JP7182135B2 - DC/DC converter - Google Patents

Description

The present disclosure relates to a DC/DC converter that converts DC power into DC power of another voltage.

Power conditioners connected to storage batteries, solar cells, fuel cells, etc. use a DC/DC converter and an inverter. DC/DC converters and inverters are desired to have highly efficient power conversion and compact design. As a DC/DC converter for realizing this, a flying capacitor circuit (four switching elements connected in series, and a flying capacitor connected in parallel to the second switching element and the third switching element) is installed after the reactor. ) are connected and the voltage at the connection point of the reactor and the flying capacitor circuit is set to three levels (see, for example, Patent Document 1).

A multi-level power converter can reduce the voltage applied to each switching element, thereby reducing switching loss and realizing highly efficient power conversion. In the multi-level power conversion device using the above flying capacitor circuit, the voltage applied to each switching element constituting the flying capacitor circuit can be reduced to 1/2 times the DC bus voltage by using three levels. .

As a result, it is possible to use switching elements with a relatively low withstand voltage (eg, 300 V) instead of switching elements with a relatively high withstand voltage (eg, 600 V) used in the full-bridge section of the inverter. Become. A switching element with a low withstand voltage is less expensive than a switching element with a high withstand voltage, and has less conduction loss, switching loss, etc. during power conversion, and contributes to higher efficiency.

JP 2019-92242 A

In the multi-level power converter as described above, overcurrent may occur when the converter is started in a state in which the precharge of the flying capacitor is insufficient.

The present disclosure has been made in view of such circumstances, and an object thereof is to provide a DC/DC converter using flying capacitors that can be started safely.

In order to solve the above problems, a DC/DC converter according to one aspect of the present disclosure includes a high-voltage side capacitor connected in parallel to a high-voltage side DC power supply, a plurality of switching elements connected in series, and at least one flying capacitor. wherein the high-voltage side DC power supply is connected to both ends of the plurality of series-connected switching elements, and the low-voltage side DC power supply is connected to both ends of some of the plurality of switching elements constituting the plurality of series-connected switching elements are connected, both ends of the low-voltage side DC power supply, at least one reactor inserted in a path between both ends of the plurality of switching elements of the part, the low-voltage side DC power supply and the reactor a voltage sensor for measuring each voltage of the flying capacitor; a plurality of switching elements connected in series; and a controller for controlling the low-voltage side switch circuit. . When the DC/DC converter is started, the control unit turns on the low-voltage side switch circuit, and then determines the timing of starting the switching operation of the flying capacitor unit based on the voltage of the flying capacitor. do.

According to the present disclosure, it is possible to safely start a DC/DC converter using flying capacitors.

1 is a diagram for explaining the configuration of a DC/DC conversion device according to Embodiment 1; FIG. 2 is a diagram summarizing switching patterns of a first switching element to an eighth switching element of the DC/DC converter shown in FIG. 1; FIG. FIGS. 3(a) to 3(d) are circuit diagrams showing current paths of respective switching patterns during boosting operation. FIGS. 4A to 4D are circuit diagrams showing current paths of respective switching patterns during step-down operation. FIG. 10 is a timing chart showing an example of switching patterns of the first switching element to the eighth switching element when the step-up ratio is 2 or more; FIG. FIG. 10 is a timing chart showing an example of switching patterns of the first switching element to the eighth switching element when the step-up ratio is less than 2. FIG. FIG. 10 is a diagram showing exemplary waveforms of the voltage of the high-voltage side capacitor, the voltage of the flying capacitor, and the reactor current at the start-up of the DC/DC converter according to the comparative example; FIG. 4 is a diagram showing an example of waveforms of a voltage of a high-voltage side capacitor, a voltage of a flying capacitor, and a reactor current at startup of the DC/DC converter according to the embodiment; FIG. 10 is a diagram for explaining the configuration of a DC/DC conversion device according to Embodiment 2; FIG. 2 is a diagram showing an N (N is a natural number) stage flying capacitor circuit; 2 is a diagram showing a configuration in which an inrush current prevention circuit is connected in parallel with a low-voltage side relay of the DC/DC converter shown in FIG. 1; FIG. FIG. 3 is a diagram for explaining the configuration of a DC/DC conversion device according to a modification of Embodiment 1; FIG.

FIG. 1 is a diagram for explaining the configuration of a DC/DC conversion device 3 according to Embodiment 1. FIG. The DC/DC converter 3 according to Embodiment 1 is a bidirectional step-up/step-down DC/DC converter. The DC/DC converter 3 can step up the DC power supplied from the low voltage side DC power supply 2 and supply it to the high voltage side DC power supply 1 . Further, the DC/DC converter 3 can step down the DC power supplied from the high-voltage side DC power supply 1 and supply it to the low-voltage side DC power supply 2 .

The low-voltage side DC power supply 2 corresponds to, for example, a storage battery, an electric double layer capacitor, or the like. The high-voltage DC power supply 1 corresponds to, for example, a DC bus to which a bidirectional DC/AC inverter is connected. The AC side of the bi-directional DC/AC inverter is connected to the utility grid and AC load for storage system applications. For electric vehicle applications, it is connected to a motor (with regenerative function). In the application of the power storage system, a DC/DC converter for solar cells or a DC/DC converter for other storage batteries may be further connected to the DC bus.

The DC/DC conversion device 3 includes a first low-voltage side capacitor C1, a low-voltage side relay RY2, a second low-voltage side capacitor C2, a reactor L1, a flying capacitor section 30, a first divided capacitor C4a, a second divided capacitor C4b, and a high-voltage side capacitor. C3, high voltage side relay RY1, first voltage sensor 41, second voltage sensor 42, third voltage sensor 43, and control unit 40 are included.

A first low-voltage side capacitor C1 and a second low-voltage side capacitor C2 are connected in parallel with the low-voltage side DC power supply 2 . The first low-voltage side capacitor C<b>1 and the second low-voltage side capacitor C<b>2 act as smoothing capacitors that stabilize the voltage of the low-voltage side DC power supply 2 . A low-voltage relay RY2 is inserted in a path connecting the positive terminal of the first low-voltage capacitor C1 and the positive terminal of the second low-voltage capacitor C2.

A high-voltage side capacitor C3 is connected in parallel with the high-voltage side DC power supply 1 . The high voltage side capacitor C3 acts as a smoothing capacitor for stabilizing the voltage of the high voltage side DC power supply 1 . A high voltage side relay RY1 is provided between the high voltage side capacitor C3 and the high voltage side DC power supply 1. As shown in FIG. In the example shown in FIG. 1, a high voltage capacitor is connected to the portion of the positive DC bus between the node to which the positive terminal of the high voltage capacitor C3 is connected and the node to which the positive terminal of the high voltage DC power supply 1 is connected. A side relay RY1 is inserted.

A first split capacitor C4a and a second split capacitor C4b are connected in series between a positive side DC bus and a negative side DC bus connected to both ends of the high voltage side DC power supply 1 respectively. The first dividing capacitor C4a and the second dividing capacitor C4b function to divide the voltage E of the high-voltage side DC power supply 1 by half, and function as a snubber capacitor for suppressing the surge voltage generated in the flying capacitor section 30. have

A flying capacitor section 30 is connected between the positive DC bus and the negative DC bus. In the circuit configuration shown in FIG. 1, the flying capacitor section 30 is configured by connecting a first flying capacitor circuit on the upper side and a second flying capacitor circuit on the lower side in series. Reactor L1 is connected between the positive terminal of low-voltage DC power supply 2 and the middle point of the first flying capacitor circuit. The negative terminal of the low-voltage DC power supply 2 and the middle point of the second flying capacitor circuit are connected. The connection point between the first flying capacitor circuit and the second flying capacitor circuit is connected to the intermediate potential point M of the high-voltage side DC power supply 1 (the voltage dividing point of the first dividing capacitor C4a and the second dividing capacitor C4b). .

The low-voltage side relay RY2 described above is provided between the low-voltage side DC power supply 2 and the first low-voltage side capacitor C1, and the second low-voltage side capacitor C2 and the reactor L1, and can electrically cut them off.

The upper first flying capacitor circuit includes a first switching element S1, a second switching element S2, a third switching element S3, a fourth switching element S4 and a first flying capacitor C5a. The lower second flying capacitor circuit includes a fifth switching element S5, a sixth switching element S6, a seventh switching element S7, an eighth switching element S8 and a second flying capacitor C5b. The first switching element S1 to the eighth switching element S8 are connected in series between the positive DC bus and the negative DC bus.

The first flying capacitor C5a is connected between a connection point between the first switching element S1 and the second switching element S2 and a connection point between the third switching element S3 and the fourth switching element S4. S1—charged and discharged by the fourth switching element S4. The second flying capacitor C5b is connected between a connection point between the fifth switching element S5 and the sixth switching element S6 and a connection point between the seventh switching element S7 and the eighth switching element S8. S5—charged and discharged by the eighth switching element S8.

At the midpoint of the first flying capacitor circuit, the voltage E [V] of the high-voltage side DC power supply 1 applied to the upper terminal of the first switching element S1 and the voltage 1 applied to the lower terminal of the fourth switching element S4. A potential in the range between /2E [V] is generated. The first flying capacitor C5a is initially charged (precharged) to a voltage of 1/4E [V], and charging and discharging are repeated centering on the voltage of 1/4E [V]. Therefore, at the midpoint of the first flying capacitor circuit, approximately three levels of potentials of E [V], 3/4 E [V], and 1/2 E [V] are generated.

Between 1/2E [V] applied to the upper terminal of the fifth switching element S5 and 0 [V] applied to the lower terminal of the eighth switching element S8, A potential in the range of . The second flying capacitor C5b is initially charged (precharged) to a voltage of 1/4E [V], and charging and discharging are repeated centering on the voltage of 1/4E [V]. Therefore, approximately three levels of potential of 1/2E [V], 1/4E [V], and 0 [V] are generated at the midpoint of the second flying capacitor circuit.

A first diode D1 to an eighth diode D8 are formed/connected in anti-parallel to the first switching element S1 to the eighth switching element S8, respectively. Switching elements having a withstand voltage lower than the voltage of the high-voltage side DC power supply 1 are used for the first switching element S1 to the eighth switching element S8. Hereinafter, in the present embodiment, an example in which 150V withstand voltage N-channel MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are used for the first switching element S1 to the eighth switching element S8 is assumed. In an N-channel MOSFET, a parasitic diode is formed in the direction from the source to the drain.

An IGBT (Insulated Gate Bipolar Transistor) or a bipolar transistor may be used for the first switching element S1 to the eighth switching element S8. In that case, parasitic diodes are not formed in the first switching element S1 to the eighth switching element S8, and external diodes are connected in antiparallel to the first switching element S1 to the eighth switching element S8, respectively. In addition to general silicon (Si) semiconductors, wide bandgap semiconductors using silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga2O3), diamond (C), or the like may be used.

The first voltage sensor 41 measures the voltage across the first flying capacitor C5a and outputs the measured voltage value to the controller 40 . The second voltage sensor 42 measures the voltage across the second flying capacitor C5b and outputs the measured voltage value to the controller 40 . The third voltage sensor 43 measures the voltage across the high voltage side capacitor C3 and outputs the measured voltage value to the controller 40 . Although not shown in FIG. 1, a voltage sensor for measuring the voltage across the second low-voltage side capacitor C2 and a current sensor for measuring the current flowing through the reactor L1 are provided, and the measured values of each are sent to the control unit 40. output.

The control unit 40 can control the first flying capacitor circuit and the second flying capacitor circuit to transmit DC power from the low-voltage side DC power supply 2 to the high-voltage side DC power supply 1 by boosting operation. In addition, DC power can be transmitted from the high-voltage side DC power supply 1 to the low-voltage side DC power supply 2 in step-down operation. More specifically, the control unit 40 supplies a driving signal (PWM (Pulse Width Modulation) signal) to the gate terminals of the first switching element S1 to the eighth switching element S8, thereby switching the first switching element S1 to the eighth switching element S1 to the eighth switching element S8. Power can be transmitted bi-directionally in step-up or step-down operation by controlling on/off of the switching element S8.

The configuration of the control unit 40 can be realized by cooperation of hardware resources and software resources, or only by hardware resources. Analog devices, microcomputers, DSPs, ROMs, RAMs, FPGAs, ASICs, and other LSIs can be used as hardware resources. Programs such as firmware can be used as software resources.

FIG. 2 is a diagram summarizing switching patterns of the first switching element S1 to the eighth switching element S8 of the DC/DC converter 3 shown in FIG. In the switching pattern shown in FIG. 2, the set of the first switching element S1 and the eighth switching element S8 and the set of the fourth switching element S4 and the fifth switching element S5 have a complementary relationship. Also, the set of the second switching element S2 and the seventh switching element S7 and the set of the third switching element S3 and the sixth switching element S6 have a complementary relationship.

The controller 40 performs step-up or step-down operation using four modes.
In mode a, the control unit 40 turns on the second switching element S2, the fourth switching element S4, the fifth switching element S5, and the seventh switching element S7, and turns on the first switching element S1, the third switching element S3, and the sixth switching element S3. The switching element S6 and the eighth switching element S8 are controlled to be turned off. In mode a, the voltage between the midpoint of the first flying capacitor circuit and the midpoint of the second flying capacitor circuit (that is, the input/output voltage V _L on the low voltage side of the flying capacitor section 30) is ½E.

In mode b, the control unit 40 turns on the first switching element S1, the third switching element S3, the sixth switching element S6, and the eighth switching element S8, and turns on the second switching element S2, the fourth switching element S4, and the fifth switching element S4. The switching element S5 and the seventh switching element S7 are controlled to be turned off. In mode b, the input/output voltage _VL on the low voltage side of the flying capacitor section 30 is 1/2E.

In mode c, the control unit 40 turns on the first switching element S1, the second switching element S2, the seventh switching element S7, and the eighth switching element S8, and turns on the third switching element S3, the fourth switching element S4, and the fifth switching element S4. The switching element S5 and the sixth switching element S6 are controlled to be turned off. In mode c, the input/output voltage _VL on the low voltage side of the flying capacitor section 30 is E.

In mode d, the control unit 40 turns on the third switching element S3, the fourth switching element S4, the fifth switching element S5, and the sixth switching element S6, and turns on the first switching element S1, the second switching element S2, and the seventh switching element S2. The switching element S7 and the eighth switching element S8 are controlled to be turned off. In mode d, the input/output voltage _VL on the low voltage side of the flying capacitor section 30 is zero.

FIGS. 3(a) to 3(d) are circuit diagrams showing current paths of respective switching patterns during boosting operation. FIGS. 4A to 4D are circuit diagrams showing current paths of respective switching patterns during step-down operation. For the sake of simplification of the drawings, MOSFETs are drawn with simple switch symbols.

3(a) shows the current path in mode a during boosting operation, FIG. 3(b) shows the current path in mode b during boosting operation, and FIG. 3(c) shows the current in mode c during boosting operation. FIG. 3(d) shows the current path in mode d during boosting operation. Similarly, FIG. 4(a) shows the current path in mode a during step-down operation, FIG. 4(b) shows the current path in mode b in step-down operation, and FIG. FIG. 4(d) shows the current path of mode d during step-down operation.

The direction of the current is opposite between the step-up operation and the step-down operation. In mode a, the first flying capacitor C5a and the second flying capacitor C5b are charged during the step-up operation as shown in FIG. 3(a). The flying capacitor C5a and the second flying capacitor C5b are discharged. In mode b, the first flying capacitor C5a and the second flying capacitor C5b discharge during the step-up operation as shown in FIG. 3(b). The flying capacitor C5a and the second flying capacitor C5b are charged.

When power is transmitted from the low-voltage DC power supply 2 to the high-voltage DC power supply 1 in a step-up operation, the control unit 40 sets a current command value in the positive direction, and the measured value of the current flowing through the reactor L1 is the current in the positive direction. The duty ratio (ON time) of the first switching element S1 to the eighth switching element S8 is controlled so as to maintain the command value. Conversely, when power is transmitted from the high-voltage DC power supply 1 to the low-voltage DC power supply 2 in step-down operation, the control unit 40 sets a current command value in the negative direction, and the measured value of the current flowing through the reactor L1 is set to the negative direction. The duty ratio (ON time) of the first switching element S1 to the eighth switching element S8 is controlled so as to maintain the direction current command value.

Also, when the ratio of the voltage of the low-voltage DC power supply 2 and the voltage of the high-voltage DC power supply 1 is smaller than the set value, the control unit 40 transmits power using mode a, mode b, and mode c. Also, the control unit 40 transmits power using mode a, mode b, and mode d when the ratio is greater than the set value. Also, when the ratio matches the set value, the control unit 40 transmits power using mode a and mode b.

The set value is set according to the ratio between the total voltage 1/2E of the voltage of the first flying capacitor C5a and the voltage of the second flying capacitor C5b and the voltage E of the high voltage side DC power supply 1. The set value is set to 2 in this embodiment.

The control unit 40 generates a duty ratio such that the current command value matches the measured value of the current flowing through the reactor L1, and the voltages of the first flying capacitor C5a and the second flying capacitor C5b are each 1/4E. do. Specifically, the control unit 40 increases the duty ratio as the measured value of the current flowing through the reactor L1 is smaller than the current command value, and decreases the duty ratio as the measured value increases.

FIG. 5 is a timing chart showing an example of switching patterns of the first switching element S1 to the eighth switching element S8 when the step-up ratio is two times or more. FIG. 6 is a timing chart showing an example of switching patterns of the first switching element S1 to the eighth switching element S8 when the step-up ratio is less than two. The control examples shown in FIGS. 5 and 6 show control examples using the double carrier drive method. The double carrier drive method uses two carrier signals (triangular waves in FIGS. 5 and 6) that are 180° out of phase. The duty ratio duty serves as a threshold to be compared with the two carrier signals. When the boost ratio is 2 times or more, the duty ratio takes a value in the range of 0.5 to 1.0. take a value.

A first gate signal supplied to the first switching element S1 and the eighth switching element S8 and a fourth gate signal supplied to the fourth switching element S4 and the fifth switching element S5 are obtained by comparing the thick line carrier signal and the duty ratio duty. Generate a signal. Specifically, in a region where the thick line carrier signal is higher than the duty ratio duty, the first gate signal is turned on and the fourth gate signal is turned off. In a region in which the heavy-line carrier signal is lower than the duty ratio duty, the first gate signal is turned off and the fourth gate signal is turned on. The first gate signal and the fourth gate signal are complementary. A dead time period is set in which the first gate signal and the fourth gate signal are turned off at the same time when the first gate signal and the fourth gate signal are switched on/off.

The second gate signal supplied to the second switching element S2 and the seventh switching element S7 and the third gate signal supplied to the third switching element S3 and the sixth switching element S6 are determined by comparing the thin line carrier signal and the duty ratio duty. Generate a signal. Specifically, in a region where the thin line carrier signal is higher than the duty ratio duty, the second gate signal is turned on and the third gate signal is turned off. In a region in which the thin line carrier signal is lower than the duty ratio duty, the second gate signal is turned off and the third gate signal is turned on. The second gate signal and the third gate signal are complementary. A dead time period is set in which the second gate signal and the third gate signal are turned off at the same time when the second gate signal and the third gate signal are switched on/off.

When the step-up ratio is twice or more, the control unit 40 alternately switches between mode a and mode b, and inserts mode d while switching between the two. That is, the control unit 40 switches modes in the order of mode a→mode d→mode b→mode d→mode a→mode d→mode b→mode d. While the duty ratio duty does not change, the periods of mode a and mode b are equal, and the voltages of the first flying capacitor C5a and the second flying capacitor C5b are maintained at 1/4E, respectively. When the step-up ratio is twice or more, the higher the duty ratio, the longer the period of mode d with respect to the periods of mode a and mode b, increasing the amount of energy to be transmitted.

If the step-up ratio is less than 2 times, the control unit 40 alternately switches between mode a and mode b, and inserts mode c between the two. That is, the control unit 40 switches modes in the order of mode a→mode c→mode b→mode c→mode a→mode c→mode b→mode c. While the duty ratio duty does not change, the periods of mode a and mode b are equal, and the voltages of the first flying capacitor C5a and the second flying capacitor C5b are maintained at 1/4E, respectively. When the boost ratio is less than 2 times, the higher the duty ratio duty, the shorter the period of mode c with respect to the periods of mode a and mode b, increasing the amount of transmitted energy.

If the step-up ratio ideally maintains 2 times and the voltages of the first flying capacitor C5a and the second flying capacitor C5b ideally maintain 1/4E, the duty ratio duty maintains 0.5.

When the total voltage of the voltage of the first flying capacitor C5a and the voltage of the second flying capacitor C5b is less than 1/2E, the control unit 40 increases the charging time of the mode a or the mode b to increase the charging time. Bring the total voltage closer to 1/2E. Conversely, when the total voltage of the voltage of the first flying capacitor C5a and the voltage of the second flying capacitor C5b exceeds 1/2E, the control unit 40 increases the time of the discharging mode of mode a and mode b. to bring the total voltage close to 1/2E.

Note that the control unit 40 causes the DC/DC converter 3 to operate as a normal boost chopper by alternately switching between the mode c and the mode d without using the first flying capacitor C5a and the second flying capacitor C5b. is also possible. In this case, the operation mode switching does not occur depending on the boost ratio.

In the circuit configuration shown in FIG. 1, relatively low-capacity capacitors are used for the high-voltage side capacitor C3, the first dividing capacitor C4a, and the second dividing capacitor C4b. These capacitors do not need to strictly maintain the voltage, the high-voltage side capacitor C3 only needs to have a smoothing function, and the first dividing capacitor C4a and the second dividing capacitor C4b only need to have a voltage dividing function.

On the other hand, relatively large-capacity capacitors are used for the first flying capacitor C5a and the second flying capacitor C5b. In each charged state of the first flying capacitor C5a and the second flying capacitor C5b, it is necessary to maintain the voltage of 1/4E as accurately as possible.

Film capacitors having the same capacity may be used for the high-voltage side capacitor C3, the first divided capacitor C4a, the second divided capacitor C4b, the first flying capacitor C5a, and the second flying capacitor C5b. In that case, each of the first flying capacitor C5a and the second flying capacitor C5b has a plurality of film capacitors connected in parallel to increase the capacity.

The operation of the DC/DC converter 3 shown in FIG. 1 at startup will be described below. An example in which the voltage of the low-voltage side DC power supply 2 is 100V and the voltage of the high-voltage side DC power supply 1 is 300V will be described below.

First, the operation at startup according to the comparative example will be described. In the initial state before the DC/DC converter 3 is activated, the controller 40 controls the low-voltage side relay RY2, the high-voltage side relay RY1, and the first switching element S1 to the eighth switching element S8 to be off. In this state, the voltages of the high voltage side capacitor C3, the first dividing capacitor C4a, the second dividing capacitor C4b, the first flying capacitor C5a and the second flying capacitor C5b are all 0V.

Next, in order to start the DC/DC converter 3, the controller 40 turns on the low-voltage side relay RY2. When the low side relay RY2 is turned on, the high side capacitor C3, the first dividing capacitor C4a and the second dividing capacitor C4b are immediately connected through the first diode D1, the second diode D2, the seventh diode D7 and the eighth diode D8. charged. The voltage of the high-voltage side capacitor C3 becomes 100V, and the voltages of the first divided capacitor C4a and the second divided capacitor C4b become 50V, respectively.

The first flying capacitor C5a is charged through the second diode D2 and the third diode D3. The second switching element S2 and the third switching element S3 have a resistance component (about 100 kΩ in this embodiment). As described above, the capacitance of the first flying capacitor C5a is set larger than the capacitances of the high voltage side capacitor C3, the first split capacitor C4a and the second split capacitor C4b.

Therefore, the time constant τ (=R×C) when charging the first flying capacitor C5a becomes a large value, and the charging of the first flying capacitor C5a is performed by the high-voltage side capacitor C3, the first split capacitor C4a, and the second split capacitor It lags behind the charging of C4b. Similarly, the charging of the second flying capacitor C5b also lags behind the charging of the high voltage side capacitor C3, the first split capacitor C4a and the second split capacitor C4b. When the charging of the first flying capacitor C5a and the second flying capacitor C5b is completed, the voltages of the first flying capacitor C5a and the second flying capacitor C5b become 25V, respectively.

The control unit 40 controls the first switching element S1 to the eighth switching element S8 to start boosting after a predetermined time has passed since the low-voltage side relay RY2 was turned on. For example, the predetermined time is set to about five times the time constant τ. In this comparative example, it is set to about 1 second. Control unit 40 boosts the voltage of low-voltage side DC power supply 2 (100 V in this embodiment) to the voltage of high-voltage side DC power supply 1 (300 V in this embodiment).

When the voltage inside the high-voltage side relay RY1 of the DC bus reaches the voltage of the high-voltage side DC power supply 1, the control unit 40 ends boosting. In this state, the voltage of the high-voltage side capacitor C3 is 300V, the voltage of each of the first dividing capacitor C4a and the second dividing capacitor C4b is 150V, and the voltage of each of the first flying capacitor C5a and the second flying capacitor C5b is 75V. .

The control unit 40 turns on the high-voltage side relay RY1 after the completion of boosting. The control unit 40 controls the first switching element S1 to the eighth switching element S8 so that the voltage of the low-voltage side DC power supply 2 and the voltage of the DC bus are kept in a balanced state. After that, the control unit 40 starts charging control or discharging control of the low-voltage side DC power supply 2 .

In the startup sequence described above, after the low-voltage side relay RY2 is turned on, the voltage of at least one of the first flying capacitor C5a and the second flying capacitor C5b reaches the voltage of 1/4E when boosting is started. Otherwise, overcurrent may occur.

First, the voltage of the first dividing capacitor C4a is 1/2E, the voltage of the first flying capacitor C5a is 1/4E, the voltage of the first switching element S1 is 1/8E, the voltage of the second switching element S2 is 1/8E, Consider a state where the voltage of the third switching element S3 is 1/8E and the voltage of the fourth switching element S4 is 1/8E. In this state, the voltages of the first switching element S1 to the fourth switching element S4 are uniform, and when signals having the same duty width are input to the respective gate terminals of the first switching element S1 to the fourth switching element S4, The currents flowing through the first switching element S1 to the fourth switching element S4 become equal.

Next, the voltage of the first dividing capacitor C4a is 1/2E, the voltage of the first flying capacitor C5a is 1/8E, the voltage of the first switching element S1 is 3/16E, and the voltage of the second switching element S2 is 1/16E. , the voltage of the third switching element S3 is 1/16E, and the voltage of the fourth switching element S4 is 3/16E. In this state, the voltages of the first switching element S1 to the fourth switching element S4 are unequal, and if signals with the same duty width are input to the respective gate terminals of the first switching element S1 to the fourth switching element S4, However, the currents flowing through the first switching element S1 to the fourth switching element S4 become uneven.

The latter example is one of the situations in which the control unit 40 starts boosting before the voltages of the first flying capacitor C5a and the second flying capacitor C5b reach 1/4E after the low-voltage side relay RY2 is turned on. corresponds to one. In the latter example, the current control is unexpected, and an unexpected overcurrent may occur. Furthermore, at least one of the first switching element S1 and the fourth switching element S4 may exceed the withstand voltage.

FIG. 7 is a diagram showing waveform examples of the voltage Vc3 of the high-voltage side capacitor C3, the voltage Vc5 of the flying capacitor C5, and the reactor current _IL when the DC/DC converter 3 is started, according to the comparative example. In the example shown in FIG. 7, the voltage Vc5 of the flying capacitor C5 has not reached 25 V at the start of boosting, and a large spike occurs in the reactor current _IL at the start of boosting.

Next, the operation of the DC/DC converter 3 according to the embodiment at startup will be described. In the embodiment, after turning on the low-voltage side relay RY2, the control unit 40 causes the measured value of the voltage of the first flying capacitor C5a to reach the target value (1/4E) and the voltage of the second flying capacitor C5b to reach the target value (1/4E). After the measured value reaches the target value (1/4E), the first switching element S1 to the eighth switching element S8 are controlled to start boosting. A value obtained by adding a margin to 1/4E may be used as the target value. Other controls are the same as in the comparative example.

FIG. 8 is a diagram showing waveform examples of the voltage Vc3 of the high-voltage side capacitor C3, the voltage Vc5 of the flying capacitor C5, and the reactor current _IL when the DC/DC converter 3 is started, according to the embodiment. In the example shown in FIG. 8, the voltage Vc5 of the flying capacitor C5 reaches 25 V at the start of boosting, and the transient fluctuation of the reactor current _IL at the start of boosting is suppressed as compared with FIG.

In the first embodiment, as the flying capacitor section 30, a configuration in which two flying capacitor circuits with 3-level output are connected in series has been described. In this regard, the flying capacitor section 30 includes a plurality of switching elements connected in series and at least one flying capacitor, and the high-voltage side DC power supply 1 is connected to both ends of the plurality of switching elements, and the plurality of switching elements are connected to each other. The configuration is not limited to that of the first embodiment as long as the configuration is such that the low-voltage side DC power supply 2 is connected to both ends of some of the plurality of switching elements.

FIG. 9 is a diagram for explaining the configuration of the DC/DC converter 3 according to the second embodiment. In the second embodiment, the upper first flying capacitor circuit includes a first switching element S1, a second switching element S2, a third switching element S3, a fourth switching element S4, a fifth switching element S5, and a sixth switching element S6. , a seventh switching element S7, an eighth switching element S8, a first flying capacitor C5a, a third flying capacitor C6a, and a fifth flying capacitor C7a.

The first flying capacitor C5a is connected between a connection point between the first switching element S1 and the second switching element S2 and a connection point between the seventh switching element S7 and the eighth switching element S8. The third flying capacitor C6a is connected between a connection point between the second switching element S2 and the third switching element S3 and a connection point between the sixth switching element S6 and the seventh switching element S7. The fifth flying capacitor C7a is connected between a connection point between the third switching element S3 and the fourth switching element S4 and a connection point between the fifth switching element S5 and the sixth switching element S6. The first flying capacitor C5a, the third flying capacitor C6a, and the fifth flying capacitor C7a are charged and discharged by the first switching element S1 to the eighth switching element S8.

The first outer flying capacitor C5a is controlled to maintain a voltage of 3/8E, the third middle flying capacitor C6a is controlled to maintain a voltage of 2/8E, and the fifth inner flying capacitor C7a is controlled to maintain a voltage of 2/8E. It is controlled to maintain a voltage of 1/8E.

The second flying capacitor circuit on the lower side includes a ninth switching element S9, a tenth switching element S10, an eleventh switching element S11, a twelfth switching element S12, a thirteenth switching element S13, a fourteenth switching element S14, and a fifteenth switching element. S15, a sixteenth switching element S16, a second flying capacitor C5b, a fourth flying capacitor C6b, and a sixth flying capacitor C7b.

The second flying capacitor C5b is connected between a connection point between the ninth switching element S9 and the tenth switching element S10 and a connection point between the fifteenth switching element S15 and the sixteenth switching element S16. The fourth flying capacitor C6b is connected between a connection point between the tenth switching element S10 and the eleventh switching element S11 and a connection point between the fourteenth switching element S14 and the fifteenth switching element S15. The sixth flying capacitor C7b is connected between a connection point between the 11th switching element S11 and the 12th switching element S12 and a connection point between the 13th switching element S13 and the 14th switching element S14. The second flying capacitor C5b, the fourth flying capacitor C6b, and the sixth flying capacitor C7b are charged and discharged by the ninth switching element S9 to the sixteenth switching element S16.

The second outer flying capacitor C5b is controlled to maintain a voltage of 3/8E, the fourth middle flying capacitor C6b is controlled to maintain a voltage of 2/8E, and the sixth inner flying capacitor C7b is controlled to maintain a voltage of 2/8E. It is controlled to maintain a voltage of 1/8E.

In order to simplify the drawing in FIG. 9, the voltage sensor for the flying capacitor is omitted. In practice, a voltage sensor that measures voltages of the first flying capacitor C5a, the second flying capacitor C5b, the third flying capacitor C6a, the fourth flying capacitor C6b, the fifth flying capacitor C7a, and the sixth flying capacitor C7b is be provided. Each voltage sensor outputs the measured voltage value to the control unit 40 .

Using a three-stage flying capacitor circuit as shown in FIG. 9, seven levels (E, 3/4E, 5/8E, 1/2E, 3/8E, 1/4E, 0) voltages can be generated.

In the second embodiment, after turning on the low-voltage side relay RY2, the control unit 40 sets the measured value of the voltage of the first flying capacitor C5a to the target value (3/8E), and measures the voltage of the second flying capacitor C5b. value is the target value (3/8E), the measured value of the voltage of the third flying capacitor C6a is the target value (2/8E), and the measured value of the voltage of the fourth flying capacitor C6b is the target value (2/8E). , after the measured value of the voltage of the fifth flying capacitor C7a reaches the target value (1/8E) and the measured value of the voltage of the sixth flying capacitor C7b reaches the target value (1/8E), the first switching element S1--The sixteenth switching element S16 is controlled to start boosting. Other controls are the same as in the first embodiment.

FIG. 10 shows an N (N is a natural number) stage flying capacitor circuit. , S13, S12, S1, S2, S3, S4, S42, S43, . of flying capacitors C11, C12, C13, . . . , C1n. The innermost flying capacitor C11 is connected in parallel with the two switching elements S2 and S3 and controlled to maintain a voltage of 1/(2N+2)E. The second flying capacitor C12 from the inside is connected in parallel to the four switching elements S1, S2, S3 and S4 and controlled to maintain a voltage of 2/(2N+2)E. The third flying capacitor C13 from the inside is connected in parallel to the six switching elements S12, S1, S2, S3, S4 and S42 and controlled to maintain a voltage of 3/(2N+2)E. The outermost flying capacitor C1n is for 2N S1(n-1), . are connected in parallel and controlled to maintain a voltage of N/(2N+2)E.

Using an N-stage flying capacitor circuit as shown in FIG. 10, it is possible to generate a voltage of (2N+1) levels between the midpoint of the first flying capacitor circuit and the midpoint of the second flying capacitor circuit. Become.

As the number of stages of the flying capacitor circuit increases, inexpensive switching elements with low withstand voltage can be used, but the number of switching elements used increases. Therefore, the designer should determine the optimal number of stages of the flying capacitor circuit in consideration of the total cost and total conversion efficiency. Also, in applications where the voltage of the high voltage side DC power supply 1 exceeds 1000V or 10000V, it is effective to increase the number of stages of the flying capacitor circuit in order to lower the breakdown voltage of each switching element.

FIG. 11 is a diagram showing a configuration in which an inrush current prevention circuit is connected in parallel with the low voltage side relay RY2 of the DC/DC converter 3 shown in FIG. The inrush current prevention circuit is configured by serially connecting a limiting resistor Ri, a diode Di, and a switching element S20. In the example shown in FIG. 11, the switching element S20 is composed of an N-channel MOSFET. The diode Di has an anode terminal connected to the limiting resistor Ri and a cathode terminal connected to the drain terminal of the switching element S20. A source terminal of the switching element S20 is connected to a connection point between the low voltage side relay RY2 and the reactor L1. A relay may be used instead of the N-channel MOSFET.

When activating the DC/DC converter 3, the control unit 40 turns on the switching element S20 before turning on the low voltage side relay RY2. In this state, the second low-voltage side capacitor C2, the high-voltage side capacitor C3, the first divided capacitor C4a, the second divided capacitor C4b, the first flying capacitor C5a, and the second flying capacitor C5b are driven by the current limited by the limiting resistor Ri. charged respectively.

After the switching element S20 is turned on, the control unit 40 turns on the low-voltage side relay RY2 when the first set time elapses. The control unit 40 turns off the switching element S20 when the second set time elapses after turning on the low-voltage side relay RY2. As a result, it is possible to prevent an inrush current when the DC/DC converter 3 is started.

In the above description, when the DC/DC converter 3 is started, after the control unit 40 turns on the low-voltage side relay RY2, after the measured values of the voltages of the flying capacitors C5a and C5b reach the target values, , an example of starting the switching operation (that is, boosting operation) of the flying capacitor unit 30 has been described. In this regard, the control unit 40 may start the switching operation of the flying capacitor unit 30 after a waiting time longer than the originally necessary waiting time derived from the time constant has elapsed after turning on the low-voltage side relay RY2. .

The charging time constant τ of the first flying capacitor C5a is defined by the product of the resistance components of the second switching element S2 and the third switching element S3 and the capacitance of the first flying capacitor C5a. The charging time constant τ of the second flying capacitor C5b is defined by the product of the resistance components of the sixth switching element S6 and the seventh switching element S7 and the capacitance of the second flying capacitor C5b.

The originally necessary standby time derived from the time constant after turning on the low-voltage side relay RY2 is about five times the time constant. A waiting time longer than the originally required waiting time derived from the time constant is set to a time (for example, 2 seconds in the embodiment) that is 1.5 times or more the originally required waiting time derived from the time constant. good too. As a result, it is possible to suppress the start of boosting before the measured values of the voltages of the flying capacitors C5a and C5b reach the respective target values, thereby suppressing the occurrence of overcurrent.

(a) the condition that the measured values of the voltages of the flying capacitors C5a and C5b reach the respective target values, and (b) the condition that the waiting time longer than the originally required waiting time derived from the time constant has elapsed, are ANDed. may be used. In this case, after turning on the low voltage side relay RY2, the control unit 40 starts the switching operation of the flying capacitor unit 30 after satisfying both the condition (a) and the condition (b).

After turning on the low-voltage side relay RY2, if the measured voltage values of the flying capacitors C5a and C5b do not reach the respective target values after the set time has elapsed, the control unit 40 turns on the DC/DC converter 3. It may be determined that an abnormality has occurred. As the set time, the originally required waiting time derived from the time constant can be used. As the abnormality, an abnormality (for example, welding) of the low-voltage side relay RY2, an open failure of at least one of the first switching element S1 to the eighth switching element S8, and the like can be considered.

As described above, in the present embodiment, after turning on the low-voltage side relay RY2, the timing for starting the switching operation of the flying capacitor section 30 is determined based on the voltages of the flying capacitors C5a and C5b. Thereby, the DC/DC converter 3 can be started safely.

The present disclosure has been described above based on the embodiments. It is to be understood by those skilled in the art that the embodiment is an example, and that various modifications are possible in the combination of each component and each treatment process, and such modifications are also within the scope of the present disclosure. .

FIG. 12 is a diagram for explaining the configuration of the DC/DC conversion device 3 according to the modification of the first embodiment. In the DC/DC converter 3 shown in FIG. 1, the reactor L1 is connected between the positive terminal of the low-voltage DC power supply 2 and the middle point of the first flying capacitor circuit. In this regard, in the modification shown in FIG. 12, the first reactor L1 is connected between the positive terminal of the low-voltage DC power supply 2 and the midpoint of the first flying capacitor circuit, and the negative terminal of the low-voltage DC power supply 2 and the first reactor L1 are connected. A second reactor L2 is connected between the midpoints of the two flying capacitor circuits. The first reactor L1 and the second reactor L2 may be composed of magnetically coupled reactors having a common core. In this case, the magnetic fluxes of the first reactor L1 and the second reactor L2 can be strengthened each other during energization.

Reactor L1 may be connected between the negative terminal of low-voltage DC power supply 2 and the middle point of the second flying capacitor circuit. In this way, the reactor L1 includes a path connecting between the positive terminal of the low-voltage DC power supply 2 and the midpoint of the first flying capacitor circuit, a negative terminal of the low-voltage DC power supply 2 and the middle point of the second flying capacitor circuit. It is sufficient if it is inserted in at least one of the routes connecting the points.

The high-voltage side relay RY1 described above is an example of a switch circuit provided between the high-voltage side capacitor C3 and the high-voltage side DC power supply 1 . For example, an N-channel MOSFET whose source terminal is connected to the high voltage side capacitor C3 and whose drain terminal is connected to the high voltage side DC power supply 1 may be used as the switch circuit. In this case, the voltage of the high voltage side capacitor C3 can be clamped so that the voltage of the high voltage side capacitor C3 does not become higher than the voltage E of the high voltage side DC power supply 1 . Also, a bidirectional switch in which two N-channel MOSFETs are connected in series in opposite directions may be used as the switch circuit. In this case, in the off state, current from either direction can be cut off, similar to the case of using a relay.

The low-voltage side relay RY2 described above is also an example of a switch circuit provided between the low-voltage side DC power supply 2 and the reactor L1. As the switch circuit, an N-channel MOSFET or a bidirectional switch may be used as in the high voltage side relay RY1.

Also, like the switch circuit on the low-voltage side in FIG. 11, a rush current prevention circuit may be connected in parallel to the switch circuit on the high-voltage side.

Note that the embodiment may be specified by the following items.

[Item 1]
A high-voltage side capacitor (3) connected in parallel with the high-voltage side DC power supply (1);
A plurality of switching elements (S1-S8) connected in series and at least one flying capacitor (C5a, C5b) are included, and the high-voltage side DC power supply is connected across the plurality of switching elements (S1-S8) connected in series. (1) is connected, and a low-voltage side DC power supply (2) is connected to both ends of some of the plurality of switching elements (S3-S6) constituting the plurality of switching elements (S1-S8) connected in series. , a flying capacitor section (30);
At least one reactor (L1) inserted in a path between both ends of the low-voltage side DC power supply (2) and both ends of the plurality of switching elements (S3-S6),
a low-voltage side switch circuit (RY2) provided between the low-voltage side DC power supply (2) and the reactor (L1);
voltage sensors (41, 42) for measuring voltages of the flying capacitors (C5a, C5b);
A plurality of switching elements (S1-S8) connected in series, and a control section (40) for controlling the low-voltage side switch circuit (RY2),
When the DC/DC converter (3) is activated, the control section (40) turns on the low-voltage side switch circuit (RY2), and then, based on the voltages of the flying capacitors (C5a, C5b), , determining the timing to start the switching operation of the flying capacitor section (30);
DC/DC converter (3).
According to this, the DC/DC converter (3) can be started safely.
[Item 2]
After turning on the low-voltage side switch circuit (RY2), the control section (40) controls the flying capacitor section ( 30) to initiate the switching operation;
A DC/DC converter (3) according to item 1.
According to this, it is possible to suppress the occurrence of overcurrent due to insufficient voltage of the flying capacitors (C5a, C5b) at the start of the switching operation of the flying capacitor section (30).
[Item 3]
further comprising a high voltage side switch circuit (RY1) provided between the high voltage side capacitor (3) and the high voltage side DC power supply (1),
After starting the switching operation of the flying capacitor section (30), the control section (40) increases the voltage of the high-voltage side capacitor (3) to a voltage corresponding to the voltage of the high-voltage side DC power supply (1). After that, the high voltage side switch circuit (RY1) is turned on,
A DC/DC converter (3) according to item 2.
According to this, it is possible to suppress the occurrence of overcurrent when the high voltage side switch circuit (RY1) is turned on.
[Item 4]
The flying capacitor section (30) is
A first switching element (S1), a second switching element (S2), a third switching element (S3), a fourth switching element (S4), a fifth switching element (S5), and a sixth switching element ( S6), a seventh switching element (S7) and an eighth switching element (S8);
A first switch connected between a connection point between the first switching element (S1) and the second switching element (S2) and a connection point between the third switching element (S3) and the fourth switching element (S4). a flying capacitor (C5a);
A second switching element connected between a connection point between the fifth switching element (S5) and the sixth switching element (S6) and a connection point between the seventh switching element (S7) and the eighth switching element (S8). a flying capacitor (C5b);
a positive terminal of the low-voltage side DC power supply (2) is electrically connected to a connection point between the second switching element (S2) and the third switching element (S3);
a negative terminal of the low-voltage DC power supply (2) is electrically connected to a connection point between the sixth switching element (S6) and the seventh switching element (S7);
Diodes (D1-D8) are formed or connected in antiparallel to the first switching element (S1) to the eighth switching element (S8), respectively,
When the DC/DC conversion device (3) is activated, the control unit (40)
turning on the low-voltage side switch circuit (RY2) while the first switching element (S1) to the eighth switching element (S8) and the high-voltage side switch circuit (RY1) are in an OFF state;
After each of the measured values of the voltage of the first flying capacitor (C5a) and the voltage of the second flying capacitor (C5b) reaches a voltage corresponding to 1/4 of the voltage of the low-voltage side DC power supply (2), Starting switching control of the first switching element (S1) - the eighth switching element (S8);
A DC/DC converter (3) according to item 3.
According to this, the 3-level multi-level DC/DC converter (3) can be started safely.
[Item 5]
The control unit (40)
The second switching element (S2), the fourth switching element (S4), the fifth switching element (S5), and the seventh switching element (S7) are turned on, and the first switching element (S1), the a first mode in which the third switching element (S3), the sixth switching element (S6) and the eighth switching element (S8) are controlled to be off;
The first switching element (S1), the third switching element (S3), the sixth switching element (S6) and the eighth switching element (S8) are turned on, and the second switching element (S2) and the second switching element (S2) are switched on. a second mode for controlling the four switching elements (S4), the fifth switching element (S5) and the seventh switching element (S7) to be in an off state;
The first switching element (S1), the second switching element (S2), the seventh switching element (S7) and the eighth switching element (S8) are turned on, and the third switching element (S3) and the third switching element (S3) are switched on. a third mode for controlling the four switching elements (S4), the fifth switching element (S5) and the sixth switching element (S6) to be in an off state;
The third switching element (S3), the fourth switching element (S4), the fifth switching element (S5) and the sixth switching element (S6) are turned on, and the first switching element (S1) and the first switching element (S1) are switched on. Among the four modes of the second switching element (S2) and the fourth mode in which the seventh switching element (S7) and the eighth switching element (S8) are turned off,
The voltage of the low-voltage side DC power supply (2) is boosted using the three modes of the first mode, the second mode, and the third mode or the fourth mode,
A DC/DC converter (3) according to item 4.
According to this, the three modes can be combined to enable a highly efficient boosting operation.
[Item 6]
After turning on the low-voltage side switch circuit (RY2), the control unit (40) causes the measured values of the voltages of the flying capacitors (C5a, C5b) to reach the target values even after the set time has passed. If not, it is determined that an abnormality has occurred.
A DC/DC converter (3) according to any one of items 1 to 5.
According to this, failure detection can be performed when the DC/DC converter (3) is activated.

1 high-voltage side DC power supply, 2 low-voltage side DC power supply, 3 DC/DC converter, 30 flying capacitor section, 40 control section, 41-43 voltage sensor, S1-S16, S20 switching element, D1-D16, Di diode, C1 First low voltage side capacitor C2 Second low voltage side capacitor C3 High voltage side capacitor C4a First dividing capacitor C4b Second dividing capacitor C5a First flying capacitor C5b Second flying capacitor C6a Third flying capacitor C6b Second 4 flying capacitors, C7a fifth flying capacitor, C7b sixth flying capacitor, L1 reactor, Ri limiting resistor, RY1 high voltage side relay, RY2 low voltage side relay.

Claims (6)

a high-voltage side capacitor connected in parallel to a high-voltage side DC power supply;
It includes a plurality of series-connected switching elements and at least one flying capacitor, and the high-voltage side DC power supply is connected to both ends of the plurality of series-connected switching elements to form the plurality of series-connected switching elements. a flying capacitor unit in which a low-voltage side DC power supply is connected to both ends of some of the plurality of switching elements;
at least one reactor inserted in a path between both ends of the low-voltage side DC power supply and between both ends of the plurality of switching elements;
a low-voltage side switch circuit provided between the low-voltage side DC power supply and the reactor;
a voltage sensor that measures each voltage of the flying capacitor;
a plurality of switching elements connected in series; and a control unit that controls the low voltage side switch circuit,
When the DC/DC converter is started, the control unit turns on the low-voltage side switch circuit, and then determines the timing of starting the switching operation of the flying capacitor unit based on the voltage of the flying capacitor. do,
DC/DC converter. After turning on the low-voltage side switch circuit, the control unit starts the switching operation of the flying capacitor unit after the measured value of each voltage of the flying capacitor reaches each target value.
The DC/DC conversion device according to claim 1. further comprising a high voltage side switch circuit provided between the high voltage side capacitor and the high voltage side DC power supply;
After starting the switching operation of the flying capacitor unit, the control unit turns on the high-voltage side switch circuit after the voltage of the high-voltage side capacitor rises to a voltage corresponding to the voltage of the high-voltage side DC power supply.
The DC/DC converter according to claim 2. The flying capacitor section is
a first switching element, a second switching element, a third switching element, a fourth switching element, a fifth switching element, a sixth switching element, a seventh switching element, and an eighth switching element connected in series;
a first flying capacitor connected between a connection point between the first switching element and the second switching element and a connection point between the third switching element and the fourth switching element;
a second flying capacitor connected between a connection point between the fifth switching element and the sixth switching element and a connection point between the seventh switching element and the eighth switching element;
a positive terminal of the low-voltage DC power supply is electrically connected to a connection point between the second switching element and the third switching element;
a negative terminal of the low-voltage DC power supply is electrically connected to a connection point between the sixth switching element and the seventh switching element;
a diode is formed or connected in inverse parallel to each of the first switching element and the eighth switching element,
When the DC/DC conversion device is activated, the control unit is configured to:
turning on the low-voltage side switch circuit when the first switching element, the eighth switching element, and the high-voltage side switch circuit are in an off state;
After each of the measured values of the voltage of the first flying capacitor and the voltage of the second flying capacitor reaches a voltage corresponding to 1/4 of the voltage of the low-voltage side DC power supply, the first switching element-the eighth initiating switching control of the switching element;
The DC/DC converter according to claim 3. The control unit
turning on the second switching element, the fourth switching element, the fifth switching element and the seventh switching element, and the first switching element, the third switching element, the sixth switching element and the eighth switching element; a first mode that controls the element to an off state;
turning on the first switching element, the third switching element, the sixth switching element and the eighth switching element and the second switching element, the fourth switching element, the fifth switching element and the seventh switching element a second mode that controls the off state of
turning on the first switching element, the second switching element, the seventh switching element and the eighth switching element and the third switching element, the fourth switching element, the fifth switching element and the sixth switching element a third mode that controls the off state of
turning on the third switching element, the fourth switching element, the fifth switching element and the sixth switching element and the first switching element, the second switching element, the seventh switching element and the eighth switching element of the four modes, a fourth mode for controlling the
The voltage of the low-voltage side DC power supply is boosted using the three modes of the first mode, the second mode, and the third mode or the fourth mode,
The DC/DC converter according to claim 4. After turning on the low-voltage side switch circuit, the control unit determines that an abnormality has occurred when the measured values of the voltages of the flying capacitors do not reach the respective target values within a set time. do,
A DC/DC converter according to any one of claims 1 to 5.

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