control unit 40 determines the phase difference between the legs in the primary bridge circuit 10 (first leg phase difference φL1) 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 positive or negative value of this value changes according to the control mode.

The control unit 40 calculates the power Pout of the DC-DC converter 1 from the secondary voltage E2 and the secondary current I2, and feeds back the deviation between the power Pout and the target power Pref_n by PI control. The control unit 40 determines the phase difference φL1 between the first legs through a limiter that limits the absolute value of the control amount calculated by PI control to an appropriate value that does not saturate.

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

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.

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

In the reference operation example, the phase difference between the first legs φL1 and the phase difference between the second legs φL2 are determined by feedback control, but this is not limiting. In other words, the control unit 40 multiplies the phase difference between the first legs φL1, which is the phase difference between the first leg 11 and the second leg 12, by the ratio of the second voltage to the first voltage to determine the phase difference between the second legs φL2, which is the phase difference between the third leg 21 and the fourth leg 22.

(Phase difference between bridges φB)
The 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 in accordance with the primary voltage E1 and the secondary voltage E2. The inter-bridge phase difference φB is a positive value when the primary bridge circuit 10 leads the secondary bridge circuit 20.

…（３）。
ここで、Ｄｉｆｆは基準電圧であり、ユーザがＤＣ－ＤＣコンバータ１の特性に応じて設定する定数である。基準電圧Ｄｉｆｆの値によって、変数ａが決定し、ブリッジ間位相差φＢが決定する。そのために、基準電圧Ｄｉｆｆの値によって、ＤＣ－ＤＣコンバータ１の応答性や制御性が決定される。変数ａは、第１電圧および第２電圧の電圧差に応じて決定される値であり、基準電圧から、第１電圧に対する第２電圧の電圧差を引いた値を基準電圧で除した値である。 The control unit 40 determines the variable a according to the following formula.

…(3).
Here, Diff is a reference voltage, and is a constant set by the user in accordance with the characteristics of the DC-DC converter 1. The value of the reference voltage Diff determines the variable a, and thus the bridge-to-bridge phase difference φB. Therefore, the value of the reference voltage Diff determines the responsiveness and controllability of the DC-DC converter 1. The variable a is a value determined in accordance with the voltage difference between the first voltage and the second voltage, and is a value obtained by subtracting the voltage difference between the second voltage and the first voltage from the reference voltage and dividing the value by the reference voltage.

Then, the control unit 40 checks whether the variable a is equal to or less than 0, and if it is, performs limit processing to set the variable a to 0. In other words, if the variable a is equal to or less than 0, the inter-bridge phase difference is set to 0. If the variable a exceeds 1, limit processing is performed to set the variable a to 1. Furthermore, the control unit 40 determines the inter-bridge phase difference φB by multiplying the variable a by the maximum inter-bridge phase difference value φB_max, which is a constant specific to the DC-DC converter 1.

In other words, the bridge phase difference φB is the phase difference between the primary bridge circuit 10 and the secondary bridge circuit 20, which is determined according to the voltage difference between the first voltage and the second voltage. The bridge phase difference φB does not depend on the output power. When the variable a is 1, the first voltage and the second voltage are equal.

(Control Mode)
The DC-DC converter 1 according to the reference operation example operates in three types of control modes. The three types of control modes are rated voltage operation, voltage step-up operation, and voltage step-down operation. Of these, the voltage step-up operation and voltage step-down operation are operation modes in which the relationship between the primary voltage E1 and the secondary voltage E2 is significantly different from the balanced state determined by the winding ratio n of the transformer Tr. In this case, the operation according to the reference operation example of the DC-DC converter 1 is limited so that the power of the voltage step-up operation becomes power running and the power of the voltage step-down operation becomes regenerative power.

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

(rated voltage operation)
The rated voltage operation is when the variable a is a positive value. In other words, it is when the voltage difference between the second voltage (=nE2) and the first voltage (=E1) is less than the reference voltage Diff. In the 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. The rated voltage operation is broadly divided into a low power region and a high power region.

Figure 2 is a timing chart showing an example of operation in rated voltage operation (powering) in the low power region when the variable a is equal to 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 rated voltage operation (powering) in the low power region when the variable a is equal to 1>a>0. As shown in Figures 2 and 3, in rated voltage operation in the low power region, the currents in the second leg 12 and the third leg 21 are zero when the second leg 12 and the third leg 21 are switched. 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.

The low output region is 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 either the bridge-to-bridge phase difference φB>the first leg phase difference φL1 or the bridge-to-bridge phase difference φB>the second leg phase difference φL2 holds.

…（４）。
上式より、１次側交流電圧Ｖａｃ１および２次側交流電圧Ｖａｃ２のパルス幅のみが電力Ｐｏｕｔに影響する。 The output power in the low output region can be expressed by the following equation:

… (4).
From the above equation, only the pulse widths of the primary AC voltage Vac1 and the secondary AC voltage Vac2 affect the power Pout.

Figure 4 is a timing chart showing an example of operation in rated voltage operation (powering) in the high power region when the variable a is 1. Figure 5 is a timing chart showing an example of operation in rated voltage operation (powering) in the high power region when the variable a is 1>a>0. As shown in Figures 4 and 5, in rated voltage operation in the high power region, the second leg 12 and the third leg 21 are capable of ZCS.

Note that in Figures 4 and 5, the state of only the odd-numbered switching elements (upper arms) from each leg is shown as a representative, but the state of the even-numbered switching elements (lower arms) is 180° out of phase with the state of the odd-numbered switching elements in the same leg. This relationship is the same in the other drawings.

The high output region is 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 the phase difference between the bridges φB < the phase difference between the first legs φL1 or the phase difference between the bridges φB < the phase difference between the second legs φL2.

…（５）。 The output power in the high output region can be expressed by the following equation:

…(5).

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 output region and the high output region are continuous, allowing seamless mode switching.

In both the low-power and high-power regions, the bridge-to-bridge phase difference φB, the first-leg phase difference φL1, and the second-leg phase difference φL2 are determined by the above-described procedure. Therefore, the bridge-to-bridge phase difference φB, the first-leg phase difference φL1, and the second-leg phase difference φL2 each change continuously, and the values do not fluctuate discontinuously. As a result, the power also changes continuously.

The above-mentioned examples of rated voltage operation are for powering (Pout>0), but similar trends are obtained for regeneration (Pout<0). Figure 7 is a timing chart showing an example of operation in rated voltage operation (regeneration) in the low output region when the variable a is 1. Figure 8 is a timing chart showing an example of operation in rated voltage operation (regeneration) in the high output region when the variable a is 1. Note that in regeneration, the phase difference φL1 between the first legs and the phase difference φL2 between the second legs are negative.

As shown in Figures 7 and 8, ZCS is possible in the second leg 12 and the third leg 21 not only in power running but also in regeneration. In other words, in rated voltage operation (when the voltage is balanced), ZCS is achieved in two legs to reduce switching loss, resulting in a higher switching loss reduction effect than the method of Patent Document 1.

(Boost operation)
The boost operation is performed when the variable a is a negative value. In other words, the voltage difference between the first voltage (E1) and the second voltage (nE2) is equal to or greater than the reference voltage Diff. In the 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.

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

Incidentally, even in boost operation, the phase difference φL1 between the first legs is first calculated, and this value is used to calculate the phase difference φL2 between the second legs. This has the effect of equalizing the time products of the AC voltages applied to the transformer Tr of the conversion unit 30. By making the time products of the AC voltages on the primary and secondary sides equal, the effect of preventing reactive current from flowing in the circuit is obtained.

As shown in FIG. 9, in boost operation, the current when the switching elements of the first leg 11, second leg 12, and third leg 21 switch is zero, so ZCS is possible. In other words, in boost operation, switching loss occurs only in the fourth leg 22. Therefore, power can be run with low loss, and the cooling mechanism can be reduced in size.

…（６）。
式（５）および式（６）を比較すると、式（６）は、式（５）においてブリッジ間位相差φＢを０とした数式であることがわかる。そのため、定格電圧動作と昇圧動作とを遷移する際に、各位相差を不連続に変動させる必要がなく、シームレスな制御が実現できる。 In this case, the output power Pout can be expressed by the following equation.

… (6).
Comparing equation (5) and equation (6), it can be seen that equation (6) is an equation in which the inter-bridge phase difference φB in equation (5) is set to 0. Therefore, when transitioning between rated voltage operation and boost operation, it is not necessary to vary each phase difference discontinuously, and seamless control can be realized.

(Step-down operation)
The step-down operation is performed when the variable a is a negative value. In other words, the voltage difference between the first voltage (=E1) and the second voltage (=nE2) is equal to or greater than the reference voltage Diff. In the 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.

Figure 10 is a timing chart showing an example of step-down operation. In step-down operation, the variable a becomes 0 due to limit processing, and the bridge-to-bridge phase difference φB also becomes 0. Therefore, in step-down operation, the power, voltage, and current are controlled by the first leg phase difference φL1 and the second leg phase difference φL2.

In addition, in the step-down operation, as in the step-up operation, no reactive current is allowed to flow within the circuit.

As shown in FIG. 10, in the step-down operation, the current when the switching elements of the first leg 11, the second leg 12, and the third leg 21 switch is zero, so ZCS is possible. In other words, in the step-down operation, switching loss occurs only in the fourth leg 22. Therefore, power can be regenerated with low loss, and the cooling mechanism can be reduced in size.

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 vary each phase difference discontinuously, and seamless control can be achieved.

(Brief Summary)
The control mode of rated voltage operation, step-up operation, and step-down operation can be changed seamlessly. At this time, since a large heat loss occurs due to ZCS not being possible only for some of the switching elements S1 to S8, it is only necessary to strengthen the cooling of only those switching elements. Therefore, it is only necessary to perform sufficient heat dissipation for only some of the switching elements S1 to S8, which makes the thermal design easier and reduces costs. In addition, since each control parameter is continuous, there is no bias magnetization of the transformer.

In addition, regardless of the type of operation in the step-up operation or step-down operation, even if the voltage difference between the first voltage and the second voltage is small, a large output can be obtained. Furthermore, losses can be reduced more than in the control method described in Patent Document 1.

[Embodiment 1]
Next, a description will be given of the operation of leveling out heat loss in the DC-DC converter 1 according to embodiment 1. The DC-DC converter 1 according to embodiment 1 has the same circuit as the DC-DC converter 1 according to the reference operation example.

First, an overview of the first embodiment will be described. In the reference operation example, ZCS was possible in the second leg 12 and the third leg 21 during rated voltage operation. In other words, heat loss occurs in the first leg 11 and the fourth leg 22 during rated voltage operation.

In contrast, in the first embodiment, the operation is performed to level (uniform) the legs in which heat loss occurs. To achieve this, the operation of each leg in the reference operation example is switched between the first leg 11 and the second leg 12 and between the third leg 21 and the fourth leg 22 for each switching cycle, thereby leveling out the heat generation of each leg and leveling out the heat loss. Leveling out the heat loss makes it easier to design the thermal design, since it is sufficient to dissipate heat uniformly from the DC-DC converter 1.

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 manner as the first leg 11, the second leg 12, the third leg 21, and the fourth leg 22 in the reference operation example, respectively. Furthermore, each switching element is controlled to alternately execute a first operation in which the first leg 11 is regarded as the first virtual leg Q1, the second leg 12 is regarded as the second virtual leg Q2, the third leg 21 is regarded as the third virtual leg Q3, and the fourth leg 22 is regarded as the fourth virtual leg Q4, and a second operation in which the second leg 12 is regarded as the first virtual leg Q1, the first leg 11 is regarded as the second virtual leg Q2, the fourth leg 22 is regarded as the third virtual leg Q3, and the third leg 21 is regarded as the fourth virtual leg Q4, for each switching period.

(Changes in timing chart during rated voltage operation)
FIG. 11 is another timing chart showing an example of operation in rated voltage operation (power running) in the low output region when 1>a>0 for the variable a in the reference operation example. FIG. 12 is a timing chart showing an example of operation in rated voltage operation (power running) in the low output region when 1>a>0 for the variable a in the first embodiment. FIG. 13 is another timing chart showing an example of operation in rated voltage operation (power running) in the high output region when 1>a>0 for the variable a in the reference operation example. FIG. 14 is a timing chart showing an example of operation in rated voltage operation (power running) in the high output region when 1>a>0 for the variable a in the first embodiment. FIG. 11 corresponds to FIG. 3, and FIG. 13 corresponds to FIG. 5, and the range shown in each timing chart is increased.

In FIG. 11, it can be seen that the primary side switching element S3 is always leading with respect to the primary side switching element S1. It can also be seen that the secondary side switching element S7 is always leading with respect to the secondary side switching element S5. In contrast, in FIG. 12, it can be seen that the primary side switching element S3 alternates between leading and lagging with respect to the primary side switching element S1 every switching period. It can also be seen that the secondary side switching element S7 alternates between leading and lagging with respect to the secondary side switching element S5 every switching period.

Also, in FIG. 13, it can be seen that the primary side switching element S3 is always leading with respect to the primary side switching element S1. Also, it can be seen that the secondary side switching element S7 is always leading with respect to the secondary side switching element S5. In contrast, in FIG. 14, it can be seen that the primary side switching element S3 alternates between leading and lagging with respect to the primary side switching element S1 every switching period. Also, it can be seen that the secondary side switching element S7 alternates between leading and lagging with respect to the secondary side switching element S5 every switching period.

In other words, in the first embodiment, 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 the first embodiment, 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 the first embodiment, the operations of the first leg 11 to the fourth leg 22 in the reference operation example are respectively referred to as the first virtual leg Q1 to the fourth virtual leg Q4, and the legs controlled by the first virtual leg Q1 to the fourth virtual leg Q4 are appropriately switched among the first leg 11 to the fourth leg 22.

As a result, in rated voltage operation, ZCS is possible in the second virtual leg Q2 and the third virtual leg Q3. That is, 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. By changing the leg in which ZCS is possible for each switching period, heat loss is leveled out. Therefore, the heat generated by each switching element because ZCS cannot be achieved is also leveled out, and it is sufficient to dissipate heat uniformly from the DC-DC converter 1.

Comparing Figures 11 and 12 with Figures 13 and 14, in the reference operation example (Figures 11 and 13), the primary AC voltage Vac1, the secondary AC voltage Vac2, the primary AC current Iac1, and the secondary AC current Iac2 each have an oscillatory waveform in which the positive and negative polarities are reversed every half cycle.

In contrast, in the first embodiment (FIGS. 12 and 14), the primary AC voltage Vac1, the secondary AC voltage Vac2, the primary AC current Iac1, and the secondary AC current Iac2 each have an oscillating waveform in which the positive and negative are reversed every cycle. However, the oscillating waveform oscillates within a range in which the positive and negative are not reversed every half cycle, and as a result, in one cycle, the waveform oscillates twice within a range in which the positive and negative are not reversed.

Note that even when switching between the first operation and the second operation, the output power Pout does not change. In other words, the secondary voltage E2 and the secondary current I2 do not change between the first embodiment and the reference operation example.

(Block Diagram)
Fig. 15 is a circuit diagram and a block diagram of the DC-DC converter 1 according to the embodiment 1. Fig. 15 according to the embodiment 1 differs from Fig. 1 according to the reference operation example in that the processes in block diagrams 44, 45, and 46 are added.

Block diagram 44 shows the logic for determining the first virtual leg Q1 to the fourth virtual leg Q4 when the duty ratio is 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.

(Switching signal SEL)
A block diagram 45 shows a process for generating a switching signal SEL. The switching signal SEL switches between a first operation and a second operation. The switching signal SEL is a signal that switches every cycle.

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 control signals for each switching element S1 to S8 are determined by a logic circuit.

Figure 16 is an example of a timing chart for controlling 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.

The timing chart for controlling the third leg 21 and the fourth leg 22 by the third virtual leg Q3, the fourth virtual leg Q4, and the switching signal SEL is obtained by replacing the first virtual leg Q1 with the third virtual leg Q3, the second virtual leg Q2 with the fourth virtual leg Q4, the control signal S1 with the control signal S5, and the control signal S3 with the control signal S7 in FIG. 16. Note that the control signals S2, S4, S6, and S8 have the inverse logic of the control signals S1, S3, S5, and S7, respectively.

(Brief Summary)
Therefore, when transmitting power in the DC-DC converter 1, the switching elements that perform ZCS are periodically replaced to level out the large heat generated by the switching elements S1 to S8 that do not achieve ZCS while maintaining the output of the DC-DC converter 1. This improves the heat dissipation efficiency of the switching elements S1 to S8 that constitute the DC-DC converter 1.

[Modifications]
(Boost operation/Step-down operation)
In the first embodiment, only the rated voltage operation has been described, but the present invention is not limited to this. That is, the voltage step-up operation and the voltage step-down operation shown in the reference operation example may be realized by the control method shown in the first embodiment.

In other words, in the step-up and step-down operations, the heat generated by the fourth leg 22 not achieving ZCS alone is equalized across the third leg 21 and the fourth leg 22, reducing the amount of heat generated by each leg. As a result, thermal design is simplified when considering the DC-DC converter 1 as a whole.

(Regarding the switching signal SEL)
In the first embodiment, the switching signal SEL is switched every one period, but this is not limiting. For example, the switching signal SEL may be switched every integer period. In addition, the temperature of each switching element may be measured, and the switching signal SEL may be controlled so that a switching element with a low temperature generates heat.

〔summary〕
In order to solve the above problems, a DC-DC converter according to a first aspect of the present invention is a DC-DC converter including: a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg; a secondary bridge circuit including a plurality of secondary switching elements and having a third leg and a fourth leg; a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit; and a control unit that controls switching of the primary switching elements and the secondary switching elements, wherein the control unit performs a first operation of regarding the first leg as a first virtual leg, the second leg as a second virtual leg, the third leg as a third virtual leg, and the fourth leg as a fourth virtual leg, a second operation in which the first leg is regarded as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg are alternately executed, and a bridge-to-bridge phase difference between the primary-side bridge circuit and the secondary-side bridge circuit is determined in accordance with a 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 a primary-side converted voltage of a voltage applied to the secondary-side bridge circuit from outside the DC-DC converter, and a first leg phase difference between the first virtual leg and the second virtual leg and a second leg phase difference between the third virtual leg and the fourth virtual leg are controlled in accordance with power transported from the primary side to the secondary side or from the secondary side to the primary side.

The above configuration makes it possible to equalize the legs in which ZCS cannot be achieved and switching losses occur within each bridge circuit. This allows the heat generated in the switching elements to be distributed throughout the DC-DC converter, making thermal design easier.

In the DC-DC converter according to aspect 2 of the present invention, in the above aspect 1, the control unit may obtain a variable by subtracting the voltage difference from a reference voltage and dividing the value by the reference voltage, and may determine the phase difference between the first leg and the phase difference between the second leg according to the variable.

With the above configuration, in rated voltage operation, rated voltage operation, boost operation, and buck operation can each be appropriately selected and controlled.

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

The above configuration allows for seamless and continuous control of the phase difference, which is a control parameter, when transitioning from rated voltage operation to step-up operation or step-down operation, or vice versa. Therefore, because each control parameter is continuous, the transformer does not become biased.

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

The above configuration allows the phase difference between the bridges to be kept within the range that can be controlled by the DC-DC converter, achieving stable control.

In a DC-DC converter according to aspect 5 of the present invention, in any one of aspects 1 to 4 above, the control unit may determine the phase difference between the second legs by multiplying the phase difference between the first legs by the ratio of the second voltage to the first voltage.

The above configuration allows for seamless and continuous control of the phase difference, which is a control parameter, for output power during rated voltage operation, step-up operation, and step-down operation. As a result, the control parameters are continuous, and the transformer does not become biased.

In order to solve the above problems, the DC-DC converter according to aspect 6 of the present invention is a DC-DC converter including a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg, a secondary bridge circuit including a plurality of secondary switching elements and having a third leg and a fourth leg, a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit, and a control unit that controls switching of the primary switching elements and the secondary switching elements, wherein the control unit refers to 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 present invention is characterized in that a first operation in which the second leg is regarded as the first virtual leg, the fourth leg as the fourth virtual leg, and a second operation in which the second leg is regarded as the first virtual leg, the first leg as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg are alternately executed, and a first inter-leg phase difference is provided between the first virtual leg and the second virtual leg, a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg, and a bridge phase difference is provided between the primary bridge circuit and the secondary bridge circuit, thereby controlling switching of each of the primary side switching elements and each of the secondary side switching elements.

The above configuration allows the roles of each leg to be equalized within the bridge, which in turn allows for equalized heat loss.

The DC-DC converter according to aspect 7 of the present invention is any one of aspects 1 to 5 above, in which the turns ratio of the transformer may be the ratio between the maximum value in the range of the voltage of the primary bridge circuit and the minimum value in the range of the voltage of the secondary bridge circuit.

The DC-DC converter according to aspect 8 of the present invention is any one of aspects 1 to 5 above, in which the maximum value of the first voltage range and the minimum value of the second voltage range may be equal.

The above configuration allows seamless and continuous control of the inter-bridge phase difference even when the first voltage and the second voltage are different.

The DC-DC converter according to aspect 9 of the present invention is any one of aspects 1 to 8 above, in which the duty ratios of the first virtual leg, the second virtual leg, the third virtual leg, and the fourth virtual leg may be 0.5.

With the above configuration, there is no need to control the duty ratio, making it easy to control the DC-DC converter.

The DC-DC converter according to aspect 10 of the present invention may be any of the above aspects 1 to 9, in which the first operation and the second operation are switched every cycle.

With the above configuration, the leg to be ZCSed can be switched every cycle.

The DC-DC converter according to aspect 11 of the present invention may be configured in any one of aspects 1 to 9 above to switch between the first operation and the second operation every N periods (N is a natural number equal to or greater than 2).

With the above configuration, the leg to be ZCSed can be switched at a period N times the cycle.

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

REFERENCE SIGNS LIST 1 DC-DC converter 10 Primary side bridge circuit 11 First leg 12 Second leg 20 Secondary side bridge circuit 21 Third leg 22 Fourth leg 30 Conversion section 40 Control section S1 to 4 Primary side switching elements S5 to 8 Secondary side switching elements

Claims (11)

a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg;
a secondary bridge circuit including a plurality of secondary switching elements and having a third leg and a fourth leg;
a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit;
A control unit that controls switching of the primary side switching element and the secondary side switching element,
The control unit is
a first operation of regarding the first leg as a first virtual leg, the second leg as a second virtual leg, the third leg as a third virtual leg, and the fourth leg as a fourth virtual leg;
a second operation of regarding the second leg as 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, alternately; and
a first virtual leg and a second virtual leg, and a second virtual leg, respectively, in response to a voltage difference between a first voltage, which is a voltage applied to the primary bridge circuit from outside the DC-DC converter, and a second voltage, which is a primary-side converted voltage of a voltage applied to the secondary bridge circuit from outside the DC-DC converter; and a first virtual leg and a second virtual leg and a second virtual leg and a fourth virtual leg, respectively, in response to power transported from the primary side to the secondary side or from the secondary side to the primary side. the control unit determines a variable by dividing a value obtained by subtracting the voltage difference from a reference voltage by the reference voltage;
2. The DC-DC converter according to claim 1, wherein the first inter-leg phase difference and the second inter-leg phase difference are determined in response to the variable. The DC-DC converter according to claim 2, wherein the control unit sets the inter-bridge phase difference to 0 when the variable is equal to or less than 0. 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 value when the variable is positive. The DC-DC converter of claim 1, wherein the control unit determines the phase difference between the second legs by multiplying the phase difference between the first legs by the ratio of the second voltage to the first voltage. a primary bridge circuit including a plurality of primary switching elements and having a first leg and a second leg;
a secondary bridge circuit including a plurality of secondary switching elements and having a third leg and a fourth leg;
a conversion unit having a transformer and connected between the primary bridge circuit and the secondary bridge circuit;
A control unit that controls switching of the primary side switching element and the secondary side switching element,
The control unit is
a first operation of regarding the first leg as a first virtual leg, the second leg as a second virtual leg, the third leg as a third virtual leg, and the fourth leg as a fourth virtual leg;
a second operation of regarding the second leg as 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, alternately executing the second operation of regarding the second leg as the first virtual leg, the first leg as the second virtual leg, the fourth leg as the third virtual leg, and the third leg as the fourth virtual leg;
providing a first inter-leg phase difference between the first virtual leg and the second virtual leg;
a second inter-leg phase difference is provided between the third virtual leg and the fourth virtual leg;
a bridge-to-bridge phase difference is provided between the primary bridge circuit and the secondary bridge circuit, and switching of each of the primary side switching elements and each of the secondary side switching elements is controlled. A DC-DC converter according to any one of claims 1 to 5, wherein the winding ratio of the transformer is the ratio between the maximum value of the range of voltages of the primary bridge circuit and the minimum value of the range of voltages of the secondary bridge circuit. A DC-DC converter as claimed in any one of claims 1 to 5, in which the maximum value of the first voltage range is equal to the minimum value of the second voltage range. A DC-DC converter according to any one of claims 1 to 6, wherein the duty ratios of the first virtual leg, the second virtual leg, the third virtual leg, and the fourth virtual leg are 0.5. A DC-DC converter as described in any one of claims 1 to 6, characterized in that the first operation and the second operation are switched every cycle. 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 (N is a natural number equal to or greater than 2).

Images