POWER CONVERSION DEVICE, ON-BOARD CHARGER, AND CONTROL METHOD

Abstract

This power conversion apparatus is an insulated-type power conversion apparatus, and includes: a transformer; a first bridge circuit that includes a plurality of switching elements and that is provided upstream of the transformer; and a second bridge circuit that includes a plurality of switching elements and that is provided downstream of the transformer; and a control unit for controlling switching of the first bridge circuit and the second bridge circuit. The control unit, based on the output voltage and the output power of the power conversion apparatus, switches the operation frequencies of the first bridge circuit and the second bridge circuit to frequencies at which soft switching is established for the control of the first bridge circuit and the second bridge circuit.

Description

[Technical field]

[0001] The present disclosure relates to a power conversion device, an on-board charger, and a control method. [Background technology]

[0002] A dual active bridge (DAB) type DC / DC converter (hereinafter referred to as "DAB type"), which is a type of isolated power conversion device, is known. A DAB type DC / DC converter has a structure in which a bridge circuit is provided on each of the primary side and secondary side of a transformer. Each bridge circuit includes a plurality of switching elements connected in a bridge configuration. A DAB type DC / DC converter transmits power from the primary side of a transformer to the secondary side, or in the reverse direction, depending on the phase difference between the voltage of the primary side bridge circuit and the voltage of the secondary side bridge circuit.

[0003] DAB DC / DC converters are capable of soft switching by utilizing the resonance phenomenon. Compared to hard switching, which forcibly cuts off the current by simply turning the switching elements on and off, soft switching has the advantage of reducing switching losses. However, DAB DC / DC converters may not be able to implement soft switching if the difference between the input voltage and output voltage becomes large. In this case, some switching elements will be forced into hard switching, increasing switching losses and reducing power conversion efficiency.

[0004] Patent Document 1, which will be described later, discloses a technique for solving such problems. Specifically, Patent Document 1 discloses a DC / DC converter that detects an output current of a secondary bridge circuit, and when the detected output current exceeds a secondary current threshold value that is greater than 0, switches the open / closed state of a switching element in the secondary bridge circuit, thereby controlling the phase difference between the switching of the open / closed state of the switching element in the primary bridge circuit and the switching of the open / closed state of the switching element in the secondary bridge circuit. The secondary current threshold value is calculated based on the output value of a PI calculation unit that performs proportional-integral calculation on the deviation between a target value of the output voltage and the output voltage.

[0005] The output current of the secondary bridge circuit changes between positive and negative depending on the control state of the DC / DC converter. Therefore, in Patent Document 1, as described above, the timing of switching the open / closed state of the switching elements in the secondary bridge circuit is controlled based on whether or not the detected value of the output current exceeds the secondary current threshold. With this configuration, the DC / DC converter suppresses hard recovery in the secondary bridge circuit regardless of the input voltage and output voltage, and expands the voltage range in which soft switching is possible.

[0006] Patent Document 1 further discloses a configuration for changing the control frequency of the bridge circuit based on the output value of the PI calculation unit in the above configuration. When the output voltage deviates from the target value and the deviation increases, the time in the freewheel mode becomes longer. In that case, there is a concern that hard recovery may occur in the primary bridge circuit when the switch is switched at the end of the freewheel mode. Therefore, in Patent Document 1, when the output voltage deviates from the target value and the deviation increases, the control frequency is increased based on the deviation to shorten the time in the freewheel mode. By adding such a configuration, the DC / DC converter in Patent Document 1 suppresses the occurrence of hard recovery in the primary bridge circuit at the end of the freewheel mode. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2016-152687 A Summary of the Invention

[0008] A power conversion device according to an aspect of the present disclosure is an isolated power conversion device including a transformer, a first bridge circuit including a plurality of switching elements and provided on a primary side of the transformer, a second bridge circuit including a plurality of switching elements and provided on a secondary side of the transformer, and a control unit that controls switching of the first bridge circuit and the second bridge circuit, and the control unit switches the operating frequencies of the first bridge circuit and the second bridge circuit to frequencies at which soft switching is established in the control of the first bridge circuit and the second bridge circuit based on an output voltage and an output power of the power conversion device.

[0009] An on-board charger according to another aspect of the present disclosure includes a conversion circuit that converts AC power into DC power and outputs the DC power, and the power conversion device to which the DC power converted by the conversion circuit is input.

[0010] A control method according to yet another aspect of the present disclosure is a control method for a power conversion device including a transformer, a first bridge circuit including a plurality of switching elements and provided on a primary side of the transformer, and a second bridge circuit including a plurality of switching elements and provided on a secondary side of the transformer, the control method including the steps of: controlling switching of the first bridge circuit and the second bridge circuit so that a phase difference is generated between the first bridge circuit and the second bridge circuit; and switching the operating frequencies of the first bridge circuit and the second bridge circuit to frequencies at which soft switching is established in the control of the first bridge circuit and the second bridge circuit based on an output voltage and an output power of the power conversion device.

[0011] The present disclosure can be realized not only as a power conversion device, an on-board charger, or a control method including such a characteristic configuration, but also as a program for causing a computer to execute the characteristic steps executed by the power conversion device, the on-board charger, or the control method, and a recording medium on which the program is recorded. Furthermore, the present disclosure can be realized as other systems including the power conversion device or the on-board charger. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing a circuit configuration of a power conversion device according to a first embodiment. [Diagram 2] FIG. 2 is a waveform diagram for explaining an example of a soft switching operation in a DC / DC converter using the DAB method. [Diagram 3] FIG. 3 is a circuit diagram for explaining an example of a soft switching operation in a DAB DC / DC converter. [Figure 4] FIG. 4 is a circuit diagram for explaining an example of the soft switching operation in a DAB DC / DC converter. [Diagram 5] FIG. 5 is a circuit diagram for explaining an example of a soft switching operation in a DAB DC / DC converter. [Figure 6] FIG. 6 is a waveform diagram for explaining an example of the hard switching operation in a DAB DC / DC converter. [Figure 7] FIG. 7 is a circuit diagram for explaining an example of the hard switching operation in a DAB DC / DC converter. [Figure 8] FIG. 8 is a circuit diagram for explaining an example of the hard switching operation in a DAB DC / DC converter. [Figure 9] FIG. 9 is a circuit diagram for explaining an example of the hard switching operation in a DAB DC / DC converter. [Figure 10]FIG. 10 is a diagram for explaining the conditions under which soft switching is established in a DAB DC / DC converter. [Figure 11] FIG. 11 is a flowchart showing an example of a control structure of a program executed by the power conversion device shown in FIG. [Figure 12] FIG. 12 is a flowchart showing an example of a control structure of a program executed by the power conversion device shown in FIG. [Figure 13] FIG. 13 is a diagram showing verification results of the effects of the power conversion device shown in FIG. [Figure 14] FIG. 14 is a diagram showing verification results of the effects of the power conversion device shown in FIG. [Figure 15] FIG. 15 is a diagram showing verification results of the effects of the power conversion device shown in FIG. [Figure 16] FIG. 16 is a diagram illustrating a circuit configuration of a power conversion device according to the second embodiment. [Figure 17] FIG. 17 is a block diagram showing a functional configuration of the control unit shown in FIG. [Figure 18] FIG. 18 is a diagram showing an example of a map (table) used in the power conversion device according to the third embodiment. [Figure 19] FIG. 19 is a flowchart showing an example of a control structure of a program executed by the power conversion device according to the third embodiment. [Figure 20] FIG. 20 is a diagram showing an example of a map (table) for determining whether soft switching is possible. [Figure 21] FIG. 21 is a diagram showing an example of a map (table) for determining whether soft switching is possible. [Figure 22] FIG. 22 is a diagram showing an example of a map (table) for determining whether soft switching is possible. [Diagram 23] FIG. 23 is a flowchart showing an example of a control structure of a program executed by the power conversion device according to the fourth embodiment. [Figure 24]FIG. 24 is a detailed flow of step S2130 in FIG. [Diagram 25] FIG. 25 is a diagram for explaining the switching operation of the operating frequency in the power conversion device according to the fourth embodiment. [Figure 26] FIG. 26 is a diagram illustrating a circuit configuration of a power conversion device according to the fifth embodiment. As shown in FIG. [Figure 27] FIG. 27 is a flowchart showing an example of a control structure of a program executed by the power conversion device shown in FIG. [Figure 28] FIG. 28 is a detailed flow of step S2132 in FIG. [Figure 29] FIG. 29 is a diagram illustrating a circuit configuration of a power conversion device according to the sixth embodiment. As shown in FIG. [Diagram 30] FIG. 30 is a diagram showing a configuration of an on-board charger according to the seventh embodiment. [Diagram 31] FIG. 31 is a schematic diagram showing a vehicle equipped with the on-board charger shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] [Problem that this disclosure aims to solve] In Patent Document 1, the output current of the secondary bridge circuit is detected, and the timing of switching the open / closed state of the switching element in the secondary bridge circuit is controlled according to whether the output current is positive or negative. Therefore, it is necessary to detect the instantaneous value of the current that fluctuates greatly within a switching period as the output current, and a current sensor capable of detecting high-frequency current with high accuracy is required. Such current sensors are expensive, and therefore the manufacturing cost increases. In addition, considering the delay of the current sensor, it is difficult to increase the control frequency. Therefore, even when changing the control frequency of the bridge circuit based on the output value of the PI calculation unit, the change range is limited.

[0014] Furthermore, in order to control the switching timing of the open / closed state of the switching element based on the output current, it is necessary to detect a current that matches the reactor current as the output current. That is, in Patent Document 1, it is assumed that the detected output current matches the reactor current (after taking the absolute value). Therefore, a capacitor or the like that prevents the output current from matching the reactor current cannot be disposed between the current sensor for detecting the output current and the secondary bridge circuit. Such a capacitor is often used as a snubber capacitor for surge countermeasures, noise countermeasures, etc. Therefore, with the configuration described in Patent Document 1, it is difficult to implement surge countermeasures, noise countermeasures, etc.

[0015] Thus, there is a problem in that the range of application of the technology described in Patent Document 1 is limited, and therefore, when the technology described in Patent Document 1 is used, there is a problem in that the degree of freedom in design is reduced.

[0016] Therefore, one object of the present disclosure is to provide a power conversion device and an on-board charger that have a high degree of design freedom and are capable of suppressing a decrease in power conversion efficiency caused by hard switching.

[0017] Another object of the present disclosure is to provide a control method for a power conversion device that has a high degree of design freedom and is capable of suppressing a decrease in power conversion efficiency caused by hard switching.

[0018] [Effects of this disclosure] According to the present disclosure, it is possible to provide a power conversion device and an on-board charger that have a high degree of design freedom and are capable of suppressing a decrease in power conversion efficiency caused by hard switching.

[0019] The present disclosure further provides a control method for a power conversion device that has a high degree of design freedom and is capable of suppressing a decrease in power conversion efficiency caused by hard switching.

[0020] [Description of the embodiments of the present disclosure] Preferred embodiments of the present disclosure will be described below. At least a part of the embodiments described below may be arbitrarily combined.

[0021] (1) A power conversion device according to a first aspect of the present disclosure is an isolated power conversion device. The power conversion device includes a transformer, a first bridge circuit including a plurality of switching elements and provided on a primary side of the transformer, a second bridge circuit including a plurality of switching elements and provided on a secondary side of the transformer, and a control unit that controls switching of the first bridge circuit and the second bridge circuit. The control unit switches the operating frequencies of the first bridge circuit and the second bridge circuit to frequencies at which soft switching is established in the control of the first bridge circuit and the second bridge circuit, based on the output voltage and output power of the power conversion device.

[0022] The control unit switches the operating frequency of the first bridge circuit and the second bridge circuit to a frequency at which soft switching is established based on the output voltage and output power of the power conversion device. That is, by switching the operating frequency based on the output voltage and output power, the switching element is prevented from being hard-switched. This reduces switching loss caused by hard switching and suppresses a decrease in power conversion efficiency. In addition, by configuring the operating frequency to be switched based on the output voltage and output power, soft switching can be achieved even when the input / output voltage or the output power changes. Furthermore, in the configuration in which the operating frequency is switched, unlike the case in which the switching timing of the open / closed state of the switching element is controlled, there is no need to detect the instantaneous value of the output current that coincides with the reactor current. Therefore, the operating frequency can be easily made high, and by providing an element such as a capacitor, surge countermeasures or noise countermeasures can be implemented. In this way, by adopting the above configuration, it is possible to both suppress a decrease in design freedom and suppress hard switching.

[0023] (2) Preferably, the power conversion device further includes a current sensor and a voltage sensor for detecting an output current and an output voltage of the power conversion device, respectively, and the control unit switches the operating frequency of the first bridge circuit and the second bridge circuit based on the output voltage detected by the voltage sensor and the output power calculated using the output current and the output voltage detected by the current sensor and the voltage sensor, respectively. Since it is not necessary to detect the instantaneous value of the output current that coincides with the reactor current by the current sensor, an inexpensive current sensor can be used. Since the options for the current sensor are expanded, the degree of freedom in design can be further increased. In addition, even if there is a delay in the current sensor, the operating frequency can be made higher.

[0024] (3) More preferably, the power conversion device further includes a circuit that applies a frequency limit to the output current detected by the current sensor. The current information used to calculate the output power may be information that has been frequency-limited (e.g., information in which high-frequency components have been cut). Therefore, the power conversion device may be configured to include a circuit that applies a frequency limit. In this case, the calculation of the output power becomes easier.

[0025] (4) More preferably, the circuit that applies the frequency limit includes a filter circuit disposed between the second bridge circuit and the current sensor. This makes it easier to calculate the output power.

[0026] (5) More preferably, the power conversion device further includes a capacitor disposed in a region between the second bridge circuit and the current sensor and adjacent to the second bridge circuit, thereby providing a measure against surges or noise, thereby improving the reliability of the power conversion device.

[0027] (6) More preferably, the power conversion device further includes a voltage sensor provided on the secondary side of the transformer for detecting an output voltage of the power conversion device, and an acquisition unit for acquiring the output power as an instruction value from an outside, and the control unit switches the operating frequencies of the first bridge circuit and the second bridge circuit based on the output voltage detected by the voltage sensor and the output power acquired by the acquisition unit. Since there is no need to calculate the output power, it is possible to easily suppress a decrease in power conversion efficiency caused by hard switching.

[0028] (7) More preferably, the control unit includes a storage unit that stores a judgment condition for judging whether soft switching is established, judges whether the switching of the first bridge circuit and the second bridge circuit is soft switching or not using the output voltage, the output power and the judgment condition, and switches the operating frequencies of the first bridge circuit and the second bridge circuit according to the judgment result. This makes it possible to easily switch the operating frequencies of the first bridge circuit and the second bridge circuit to a frequency at which soft switching is established even if the input / output voltage or the output power changes.

[0029] (8) More preferably, the storage unit stores a table indicating whether or not various output voltage values ​​and output power values ​​satisfy the conditions for soft switching as the judgment conditions, and the control unit judges whether or not the switching of the first bridge circuit and the second bridge circuit is soft switching by referring to the table for the output voltage and the output power, and switches the operating frequencies of the first bridge circuit and the second bridge circuit according to the judgment result. This makes it easier to switch the operating frequencies of the first bridge circuit and the second bridge circuit to frequencies at which soft switching is established.

[0030] (9) An on-board charger according to a second aspect of the present disclosure includes a conversion circuit that converts AC power into DC power and outputs the DC power, and the power conversion device according to the first aspect, to which the DC power converted by the conversion circuit is input. This makes it possible to suppress a decrease in power conversion efficiency caused by hard switching, and provides an on-board charger with high design freedom.

[0031] (10) A control method according to a third aspect of the present disclosure is a control method for a power conversion device including a transformer, a first bridge circuit including a plurality of switching elements and provided on the primary side of the transformer, and a second bridge circuit including a plurality of switching elements and provided on the secondary side of the transformer. This control method includes the steps of controlling the switching of the first bridge circuit and the second bridge circuit so that a phase difference occurs between the first bridge circuit and the second bridge circuit, and switching the operating frequencies of the first bridge circuit and the second bridge circuit to frequencies at which soft switching is established in the control of the first bridge circuit and the second bridge circuit based on the output voltage and output power of the power conversion device. By using such a control method, it is possible to suppress a decrease in power conversion efficiency caused by hard switching and also to suppress a decrease in design freedom.

[0032] [Details of the embodiment of the present disclosure] In the following embodiments, the same parts are denoted by the same reference numerals, and their names and functions are also the same, so detailed description thereof will not be repeated.

[0033] (First embodiment) [Overall configuration] Referring to FIG. 1, the power conversion device 50 according to the present embodiment is an insulating type power conversion device including a transformer TR. More specifically, the power conversion device 50 is a DC / DC converter of the DAB system. The power conversion device 50 is disposed between a first DC power source 40, which is a DC voltage source, and a second DC power source 42, such as a secondary battery, and converts the power supplied from the first DC power source 40 and outputs it to the second DC power source 42. In this case, the input voltage of the power conversion device 50 input to the primary side is E1, and the output voltage of the power conversion device 50 output from the secondary side is E2. Since the DC / DC converter of the DAB system is a bidirectional converter, the power conversion device 50 can also output the power supplied from the second DC power source 42 to the first DC power source 40.

[0034] The transformer TR includes a primary coil 60, a secondary coil 62, and a core member 64. The turns ratio between the primary coil 60 and the secondary coil 62 is, for example, 1:1. However, the turns ratio of the transformer TR is not limited to this. The turns ratio can be set to any desired ratio.

[0035] The power conversion device 50 includes, in addition to a transformer TR, a primary side bridge circuit 100 provided on the primary side of the transformer TR, a secondary side bridge circuit 120 provided on the secondary side of the transformer TR, a current sensor 140 that detects an output current, a voltage sensor 150 that detects an output voltage, and a control unit 160 that controls the primary side bridge circuit 100 and the secondary side bridge circuit 120. A smoothing capacitor 110 is provided on the input side of the primary side bridge circuit 100, and a smoothing capacitor 130 is provided on the output side of the secondary side bridge circuit 120.

[0036] The primary-side bridge circuit 100 is a switching circuit including a plurality of switching elements Q1 to Q4. These switching elements Q1 to Q4 are full-bridge connected to form an inverter circuit. Specifically, the switching element Q1 and the switching element Q2 are connected in series to form a first leg. Similarly, the switching element Q3 and the switching element Q4 are connected in series to form a second leg. The first leg and the second leg are connected in parallel to each other.

[0037] The contacts of the switching elements Q1 and Q2 in the first leg are connected to one end (first end) of the primary coil 60 of the transformer TR. The other end (second end) of the primary coil 60 of the transformer TR is connected to the contacts of the switching elements Q3 and Q4 in the second leg.

[0038] The switching elements Q1 to Q4 are, for example, MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). The switching elements Q1 to Q4 may be power semiconductor devices other than MOSFETs, such as IGBTs (Insulated Gate Bipolar Transistors) or HEMTs (High Electron Mobility Transistors).

[0039] The secondary bridge circuit 120 has the same configuration as the primary bridge circuit 100. That is, the secondary bridge circuit 120 is a switching circuit including a plurality of switching elements Q5 to Q8. These switching elements Q5 to Q8 are full-bridge connected to form an inverter circuit. Specifically, the switching element Q5 and the switching element Q5 are connected in series to form a third leg. Similarly, the switching element Q7 and the switching element Q8 are connected in series to form a fourth leg. The third leg and the fourth leg are connected in parallel to each other.

[0040] The contacts of the switching elements Q5 and Q6 in the third leg are connected to one end (first end) of the secondary coil 62 of the transformer TR. The other end (second end) of the secondary coil 62 of the transformer TR is connected to the contacts of the switching elements Q7 and Q8 in the second leg.

[0041] Like the switching elements Q1 to Q4, the switching elements Q5 to Q8 are also formed of, for example, MOSFETs. Note that the switching elements Q5 to Q8 may also be power semiconductor devices other than MOSFETs, such as IGBTs or HEMTs.

[0042] The switching elements Q1 to Q8 are provided with free wheel diodes D1 to D8, respectively. The primary side smoothing capacitor 110 and the secondary side smoothing capacitor 130 each have a function of smoothing voltage fluctuations such as ripples. Since the power conversion device 50 is a DAB type DC / DC converter, reactors L1 and L2 are provided between the primary side bridge circuit 100 and the secondary side bridge circuit 120. The reactors L1 and L2 may be leakage inductance of the transformer TR or may be external reactors.

[0043] The current sensor 140 detects the output current flowing through the secondary side, and provides the current detection value to the control unit 160. The voltage sensor 150 detects the output voltage E2 of the power conversion device 50, and provides the voltage detection value to the control unit 160.

[0044] Each of the switching elements Q1 to Q8 is connected to the control unit 160 and is on / off controlled by a gate signal from the control unit 160. The on / off control of each of the switching elements Q1 to Q8 controls the phase difference between the output voltage V1 of the primary side bridge circuit 100 and the output voltage V2 of the secondary side bridge circuit 120 (i.e., the phase difference of the transformer voltage). This phase difference allows power to be transmitted from the primary side to the secondary side, or in the reverse direction. That is, the power conversion device 50 adjusts the output power by controlling the phase difference between the bridge circuits. Note that, in order to avoid confusion with the output powers of the primary side bridge circuit 100 and the secondary side bridge circuit 120, the output power E2 of the power conversion device 50 may be referred to as the "DC output voltage."

[0045] Each function in the control unit 160 can be realized by one or more processing circuits. More specifically, the control unit 160 is a microcomputer including, for example, a CPU (Central Processing Unit) 162 and a storage unit 164, and controls the driving of the primary side bridge circuit 100 and the secondary side bridge circuit 120. The control unit 160 outputs gate signals to the primary side bridge circuit 100 and the secondary side bridge circuit 120 to perform PWM (Pulse Width Modulation) control of the operation of each of the switching elements Q1 to Q8. More specifically, the control unit 160 drives the primary side bridge circuit 100 and the secondary side bridge circuit 120 at a predetermined operating frequency (switching frequency). At that time, the control unit 160 drives the secondary side bridge circuit 120 so as to output a voltage V2 having a phase difference with respect to the voltage V1 of the primary side bridge circuit 100.

[0046] The storage unit 164 stores a computer program for controlling the operation of each of the switching elements Q1 to Q4 of the primary side bridge circuit 100 and each of the switching elements Q5 to Q8 of the secondary side bridge circuit 120. When the CPU 162 executes this computer program, the control unit 160 executes a process for controlling the driving of the primary side bridge circuit 100 and the secondary side bridge circuit 120 under conditions that establish soft switching, based on the DC output voltage and the output power.

[0047] The control unit 160 may be a control IC such as an analog IC (Integrated Circuit), a dedicated LSI (Large-Scale Integration), or an FPGA (Field-Programmable Gate Array), or a part or all of it may be configured by a hardware circuit.

[0048] In a DAB DC / DC converter, when the difference between the input voltage and the output voltage (DC output voltage) is large and the output power is small, some of the switching elements are hard-switched. Therefore, the power conversion device 50 according to the present embodiment judges whether or not soft switching is being performed based on the DC output voltage and the output power, and when the judgment result is negative (when hard switching is being performed), the hard switching is suppressed by changing the operating frequency.

[0049] [Soft switching operation example] FIG. 2 shows the waveforms of voltage and current when soft switching is established. The upper part of FIG. 2 shows the waveforms of the output voltage V1 of the primary bridge circuit and the output voltage V2 of the secondary bridge circuit, and the lower part shows the waveform of the reactor current I2. In this operation example, the primary bridge circuit and the secondary bridge circuit are controlled so that the output voltage V1 and the output voltage V2 are square waves with a duty of 50%, respectively. The period from the rise of the output voltage V1 to the rise of the output voltage V2 is defined as period A, and the period from the rise of the output voltage V2 to the fall of the output voltage V1 is defined as period B. A dead time is provided between periods A and B. Note that FIG. 2 shows an example in which the output voltage V1 is larger than the output voltage V2.

[0050] In the periods A and B, the switching elements Q1 to Q8 are controlled (switched) as shown in Table 1 below.

[0051] [Table 1]

[0052] Referring to FIG. 2, the reactor current I2 increases during period A, and immediately before the end of period A, the reactor current I2 has a "positive" value. The direction of flow of the reactor current I2 is defined as the positive direction when the current flows from the secondary coil 62 of the transformer TR to the reactor L2. There is a dead time before the transition to period B. During period B, power is transmitted (conveyed) from the primary side to the secondary side. As shown by the dashed line a, the value of the reactor current I2 when switching from period A to period B is "positive", and the reactor current I2 flows from the secondary coil 62 of the transformer TR toward the reactor L2.

[0053] 3, immediately before the end of period A, in the primary bridge circuit 100, switching elements Q1 and Q4 are turned on, and switching elements Q2 and Q3 are turned off. In the secondary bridge circuit 120, switching elements Q6 and Q7 are turned on, and switching elements Q5 and Q8 are turned off. Immediately before the end of period A, the value of reactor current I2 is "positive," so the current flows as indicated by the thick arrow.

[0054] 4, during the dead time, all of the switching elements Q5 to Q8 of the secondary bridge circuit 120 are turned off. At this time, the freewheel diode D5 of the switching element Q5 is conductive, and the freewheel diode D8 of the switching element Q8 is conductive. Therefore, even during the dead time, the current continues to flow along the path indicated by the thick arrow.

[0055] Referring to FIG. 5, in period B, switching element Q5 and switching element Q8 are turned on. Switching element Q5 is turned on when freewheel diode D5 is conductive, i.e., when the voltage is 0V. Therefore, the switching to turn on switching element Q5 is ZVS (Zero Voltage Switching). Similarly, switching element Q8 is turned on when freewheel diode D8 is conductive, i.e., when the voltage is 0V. Therefore, the switching to turn on switching element Q8 is also ZVS. Therefore, both switching element Q5 and switching element Q8 for transitioning to period B are soft switched.

[0056] [Example of hard switching operation] FIG. 6 shows the waveforms of voltage and current when soft switching is not established, that is, when hard switching occurs. The upper part of FIG. 6 shows the waveforms of the output voltage V1 of the primary bridge circuit and the output voltage V2 of the secondary bridge circuit, and the lower part shows the waveform of the reactor current I2. In this operation example, similar to the operation example of soft switching, the primary bridge circuit and the secondary bridge circuit are controlled so that the output voltage V1 and the output voltage V2 are rectangular waves with a duty of 50%, respectively. Period A is the period from the rise of the output voltage V1 to the rise of the output voltage V2, and period B is the period from the rise of the output voltage V2 to the fall of the output voltage V1. Similarly, a dead time is provided between periods A and B. FIG. 6 also shows an example in which the output voltage V1 is larger than the output voltage V2.

[0057] Each of the switching elements Q1 to Q8 is controlled in the same manner as in the example of the soft switching operation in the periods A and B. That is, each of the switching elements Q1 to Q8 is controlled as shown in Table 1.

[0058] 6, although the reactor current I2 increases during period A, the reactor current I2 has a negative value immediately before the end of period A. There is a dead time before transitioning to period B. During period B, power is transmitted from the primary side to the secondary side. As a result, as shown by the dashed line b, the value of the reactor current I2 when switching from period A to period B is negative, and the reactor current I2 flows from the reactor L2 toward the secondary coil 62 of the transformer TR.

[0059] 7, the value of reactor current I2 is negative just before the end of period A, so the current flows as indicated by the thick arrow. That is, the current flows in the opposite direction to the case of soft switching shown in FIG.

[0060] 8, during the dead time, all of the switching elements Q5 to Q8 of the secondary bridge circuit 120 are turned off. At this time, the freewheel diode D6 of the switching element Q6 is conductive, and the freewheel diode D7 of the switching element Q7 is conductive. Therefore, even during the dead time, a current continues to flow in the same direction as during the period A along the path indicated by the thick arrow.

[0061] 9, in period B, switching element Q5 and switching element Q8 are turned on. However, switching element Q5 is not in ZVS because switching element Q5 is turned on while the freewheel diode D6 of switching element Q6 is conductive, i.e., the voltage of switching element Q5 is not 0V. Similarly, switching element Q8 is not in ZVS because switching element Q5 is turned on while the freewheel diode D7 is conductive, i.e., the voltage of switching element Q8 is not 0V. Therefore, both switching element Q5 and switching element Q8 for transitioning to period B are hard-switched.

[0062] As described above, it is possible to determine whether soft switching or hard switching will occur depending on whether the value of reactor current I2 just before the end of period A (when switching from period A to period B) is "positive" or "negative."

[0063] [Control by control unit 160] The control unit 160 has a function of determining whether the switching of each bridge circuit is soft switching or not, based on the DC output voltage and output power of the power conversion device 50. When the switching of the primary side bridge circuit 100 and the secondary side bridge circuit 120 is not soft switching, i.e., when it is hard switching, the control unit 160 changes the operating frequency to a frequency at which soft switching is established. This configuration will be described in more detail.

[0064] Since the power conversion device 50 is a DC / DC converter of the DAB system, the output power P is expressed by the following equation (1).

[0065]

number

[0066] The output power P can be rewritten in the form of equation (2).

[0067]

number

[0068] From this equation (2), the phase difference T pis expressed by equation (3).

[0069]

number

[0070] 10, for example, when the input voltage E1 (FIG. 1) of the power conversion device is higher than the output voltage E2 (FIG. 1) (E1>E2), the reactor current I2 immediately before the end of the period A can be expressed by equation (4). Note that the reactor current I2 immediately before the end of the period B can be expressed by equation (5).

[0071]

number

[0072]

number

[0073] As mentioned above, whether soft switching occurs or not is determined by whether the value of the reactor current I2 just before the end of period A (when switching from period A to period B) is "positive" or "negative." When the value of the reactor current I2 is "positive" (>0), soft switching occurs, and when it is "negative" (<0), hard switching occurs. The amount of change ΔI in the reactor current during period A is determined by the phase difference T p (i.e., it depends on the output power P). Therefore, whether or not soft switching is established also depends on the output power P.

[0074] The boundary condition between hard switching and soft switching can be expressed by the following formula (6) when the input voltage E1 of the power conversion device is greater than the output voltage E2 (E1>E2). Formula (6) can be obtained by substituting formula (3) into formula (4).

[0075]

number

[0076] When the input voltage E1 is smaller than the output voltage E2 (E1 < E2), the boundary condition between hard switching and soft switching can be expressed by the following formula (7).

[0077]

Equation

[0078] When formula (6) or formula (7) is satisfied, soft switching is established; when it is not satisfied, hard switching occurs. Note that formulas (6) and (7) show the case where the turns ratio of the transformer is 1:1. When the turns ratio of the transformer is other than 1:1, the output voltage E2 may be set to the output voltage E2' considering the turns ratio of the transformer.

[0079] From formulas (6) and (7), the parameters when the input voltage E1 is constant (fixed) are the output voltage E2, the output power P, and the switching period T (i.e., the operating frequency). Therefore, by monitoring the output voltage E2 and the output power P, it is possible to determine whether the switching is soft switching or hard switching when driving the bridge circuit at the switching period T. When soft switching is not established, that is, when it is determined to be hard switching, the switching period T (operating frequency) is changed. As a result, formula (6) or formula (7) is satisfied, and soft switching can be performed.

[0080] In this way, the power conversion device 50 (control unit 160) according to the present embodiment does not measure the instantaneous value of the reactor current I2 (output current that coincides with the reactor current I2) immediately before the end of period A to determine whether soft switching is established, but determines whether the value of the reactor current I2 immediately before the end of period A is "positive" or "negative" based on the output voltage E2 and the output power P of the power conversion device 50, and thereby determines whether soft switching is established.

[0081] 1 again, for example, an operating frequency other than the reference operating frequency (switching frequency) is stored in the memory unit 164 of the control unit 160. For example, when the DC output voltage and output power of the power conversion device 50 are changed, the stored operating frequency is preferably determined by using the above formulas (6) and (7) as an operating frequency that can realize soft switching at the DC output voltage and output power. When it is determined that soft switching is not established, the control unit 160 switches the operating frequency of the primary side bridge circuit 100 and the secondary side bridge circuit 120 to another operating frequency stored in the memory unit 164.

[0082] [Software configuration] 11 and 12, a control structure of a computer program executed by the control unit 160 (see FIG. 1) to control the driving of the primary side bridge circuit 100 and the secondary side bridge circuit 120 under conditions where soft switching is established based on the output voltage (DC output voltage) and output power of the power conversion device 50 will be described. This program starts in response to an instruction to start operation of the power conversion device 50. The program shown in FIG. 11 is a program that controls the switching of the primary side bridge circuit 100 and the secondary side bridge circuit 120, and the program shown in FIG. 12 is a program that determines the switching during driving of each bridge circuit and changes the operating frequency according to the determination result. The program in FIG. 11 and the program in FIG. 12 are executed in parallel.

[0083] 11, this program includes step S1000 for initializing the operating frequency (setting a reference frequency) and initializing a Flag (Flag=0) in the shared area of ​​the storage unit 164, step S1010 for reading the operating frequency, step S1030 for switching the primary side bridge circuit 100 and the secondary side bridge circuit 120 at the read operating frequency to drive and control each bridge circuit, step S1020 for repeating the switching of each bridge circuit a predetermined number of times, with one cycle of switching being one time, step S1040 for setting Flag to 1, and step S1050 for determining whether an instruction to stop the operation of the power conversion device 50 has been issued and branching the flow of control according to the determination result in step S1040. If it is determined in step S1050 that an instruction to stop the operation has not been issued, the control returns to step S1010. If it is determined in step S1050 that an instruction to stop operation has been issued, the program ends.

[0084] 12, this program includes step S2000 of monitoring the shared area of ​​storage unit 164 and waiting until Flag is set to 1, step S2010 of detecting the output current and the DC output voltage via current sensor 140 and voltage sensor 150 when Flag is set to 1, step S2020, which is executed after step S2010, of calculating the output power using the output current and the DC output voltage, and step S2020, which is executed after step S2020, of determining whether the switching of the bridge circuit is soft switching based on the DC output voltage and the output power, and controlling the switching in accordance with the determination result. The program includes step S2030 for branching the control flow, step S2040 for changing the operating frequency, which is executed when it is determined in step S2030 that the switching is not soft switching (hard switching), step S2050 for setting Flag to 0, which is executed when it is determined in step S2030 that the switching is soft switching or after step S2040, and step S2060 for determining whether or not an instruction to stop the operation of the power conversion device 50 has been issued, and branching the control flow according to the determination result. If it is determined in step S2060 that an instruction to stop the operation has not been issued, the control returns to step S2000. If it is determined in step S2060 that an instruction to stop the operation has been issued, the program ends.

[0085] [Operation] The power conversion device 50 according to this embodiment operates as follows.

[0086] Referring to FIG. 1, when the control unit 160 of the power conversion device 50 receives an instruction to start operation, it initializes the operating frequency and also initializes the Flag. Specifically, the control unit 160 sets a reference operating frequency and sets the Flag to 0 (step S1000 in FIG. 11). The control unit 160 reads the set operating frequency (step S1010) and drives each bridge circuit at the operating frequency (step S1030). At that time, the control unit 160 controls switching of the primary side bridge circuit 100 and the secondary side bridge circuit 120 so that a phase difference in voltage occurs between the primary side bridge circuit 100 and the secondary side bridge circuit 120. In this embodiment, the control unit 160 operates the primary side bridge circuit 100 and the secondary side bridge circuit 120 with the input voltage set to a predetermined constant voltage.

[0087] When a predetermined number of switching operations (e.g., 10 times) have been performed, with one cycle of switching being regarded as one operation, the control unit 160 sets the flag to 1. The control unit 160 repeats the processes of steps S1010 to S1040 until an instruction to stop operation is received. The control unit 160 executes a process of determining whether or not soft switching is being performed in parallel with the switching control of the primary side bridge circuit 100 and the secondary side bridge circuit 120. The control unit 160 executes this process when the flag is set to 1 (YES in step S2000 in FIG. 12). That is, this process is executed every time the switching of the bridge circuit is performed a predetermined number of cycles (e.g., 10 cycles).

[0088] Specifically, the control unit 160 detects the output voltage and the output current via the current sensor 140 and the voltage sensor 150 (step S2010 in FIG. 12), and calculates the output power from the detected output voltage and output current (step S2020). The output current used to calculate the output power may be, for example, an average current per switching period. The control unit 160 determines whether the above formula (6) or formula (7) is satisfied based on the DC output voltage and output power of the power conversion device 50. Which of formula (6) and formula (7) is used is determined by comparing the output voltage E2 with the input voltage E1. If formula (6) or formula (7) is satisfied, the control unit 160 determines that soft switching is being performed (YES in step S2030). On the other hand, if formula (6) or formula (7) is not satisfied, the control unit 160 determines that soft switching is not being performed, that is, hard switching is being performed (NO in step S2030).

[0089] When the control unit 160 determines that soft switching is not being performed, it changes the operating frequency (step S2040). The control unit 160 sets the flag to 0 (step S2050) and waits again until the flag is set to 1. When the operating frequency is changed, the control unit 160 reads the changed operating frequency (step S1010 in FIG. 11) and drives each bridge circuit at the changed operating frequency (step S1030).

[0090] When the instruction to stop the operation is given, the control unit 160 stops the operation of the primary side bridge circuit 100 and the secondary side bridge circuit 120 (YES in step S1050 in FIG. 11, and YES in step S2060 in FIG. 12).

[0091] [Advantages of this embodiment] As is clear from the above description, the power conversion device 50 according to this embodiment provides the following advantages.

[0092] The control unit 160 switches the operating frequency of the primary side bridge circuit 100 and the secondary side bridge circuit 120 to a frequency at which soft switching is established based on the output voltage (DC output voltage) and output power of the power conversion device 50. That is, by switching the operating frequency based on the output voltage and output power, the switching elements are prevented from switching hard. This reduces switching loss caused by hard switching and suppresses a decrease in power conversion efficiency. In addition, by configuring the operating frequency to be switched based on the output voltage and output power, soft switching can be achieved even when the input / output voltage or the output power changes. Furthermore, in the configuration in which the operating frequency is switched, unlike the case in which the switching timing of the open / closed state of the switching elements is controlled, there is no need to detect the instantaneous value of the output current that coincides with the reactor current. Therefore, the operating frequency can be easily made high, and by providing an element such as a capacitor, surge countermeasures or noise countermeasures can be implemented. In this way, by adopting the above configuration, it is possible to simultaneously suppress a decrease in design freedom and suppress hard switching.

[0093] In the case of a method in which the instantaneous value of the reactor current just before the end of period A is measured and the timing of switching the open / closed state of the switching elements in the secondary bridge circuit is controlled depending on whether the instantaneous value is positive or negative, it becomes difficult to increase the control frequency to, for example, 100 kHz or higher, taking into account delays in the current sensor, etc.

[0094] The power conversion device 50 includes a current sensor 140 and a voltage sensor 150 that detect the output current and output voltage (DC output voltage) on the secondary side, respectively. The control unit 160 switches the operating frequency of the primary side bridge circuit 100 and the secondary side bridge circuit 120 based on the output voltage detected by the voltage sensor 150 and the output power calculated from the output current and output voltage. The output current used to calculate the output power may be, for example, an average current per switching period, and it is not necessary to detect the instantaneous value of the output current that coincides with the reactor current by the current sensor. Therefore, an inexpensive current sensor can be used as the current sensor 140. Since the options for the current sensor 140 are expanded, the degree of freedom in design can be further increased. In addition, even if there is a delay in the current sensor 140, the operating frequency can be made high. As a result, even if the output power is small, the frequency can be easily increased to realize soft switching.

[0095] [verification] The results of a test carried out to confirm the effects of the power conversion device 50 according to this embodiment will be described.

[0096] In this verification, the input voltage E1 was 400V, the output voltage E2 was 300V, and the inductor L value was 33μH.

[0097] Fig. 13 shows the operation result when the power conversion device is operated at an operating frequency of 30 kHz and an output power of 3.6 kW. Referring to Fig. 13, as shown in Figs. 2 to 5, switching element Q6 turns off the gate when reactor current I2 is positive, so that the charge of the parallel capacitance component of switching element Q5 is discharged by the energy stored in the inductor. Therefore, the drain-source voltage V of switching element Q5 ds_Q5 After the gate-source voltage V gs_Q5 Therefore, the switching element Q5 is soft-switched.

[0098] When lowering the output power to 1.8kW from this state, it is possible to reduce the phase difference while maintaining the operating frequency at 30kHz. DAB DC / DC converters are usually controlled in this way. Figure 14 shows the results of operation in this way.

[0099] Referring to FIG. 14, by reducing the output power to 1.8 kW, the value of reactor current I2 immediately before the end of period A (the area surrounded by the dashed circle) becomes negative. Switching element Q6 turns off the gate when reactor current I2 is negative. As shown in FIGS. 6 to 9, switching element Q6 includes a freewheeling diode that allows current to flow in the reverse direction, so the current path of the inductor does not change even when the gate is turned off. Therefore, the charge of switching element Q5 is not discharged, and the drain-source voltage V of switching element Q5 remains constant. ds_Q5 Before the gate-source voltage V gs_Q5 Therefore, in this case, the switching element Q5 is hard-switched, and a large switching loss occurs.

[0100] In this embodiment, when the output power is reduced to 1.8 kW, the operating frequency is changed before driving each bridge circuit. At that time, the phase difference may be slightly adjusted by feedback control.

[0101] FIG. 15 shows the operation result when the power conversion device is operated at an operating frequency of 60 kHz and an output power of 1.8 kW. Referring to FIG. 15, even if the output power is reduced to 1.8 kW, the value of reactor current I2 immediately before the end of period A (part surrounded by a dashed circle) becomes positive by increasing the operating frequency to 60 kHz. This allows switching element Q6 to turn off the gate while reactor current I2 is positive, thereby allowing switching element Q5 to discharge. Therefore, switching element Q5 is soft-switched, and soft-switching can be maintained even when the output power is small.

[0102] From the above, it has been confirmed that, according to the power conversion device 50 of this embodiment, soft switching can be maintained even when the output power is small.

[0103] (Second embodiment) Referring to FIG. 16, power conversion device 50A according to this embodiment differs from the first embodiment in that it receives a control command from external device 200 and determines whether or not soft switching is required using an instruction value of the output power included in the control command.

[0104] The power conversion device 50A includes a control unit 160A instead of the control unit 160 (FIG. 1). This control unit 160A controls the driving of the primary side bridge circuit 100 and the secondary side bridge circuit 120 based on a control command from the external device 200. The control command includes an instruction value for the output power, and the control unit 160A controls the primary side bridge circuit 100 and the secondary side bridge circuit 120 so that the output power becomes the output power indicated by the instruction value.

[0105] Referring to FIG. 17, the control unit 160A includes, as functional units, a drive control unit 170 that controls the drive of the primary side bridge circuit 100 and the secondary side bridge circuit 120, an acquisition unit 172 that acquires the output power as an instruction value from outside, a determination unit 174 that determines whether or not soft switching is being performed, and a frequency change unit 176 that changes the operating frequency depending on the determination result of the determination unit 174.

[0106] When determining whether soft switching is being performed, the control unit 160A uses the output power indicated by the instruction value. That is, the control unit 160A performs the determination based on the output power indicated by the instruction value and the output voltage (DC output voltage) detected via the voltage sensor 150. Since there is no need to calculate the output power, it is possible to easily determine whether soft switching is being performed. Therefore, according to this embodiment, it is possible to easily suppress a decrease in power conversion efficiency caused by hard switching.

[0107] (Third embodiment) The power conversion device according to this embodiment differs from the first embodiment in that it determines whether or not to switch the operating frequency using a table (hereinafter referred to as a "map") that determines whether or not soft switching is possible at a reference operating frequency. The map indicates whether or not the conditions for establishing soft switching are satisfied for various DC output voltage values ​​and output power values.

[0108] Fig. 18 shows an example of a map when the reference operating frequency is 250 kHz. Referring to Fig. 18, this map is in the form of a matrix with the vertical axis (rows) representing output power (W) and the horizontal axis (columns) representing DC output voltage (V). The map shows the results of calculating whether soft switching is possible for each DC output voltage and output power using the above-mentioned formulas (6) and (7). The input voltage used in the calculation is 400 V, and the total value L of the inductor components connected in series with the transformer is 1. r is 40 μH. In the map, "TRUE" indicates that soft switching is possible, and "FALSE" indicates that soft switching is not possible (hard switching). For example, when the output power is 1600 W and the DC output voltage is 300 V, the result is "TRUE", so in this case it is determined that soft switching is possible. On the other hand, when the output power is 1000 W and the DC output voltage is 300 V, the result is "FALSE", so in this case it is determined that hard switching is occurring. In FIG. 18, for the sake of convenience, "FALSE" is divided into "FALSE1" and "FALSE2" to indicate that the operating frequencies to be switched are different. Both FALSE1 and FALSE2 are the same in that they both indicate that soft switching is not possible (hard switching).

[0109] A map as shown in FIG. 18 is stored in advance in the control unit (storage unit), and the power conversion device according to this embodiment refers to the map stored in the storage unit when determining whether soft switching is possible. "FALSE1" and "FALSE2" in the map shown in FIG. 18 are associated with an operating frequency to be switched (a predetermined operating frequency at which soft switching is possible). For example, an operating frequency of 500 kHz is associated with FALSE1, and a predetermined operating frequency higher than 500 kHz (for example, 800 kHz) is associated with FALSE2. That is, since the condition for FALSE2 is that soft switching is not possible even at an operating frequency of 500 kHz, an operating frequency higher than 500 kHz is associated with FALSE2.

[0110] [Software configuration] In the power conversion device according to the present embodiment, a program shown in Fig. 19 is executed in place of the program shown in Fig. 12. The program shown in Fig. 19 is executed in parallel with the program in Fig. 11.

[0111] The program in Fig. 19 includes steps S2032 and S2034 instead of step S2030 in the program in Fig. 12. The processes in steps S2000 to S2020 and steps S2040 to S2060 in Fig. 19 are the same as the processes in the respective steps shown in Fig. 12. The different parts will be described below.

[0112] 19, this program includes step S2032, which is executed after step S2020, for referring to a map, and step S2034, which is executed after step S2032, for determining whether the switching of the bridge circuit is soft switching or not based on the map, the DC output voltage, and the output power, and for branching the flow of control according to the determination result. If it is determined in step S2034 that the switching is not soft switching (is hard switching), control proceeds to step S2040. If it is determined in step S2034 that the switching is soft switching, control proceeds to step S2050.

[0113] [Operation] The power conversion device according to the present embodiment operates as follows. Note that the operations other than the operation of determining whether soft switching is possible or not by referring to the map are the same as those in the first embodiment. Therefore, detailed description of the similar operations will not be repeated.

[0114] The power conversion device refers to the map and extracts a truth value corresponding to the DC output voltage and the output power. When the extracted truth value is "TRUE" (YES in step S2034), the power conversion device determines that soft switching is being performed and maintains the currently performed operating frequency (e.g., 250 kHz). On the other hand, when the extracted truth value is "FALSE" (NO in step S2034), the power conversion device determines that soft switching is not being performed (hard switching is being performed) and changes the operating frequency (step S2040). Specifically, when the extracted truth value is "FALSE (FALSE1)" and the currently performed operating frequency is 250 kHz, the power conversion device switches the operating frequency to, for example, 500 kHz. When the extracted truth value is "FALSE (FALSE2)", the power conversion device switches the operating frequency to a predetermined frequency (e.g., 800 kHz) at which soft switching is possible.

[0115] The power conversion device may store maps corresponding to a plurality of operating frequencies, and after switching the operating frequency, may refer to the map corresponding to the switched operating frequency to determine whether soft switching is possible. For example, after switching the operating frequency to 500 kHz, may refer to the map corresponding to 500 kHz to determine whether soft switching is possible. In this case, it is not necessary to classify the truth values ​​of the map into FALSE1 and FALSE2, and they may simply be "TRUE" and "FALSE".

[0116] Furthermore, for example, even after switching the operating frequency to 500kHz, the operating frequency may be returned to 250kHz when it is determined that soft switching is possible by referring to the map of 250kHz. In a DAB DC / DC converter, the upper limit of the power that can be output is reduced when the operating frequency is increased. Therefore, by returning the operating frequency to 250kHz when it is determined that soft switching is possible, the range of the power that can be output can be increased.

[0117] In this way, by referring to the map (table) when determining whether soft switching is possible, it becomes easier to determine whether soft switching is possible, and therefore it is easier to switch the operating frequencies of the primary side bridge circuit and the secondary side bridge circuit to a frequency at which soft switching is possible.

[0118] (Fourth embodiment) In this embodiment, a threshold value of the output power for switching the operating frequency is obtained based on a map indicating whether or not soft switching is established, and the operating frequency is switched according to the result of comparing the output power with the threshold value. The other configurations are the same as those of the above-mentioned embodiment.

[0119] FIG. 20 shows an example of a map when the operating frequency is 250 kHz. FIG. 21 shows an example of a map when the operating frequency is 500 kHz. As with the map shown in FIG. 18, these maps are in the form of a matrix with the vertical axis (row) representing output power (W) and the horizontal axis (column) representing DC output voltage (V). The maps shown in FIG. 20 and FIG. 21 show the results of calculating whether soft switching is possible for each DC output voltage and output power using the above-mentioned equations (6) and (7). The input voltage used in the calculation was 400 V, and the total value L of the inductor components connected in series with the transformer was 1. r is 40μH.

[0120] Similarly, "TRUE" in the map indicates that soft switching is possible, and "FALSE" indicates that soft switching is not possible (hard switching). In the map of FIG. 21, areas where no truth value is shown are areas where output is not possible (DISABLE) and operation is not possible under those conditions. Therefore, combinations of DC output voltage and output power that correspond to areas where output is not possible are not detected.

[0121] The map shown in FIG. 22 is obtained by combining the map in FIG. 20 and the map in FIG. 21. Referring to FIG. 22, as an example, the map (FIG. 20) with an operating frequency of 250 kHz is applied to the region where the output power is 1800 W or more, and the map (FIG. 21) with an operating frequency of 500 kHz is applied to the region where the output power is 1600 W or less. That is, the map in FIG. 20 and the map in FIG. 21 are combined to obtain an output power threshold value for switching the operating frequency. In the example of the map in FIG. 22, two threshold values ​​of 1600 W and 1800 W are obtained as the output power threshold value. The threshold value of 1600 W can be, for example, the threshold value when the operating frequency is switched from 250 kHz to 500 kHz, and the threshold value of 1800 W can be, for example, the threshold value when the operating frequency is switched from 500 kHz to 250 kHz.

[0122] [Software configuration] In the power conversion device according to the present embodiment, a program shown in Fig. 23 is executed in place of the program shown in Fig. 12. The program shown in Fig. 23 is executed in parallel with the program in Fig. 11.

[0123] The program in Fig. 23 includes step S2130 instead of step S2030 and step S2040 in the program in Fig. 12. The processes in steps S2000 to S2020, step S2050, and step S2060 in Fig. 23 are the same as the processes in the respective steps shown in Fig. 12. The different parts will be described below.

[0124] 23, this program is executed after step S2020 and includes step S2130 for switching the operating frequency.

[0125] Fig. 24 is a detailed flow of step S2130 in Fig. 23. Referring to Fig. 24, this routine includes step S2200 for determining whether or not the output power is less than 1600 W and branching the control flow in accordance with the determination result, step S2210, which is executed if it is determined in step S2200 that the output power is less than 1600 W, for determining whether or not the operating frequency being executed is 250 kHz and branching the control flow in accordance with the determination result, and step S2220, which is executed if it is determined in step S2210 that the operating frequency being executed is 250 kHz and switching the operating frequency to 500 kHz.

[0126] This routine further includes step S2230, which is executed if it is determined in step S2200 that the output power is not less than 1600 W, for determining whether the output power is 1800 W or greater and branching the control flow in accordance with the determination result; step S2240, which is executed if it is determined in step S2230 that the output power is 1800 W or greater, for determining whether the operating frequency currently being executed is 500 kHz or greater and branching the control flow in accordance with the determination result; and step S2250, which is executed if it is determined in step S2240 that the operating frequency currently being executed is 500 kHz, for switching the operating frequency to 250 kHz.

[0127] This routine ends if it is determined in step S2210 that the operating frequency in progress is not 250 kHz, if it is determined in step S2230 that the output power is not 1800 W or more, or if it is determined in step S2240 that the operating frequency in progress is not 500 kHz. Furthermore, this routine ends when the processing of step S2220 or step S2250 ends.

[0128] [Operation] The power conversion device according to the present embodiment operates as follows. Note that the operations other than the operation of switching the operating frequency are similar to those of the first embodiment. Therefore, detailed description of the similar operations will not be repeated.

[0129] With reference to Fig. 25, for example, it is assumed that the power conversion device is operating at an operating frequency of 250 kHz. At this time, if the output power falls below 1600 W (YES in step S2200 and YES in step S2210 in Fig. 24), the control unit of the power conversion device switches the operating frequency to 500 kHz (step S2220). Furthermore, it is assumed that the power conversion device is operating at an operating frequency of 500 kHz, for example. At this time, if the output power exceeds 1800 W (YES in step S2230 and YES in step S2240), the control unit of the power conversion device switches the operating frequency to 250 kHz (step S2250).

[0130] If the output power is equal to or greater than 1600 W and less than 1800 W (NO in step S2200 and NO in step S2230 in FIG. 24), the operating frequency currently in progress is maintained. Even if the output power is equal to or greater than 1800 W (YES in step S2230) and the operating frequency currently in progress is 250 kHz (NO in step S2240), the operating frequency is maintained. Furthermore, even if the output power is less than 1600 W (YES in step S2200) and the operating frequency currently in progress is 500 kHz (NO in step S2210), the operating frequency is maintained.

[0131] [effect] In this embodiment, instead of judging whether soft switching occurs, a threshold value calculated based on a map for judging whether soft switching occurs is used, and the operating frequency is switched according to the result of comparing the output power with the threshold value. This makes it possible to switch the operating frequency before hard switching occurs, so that hard switching can be more reliably suppressed. This makes it possible to more easily suppress the decrease in power conversion efficiency caused by hard switching.

[0132] In this embodiment, the operating frequency is switched at a constant output power regardless of the output voltage (DC output voltage) of the power conversion device, but the threshold of the output power may be changed depending on the output voltage. For example, when the DC output voltage is 400V, the operation at 250kHz may be maintained up to the output power of 0W, while when the DC output voltage is 500V, the operation may be switched between 250kHz and 500kHz at the output power of 1600W.

[0133] Furthermore, the operating frequency may be switched according to the DC output voltage. For example, when operating at a constant output power of 1000 W, the soft switching operation can be maintained by configuring the inverter to operate at 250 kHz for DC output voltages of 360 V to 440 V, and at 500 kHz for DC output voltages of 300 V to 360 V and for DC output voltages of 440 V to 500 V.

[0134] (Fifth embodiment) The power conversion device according to the present embodiment differs from the first embodiment in that it also uses the input voltage on the primary side to determine whether or not soft switching is required.

[0135] 26, power conversion device 50B according to the present embodiment further includes a voltage sensor 152 that detects an input voltage on the primary side. Power conversion device 50B includes a control unit 160B instead of control unit 160 (see FIG. 1). Control unit 160B differs from control unit 160 in the program stored in storage unit 164.

[0136] [Software configuration] In the power conversion device 50B according to the present embodiment, a program shown in Fig. 27 is executed instead of the program shown in Fig. 12. The program shown in Fig. 27 is executed in parallel with the program in Fig. 11. In the program shown in Fig. 27, the reference operating frequency that is set as an initial value is, for example, 250 kHz.

[0137] The program in Fig. 27 includes step S2012 instead of step S2010, and step S2132 instead of step S2030 and step S2040 in the program in Fig. 12. The processes in steps S2000, S2020, S2050, and S2060 in Fig. 27 are the same as the processes in the respective steps shown in Fig. 12. The different parts will be described below.

[0138] Referring to FIG. 27, this program is executed when it is determined in step S2000 that the flag is 1, and includes step S2012 for detecting the input voltage, output voltage (DC output voltage), and output current of the power conversion device 50B, and step S2132 for performing an operating frequency switching process after step S2020.

[0139] Fig. 28 is a detailed flow of step S2132 in Fig. 27. Referring to Fig. 28, this routine includes step S2300 for determining whether or not the input voltage E1 is greater than the output voltage E2 and branching the control flow depending on the determination result, step S2310, which is executed when it is determined in step S2300 that the input voltage E1 is greater than the output voltage E2, for determining whether or not the above-mentioned formula (6) is satisfied and for branching the control flow depending on the determination result, step S2320, which is executed when it is determined in step S2310 that the above-mentioned formula (6) is satisfied and for maintaining the operating frequency at 250 kHz, and step S2330, which is executed when it is determined in step S2310 that the above-mentioned formula (6) is not satisfied and for changing the operating frequency to 500 kHz.

[0140] This routine further includes step S2340, which is executed if it is determined in step S2300 that the input voltage E1 is not greater than the output voltage E2, i.e., that the output voltage E2 is equal to or greater than the input voltage E1, and which determines whether the above-mentioned equation (7) is satisfied and branches the control flow depending on the determination result; step S2350, which is executed if it is determined in step S2340 that the above-mentioned equation (7) is satisfied, and which maintains the operating frequency at 250 kHz; and step S2360, which is executed if it is determined in step S2340 that the above-mentioned equation (7) is not satisfied, and which changes the operating frequency to 500 kHz.

[0141] When the processing of step S2320, step S2330, step S2350, or step S2360 is completed, this routine ends.

[0142] As described above, the power conversion device 50B according to the present embodiment also uses the primary input voltage to determine whether or not soft switching is required. Therefore, even if the primary input voltage changes, soft switching can be realized. Therefore, switching loss can be effectively reduced, and a decrease in power conversion efficiency caused by hard switching can be more easily suppressed.

[0143] (Sixth embodiment) 29, a power conversion device 50C according to the present embodiment differs from the first embodiment in that a filter circuit 180 and a snubber capacitor 190 are further included.

[0144] The filter circuit 180 is disposed between the secondary bridge circuit 120 and the current sensor 140, and applies a frequency limit to the output current detected by the current sensor 140. The filter circuit 180 is an LC filter including a reactor 182 and a capacitor 184.

[0145] The snubber capacitor 190 is disposed in a region between the secondary bridge circuit 120 and the current sensor 140, adjacent to the secondary bridge circuit 120. In other words, the snubber capacitor 190 is disposed in a region closer to the secondary bridge circuit 120 than the filter circuit 180.

[0146] As described above, the current information used to calculate the output power may be frequency-limited information (for example, information with high-frequency components cut off). By providing a filter circuit 180 that imposes a frequency limit on the current information of the current sensor 140, the output power can be easily calculated when the output power is calculated using the output current and the output voltage (DC output voltage).

[0147] Furthermore, by providing the snubber capacitor 190, surge protection, noise protection, and the like can be achieved, thereby improving the reliability of the power conversion device 50C. Even when the filter circuit 180 and the snubber capacitor 190 are provided, it is possible to determine whether or not soft switching is occurring in the same way as when they are not provided.

[0148] In the present embodiment, an example in which both the filter circuit 180 and the snubber capacitor 190 are provided has been shown, but the present disclosure is not limited to this configuration. For example, either the filter circuit 180 or the snubber capacitor 190 may be provided in the power conversion device.

[0149] (Seventh embodiment) In this embodiment mode, an example in which the power conversion device described in the above embodiment mode is applied to an on-board charger will be described.

[0150] 30, an on-board charger 300 according to this embodiment includes an AC / DC conversion circuit 310 that converts AC power from a power system 340 into DC power and outputs the DC power, and a power conversion device 320 to which the DC power converted by the AC / DC conversion circuit 310 is input. A smoothing capacitor 330 is provided between the AC / DC conversion circuit 310 and the power conversion device 320. Note that in FIG. 30, a control unit that controls the AC / DC conversion circuit 310 and a control unit that controls the power conversion device 320 are omitted.

[0151] The AC / DC conversion circuit 310 converts AC 200V alternating current power from the power system 340 into DC 400V, for example.

[0152] The power conversion device 320 is a DC / DC converter of the DAB system. Any of the power conversion devices shown in the above embodiments can be used for the power conversion device 320. The power conversion device 320 converts the DC 400V direct current power output from the AC / DC conversion circuit 310 into, for example, DC 300V direct current power and outputs it to the high-voltage battery 350. The high-voltage battery 350 is charged with the power converted by the on-board charger 300.

[0153] [Installation in vehicle] 31, vehicle 400 is an electric vehicle such as a plug-in hybrid electric vehicle (PHEV) or an electric vehicle (EV). Vehicle 400 includes a charging socket 410, an on-board charger 300, a high-voltage battery 350, an inverter 420, and a drive device 430.

[0154] A charging plug of a charger (not shown) is connected to the charging socket 410 when charging the high-voltage battery 350. The charger outputs AC 200V alternating current power from the power grid. The charging output from the charger is input to the on-board charger 300 via the charging socket 410. The on-board charger 300 converts the AC 200V alternating current power into, for example, DC 300V direct current power and outputs it to the high-voltage battery 350.

[0155] The inverter 420 controls the supply of electricity to the drive device 430 according to a command from a vehicle controller (not shown). The drive device 430 includes a motor, and generates a driving force from the power supplied from the high-voltage battery 350. That is, the drive device 430 generates a driving force for turning the wheels from the AC power converted by the inverter 420.

[0156] (Modification) In the above embodiment, an example of a two-port DAB DC / DC converter has been shown, but the present disclosure is not limited to such an embodiment. The number of ports of the power conversion device that is a DAB DC / DC converter may be three or more.

[0157] In the above embodiment, an example has been shown in which the output voltage of each bridge circuit is a rectangular wave with a duty of 50%, but the present disclosure is not limited to such an embodiment. The duty of the output voltage may be other than 50%. Furthermore, the duty of the primary side bridge circuit and the duty of the secondary side bridge circuit may be different values. The technology of the present disclosure can be applied to any circuit whose output changes depending on the phase difference and the operating frequency.

[0158] In the above embodiment, the configuration in which the operating frequency is switched between two values, 30 kHz and 60 kHz, is shown, but the present disclosure is not limited to such an embodiment. For example, the operating frequency may be switched stepwise by dividing the range between 30 kHz and 60 kHz into smaller ranges, such as 30 kHz, 40 kHz, and 50 kHz, or the operating frequency may be changed continuously between 30 kHz and 60 kHz. In the above embodiment, the configuration in which the operating frequency is switched between two values, 250 kHz and 500 kHz, is also shown. In this case, similarly to the above, the operating frequency may be switched stepwise between 250 kHz and 500 kHz, or the operating frequency may be changed continuously between 250 kHz and 500 kHz.

[0159] In the above embodiment, as shown in the map of Fig. 18, when soft switching is not possible ("FALSE"), two operating frequencies ("FALSE1" and "FALSE2") are set according to the output voltage and output power, but the present disclosure is not limited to such a configuration. For example, three or more operating frequencies may be set according to the switching operating frequency.

[0160] In the above embodiment, a map using truth values ​​of "TRUE" or "FALSE" has been shown as an example of a map for determining whether soft switching is possible, but the present disclosure is not limited to such an embodiment. The map may be configured to determine whether soft switching is possible using a value other than truth values. For example, instead of truth values, a map indicating an operating frequency at which soft switching is possible may be used. In this map, for example, an operating frequency is indicated for a combination of output voltage (DC output voltage) and output power for which soft switching is possible, and a frequency to be changed is indicated for a combination of output voltage (DC output voltage) and output power for which soft switching is not possible. In this case, by referring to the map, it is possible to easily switch to an operating frequency at which soft switching is possible.

[0161] The values ​​of the operating frequency and the output power shown in the above embodiment are examples and are not limited thereto. The maps shown in the above embodiment are also examples and can be changed as appropriate. The output voltage (horizontal axis) and the output power (vertical axis) shown in the map can also indicate their respective ranges. For example, the output voltage 320V shown in the map can be 320V or more and less than 340V (or more than 320V and less than 340V). In this case, if the detected value of the output voltage is, for example, 330V, the truth value of the output voltage 320V in the map can be referred to. Similarly, for example, the output power of 1600W can be 1600W or more and less than 1800W (or more than 1600W and less than 1800W). In this case, if the value of the output power is, for example, 1650W, the truth value of the output power of 1600W in the map can be referred to.

[0162] In addition, embodiments obtained by appropriately combining the techniques disclosed above are also included in the technical scope of the present disclosure.

[0163] The embodiments disclosed herein are merely examples, and the present disclosure is not limited to the above-described embodiments. The scope of the present disclosure is indicated by the claims of the claims in consideration of the description of the detailed description of the invention, and includes all modifications within the meaning and scope equivalent to the wording described therein. [Explanation of symbols]

[0164] 40 First DC power source 42 Second DC power source 50, 50A~50C, 320 Power converter 60 Primary coil 62 Secondary coil 64 Core member 100 Primary side bridge circuit 110, 130, 330 Smoothing capacitor 120 Secondary bridge circuit 140 Current Sensor 150, 152 Voltage sensor 160, 160A, 160B Control section 162 CPU 164 Storage section 170 Drive control unit 172 Acquisition Department 174 Judgment section 176 Frequency change unit 180 Filter Circuit 184 Capacitor 190 Snubber capacitor 200 External device 300 On-board charger 310 AC / DC conversion circuit 340 Power System 350 High Voltage Battery 400 vehicles 410 Charging socket 420 Inverter 430 Drive Unit D1~D8 Freewheeling diodes L1, L2, 182 reactor Q1~Q8 Switching elements TR Transformer V1, V2 bridge circuit output voltage E1 input voltage E2 output voltage (DC output voltage) V ds_Q5 Drain-Source Voltage V gs_Q5 Gate-Source Voltage

Claims

1. An insulation type power conversion device, Transformer and a first bridge circuit including a plurality of switching elements and provided on a primary side of the transformer; a second bridge circuit including a plurality of switching elements and provided on a secondary side of the transformer; a control unit that controls switching of the first bridge circuit and the second bridge circuit, the control unit switches the operating frequencies of the first bridge circuit and the second bridge circuit to a frequency at which soft switching is established in control of the first bridge circuit and the second bridge circuit, based on an output voltage and an output power of the power conversion device; a voltage sensor provided on the secondary side of the transformer to detect an output voltage of the power conversion device; and an acquisition unit that acquires the output power as an instruction value from an external source, The control unit switches operating frequencies of the first bridge circuit and the second bridge circuit based on the output voltage detected by the voltage sensor and the output power acquired by the acquisition unit.

2. a current sensor for detecting an output current of the power conversion device; 2. The power conversion device according to claim 1, wherein the control unit switches operating frequencies of the first bridge circuit and the second bridge circuit based on the output power calculated using the output voltage detected by the voltage sensor, and the output current and output voltage detected by the current sensor and the voltage sensor, respectively.

3. The power conversion device according to claim 2 , further comprising a circuit for imposing a frequency limit on the output current detected by the current sensor.

4. The power conversion device according to claim 3 , wherein the circuit that applies the frequency limit includes a filter circuit disposed between the second bridge circuit and the current sensor.

5. The power conversion device according to claim 2 , further comprising a capacitor disposed in a region between the second bridge circuit and the current sensor, the region being adjacent to the second bridge circuit.

6. An isolated power conversion device, Transformer and a first bridge circuit including a plurality of switching elements and provided on a primary side of the transformer; a second bridge circuit including a plurality of switching elements and provided on a secondary side of the transformer; a control unit that controls switching of the first bridge circuit and the second bridge circuit, the control unit switches the operating frequencies of the first bridge circuit and the second bridge circuit to a frequency at which soft switching is established in control of the first bridge circuit and the second bridge circuit, based on an output voltage and an output power of the power conversion device; The control unit includes a memory unit that stores judgment conditions for judging whether or not soft switching is established, and judges whether or not the switching of the first bridge circuit and the second bridge circuit is soft switching using the output voltage, the output power, and the judgment conditions, and switches operating frequencies of the first bridge circuit and the second bridge circuit depending on the judgment result.

7. the storage unit stores, as the determination condition, a table indicating whether or not a condition for establishing soft switching is satisfied for various output voltage values ​​and output power values; 7. The power conversion device according to claim 6, wherein the control unit determines whether switching of the first bridge circuit and the second bridge circuit is soft switching or not by referring to the table for the output voltage and the output power, and switches operating frequencies of the first bridge circuit and the second bridge circuit depending on a determination result.

8. A conversion circuit that converts AC power into DC power and outputs the DC power; 8. An on-board charger comprising: the power conversion device according to claim 1, to which the DC power converted by the conversion circuit is input.

9. A control method for an isolated power conversion device comprising: a transformer; a first bridge circuit including a plurality of switching elements and provided on a primary side of the transformer; a second bridge circuit including a plurality of switching elements and provided on a secondary side of the transformer; a voltage sensor provided on the secondary side of the transformer for detecting an output voltage of the power conversion device; and an acquisition unit for acquiring an output power of the power conversion device as an instruction value from an outside, comprising: controlling switching of the first bridge circuit and the second bridge circuit so that a phase difference is generated between the first bridge circuit and the second bridge circuit; switching an operating frequency of the first bridge circuit and the second bridge circuit to a frequency at which soft switching is established in control of the first bridge circuit and the second bridge circuit, based on an output voltage and an output power of the power conversion device; switching operating frequencies of the first bridge circuit and the second bridge circuit based on the output voltage detected by the voltage sensor and the output power acquired by the acquisition unit.

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