Power conversion device

Abstract

This power conversion device is a power conversion device that converts AC power into DC power and outputs the DC power. An output voltage is variable. The power conversion device includes: an ACDC conversion unit; a DCDC conversion unit; and a control unit. The DCDC conversion unit includes: a transformer; a primary-side bridge circuit which is connected to a primary-side coil of the transformer via the ACDC conversion unit and a first inductor and constitutes a first full bridge by a plurality of first switching elements; and a secondary-side bridge circuit which is connected to a secondary-side coil of the transformer via a second inductor and constitutes a second full bridge by a plurality of second switching elements. The control unit transmits power from the primary side to the secondary side by controlling a phase difference between on / off switching of the first switching element and on / off switching of the second switching element. A first DC voltage is variable by control by the control unit.

Description

Power Conversion Device

[0001] The present disclosure relates to a power conversion device.

[0002] Techniques relating to an isolated DC-DC converter of a DAB (Dual Active Bridge) system using two bridge circuits have been disclosed (see, for example, Patent Documents 1 and 2).

[0003] JP 2015-27196 A JP 2016-152687 A

[0004] A power conversion device according to the present disclosure converts AC power to DC power and outputs the converted DC power. The output voltage is variable. The power conversion device includes an A / C converter that converts an input AC voltage into a first DC voltage, a D / C converter that converts the first DC voltage converted by the A / C converter into a second DC voltage and outputs the second DC voltage, and a control unit that controls the operation of the A / C converter and the D / C converter. The D / C converter includes a transformer, a primary-side bridge circuit connected to a primary coil of the transformer via the A / C converter and a first inductor and forming a first full bridge with multiple first switching elements, and a secondary-side bridge circuit connected to a secondary coil of the transformer via a second inductor and forming a second full bridge with multiple second switching elements. The control unit transfers power from the primary side to the secondary side by controlling the phase difference between the on / off switching of the first switching elements and the on / off switching of the second switching elements. The first DC voltage is variable under control of the control unit.

[0005] FIG. 1 is a schematic diagram illustrating the configuration of a power conversion device according to a first embodiment of the present disclosure. FIG. 2 is a diagram illustrating an overview of the internal configuration of a DCDC conversion unit. FIG. 3 is a graph illustrating the displacement of a first DC voltage and a second DC voltage and the control of the on / off switching of each switching element. FIG. 4 is a graph illustrating the relationship between the output voltage VHDC and the output power when Vbus is fixed at 400 V. FIG. 5 is a graph illustrating the relationship between the output voltage VHDC and the output power when Vbus is variably controlled between 300 V and 500 V. FIG. 6 is a graph illustrating the relationship between Vbus and VHDC′.

[0006] [Problem to be Solved by the Present Disclosure] In a power conversion device that converts AC (alternating current) power to DC (direct current) power, the output voltage may be variable. In the power conversion device, it is required to improve the conversion efficiency when converting power, particularly in a DCDC conversion unit that converts the output voltage.

[0007] Therefore, one of the objects is to provide a power conversion device that can improve conversion efficiency.

[0008] [Effects of the Present Disclosure] According to the power conversion device, it is possible to improve conversion efficiency.

[0009] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described. A power conversion device according to the present disclosure (1) is a power conversion device that converts AC power into DC power and outputs the DC power. The output voltage is variable. The power conversion device includes an A / C converter that converts an input AC voltage into a first DC voltage, a D / C converter that converts the first DC voltage converted by the A / C converter into a second DC voltage and outputs the second DC voltage, and a control unit that controls the operation of the A / C converter and the D / C converter. The D / C converter includes a transformer, a primary-side bridge circuit connected to the primary coil of the transformer via the A / C converter and a first inductor and forming a first full bridge with multiple first switching elements, and a secondary-side bridge circuit connected to the secondary coil of the transformer via a second inductor and forming a second full bridge with multiple second switching elements. The control unit transfers power from the primary side to the secondary side by controlling the phase difference between the on / off switching of the first switching elements and the on / off switching of the second switching elements. The first DC voltage is variable under control by the control unit.

[0010] The power conversion device according to the present disclosure includes a DC-DC conversion unit including a transformer, a primary-side bridge circuit, and a secondary-side bridge circuit. Therefore, by adopting the DAB method, soft switching can be achieved under the control of a control unit. Here, if hard switching rather than soft switching accounts for a large proportion of on / off switching of switching elements, losses during conversion increase. According to the power conversion device, the first DC voltage, which is an intermediate voltage, is variable under the control of the control unit, so that the first DC voltage can be changed according to the second DC voltage, which is the output voltage. This allows for a larger proportion of soft switching compared to when the first DC voltage is a constant voltage. Therefore, a decrease in losses during conversion can be suppressed, improving conversion efficiency.

[0011] (2) In the above (1), if the first DC voltage is Vbus and the value of the second DC voltage converted to the primary side is VHDC', Vbus may be controlled to approach VHDC'. By doing so, it is possible to further suppress the decrease in loss during conversion and further improve the conversion efficiency.

[0012] (3) In the above (1) or (2), the control unit may control Vbus so that Vbus is equal to VHDC', where Vbus is the first DC voltage and VHDC' is the value obtained by converting the second DC voltage to the primary side. This can further suppress the decrease in loss during conversion and further improve the conversion efficiency.

[0013] (4) In any of the above (1) to (3), the control unit may control the first DC voltage to be a constant value under predetermined conditions. This allows control to be performed separately for when the first DC voltage is a constant value and when the first DC voltage is variable, thereby enabling more efficient control.

[0014] (5) In any one of (1) to (4) above, the power conversion device may further include a capacitor connected between the ACDC conversion unit and the DCDC conversion unit. The upper limit value of the first DC voltage may be determined based on the withstand voltage of the capacitor. This prevents damage to the capacitor and ensures stable operation of the power conversion device.

[0015] (6) In any of the above (1) to (5), the lower limit of the first DC voltage may be determined based on the AC voltage. This allows the lower limit to be set more appropriately, ensuring stable operation of the power conversion device.

[0016] [Details of the embodiment of the present disclosure] Next, an embodiment of the power conversion device of the present disclosure will be described with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and the description thereof will not be repeated.

[0017] (Embodiment 1) The configuration of a power conversion device according to embodiment 1 of the present disclosure will be described. FIG. 1 is a schematic diagram illustrating the configuration of a power conversion device according to embodiment 1 of the present disclosure. Referring to FIG. 1, a power conversion device 11a according to embodiment 1 is used in a power conversion system 10a that converts AC power 14a input from a home 12a or a power grid into DC power 15a and outputs the DC power to, for example, an electric vehicle 13a. The output voltage is variable. For example, the power conversion device 11a converts an input of 200V AC into a DC voltage of 50V to 450V DC and outputs the DC power. With this configuration, when charging various electric vehicles with different charging voltages, the output voltage can be changed according to the charging voltage of the electric vehicle.

[0018] The power conversion device 11a includes an ACDC conversion unit 21a, a DCDC conversion unit 22a, a capacitor 23a, and a control unit 24a. The capacitor 23a is connected between the ACDC conversion unit 21a and the DCDC conversion unit 22a. The ACDC conversion unit 21a converts the input AC voltage into a first DC voltage, which is an intermediate voltage. The first DC voltage is the potential of the capacitor 23a, called Vbus. The DCDC conversion unit 22a converts the first DC voltage converted by the ACDC conversion unit 21a into a second DC voltage and outputs it. The second DC voltage is an output voltage called VHDC. The second DC voltage, which is the output voltage, is variable. In this embodiment, it is variable between 50V and 450V. The value of the second DC voltage, which is the output voltage, is a value that changes depending on, for example, the specifications of the electric vehicle to be charged. The control unit 24a controls the operation of the ACDC conversion unit 21a and the DCDC conversion unit 22a. The first DC voltage is variable under the control of the control unit 24. The control unit 24 can change the first DC voltage depending on the second DC voltage, which is the output voltage. In this embodiment, the first DC voltage is variable between 300 V and 500 V. Control of the operation of the DCDC conversion unit 22a in the control unit 24a will be described later.

[0019] FIG. 2 is a diagram showing an outline of the internal configuration of the DCDC conversion unit 22a. Also referring to FIG. 2, the DCDC conversion unit 22a includes a transformer 25a, a primary-side bridge circuit 26a, a secondary-side bridge circuit 27a, a first inductor 28a, and a second inductor 29a. In this embodiment, the transformer 25a is an isolation transformer. The transformer 25a includes a primary-side coil 31a and a secondary-side coil 32a. Note that the first inductor 25a may not be an independent component, and the leakage inductance of the transformer 25a may be utilized.

[0020] The primary bridge circuit 26a is connected to the primary coil 31a of the transformer 25a via the AC-DC converter 21a and a first inductor 28a. The primary bridge circuit 26a configures a first full bridge with a plurality of first switching elements. In this embodiment, the primary bridge circuit 26a includes four first switching elements 41a (Q １), first switching element 42a (Q ２ ), the first switching element 43a (Q ３ ), the first switching element 44a (Q ４ Each of the first switching elements 41a, 42a, 43a, and 44a includes a body diode. The primary-side bridge circuit 26a also includes a capacitor 35a connected to the first input terminal 33a and the second input terminal 34a. The voltage of the capacitor 35a serves as the intermediate voltage Vbus.

[0021] The secondary bridge circuit 27a is connected to the secondary coil 32a of the transformer 25a via a second inductor 29a. The secondary bridge circuit 27a configures a second full bridge with a plurality of second switching elements. In this embodiment, the secondary bridge circuit 27a includes four second switching elements 51a (Q ５ ), second switching element 52a (Q ６ ), second switching element 53a (Q ７ ), second switching element 54a (Q ８ Each of the second switching elements 51 a, 52 a, 53 a, and 54 a includes a body diode. The secondary-side bridge circuit 27 a also includes a capacitor 38 a, a capacitor 39 a, and a resistor 40 a connected to the first output terminal 36 a and the second output terminal 37 a.

[0022] In the present embodiment, the first switching element 41a, the first switching element 42a, the first switching element 43a, the first switching element 44a, the second switching element 51a, the second switching element 52a, the second switching element 53a, and the second switching element 54a are, for example, metal-oxide-semiconductor field effect transistors (MOSFETs). Note that the first switching element 41a, the first switching element 42a, the first switching element 43a, the first switching element 44a, the second switching element 51a, the second switching element 52a, the second switching element 53a, and the second switching element 54a may also be insulated gate bipolar transistors (IGBTs).

[0023] The control unit 24a controls the operation of the DCDC conversion unit 22a. Specifically, the control unit 24a controls the phase difference between the on / off switching of the first switching element 41a, the first switching element 42a, the first switching element 43a, and the first switching element 44a and the on / off switching of the second switching element 51a, the second switching element 52a, the second switching element 53a, and the second switching element 54a, thereby transmitting power from the primary side to the secondary side.

[0024] 3 is a graph showing the displacement of the first DC voltage and the second DC voltage and the control of the on / off switching of each switching element. In FIG. 3, the vertical axis represents the displacement of the first DC voltage and the second DC voltage and the on / off state, and the horizontal axis represents the elapsed time. FIG. 3 also shows the displacement of the current flowing through the first inductor 28a. The line 61a represents the voltage V １ The transition of V １ represents the voltage at the midpoint of the primary bridge circuit 26a, as shown in FIG. ２ The transition of V ２ represents the voltage at the midpoint of the secondary bridge circuit 27a, as shown in FIG. １1 shows a transition of the current (current flowing through the first inductor 28a). A line 64a shows a transition of the on / off switching of the first switching elements 41a and 44a. A line 65a shows a transition of the on / off switching of the first switching elements 42a and 43a. A line 66a shows a transition of the on / off switching of the second switching elements 51a and 54a. A line 67a shows a transition of the on / off switching of the second switching elements 52a and 53a.

[0025] 3, the capacitor 35a operates in a state where the first DC voltage (Vbus) is always applied. Under the control of the control unit 24, the first switching element 41a and the first switching element 44a are simultaneously turned on in the DCDC conversion unit 22a. Then, V １ A positive Vbus is applied to the VHDC terminal. After a time Tp has elapsed, the second switching elements 51a and 54a are turned on. As a result, VHDC is output as the output voltage. After a further time Tx has elapsed, the first switching elements 41a and 44a are turned off at the same time, and then the first switching elements 42a and 43a are turned on at the same time. At this time, V １ A negative Vbus voltage is applied to the second switching element 51a. After a time Tp has elapsed, the second switching element 51a and the second switching element 54a are simultaneously turned off, and then the second switching element 52a and the second switching element 53a are simultaneously turned on. In the above control, the phase difference (Tp) is increased or decreased to switch on and off each switching element, thereby transmitting power.

[0026] Here, switching of switching elements can be classified into hard switching and soft switching. In order to improve conversion efficiency, it is preferable to avoid hard switching, which switches while voltage remains, as much as possible and to achieve soft switching as much as possible. In switching elements, this tendency, i.e., the tendency to become hard switching, is particularly pronounced in the low output range.

[0027] Let P be the output power, and let N be the ratio of the number of turns on the input side (primary coil) to the output side (secondary coil). １ :N ２ and the total capacitance of the inductance converted into the input side is L, the output power P is expressed by the following equation (1).

[0028]

[0029] and the current I １ The gradient of is expressed by the following equation (2).

[0030]

[0031] The soft switching conditions can be realized when both the following formula (3) and formula (4) are satisfied. In the formulas, V'HDC, Tp, and Vbus are variables. Note that symbols such as Lsum in the formulas are represented by the following formulas (5), (6), and (7).

[0032]

[0033]

[0034]

[0035]

[0036]

[0037] That is, when both the formula (3) and the formula (4) are satisfied, soft switching is realized.

[0038] Here, when Vbus is a constant value and the output voltage is variable, the proportions of hard switching and soft switching are as shown in Figure 4. Figure 4 is a graph showing the relationship between the output voltage VHDC and the output power when Vbus is fixed at 400 V. In Figure 4, the horizontal axis represents VHDC (V) and the vertical axis represents output power (W). The same applies to the relationship between the horizontal and vertical axes in Figure 5 below.

[0039] 4, the region surrounded by line 71a is the region where hard switching occurs because equation (3) above is not satisfied, and the region surrounded by line 72a is the region where hard switching occurs because equation (4) above is not satisfied. Furthermore, the region where desired output voltage is variable is the region surrounded by line 73a (50 V to 450 V). As can be seen from FIG. 4, when such control is performed, i.e., when Vbus is controlled to a constant value of 400 V (when the first DC voltage is fixed and controlled to a constant value), the region where hard switching occurs occupies a large proportion of the desired output region, and this tendency is particularly noticeable on the low output side.

[0040] In contrast, in the power conversion device 11 a according to the first embodiment of the present disclosure, the first DC voltage is variable under the control of the control unit 24 a. Fig. 5 is a graph showing the relationship between the output voltage VHDC and the output power when Vbus is variably controlled between 300 V and 500 V.

[0041] Referring to Figure 5, the region surrounded by line 74a is the region where hard switching occurs because the above formula (3) is not satisfied. The region where hard switching occurs because the above formula (4) is not satisfied is not shown in Figure 5. Furthermore, the region where output voltage is desired when the output voltage is variable is the region surrounded by line 75a (50 V to 450 V). As can be seen from Figure 5, the proportion of the region where hard switching occurs within the desired output region is smaller than in the case shown in Figure 4, and there is a region where hard switching can be avoided even on the low output side.

[0042] In the power conversion device 11a, the intermediate voltage Vbus (first DC voltage) is variable under the control of the control unit 24a, so the first DC voltage can be changed according to the output voltage VHDC (second DC voltage). This allows for a higher proportion of soft switching compared to when the first DC voltage is a constant voltage. This reduces loss during conversion and improves conversion efficiency.

[0043] In this embodiment, the control range of the variable Vbus is as follows. FIG. 6 is a graph showing the relationship between Vbus and VHDC'. In FIG. 6, the horizontal axis represents VHDC' (V) and the vertical axis represents Vbus (V). VHDC' is a value obtained by converting the second DC voltage into the first DC voltage. The ratio of the number of turns on the input side (primary coil) of the transformer 25a to the number of turns on the output side (secondary coil) is set to N. １ :N ２ In this case, VHDC′=VHDC×N １ / N ２ It is calculated as follows.

[0044] 6, when VHDC' is 300V or less, Vbus is set to 300V, and when VHDC' is 500V or more, Vbus is set to 500V. When VHDC' is between 300V and 500V, VHDC' and Vbus are controlled to be the same value. The upper limit of Vbus is determined by the withstand voltage of capacitor 23a, which is 500V in this case. The lower limit of Vbus is determined by the system voltage, in this case, √2 times the input AC voltage (root double), which is 300V in this case.

[0045] In this embodiment, the upper limit of the first DC voltage is determined according to the withstand voltage of the capacitor, thereby preventing damage to the capacitor and ensuring stable operation of the power conversion device.

[0046] In this embodiment, the lower limit of the first DC voltage is determined in accordance with the AC voltage, so that the lower limit can be set more appropriately to ensure stable operation of the power conversion device.

[0047] In the above-described embodiment, the control unit may control Vbus to approach VHDC', where Vbus is the first DC voltage and VHDC' is the value obtained by converting the second DC voltage to the primary side. This can further reduce the decrease in loss during conversion and improve the conversion efficiency.

[0048] In the above-described embodiment, the control unit may control Vbus so that Vbus is equal to VHDC', where Vbus is the first DC voltage and VHDC' is the value obtained by converting the second DC voltage to the primary side. This can further reduce the decrease in loss during conversion and further improve the conversion efficiency.

[0049] In the above embodiment, the control unit may control the first DC voltage to be a constant value under predetermined conditions, thereby enabling control to be performed separately between a case where the first DC voltage is a constant value and a case where the first DC voltage is variable, thereby enabling more efficient control.

[0050] It should be understood that the embodiments disclosed herein are illustrative in all respects and are not limiting in any respect. The scope of the present disclosure is defined not by the above description but by the scope of the claims, and it is intended to include all modifications within the meaning and scope of the claims.

[0051] REFERENCE SIGNS LIST 10a Power conversion system 11a Power conversion device 12a Home 13a Electric vehicle 14a AC power 15a DC power 21a AC-DC conversion unit 22a DCDC conversion unit 23a, 35a, 38a, 39a Capacitor 24a Control unit 25a Transformer 26a Primary bridge circuit 27a Secondary bridge circuit 28a First inductor 29a Second inductor 31a Primary coil 32a Secondary coil 33a First input terminal 34a Second input terminal 36a First output terminal 37a Second output terminal 40a Resistor 41a, 42a, 43a, 44a First switching element 51a, 52a, 53a, 54a Second switching element

Claims

1. A power conversion device that converts AC power to DC power and outputs it, wherein the output voltage is variable, the power conversion device comprising: an ACDC conversion unit that converts an input AC voltage into a first DC voltage; a DCDC conversion unit that converts the first DC voltage converted by the ACDC conversion unit into a second DC voltage and outputs the second DC voltage; and a control unit that controls the operation of the ACDC conversion unit and the operation of the DCDC conversion unit, wherein the DCDC conversion unit includes: a transformer; a primary side bridge circuit that is connected to a primary coil of the transformer via the ACDC conversion unit and a first inductor and forms a first full bridge with a plurality of first switching elements; and a secondary side bridge circuit that is connected to a secondary coil of the transformer via a second inductor and forms a second full bridge with a plurality of second switching elements, the control unit transfers power from the primary side to the secondary side by controlling the phase difference between the on / off switching of the first switching element and the on / off switching of the second switching element, and the first DC voltage is variable under the control of the control unit.

2. The power conversion device according to claim 1, wherein the control unit controls Vbus to approach VHDC', where Vbus is the first DC voltage and VHDC' is a value converted to the primary side of the second DC voltage.

3. The power conversion device according to claim 1 or 2, wherein the control unit controls Vbus to be the same as VHDC', where Vbus is the first DC voltage and VHDC' is a value converted to the primary side of the second DC voltage.

4. The power conversion device according to claim 1 or 2, wherein the control unit controls the first DC voltage to be a constant value under predetermined conditions.

5. The power conversion device according to claim 1 or 2, further comprising a capacitor connected between the ACDC conversion unit and the DCDC conversion unit, wherein the upper limit value of the first DC voltage is determined according to a withstand voltage of the capacitor.

6. The power conversion device according to claim 1 or 2, wherein the lower limit of the first DC voltage is determined according to the AC voltage.

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