JP7503766B2 - Power Conversion Equipment - Google Patents

Description

The present invention relates to a power conversion device equipped with a DC/DC converter.

DC/DC converters and inverters are used in power conditioners connected to solar cells, storage batteries, fuel cells, etc. DC/DC converters and inverters are desired to be highly efficient, compact, and low cost. As a DC/DC converter to achieve this, a power conversion device has been disclosed in which a switched capacitor circuit (composed of four switching elements connected in series and two capacitors connected in series connected in parallel to the load) is connected to the rear of a reactor (see, for example, Patent Document 1).

In this power conversion device, the voltage applied to the power device is half the load voltage, which allows the withstand voltage of the switching elements to be lowered, making it possible to use low on-resistance metal-oxide-semiconductor field-effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs) with a low saturation voltage _Vsat . The use of these switching elements leads to high efficiency.

JP 2010-4726 A

However, for example, in power conditioners connected to a power grid, it is required by law to suppress leakage current. It is also necessary to reduce common mode noise. In the power conversion device described above, leakage current and common mode noise are likely to occur during the switching pattern in which the two capacitors in the switched capacitor circuit are charged.

Therefore, it is desirable to have a circuit configuration and switching pattern that is less likely to generate leakage current and common mode noise. If leakage current and common mode noise can be reduced, the number of countermeasure components required to suppress them can be reduced, leading to high efficiency, compact size, and low cost.

This disclosure has been made in light of these circumstances, and its purpose is to provide a highly efficient, compact, and low-cost power conversion device.

In order to solve the above problem, a power conversion device according to one aspect of the present disclosure includes a first flying capacitor circuit and a second flying capacitor circuit connected in series in parallel with a high-voltage side DC section, a positive terminal of a low-voltage side DC section and a first reactor connected between the midpoints of the first flying capacitor circuit, and a negative terminal of the low-voltage side DC section and a second reactor connected between the midpoints of the second flying capacitor circuit. The connection point between the first flying capacitor circuit and the second flying capacitor circuit is connected to the intermediate potential point of the high-voltage side DC section.

This disclosure makes it possible to realize a highly efficient, compact, and low-cost power conversion device.

1 is a diagram for explaining a configuration of a power conversion device according to a first embodiment; 11 is a diagram summarizing switching patterns of the first switching element to the eighth switching element of the power conversion device according to the first embodiment. FIG. 3(a) to 3(d) are circuit diagrams showing current paths for each switching pattern during boost operation. 4(a) to 4(d) are circuit diagrams showing current paths for each switching pattern during step-down operation. 11 is a timing chart showing an example of a switching pattern of a first switching element-an eighth switching element when a boost ratio is greater than two. 11 is a timing chart showing an example of a switching pattern of a first switching element-an eighth switching element when a boost ratio is smaller than two. 1 is a diagram for explaining a configuration of a power conversion device according to an application example of the first embodiment. FIG. FIG. 11 is a diagram for explaining the configuration of a power conversion device according to a second embodiment. 13 is a diagram for explaining the configuration of a power conversion device according to an application example of embodiment 2. FIG. 10A to 10C are diagrams showing configuration examples of flying capacitor circuits. FIG. 1 is a diagram showing a flying capacitor circuit having N stages (N is a natural number). FIG. 10 is a diagram for explaining the configuration of a power conversion device according to a first modified example. FIG. 11 is a diagram for explaining the configuration of a power conversion device according to Modification 2.

(Embodiment 1)
1 is a diagram for explaining the configuration of a power conversion device 3 according to the first embodiment. The power conversion device 3 according to the first embodiment is a bidirectional step-up/step-down DC/DC converter. The power conversion device 3 can step up the DC power supplied from the second DC power source 2 and supply it to the first DC power source 1. The power conversion device 3 can also step down the DC power supplied from the first DC power source 1 and supply it to the second DC power source 2. In this specification, it is assumed that the second DC power source 2 is a power source with a lower voltage than the first DC power source 1.

The second DC power source 2 may be, for example, a storage battery or an electric double-layer capacitor. The first DC power source 1 may be, for example, a DC bus to which a bidirectional DC/AC inverter is connected. In a power storage system application, the AC side of the bidirectional DC/AC inverter is connected to a commercial power system and an AC load. In an electric vehicle application, it is connected to a motor (with regenerative function). In a power storage system application, a DC/DC converter for a solar cell or a DC/DC converter for another storage battery may be further connected to the DC bus.

A first Y capacitor C6 is connected between the positive line of the second DC power supply 2 and the earth, and a second Y capacitor C7 is connected between the negative line of the second DC power supply 2 and the earth. The first Y capacitor C6 and the second Y capacitor C7 have the effect of reducing common mode noise by passing the common mode noise to the earth. Note that some second DC power supplies 2, such as solar panels, have parasitic capacitance to the earth, and this parasitic capacitance also has the effect of reducing common mode noise.

The power conversion device 3 according to the first embodiment includes a DC/DC conversion unit 30 and a control unit 40. The DC/DC conversion unit 30 includes an input capacitor C5, a first reactor L1, a second reactor L2, a first flying capacitor circuit 31, a second flying capacitor circuit 32, a first dividing capacitor C3, and a second dividing capacitor C4.

An input capacitor C5 is connected in parallel with the second DC power supply 2. A first dividing capacitor C3 and a second dividing capacitor C4 are connected in series between the positive bus and the negative bus of the first DC power supply 1. The first dividing capacitor C3 and the second dividing capacitor C4 divide the voltage E of the first DC power supply 1 in half and act as snubber capacitors to suppress surge voltages generated within the DC/DC conversion unit 30. In this specification, the configuration preceding the input capacitor C5 is called the low-voltage DC section, and the configuration following the first dividing capacitor C3 and the second dividing capacitor C4 is called the high-voltage DC section.

The first flying capacitor circuit 31 and the second flying capacitor circuit 32 are connected in series in parallel with the high-voltage side DC section. The first reactor L1 is connected in series between the positive terminal of the low-voltage side DC section and the midpoint of the first flying capacitor circuit 31. The second reactor L2 is connected in series between the negative terminal of the low-voltage side DC section and the midpoint of the second flying capacitor circuit 32. In this embodiment, reactors of the same specifications are used for the first reactor L1 and the second reactor L2. The connection point between the first flying capacitor circuit 31 and the second flying capacitor circuit 32 is connected to the intermediate potential point M of the high-voltage side DC section (the voltage division point of the first dividing capacitor C3 and the second dividing capacitor C4).

The first flying capacitor circuit 31 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 C1. The first switching element S1, the second switching element S2, the third switching element S3, and the fourth switching element S4 are connected in series and connected between the positive side bus of the high voltage DC section and the intermediate potential point M. The first flying capacitor C1 is connected between the connection point between the first switching element S1 and the second switching element S2 and the connection point between the third switching element S3 and the fourth switching element S4, and is charged and discharged by the first switching element S1-fourth switching element S4.

At the midpoint of the first flying capacitor circuit 31, a potential is generated in the range between the voltage E [V] of the first DC power supply 1 applied to the upper terminal of the first switching element S1 and 1/2E [V] applied to the lower terminal of the fourth switching element S4. The first flying capacitor C1 is initially charged (precharged) to a voltage of 1/4E [V], and charging and discharging are repeated with the voltage of 1/4E [V] at the center. Therefore, three levels of potential, roughly E [V], 3/4E [V], and 1/2E [V], are generated at the midpoint of the first flying capacitor circuit 31.

The second flying capacitor circuit 32 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 C2. The fifth switching element S5, the sixth switching element S6, the seventh switching element S7, and the eighth switching element S8 are connected in series and connected between the intermediate potential point M of the high voltage DC section and the negative bus. The second flying capacitor C2 is connected between the connection point between the fifth switching element S5 and the sixth switching element S6 and the connection point between the seventh switching element S7 and the eighth switching element S8, and is charged and discharged by the fifth switching element S5-eighth switching element S8.

At the midpoint of the second flying capacitor circuit 32, a potential is generated in the range 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. The second flying capacitor C2 is initially charged (precharged) to a voltage of 1/4E [V], and charging and discharging are repeated with the voltage of 1/4E [V] at the center. Therefore, three levels of potential, roughly 1/2E [V], 1/4E [V], and 0 [V], are generated at the midpoint of the second flying capacitor circuit 32.

The first diode D1 to the eighth diode D8 are formed/connected in anti-parallel to the first switching element S1 to the eighth switching element S8, respectively.

For the first switching element S1 to the eighth switching element S8, it is preferable to use switching elements with a lower withstand voltage than the voltage of the first DC power supply 1 and the second DC power supply 2. In the following embodiment, an example is assumed in which an N-channel MOSFET with a withstand voltage of 150 V is used for the first switching element S1 to the eighth switching element S8. In an N-channel MOSFET, a parasitic diode is formed from the source to the drain.

Note that IGBTs or bipolar transistors may be used for the first switching element S1 to the eighth switching element S8. In that case, no parasitic diodes are formed in the first switching element S1 to the eighth switching element S8, and external diodes are connected in inverse parallel to the first switching element S1 to the eighth switching element S8, respectively.

Although not shown in FIG. 1, a first voltage sensor that detects the voltage across the low-voltage DC section, a current sensor that detects the current flowing through the first reactor L1 or the second reactor L2, and a second voltage sensor that detects the voltage across the high-voltage DC section are provided, and the respective measured values are output to the control unit 40.

The control unit 40 controls the first flying capacitor circuit 31 and the second flying capacitor circuit 32 to transmit DC power from the low-voltage side DC section to the high-voltage side DC section by step-up operation. It can also transmit DC power from the high-voltage side DC section to the low-voltage side DC section by step-down operation. More specifically, the control unit 40 supplies a drive signal (PWM (Pulse Width Modulation) signal) to the gate terminals of the first switching element S1 to the eighth switching element S8 to control the on/off of the first switching element S1 to the eighth switching element S8, thereby transmitting power in both directions by step-up operation or step-down operation.

The configuration of the control unit 40 can be realized by a combination of hardware and software resources, or by hardware resources alone. Analog elements, 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.

Figure 2 is a diagram summarizing the switching patterns of the first switching element S1 to the eighth switching element S8 of the power conversion device 3 according to the first embodiment. In the switching patterns shown in Figure 2, the pair of the first switching element S1 and the eighth switching element S8 is complementary to the pair of the fourth switching element S4 and the fifth switching element S5. Also, the pair of the second switching element S2 and the seventh switching element S7 is complementary to the pair of the third switching element S3 and the sixth switching element S6.

The control unit 40 executes the voltage step-up operation or the voltage step-down operation using four modes.
In mode a, the control unit 40 controls the second switching element S2, the fourth switching element S4, the fifth switching element S5, and the seventh switching element S7 to be in the on state, and the first switching element S1, the third switching element S3, the sixth switching element S6, and the eighth switching element S8 to be in the off state. In mode a, the voltage between the midpoint of the first flying capacitor circuit 31 and the midpoint of the second flying capacitor circuit 32 (i.e., the input/output voltage _VL on the low-voltage side) becomes 1/2E.

In mode b, the control unit 40 controls the first switching element S1, the third switching element S3, the sixth switching element S6, and the eighth switching element S8 to be in the ON state, and the second switching element S2, the fourth switching element S4, the fifth switching element S5, and the seventh switching element S7 to be in the OFF state. In mode b, the input/output voltage _VL on the low-voltage side of the first flying capacitor circuit 31 and the second flying capacitor circuit 32 becomes 1/2E.

In mode c, the control unit 40 controls the first switching element S1, the second switching element S2, the seventh switching element S7, and the eighth switching element S8 to be in the ON state, and the third switching element S3, the fourth switching element S4, the fifth switching element S5, and the sixth switching element S6 to be in the OFF state. In mode c, the input/output voltage _VL on the low-voltage side of the first flying capacitor circuit 31 and the second flying capacitor circuit 32 becomes E.

In mode d, the control unit 40 controls the third switching element S3, the fourth switching element S4, the fifth switching element S5, and the sixth switching element S6 to be in the ON state, and the first switching element S1, the second switching element S2, the seventh switching element S7, and the eighth switching element S8 to be in the OFF state. In mode d, the input/output voltage _VL on the low-voltage side of the first flying capacitor circuit 31 and the second flying capacitor circuit 32 becomes 0.

Figures 3(a)-(d) are circuit diagrams showing the current paths of each switching pattern during boost operation. Figures 4(a)-(d) are circuit diagrams showing the current paths of each switching pattern during buck operation. Note that to simplify the drawings, MOSFETs are depicted with simple switch symbols.

Figure 3(a) shows the current path in mode a during boost operation, Figure 3(b) shows the current path in mode b during boost operation, Figure 3(c) shows the current path in mode c during boost operation, and Figure 3(d) shows the current path in mode d during boost operation. Similarly, Figure 4(a) shows the current path in mode a during buck operation, Figure 4(b) shows the current path in mode b during buck operation, Figure 4(c) shows the current path in mode c during buck operation, and Figure 4(d) shows the current path in mode d during buck operation.

The direction of the current is opposite during boost operation and step-down operation. In mode a, the first flying capacitor C1 and the second flying capacitor C2 are charging during boost operation as shown in FIG. 3(a), but the first flying capacitor C1 and the second flying capacitor C2 are discharging during step-down operation as shown in FIG. 4(a). In mode b, the first flying capacitor C1 and the second flying capacitor C2 are discharging during boost operation as shown in FIG. 3(b), but the first flying capacitor C1 and the second flying capacitor C2 are charging during step-down operation as shown in FIG. 4(b).

When transmitting power from the low voltage DC section to the high voltage DC section by step-up operation, the control unit 40 sets a positive current command value and controls the duty ratio (on time) of the first switching element S1 to the eighth switching element S8 so that the measured value of the current flowing through the first reactor L1 (which may be the second reactor L2) maintains the positive current command value. Conversely, when transmitting power from the high voltage DC section to the low voltage DC section by step-down operation, the control unit 40 sets a negative current command value and controls the duty ratio (on time) of the first switching element S1 to the eighth switching element S8 so that the measured value of the current flowing through the first reactor L1 maintains the negative current command value.

When the ratio between the voltage of the low-voltage side DC section and the voltage of the high-voltage side DC section (hereinafter defined as the step-up ratio) is smaller than a set value, the control unit 40 transmits power using modes a, b, and c. When the step-up ratio is larger than the set value, the control unit 40 transmits power using modes a, b, and d. When the step-up ratio matches the set value, the control unit 40 transmits power using modes a and b.

The voltage of the low-voltage side DC section and the voltage of the high-voltage side DC section are measured by voltage sensors. The above set value is set according to the ratio of the total voltage 1/2E of the voltages of the first flying capacitor C1 and the second flying capacitor C2 to the voltage E of the first DC power source 1. In this embodiment, the above set value is set to 2. Note that when the ratio of the voltage of the low-voltage side DC section and the voltage of the high-voltage side DC section is defined as a step-down ratio, the above set value is set to 1/2.

The control unit 40 generates a duty ratio so that the current command value matches the measured value of the current flowing through the first reactor L1 and the voltages of the first flying capacitor C1 and the second flying capacitor C2 are each 1/4E. Specifically, the control unit 40 increases the duty ratio as the current flowing through the first reactor L1 is smaller than the current command value, and decreases the duty ratio as the current flowing through the first reactor L1 is larger than the current command value.

Figure 5 is a timing chart showing an example of the switching pattern of the first switching element S1 to the eighth switching element S8 when the step-up ratio is greater than 2. Figure 6 is a timing chart showing an example of the switching pattern of the first switching element S1 to the eighth switching element S8 when the step-up ratio is less than 2. The control examples shown in Figures 5 and 6 show a control example using a double carrier drive method. In the double carrier drive method, two carrier signals (triangular waves in Figures 5 and 6) that are 180 degrees out of phase with each other are used. The duty ratio duty is a threshold value that is compared with the two carrier signals. When the step-up ratio is greater than 2, the duty ratio duty takes a value in the range of 0.5 to 1.0, and when the step-up ratio is less than 2, the duty ratio duty takes a value in the range of 0.0 to 0.5.

The first gate signal to be supplied to the first switching element S1 and the eighth switching element S8, and the fourth gate signal to be supplied to the fourth switching element S4 and the fifth switching element S5 are generated based on the result of comparing the thick carrier signal with the duty ratio (duty). Specifically, in the region where the thick carrier signal is higher than the duty ratio (duty), the first gate signal is on and the fourth gate signal is off. In the region where the thick carrier signal is lower than the duty ratio (duty), the first gate signal is off and the fourth gate signal is on. The first gate signal and the fourth gate signal are complementary. Note that when the first gate signal and the fourth gate signal switch on/off, a dead time period is set during which the first gate signal and the fourth gate signal are simultaneously off.

Based on the comparison result between the carrier signal of the thin line and the duty ratio (duty), a second gate signal to be supplied to the second switching element S2 and the seventh switching element S7, and a third gate signal to be supplied to the third switching element S3 and the sixth switching element S6 are generated. Specifically, in an area where the carrier signal of the thin line is higher than the duty ratio (duty), the second gate signal is on and the third gate signal is off. In an area where the carrier signal of the thin line is lower than the duty ratio (duty), the second gate signal is off and the third gate signal is on. The second gate signal and the third gate signal are complementary. Note that when the second gate signal and the third gate signal are switched on/off, a dead time period is set during which the second gate signal and the third gate signal are simultaneously off.

When the boost ratio is greater than 2, the control unit 40 alternates between modes a and b, inserting mode d between the two. That is, the control unit 40 switches between modes in the following order: mode a → mode d → mode b → mode d → mode a → mode d → mode b → mode d... While the duty ratio does not change, the periods of mode a and mode b are equal, and the voltages of the first flying capacitor C1 and the second flying capacitor C2 are each maintained at 1/4E. When the boost ratio is greater than 2, the higher the duty ratio, the longer the period of mode d relative to the periods of mode a and mode b, and the amount of energy transmitted increases.

When the boost ratio is less than 2, the control unit 40 alternates between modes a and b, inserting mode c between the two. That is, the control unit 40 switches between modes in the following order: 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 C1 and the second flying capacitor C2 are each maintained at 1/4E. When the boost ratio is less than 2, the higher the duty ratio (duty), the shorter the period of mode c relative to the periods of mode a and mode b, and the greater the amount of energy transmitted.

If the boost ratio is ideally maintained at 2x and the voltages of the first flying capacitor C1 and the second flying capacitor C2 are ideally maintained at 1/4E, respectively, the duty ratio duty will be maintained at 0.5.

When the total voltage of the voltage of the first flying capacitor C1 and the voltage of the second flying capacitor C2 falls below 1/2E, the control unit 40 increases the time of the charging mode of mode a or mode b to bring the total voltage closer to 1/2E. Conversely, when the total voltage of the voltage of the first flying capacitor C1 and the voltage of the second flying capacitor C2 exceeds 1/2E, the control unit 40 increases the time of the discharging mode of mode a or mode b to bring the total voltage closer to 1/2E.

The control unit 40 can also cause the DC/DC conversion unit 30 to operate as a normal boost chopper by alternately switching between mode c and mode d without using the first flying capacitor C1 and the second flying capacitor C2. In this case, the operation mode is not switched according to the boost ratio.

Figure 7 is a diagram for explaining the configuration of the power conversion device 3 according to an application example of the first embodiment. In the application example of the first embodiment, the first reactor L1 and the second reactor L2 are configured as the first magnetic coupling reactor Lc1 with a common core. The first reactor L1 and the second reactor L2 are installed on a common core so that a closed magnetic circuit is formed in a direction in which the magnetic fluxes are mutually strengthened when the first reactor L1 and the second reactor L2 are energized. During the step-up operation shown in Figures 3(a)-(d) or the step-down operation shown in Figures 4(a)-(d), the current flowing through the first reactor L1 and the current flowing through the second reactor L2 generate mutual inductance so as to mutually strengthen the magnetic fluxes.

As described above, according to the first embodiment, in a DC/DC conversion device using the first flying capacitor circuit 31 and the second flying capacitor circuit 32 connected in series, the circuit configuration is symmetrical above and below with respect to the earth and the midpoint M, thereby reducing leakage current and common mode noise. Furthermore, by making the first reactor L1 and the second reactor L2 the same specifications, the potential of the first Y capacitor C6 and the second Y capacitor C7 with respect to the earth can be made the same, and leakage current and common mode noise can be reduced. This makes it possible to reduce the number of parts for preventing leakage current and common mode noise, and to achieve high efficiency, miniaturization, and cost reduction of the DC/DC conversion device. Furthermore, by making the first reactor L1 and the second reactor L2 the same specifications, mass production effects are generated, and the unit price of the reactor can be reduced.

In addition, since low-voltage switching elements (e.g., MOSFETs with a voltage resistance of 150V) can be used for the first switching element S1 through the eighth switching element S8, the conduction loss of the switching elements can be reduced. Furthermore, the use of low-voltage switching elements reduces heat generation, allowing the heat dissipation components to be made smaller. Furthermore, the use of low-voltage switching elements allows for high frequencies with low switching loss, and passive components can also be made smaller.

In addition, if the first magnetic coupling reactor Lc1 is configured so that the internal magnetic flux is strengthened during voltage step-up or voltage step-down operation, rather than configuring the first reactor L1 and the second reactor L2 as separate bodies, the reactor can be made smaller, lighter, and less expensive.

(Embodiment 2)
8 is a diagram for explaining the configuration of the power conversion device 3 according to the second embodiment. The power conversion device 3 according to the second embodiment further includes a DC/AC conversion unit 35. The DC/AC conversion unit 35 includes a DC bus capacitor C8, a DC/AC inverter 36, and an output filter. The output filter is an LC filter including a third reactor L3, a fourth reactor L4, and an output capacitor C9.

The DC side of the DC/AC conversion unit 35 is connected to the DC/DC conversion unit 30 via a DC bus. A DC bus capacitor C8 is connected to the DC bus, and the voltage of the DC bus is stabilized. The voltage of the DC bus becomes the voltage E of the high-voltage side DC section described above.

The AC side of the DC/AC conversion unit 35 is connected to the single-phase three-wire system 4 via three wires: a first voltage wire (U phase), a neutral wire (O phase), and a second voltage wire (V phase). The neutral wire (O phase) is connected to earth.

The single-phase three-wire system 4 is divided into a first system power supply 4a (e.g., AC 100V) and a second system power supply 4b (e.g., AC 100V) by the neutral line (O phase). A third Y capacitor C10 is connected between the first voltage line (U phase) and the neutral line (O phase), and a fourth Y capacitor C21 is connected between the neutral line (O phase) and the second voltage line (V phase). The third Y capacitor C10 and the fourth Y capacitor C20 have the effect of reducing common mode noise by passing the common mode noise to earth.

The third reactor L3 constituting the output filter is inserted in the first voltage line (U phase), and the fourth reactor L4 is inserted in the second voltage line (V phase). The output capacitor C9 is connected between the first voltage line (U phase) and the second voltage line (V phase). In this embodiment, reactors of the same specifications are used for the third reactor L3, the fourth reactor L4, the first reactor L1, and the second reactor L2.

The DC/AC inverter 36 is a bidirectional inverter that converts the DC power input from the DC/DC conversion unit 30 via the DC bus into AC power. The output filter attenuates harmonic components of the voltage and current output from the DC/AC inverter 36, and outputs them closer to the sine wave of system 4. The DC/AC inverter 36 also converts the AC power input from system 4 via the output filter into DC power and outputs it to the DC bus.

Figure 9 is a diagram for explaining the configuration of the power conversion device 3 according to an application example of the second embodiment. In the application example of the second embodiment, the third reactor L3 and the fourth reactor L4 are configured with the second magnetic coupling reactor Lc2 that shares a core. The third reactor L3 and the fourth reactor L4 are installed on a common core so that a closed magnetic circuit is formed in a direction in which the magnetic fluxes are mutually strengthened when the third reactor L3 and the fourth reactor L4 are energized. In the second embodiment, magnetic coupling reactors of the same specifications are used for the second magnetic coupling reactor Lc2 included in the DC/AC conversion unit 35 and the first magnetic coupling reactor Lc1 included in the DC/DC conversion unit 30.

As described above, according to the second embodiment, the same effects as those of the first embodiment are achieved. Furthermore, by standardizing the reactor used in the DC/DC conversion unit 30 and the reactor used in the DC/AC conversion unit 35, mass production efficiency is further increased, and the unit price of the reactor can be further reduced.

The present disclosure has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of each component and each processing process, and that such modifications are also within the scope of the present disclosure.

In the above-mentioned first and second embodiments, a one-stage flying capacitor circuit using four switching elements connected in series and one flying capacitor is given as an example of a configuration of a flying capacitor circuit. In this regard, a flying capacitor circuit with more stages can also be used.

Figures 10(a)-(c) are diagrams showing configuration examples of a flying capacitor circuit. Figure 10(a) shows a one-stage flying capacitor circuit. The flying capacitor circuit shown in Figure 10(a) has the same circuit configuration as that described in the first and second embodiments above.

Figure 10(b) shows a two-stage flying capacitor circuit. The two-stage flying capacitor circuit includes six switching elements S12, S1, S2, S3, S4, and S42 connected in series, and two flying capacitors C11 and C12. The innermost flying capacitor C11 is connected in parallel to the two switching elements S2 and S3, and is controlled to maintain a voltage of 1/6E. In this specification, E represents the voltage of the high-voltage side DC section. The second innermost flying capacitor C12 is connected in parallel to the four switching elements S1, S2, S3, and S4, and is controlled to maintain a voltage of 1/6E.

Figure 10(c) shows a three-stage flying capacitor circuit. The three-stage flying capacitor circuit includes six switching elements S13, S12, S1, S2, S3, S4, S42, and S43 connected in series, and three flying capacitors C11, C12, and C13. The innermost flying capacitor C11 is connected in parallel to the two switching elements S2 and S3, and is controlled to maintain a voltage of 1/8E. The second innermost flying capacitor C12 is connected in parallel to the four switching elements S1, S2, S3, and S4, and is controlled to maintain a voltage of 2/8E. The third innermost flying capacitor C13 is connected in parallel to the six switching elements S12, S1, S2, S3, S4, and S42, and is controlled to maintain a voltage of 3/8E.

Figure 11 shows an N-stage (N is a natural number) flying capacitor circuit. The N-stage flying capacitor circuit includes (2N+2) switching elements S1n, ..., S13, S12, S1, S2, S3, S4, S42, S43, ..., S4n connected in series, and N flying capacitors C11, C12, C13, ..., C1n. The innermost flying capacitor C11 is connected in parallel to the two switching elements S2 and S3, and is controlled to maintain a voltage of 1/(2N+2)E. The second innermost flying capacitor C12 is connected in parallel to the four switching elements S1, S2, S3, and S4, and is controlled to maintain a voltage of 2/(2N+2)E. The third innermost flying capacitor C13 is connected in parallel to the six switching elements S12, S1, S2, S3, S4, and S42, and is controlled to maintain a voltage of 3/(2N+2)E. The outermost flying capacitor C1n is connected in parallel to 2N switching elements S1(n-1), ..., S13, S12, S1, S2, S3, S4, S42, S43, ..., S4(n-1), and is controlled to maintain a voltage of N/(2N+2)E.

The first flying capacitor circuit 31 and the second flying capacitor circuit 32 shown in FIG. 1, FIG. 7-FIG. 9 use a one-stage flying capacitor circuit shown in FIG. 10(a). When a one-stage flying capacitor circuit is used, it is possible to generate a voltage of three levels (E, 1/2E, 0) between the midpoint of the first flying capacitor circuit 31 and the midpoint of the second flying capacitor circuit 32. When a two-stage flying capacitor circuit shown in FIG. 10(b) is used, it is possible to generate a voltage of five levels (E, 2/3E, 1/2E, 1/3E, 0) between the midpoint of the first flying capacitor circuit 31 and the midpoint of the second flying capacitor circuit 32. When a three-stage flying capacitor circuit shown in FIG. 10(c) is used, it is possible to generate a voltage of seven levels (E, 3/4E, 5/8E, 1/2E, 3/8E, 1/4E, 0) between the midpoint of the first flying capacitor circuit 31 and the midpoint of the second flying capacitor circuit 32. By using the N-stage flying capacitor circuit shown in FIG. 11, it is possible to generate (2N+1) levels of voltage between the midpoint of the first flying capacitor circuit 31 and the midpoint of the second flying capacitor circuit 32.

Increasing the number of stages in a flying capacitor circuit allows the use of cheaper switching elements with lower voltage resistance, but it also increases the number of switching elements used. Therefore, designers need only consider the total cost and total conversion efficiency when determining the optimal number of stages in the flying capacitor circuit. In addition, in applications where the voltage of the high-voltage side DC section exceeds 1000V or exceeds 10,000V, it is effective to increase the number of stages in the flying capacitor circuit in order to reduce the voltage resistance of each switching element.

Figure 12 is a diagram for explaining the configuration of the power conversion device 3 according to the first modification. The power conversion device 3 according to the first modification is a unidirectional step-down DC/DC converter, and cannot transmit power from the low-voltage side DC section to the high-voltage side DC section. In the power conversion device 3 according to the first modification, four diode elements (the third diode D3, the fourth diode D4, the fifth diode D5, and the sixth diode D6) are used instead of the third switching element S3, the fourth switching element S4, the fifth switching element S5, and the sixth switching element S6. This allows the number of switching elements and drivers to be reduced, and costs to be reduced. The power conversion device 3 according to the first modification can be used, for example, as a step-down circuit that generates a reference voltage (e.g., DC 12V, DC 24V, DC 48V) from the first DC power source 1.

Figure 13 is a diagram for explaining the configuration of the power conversion device 3 according to the modified example 2. The power conversion device 3 according to the modified example 2 is a unidirectional step-up DC/DC converter, and cannot transmit power from the high-voltage side DC section to the low-voltage side DC section. In the power conversion device 3 according to the modified example 2, four diode elements (first diode D1, second diode D2, seventh diode D7, and eighth diode D8) are used instead of the first switching element S1, the second switching element S2, the seventh switching element S7, and the eighth switching element S8. This allows the number of switching elements and drivers to be reduced, and costs to be reduced. The power conversion device 3 according to the modified example 2 can be used, for example, as a step-up circuit for a solar cell.

The embodiment may be specified by the following:

[Item 1]
a first flying capacitor circuit (31) and a second flying capacitor circuit (32) connected in series in parallel with the high voltage side DC section;
a first reactor (L1) connected between a positive terminal of the low-voltage side DC unit and a midpoint of the first flying capacitor circuit (31);
a second reactor (L2) connected between a negative terminal of the low voltage side DC unit and a midpoint of the second flying capacitor circuit (32);
A power conversion device (3) characterized in that a connection point between the first flying capacitor circuit (31) and the second flying capacitor circuit (32) is connected to an intermediate potential point of the high voltage side DC section.
By configuring the circuit symmetrically above and below the midpoint, leakage current and common mode noise can be reduced.
[Item 2]
2. The power conversion device (3) according to claim 1, further comprising a control unit (40) capable of controlling the first flying capacitor circuit (31) and the second flying capacitor circuit (32) to perform at least one of power transmission from the low-voltage side DC section to the high-voltage side DC section by a step-up operation and power transmission from the high-voltage side DC section to the low-voltage side DC section by a step-down operation.
This allows power to be transmitted in both directions.
[Item 3]
The first flying capacitor circuit (31) comprises:
a first switching element (S1), a second switching element (S2), a third switching element (S3) and a fourth switching element (S4) connected in series;
a first flying capacitor (C1) 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);
The second flying capacitor circuit (32) comprises:
a fifth switching element (S5), a sixth switching element (S6), a seventh switching element (S7) and an eighth switching element (S8) connected in series;
The power conversion device (3) described in item 2, further comprising: a second flying capacitor (C2) 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).
By connecting eight switching elements (S1-S8) in series in parallel with the high voltage DC section, it becomes possible to use switching elements with lower withstand voltage than before.
[Item 4]
The control unit (40)
a first mode in which the second switching element (S2), the fourth switching element (S4), the fifth switching element (S5), and the seventh switching element (S7) are controlled to an on state, and the first switching element (S1), the third switching element (S3), the sixth switching element (S6), and the eighth switching element (S8) are controlled to an off state;
a second mode in which the first switching element (S1), the third switching element (S3), the sixth switching element (S6), and the eighth switching element (S8) are controlled to an ON state and the second switching element (S2), the fourth switching element (S4), the fifth switching element (S5), and the seventh switching element (S7) are controlled to an OFF state;
a third mode in which the first switching element (S1), the second switching element (S2), the seventh switching element (S7), and the eighth switching element (S8) are controlled to an ON state, and the third switching element (S3), the fourth switching element (S4), the fifth switching element (S5), and the sixth switching element (S6) are controlled to an OFF state;
a fourth mode in which the third switching element (S3), the fourth switching element (S4), the fifth switching element (S5), and the sixth switching element (S6) are controlled to an ON state, and the first switching element (S1), the second switching element (S2), the seventh switching element (S7), and the eighth switching element (S8) are controlled to an OFF state;
The power conversion device (3) according to item 3, characterized in that the step-up operation or the step-down operation is performed using four modes.
By using the four modes in combination, various controls are possible.
[Item 5]
The control unit (40)
5. The power conversion device (3) according to item 4, characterized in that when a ratio between a voltage of the low-voltage side DC section and a voltage of the high-voltage side DC section is smaller than a set value, the first mode, the second mode, and the third mode are used, and when the ratio is larger than the set value, the first mode, the second mode, and the fourth mode are used.
By switching the mode according to this ratio, the amount of energy stored in the reactors (L1, L2) can be changed.
[Item 6]
The power conversion device (3) according to any one of items 1 to 5, characterized in that the first reactor (L1) and the second reactor (L2) are reactors of the same specifications.
This ensures symmetry between the top and bottom, and reduces leakage current and common mode noise.
[Item 7]
The power conversion device (3) according to any one of items 1 to 6, characterized in that the first reactor (L1) and the second reactor (L2) form a magnetic coupling reactor (Lc1), and mutually strengthen magnetic fluxes during a voltage step-up operation for transmitting power from the low-voltage side DC section to the high-voltage side DC section or during a voltage step-down operation for transmitting power from the high-voltage side DC section to the low-voltage side DC section.
This allows for size reduction, weight reduction, and cost reduction.
[Item 8]
The voltage of the high-voltage side DC section is the voltage of a DC bus connected to a DC terminal of a DC/AC inverter (36) that converts DC power into AC power,
a third reactor (L3) is inserted in a first voltage line between the DC/AC inverter (36) and a single-phase three-wire system (4), and a fourth reactor (L4) is inserted in a second voltage line;
The power conversion device (3) according to any one of items 1 to 7, characterized in that the first reactor (L1), the second reactor (L2), the third reactor (L3), and the fourth reactor (L4) are reactors of the same specification.
This can improve the efficiency of mass production of the reactors (L1-L4).
[Item 9]
9. The power conversion device (3) according to item 8, characterized in that a magnetic coupling reactor (Lc1) formed by the first reactor (L1) and the second reactor (L2), and a magnetic coupling reactor (Lc2) formed by the third reactor (L3) and the fourth reactor (L4) are magnetic coupling reactors of the same specification.
This allows for size reduction, weight reduction, and cost reduction.

1 First DC power source, 2 Second DC power source, 3 Power conversion device, 4 System, 4a First system power source, 4b Second system power source, 30 DC/DC conversion unit, 31, 32 Flying capacitor circuit, 35 DC/AC conversion unit, 36 DC/AC inverter, 40 Control unit, S1-S8 Switching elements, D1-D8 Diodes, C1, C2 Flying capacitors, C3, C4 Split capacitors, C5 Input capacitors, C6, C7, C10, C20 Y capacitors, C8 DC bus capacitors, C9 Output capacitors, L1-L4 Reactors, Lc1, Lc2 Magnetic coupling reactors.

Claims (6)

a first flying capacitor circuit and a second flying capacitor circuit connected in series in parallel with the high voltage side DC unit;
a first reactor connected between a positive terminal of a low-voltage side DC unit and a midpoint of the first flying capacitor circuit;
a second reactor connected between a negative terminal of the low voltage side DC unit and a midpoint of the second flying capacitor circuit;
a control unit capable of controlling the first flying capacitor circuit and the second flying capacitor circuit to perform at least one of power transfer from the low-voltage side DC unit to the high-voltage side DC unit by a step-up operation and power transfer from the high-voltage side DC unit to the low-voltage side DC unit by a step-down operation,
a connection point between the first flying capacitor circuit and the second flying capacitor circuit is connected to an intermediate potential point of the high voltage side DC section ;
The first flying capacitor circuit includes:
a first switching element, a second switching element, a third switching element and a fourth 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,
The second flying capacitor circuit includes:
a fifth switching element, a sixth switching element, a seventh switching element, and an eighth switching element connected in series;
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,
The control unit is
a first mode in which the second switching element, the fourth switching element, the fifth switching element, and the seventh switching element are controlled to an on state, and the first switching element, the third switching element, the sixth switching element, and the eighth switching element are controlled to an off state;
a second mode in which the first switching element, the third switching element, the sixth switching element, and the eighth switching element are controlled to an ON state and the second switching element, the fourth switching element, the fifth switching element, and the seventh switching element are controlled to an OFF state;
a third mode in which the first switching element, the second switching element, the seventh switching element, and the eighth switching element are controlled to be in an on state and the third switching element, the fourth switching element, the fifth switching element, and the sixth switching element are controlled to be in an off state;
a fourth mode in which the third switching element, the fourth switching element, the fifth switching element, and the sixth switching element are controlled to an on state and the first switching element, the second switching element, the seventh switching element, and the eighth switching element are controlled to an off state;
The power conversion device performs the voltage step-up operation or the voltage step-down operation using four modes . The control unit is
2. The power conversion device according to claim 1, wherein when a ratio between a voltage of the low-voltage side DC section and a voltage of the high-voltage side DC section is smaller than a set value, the first mode, the second mode, and the third mode are used, and when the ratio is larger than the set value, the first mode, the second mode, and the fourth mode are used. 3. The power conversion device according to claim 1, wherein the first reactor and the second reactor are reactors of the same specification. 4. The power conversion device according to claim 1, wherein the first reactor and the second reactor form a magnetic coupling reactor, and mutually strengthen magnetic fluxes during a voltage step-up operation for transmitting power from the low-voltage side DC section to the high-voltage side DC section or during a voltage step-down operation for transmitting power from the high-voltage side DC section to the low- voltage side DC section. the voltage of the high-voltage side DC section is a voltage of a DC bus connected to a DC terminal of a DC/AC inverter that converts DC power into AC power;
a third reactor is inserted in a first voltage line between the DC/AC inverter and a single-phase three-wire system, and a fourth reactor is inserted in a second voltage line;
5. The power conversion device according to claim 1, wherein the first reactor, the second reactor, the third reactor, and the fourth reactor are reactors of the same specification. 6. The power conversion device according to claim 5, wherein a magnetic coupling reactor formed by the first reactor and the second reactor, and a magnetic coupling reactor formed by the third reactor and the fourth reactor are magnetic coupling reactors of the same specification.

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