Power conversion device and power system

Abstract

A potential intermediate between the potentials at the first and second input terminals (P1, P2) is generated at a first node (N1). A second node (N2) is connected to terminals (b1-b3) of primary windings (w1, w4, w7) of transformers (T1-T3) of a plurality of LLC resonant converters (11-13). A switching circuit (SW) is connected between the first and second nodes (N1, N2). A control circuit (15) turns on the switching circuit (SW) when a load current of a load device (6) connected to first and second output terminals (P3, P4) is below a predetermined reference, and turns off the switching circuit (SW) when the load current of the load device (6) is greater than the reference.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a power conversion device provided with a plurality of LLC resonant converters. In addition, the present disclosure relates to a power system provided with such a power conversion device. BACKGROUND

[0002] If the current flowing to the power conversion device is increased in order to increase the power supplied from the power conversion device such as a DC / DC converter device to a load device, the heat in the power conversion device also increases. Therefore, it is known that in order to reduce the heat in the power conversion device, a power conversion device provided with multiplexed constituent elements, such as a plurality of LLC resonant converters operating at mutually different phases.

[0003] When a power conversion circuit is provided with a plurality of LLC resonant converters, the transformers, inductors, and capacitors, etc. of each LLC resonant converter can have mutually different differences due to their design values. Due to these differences, the peak value (amplitude), effective value, waveform, etc. of the current of each phase can become uneven. In response to this, by not connecting one set of the primary windings of the transformers of each LLC resonator to each other, but connecting them to the inverter circuit on the primary side, that is, adopting a floating star configuration junction (Y junction when three phases), it is possible to improve the current imbalance caused by the differences in the circuit elements.

[0004] For example, Patent Literature 1 discloses a power conversion circuit provided with three-phase parallel LLC converters.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: U.S. Patent Application Publication No. 2016 / 0254756 Specification SUMMARY OF THE INVENTION

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] The output voltage of an LLC resonant converter varies depending on the switching frequency of the inverter circuit, and also varies depending on the load current of the load device connected to the output terminal. Generally, near the resonant frequency of the LLC resonant converter, if the switching frequency or the load current increases, the output voltage decreases, and if the switching frequency or the load current decreases, the output voltage increases.

[0010] When a plurality of LLC resonant converters are provided in a power conversion circuit and the primary winding of the transformer of each LLC resonator has a star configuration junction that is floating, the output voltage of the power conversion device greatly changes depending on the load current, as compared to when there is no star configuration junction. In particular, when the load current is small, the inverter circuit sometimes cannot achieve the desired output voltage within the range of the switching frequency at which it can operate. Therefore, there is a demand for a power conversion device that has a plurality of LLC resonant converters and can achieve the desired output voltage even if the magnitude of the load current changes.

[0011] An object of the present disclosure is to provide a power conversion device that has a plurality of LLC resonant converters and can achieve the desired output voltage even if the magnitude of the load current changes. In addition, an object of the present disclosure is to provide a power system that has such a power conversion device.

[0012] Solution for solving the technical problem

[0013] One aspect of the present disclosure relates to a power conversion device that has three or more LLC resonant converters,

[0014] Each of the LLC resonant converters has a transformer having a primary winding and a secondary winding, an inverter circuit connected to the primary winding, a first capacitor connected to the primary winding, and a rectifier circuit connected to the secondary winding, the primary winding having a first end connected to the inverter circuit and a second end not connected to the inverter circuit but connected to the primary winding of another LLC resonant converter,

[0015] The power conversion device further has:

[0016] a first input terminal and a second input terminal connected to each of the inverter circuits of each of the LLC resonant converters;

[0017] a first node that generates a potential that is intermediate between each of the potentials of the first input terminal and the second input terminal;

[0018] a second node connected to the second end of each of the primary windings of each of the LLC resonant converters;

[0019] a switching circuit connected between the first node and the second node;

[0020] a drive circuit that causes each of the inverter circuits of each of the LLC resonant converters to operate at a predetermined switching frequency and at mutually different phases;

[0021] a control circuit that controls the switching circuit and the drive circuit; and

[0022] a first output terminal and a second output terminal connected to each of the rectifier circuits of each of the LLC resonant converters,

[0023] The control circuit turns on the switching circuit when a load current of a load device connected to the first output terminal and the second output terminal is below a predetermined reference, and turns off the switching circuit when the load current of the load device is greater than the reference.

[0024] Thus, even if the magnitude of the load current varies, the desired output voltage can be achieved.

[0025] An electric power conversion device according to an aspect of the present disclosure,

[0026] The electric power conversion device further includes a current sensor that measures an output current of the first or second output terminal,

[0027] The control circuit determines that the load current of the load device is below the reference when the output current is below a first threshold, and determines that the load current of the load device is greater than the reference when the output current is greater than the first threshold.

[0028] Thus, the control circuit can determine whether the load current of the load device is below the predetermined reference based on the output current.

[0029] An electric power conversion device according to an aspect of the present disclosure,

[0030] The control circuit determines that the load current of the load device is below the reference when the switching frequency is above a second threshold, and determines that the load current of the load device is greater than the reference when the switching frequency is below a third threshold that is smaller than the second threshold.

[0031] Thus, the control circuit can determine whether the load current of the load device is below the predetermined reference based on the switching frequency.

[0032] An electric power conversion device according to an aspect of the present disclosure,

[0033] The electric power conversion device further includes a voltage sensor that measures an output voltage of the first output terminal and the second output terminal,

[0034] The control circuit controls the drive circuit to vary the switching frequency so that the output voltage approaches a target voltage that is determined in advance.

[0035] Thus, the desired output voltage can be achieved.

[0036] The power conversion device according to an aspect of the present disclosure,

[0037] The switching circuit is configured to allow current to flow in both directions when on and to block current in either direction when off.

[0038] Thus, it is possible to make it difficult to generate current imbalance and voltage offset. In addition, it is possible to make it difficult to significantly affect the waveforms of voltage and current when the switching circuit is shifted from on to off and from off to on.

[0039] The power conversion device according to an aspect of the present disclosure,

[0040] The power conversion device further includes a pair of second capacitors connected in series with each other across the first input terminal and the second input terminal, the pair of second capacitors having equal capacitances,

[0041] The first node is provided between the pair of second capacitors.

[0042] Thus, it is possible to generate a potential intermediate between the potential of the first input terminal and the potential of the second input terminal.

[0043] The power system according to an aspect of the present disclosure includes:

[0044] A power supply device that supplies a first direct-current voltage;

[0045] The power conversion device described above converts the first direct-current voltage into a second direct-current voltage; and

[0046] A load device that operates by the second direct-current voltage.

[0047] Thus, even if the magnitude of the load current varies, it is possible to achieve a desired output voltage.

[0048] The power system according to an aspect of the present disclosure,

[0049] The power supply device includes:

[0050] A noise filter device that reduces at least one of a normal-mode noise signal and a common-mode noise signal;

[0051] A rectifier that converts an alternating-current voltage into the first direct-current voltage; and

[0052] A power factor regulator that makes the power factor of the first direct-current voltage close to 1.

[0053] Thus, the power system is able to operate in a state in which noise is low and the power factor is high.

[0054] Effects of the Invention

[0055] One aspect of this disclosure relates to a power conversion device that can achieve the desired output voltage even if the magnitude of the load current changes. Attached Figure Description

[0056] Figure 1 A block diagram illustrating an example configuration of a power system including the power conversion device 5 according to the first embodiment is shown.

[0057] Figure 2 To illustrate Figure 1 A circuit diagram illustrating an example of the configuration of the power conversion device 5.

[0058] Figure 3 To illustrate Figure 2 The circuit diagram is an example of the configuration of the switching circuit SW.

[0059] Figure 4 To illustrate Figure 2 The circuit diagram of a modified example of the switching circuit SW.

[0060] Figure 5 for Figure 2 The equivalent circuit diagram of the power conversion device 5.

[0061] Figure 6 A graph illustrating the characteristics of the output voltage Vout at the switching frequency fsw of the power conversion device without a floating Y-junction involved in the first comparative example.

[0062] Figure 7 A graph is shown to illustrate the characteristics of the output voltage Vout at the switching frequency fsw of a power conversion device with a floating Y junction but no switching circuit SW involved in the second comparative example.

[0063] Figure 8 To show the output voltage Vout at Figure 1 A graph showing the characteristics of the power conversion device 5 at the switching frequency fsw.

[0064] Figure 9 To show the passage Figure 2 The flowchart of the power conversion process performed by the control circuit 15.

[0065] Figure 10 To illustrate schematically, in connection with Figure 1 A diagram showing the operation of the power conversion device 5 during the hourly load current of the load device 6 of the load device 5.

[0066] Figure 11 To illustrate schematically, in connection with Figure 1A chart showing the operation of the power conversion device 5 when the load current of the load device 6 of the power conversion device 5 is in the middle degree or more.

[0067] Figure 12 A chart showing exemplary waveforms of the voltage and current of the resonance capacitors Cr1 to Cr3 when the switching circuit SW of the power conversion device 5 according to the first embodiment is shifted from on to off. Figure 2

[0068] Figure 13 A chart showing exemplary waveforms of the voltage and current of the resonance capacitors Cr1 to Cr3 when the switching circuit SW of the power conversion device 5 according to the first embodiment is shifted from off to on. Figure 2

[0069] Figure 14 A chart showing exemplary waveforms of the voltage and current of the resonance capacitors Cr1 to Cr3 when the switching circuit SW of the power conversion device 5 according to the third comparative example is shifted from off to on. Figure 2

[0070] Figure 15 A circuit diagram schematically showing a configuration example of the power conversion device 5B according to the first modification of the first embodiment.

[0071] Figure 16 A circuit diagram schematically showing a configuration example of the power conversion device 5C according to the second modification of the first embodiment.

[0072] Figure 17 A circuit diagram schematically showing a configuration example of the power conversion device 5D according to the second embodiment.

[0073] Figure 18 A flowchart showing the power conversion process performed by the control circuit 15D of the power conversion device 5D according to the second embodiment. Figure 17

[0074] A chart schematically showing the operation of the power conversion device 5D when the switching frequency fsw exceeds the upper threshold fth1 in step S4A of the power conversion process according to the second embodiment. Figure 19 Figure 18 A chart schematically showing the operation of the power conversion device 5D when the switching frequency fsw is less than the lower threshold fth2 in step S7A of the power conversion process according to the second embodiment.

[0075] Figure 20 Figure 18 DETAILED DESCRIPTION

[0076] Embodiments according to an aspect of the present disclosure will be described below based on the drawings. In the drawings, the same reference numerals denote the same constituent elements. ​​​​​​

[0077] [Application Example]

[0078] Figure 1 A block diagram for schematically showing a configuration example of a power system including the power conversion device 5 related to the first embodiment is shown. Figure 1 The power system 1 has, for example, an alternating current power supply device 1, a noise filter device 2, a rectifier 3, a power factor regulator 4, a power conversion device 5, and a load device 6.

[0079] The noise filter device 2, the rectifier 3, and the power factor regulator 4 receive supply of alternating current power from the alternating current power supply device 1 and generate direct current power having a first direct current voltage. The noise filter device 2, the rectifier 3, and the power factor regulator 4 (or these constituent elements and the alternating current power supply device 1) are an example of a "power supply device" that supplies the first direct current voltage. The power conversion device 5 is a DC / DC converter that converts the first direct current voltage to a second direct current voltage. The load device 6 operates by the second direct current voltage.

[0080] Figure 2 A circuit diagram for schematically showing a configuration example of the power conversion device 5 is shown. Figure 1 The power conversion device 5 has at least input terminals P1, P2, output terminals P3, P4, LLC resonant converters 11 to 13, capacitors C1, C2, nodes N1, N2, a switching circuit SW, a drive circuit 14, and a control circuit 15.

[0081] The input terminals P1, P2 are connected to the power factor regulator 4 and further connected to inverter circuits 21, 23, 25 (described later) of the LLC resonant converters 11 to 13, respectively. The LLC resonant converters 11 to 13 receive supply of a direct current input voltage Vin from the power factor regulator 4 via the input terminals P1, P2.

[0082] The capacitors C1, C2 have equal capacitances to each other and are connected in series between the input terminals P1, P2. Therefore, a potential intermediate between positive and negative potentials of the input terminals P1, P2 is generated at the node N1 between the capacitors C1, C2.

[0083] The LLC resonant converter 11 has a transformer T1, an inverter circuit 21, a resonant capacitor Cr1, and a rectifier circuit 22. The transformer T1 has a primary winding w1 and secondary windings w2, w3, and further has a magnetizing inductance Lm1 and a leakage inductance Lr1. The inverter circuit 21 and the resonant capacitor Cr1 are connected to the primary winding w1. The resonant capacitor Cr1, the magnetizing inductance Lm1, and the leakage inductance Lr1 constitute an LLC resonant circuit. Further, the rectifier circuit 22 is connected to the secondary windings w2, w3.

[0084] The LLC resonant converter 12 has a transformer T2, an inverter circuit 23, a resonant capacitor Cr2, and a rectifier circuit 24. The transformer T2 has primary windings w4 and secondary windings w5 and w6, and has an excitation inductance Lm2 and a leakage inductance Lr2. The inverter circuit 23 and the resonant capacitor Cr2 are connected to the primary winding w4. The resonant capacitor Cr2, the excitation inductance Lm2, and the leakage inductance Lr2 constitute an LLC resonant circuit. Further, the rectifier circuit 24 is connected to the secondary windings w5 and w6.

[0085] The LLC resonant converter 13 has a transformer T3, an inverter circuit 25, a resonant capacitor Cr3, and a rectifier circuit 26. The transformer T3 has primary windings w7 and secondary windings w8 and w9, and has an excitation inductance Lm3 and a leakage inductance Lr3. The inverter circuit 25 and the resonant capacitor Cr3 are connected to the primary winding w7. The resonant capacitor Cr3, the excitation inductance Lm3, and the leakage inductance Lr3 constitute an LLC resonant circuit. Further, the rectifier circuit 26 is connected to the secondary windings w8 and w9.

[0086] The primary winding w1 of the transformer T1 has terminals a1 and b1. The terminal a1 is connected to the inverter circuit 21. The terminal b1 is not connected to the inverter circuits 21, 23, and 25, but is connected to the primary winding w4 of the other LLC resonant converter 12 via the resonant capacitor Cr1 to Cr3. Similarly, the primary winding w4 of the transformer T2 has terminals a2 and b2. The terminal a2 is connected to the inverter circuit 23. The terminal b2 is not connected to the inverter circuits 21, 23, and 25, but is connected to the primary winding w1 of the other LLC resonant converter 11 via the resonant capacitor Cr1 to Cr3. Similarly, the primary winding w7 of the transformer T3 has terminals a3 and b3. The terminal a3 is connected to the inverter circuit 25. The terminal b3 is not connected to the inverter circuits 21, 23, and 25, but is connected to the primary winding w1 of the other LLC resonant converter 11 via the resonant capacitor Cr1 to Cr3.

[0087] The terminals b1 to b3 of the primary windings w1, w4, and w7 of the respective LLC resonant converters 11 to 13 are connected to the node N2 via the resonant capacitors Cr1 to Cr3.

[0088] The output terminals P3 and P4 are connected to the respective rectifier circuits 22, 24, and 26 of the LLC resonant converters 11 to 13, and are also connected to the load device 6. The output voltage Vout and the output current Iout of the direct current converted by the power conversion device 5 are supplied to the load device 6 via the output terminals P3 and P4.

[0089] The switching circuit SW is connected between the nodes N1, N2 and is turned on / off under the control of the control circuit 15. The switching circuit SW is configured to allow current to flow in both directions when turned on and to block current in either direction when turned off, for example.

[0090] The drive circuit 14 causes the inverter circuits 21, 23, 25 of the LLC resonant converters 11 to 13 to operate at a predetermined switching frequency fsw and at mutually different phases (e.g., different phases each differing by 120 degrees) under the control of the control circuit 15.

[0091] The control circuit 15 turns on the switching circuit SW when the load current of the load device 6 connected to the output terminals P3, P4 is below a predetermined reference and turns off the switching circuit SW when the load current of the load device 6 is greater than the reference. In addition, the control circuit 15 sets the switching frequency fsw of the inverter circuits 21, 23, 25 in the drive circuit 14. The control circuit 15 controls the drive circuit 14 to vary the switching frequency fsw so as to cause the output voltage Vout to approach a predetermined target voltage.

[0092] When the switching circuit SW is turned off, in other words, when the primary windings w1, w4, w7 of the transformers T1 to T3 have floating Y junctions, the output voltage Vout greatly depends on the load current of the load device 6 as described later. In particular, when the load current of the load device 6 is small, the difference between the output voltage Vout and the target voltage sometimes increases. Therefore, when the load current of the load device 6 is small, the power conversion device 5 is able to cause the inverter circuits 21, 23, 25 to operate within a range of the switching frequency fsw at which operation is possible to achieve a desired output voltage by turning on the switching circuit SW; on the other hand, when the load current of the load device 6 is moderate or greater, the primary windings w1, w4, w7 of the transformers T1 to T3 have floating Y junctions by turning off the switching circuit SW. Thus, when the switching circuit SW is turned off, current imbalance caused by differences in circuit elements of the power conversion device 5 can be improved compared to when the switching circuit SW is turned on. In addition, when the load current of the load device 6 is moderate or greater, the inverter circuits 21, 23, 25 are able to operate within a range of the switching frequency fsw at which operation is possible to achieve a desired output voltage regardless of whether the switching circuit SW is turned on or off. Thus, the power conversion device 5 is able to achieve a desired output voltage even if the magnitude of the load current varies.

[0093] [First Embodiment]

[0094] A power system provided with the power conversion device according to the first embodiment will be described further below.

[0095] [Configuration Example of the First Embodiment]

[0096] Reference Figure 1 Further explanation of the various components of the power system.

[0097] The AC power supply unit 1 supplies AC power at a predetermined voltage and frequency. The AC power supply unit 1 can be a power supply device from a commercial power grid, or alternatively, it can include a DC power supply unit and an inverter.

[0098] The noise filter device 2 is configured to reduce at least one of the constant-mode noise signal and the common-mode noise signal propagating through the wire. The noise filter device 2 includes at least one of an active filter that generates an inverted signal having a polarity opposite to that of the noise signal and a passive filter composed of passive elements such as capacitors and inductors.

[0099] The rectifier 3 converts the AC power supplied from the AC power supply unit 1 via the noise filter unit 2 into DC power. The rectifier 3 can also be a rectifier circuit with a diode bridge. Alternatively, the rectifier 3 can be a synchronous rectifier circuit with switching elements that operate in phase matching the input AC voltage or AC current.

[0100] The power factor regulator 4 improves the power factor (i.e., close to "1") of the DC power output from the rectifier 3. The power factor regulator 4 may also include passive components such as inductors and / or capacitors, and may further include active components such as transistors and diodes.

[0101] The power conversion device 5 converts the first DC voltage output from the power factor regulator 4 into a second DC voltage.

[0102] The load device 6 operates by receiving DC power from the power conversion device 5 to perform certain tasks. The load device 6 may include, for example, a motor, a battery, a sensor, a communication device, etc.

[0103] Reference Figure 2 The components of the power conversion device 5 will be further explained.

[0104] exist Figure 2 In the example, inverter circuits 21, 23, and 25 constitute a half-bridge inverter with switching element pairs Q1, Q2, Q3, Q4, and Q5, Q6, respectively. Node N11 between switching elements Q1 and Q2 is connected to terminal a1 of the primary winding w1. Node N12 between switching elements Q3 and Q4 is connected to terminal a2 of the primary winding w4. Node N13 between switching elements Q5 and Q6 is connected to terminal a3 of the primary winding w7. Figure 2In the example of FIG. 1, a leakage inductance Lrl is shown between the node Nll and the terminal al, a leakage inductance Lr2 is shown between the node N12 and the terminal a2, and a leakage inductance Lr3 is shown between the node N13 and the terminal a3.

[0105] In Figure 2 In the example of FIG. 1, the rectifier circuit 22 includes diodes Dl, D2. The secondary windings w2, w3 of the transformer Tl are connected to the positive output terminal P3 of the power conversion device 5 via the diodes Dl, D2, respectively, and the center taps of the secondary windings w2, w3 are connected to the negative output terminal P4 of the power conversion device 5. In addition, the rectifier circuit 24 includes diodes D3, D4. The secondary windings w5, w6 of the transformer T2 are connected to the positive output terminal P3 of the power conversion device 5 via the diodes D3, D4, respectively, and the center taps of the secondary windings w5, w6 are connected to the negative output terminal P4 of the power conversion device 5. In addition, the rectifier circuit 26 includes diodes D5, D6. The secondary windings w8, w9 of the transformer T3 are connected to the positive output terminal P3 of the power conversion device 5 via the diodes D5, D6, respectively, and the center taps of the secondary windings w8, w9 are connected to the negative output terminal P4 of the power conversion device 5. Such rectifier circuits 22, 24, 26 using the center taps of the secondary windings are examples of the rectifier circuits involved in the embodiments.

[0106] In order to smooth the output power of the LLC resonant converters 11-13, the power conversion device 5 can further include a capacitor C3 connected between the output terminals P3, P4.

[0107] In addition, as described above, the LLC resonant converters 11-13 operate at mutually different phases. Therefore, the drive circuit 14 supplies control signals to the switching elements Ql, Q2 in such a manner that the switching element Ql is turned on and the switching element Q2 is turned off in the first half of each cycle, and the switching element Ql is turned off and the switching element Q2 is turned on in the second half of each cycle. In addition, the drive circuit 14 supplies control signals to the switching elements Q3, Q4 in such a manner that the switching element Q3 is turned on and the switching element Q4 is turned off in the first half of each cycle that is delayed by 120 degrees from the beginning of the cycle in which the switching elements Ql, Q2 operate, and the switching element Q3 is turned off and the switching element Q4 is turned on in the second half of each cycle. In addition, the drive circuit 14 supplies control signals to the switching elements Q5, Q6 in such a manner that the switching element Q5 is turned on and the switching element Q6 is turned off in the first half of each cycle that is delayed by 240 degrees from the beginning of the cycle in which the switching elements Ql, Q2 operate, and the switching element Q5 is turned off and the switching element Q6 is turned on in the second half of each cycle. As a result, the LLC resonant converters 11-13 generate three-phase power having mutually different phases that are each different by 120 degrees.

[0108] As described above, the output voltage Vout of each LLC resonant converter 11 to 13 depends on the switching frequency fsw of the switching elements Ql to Q6. Therefore, the drive circuit 14 varies the switching frequency fsw of the switching elements Ql to Q6 in accordance with the desired output voltage Vout of the power conversion device 5.

[0109] In Figure 2 the example, the power conversion device 5 further has a current sensor 17 that measures the output current Iout of the output terminals P3, P4. When the output voltage Vout is generated to match the target voltage decided in advance, if the load current of the load device 6 increases, the output current Iout also increases, and if the load current decreases, the output current Iout also decreases. Thus, the control circuit 15 can judge whether the load current of the load device 6 is below a predetermined reference based on the output current Iout. When the output current Iout is below a threshold value Ith, the control circuit 15 turns on the switching circuit SW, and when the output current Iout is greater than the threshold value Ith, the control circuit 15 turns off the switching circuit SW.

[0110] In Figure 2 the example, the power conversion device 5 further has a voltage sensor 16 that measures the output voltage Vout of the output terminals P3, P4. The control circuit 15 controls the drive circuit 14 to vary the switching frequency fsw so that the output voltage Vout approaches the target voltage decided in advance.

[0111] The control circuit 15 can be a dedicated circuit that performs the power conversion processing described later with reference to Figure 9 or Figure 18 a general-purpose processor that executes a predetermined program.

[0112] The drive circuit 14 and the control circuit 15 can be configured as independent circuits or as an integrated circuit.

[0113] Figure 3 A circuit diagram that schematically shows a configuration example of the switching circuit SW. Figure 2 Figure 3 ​The switching circuit SW includes switching elements Q101 and Q102, and diodes D101 and D102. Switching elements Q101 and Q102 are, for example, metal-oxide-semiconductor field-effect transistors (MOSFETs) including a body diode. Switching element Q101 and diode D101 are connected in series, with the anode (or cathode) of the body diode of switching element Q101 facing each other. Similarly, switching element Q102 and diode D102 are connected in series, with the cathode (or anode) of the body diode of switching element Q102 facing each other. The series circuit of switching element Q102 and diode D102 is connected in parallel to the series circuit of switching element Q101 and diode D101. Switching elements Q101 and Q102 are simultaneously turned on or simultaneously turned off according to the control signal from control circuit 15. Thus, the switching circuit SW is configured to allow current to flow bidirectionally when on and to block current in either direction when off.

[0114] Figure 4 To illustrate Figure 2 The circuit diagram of a modified example of the switching circuit SW. Figure 2 The power conversion device 5 can also be equipped with Figure 4 The switching circuit SWA is used to replace Figure 3 The switching circuit SW. Figure 4 The switching circuit SWA includes switching elements Q111 and Q112. Switching elements Q111 and Q112 are, for example, metal-oxide-semiconductor field-effect transistors (MOSFETs) including body diodes. Switching elements Q111 and Q112 are connected in series and with the positive (or negative) terminals of these body diodes facing each other. Switching elements Q111 and Q112 are simultaneously turned on or off according to a control signal from control circuit 15. Thus, the switching circuit SWA is configured to allow current to flow bidirectionally when on and to block current in either direction when off.

[0115] [Operational Example of the First Embodiment]

[0116] Next, refer to Figures 5-14 An operation example of the power conversion device 5 according to the first embodiment will be described.

[0117] Figure 5 for Figure 2 The equivalent circuit diagram of the power conversion device 5.Figure 5 For the sake of simplicity, the magnetizing inductors Lm1 to Lm3 and the leakage inductors Lr1 to Lr3 are omitted. As mentioned earlier, a potential Vin / 2, which is half the input voltage, is generated at node N1, which is the midpoint between the positive and negative potentials of the input terminals P1 and P2. Using the potential of node N1 as a reference, AC voltages V(N11) to V(N13) with different phases, each differing by 120 degrees, are generated at nodes N11 to N13. When the switching circuit SW is open, the primary windings w1, w4, and w7 of transformers T1 to T3 have floating Y-junctions; on the other hand, when the switching circuit SW is closed, the primary windings w1, w4, and w7 of transformers T1 to T3 do not have floating Y-junctions and are connected to the voltage source at node N1, i.e., the potential Vin / 2.

[0118] Figure 6 To illustrate the characteristics of the output voltage Vout at the switching frequency fsw of the power conversion device without a floating Y-junction involved in the first comparative example. Figure 6 In China, targeting those with... Figure 2 The power conversion device 5, consisting of capacitors C1 and C2 removed and the switching circuit SW removed, with node N2 connected to input terminal P2, demonstrates the characteristics of the output voltage Vout at the switching frequency fsw as the load resistance of the load device 6 changes. When generating the output voltage Vout to match a predetermined target voltage, generally, if the load resistance increases, the load current decreases, and if the load resistance decreases, the load current increases. According to... Figure 6 It can be seen that when the power conversion device operates at a certain switching frequency fsw, even if the load resistance changes, the change in output voltage Vout remains small. Therefore, even if the load current changes, near the resonant frequency of the LLC resonant converter, a small change in the switching frequency fsw can make the output voltage Vout match the target voltage.

[0119] Figure 7 A graph illustrating the characteristics of the output voltage Vout at the switching frequency fsw of the power conversion device with a floating Y-junction but without a switching circuit SW involved in the second comparative example. Figure 7 In China, targeting those with... Figure 2 The power conversion device 5, which is a power conversion device consisting of capacitors C1 and C2 and switching circuit SW, is shown. The characteristics of the output voltage Vout at the switching frequency fsw are shown when the load resistance of the load device 6 changes. According to Figure 7 It can be seen that when the power conversion device operates at a certain switching frequency fsw, if the load resistance changes, compared to Figure 6 In this case, the output voltage Vout changes more significantly. Especially in... Figure 7In the example, when the load resistance is large (15.5Ω), the difference between the output voltage Vout and the target voltage increases. Furthermore, there is no switching frequency fsw near the resonant frequency of the LLC resonant converter that can achieve the target voltage. It should be noted that, according to... Figure 7 It is believed that even with a large load resistance, increasing the switching frequency fsw may reduce the output voltage Vout to the target voltage. However, in this case, the difference between the switching frequency fsw and the resonant frequency of the LLC resonant converter increases. Furthermore, there are cases where the switching frequency fsw exceeds the range of the operable switching frequency fsw, thus potentially failing to achieve the target voltage. Additionally, parasitic capacitances of diodes and transformer windings can cause the output voltage Vout to rise in the high-frequency band, again potentially preventing the target voltage from being reached. Thus, when the power conversion device has a floating Y-junction on the primary side of the transformer, the output voltage Vout's characteristics at the switching frequency fsw change significantly with varying load resistance, making it particularly difficult to achieve the target voltage when the load resistance is high (i.e., the load current is low).

[0120] according to Figure 7 When the load resistance is large (15.5Ω), a higher switching frequency fsw is required to achieve the same output voltage Vout as when the load resistance is small (5Ω). The reason for this is that the current flowing from the primary winding of one transformer through node N2 to the primary winding of another transformer will generate an uncancellable magnetizing current in the secondary winding of the latter transformer.

[0121] Figure 8 To show the output voltage Vout at Figure 1 A graph showing the characteristics of the power conversion device 5 at the switching frequency fsw. When the switching circuit SW is open and the load current is small, the following can be obtained: Figure 8 The characteristics are shown by the dashed line. At this point, there is a significant difference between the output voltage Vout and the target voltage, making it difficult to bring the output voltage Vout close to the target voltage. To address this, by turning on the switching circuit SW, the desired voltage can be obtained... Figure 8 The solid line illustrates the characteristics shown. At this point, the target voltage can be easily achieved near the resonant frequency of the LLC resonant converter.

[0122] Figure 9 To show the passage Figure 2 The flowchart of the power conversion process performed by the control circuit 15.

[0123] When the power conversion process begins, the switching circuit SW can be disconnected in the initial state.

[0124] In step S1, the control circuit 15 starts the LLC resonant converters 11 to 13 by controlling the drive circuit 14 to start transmission of the control signals of the switching elements Q1 to Q6. At this time, the control circuit 15 can also soft-start the LLC resonant converters 11 to 13. When the capacitor C3 is charged, an inrush current can flow through the switching elements Q1 to Q6. By soft-starting the LLC resonant converters 11 to 13, it is possible to make it difficult to generate an inrush current.

[0125] In step S2, the control circuit 15 turns off the switching circuit SW.

[0126] In step S3, the control circuit 15 controls the switching frequency fsw based on the current output voltage Vout acquired from the voltage sensor 16 so as to cause the output voltage Vout to approach the target voltage to generate the target voltage.

[0127] In step S4, the control circuit 15 determines whether the current output current Iout acquired from the current sensor 17 is below a threshold value Ith decided in advance, and when YES, proceeds to step S5, and when NO, returns to step S3. The threshold value Ith can also be set to, for example, 10% of the maximum current flowing in the load device 6.

[0128] In step S5, the control circuit 15 turns on the switching circuit SW.

[0129] Figure 10 A graph of the operation of the power conversion device 5 when the load current of the load device 6 connected to the power conversion device 5 is small is shown schematically. When the output current Iout is below the threshold value Ith (YES in step S4), it is determined that the load current is small. When the switching circuit SW is turned off and the load current is small, for example, the characteristic shown by the dotted line of FIG. 6 is obtained. At this time, in a frequency range below the maximum value fmax of the operable switching frequency fsw, there is a large difference between the output voltage Vout and the target voltage. Also at this time, in order to achieve the target voltage, it is considered that a higher frequency f2 than the maximum value fmax of the operable switching frequency fsw is required. Therefore, by turning on the switching circuit SW in step S5, for example, the characteristic shown by the solid line of FIG. 6 is obtained. At this time, at a frequency f1 included in the range of the operable switching frequency fsw, it is possible to achieve the target voltage. Figure 1 Figure 10 Figure 10

[0130] In step S6 of FIG. 7, Figure 9 In step S6 of FIG. 7, the control circuit 15 controls the switching frequency fsw based on the current output voltage Vout acquired from the voltage sensor 16 so as to cause the output voltage Vout to approach the target voltage to generate the target voltage.

[0131] ​​​In step S7, the control circuit 15 determines whether the current output current Iout obtained from the current sensor 17 is greater than the threshold Ith. If yes, it returns to step S2; otherwise, it returns to step S6.

[0132] Figure 11 To illustrate schematically, in connection with Figure 1 A graph showing the operation of the power conversion device 5 when the load current of the load device 6 is at or above a moderate level. When the output current Iout is greater than the threshold Ith (yes in step S7), it is determined that the load current is at or above a moderate level. When the load current is at or above a moderate level, if... Figure 11 As shown by the solid and dashed lines, the output voltage Vout exhibits minimal variation at the switching frequency fsw regardless of whether the switching circuit SW is on or off. The target voltage can be achieved at frequencies f3 and f4, which fall within the frequency range below the maximum value fmax of the operating switching frequency fsw. Therefore, by disconnecting the switching circuit SW in step S2, the current imbalance caused by differences in the circuit elements of the power conversion device 5 can be improved while simultaneously generating an output voltage Vout near the target voltage.

[0133] Figure 12 To show that Figure 2 A graph showing exemplary waveforms of the voltage and current of resonant capacitors Cr1 to Cr3 when the switching circuit SW switches from ON to OFF. Figure 13 To show that Figure 2 A graph illustrating exemplary waveforms of the voltage and current across resonant capacitors Cr1 to Cr3 when the switching circuit SW transitions from open to closed. V(Cr1), V(Cr2), and V(Cr3) represent the voltages across each of the resonant capacitors Cr1 to Cr3, i.e., the potential of node N2 relative to the potentials of terminals b1 to b3 of each winding w1, w4, and w7. Additionally, I(Cr1), I(Cr2), and I(Cr3) represent the current flowing from terminals b1 to b3 towards node N2 via the resonant capacitors Cr1 to Cr3. As previously described, the switching circuit SW is configured to allow bidirectional current flow when closed and to block current in either direction when closed. By using such a switching circuit SW, the waveforms of voltage and current are not significantly affected when the switching circuit SW transitions from closed to open or from open to closed.

[0134] Figure 14 To illustrate, in the third comparative example, a switching element is used instead of... Figure 2This is a graph illustrating exemplary waveforms of the voltage and current of resonant capacitors Cr1 to Cr3 when the switching element in the power conversion device of the switching circuit SW is switched from open to closed. Here, the switching element is, for example, a metal-oxide-semiconductor field-effect transistor (MOSFET). A MOSFET has a body diode, and when open, current flows in one direction through the body diode. Therefore, in this case, when the switching element is open, it is impossible to prevent either the positive or negative current flowing through the switching element between nodes N1 and N2, resulting in a current imbalance. Figure 14 When the switching element is disconnected, a deflection of approximately -100V is generated in the voltage across each of the resonant capacitors Cr1 to Cr3. Furthermore, according to... Figure 14 After the switching element is switched from open to closed, the voltage and current waveforms change dramatically immediately. On the other hand, according to the power conversion device 5 of the embodiment, by using a switching circuit SW configured to allow current to flow bidirectionally when closed and to block current in either direction when closed, it is difficult to generate current imbalance and voltage deviation. In addition, according to the power conversion device 5 of the embodiment, as referred to Figure 12 and Figure 13 As explained, switching the circuit SW from on to off, or from off to on, does not significantly affect the waveforms of voltage and current.

[0135] [Modifications of the First Embodiment]

[0136] Figure 15 A circuit diagram illustrating a configuration example of the power conversion device 5B according to a first variation of the first embodiment is shown. Figure 15 The power conversion device 5B is equipped with LLC resonant converters 11B to 13B to replace Figure 2 LLC resonant converters 11-13.

[0137] The LLC resonant converter 11B uses a transformer T11 and an inductor Lr11 instead of Figure 2 The transformer T1 is equipped with a rectifier circuit 22B to replace it. Figure 2 The rectifier circuit 22. Additionally, the LLC resonant converter 12B uses a transformer T12 and an inductor Lr12 to replace... Figure 2 The transformer T2, and equipped with rectifier circuit 24B to replace Figure 2the transformer T3 of the LLC resonant converter 13A, and has a rectification circuit 26B instead of the rectification circuit 26 of the LLC resonant converter 13A. Figure 2 Figure 2

[0138] The LLC resonant circuits of the LLC resonant converters 11B to 13B can include, as discrete components, the inductors Lrl 1 to Lrl 3 instead of the leakage inductances Lrl to Lr3 of the transformers Tl to T3, respectively.

[0139] The transformer Tll has a primary winding wl and a secondary winding wl l. The rectification circuit 22B is a full-bridge rectification circuit composed of diodes Dl l to Dl 4. The secondary winding wl l is connected to the output terminals P3, P4 of the power conversion device 5B via the rectification circuit 22B. In addition, the transformer Tl 2 has a primary winding w4 and a secondary winding wl 2. The rectification circuit 24B is a full-bridge rectification circuit composed of diodes Dl 5 to Dl 8. The secondary winding wl 2 is connected to the output terminals P3, P4 of the power conversion device 5B via the rectification circuit 24B. In addition, the transformer Tl 3 has a primary winding w7 and a secondary winding wl 3. The rectification circuit 26B is a full-bridge rectification circuit composed of diodes Dl 9 to D22. The secondary winding wl 3 is connected to the output terminals P3, P4 of the power conversion device 5B via the rectification circuit 26B.

[0140] Figure 16 Fig. 16 is a circuit diagram schematically showing a configuration example of the power conversion device 5C related to the second modification of the first embodiment. Figure 16 The power conversion device 5C has LLC resonant converters 11C to 13C instead of the LLC resonant converters 11 to 13 of the power conversion device 5A, and further has the capacitor C3 of the power conversion device 5A removed. Figure 2 Figure 15 The LLC resonant converter 11C has a rectification circuit 22C instead of the rectification circuit 22B of the LLC resonant converter 11A. In addition, the LLC resonant converter 12C has a rectification circuit 24C instead of the rectification circuit 24B of the LLC resonant converter 12A. In addition, the LLC resonant converter 13C has a rectification circuit 26C instead of the rectification circuit 26B of the LLC resonant converter 13A.

[0141] The LLC resonant converter 11C has a rectification circuit 22C instead of the rectification circuit 22B of the LLC resonant converter 11A. In addition, the LLC resonant converter 12C has a rectification circuit 24C instead of the rectification circuit 24B of the LLC resonant converter 12A. In addition, the LLC resonant converter 13C has a rectification circuit 26C instead of the rectification circuit 26B of the LLC resonant converter 13A. Figure 15 Figure 15 Figure 15

[0142] ​​​​​​The rectifier circuit 22C includes diodes D31, D32 and capacitors C31, C32. The secondary winding w11 of the transformer T11 is connected to the output terminals P3, P4 of the power conversion device 5C via a voltage-doubling rectifier circuit composed of the diodes D31, D32 and the capacitors C31, C32. In addition, the rectifier circuit 24C includes diodes D33, D34 and capacitors C33, C34. The secondary winding w12 of the transformer T12 is connected to the output terminals P3, P4 of the power conversion device 5C via a voltage-doubling rectifier circuit composed of the diodes D33, D34 and the capacitors C33, C34. In addition, the rectifier circuit 26C includes diodes D35, D36 and capacitors C35, C36. The secondary winding w13 of the transformer T13 is connected to the output terminals P3, P4 of the power conversion device 5C via a voltage-doubling rectifier circuit composed of the diodes D35, D36 and the capacitors C35, C36.

[0143] Only a part of the configuration shown in Figure 15 may be applied to the power conversion device 5 of Figure 16 For example, the LLC resonant converters 11 to 13 of Figure 2 may each include the inductors Lr11 to Lr13 of Figure 2 In addition, the LLC resonant converters 11 to 13 of Figure 15 may include the transformers T11 to T13 and the rectifier circuits 22B, 24B, 26B of Figure 2 instead of the transformers T1 to T3 and the rectifier circuits 22, 24, 26. In addition, the LLC resonant converters 11 to 13 of Figure 15 may include the transformers T11 to T13 and the rectifier circuits 22C, 24C, 26C of Figure 2 instead of the transformers T1 to T3 and the rectifier circuits 22, 24, 26. Thereby, the degree of freedom in design of the power conversion device can be improved. Figure 16

[0144] According to the power conversion device 5, 5B, 5C related to the embodiments, by providing the Y junction (i.e., the node N2) only on the primary side of the transformers T1 to T3, various rectifier circuits can be employed on the secondary side of the transformers T1 to T3. For example, the rectifier circuit can be selected to match the product specifications such as input and output power.

[0145] [Effects of the first embodiment]

[0146] ​According to the power conversion device 5 according to the embodiment, when the load current of the load device 6 is small, the inverter circuits 21, 23, 25 can be operated in the range of the operable switching frequency fsw to achieve the desired output voltage Vout by turning on the switching circuit SW. In addition, according to the power conversion device 5 according to the embodiment, when the load current of the load device 6 is equal to or larger than the intermediate level, the primary windings w1, w4, w7 of the transformers T1 to T3 have a floating Y-junction by turning off the switching circuit SW, so that current imbalance caused by a difference in circuit elements of the power conversion device 5 can be improved. In addition, when the load current of the load device 6 is equal to or larger than the intermediate level, the inverter circuits 21, 23, 25 can be operated in the range of the operable switching frequency fsw to achieve the desired output voltage Vout. As such, even if the magnitude of the load current varies, the power conversion device 5 according to the embodiment can achieve the desired output voltage Vout.

[0147] According to the power conversion device 5 according to the embodiment, by using the switching circuit SW configured to allow bidirectional current flow when turned on and to block current in either direction when turned off, it is possible to make it difficult to cause current imbalance and voltage offset. In addition, according to the power conversion device 5 according to the embodiment, when the switching circuit SW is shifted from on to off and from off to on, it is possible to make it difficult to significantly affect the waveforms of voltage and current.

[0148] The power conversion device 5 according to the embodiment can disperse heat sources in the housing of the power conversion device 5 by having a plurality of LLC resonant converters 11 to 13. Therefore, for example, it is possible to provide a DC / DC converter device that is large in power and high in power density without an air cooling fan.

[0149] [Second Embodiment]

[0150] A power system having the power conversion device according to the second embodiment will be further described below.

[0151] [Configuration Example of Second Embodiment]

[0152] Figure 17 A configuration example circuit diagram of the power conversion device 5D according to the second embodiment will be schematically shown. Figure 17 The power conversion device 5D according to the second embodiment has a control circuit 15D instead of the control circuit 15 and the current sensor 17 of the power conversion device 5 according to the first embodiment. Figure 2

[0153] ​As an alternative to the output current Iout, the control circuit 15D determines whether the load current of the load device 6 is below a predetermined reference based on the current switching frequency fsw of the inverter circuits 21, 23, and 25 set in the drive circuit 14. The switching frequency fsw set in the drive circuit 14 by the control circuit 15D is also stored in the internal memory (not shown) of the control circuit 15D. (See reference...) Figure 6 and Figure 7 As explained, the characteristics of the output voltage Vout at the switching frequency fsw depend on the load current (or load resistance) of the load device 6. Near the resonant frequencies of the LLC resonant converters 11-13, the switching frequency fsw, which achieves the predetermined target voltage, increases as the load current decreases and decreases as the load current increases. Therefore, when the switching frequency fsw is above the threshold fth1, the control circuit 15D turns on the switching circuit SW; when the switching frequency fsw is below the threshold fth2 (which is less than the threshold fth1), the control circuit 15D turns off the switching circuit SW.

[0154] [Operational Example of the Second Embodiment]

[0155] Figure 18 To show the passage Figure 17 The flowchart shows the power conversion process performed by the control circuit 15D. Figure 18 In the flowchart, steps S4A and S7A are performed instead of Figure 9 Steps S4 and S7.

[0156] In step S4A, the control circuit 15D determines whether the current switching frequency fsw is above a predetermined threshold fth1. If yes, it proceeds to step S5; otherwise, it returns to step S3. The threshold fth1 can be set, for example, to the maximum value fmax of the operable switching frequency fsw, or it can be set to the resonant frequency of the LLC resonant converters 11-13.

[0157] Figure 19 To illustrate in Figure 18 The graph shows the operation of the power conversion device 5D when the switching frequency fsw exceeds the upper limit threshold fth1 in step S4A. When the switching frequency fsw is above the threshold fth1 (yes in step S4A), it is determined that the load current is small. When the switching circuit SW is open and the load current is small, for example, it is possible to obtain... Figure 19 The characteristics are shown by the dashed line. At this point, in order to achieve the target voltage, it is considered that a frequency f6 higher than the maximum value fmax of the operating switching frequency fsw is required. Therefore, by turning on the switching circuit SW in step S5, for example, a voltage can be obtained through... Figure 19The solid line illustrates the characteristics. At this point, the target voltage can be achieved at a frequency f5, which is within a frequency range including the maximum value fmax of the operating switching frequency fsw.

[0158] exist Figure 18 In step S7A, the control circuit 15D determines whether the current switching frequency fsw is below a predetermined threshold fth2. If yes, it returns to step S2; otherwise, it returns to step S6. For example, when the switching circuit SW is turned on and the load device 6 has a predetermined desired load current (e.g., 25% of the rated current), the threshold fth2 can also be set to the switching frequency that achieves the target voltage.

[0159] Figure 20 To illustrate in Figure 18 The graph shows the operation of the power conversion device 5D for an hour when the switching frequency fsw is lower than the threshold fth2 in step S7A. When the switching frequency fsw is below the threshold fth2 (yes in step S7A), it is determined that the load current is above the medium level. When the load current is above the medium level, such as Figure 20 As shown by the solid and dashed lines, the output voltage Vout exhibits minimal variation at the switching frequency fsw regardless of whether the switching circuit SW is on or off. The target voltage can be achieved at frequencies f7 and f8, which fall within the frequency range below the maximum value fmax of the operating switching frequency fsw. Therefore, by disconnecting the switching circuit SW in step S2, the current imbalance caused by differences in the circuit components of the power conversion device 5D can be improved while simultaneously generating an output voltage Vout near the target voltage.

[0160] [Effects of the Second Implementation]

[0161] As an alternative to the output current Iout, the power conversion device 5D according to the second embodiment determines whether the load current of the load device 6 is below a predetermined reference based on the switching frequency fsw. Therefore, like the power conversion device 5D according to the first embodiment, the power conversion device 5D according to the second embodiment can achieve the desired output voltage Vout even if the magnitude of the load current changes.

[0162] [Variation Example]

[0163] The embodiments of this disclosure have been described in detail above; however, the foregoing description is merely illustrative in all respects. Various modifications and variations can be made without departing from the scope of this disclosure. For example, the following changes are possible. It should be noted that, in the following text, the same reference numerals are used for the same constituent elements as in the above embodiments, and descriptions of the same points as in the above embodiments are appropriately omitted. The following variations can be appropriately combined.

[0164] The power conversion device according to the embodiment can also be provided with a switching circuit including a relay instead of the switching circuit SW. Figure 3 and Figure 4 A switching circuit including a switching element such as a metal-oxide-semiconductor field-effect transistor (MOSFET) and the like as shown.

[0165] The power conversion device according to the embodiment can also be configured to have four or more LLC resonant converters, thereby generating four-phase or more alternating current power, and is not limited to three-phase. In this case, each LLC resonant converter is provided with a transformer having a primary winding and a secondary winding, an inverter circuit connected to the primary winding, a resonant capacitor connected to the primary winding, and a rectifier circuit connected to the secondary winding. Each primary winding has a first end connected to the corresponding inverter circuit and a second end not connected to the inverter circuit but connected to the primary winding of the other LLC resonant converter. In this case, the switching circuit is connected between a node at which a potential intermediate between positive and negative potentials at the input terminal is generated and a node at which star-configuration junctions of the primary windings of the transformers of the respective LLC resonant converters are connected to each other. The switching circuit is controlled in the same manner as the switching circuit SW of the power conversion device according to the first or second embodiment described above.

[0166] The power system can also be provided with a direct-current power source device instead of the alternating-current power source device 1 and the rectifier 3. In addition, the power system can also be provided with an inverter and an alternating-current load device instead of the direct-current load device 6. The power conversion device according to the embodiment can also be used in these cases.

[0167] [SUMMARY]

[0168] The power conversion device and the power system according to aspects of the present disclosure can also be configured as follows.

[0169] One aspect of the present disclosure relates to a power conversion device 5 provided with three or more LLC resonant converters 11 to 13. Each LLC resonant converter 11 to 13 is provided with a transformer T1 to T3 having a primary winding and a secondary winding, an inverter circuit 21, 23, 25 connected to the primary winding, a resonant capacitor Cr1 to Cr3 connected to the primary winding, and a rectifier circuit 22, 24, 26 connected to the secondary winding. The primary winding has a first end portion al to a3 connected to the inverter circuit 21, 23, 25 and a second end portion bl to b3 not connected to the inverter circuit 21, 23, 25 but connected to the primary winding of the other LLC resonant converter. The power conversion device 5 is further provided with a first input terminal P1 and a second input terminal P2, a first node N1, a second node N2, a switching circuit SW, a drive circuit 14, a control circuit 15, and a first output terminal P3 and a second output terminal P4. The first input terminal P1 and the second input terminal P2 are connected to the inverter circuit 21, 23, 25 of each LLC resonant converter 11 to 13. The first node N1 generates a potential intermediate between the potential of the first input terminal P1 and the potential of the second input terminal P2. The second node N2 is connected to the second end portion bl to b3 of the primary winding of each LLC resonant converter 11 to 13. The switching circuit SW is connected between the first node N1 and the second node N2. The drive circuit 14 causes the inverter circuit 21, 23, 25 of each LLC resonant converter 11 to 13 to operate at a predetermined switching frequency fsw and at phases different from each other, respectively. The control circuit 15 controls the switching circuit SW and the drive circuit 14. The first output terminal P3 and the second output terminal P4 are connected to the rectifier circuit 22, 24, 26 of each LLC resonant converter 11 to 13. When the load current of a load device 6 connected to the first output terminal P3 and the second output terminal P4 is below a predetermined reference, the control circuit 15 turns on the switching circuit SW, and when the load current of the load device 6 is greater than the reference, the control circuit 15 turns off the switching circuit SW.

[0170] One aspect of the present disclosure relates to the power conversion device 5 further provided with a current sensor 17 that measures an output current Iout of the first output terminal P3 or the second output terminal P4. When the output current Iout is below a first threshold Ith, the control circuit 15 determines that the load current of the load device 6 is below the reference and turns on the switching circuit SW, and when the output current Iout is greater than the first threshold Ith, the control circuit 15 determines that the load current of the load device 6 is greater than the reference and turns off the switching circuit SW.

[0171] According to an aspect of the present disclosure, in the power conversion device 5, when the switching frequency fsw is equal to or higher than a second threshold value fth1, the control circuit 15D determines that the load current of the load device 6 is lower than a reference, and turns on the switching circuit SW, and when the switching frequency fsw is lower than a third threshold value fth2 that is lower than the second threshold value fth1, the control circuit 15D determines that the load current of the load device 6 is greater than the reference, and turns off the switching circuit SW.

[0172] According to an aspect of the present disclosure, the power conversion device 5 further includes a voltage sensor 16 that measures an output voltage Vout of the first output terminal P3 and the second output terminal P4. The control circuit 15 controls the drive circuit 14 to change the switching frequency fsw so that the output voltage Vout approaches a target voltage that is determined in advance.

[0173] According to an aspect of the present disclosure, the power conversion device 5 includes a switching circuit SW configured to allow current to flow in both directions when turned on and to block current in either direction when turned off.

[0174] According to an aspect of the present disclosure, the power conversion device 5 further includes a pair of second capacitors C1, C2 connected in series with each other across the first input terminal P1 and the second input terminal P2. The pair of second capacitors C1, C2 have equal capacitances. The first node N1 is provided between the pair of second capacitors C1, C2.

[0175] According to an aspect of the present disclosure, the power system includes: a power source device that supplies a first direct-current voltage; a power conversion device 5 that converts the first direct-current voltage into a second direct-current voltage; and a load device 6 that operates by the second direct-current voltage.

[0176] According to an aspect of the present disclosure, the power system includes: a power source device that supplies a first direct-current voltage; a power conversion device 5 that converts the first direct-current voltage into a second direct-current voltage; and a load device 6 that operates by the second direct-current voltage.

[0177] Industrial Applicability

[0178] According to an aspect of the present disclosure, the power conversion device can be applied to a power system that operates as a power source system to which alternating-current power is input and that outputs direct-current power of a size of 2 kW or less.

[0179] Explanation of Reference Signs

[0180] 1 AC power source device

[0181] 2 Noise filter device

[0182] 3 Rectifier

[0183] 4 power factor regulator

[0184] 5, 5B-5D power conversion device

[0185] 6 load device

[0186] 11-13, 11B-13B, 11C-13C LLC resonant converter

[0187] 14 drive circuit

[0188] 15, 15D control circuit

[0189] 16 voltage sensor

[0190] 17 current sensor

[0191] 21, 23, 25 inverter circuit

[0192] 22, 24, 26, 22B, 24B, 26B, 22C, 24C, 26C rectifier circuit

[0193] Cr1-Cr3 resonant capacitor

[0194] C1-C3, C31-C36 capacitor

[0195] D1-D6, D11-D22, D31-D36, D101, D102 diode

[0196] Lm1-Lm3 magnetizing inductance

[0197] Lr1-Lr3 leakage inductance

[0198] Lr11-Lr13 inductor

[0199] N1, N2, N11-N13 node

[0200] P1, P2 input terminal

[0201] P3, P4 output terminal

[0202] Q1-Q6, Q101, Q102, Q111, Q112 switching element

[0203] SW, SWA switching circuit

[0204] T1-T3, T11-T13 transformer

[0205] w1-w9, w11-w13 winding

Claims

1. A power conversion device provided with three or more LLC resonant converters, wherein each of the LLC resonant converters is provided with a transformer having a primary winding and a secondary winding, an inverter circuit connected to the primary winding, a first capacitor connected to the primary winding, and a rectifier circuit connected to the secondary winding, the primary winding having a first end connected to the inverter circuit and a second end not connected to the inverter circuit but connected to a primary winding of another LLC resonant converter, the power conversion device is further provided with: first and second input terminals connected to the inverter circuit of each of the LLC resonant converters; a first node generating a potential intermediate between the potentials of the first and second input terminals; a second node connected to the second end of the primary winding of each of the LLC resonant converters; a switching circuit connected between the first and second nodes; a drive circuit causing the inverter circuit of each of the LLC resonant converters to operate at a predetermined switching frequency and at phases different from each other; a control circuit controlling the switching circuit and the drive circuit; and first and second output terminals connected to the rectifier circuit of each of the LLC resonant converters, the control circuit turns on the switching circuit when a load current of a load device connected to the first and second output terminals is below a predetermined reference, and turns off the switching circuit when the load current of the load device is greater than the reference.

2. The power conversion device according to claim 1, wherein the power conversion device is further provided with a current sensor that measures an output current of the first or second output terminal, the control circuit determines that the load current of the load device is below the reference when the output current is below a first threshold, and turns on the switching circuit, and determines that the load current of the load device is greater than the reference when the output current is greater than the first threshold, and turns off the switching circuit.

3. The power conversion device according to claim 1, wherein the control circuit determines that the load current of the load device is below the reference when the switching frequency is above a second threshold, and turns on the switching circuit, and determines that the load current of the load device is greater than the reference when the switching frequency is below a third threshold smaller than the second threshold, and turns off the switching circuit.

4. The power conversion device according to any one of claims 1 to 3, wherein the power conversion device is further provided with a voltage sensor that measures an output voltage of the first and second output terminals, the control circuit controls the drive circuit to cause the switching frequency to change so that the output voltage approaches a target voltage decided in advance.

5. The power conversion device according to any one of claims 1 to 3, wherein ​ The switching circuit is configured to allow current to flow in both directions when turned on and to block current in either direction when turned off.

6. The power conversion device according to any one of claims 1 to 3, wherein The power conversion device further includes a pair of second capacitors connected in series with each other across the first input terminal and the second input terminal, the pair of second capacitors having equal capacitances, The first node is provided between the pair of second capacitors.

7. A power system comprising: a power supply device that supplies a first direct-current voltage; the power conversion device according to any one of claims 1 to 6 that converts the first direct-current voltage into a second direct-current voltage; and a load device that operates by the second direct-current voltage.

8. The power system according to claim 7, wherein The power supply device includes: a noise filter device that reduces at least one of a normal-mode noise signal and a common-mode noise signal; a rectifier that converts an alternating-current voltage into the first direct-current voltage; and a power factor regulator that makes a power factor of the first direct-current voltage close to 1.

Images