Power conversion device and control method

Abstract

The present invention suppresses fluctuations in an output voltage of a power conversion device which occur when switching is made from a full bridge operation mode to a half bridge operation mode. A power conversion device (1) comprises: a first H switch (AH), a first L switch (AL), a second H switch (BH), and a second L switch (BL); a transformer (T1); a capacitor (Cr) and an inductor (Lr); a voltage measurement circuit (20) that measures the voltage across both ends of the capacitor (Cr); and a control circuit (10). The control circuit (10) is capable of switching between a full bridge operation mode and a half bridge operation mode. When switching is made from the full bridge operation mode to the half bridge operation mode, the first H switch (AH) is turned on or an on state thereof is maintained. When the voltage across both ends the capacitor (Cr) has reached a threshold voltage within a prescribed voltage range, the first H switch (AH) is turned off and then, from a phase at 0.5 cycles in the half bridge operation mode, driving of the first H switch (AH) in the half bridge operation mode starts.

Description

Power conversion device and control method

[0001] The present disclosure relates to a power conversion device that steps up or steps down an input voltage to a predetermined voltage and outputs the resulting voltage, and a method for controlling the power conversion device.

[0002] Patent Document 1 describes controlling a full-bridge circuit on the primary side of an isolated power conversion device by switching between a full-bridge operation mode and a half-bridge operation mode, and describes that the drive frequency for each mode is acquired in advance. As a result, by switching the drive frequency to the previously acquired drive frequency at the same time as switching the operation mode, it is possible to adjust the output voltage to a target value.

[0003] JP 2013-188084 A

[0004] However, in the technology disclosed in Patent Document 1, when switching from full-bridge operation mode (hereinafter referred to as FB operation mode) to half-bridge operation mode (hereinafter referred to as HB operation mode), the output voltage fluctuates, and there is a risk that, for example, an excessive load will be placed on auxiliary equipment connected to the output terminal.

[0005] Therefore, the present disclosure provides a power conversion device and the like that can suppress fluctuations in output voltage that occur when switching from the FB operation mode to the HB operation mode.

[0006] A power conversion device according to the present disclosure is a power conversion device that boosts or drops an input voltage to a predetermined voltage and outputs the voltage, and includes a first high-side switch, a first low-side switch, a second high-side switch, a second low-side switch, an isolation transformer, a resonant capacitor, a resonant inductor, a voltage measurement circuit, and a control circuit. The first high-side switch is provided on a first path connecting a first input terminal and a second input terminal. The first low-side switch is provided on the first path and connected in series with the first high-side switch. The second high-side switch is provided on a second path connecting the first input terminal and the second input terminal, which is different from the first path. The second low-side switch is provided on the second path and connected in series with the second high-side switch. The isolation transformer has a primary winding, and the primary winding is connected between a first node on the first path between the first high-side switch and the first low-side switch and a second node on the second path between the second high-side switch and the second low-side switch. The resonant capacitor and the resonant inductor are connected between a first node and the primary winding or between a second node and the primary winding. A voltage measurement circuit measures the voltage across the resonant capacitor. The control circuit controls switching of the first high-side switch, the first low-side switch, the second high-side switch, and the second low-side switch. The first high-side switch and the second low-side switch form a first switch group, the first low-side switch and the second high-side switch form a second switch group, the first high-side switch and the first low-side switch form a first leg, and the second high-side switch and the second low-side switch form a second leg. In this case, the control circuit can switch between a full-bridge operation mode and a half-bridge operation mode. The full-bridge operation mode is an operation mode in which the first switch group is turned on, the first switch group is turned off after a certain period, then the second switch group is turned on, and the second switch group is turned off after a certain period, and this is repeated.The half-bridge operation mode is an operation mode in which the high-side switch of one of the first and second legs is fixed off and the low-side switch of the other leg is fixed on, and the following operation is repeated: the high-side switch of the other of the first and second legs is turned on, and after a certain period of time, the high-side switch is turned off, and then the low-side switch of the other leg is turned on, and after a certain period of time, the low-side switch is turned off, and this process is repeated. When switching from the full-bridge operation mode to the half-bridge operation mode, the high-side switch of the other leg is turned on or maintained in an on state, and the high-side switch is turned off when the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range. This cycle is defined as one period in which the high-side switch is driven from the on state to the off state in the half-bridge operation mode. In this case, the high-side switch starts to be driven in the half-bridge operation mode from the 0.5-cycle phase.

[0007] A control method according to the present disclosure is a control method for a power conversion device that boosts or bucks an input voltage to a predetermined voltage and outputs the boosted voltage. The power conversion device includes a first high-side switch, a first low-side switch, a second high-side switch, a second low-side switch, an isolation transformer, a resonant capacitor, a resonant inductor, a voltage measurement circuit, and a control circuit. The first high-side switch is provided on a first path connecting a first input terminal and a second input terminal. The first low-side switch is provided on the first path and connected in series with the first high-side switch. The second high-side switch is provided on a second path connecting the first input terminal and the second input terminal, which is different from the first path. The second low-side switch is provided on the second path and connected in series with the second high-side switch. The isolation transformer has a primary winding, and the primary winding is connected between a first node on the first path between the first high-side switch and the first low-side switch and a second node on the second path between the second high-side switch and the second low-side switch. A resonant capacitor and a resonant inductor are connected between a first node and the primary winding or between a second node and the primary winding. A voltage measurement circuit measures the voltage across the resonant capacitor. The first high-side switch and the second low-side switch are defined as a first switch group, the first low-side switch and the second high-side switch are defined as a second switch group, the first high-side switch and the first low-side switch are defined as a first leg, and the second high-side switch and the second low-side switch are defined as a second leg. In this case, the control method can switch between a full-bridge operation mode and a half-bridge operation mode. In the full-bridge operation mode, the first switch group is turned on, and after a certain period of time, the first switch group is turned off, and then the second switch group is turned on, and after a certain period of time, the second switch group is turned off, and this is repeated. In the half-bridge operation mode, the high-side switch of one of the first and second legs is fixed off, and the low-side switch of the other leg is fixed on, and the following is repeated.That is, the high-side switch of the other of the first and second legs is turned on, and after a certain period of time, the high-side switch is turned off. Then, the low-side switch of the other leg is turned on, and after a certain period of time, the low-side switch is turned off. This process is repeated. When switching from full-bridge operation mode to half-bridge operation mode, the high-side switch of the other leg is turned on or maintained in the on state. When the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range, the high-side switch is turned off. This cycle is defined as one cycle in which the high-side switch is driven in the half-bridge operation mode from the on state to the off state. In this case, the high-side switch starts to be driven in the half-bridge operation mode from the 0.5-cycle phase.

[0008] These comprehensive or specific aspects may be realized as a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium such as a CD-ROM, or may be realized as any combination of a system, a method, an integrated circuit, a computer program, and a recording medium.

[0009] According to a power conversion device according to an aspect of the present disclosure, fluctuations in output voltage that occur when switching from the FB operation mode to the HB operation mode can be suppressed.

[0010] FIG. 1 is a circuit configuration diagram showing an example of a power conversion device according to an embodiment. FIG. 2 is a diagram for explaining switching from an FB operation mode to an HB operation mode. FIG. 3 is a diagram for explaining consideration of fluctuations in output voltage that occur when switching from the FB operation mode to the HB operation mode. FIG. 4 is a diagram for explaining control details of a high-side switch of the other leg when switching from the FB operation mode to the HB operation mode. FIG. 5 is a diagram for explaining control details of a high-side switch of the other leg when switching from the FB operation mode to the HB operation mode. FIG. 6 is a diagram for explaining an example of a predetermined voltage range. FIG. 7 is a circuit configuration diagram showing an example of a voltage measurement circuit according to an embodiment. FIG. 8 is a circuit configuration diagram showing an example of a voltage measurement circuit according to an embodiment. FIG. 9 is a flowchart showing an example of a control method according to another embodiment.

[0011] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0012] The embodiments described below are all comprehensive or specific examples, and the numerical values, shapes, materials, components, arrangement and connection of the components, steps, and order of steps shown in the following embodiments are merely examples and are not intended to limit the present disclosure.

[0013] (Embodiment) Hereinafter, a power conversion device according to an embodiment will be described.

[0014] FIG. 1 is a circuit configuration diagram showing an example of a power conversion device 1 according to an embodiment.

[0015] The power conversion device 1 is an isolated DC-DC converter that boosts or bucks an input voltage to a predetermined voltage and outputs it. For example, the power conversion device 1 is an LLC converter that utilizes LLC resonance due to the leakage inductance, excitation inductance, and resonant capacitor of a transformer. The LLC converter performs frequency control to change the switching frequency of each switch on the primary side and phase shift control to change the phase difference between the switching of each switch on the primary side, thereby changing the input / output voltage ratio (Gain), and thereby enabling the desired power output.

[0016] The power conversion device 1 has terminals t1, t2, t3, and t4. Terminal t1 is an example of a first input terminal. Terminal t2 is an example of a second input terminal, specifically a ground terminal. Terminal t3 is an output terminal, and terminal t4 is a ground terminal. Note that, since the power conversion device 1 is an isolated DC-DC converter, terminals t2 and t4 are electrically isolated. An input voltage and an input current are input to terminal t1. The input voltage is the voltage between terminals t1 and t2. An output voltage and an output current are output from terminal t3. The output voltage is the voltage between terminals t3 and t4.

[0017] For example, the power conversion device 1 is mounted on a vehicle and applied to an electric vehicle system that drives auxiliary equipment. For example, a high-voltage lithium-ion battery or the like is connected to terminals t1 and t2, and a low-voltage lead-acid battery and auxiliary equipment are connected to terminals t3 and t4. For example, the voltage of a lithium-ion battery is 250 V to 450 V, and the voltage of a lead-acid battery is 10 V to 16 V. For example, to convert a high voltage of 250 V to 450 V to a low voltage of 10 V to 16 V, a power conversion device 1 such as an LLC converter that supports a wide input / output voltage ratio is used.

[0018] The power conversion device 1 includes switches AH, AL, BH, BL, CH, CL, DH, and DL, a transformer T1, a capacitor Cr, an inductor Lr, a control circuit 10, and a voltage measurement circuit 20.

[0019] The switch AH is a switch provided on a path P1 connecting the terminal t1 and the terminal t2. The switch AH is an example of a first high-side switch. The path P1 is an example of a first path. The switch AH is, for example, an N-channel metal oxide semiconductor field effect transistor (MOSFET). The drain of the switch AH is connected to the terminal t1, and the source of the switch AH is connected to the drain of the switch AL.

[0020] The switch AL is provided on the path P1 and is connected in series with the switch AH. The switch AL is an example of a first low-side switch. The switch AL is, for example, an N-channel MOSFET. The drain of the switch AL is connected to the source of the switch AH, and the source of the switch AL is connected to the terminal t2.

[0021] The switch BH is a switch provided on a path P2 that connects the terminal t1 and the terminal t2 and is different from the path P1. The switch BH is an example of a second high-side switch. The path P2 is an example of a second path. The switch BH is, for example, an N-channel MOSFET. The drain of the switch BH is connected to the terminal t1, and the source of the switch BH is connected to the drain of the switch BL.

[0022] The switch BL is provided on the path P2 and is connected in series with the switch BH. The switch BL is an example of a second low-side switch. The switch BL is, for example, an N-channel MOSFET. The drain of the switch BL is connected to the source of the switch BH, and the source of the switch BL is connected to the terminal t2.

[0023] The switches AH and AL are also referred to as a first leg, and the switches BH and BL are also referred to as a second leg. The switches AH and BL are also referred to as a first switch group, and the switches AL and BH are also referred to as a second switch group.

[0024] The transformer T1 is an example of an isolation transformer, and has a primary winding and a secondary winding that are insulated from each other.

[0025] The primary winding of the transformer T1 is connected between a node N1 on the path P1 between the switch AH and the switch AL, and a node N2 on the path P2 between the switch BH and the switch BL. The node N1 is an example of a first node, and the node N2 is an example of a second node.

[0026] The capacitor Cr and the inductor Lr are connected between the node N1 and the primary winding of the transformer T1 or between the node N2 and the primary winding of the transformer T1. The capacitor Cr is an example of a resonant capacitor, and the inductor Lr is an example of a resonant inductor. For example, the capacitor Cr is connected to the node N1, the inductor Lr is connected to the node N1 via the capacitor Cr, and the capacitor Cr and the inductor Lr are connected in series. The inductor Lr may be provided as a separate inductor, or may be provided by utilizing the leakage inductance of the transformer T1.

[0027] The capacitor Cr and the inductor Lr may be connected to the node N2. In this case, one end of the primary winding of the transformer T1 is connected to the node N1, and the other end of the primary winding of the transformer T1 is connected to the node N2 via the capacitor Cr and the inductor Lr. Although the example in which the capacitor Cr is connected to the transformer T1 via the inductor Lr has been shown, the inductor Lr may be connected to the transformer T1 via the capacitor Cr. For example, the capacitor Cr and the inductor Lr shown in FIG. 1 may be interchanged. However, if the inductor Lr is provided by utilizing the leakage inductance of the transformer T1, the inductor Lr is placed closer to the transformer T1 than the capacitor Cr (in other words, the capacitor Cr is connected to the transformer T1 via the inductor Lr).

[0028] Alternatively, the capacitor Cr may be connected to the node N1 and the inductor Lr may be connected to the node N2, or the capacitor Cr may be connected to the node N2 and the inductor Lr may be connected to the node N1. In other words, the primary winding of the transformer T1 may be connected between the capacitor Cr and the inductor Lr.

[0029] In this way, the order in which the capacitor Cr, the inductor Lr, and the primary winding of the transformer T1 are connected between the node N1 and the node N2 is not particularly limited.

[0030] In FIG. 1, the excitation inductance of the transformer T1 is indicated by an inductor Lm.

[0031] The switch CH is provided on the path P3 connecting the terminal t3 and the terminal t4. The switch CH is, for example, an N-channel MOSFET. The drain of the switch CH is connected to the terminal t3, and the source of the switch CH is connected to the drain of the switch CL.

[0032] The switch CL is provided on the path P3 and is connected in series with the switch CH. The switch CL is, for example, an N-channel MOSFET. The drain of the switch CL is connected to the source of the switch CH, and the source of the switch CL is connected to the terminal t4.

[0033] The switch DH is a switch provided on a path P4 that connects the terminal t3 and the terminal t4 and is different from the path P3. The switch DH is, for example, an N-channel MOSFET. The drain of the switch DH is connected to the terminal t3, and the source of the switch DH is connected to the drain of the switch DL.

[0034] The switch DL is provided on the path P4 and is connected in series with the switch DH. The switch DL is, for example, an N-channel MOSFET. The drain of the switch DL is connected to the source of the switch DH, and the source of the switch DL is connected to the terminal t4.

[0035] The secondary winding of the transformer T1 is connected between a node N3 on the path P3 between the switch CH and the switch CL, and a node N4 on the path P4 between the switch DH and the switch DL.

[0036] 1 shows the parasitic capacitance of each switch, and each parasitic capacitance is connected in parallel to the corresponding switch in the equivalent circuit. Also, FIG. 1 shows the body diode of each switch, and each body diode is connected in parallel to the corresponding switch in the equivalent circuit. Specifically, the anode of each body diode is connected to the source of the corresponding switch in the equivalent circuit, and the cathode is connected to the drain of the corresponding switch.

[0037] The power conversion device 1 does not necessarily have to include the switches CH, CL, DH, and DL. Furthermore, the circuit connected to the secondary winding of the transformer T1 may be any circuit having a rectifying function. For example, diodes may be provided instead of the switches CH, CL, DH, and DL.

[0038] The control circuit 10 controls the switching of the switches AH, AL, BH, and BL. The control circuit 10 also controls the switching of the switches CH, CL, DH, and DL. For example, the control circuit 10 performs synchronous rectification control by controlling the switching of the switches CH, CL, DH, and DL. For example, the control circuit 10 controls the switching of the switches AH, AL, BH, BL, CH, CL, DH, and DL by controlling a gate drive circuit (not shown) connected to the gates of the switches AH, AL, BH, BL, CH, CL, DH, and DL via a PWM generator (not shown) or the like.

[0039] The control circuit 10 is realized by, for example, a computer including a processor (microprocessor) and a memory. The memory may be a read-only memory (ROM) or a random access memory (RAM), and can store programs executed by the processor. For example, the control circuit 10 is realized by a microcontroller.

[0040] The voltage measurement circuit 20 measures the voltage across the capacitor Cr. Details of the voltage measurement circuit 20 will be described later.

[0041] The control circuit 10 has an FB operation mode and an HB operation mode, and is capable of switching between the FB operation mode and the HB operation mode.

[0042] The FB operation mode is a mode in which (1) the first switch group (switches AH and BL) is turned on, (2) the first switch group is turned off after a certain period of time, (3) thereafter the second switch group (switches AL and BH) is turned on, and (4) the second switch group is turned off after a certain period of time, and the following is repeated. In other words, in the FB operation mode, the above steps (1) to (4) are repeated.

[0043] The HB operation mode is a mode in which, with the high-side switch of one of the first and second legs fixed off and the low-side switch of the other leg fixed on, (5) the high-side switch of the other of the first and second legs is turned on, (6) the high-side switch is turned off after a certain period of time, (7) thereafter the low-side switch of the other leg is turned on, and (8) the low-side switch is turned off after a certain period of time, and the following is repeated. In other words, in the HB operation mode, with the high-side switch of one of the first and second legs fixed off and the low-side switch of the other leg fixed on, the above steps (5) to (8) are repeated.

[0044] When one leg is the second leg and the other leg is the first leg, in the HB operation mode, the switch BH is fixed off and the switch BL is fixed on, and the switch AH is turned on, and the switch AH is turned off after a certain period of time, and then the switch AL is turned on, and the switch AL is turned off after a certain period of time, and this is repeated.Also, when one leg is the first leg and the other leg is the second leg, in the HB operation mode, the switch AH is fixed off and the switch AL is fixed on, and the switch BH is turned on, and the switch BH is turned off after a certain period of time, and then the switch BL is turned on, and the switch BL is turned off after a certain period of time, and this is repeated.

[0045] When switching from the FB operation mode to the HB operation mode, the control circuit 10 turns on the high-side switch of the other leg or keeps it in the on state, and turns off the high-side switch when the voltage across the capacitor Cr reaches a threshold voltage within a predetermined voltage range. Thereafter, when one cycle is defined as the high-side switch being driven from the on state to the off state in the HB operation mode, the control circuit 10 starts driving the high-side switch in the HB operation mode from a phase of 0.5 cycles.

[0046] When one leg is the second leg and the other leg is the first leg, the control circuit 10 turns on or maintains the on state of the switch AH when switching from the FB operation mode to the HB operation mode, turns off the switch AH when the voltage across the capacitor Cr reaches a threshold voltage within a predetermined voltage range, and then starts driving the switch AH in the HB operation mode from phase 0.5 of a cycle in which the switch AH is driven from the on state to the off state in the HB operation mode. When one leg is the first leg and the other leg is the second leg, the control circuit 10 turns on or maintains the on state of the switch BH when switching from the FB operation mode to the HB operation mode, turns off the switch BH when the voltage across the capacitor Cr reaches a threshold voltage within a predetermined voltage range, and then starts driving the switch BH in the HB operation mode from phase 0.5 of a cycle in which the switch BH is driven from the on state to the off state in the HB operation mode.

[0047] The operation of the control circuit 10 will be described in detail later.

[0048] Next, the reason why switching from the FB operation mode to the HB operation mode is necessary will be explained with reference to FIG.

[0049] 2 is a diagram for explaining switching from the FB operation mode to the HB operation mode, and shows graphs illustrating the relationship between the switching frequency and the gain in the FB operation mode and the relationship between the switching frequency and the gain in the HB operation mode.

[0050] For example, when the gain is reduced in the FB operation mode, it is necessary to increase the switching frequency and the phase shift amount. Therefore, when the gain is reduced in the FB operation mode, the number of switching operations increases and the reactive power period due to the phase shift becomes longer, resulting in a decrease in efficiency. Therefore, when the gain is reduced, the FB operation mode is switched to the HB operation mode. As shown in FIG. 2 , in the HB operation mode, the switching frequency can be reduced while maintaining the same desired gain value as in the FB operation mode. Furthermore, in the HB operation mode, the high-side switch of one of the first and second legs is fixed off, and the low-side switch of the other leg is fixed on, eliminating phase shift control and shortening the reactive power period. Therefore, when the gain is reduced, switching from the FB operation mode to the HB operation mode reduces the number of switching operations and shortens the reactive power period, thereby improving efficiency.

[0051] For example, the determination of whether to switch the operation mode may be made by the control circuit 10 or by a component other than the control circuit 10. When the determination of whether to switch the operation mode is made by a component other than the control circuit 10, the control circuit 10 receives an instruction from that component to switch the operation mode, and controls each switch with control content according to the operation mode after switching.

[0052] However, when switching from the FB operation mode to the HB operation mode, the output voltage may fluctuate. Here, a consideration of the output voltage fluctuation that occurs when switching from the FB operation mode to the HB operation mode will be explained using Fig. 3. In Fig. 3, it is assumed that one leg is the second leg and the other leg is the first leg.

[0053] 3 is a diagram for explaining consideration of the fluctuation in output voltage that occurs when switching from the FB operation mode to the HB operation mode. The left side of Fig. 3 shows the resonant current, the voltage across the capacitor Cr, the drive signal for the switch AH, the operation mode, and the output voltage when the fluctuation in output voltage that occurs when switching from the FB operation mode to the HB operation mode is large. The right side of Fig. 3 shows the resonant current, the voltage across the capacitor Cr, the drive signal for the switch AH, the operation mode, and the output voltage when the fluctuation in output voltage that occurs when switching from the FB operation mode to the HB operation mode is small.

[0054] As shown in FIG. 3 , it was found that the fluctuation in the output voltage occurring when switching from the FB operation mode to the HB operation mode increases or decreases depending on the situation. The inventors discovered that, as shown in FIG. 3 , when switching from the FB operation mode to the HB operation mode, the output voltage fluctuation is large when the voltage across capacitor Cr increases significantly, and the output voltage fluctuation is small when the increase in the voltage across capacitor Cr is suppressed. Because the output voltage fluctuates depending on the resonant current, and the resonant current fluctuates depending on the voltage across capacitor Cr, it is believed that a large increase in the voltage across capacitor Cr causes a large fluctuation in the resonant current, which in turn causes a corresponding fluctuation in the output voltage. Furthermore, the inventors discovered that, as shown in FIG. 3 , the increase in the voltage across capacitor Cr is suppressed depending on the timing of the first turn-off of switch AH (or switch BH if one leg is the first leg and the other leg is the second leg) after switching from the FB operation mode to the HB operation mode.

[0055] From this, the inventors have come up with the idea that by measuring the voltage across capacitor Cr, and switching from the FB operation mode to the HB operation mode, and then turning off switch AH (note that if one leg is the first leg and the other leg is the second leg, switch BH) when the voltage across capacitor Cr reaches a threshold voltage within a predetermined voltage range, it is possible to suppress the increase in the voltage across capacitor Cr after the voltage across capacitor Cr reaches the threshold voltage, and ultimately to suppress fluctuations in the output voltage.

[0056] The operation of the control circuit 10 will be described in detail below, taking as an example a case where one leg is the second leg and the other leg is the first leg.

[0057] 4A and 4B are diagrams illustrating the control of the high-side switch of the other leg (switch AH in this case) when switching from the FB operation mode to the HB operation mode. Fig. 4A shows the time waveform of capacitor Cr. Fig. 4B shows an example of a drive signal in the FB operation mode, an example of a drive signal in the HB operation mode, and an example of the drive signal of switch AH before and after switching from the FB operation mode to the HB operation mode.

[0058] When switching from the FB operation mode to the HB operation mode, the control circuit 10 fixes the switch BH to the off state and fixes the switch BL to the on state. Furthermore, when switching from the FB operation mode to the HB operation mode, the control circuit 10 turns on the switch AH or maintains it in the on state. Specifically, when switching from the FB operation mode to the HB operation mode, the control circuit 10 turns on the switch AH if it is in the off state, and maintains the switch AH in the on state if it is in the on state. As a result, when the switches AH and BL are in the on state, a voltage is applied to one end of the capacitor Cr, and as shown in FIG. 4A , the voltage across the capacitor Cr increases after switching from the FB operation mode to the HB operation mode.

[0059] After switching from the FB operation mode to the HB operation mode, the control circuit 10 turns off the switch AH once when the voltage across the capacitor Cr measured by the voltage measurement circuit 20 reaches a threshold voltage within a predetermined voltage range, as shown in Fig. 4A. A specific example of the predetermined voltage range will be described later.

[0060] When one cycle is defined as the driving of the switch AH from the ON state to the OFF state in the HB operation mode, the control circuit 10 starts driving the switch AH in the HB operation mode from the phase of the 0.5 cycle. Figure 4B shows one cycle in the HB operation mode and the timing of the phase 0.5 cycle in the HB operation mode. The timing of the phase 0.5 cycle in the HB operation mode is the timing at which the switch AH, which has been in the ON state for the first 0.5 cycle of one cycle in the HB operation mode, turns off.

[0061] As shown in the graph of the drive signal for switch AH in Figure 4B, until the timing of switching from the FB operation mode to the HB operation mode, switch AH is controlled by the drive signal in the FB operation mode at a switching frequency higher than the switching frequency in the HB operation mode. In Figure 4B, switching from the FB operation mode to the HB operation mode occurs at a timing when switch AH controlled by the drive signal in the FB operation mode is in the OFF state, so the control circuit 10 turns on switch AH at that timing. Note that if switching from the FB operation mode to the HB operation mode occurs at a timing when switch AH controlled by the drive signal in the FB operation mode is in the ON state, the control circuit 10 maintains switch AH in the ON state.

[0062] 4A, the control circuit 10 turns off the switch AH once when the voltage across the capacitor Cr reaches a threshold voltage within a predetermined voltage range. Then, at the timing when the control circuit 10 turns off the switch AH, the control circuit 10 starts driving the switch AH in the HB operation mode from the phase of 0.5 cycles in the HB operation mode described above.

[0063] Next, specific examples of the predetermined voltage range will be described with reference to FIGS.

[0064] 5 and 6 are diagrams for explaining an example of the predetermined voltage range, each showing a time waveform of the capacitor Cr.

[0065] As shown in FIG. 5, for example, the predetermined voltage range may be a range equal to or less than the average value of the voltage across capacitor Cr when the voltage across capacitor Cr reaches a steady state after switching from the FB operation mode to the HB operation mode.

[0066] The inventors conducted simulations under various conditions and discovered that the condition under which the fluctuation in output voltage that occurs when switching from the FB operation mode to the HB operation mode tends to be small is when the voltage across the capacitor Cr is in a range equal to or less than the average value of the voltage across the capacitor Cr when the voltage across the capacitor Cr is in a steady state. Therefore, by turning off the switch AH when the voltage across the capacitor Cr reaches a threshold voltage that is within the range equal to or less than the average value, the fluctuation in output voltage that occurs when switching from the FB operation mode to the HB operation mode can be suppressed.

[0067] Also, as shown in FIG. 6, for example, the predetermined voltage range may be a range of less than or equal to the average value and more than half of the average value.

[0068] The inventors conducted simulations under various conditions and discovered that the tendency of the condition for further reducing the fluctuation in output voltage that occurs when switching from the FB operation mode to the HB operation mode is that the switch AH turns off when the voltage across the capacitor Cr is in a range of less than or equal to the average value of the voltage across the capacitor Cr when the voltage across the capacitor Cr is in a steady state and more than half of that average value. Therefore, by turning off the switch AH when the voltage across the capacitor Cr reaches a threshold voltage that is less than or equal to the average value and more than half of that average value, it is possible to further suppress the fluctuation in output voltage that occurs when switching from the FB operation mode to the HB operation mode.

[0069] Next, the voltage measurement circuit 20 will be described in detail with reference to FIGS.

[0070] 7 to 9 are circuit configuration diagrams showing an example of a voltage measurement circuit 20 according to an embodiment. In addition to the voltage measurement circuit 20, a capacitor Cr and a control circuit 10 are also shown in FIGS.

[0071] As shown in FIG. 7, the voltage measurement circuit 20 may include a voltage conversion circuit 21 and a comparator 22 .

[0072] The voltage conversion circuit 21 is a circuit that outputs a voltage corresponding to the voltage across the capacitor Cr. For example, the voltage conversion circuit 21 is a voltage divider circuit. For example, the voltage corresponding to the voltage across the capacitor Cr is a voltage within a voltage range handled by the control circuit 10 (for example, approximately 3 V to 5 V). The voltage across the capacitor Cr may be a high voltage (for example, approximately 250 V to 450 V) that cannot be handled by the control circuit 10. Therefore, by converting the voltage across the capacitor Cr using the voltage conversion circuit 21, the voltage across the capacitor Cr can be handled by the control circuit 10.

[0073] The comparator 22 has a first input terminal and a second input terminal. For example, the first input terminal is a positive input terminal, and the second input terminal is a negative input terminal. The comparator 22 receives the voltage output from the voltage conversion circuit 21 at its first input terminal and a reference voltage corresponding to the threshold voltage at its second input terminal, and outputs a comparison result between the voltage output from the voltage conversion circuit 21 and the reference voltage to the control circuit 10. By using the comparator 22, it is possible to determine whether the voltage output from the voltage conversion circuit 21 has reached the reference voltage, i.e., whether the voltage across the capacitor Cr has reached the threshold voltage.

[0074] As shown in FIG. 8 , the control circuit 10 may output a reference voltage to the second input terminal. For example, the control circuit 10 may calculate the reference voltage based on the input voltage. Because the optimal threshold voltage varies depending on the input voltage to the power conversion device 1, for example, by preparing a lookup table or an arithmetic expression indicating the correspondence between the input voltage and the reference voltage in advance, the optimal reference voltage can be calculated depending on the input voltage to the power conversion device 1. For example, by performing a simulation or the like to obtain in advance the average value of the voltage across the capacitor Cr when the voltage across the capacitor Cr is in a steady state for each input voltage, it is possible to calculate a reference voltage corresponding to a threshold voltage within a range equal to or less than the average value, or a reference voltage corresponding to a threshold voltage within a range equal to or less than the average value and at least half of the average value.

[0075] The reference voltage does not have to be calculated by the control circuit 10, but may be a predetermined voltage.

[0076] The voltage measurement circuit 20 may further include an isolator provided on at least one of the nodes between the voltage conversion circuit 21 and the first input terminal and the node between the control circuit 10 and the second input terminal. FIG. 8 shows an example in which the voltage measurement circuit 20 includes an isolator 23a provided on the node between the voltage conversion circuit 21 and the first input terminal and an isolator 23b provided on the node between the control circuit 10 and the second input terminal. The isolators 23a and 23b are, for example, isolation amplifiers or photocouplers. Because a high voltage may be handled in the path in which the capacitor Cr is disposed, providing the isolator 23a or 23b can prevent the high voltage from being input to the control circuit 10.

[0077] The voltage measurement circuit 20 may further include a relay provided on at least one of the multiple nodes connected to the voltage conversion circuit 21, and the control circuit 10 may turn the relay off in the FB operation mode and turn the relay on in the HB operation mode. Figure 9 shows an example in which the voltage measurement circuit 20 includes a relay 24a provided on a node connected between the voltage conversion circuit 21 and one end of the capacitor Cr, a relay 24b provided on a node connected between the voltage conversion circuit 21 and the other end of the capacitor Cr, and a relay 24c provided on a node connected between the voltage conversion circuit 21 and the isolator 23a. Because high voltages may be handled in the path in which the capacitor Cr is located, the relay is turned off in the FB operation mode, in which the measurement result of the voltage across the capacitor Cr is not required, and the relay is turned on in the HB operation mode, in which the measurement result of the voltage across the capacitor Cr is required, thereby preventing high voltages from being input to the control circuit 10.

[0078] 9 shows an example in which the voltage measurement circuit 20 includes the isolators 23a and 23b, but the voltage measurement circuit 20 does not have to include the isolators 23a and 23b in FIG. 9. In other words, the voltage measurement circuit 20 does not have to include at least one of the isolators 23a and 23b and at least one of the relays 24a, 24b, and 24c.

[0079] As described above, the inventors have come up with the idea that by measuring the voltage across capacitor Cr and switching from the FB operation mode to the HB operation mode, and then turning off switch AH when the voltage across capacitor Cr reaches a threshold voltage within a predetermined voltage range, the increase in the voltage across capacitor Cr can be suppressed after the voltage across capacitor Cr reaches the threshold voltage, and thus fluctuations in the output voltage can be suppressed. Note that with this method, there is no need to provide a transition period for switching from the FB operation mode to the HB operation mode, as described in, for example, Japanese Patent Application Laid-Open No. 2022-183908.

[0080] Furthermore, the phase of 0.5 cycles in the HB operation mode is the timing at which the switch AH is turned off. Therefore, by starting to drive the switch AH in the HB operation mode from the phase of 0.5 cycles in the HB operation mode, the switch AH can be turned off when the voltage across the capacitor Cr reaches a threshold voltage within a predetermined voltage range, and then the switch AH can be maintained in the off state. In other words, the off state of the switch AH is maintained for a while even after the switch AH is turned off, so that the increase in the voltage across the capacitor Cr can continue to be suppressed.

[0081] In this way, it is possible to suppress fluctuations in the output voltage that occur when switching from the FB operation mode to the HB operation mode. Specifically, it is possible to prevent the output voltage from becoming excessively high when switching from the FB operation mode to the HB operation mode, and to prevent excessive loads from being applied to the auxiliary equipment connected to the terminal t3.

[0082] (Other Embodiments) As described above, the embodiments have been described as examples of the technology according to the present disclosure. However, the technology according to the present disclosure is not limited to these, and can be applied to embodiments in which modifications, substitutions, additions, omissions, etc. are made as appropriate. For example, the following modifications are also included in one embodiment of the present disclosure.

[0083] For example, although the power conversion device 1 is an LLC converter in the above embodiment, the present invention is not limited to this. For example, the power conversion device 1 may be a resonant converter including a resonator configured with a series-parallel LC, such as an LCC converter or a CLLC converter.

[0084] For example, the present disclosure can be realized not only as the power conversion device 1 but also as a control method including steps (processing) performed by components (for example, the control circuit 10) that make up the power conversion device 1.

[0085] FIG. 10 is a flowchart showing an example of a control method according to another embodiment.

[0086] The control method is a control method for a power conversion device 1 that boosts or bucks an input voltage to a predetermined voltage and outputs the voltage. The power conversion device 1 includes a first high-side switch, a first low-side switch, a second high-side switch, a second low-side switch, an isolation transformer, a resonant capacitor, a resonant inductor, and a voltage measurement circuit. The first high-side switch is provided on a first path connecting a first input terminal and a second input terminal. The first low-side switch is provided on the first path and connected in series with the first high-side switch. The second high-side switch is provided on a second path connecting the first input terminal and the second input terminal, which is different from the first path. The second low-side switch is provided on the second path and connected in series with the second high-side switch. The isolation transformer has a primary winding, and a primary winding is connected between a first node on the first path between the first high-side switch and the first low-side switch and a second node on the second path between the second high-side switch and the second low-side switch. The resonant capacitor and the resonant inductor are connected between the first node and the primary winding or between the second node and the primary winding. The voltage measurement circuit measures the voltage across the resonant capacitor. The first high-side switch and the second low-side switch are defined as a first switch group, the first low-side switch and the second high-side switch are defined as a second switch group, the first high-side switch and the first low-side switch are defined as a first leg, and the second high-side switch and the second low-side switch are defined as a second leg. In this case, the control method can switch between a full-bridge operation mode and a half-bridge operation mode. In the full-bridge operation mode, the first switch group is turned on, and after a certain period of time, the first switch group is turned off, and then the second switch group is turned on, and after a certain period of time, the second switch group is turned off, and this process is repeated. In the half-bridge operation mode, the high-side switch of one of the first leg and the second leg is fixed to off, and the following process is repeated.That is, while the low-side switch of one leg is fixed on, the high-side switch of the other of the first and second legs is turned on, and after a certain period of time, the high-side switch is turned off. Then, the low-side switch of the other leg is turned on, and after a certain period of time, the low-side switch is turned off. This process is repeated. As shown in Fig. 10 , when switching from full-bridge operation mode to half-bridge operation mode, the high-side switch of the other leg is turned on or maintained in the on state (step S11). After the high-side switch is turned off when the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range (step S12), the high-side switch is driven in the half-bridge operation mode in sequence from the on state to the off state, which constitutes one cycle. In this case, the high-side switch starts to be driven in the half-bridge operation mode from the 0.5-cycle phase (step S13).

[0087] For example, the present disclosure can be realized as a program for causing a computer (processor) to execute steps included in the control method. Furthermore, the present disclosure can be realized as a non-transitory computer-readable recording medium, such as a CD-ROM, on which the program is recorded.

[0088] For example, when the present disclosure is realized as a program (software), each step is performed by running the program using hardware resources such as a computer's CPU, memory, input / output circuits, etc. In other words, each step is performed by the CPU acquiring data from memory or input / output circuits, etc., performing calculations, and outputting the calculation results to memory or input / output circuits, etc.

[0089] In the above embodiment, each component included in the power conversion device 1 may be configured with dedicated hardware, or may be realized by executing a software program suitable for each component. Each component may be realized by a program execution unit such as a CPU or processor reading and executing a software program recorded on a recording medium such as a hard disk or semiconductor memory.

[0090] Some or all of the functions of the power conversion device 1 according to the above embodiment are typically realized as an LSI, which is an integrated circuit. These may be individually integrated into single chips, or may be integrated into a single chip that includes some or all of the functions. Furthermore, the integrated circuit is not limited to an LSI, and may be realized using a dedicated circuit or a general-purpose processor. It is also possible to use an FPGA (Field Programmable Gate Array) that can be programmed after LSI manufacturing, or a reconfigurable processor that can reconfigure the connections and settings of circuit cells within the LSI.

[0091] Furthermore, if an integrated circuit technology that can replace LSI emerges due to advances in semiconductor technology or other derived technologies, it is natural that each component included in the power conversion device 1 can be integrated using that technology.

[0092] In addition, this disclosure also includes forms obtained by making various modifications to the embodiments that a person skilled in the art would think of, and forms realized by arbitrarily combining the components and functions in each embodiment within the scope of the present disclosure.

[0093] (Additional Notes) The above description of the embodiments discloses the following techniques.

[0094] (Technology 1) A power conversion device that steps up or steps down an input voltage to a predetermined voltage and outputs the voltage, comprising: a first high-side switch provided on a first path connecting a first input terminal and a second input terminal; a first low-side switch provided on the first path and connected in series with the first high-side switch; a second high-side switch provided on a second path connecting the first input terminal and the second input terminal and different from the first path; a second low-side switch provided on the second path and connected in series with the second high-side switch; an isolation transformer having a primary winding connected between a first node on the first path between the first high-side switch and the first low-side switch and a second node on the second path between the second high-side switch and the second low-side switch; a resonant capacitor and a resonant inductor connected between the first node and the primary winding or between the second node and the primary winding; and a voltage measurement circuit that measures a voltage across the resonant capacitor. a control circuit for controlling switching of the first high-side switch, the first low-side switch, the second high-side switch, and the second low-side switch, wherein when the first high-side switch and the second low-side switch are defined as a first switch group, the first low-side switch and the second high-side switch are defined as a second switch group, the first high-side switch and the first low-side switch are defined as a first leg, and the second high-side switch and the second low-side switch are defined as a second leg, the control circuit repeats a full-bridge operation mode in which it turns on the first switch group, turns off the first switch group after a certain period of time, then turns on the second switch group, and turns off the second switch group after a certain period of time;a half-bridge operation mode in which, with the high-side switch of one of the first and second legs fixed off and the low-side switch of the one leg fixed on, the high-side switch of the other of the first and second legs is turned on, the high-side switch is turned off after a certain period of time, and then the low-side switch of the other leg is turned on and the low-side switch is turned off after a certain period of time, and this process is repeated; and when switching from the full-bridge operation mode to the half-bridge operation mode, the high-side switch of the other leg is turned on or maintained in an on state, and the high-side switch is turned off when the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range, and then, when one period is defined as the period in which the high-side switch is driven from an on state to an off state in the half-bridge operation mode, driving of the high-side switch in the half-bridge operation mode begins from a phase of 0.5 of the period.

[0095] The inventors discovered that when switching from full-bridge operation mode to half-bridge operation mode, the output voltage fluctuates significantly when the voltage across the resonant capacitor increases significantly, whereas the output voltage fluctuates less when the increase in the voltage across the resonant capacitor is suppressed. Furthermore, the increase in the voltage across the resonant capacitor is suppressed depending on the timing of the first turn-off of the high-side switch of the other leg after switching from full-bridge operation mode to half-bridge operation mode. Based on this, the inventors conceived the idea that by measuring the voltage across the resonant capacitor and, after switching from full-bridge operation mode to half-bridge operation mode, turning off the high-side switch when the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range, the increase in the voltage across the resonant capacitor can be suppressed after the voltage across the resonant capacitor reaches the threshold voltage, thereby suppressing the fluctuation in the output voltage. This technique does not require a transition period for switching from full-bridge operation mode to half-bridge operation mode, as described, for example, in Japanese Patent Application Laid-Open No. 2022-183908.

[0096] Furthermore, the phase of 0.5 cycles in the half-bridge operation mode is the timing at which the high-side switch is turned off. Therefore, by starting to drive the high-side switch in the half-bridge operation mode from the phase of 0.5 cycles in the half-bridge operation mode, the high-side switch can be turned off when the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range, and then the high-side switch can be maintained in an off state. In other words, since the high-side switch remains in an off state for a while even after being turned off, it is possible to continue to suppress an increase in the voltage across the resonant capacitor.

[0097] In this way, it is possible to suppress fluctuations in the output voltage that occur when switching from the full-bridge operation mode to the half-bridge operation mode. Specifically, it is possible to prevent the output voltage from becoming excessively high when switching from the full-bridge operation mode to the half-bridge operation mode, and to prevent excessive loads from being applied to accessories connected to the output terminals.

[0098] (Technology 2) A power conversion device according to Technology 1, wherein the predetermined voltage range is a range below the average value of the voltage across the resonant capacitor when the voltage across the resonant capacitor reaches a steady state after switching from the full-bridge operation mode to the half-bridge operation mode.

[0099] The inventors conducted simulations under various conditions and discovered that the high-side switch turns off when the voltage across the resonant capacitor is in a range equal to or less than the average value of the voltage across the resonant capacitor when the voltage across the resonant capacitor is in a steady state, which tends to reduce fluctuations in the output voltage when switching from full-bridge operation mode to half-bridge operation mode. Therefore, by turning off the high-side switch when the voltage across the resonant capacitor reaches a threshold voltage within a range equal to or less than the average value, fluctuations in the output voltage when switching from full-bridge operation mode to half-bridge operation mode can be suppressed.

[0100] (Technology 3) The power conversion device according to Technology 2, wherein the predetermined voltage range is a range equal to or less than the average value and equal to or more than half the average value.

[0101] The inventors conducted simulations under various conditions and discovered that the fluctuation in output voltage that occurs when switching from full-bridge operation mode to half-bridge operation mode tends to be further reduced when the voltage across the resonant capacitor is in a range of less than or equal to the average value of the voltage across the resonant capacitor when the voltage across the resonant capacitor is in a steady state and more than half of that average value, causing the high-side switch to turn off. Therefore, by turning off the high-side switch when the voltage across the resonant capacitor reaches a threshold voltage that is less than or equal to the average value and more than half of that average value, it is possible to further suppress the fluctuation in output voltage that occurs when switching from full-bridge operation mode to half-bridge operation mode.

[0102] (Technology 4) The voltage measurement circuit includes a voltage conversion circuit that outputs a voltage corresponding to the voltage across the two ends, and a comparator having a first input terminal and a second input terminal, wherein the comparator receives the voltage output from the voltage conversion circuit at the first input terminal, receives a reference voltage corresponding to the threshold voltage at the second input terminal, and outputs a comparison result between the voltage output from the voltage conversion circuit and the reference voltage to the control circuit. A power conversion device according to any one of technologies 1 to 3.

[0103] The voltage across the resonant capacitor may be high and may be too high for the control circuit to handle. Therefore, by converting the voltage across the resonant capacitor using a voltage conversion circuit, the voltage across the resonant capacitor can be handled by the control circuit. Furthermore, by using a comparator, it is possible to determine whether the voltage output from the voltage conversion circuit has reached the reference voltage, i.e., whether the voltage across the resonant capacitor has reached the threshold voltage.

[0104] (Technology 5) The power conversion device according to Technology 4, wherein the control circuit outputs the reference voltage to the second input terminal.

[0105] Thus, the reference voltage or threshold voltage may be calculated by the control circuit and input to the comparator.

[0106] (Technology 6) The power conversion device according to Technology 5, wherein the control circuit calculates the reference voltage based on the input voltage.

[0107] The optimum threshold voltage varies depending on the input voltage to the power conversion device. Therefore, for example, by preparing a lookup table or an arithmetic expression that indicates the correspondence between the input voltage and the reference voltage in advance, it is possible to calculate the optimum reference voltage depending on the input voltage to the power conversion device.

[0108] (Technology 7) The power conversion device according to Technology 5 or 6, wherein the voltage measurement circuit further includes an isolator provided on at least one of a node between the voltage conversion circuit and the first input terminal and a node between the control circuit and the second input terminal.

[0109] Since a high voltage can be handled in the path in which the resonant capacitor is arranged, the provision of an isolator can prevent the high voltage from being input to the control circuit.

[0110] (Technology 8) A power conversion device according to any one of Technologies 4 to 7, wherein the voltage measurement circuit further includes a relay provided on at least one node of a plurality of nodes connected to the voltage conversion circuit, and the control circuit turns off the relay in the full-bridge operation mode and turns on the relay in the half-bridge operation mode.

[0111] Since high voltages can be handled in the path in which the resonant capacitor is placed, the relay is turned off in full-bridge operation mode, where measurement results of the voltage across the resonant capacitor are not required, and the relay is turned on in half-bridge operation mode, where measurement results of the voltage across the resonant capacitor are required, thereby preventing high voltages from being input to the control circuit.

[0112] (Technology 9) A control method for a power conversion device that steps up or steps down an input voltage to a predetermined voltage and outputs the voltage, the power conversion device comprising: a first high-side switch provided on a first path connecting a first input terminal and a second input terminal; a first low-side switch provided on the first path and connected in series with the first high-side switch; a second high-side switch provided on a second path connecting the first input terminal and the second input terminal and different from the first path; a second low-side switch provided on the second path and connected in series with the second high-side switch; an isolation transformer having a primary winding connected between a first node on the first path between the first high-side switch and the first low-side switch and a second node on the second path between the second high-side switch and the second low-side switch; a resonant capacitor and a resonant inductor connected between the first node and the primary winding or between the second node and the primary winding; and a voltage measurement circuit that measures a voltage across the resonant capacitor. When the first high-side switch and the second low-side switch are defined as a first switch group, the first low-side switch and the second high-side switch are defined as a second switch group, the first high-side switch and the first low-side switch are defined as a first leg, and the second high-side switch and the second low-side switch are defined as a second leg, the control method is capable of switching between a full-bridge operation mode in which the first switch group is turned on, the first switch group is turned off after a certain period of time, and then the second switch group is turned on and the second switch group is turned off after a certain period of time, and this is repeated; and a half-bridge operation mode in which the high-side switch of one of the first and second legs is fixed off, the low-side switch of the one leg is fixed on, and the high-side switch of the other of the first and second legs is turned on, the high-side switch is turned off after a certain period of time, and then the low-side switch of the other leg is turned on and the low-side switch is turned off after a certain period of time, and this is repeated;a control method in which, when switching from the full-bridge operation mode to the half-bridge operation mode, the high-side switch of the other leg is turned on or maintained in an on state, and the high-side switch is turned off when the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range, and then, when one cycle is defined as the driving of the high-side switch from the on state to the off state in the half-bridge operation mode, driving of the high-side switch in the half-bridge operation mode begins from a phase of 0.5 cycles.

[0113] This makes it possible to provide a control method that can suppress fluctuations in output voltage that occur when switching from the full-bridge operation mode to the half-bridge operation mode.

[0114] The present disclosure can be applied to LLC converters and the like that are compatible with a wide range of input and output voltages.

[0115] REFERENCE SIGNS LIST 1 Power conversion device 10 Control circuit 20 Voltage measurement circuit 21 Voltage conversion circuit 22 Comparator 23a, 23b Isolator 24a, 24b, 24c Relay AH, AL, BH, BL, CH, CL, DH, DL Switch Cr Capacitor Lm, Lr Inductor N1, N2, N3, N4 Node P1, P2, P3, P4 Path T1 Transformer t1, t2, t3, t4 Terminal

Claims

1. A power conversion device that steps up or down an input voltage to a predetermined voltage and outputs the voltage, comprising: a first high-side switch provided on a first path connecting a first input terminal and a second input terminal; a first low-side switch provided on the first path and connected in series with the first high-side switch; a second high-side switch provided on a second path different from the first path connecting the first input terminal and the second input terminal; a second low-side switch provided on the second path and connected in series with the second high-side switch; an isolation transformer having a primary winding connected between a first node on the first path between the first high-side switch and the first low-side switch and a second node on the second path between the second high-side switch and the second low-side switch; a resonant capacitor and a resonant inductor connected between the first node and the primary winding or between the second node and the primary winding; and a voltage measurement circuit that measures the voltage across the resonant capacitor. a control circuit for controlling switching of the first high-side switch, the first low-side switch, the second high-side switch, and the second low-side switch, wherein when the first high-side switch and the second low-side switch are defined as a first switch group, the first low-side switch and the second high-side switch are defined as a second switch group, the first high-side switch and the first low-side switch are defined as a first leg, and the second high-side switch and the second low-side switch are defined as a second leg, the control circuit is configured to: operate in a full-bridge operation mode in which the control circuit turns on the first switch group, turns off the first switch group after a certain period of time, then turns on the second switch group, and turns off the second switch group after a certain period of time, and repeats this;a half-bridge operation mode in which, with the high-side switch of one of the first and second legs fixed to off and the low-side switch of the one leg fixed to on, the high-side switch of the other of the first and second legs is turned on, the high-side switch is turned off after a certain period of time, and then the low-side switch of the other leg is turned on and the low-side switch is turned off after a certain period of time, and this process is repeated; and when switching from the full-bridge operation mode to the half-bridge operation mode, the high-side switch of the other leg is turned on or maintained in an on state, and the high-side switch is turned off when the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range, and then the high-side switch starts to be driven in the half-bridge operation mode from a phase of 0.5 of a period, where one period is defined as the period in which the high-side switch is driven from an on state to an off state in the half-bridge operation mode.

2. The power conversion device according to claim 1, wherein the predetermined voltage range is a range equal to or less than the average value of the voltage across the resonant capacitor when the voltage across the resonant capacitor reaches a steady state after switching from the full-bridge operation mode to the half-bridge operation mode.

3. The power conversion device according to claim 2, wherein the predetermined voltage range is a range that is equal to or less than the average value and equal to or more than half of the average value.

4. A power conversion device according to any one of claims 1 to 3, wherein the voltage measurement circuit comprises: a voltage conversion circuit that outputs a voltage corresponding to the voltage across the two terminals; and a comparator having a first input terminal and a second input terminal, wherein the comparator receives the voltage output from the voltage conversion circuit at the first input terminal and a reference voltage corresponding to the threshold voltage at the second input terminal, and outputs a comparison result between the voltage output from the voltage conversion circuit and the reference voltage to the control circuit.

5. The power conversion device according to claim 4, wherein the control circuit outputs the reference voltage to the second input terminal.

6. The power conversion device according to claim 5, wherein the control circuit calculates the reference voltage based on the input voltage.

7. The power conversion device according to claim 5, wherein the voltage measurement circuit further comprises an isolator provided on at least one of a node between the voltage conversion circuit and the first input terminal and a node between the control circuit and the second input terminal.

8. The power conversion device according to claim 4, wherein the voltage measurement circuit further comprises a relay provided on at least one of a plurality of nodes connected to the voltage conversion circuit, and the control circuit turns the relay off in the full-bridge operation mode and turns the relay on in the half-bridge operation mode.

9. A control method for a power conversion device that steps up or steps down an input voltage to a predetermined voltage and outputs the voltage, the power conversion device comprising: a first high-side switch provided on a first path connecting a first input terminal and a second input terminal; a first low-side switch provided on the first path and connected in series with the first high-side switch; a second high-side switch provided on a second path connecting the first input terminal and the second input terminal and different from the first path; a second low-side switch provided on the second path and connected in series with the second high-side switch; an isolation transformer having a primary winding connected between a first node on the first path between the first high-side switch and the first low-side switch and a second node on the second path between the second high-side switch and the second low-side switch; a resonant capacitor and a resonant inductor connected between the first node and the primary winding or between the second node and the primary winding; and a voltage measurement circuit that measures a voltage across the resonant capacitor, When the first high-side switch and the second low-side switch are defined as a first switch group, the first low-side switch and the second high-side switch are defined as a second switch group, the first high-side switch and the first low-side switch are defined as a first leg, and the second high-side switch and the second low-side switch are defined as a second leg, the control method is capable of switching between a full-bridge operation mode in which the first switch group is turned on, the first switch group is turned off after a certain period of time, and then the second switch group is turned on and the second switch group is turned off after a certain period of time, and this is repeated; and a half-bridge operation mode in which the high-side switch of one of the first and second legs is fixed off, the low-side switch of the one leg is fixed on, and the high-side switch of the other of the first and second legs is turned on, the high-side switch is turned off after a certain period of time, and then the low-side switch of the other leg is turned on and the low-side switch is turned off after a certain period of time, and this is repeated;a control method in which, when switching from the full-bridge operation mode to the half-bridge operation mode, the high-side switch of the other leg is turned on or maintained in an on state, and the high-side switch is turned off when the voltage across the resonant capacitor reaches a threshold voltage within a predetermined voltage range, and then, when one cycle is defined as the driving of the high-side switch from the on state to the off state in the half-bridge operation mode, driving of the high-side switch in the half-bridge operation mode is started from a phase of 0.5 cycles.

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