Power conversion device

Abstract

To provide a power conversion device that realizes appropriate connection switching of an input or an output of a power converter.SOLUTION: A power conversion device 100 includes a control circuit 101. When an output of a first DC-DC converter 100A and a second DC-DC converter 100B are switched from a parallel connection to a serial connection, the control circuit 101 causes at least one insulated DC-DC converter of the first DC-DC converter 100A and the second DC-DC converter 100B to continue operation, and after cutting off an output of the stopped insulated DC-DC converter using switches S1 to S3, the control circuit causes the stopped insulated DC-DC converter to regenerate stored energy of an output-DC capacitor Co to an input-side. When a voltage between terminals of the output-DC capacitor Co of the stopped insulated DC-DC converter becomes a predetermined value or smaller, the control circuit causes an output of the stopped insulated DC-DC converter to connect by a serial connection and resumes operation of the stopped insulated DC-DC converter.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a power conversion device. [Background technology]

[0002] In recent years, power converters that convert AC to DC or DC to AC have become widely used, and these types of power converters are also used in high voltage applications. Power converters that charge and discharge batteries installed in backup power supplies and electric vehicles (EVs) must be able to charge and discharge appropriately across a wide range of battery voltages. In recent years, there has been a trend toward increasing the number of battery cells connected in series and increasing the battery cell voltage, enabling high-power output while suppressing increases in current. Therefore, power converters connected to batteries must be able to handle higher voltages than ever before. However, high-voltage power devices are generally expensive, and simply increasing the withstand voltage increases the cost of the power converter.

[0003] Patent Document 1 describes a power conversion device that can handle high voltages using inexpensive, low-voltage power devices by switching the input or output connection of two power converters between series and parallel. The power conversion device described in Patent Document 1 has three switches on the output side of two isolated DC-DC converters, and by switching these switches, the outputs of the two isolated DC-DC converters can be connected in series or in parallel. For example, if a higher output voltage is required, the outputs can be connected in series, which can increase the output voltage by approximately two times. On the other hand, if a low output voltage but a high output current is required, the outputs can be connected in parallel. This can increase the output current by approximately two times. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-170903 Summary of the Invention [Problem to be solved by the invention]

[0005] The device described in Patent Document 1 can switch between input and output connections by controlling the on / off states of three switches. However, there is no detailed description of the switching procedure for switching the connection during the operation of the power converter. For example, if a battery is connected to the output of a DC-DC converter and the output is switched from a parallel connection to a series connection, simply flipping the switch can suddenly apply approximately twice the voltage to the battery, which could cause an inrush current. Meanwhile, some EV charger specifications include a function that issues an error and shuts down the system if the charging power drops to zero for a certain period of time. For this reason, it is desirable to be able to switch between parallel and series connections while the power converter is operating.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a power conversion device that realizes appropriate connection switching of the input or output of a power converter. [Means for solving the problem]

[0007] In order to solve the above problems, the power conversion device of the present invention is a power conversion device comprising: a plurality of isolated DC-DC converters whose inputs are connected in parallel and each having a DC capacitor at its output; a changeover switch that switches between connecting the outputs of the plurality of isolated DC-DC converters in parallel or in series, or cutting off the outputs; and a control unit that controls the isolated DC-DC converters and the changeover switch, wherein when switching the outputs of the plurality of isolated DC-DC converters from parallel connection to series connection, the control unit controls at least one of the plurality of isolated DC-DC converters. the output of the stopped isolated DC-DC converter is cut off using the changeover switch, the stored energy of the DC capacitor of the stopped isolated DC-DC converter is regenerated in the isolated DC-DC converter that continues to operate, and when the voltage between the terminals of the DC capacitor of the stopped isolated DC-DC converter falls to a predetermined value or less, the output of the stopped isolated DC-DC converter is connected in series using the changeover switch, and the operation of the stopped isolated DC-DC converter is resumed. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a power conversion device that realizes appropriate connection switching of the input or output of a power converter. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram showing a circuit configuration of a power conversion device according to a first embodiment of the present invention. [Figure 2A] 1 is a conceptual diagram showing the relationship between the output connection state of the first DC-DC converter and the second DC-DC converter and the output voltage Vo when the power conversion device according to the first embodiment of the present invention has one threshold voltage. FIG. [Figure 2B]3 is a conceptual diagram showing the relationship between the output connection state of the first DC-DC converter and the second DC-DC converter and the output voltage Vo when the power conversion device according to the first embodiment of the present invention has two threshold voltages. FIG. [Figure 3] 1 is a graph showing the operation of the power conversion device according to the first embodiment of the present invention from parallel to series connection, with time taken on the horizontal axis. [Figure 4A] 4 is a diagram showing current paths in the power conversion device in each operation mode during the period from time t0 to t1 in FIG. 3. FIG. [Figure 4B] 4 is a diagram showing current paths in the power conversion device in each operation mode during the period from time t1 to time t2 in FIG. 3. FIG. [Figure 4C] 4 is a diagram showing current paths in the power conversion device in each operation mode during the period from time t2 to time t3 in FIG. 3. FIG. [Figure 4D] 4 is a diagram showing current paths in the power conversion device in each operation mode during the period from time t3 to time t4 in FIG. 3. FIG. [Figure 4E] 4 is a diagram showing current paths in the power conversion device in each operation mode during the period from time t4 to time t5 in FIG. 3. FIG. [Figure 4F] 4 is a diagram showing current paths in the power conversion device in each operation mode during the period from time t5 to time t6 in FIG. 3. FIG. [Figure 4G] 4 is a diagram showing current paths in the power conversion device in each operation mode during the period from time t6 to time t7 in FIG. 3. FIG. [Figure 4H] 4A to 4C are diagrams showing current paths in the power conversion device in each operation mode after time t7 in FIG. [Figure 5] 3 is a flowchart showing the <parallel to series connection> switching control of the power conversion device according to the first embodiment of the present invention. [Figure 6] 10 is a graph showing the operation of a power conversion device according to a second embodiment of the present invention from series to parallel connection, with time plotted on the horizontal axis. [Figure 7A] 7A and 7B are diagrams showing current paths in the power conversion device in each operation mode during the period from time t0 to t1 in FIG. 6. [Figure 7B]7 is a diagram showing current paths in the power conversion device in each operation mode during the period from time t1 to time t2 in FIG. 6. FIG. [Figure 7C] 7A to 7C are diagrams showing current paths in the power conversion device in each operation mode during the period from time t2 to time t3 in FIG. 6. [Figure 7D] 7A to 7C are diagrams showing current paths in the power conversion device in each operation mode during the period from time t3 to time t4 in FIG. 6. [Figure 7E] 7A and 7B are diagrams showing current paths in the power conversion device in each operation mode after time t4 in FIG. 6. [Figure 8] 10 is a flowchart showing the <series→parallel connection> switching control of the power conversion device according to the second embodiment of the present invention. [Figure 9] FIG. 10 is a diagram showing a circuit configuration of a power conversion device according to a third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments, and can be combined with other embodiments or modified as desired without departing from the technical spirit of the present invention. In addition, in all the drawings for explaining the following embodiments, components having the same functions are generally designated by the same reference numerals, and repeated explanations thereof will be omitted.

[0011] The present invention realizes switching of the input or output connection while the power converter is operating, without affecting loads such as batteries connected to the input or output of the power converter, when switching the output from a parallel connection to a series connection, or from a series connection to a parallel connection. The first embodiment describes an example of switching the output from a parallel connection to a series connection, and the second embodiment describes an example of switching the output from a series connection to a parallel connection. The power conversion device according to the first embodiment and the power conversion device according to the second embodiment have the same configuration.

[0012] (First embodiment) FIG. 1 is a diagram showing a circuit configuration of a power conversion device 100 according to a first embodiment of the present invention. As shown in FIG. 1, the power conversion device 100 includes a first DC-DC converter 100A and a second DC-DC converter 100B (plurality of isolated DC-DC converters) that receive DC power from input terminals P1 and N1, switches (changeover switches) S1 to S3 on the output sides of the first DC-DC converter 100A and the second DC-DC converter 100B that switch between connecting the outputs in parallel or in series, or disconnecting the outputs, and a control circuit 101 (control unit) that controls the first DC-DC converter 100A and the second DC-DC converter 100B and the switches S1 to S3.

[0013] The first DC-DC converter 100A and the second DC-DC converter 100B are a plurality of isolated DC-DC converters whose inputs are connected in parallel and whose outputs include a DC capacitor. The power output from the first DC-DC converter 100A and the second DC-DC converter 100B is sent from the output terminals P2 and N2 to any external load.

[0014] The first DC-DC converter 100A and the second DC-DC converter 100B have a common circuit configuration, and include an input DC capacitor Ci, an input-side full-bridge circuit FB1, a high-frequency transformer Tr, an output-side full-bridge circuit FB2, and an output DC capacitor Co. However, when distinguishing between the first DC-DC converter 100A and the second DC-DC converter 100B, the first DC-DC converter 100A will be referred to as an input DC capacitor Ci(A), an input-side full-bridge circuit FB1(A), a high-frequency transformer Tr(A), an output-side full-bridge circuit FB2(A), and an output DC capacitor Co(A), and the first DC-DC converter 100B will be referred to as an input DC capacitor Ci(B), an input-side full-bridge circuit FB1(B), a high-frequency transformer Tr(B), an output-side full-bridge circuit FB2(B), and an output DC capacitor Co(B).

[0015] 1, the first DC-DC converter 100A and the second DC-DC converter 100B are isolated DC-DC converters with a circuit system called DAB (Dual Active Bridge). The first DC-DC converter 100A and the second DC-DC converter 100B may also be LLC resonant converters (see FIG. 9 below).

[0016] The first DC-DC converter 100A and the second DC-DC converter 100B adjust power by shifting the switching phases of the MOSFETs provided in the input side full bridge circuit FB1 and the output side full bridge circuit FB2 between the input side full bridge circuit FB1 and the output side full bridge circuit FB2.

[0017] The high-frequency transformer Tr1 converts the high-frequency AC power output by the input-side full-bridge circuit FB1 into a voltage according to the turns ratio while isolating it, and sends it to the output-side full-bridge circuit FB2. In principle, the high-frequency transformer Tr1 has leakage inductances Lr1 and Lr2.

[0018] Regarding the switches S1 to S3 on the output side, by turning switch S1 off and switches S2 and S3 on, the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B can be connected in parallel. Also, by turning switch S1 on and switches S2 and S3 off, the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B can be connected in series. Also, the input side full bridge circuit FB1, the output side full bridge circuit FB2, and switches S1 to S3 are driven by a control circuit 101.

[0019] The control circuit 101 outputs a control signal Gsig1(A) that controls the input side full bridge circuit FB1 of the first DC-DC converter 100A and a control signal Gsig2(A) that controls the output side full bridge circuit FB2, a control signal Gsig1(B) that controls the input side full bridge circuit FB1 of the second DC-DC converter 100B and a control signal Gsig2(B) that controls the output side full bridge circuit FB2, and a switching control signal GsigSW for the switches S1 to S3.

[0020] In the case of switching the output from parallel connection to series connection <parallel ⇒ series connection>, the control circuit 101 performs the following control. When switching the outputs of first DC-DC converter 100A and second DC-DC converter 100B (multiple isolated DC-DC converters) from a parallel connection to a series connection, control circuit 101 continues operation of at least one of first DC-DC converter 100A and second DC-DC converter 100B, stops the other isolated DC-DC converters, and uses switches (changeover switches) S1 to S3 to disconnect the output of the stopped isolated DC-DC converter, then causes the stopped isolated DC-DC converter to regenerate the stored energy in the output DC capacitor Co to the input side (of the operating isolated DC-DC converter), and when the voltage across the terminals of the output DC capacitor Co of the stopped isolated DC-DC converter falls below a predetermined value, uses switches (changeover switches) S1 to S3 to reconnect the output of the stopped isolated DC-DC converter in series, thereby restarting the operation of the stopped isolated DC-DC converter. Note that the predetermined value is set appropriately depending on the purpose, etc. The above phrase "regenerating the stored energy of the output DC capacitor Co to the isolated DC-DC converter that continues to operate" means that the stored energy of the output DC capacitor Co is used to charge the input DC capacitor Ci of the isolated DC-DC converter that continues to operate.

[0021] If the voltage across the terminals of the DC capacitor of the stopped isolated DC-DC converter does not drop below a predetermined value, the control circuit 101 consumes the stored energy of the DC capacitor as switching loss of the power device provided in the isolated DC-DC converter.

[0022] When restarting the operation of the stopped isolated DC-DC converter, the control circuit 101 calculates output voltage command values ​​for the operating isolated DC-DC converter and the stopped isolated DC-DC converter so that the total output voltage obtained by adding up the output voltages of the operating isolated DC-DC converter and the stopped isolated DC-DC converter remains constant.

[0023] In FIG. 1, power MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are used as the power devices of the input side full bridge circuit FB1 and the output side full bridge circuit FB2, but this embodiment is not limited to this, and other power devices such as IGBTs (Insulated Gate Bipolar Transistors) may also be used.

[0024] The following describes the operation of the power conversion device 100 configured as described above. The first embodiment is a <parallel to series connection> operation in which the output is switched from a parallel connection to a series connection.

[0025] 2A and 2B are conceptual diagrams showing the relationship between the output connection state of the first DC-DC converter 100A and the second DC-DC converter 100B and the output voltage Vo. Figure 2A shows a transistor with a single threshold voltage Vth.<w / o hysteresis > This indicates that the parallel and series configurations switch at the threshold voltage Vth (see the facing arrows in Figure 2A). When the output voltage Vo is below the threshold voltage Vth, the outputs are connected in parallel, and when it exceeds the threshold voltage Vth, the outputs are connected in series.

[0026] <Output connection status and output voltage Vo> Figure 2B shows a case where there are two threshold voltages Vth.<w / hysteresis> In this case, there are two threshold voltages: a first threshold voltage Vths-p and a second threshold voltage Vthp-s. The device initially operates in parallel connection, but when the output voltage Vo increases and exceeds the second threshold voltage Vths-s, it switches to series connection. On the other hand, when the output voltage Vo decreases from the series connection and falls below the first threshold voltage Vths-p, it switches to parallel connection (see the circular arrow in Figure 2B). Here, chattering can be prevented by setting the first threshold voltage Vths-p lower than the second threshold voltage Vths-s, i.e., by providing hysteresis.

[0027] <Transition from parallel to series connection> 3 is a graph showing the operation of the power conversion device 100 according to the first embodiment from parallel to series connection, with time taken as the horizontal axis. FIG. 3 shows the transition of the power conversion device 100 from parallel to series connection. From top to bottom in Figure 3, these are the states of switches S1 to S3, the total output power of first DC-DC converter 100A and second DC-DC converter 100B, the average output current Io(A) of first DC-DC converter 100A, the average output current Io(B) of second DC-DC converter 100B, the total output voltage Vo, the output voltage Vo(A) of first DC-DC converter 100A, and the output voltage Vo(B) of second DC-DC converter 100B.

[0028] The circuit operation at each time and period will be explained below. Initial state: The converter outputs are connected in parallel. -Period up to time t1 In the period up to time t1, switch S1 is off and switches S2 and S3 are on, so the outputs of the first DC-DC converter 100A and second DC-DC converter 100B are connected in parallel. Just before time t1, the control circuit 101 determines to switch the output connection state from parallel to series, and at time t1, only the output of the second DC-DC converter 100B is stopped, while the operation of the first DC-DC converter 100A continues. At time t1, a parallel-series switching command is received from the control circuit 101, so the output of the second DC-DC converter 100B is stopped, but the first DC-DC converter 100A continues to operate. As a result, the average output current Io(B) of the second DC-DC converter 100B becomes zero.

[0029] ·Time t2 As a result, the total output power of the first DC-DC converter 100A and the second DC-DC converter 100B is halved. When the control circuit 101 stops the second DC-DC converter 100B, at time t2, only switch S2 is turned off. At time t2, after the output of the second DC-DC converter 100B has stopped, the control circuit 101 turns off switch S2. This disconnects the output terminal of the second DC-DC converter 100B from output terminal P2.

[0030] ·Time t3 When switch S2 is turned off, at time t3 the control circuit 101 starts operation of the second DC-DC converter 100B so as to discharge the output voltage Vo(B) of the second DC-DC converter 100B. At this time, the second DC-DC converter 100B is controlled to send power from the output to the input. The power sent to the input side is supplied to the load from the operating second DC-DC converter 100A. In other words, the energy stored in the output DC capacitor Co(B) of the second DC-DC converter 100B is supplied to the load rather than being converted to heat by a discharge resistor or the like. Thus, at time t3, the control circuit 101 controls the second DC-DC converter 100B to send power to the input (for example, by controlling Io(B) to be negative), and starts discharging the output DC capacitor Co(B).

[0031] ·Time t4 When the output voltage Vo(B) of the second DC-DC converter 100B reaches approximately zero (discharge completion) at time t4, the control circuit 101 turns on the switch S1. Depending on the circuit configuration of the isolated DC-DC converter, it may become impossible to send power to the input side before the output voltage Vo(B) reaches zero. In this case, for example, if the output voltage falls below a certain voltage, the upper or lower arms of each leg of the full-bridge circuit can be switched in phase, thereby dissipating the stored energy in the output DC capacitor Co(B) as switching loss.

[0032] ·Time t5 Next, at time t5, the control circuit 101 turns off the switch S3. Through the operations up to this point, only the switch S1 is turned on, and the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B are connected in series.

[0033] ·Time t6 At time t6, the control circuit 101 begins controlling the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B so that they are balanced while maintaining the total output voltage Vo constant. That is, it controls the output voltage Vo(A) to be lowered and the output voltage Vo(B) to be higher. At time t6, the control circuit 101 begins charging the output voltage Vo(B) of the second DC-DC converter 100B, and performs voltage control to lower the output voltage Vo(A) of the first DC-DC converter 100A so that the total voltage Vo of the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B remains constant.

[0034] ·Time t7 When the output voltage Vo(A) and the output voltage Vo(B) are balanced at time t7, the control circuit 101 starts output to the load from the second DC-DC converter 100B, the total output power of the first DC-DC converter 100A and the second DC-DC converter 100B becomes equal to that before the output connection was switched, and the state transitions to a steady state. At time t7, the control circuit 101 transitions to the steady state when the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B reach the target voltage (Vo(A) = Vo(B)).

[0035] Through the above operation, the power conversion device 100 according to the first embodiment can switch the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B from a parallel connection to a series connection without stopping the output to the load. Furthermore, since the energy stored in the output DC capacitor Co(B) before the switching is discharged by being supplied to the load via the operating first DC-DC converter 100A, the switching operation can be performed with high efficiency.

[0036] In Fig. 3, the first DC-DC converter 100A is controlled to continue operating, but the first DC-DC converter 100A and the second DC-DC converter 100B have the same configuration. Therefore, the second DC-DC converter 100B may also be controlled to continue operating. Also, in Fig. 3, the output connection state is switched, but the input connection state may also be switched.

[0037] <Current paths in each operation mode> Figures 4A to 4H are diagrams showing current paths in power conversion device 100 in each operation mode in Figure 3. The same components as in Figure 1 are assigned the same reference numerals. The current paths in each operation mode are indicated on the circuit diagram by thick solid arrows and thick dashed arrows. Here, the current paths are shown in a state in which the switches of the upper arms of the left legs of the input side full-bridge circuit FB1 and the output side full-bridge circuit FB2 are conductive.

[0038] The current path shown in Figure 4A Figure 4A shows the current path during the period from time t0 to t1 in Figure 3. During this period, switch S1 is off, switches S2 and S3 are on, and the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B are connected in parallel. Current (indicated by thick solid arrows a, c, and d and thick dashed arrows b and e in Figure 4A) flows from input terminal P1 to the first DC-DC converter 100A and the second DC-DC converter 100B, and power is supplied to output DC capacitor Co via high-frequency transformer Tr. The total current from the first DC-DC converter 100A and the second DC-DC converter 100B flows to output terminal P2.

[0039] The current path shown in Figure 4B Figure 4B shows the current path during the period from time t1 to t2 in Figure 3. During this period, switching of the input-side full-bridge circuit FB1 and the output-side full-bridge circuit FB2 of the second DC-DC converter 100B is stopped. As a result, current supply from the second DC-DC converter 100B is stopped (only the thick solid arrows a and d in Figure 4B).

[0040] The current path shown in Figure 4C Figure 4C shows the current path during the period from time t2 to t3 in Figure 3. At time t2, switch S2 is turned off, but since current has already stopped flowing through switch S2 in Figure 4B, the current path (thick solid arrows a and d in Figure 4C) is the same as in Figure 4B.

[0041] The current path shown in Figure 4D Fig. 4D shows the current path during the period from time t3 to t4 in Fig. 3. From time t3, the second DC-DC converter 100B is controlled in a direction that discharges the output DC capacitor Co(B) (thick solid arrow c in Fig. 4D), so that current flows from the output side to the input side (thick solid arrow c in Fig. 4D). This current flowing into the input side flows to the input side of the first DC-DC converter 100A (thick dashed arrow b in Fig. 4D) and is supplied to the output side of the first DC-DC converter 100A (thick solid arrow a in Fig. 4D). Therefore, the energy stored in the output DC capacitor Co(B) can be supplied to the output side via the first DC-DC converter 100A.

[0042] The current path shown in Figure 4E Figure 4E shows the current path from time t4 to t5 in Figure 3. At time t4, the discharge of the output DC capacitor Co(B) is completed, and switch S1 is turned on. At this time, switches S1 and S3 form a parallel circuit, and the current from output terminal N2 is divided into current (thick solid arrow d in Figure 4E) and current (thick dashed arrow f in Figure 4E). In other words, because the voltage of the output DC capacitor Co(B) is almost zero, the parasitic diode of the MOSFET that constitutes the output-side full-bridge circuit FB2 conducts, and current flows to switch S1 (thick dashed arrow f in Figure 4E). Meanwhile, switch S3 also conducts, and current flows (thick solid arrow d in Figure 4E). The ratio of the two currents (the ratio of the current flowing to switch S1 and the current flowing to switch S3) is determined by factors such as the forward voltage of the MOSFET's parasitic diode.

[0043] The current path shown in Figure 4F Figure 4F shows the current path during the period from time t5 to t6 in Figure 3. When switch S3 turns off at time t5, the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B are connected in series. All of the current shunted in Figure 4E flows through switch S1 (thick solid arrow f in Figure 4F).

[0044] The current path shown in Figure 4G Figure 4G shows the current path during the period from time t6 to t7 in Figure 3. At time t6, the second DC-DC converter 100B starts switching and starts charging the output DC capacitor Co(B) (bold solid arrow g in Figure 4G). As the voltage of the output DC capacitor Co(B) increases, the parasitic diode of the MOSFET turns off.

[0045] The current path shown in Figure 4H Figure 4H shows the current path after time t7 in Figure 3. At time t7, the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B are connected in series (thick solid arrows a, c, and g, and thick dashed arrow b in Figure 4H), and the state transitions to a steady state. A common current flows through the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B (thick solid arrow g in Figure 4H), and is supplied to output terminal P2.

[0046] Although FIG. 4H shows a mode in which a current flows through the parasitic diode of the MOSFET, the present invention is not limited to this, and for example, the MOSFET may be turned on.

[0047] <Flowchart> 5 is a flowchart showing the <parallel to series connection> switching control of the power conversion device 100 according to the first embodiment. This flow is repeatedly executed by the control circuit 101 (FIG. 1) at predetermined timings. When the control circuit 101 starts the switching control and the calculation flow starts, in step S11 the control circuit 101 stops the output of the second DC-DC converter 100B (referred to as converter B in FIG. 5 due to space limitations).

[0048] In step S12, the control circuit 101 turns off the switch S2. In step S13, the control circuit 101 controls the second DC-DC converter 100B so that power is sent in the direction of discharging the output DC capacitor Co(B) of the second DC-DC converter 100B.

[0049] In step S14, the control circuit 101 determines whether the output voltage Vo(B) of the second DC-DC converter 100B is less than a predetermined value. If the output voltage Vo(B) of the second DC-DC converter 100B is equal to or greater than the predetermined value (S14: No), the process returns to step S13, and the discharge operation of the output DC capacitor Co(B) continues.

[0050] If the output voltage Vo(B) of the second DC-DC converter 100B is less than the predetermined value (S14: Yes), in step S15 the control circuit 101 turns on the switch S1. Next, in step S16, the control circuit 101 turns off the switch S3.

[0051] In step S17, the control circuit 101 controls the second DC-DC converter 100B in a direction to charge the output DC capacitor Co(B), and at the same time manipulates and controls the voltage command value Vo(A)ref of the first DC-DC converter 100A (referred to as converter A in Figure 5 due to space constraints) so that the total output power Vo does not change.

[0052] In step S18, the control circuit 101 determines whether the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B are equal to each other. If the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B are not equal (S18: No), the process returns to step S17 and the control of step S17 is continued.

[0053] If the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B are equal (S18: Yes), the switching to a series connection of the output is completed, and the switching control ends by transitioning to a steady state.

[0054] By performing the switching control shown in the above flowchart, the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B can be switched from parallel to series without generating inrush currents or the like in the loads connected to the output terminals P2 and N2.

[0055] (Second embodiment) The second embodiment of the present invention is an example in which the output of the power conversion device 100 of FIG. 1 is switched from a series connection to a parallel connection. In the second embodiment, control is performed to switch the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B from series to parallel in the configuration of the power conversion device 100 in Fig. 1. Below, differences from the first embodiment will be described, and explanations of similarities to the first embodiment will be omitted.

[0056] In the case of switching the output from a series connection to a parallel connection <series ⇒ parallel connection>, the control circuit 101 (FIG. 1) performs the following control. When switching the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B (multiple isolated DC-DC converters) from a series connection to a parallel connection, the control circuit 101 continues operation of at least one of the first DC-DC converter 100A and the second DC-DC converter 100B, and when the output voltage of the other isolated DC-DC converter falls below a predetermined value, uses switches S1 to S3 to switch so that the outputs of the isolated DC-DC converter that is continuing to operate and the isolated DC-DC converter that has been stopped are connected in series.

[0057] The operation of the power conversion device 100 for switching the output from a series connection to a parallel connection will be described below.

[0058] <Transition from series to parallel connection> FIG. 6 is a graph showing the operation of the power conversion device 100 according to the second embodiment from series to parallel connection, with time plotted on the horizontal axis. From top to bottom in Figure 6, these are the states of switches S1 to S3, the total output power of first DC-DC converter 100A and second DC-DC converter 100B, the average output current Io(A) of first DC-DC converter 100A, the average output current Io(B) of second DC-DC converter 100B, the total output voltage Vo, the output voltage Vo(A) of first DC-DC converter 100A, and the output voltage Vo(B) of second DC-DC converter 100B.

[0059] The circuit operation at each time and period will be explained below. -Period up to time t1 In the period up to time t1, switch S1 is on and switches S2 and S3 are off, and the outputs of the first DC-DC converter 100A and second DC-DC converter 100B are connected in series. The control circuit begins output switching control, and from time t1 begins discharging the output DC capacitor Co(B) of the second DC-DC converter 100B. At this time, the output voltage command value Vo(B)ref of the second DC-DC converter 100B is controlled to be lowered and the output voltage command value Vo(A)ref of the first DC-DC converter 100A is controlled to be raised so that the total output voltage Vo does not change. The energy stored in the output DC capacitor Co(B) is supplied to the load during this operation.

[0060] ·Time t2 At time t2, when the output voltage Vo(B) becomes zero and discharging is completed, the second DC-DC converter 100B is stopped and the switch S3 is turned on.

[0061] ·Time t3 At time t3, the switch S1 is turned off and the second DC-DC converter 100B is controlled to charge the output DC capacitor Co(B). Between times t3 and t4, the output of the second DC-DC converter 100B is disconnected from the load, and the output DC capacitor Co(B) is charged with power sent from the input side.

[0062] ·Time t4 When the output voltage Vo(B) becomes equal to the output voltage Vo(A) of the first DC-DC converter 100A at time t4, the switch S2 is turned on, completing the changeover to the parallel connection.

[0063] By the above operation, the power conversion device 100 according to the second embodiment can switch the connection from series to parallel without stopping the operation of the first DC-DC converter 100 A. Furthermore, in the discharge operation of the output DC capacitor Co(B), the accumulated energy is supplied to the load for discharge, so that switching can be performed with high efficiency.

[0064] 6, the first DC-DC converter 100A is controlled to continue operating, but the present invention is not limited to this, and for example, the second DC-DC converter 100B may be controlled to continue operating. Also, although the output connection state is switched in FIG. 6, the input connection state may be switched.

[0065] <Current paths in each operation mode> Figures 7A to 7E are diagrams showing the current paths of power conversion device 100 in each operation mode in Figure 6. The same components as in Figure 1 are assigned the same reference numerals. The current paths in each operation mode are indicated on the circuit diagram by thick solid arrows and thick dashed arrows. Here, the current paths are shown in a state in which the switches of the upper arms of the left legs of the input side full-bridge circuit FB1 and the output side full-bridge circuit FB2 are conductive.

[0066] The current path shown in Figure 7A Figure 7A shows the current path during the period from time t0 to t1 in Figure 6. During this period from time t0 to t1, switch S1 is on, switches S2 and S3 are off, and the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B are connected in series (thick solid arrows a, c, and g in Figure 7A, and thick dashed arrow b). Current flows from input terminal P1 to the first DC-DC converter 100A and the second DC-DC converter 100B (thick solid arrows a and c in Figure 7A), and power is supplied to output DC capacitor Co via high-frequency transformer Tr. A current flows through output terminal P2, which is common to the first DC-DC converter 100A and the second DC-DC converter 100B (thick solid arrow g in Figure 7A).

[0067] The current path shown in Figure 7B Figure 7B shows the current path during the period from time t1 to t2 in Figure 6. During the period from time t1 to t2, the second DC-DC converter 100B is controlled to decrease Vo(B) while maintaining the sum of the output voltages Vo(A) and Vo(B) constant. The current path is the same as in Figure 7A (thick solid arrows a, c, g and thick dashed arrow b in Figure 7B), and there are no differences between the drawings.

[0068] The current path shown in Figure 7C Figure 7C shows the current path during the period from time t2 to t3 in Figure 6. During the period from time t2 to t3, the output voltage Vo(B) of the second DC-DC converter 100B becomes almost zero, turning on the switch S3 (bold solid arrow d in Figure 7C). When the output voltage Vo(B) becomes equal to or lower than the forward voltage of the parasitic diode of the MOSFET, the parasitic diode of the MOSFET turns on, and the current to the output terminal P2 flows in parallel through the parasitic diode and the switch S3 (bold dashed arrow f in Figure 7C). The ratio of the two currents (the current flowing through the switch S3 and the current flowing through the switch S3) is determined by factors such as the forward voltage of the parasitic diode of the MOSFET.

[0069] The current path shown in Figure 7D Fig. 7D shows the current path during the period from time t3 to t4 in Fig. 6. During the period from time t3 to t4, switch S1 is turned off, and all of the current to output terminal P2 flows to switch S3 (bold solid arrow d in Fig. 7D). Also, from time t3, the second DC-DC converter 100B starts switching and starts charging the output DC capacitor Co(B) (bold solid arrow c in Fig. 7D). At this time, because switches S1 and S2 are both off, the only current flowing through the second DC-DC converter 100B is the charging current for output DC capacitor Co(B) (bold solid arrow c in Fig. 7D), and no current is supplied to output terminal P2.

[0070] The current path shown in Figure 7E Figure 7E shows the current path after time t4 in Figure 6. After time t4, when the output voltages Vo(A) and Vo(B) become approximately equal, switch S2 turns on, the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B are connected in parallel, and the state transitions to a steady state. The total current from the first DC-DC converter 100A and the second DC-DC converter 100B (indicated by the thick solid arrow d in Figure 7E) flows to output terminal P2.

[0071] <Flowchart> 8 is a flowchart showing the <series → parallel connection> switching control of the power conversion device 100 according to the second embodiment. This flow is repeatedly executed by the control circuit 101 (FIG. 1) at predetermined timings. 8, in the initial state, the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B are connected in series, that is, the switch S1 is on, and the switches S2 and S3 are off.

[0072] When the control circuit 101 (Figure 1) starts switching control and the calculation flow starts, in step S21, the output voltage command value Vo(B)ref of the second DC-DC converter 100B is controlled to be lowered and the output voltage command value Vo(A)ref of the first DC-DC converter 100A is controlled to be raised so that the total output voltage Vo does not change.

[0073] In step S22, the control circuit 101 determines whether the output voltage Vo(B) of the second DC-DC converter 100B is less than a predetermined value. If the output voltage Vo(B) of the second DC-DC converter 100B is equal to or greater than the predetermined value (S22: No), the process returns to step S21, and the operation of lowering the output voltage command value Vo(B)ref of the second DC-DC converter 100B and increasing the output voltage command value Vo(A)ref of the first DC-DC converter 100A continues.

[0074] If the output voltage Vo(B) of the second DC-DC converter 100B is less than the predetermined value (S22: Yes), the control circuit 101 turns on the switch S3 in step S23. Next, in step S24, the control circuit 101 turns off the switch S1.

[0075] In step S25, the control circuit 101 controls the second DC-DC converter 100B so as to charge the output DC capacitor Co(B).

[0076] In step S26, the control circuit 101 determines whether the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B are equal to each other. If the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B are not equal (S26: No), the process returns to step S25 and continues the control of step S25.

[0077] If the output voltage Vo(A) of the first DC-DC converter 100A and the output voltage Vo(B) of the second DC-DC converter 100B are equal (S26: Yes), the control circuit 101 turns on the switch S2 in step S27. The above flow completes the switching of the output to parallel connection, and the transition to a steady state ends the switching control.

[0078] By performing the switching control shown in the above flowchart, the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B can be switched from series to parallel without generating inrush currents or the like in the loads connected to the output terminals P2 and N2.

[0079] (Third embodiment) The third embodiment of the present invention is a modified example of the first and second embodiments. Below, differences from the first and second embodiments will be described, and explanations of similarities to the first and second embodiments will be omitted.

[0080] 9 is a diagram showing the circuit configuration of a power conversion device 200 according to a third embodiment of the present invention. The same components as those in FIG. 1 are given the same reference numerals, and explanations of overlapping parts will be omitted. In the first and second embodiments, the first DC-DC converter A and the second DC-DC converter B are of the DAB type, but in the power conversion device 200 shown in FIG. 9, capacitors Cr1 and Cr2 are inserted in series on the input side and the output side of the high-frequency transformer Tr, respectively.

[0081] The first DC-DC converter 200A and the second DC-DC converter 200B (multiple isolated DC-DC converters) adjust power by shifting the switching phases of the MOSFETs provided in the input side full bridge circuit FB1 and the output side full bridge circuit FB2 between the input side full bridge circuit FB1 and the output side full bridge circuit FB2.

[0082] The circuit configuration in FIG. 9 is called a resonant system or resonant type, and is characterized by the fact that the winding current of the high-frequency transformer Tr becomes sinusoidal because the excitation inductance and leakage inductances Lr1 and Lr2 (not shown) of the high-frequency transformer Tr and the capacitors Cr1 and Cr2 cause series current resonance.

[0083] In the resonant method, like DAB, the MOSFETs in each full-bridge circuit operate under ZVS (Zero Voltage Switching: switching performed when the voltage is zero) when turned on. Furthermore, a feature of the resonant method is that the MOSFETs turn off when the sinusoidal current drops from its peak, reducing the interruption current and suppressing switching losses.

[0084] This embodiment can also be applied to a resonant system, and has the same effect as the first and second embodiments, i.e., it is possible to switch the input or output connection while the power converter is operating, without affecting a load such as a battery connected to the input or output of the power converter.

[0085] [effect] A power conversion device 100 (FIG. 1) according to the first embodiment includes a first DC-DC converter 100A and a second DC-DC converter 100B having inputs connected in parallel and having a DC capacitor Co at the output, switches S1 to S3 on the output sides of the first DC-DC converter 100A and the second DC-DC converter 100B for switching between parallel connection and series connection of the outputs or disconnecting the outputs, and a control circuit 101 for controlling the first DC-DC converter 100A and the second DC-DC converter 100B and the switches S1 to S3. When switching the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B from parallel connection to series connection, the control circuit 101 (FIG. 1) controls the first DC-DC converter 100A and the second DC-DC converter 100B. At least one of the first DC-DC converters 100A and second DC-DC converter 100B is allowed to continue operating, the other isolated DC-DC converters are stopped, and the output of the stopped isolated DC-DC converter is cut off using switches S1 to S3. Thereafter, the stored energy in the output DC capacitor Co of the stopped isolated DC-DC converter is regenerated to the input side (the isolated DC-DC converter that continues to operate), and when the voltage across the terminals of the output DC capacitor Co of the stopped isolated DC-DC converter falls below a predetermined value, the outputs of the stopped isolated DC-DC converters are connected in series using switches S1 to S3, and the operation of the stopped isolated DC-DC converter is resumed (FIGS. 3, 4A-H, 5, 6).

[0086] With conventional technology, when switching from parallel to series, it was necessary to stop each DC-DC converter and discharge each capacitor through a resistor to prevent inrush current due to the voltage difference with the DC section, which resulted in downtime for the system. Therefore, discharging the capacitors without shutting down the system was a challenge.

[0087] In the power conversion device 100 according to the first embodiment, of the two converters (first DC-DC converter 100A and second DC-DC converter 100B), for example, only the second DC-DC converter 100B is stopped, and the stored energy in the output DC capacitor Co is discharged by being regenerated in the first DC-DC converter 100A that is in operation. After discharging, the control circuit 101 controls the switching of the switches S1 to S3 to connect them in series, and then controls the voltages to lower the output voltage of the first DC-DC converter 100A and raise the output voltage of the second DC-DC converter 100B so that the total output voltage of the two converters remains the same as before the switching. That is, when switching the series-parallel connection of the two isolated DC-DC converters 100A, 100B with the three switches S1 to S3, the control circuit 101 stops the circuit and discharges the output DC capacitor Co before switching, taking transient operation into consideration, so as to prevent inrush current from occurring due to the voltage difference with the DC section capacitor (battery).

[0088] In this way, when the power conversion device 100 (FIG. 1) switches the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B from a parallel connection to a series connection, the energy of the stopped DC-DC converter is regenerated and sent to the other DC-DC converter to switch to series. In other words, the stopped DC-DC converter is started in a direction that sends power to the primary side, and the output DC capacitor Co is discharged. The energy of the output DC capacitor Co can be regenerated, and no discharge resistor is required.

[0089] This allows the power conversion device 100 (Fig. 1) to switch from parallel to series while continuing to operate the system. In the first embodiment (Figs. 3, 4A-H, 5, and 6), the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B can be switched from parallel to series without generating an inrush current or the like in the load connected to the output terminals P2 and N2. As a result, it is possible to switch the input or output connection while the power converter is operating, without affecting the load, such as a battery, connected to the input or output of the power converter.

[0090] When switching the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B from a series connection to a parallel connection, the control circuit 101 (FIG. 1) of the power conversion device 100 (FIG. 1) of the second embodiment continues operation of at least one of the first DC-DC converter 100A and the second DC-DC converter 100B, and when the output voltage of the other isolated DC-DC converter falls below a predetermined value, switches S1 to S3 are used to switch so that the outputs of the isolated DC-DC converter that continues to operate and the isolated DC-DC converter that has been stopped are connected in series.

[0091] In this way, in the second embodiment (FIGS. 6, 7A-E, and 8), the outputs of the first DC-DC converter 100A and the second DC-DC converter 100B can be switched from series to parallel without generating an inrush current or the like in the load connected to the output terminals P2 and N2. As a result, the input or output connection can be switched while the power converter is operating, without affecting the load, such as a battery, connected to the input or output of the power converter.

[0092] Furthermore, in the power conversion device 100 (FIG. 1) according to the first embodiment, if the voltage across the terminals of the DC capacitor of the stopped isolated DC-DC converter does not drop below a predetermined value, the control circuit 101 consumes the stored energy of the DC capacitor as switching loss of the power device provided in the isolated DC-DC converter.

[0093] By doing so, when the output voltage does not drop, it can be consumed by switching loss.

[0094] Furthermore, in the power conversion device 100 (FIG. 1) according to the first embodiment, when the operation of the stopped isolated DC-DC converter is resumed, the control circuit 101 calculates output voltage command values ​​for the operating isolated DC-DC converter and the stopped isolated DC-DC converter so that the total output voltage obtained by adding up the output voltages of the operating isolated DC-DC converter and the stopped isolated DC-DC converter remains constant.

[0095] By doing this, the total voltage remains unchanged when operation is resumed, so when the output of the DC-DC converter is switched from a series connection to a parallel connection, fluctuations in the output voltage can be prevented in advance, ensuring stable system operation.

[0096] The present invention is not limited to the above-described embodiments, and includes other modifications and applications without departing from the spirit of the present invention as defined in the claims. For example, the above-described embodiments have been described in detail to facilitate understanding of the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations. For example, the arrangement and wiring of the semiconductor elements that make up the full-bridge circuit are not limited to the form shown in the figure. The semiconductor elements may be MOSFETs, unipolar devices such as JFETs (Junction Field Effect Transistors), or bipolar devices such as IGBTs. Depending on the device, the main terminals and sense terminals may be called "collector" and "emitter" instead of the "drain" and "source" mentioned above. [Explanation of symbols]

[0097] 100,200 Power conversion equipment 100A, 200A First DC-DC Converter (Multiple Isolated DC-DC Converters) 100B, 200B Second DC-DC converter (multiple isolated DC-DC converters) 101 Control circuit (control unit) P1, N1 input terminal P2, N2 output terminals Ci(A), Ci(B) Input DC capacitors FB1(A), FB1(B) Input side full bridge circuit Lr1A, Lr1B Input leakage inductance Tr(A),Tr(B) High frequency transformer Lr2(A), Lr2(B) Output leakage inductance FB2(A), FB2(B) Output side full bridge circuit Co, Co(A), Co(B) Output DC capacitors S1, S2, S3 switches (selector switches) Vo(A)ref Output voltage command value of the first DC-DC converter Vo(B)ref Output voltage command value of the second DC-DC converter Vo(A) Output voltage of the first DC-DC converter Vo(B) Output voltage of the second DC-DC converter Vo total output voltage

Claims

1. A power conversion device comprising: a plurality of isolated DC-DC converters whose inputs are connected in parallel and whose outputs are provided with DC capacitors; a changeover switch that switches between connecting the outputs of the plurality of isolated DC-DC converters in parallel or in series, or disconnecting the outputs; and a control unit that controls the isolated DC-DC converters and the changeover switch, When switching the outputs of the plurality of isolated DC-DC converters from parallel connection to series connection, the control unit: Among the plurality of isolated DC-DC converters, at least one isolated DC-DC converter is allowed to continue operating, the other isolated DC-DC converters are stopped, the output of the stopped isolated DC-DC converter is cut off using the changeover switch, and then the stored energy of the DC capacitor of the stopped isolated DC-DC converter is regenerated in the isolated DC-DC converter that continues operating, and when the voltage between the terminals of the DC capacitor of the stopped isolated DC-DC converter becomes equal to or lower than a predetermined value, the output of the stopped isolated DC-DC converter is connected in series using the changeover switch, and the operation of the stopped isolated DC-DC converter is resumed. A power conversion device characterized by:

2. A power conversion device comprising: a plurality of isolated DC-DC converters whose inputs are connected in parallel and whose outputs are provided with DC capacitors; a changeover switch that switches between connecting the outputs of the plurality of isolated DC-DC converters in parallel or in series, or disconnecting the outputs; and a control unit that controls the isolated DC-DC converters and the changeover switch, When switching the outputs of the plurality of isolated DC-DC converters from a series connection to a parallel connection, the control unit: At least one of the plurality of isolated DC-DC converters continues to operate, and when the output voltage of the other isolated DC-DC converters falls below a predetermined value, the changeover switch is used to switch the outputs of the isolated DC-DC converter that continues to operate and the isolated DC-DC converter that has stopped to be connected in series. A power conversion device characterized by:

3. When the voltage between the terminals of the DC capacitor of the stopped isolated DC-DC converter does not drop below a predetermined value, the control unit consumes the stored energy of the DC capacitor as a switching loss of a power device provided in the isolated DC-DC converter.

2. The power conversion device according to claim 1.

4. When restarting the operation of the stopped isolated DC-DC converter, the control unit calculates output voltage command values ​​for the isolated DC-DC converter that continues to operate and the stopped isolated DC-DC converter so that a total output voltage obtained by adding up the output voltages of the isolated DC-DC converter that continues to operate and the stopped isolated DC-DC converter becomes constant.

2. The power conversion device according to claim 1.

5. The isolated DC-DC converter is a DAB (Dual Active Bridge) 3. The power conversion device according to claim 1 or 2.

6. The isolated DC-DC converter is a resonant converter.

3. The power conversion device according to claim 1 or 2.

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