Power conversion device, charging device, and control method

Abstract

This power conversion device comprises: a transformer that includes first and second coils; a first bridge circuit that includes a plurality of first switching elements and that is connected to the first coil; and a second bridge circuit that includes a plurality of second switching elements and that is connected to the second coil. The first bridge circuit generates a first AC voltage and supplies the same to the first coil. The second bridge circuit converts a second AC voltage generated on the second coil and outputs the resulting voltage. The power conversion device switches, in accordance with transmission power, between phase difference control for adjusting a phase difference between the second AC voltage and the first AC voltage and burst control for adjusting a period of supplying the first AC voltage while keeping a phase difference constant and fixing one cycle of the control.

Description

Power conversion device, charging device, and control method

[0001] The present disclosure relates to a power conversion device, a charging device, and a control method.

[0002] Power conversion devices are used in various electrical devices and electrical facilities. For example, vehicles such as plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) are equipped with an on-board charger, a DC / DC converter, and multiple power conversion units. These power conversion devices convert AC power from the power grid into DC power to charge the on-board battery. When the vehicle is running, the output voltage of the on-board battery is converted to an appropriate voltage and supplied to various devices inside the vehicle.

[0003] Patent Document 1 below discloses an isolated bidirectional DC / DC converter that achieves high efficiency over a wide range of transmission power. This DC / DC converter performs phase difference control in the high-output power range and burst control in the low-output power range as a countermeasure against reduced transmission efficiency. The burst control described in Patent Document 1 performs intermittent operation by switching the DC / DC converter for a period of δ+2πm, where m is a real number that is a natural number multiple of 0.5 and n is a real number, and halting the switching of the DC / DC converter for a period of 2πn. In burst control, the transmission power is adjusted by changing n. That is, the transmission power is adjusted by changing the period during which switching is halted.

[0004] Japanese Patent Application Laid-Open No. 2017-130997

[0005] A power conversion device according to an aspect of the present disclosure includes a transformer including a first coil and a second coil, a first bridge circuit including a plurality of first switching elements and connected to the first coil, and a second bridge circuit including a plurality of second switching elements and connected to the second coil, wherein the first bridge circuit generates a first AC voltage and supplies it to the first coil, and the second bridge circuit converts and outputs a second AC voltage generated in the second coil, and switches between phase difference control that adjusts the phase difference between the second AC voltage and the first AC voltage, and burst control that keeps the phase difference constant, fixes one control cycle, and adjusts the period during which the first AC voltage is supplied, depending on the transmission power.

[0006] FIG. 1 is a graph showing changes in voltage and current when switching elements are in an on-operation state. FIG. 2 is a circuit diagram showing a configuration of a power conversion device according to a first embodiment of the present disclosure. FIG. 3 is a waveform diagram showing control signals for each switching element shown in FIG. 2. FIG. 4 is a circuit diagram showing a current flow in a first mode of the first bridge circuit shown in FIG. 2. FIG. 5 is a circuit diagram showing a current flow in a second mode of the first bridge circuit shown in FIG. 2. FIG. 6 is a circuit diagram showing a current flow in a third mode of the second bridge circuit shown in FIG. 2. FIG. 7 is a circuit diagram showing a current flow in a fourth mode of the second bridge circuit shown in FIG. 2. FIG. 8 is a graph showing changes in voltage and current in the power conversion device shown in FIG. 2. FIG. 9 is a circuit diagram showing a current flow when switching from the first mode to the second mode in the case of soft switching in the first bridge circuit shown in FIG. 2. FIG. 10 is a circuit diagram showing a current flow, different from that shown in FIG. 3, in the first mode of the first bridge circuit shown in FIG. 2. FIG. 11 is a circuit diagram showing the current flow when switching from the first mode to the second mode in the case of hard switching in the first bridge circuit shown in FIG. 2 . FIG. 12 is a graph showing changes in voltage and current when the switching operation in the power conversion device shown in FIG. 2 is hard switching. FIG. 13 is a flowchart showing the operation executed by the control unit shown in FIG. 2 . FIG. 14 is a graph showing a voltage waveform when the output power is 100% in burst control. FIG. 15 is a graph showing a voltage waveform when the output power is less than 100% in burst control. FIG. 16 is a graph showing changes in phase difference and burst rate due to switching between phase difference control and burst control. FIG. 17 is a diagram showing simulation results when only phase difference control is performed. FIG. 18 is a diagram showing simulation results when switching between phase difference control and burst control. FIG. 19 is a graph showing the correspondence between voltage waveforms and current waveforms in burst control. FIG. 20 is a block diagram showing a charging device according to a second embodiment of the present disclosure.

[0007] [Problem to be Solved by the Present Disclosure] In phase difference control, the smaller the output power from the power conversion device, the smaller the phase difference needs to be. When the phase difference is small, the switching operation of the switching elements constituting the bridge circuit included in the power conversion device becomes hard switching, which causes a problem of power loss during switching (hereinafter referred to as switching loss). It is preferable to take switching loss into consideration when controlling the on / off of the switching elements.

[0008] Referring to FIG. 1 , during a transition period when a switching element changes from an off state to an on state, the current indicated by the dashed line typically rises from 0 amperes to a predetermined current value I0, and the voltage indicated by the solid line typically falls from a predetermined voltage V0 to 0 volts. The switching loss can be calculated by integrating the product of the voltage and current over the transition period. Assume that a field effect transistor (FET) is used as the switching element. When any of the switching elements constituting a power conversion device changes from an off state to an on state, if no current flows through the parasitic diode in the off state of that switching element, a voltage is generated between the source and drain of that switching element, resulting in switching loss (i.e., hard switching). On the other hand, if current flows through the parasitic diode in the off state of the switching element, the voltage between the source and drain of that switching element is zero, and no switching loss occurs. This type of switching is called soft switching. Note that the value zero does not mean mathematical zero, but rather a value that can be interpreted as substantially zero, including the value of the forward voltage of the parasitic diode.

[0009] As mentioned above, Patent Document 1 discloses burst control in a low output range. However, the disclosed burst control adjusts transmission power by changing the period during which switching is stopped by changing n. This creates a problem in that it is difficult to adjust transmission power because one cycle of intermittent operation changes with the change in n. Furthermore, this change in one cycle of intermittent operation causes the timing at which switching starts to differ from cycle to cycle, which is inconvenient. Therefore, there is also the problem of complicated control, since adjustments to match the timing are required each time.

[0010] Therefore, an object of the present disclosure is to provide a power conversion device, a charging device, and a control method that can reduce switching loss through simple control even when the output power is small.

[0011] Effect of the Present Disclosure According to the present disclosure, it is possible to provide a power conversion device, a charging device, and a control method that can reduce switching loss through simple control even when the output power is small.

[0012] [Description of Embodiments of the Present Disclosure] The contents of the embodiments of the present disclosure will be listed and described below. At least some of the embodiments described below may be combined in any combination.

[0013] (1) A power conversion device according to a first aspect of the present disclosure includes a transformer including a first coil and a second coil, a first bridge circuit including a plurality of first switching elements and connected to the first coil, and a second bridge circuit including a plurality of second switching elements and connected to the second coil, wherein the first bridge circuit generates a first AC voltage and supplies it to the first coil, and the second bridge circuit converts and outputs a second AC voltage generated in the second coil, and switches between phase difference control that adjusts a phase difference between the first AC voltage and the second AC voltage and burst control that keeps the phase difference constant, fixes one control cycle, and adjusts a period during which the first AC voltage is supplied, depending on the transmission power. This allows each switching element of the power conversion device to perform soft switching even when the output power is low, thereby reducing switching loss due to hard switching and suppressing overheating and failure.

[0014] (2) In the above (1), the power conversion device performs phase difference control if the transmission power is greater than a predetermined threshold, and performs burst control if the transmission power is equal to or less than the threshold, thereby making it possible to easily switch between phase difference control and burst control.

[0015] (3) In the above (2), the threshold value is greater than a value corresponding to a lower limit of the phase difference, and the lower limit is the value of the phase difference at which the operation of at least some of the first switching elements and the second switching elements changes from soft switching to hard switching, thereby ensuring that each switching element performs soft switching.

[0016] (4) In any one of (1) to (3) above, the burst control changes the burst period, which is the period during which the first AC voltage is supplied, based on a predetermined period. This allows a desired output power to be output.

[0017] (5) In the above (4), N and Nb are natural numbers, and N is greater than Nb, the predetermined period is equal to N times the period of the first AC voltage, and the first AC voltage is continuously input to the first bridge circuit during the burst period, and the burst period is equal to Nb times the period, thereby facilitating burst control.

[0018] (6) In any one of (1) to (5) above, the frequency of the first AC voltage is constant during burst control. This makes it easier to perform burst control.

[0019] (7) A charging device according to a second aspect of the present disclosure includes the power conversion device according to any one of (1) to (6) above, and outputs the DC voltage converted by the second bridge circuit as charging power. This allows each switching element constituting the power conversion device to perform soft switching even when the output power is small, thereby reducing switching loss due to hard switching and suppressing overheating and failure.

[0020] (8) In the above (7), the charging device further includes an AC / DC converter, and the DC voltage output from the AC / DC converter is input to the first bridge circuit. This allows for the charging device to operate using AC power supplied from an external source (e.g., commercial AC power).

[0021] (9) In the above (7) or (8), the charging device is fixedly installed and supplies power to a storage battery mounted in an external device. This makes it possible to realize a charging device that can charge a storage battery mounted in an external device (e.g., a vehicle) using commercial AC power.

[0022] (10) In any one of (7) to (9) above, the charging power is changed according to the capacity of the storage battery to which the charging power is supplied. This allows the storage battery to be appropriately charged.

[0023] (11) A control method according to a third aspect of the present disclosure is a control method for a power conversion device including a transformer including a first coil and a second coil, a first bridge circuit including a plurality of first switching elements and connected to the first coil, and a second bridge circuit including a plurality of second switching elements and connected to the second coil, the control method including the steps of: causing the first bridge circuit to generate a first AC voltage and supply it to the first coil; causing the second bridge circuit to convert and output a second AC voltage generated in the second coil; a phase difference control step of adjusting a phase difference between the first AC voltage and the second AC voltage; a burst control step of adjusting a period during which the first AC voltage is supplied according to output power while maintaining the phase difference constant and fixing one control cycle; and a step of switching between the phase difference control step and the burst control step according to transmission power. This allows soft switching operation of each switching element constituting the power conversion device even when the output power is low, reducing switching loss due to hard switching, and suppressing overheating and failure.

[0024] [Details of the embodiments of the present disclosure] In the following embodiments, the same components are denoted by the same reference numerals, and their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[0025] 2, a power conversion device 100 according to a first embodiment of the present disclosure is an isolated power conversion device including a transformer, i.e., a DAB (Dual Active Bridge) DC / DC converter. Specifically, the power conversion device 100 includes a first bridge circuit 102, a transformer 104, a second bridge circuit 106, and a control unit 110. The power conversion device 100 may also include a capacitor C1, a capacitor C2, a capacitor C3, and a choke coil L3.

[0026] The first bridge circuit 102 is a DC / AC conversion circuit and includes switching elements Q1, Q2, Q3, and Q4. Switching elements Q1 to Q4 are bridge-connected to form a full-bridge circuit. Switching elements Q1 to Q4 are configured, for example, by field-effect transistors (FETs). FIG. 2 shows parasitic diodes (i.e., body diodes) formed inside the FETs.

[0027] The second bridge circuit 106 is an AC / DC conversion circuit and includes switching elements Q5, Q6, Q7, and Q8. Switching elements Q5 to Q8 are bridge-connected to form a full-bridge circuit. Switching elements Q5 to Q8 are configured, for example, by FETs.

[0028] The transformer 104 includes a core 120, a first coil 122 and a second coil 124 wound around the core 120, and a first inductor L1 and a second inductor L2. Note that the transformer 104 does not necessarily have to include the core 120. The first coil 122 functions as the primary coil of the transformer 104, and the second coil 124 functions as the secondary coil of the transformer 104. The first inductor L1 and the second inductor L2 may be any inductive components. In FIG. 2, the first inductor L1 and the second inductor L2 utilize the leakage inductance of the transformer 104 and are illustrated as being included in the transformer 104. The first inductor L1 and the second inductor L2 may be separate coils from the first coil 122 and the second coil 124 of the transformer 104.

[0029] The output terminal of the first bridge circuit 102 is connected to both terminals of the series-connected first inductor L1 and first coil 122. The input terminal of the second bridge circuit 106 is connected to the series-connected second inductor L2 and second coil 124. A DC voltage E1 is supplied between nodes N1 and N2 that constitute the input unit 130. The DC voltage E1 is input to the first bridge circuit 102 via a capacitor C1. The on / off of each of the switching elements Q1 to Q4 is controlled by the control unit 110, so that the first bridge circuit 102 converts the DC voltage E1 input between nodes N1 and N2 into an AC voltage and outputs it as an AC voltage V1 between nodes N5 and N6.

[0030] The AC voltage V1 is supplied to the first coil 122 of the transformer 104. The AC voltage generated in the second coil 124 is input to the second bridge circuit 106 as the AC voltage V2 between the nodes N7 and N8. The control unit 110 controls the on / off of the switching elements Q5 to Q8, converting the AC voltage V2 into a DC voltage and outputting it from the second bridge circuit 106. The output voltage of the second bridge circuit 106 is smoothed by the choke coil L3, the capacitors C2 and C3, and output as the DC voltage E2 between the nodes N3 and N4 that constitute the output unit 132. That is, the power conversion device 100 functions as a DC / DC converter as described above.

[0031] The control unit 110 includes a CPU (Central Processing Unit) 112, a memory 114, and an I / F (interface) unit 116. The memory 114 stores programs executed by the CPU 112. Under the control of the CPU 112, the I / F unit 116 outputs signals (i.e., gate voltages of each switching element) for controlling the on / off of each switching element constituting the first bridge circuit 102 and the second bridge circuit 106, as described above. Furthermore, power measuring devices (e.g., sensors) (not shown) are provided in the input unit 130 and the output unit 132. The I / F unit 116 receives measured values ​​from the input unit 130 and the output unit 132 and stores them in the memory 114. The I / F unit 116 also receives instructions, such as a target value for output power, from outside the power conversion device 100 and stores them in the memory 114. The stored measurements and instructions (e.g., target values ​​of output power) are used in controlling the first bridge circuit 102 and the second bridge circuit 106, as will be described later. The CPU 112 executes programs read from the memory 114 to perform these processes.

[0032] (Phase difference control)

[0033] Switching elements Q1 to Q8 are controlled using a phase shift method. Referring to FIG. 3, switching elements Q1 to Q8 are all controlled using the same cycle T. Switching elements Q1 and Q4 are turned on at the same timing and turned off at the same timing. Switching elements Q2 and Q3 are also controlled to be on and off at the same timing. Switching elements Q1 and Q2 are alternately turned on with a duty of 50%, and switching elements Q3 and Q4 are also alternately turned on with a duty of 50%. In other words, the pulse widths of the control signals are the same. Note that "same" means that they are within a predetermined error range.

[0034] Similarly, switching element Q5 and switching element Q8 are controlled to be turned on and off at the same timing. Switching element Q6 and switching element Q7 are also controlled to be turned on and off at the same timing. Switching element Q5 and switching element Q6 are alternately turned on with a duty of 50%, and switching element Q7 and switching element Q8 are also alternately turned on with a duty of 50%. Furthermore, switching element Q5 is turned on with a phase difference of time Tp relative to switching element Q1. The phase difference of time Tp is expressed in degrees as 2π·Tp / T (rad). As such, time Tp is treated as a phase difference, and is hereinafter referred to as phase difference Tp.

[0035] 4 to 8, the operation of the power conversion device 100 will be specifically described. Under the control of the power conversion device 100, the first bridge circuit 102 and the second bridge circuit 106 each operate in two modes. The two operation modes of the first bridge circuit 102 are a first mode m1 and a second mode m2, and the two operation modes of the second bridge circuit 106 are a third mode m3 and a fourth mode m4.

[0036] 4 shows the circuit to the left of the first coil 122 of the transformer 104 in FIG. 2. Referring to FIG. 4, in the first mode m1 of the first bridge circuit 102, the switching elements Q1 and Q4 are turned on, and the switching elements Q2 and Q3 are turned off. This causes a current to flow as shown by the arrows. That is, the current flows through the switching element Q1, the first inductor L1, the first coil 122, and the switching element Q4.

[0037] Similar to Fig. 4, Fig. 5 shows the circuit to the left of the first coil 122 of the transformer 104 in Fig. 2. Referring to Fig. 5, in the second mode m2 of the first bridge circuit 102, the switching elements Q1 and Q4 are turned off, and the switching elements Q2 and Q3 are turned on. This causes a current to flow as shown by the arrows. That is, the current flows through the switching element Q3, the first inductor L1, the first coil 122, and the switching element Q4.

[0038] 6 shows the circuit to the right of the second coil 124 of the transformer 104 in FIG. 2. Referring to FIG. 6, in the third mode m3 of the transformer 104, the switching elements Q5 and Q8 are turned on, and the switching elements Q6 and Q7 are turned off. This causes a current to flow as shown by the arrows. That is, the current flows through the switching element Q8, the second inductor L2, the second coil 124, and the switching element Q5.

[0039] Similar to Fig. 6, Fig. 7 shows the circuit to the right of the second coil 124 of the transformer 104 in Fig. 2. Referring to Fig. 7, in the fourth mode m4 of the transformer 104, the switching elements Q5 and Q8 are turned off, and the switching elements Q6 and Q7 are turned on. This causes a current to flow as shown by the arrows. That is, the current flows through the switching element Q6, the second inductor L2, the second coil 124, and the switching element Q7.

[0040] Referring to FIG. 8 , a half cycle (T / 2) is divided into four periods depending on the on / off states of switching elements Q1 to Q8. In the first period, the first bridge circuit 102 operates in the first mode m1, and the second bridge circuit 106 operates in the fourth mode m4. In the second period following the first period, the first bridge circuit 102 operates in the first mode m1, and the second bridge circuit 106 operates in the third mode m3. In the third period following the second period, the first bridge circuit 102 operates in the second mode m2, and the second bridge circuit 106 operates in the third mode m3. In the fourth period following the third period, the first bridge circuit 102 operates in the second mode m2, and the second bridge circuit 106 operates in the fourth mode m4. As a result, the output voltage of the first bridge circuit 102 (i.e., AC voltage V1), the input voltage of the second bridge circuit 106 (i.e., AC voltage V2), the AC voltage V3 of the first inductor L1, and the current I1 flowing through the first inductor L1 change as shown in Fig. 8. At this time, whether the switching operation of the switching elements is soft switching or hard switching is determined by the positive or negative state of the current I1 (i.e., the current values ​​at points A, B, C, and D) when switching elements Q1 to Q8 are switched. The current I1 is considered positive when it flows in the direction shown by the arrow in Fig. 2 (i.e., to the right), and negative when it flows in the opposite direction (i.e., to the left).

[0041] (Soft Switching) When the first bridge circuit 102 switches from the first mode m1 (see, for example, FIG. 4 ) to the second mode m2 (see, for example, FIG. 5 ), the state shown in FIG. 9 is reached before the first mode m2 is entered. Referring to FIG. 9 , the switching elements Q1 and Q4, which were on in the first mode m1, are turned off. The switching elements Q2 and Q3 are maintained off. At this time, the direction of the current flowing through the first inductor L1 is maintained, so the current flows as indicated by the arrows. That is, the current flows through the parasitic diode of the switching element Q2, the first inductor L1, the first coil 122, and the parasitic diode of the switching element Q3. In this state, the first bridge circuit 102 operates in the second mode m2, and the switching elements Q2 and Q3 are turned on. Therefore, the switching that turns on the switching elements Q2 and Q3 is soft switching.

[0042] (Hard Switching) When the first bridge circuit 102 switches from the first mode m1 to the second mode m2, the first mode m1 may be in a state such as that shown in FIG. 10 . Referring to FIG. 10 , in the first mode m1, the current I1 flows in the opposite direction to that shown in FIG. 4 (i.e., the current I1 is negative). Before switching from the first mode m1 shown in FIG. 10 to the second mode m2, the state shown in FIG. 11 , for example, is reached. Referring to FIG. 11 , the switching elements Q1 and Q4, which were on in the first mode m1, are turned off. The switching elements Q2 and Q3 are maintained off. At this time, the direction of the current flowing through the first inductor L1 is maintained, so the current flows as shown by the arrows. That is, the current flows through the parasitic diode of the switching element Q4, the first inductor L1, the first coil 122, and the parasitic diode of the switching element Q1, but no current flows through the parasitic diodes of the switching elements Q2 and Q3. In this state, the first bridge circuit 102 operates in the second mode m2, so that the switching elements Q2 and Q3 are turned on. Therefore, the switching that turns on the switching elements Q2 and Q3 is hard switching.

[0043] (Soft Switching Conditions) From the above, at the timing of switching from the first mode m1 to the second mode m2, if the current I1 is positive, soft switching occurs, and if the current I1 is negative, hard switching occurs. Similarly, at the timing of switching from the second mode m2 to the first mode m1, if the current I1 is negative, soft switching occurs, and if the current I1 is positive, hard switching occurs. Similarly, for switching between the third mode m3 and the fourth mode m4, soft switching or hard switching occurs depending on whether the current I1 is positive or negative. At the timing of switching from the third mode m3 to the fourth mode m4, if the current I1 is negative, soft switching occurs, and if the current I1 is positive, hard switching occurs. At the timing of switching from the fourth mode m4 to the third mode m3, if the current I1 is positive, soft switching occurs, and if the current I1 is negative, hard switching occurs.

[0044] That is, the soft-switching condition when the first bridge circuit 102 switches from the first mode m1 to the second mode m2 is that the current I1 is positive. The soft-switching condition when the first bridge circuit 102 switches from the second mode m2 to the first mode m1 is that the current I1 is negative. The soft-switching condition when the second bridge circuit 106 switches from the third mode m3 to the fourth mode m4 is that the current I1 is negative. The soft-switching condition when the second bridge circuit 106 switches from the fourth mode m4 to the third mode m3 is that the current I1 is positive.

[0045] The current I1 shown in FIG. 8 satisfies the soft switching condition at all timings when the first bridge circuit 102 and the second bridge circuit 106 switch modes. Therefore, in this case, phase difference control with reduced switching loss is achieved. On the other hand, when the output power of the power conversion device 100 is reduced, the phase difference Tp is controlled to be smaller, as shown in FIG. 12, for example. As a result, referring to FIG. 12, points A, B, C, and D shown in FIG. 8 change to points A', B', C', and D'. That is, the current I1 at point B' becomes negative, and the current I1 at point D' becomes positive. Point B' is the timing when the second bridge circuit 106 switches from the fourth mode m4 to the third mode m3. Because the current I1 is negative, the soft switching condition (i.e., the current I1 is positive) when switching from the fourth mode m4 to the third mode m3 is not satisfied, resulting in hard switching. Furthermore, point D' is the timing at which the second bridge circuit 106 switches from the third mode m3 to the fourth mode m4, and since the current I1 is positive, the soft switching condition (i.e., the current I1 is negative) when switching from the third mode m3 to the fourth mode m4 is not satisfied, and hard switching occurs.

[0046] (Control Method) As described above, when the current value I1 changes as shown in Fig. 12, for example, that is, when hard switching occurs due to phase difference control, the power conversion device 100 reduces switching loss by performing burst control instead of phase difference control. That is, the control unit 110 of the power conversion device 100 switches between phase difference control and burst control as shown in Fig. 13. The processing of Fig. 13 is executed by the CPU 112 of the control unit 110.

[0047] In step 300, the CPU 112 reads the target value of output power and the initial value of phase difference control from the memory 114, and starts phase difference control. Thereafter, control proceeds to step 302. For example, the target value of output power is received by the I / F unit 116 as an instruction from outside the control unit 110, as described above, and is stored in the memory 114. The initial value of phase difference control includes the period T and the initial value of phase difference Tp. Note that since the output power can be calculated using the following equation, the initial value of phase difference Tp may be calculated from the target value of output power.

[0048]

[0049] In the formula, P is the output power of the power conversion device 100, L is the total value of the inductance of the first inductor L1 and the second inductor L2 of the transformer 104, and n1 and n2 are the numbers of turns of the first coil 122 and the second coil 124 of the transformer 104, respectively. E1 and E2 are the input voltage (i.e., DC voltage E1) and the output voltage (i.e., DC voltage E2) of the power conversion device 100 described above, respectively.

[0050] In step 302, CPU 112 determines whether the output power is equal to the target value. Specifically, CPU 112 acquires the output power (hereinafter referred to as "measured power") via I / F unit 116 using a measuring device provided in output unit 132, compares the measured power with the target value, and determines whether the difference between the two (hereinafter referred to as "power difference") is within a predetermined range. For example, the absolute value of the value obtained by subtracting the measured power from the target value is taken as the power difference (i.e., power difference = |target value - measured power|). If it is determined that the power difference is within the predetermined range, control proceeds to step 314. If not, control proceeds to step 304.

[0051] In step 304, the CPU 112 determines whether the current phase difference Tp is equal to or less than the threshold value Tpth. If it is determined that Tp≦Tpth, control proceeds to step 306. Otherwise (i.e., Tp>Tpth), control proceeds to step 308.

[0052] In step 306, the CPU 112 stops the phase difference control and starts burst control. The phase difference Tp at this time is used as the phase difference for switching control in burst control. Note that a threshold value Tpth may be used as the phase difference Tp for burst control.

[0053] In burst control, each switching element is operated intermittently. In phase difference control, as described above, switching elements Q1 to Q8 are continuously switched at a constant period T. At this time, AC voltages V1 and V2 are, for example, as shown in FIG. 14 , which represents the maximum output (i.e., 100% output power) in burst control. The voltage waveforms in FIG. 14 correspond to the voltage waveforms shown in FIG. 8 and are shown over a longer period than those in FIG. 8 . In burst control, to adjust the output power, a period Ts is provided during which the switching operation is stopped, as shown in FIG. 15 , from the state shown in FIG. 14 . In FIG. 15 , the period during which the stop period Ts is repeated is represented by Ta. Also, in FIG. 15 , the number of oscillations of the voltage waveform during the period Ta (hereinafter referred to as the reference burst number) N is shown in parentheses. Similarly, the period during which the voltage is output is represented by Tb, and the corresponding number of oscillations of the voltage waveform (hereinafter referred to as the burst number) Nb is shown in parentheses. At this time, the output power is (Nb / N)×100(%) of the output power in the state shown in Fig. 14 (i.e., 100% output power). Note that the switching of the switching elements is controlled by the phase difference Tp, which realizes soft switching, and therefore switching loss is reduced.

[0054] On the other hand, if the determination result in step 304 is NO, in step 308, the CPU 112 changes the phase difference Tp from its current value. For example, if the (target value - measured power) calculated in step 302 is positive, the CPU 112 increases the phase difference Tp from its current value. For example, the CPU 112 sets the new phase difference Tp to a value obtained by adding a predetermined value ΔTp to the current phase difference Tp (i.e., Tp + ΔTp). If the (target value - measured power) is negative, the CPU 112 decreases the phase difference Tp from its current value. For example, the CPU 112 sets the new phase difference Tp to a value obtained by subtracting a predetermined value ΔTp from the current phase difference Tp (i.e., Tp - ΔTp). As a result, the CPU 112 controls the switching of switching elements Q1 to Q8 using the new phase difference Tp. Thereafter, control returns to step 302. As a result, while Tp > Tpth, the switching of switching elements Q1 to Q8 is controlled by phase difference control.

[0055] After starting burst control, in step 310, the CPU 112 determines whether the output power is equal to the target value, similar to step 302. If it is determined that the power difference between the measured power and the target value (i.e., |target value−measured power|) is within a predetermined range, control proceeds to step 314. If not, control proceeds to step 312.

[0056] In step 312, the CPU 112 changes the burst count Nb. For example, if the (target value - measured power) calculated in step 310 is positive, the CPU 112 increases the burst count Nb from its current value. For example, the CPU 112 adds a predetermined value ΔNb to the current burst count Nb and sets the resulting value (i.e., Nb + ΔNb) as the new burst count Nb. If the (target value - measured power) is negative, the CPU 112 decreases the burst count Nb from its current value. For example, the CPU 112 subtracts a predetermined value ΔNb from the current burst count Nb and sets the resulting value (i.e., Nb - ΔNb) as the new burst count Nb. As a result, the CPU 112 uses the new burst count Nb to intermittently operate switching elements Q1 to Q8. Then, control returns to step 310.

[0057] In step 314, the CPU 112 determines whether the target value of the output power has been changed. Specifically, the CPU 112 determines whether a target value different from the current target value has been received as a new command from the outside. If it is determined that the target value has been changed (i.e., a new target value has been received), control returns to step 300, and the above-described processing is executed using the new target value. If it is determined that the target value has not been changed (i.e., a new target value has not been received), control proceeds to step 316.

[0058] In step 316, the CPU 112 determines whether or not to terminate the control of the switching elements. If it is determined that the control should be terminated, the CPU 112 terminates the program. If not, the control returns to step 314. The termination instruction is issued, for example, by stopping the power supply for operating the power conversion device 100.

[0059] As a result, as shown in Fig. 16, the control unit 110 can switch between phase difference control and burst control to operate the power conversion device 100. In Fig. 16, the horizontal axis represents output power, the solid line represents changes in phase difference Tp (see the vertical axis on the right), and the dashed line represents changes in burst rate (see the vertical axis on the left). The vertical axis on the right represents the phase difference threshold value Tpth used for the determination in step 304 of Fig. 13. The threshold value Tpth is the lower limit of the phase difference for which phase difference control is executed. In other words, the phase difference is not set to a value smaller than Tpth.

[0060] 16 shows the maximum and minimum values ​​Pmax and Pmin of the output power, and the output power threshold Pth corresponding to the phase difference threshold Tpth. In burst control, the output power threshold Pth is set to 100% output power in burst control, and the burst rate (i.e., (Nb / N)×100(%)) is reduced (i.e., the number of bursts Nb is reduced).

[0061] If the target value of output power decreases to below threshold value Pth during phase difference control, i.e., if phase difference Tp≦Tpth, phase difference control is stopped and burst control is executed. In burst control, the number of bursts Nb (i.e., burst ratio) is changed to achieve the target output power. In burst control, a phase difference that achieves soft switching is maintained and switching loss is reduced. Furthermore, if the target value of output power increases to above threshold value Pth during burst control, i.e., if burst number Nb>N, burst control is stopped and phase difference control is executed. In phase difference control, the phase difference Tp is changed to achieve the target output power. In phase difference control, soft switching is achieved and switching loss is reduced.

[0062] In this way, even when the output power is small, each switching element constituting the power conversion device 100 can be made to perform soft switching operation, thereby reducing switching loss due to hard switching and suppressing overheating and failure.

[0063] As described above, the phase difference Tp is compared with the predetermined threshold value Tpth, and if the phase difference Tp is greater than the predetermined threshold value Tph, phase difference control is performed, and if the phase difference Tp is equal to or less than the threshold value Tpth, burst control is performed. This makes it possible to easily switch between phase difference control and burst control.

[0064] As described above, in phase difference control, when the phase difference Tp becomes small, hard switching occurs in the switching operation of the switching elements. If the phase difference Tp at the boundary where soft switching changes to hard switching is defined as the lower limit value Tpmin of the phase difference, it is preferable that the phase difference threshold value Tpth be set to be equal to or greater than the lower limit value Tpmin of the phase difference (i.e., Tpth≧Tpmin). Note that the lower limit value Tpmin of the phase difference may be any phase difference at which the operation of at least some of the switching elements Q1 to Q8 changes from soft switching to hard switching. This ensures that each switching element performs soft switching operation reliably.

[0065] As described above, in burst control, the burst period (i.e., period Tb) during which the AC voltage V1 is supplied is changed based on a predetermined period (i.e., cycle Ta), thereby enabling a desired output power to be output.

[0066] As described above, where N and Nb are natural numbers and N is greater than Nb, the predetermined period (i.e., the period Ta) is equal to N times the period T of the AC voltage V1, the AC voltage V1 is continuously input to the first bridge circuit 102 during the predetermined period, and the burst period (i.e., the period Tb) is equal to Nb times the period T. This facilitates burst control.

[0067] As described above, in burst control, the period T of the AC voltage V1 is constant, i.e., the frequency is constant, which makes it easier to perform burst control.

[0068] FIG. 17 shows an example of a simulation result when the switching elements constituting the power conversion device 100 are controlled by phase difference control. In FIG. 17 , the horizontal axis represents the output voltage E2, and the vertical axis represents the output power. The output voltage E2 varies from a minimum value E2min to a maximum value E2max. The output power varies from a minimum value Pmin to a maximum value Pmax. In the dotted region, the switching operation of switching elements Q1 to Q8 is soft switching. In the shaded region, the switching operation of switching elements Q1 to Q8 is hard switching. As such, when the output power of the power conversion device 100 decreases, i.e., when the phase difference Tp decreases, the switching operation of switching elements Q1 to Q8 becomes hard switching, resulting in switching losses. Note that, although the hard switching region is divided into two regions in FIG. 17 , this is not limiting. The shape of the hard switching region varies depending on the simulation conditions.

[0069] FIG. 18 shows the results of a simulation in which the switching elements constituting the power conversion apparatus 100 are controlled by switching between phase difference control and burst control as described above. The meanings of the vertical and horizontal axes in FIG. 18 and the displayed range are the same as those in FIG. 17 . In FIG. 18 , the dashed line sloping upward to the right represents a state in which the phase difference Tp is a constant value (i.e., when the phase difference Tp is a constant value, the above equation 1 becomes an equation in which P is proportional to the DC voltage E2). In the region above the dashed line, phase difference control is performed, and in the region below the dashed line, burst control is performed. Therefore, as shown in FIG. 18 , soft switching is achieved in the switching control of switching elements Q1 to Q8 in all regions. This reduces switching loss.

[0070] In burst control, hard switching occurs immediately after the start of a burst period. For example, as shown in FIG. 19 , hard switching occurs for a short period of time immediately after switching control is started after switching control has been stopped (see region 200 in FIG. 19 ). When switching control is subsequently repeated, soft switching occurs (see region 202 in FIG. 19 ). Therefore, although the repetition period Ta of the period Ts during which voltage output is stopped, i.e., the reference burst count N, is arbitrary, it is preferable to use a larger value so that switching control continues until soft switching occurs. Using a large value for the reference burst count N can prevent hard switching from being maintained and reduce switching loss. However, since increasing N too much can have disadvantages such as increasing the size of the output filter, it is preferable that N be, for example, 100 or less (N≦100).

[0071] Second Embodiment The power conversion device 100 shown in FIG. 2 can be used in a charging device. Referring to FIG. 20 , a charging device 220 according to a second embodiment of the present disclosure includes a DC / DC converter 222, an AC / DC converter 224, and a control unit 226. The charging device 220 is fixedly installed, for example, in a home or the like. The DC / DC converter 222 is configured using the power conversion device 100 shown in FIG. 2. The AC / DC converter 224 converts AC power supplied from an AC power source into DC power (specifically, DC voltage E2 shown in FIG. 2 ) and outputs the DC power. The voltage input to the AC / DC converter 224 is, for example, an AC voltage (e.g., 110 V) supplied from a commercial power source. The AC / DC converter 224 can be realized, for example, by a bridge circuit similar to the second bridge circuit 106 shown in FIG. 2. The output section of the AC / DC converter 224 and the input section of the DC / DC converter 222 are connected via a capacitor 228. 2, and controls the on / off of switching elements included in the DC / DC converter 222 and the AC / DC converter 224. The automobile 230 includes a storage battery and is, for example, a PHEV or EV.

[0072] Control unit 110 controls the switching elements of DC / DC converter 222 and AC / DC converter 224 so that DC / DC converter 222 outputs a DC voltage (i.e., DC voltage E2) appropriate for charging the storage battery included in vehicle 230. This allows charging device 220 to charge the storage battery of vehicle 230. The charging power supplied from charging device 220 is changed depending on the capacity (e.g., SOC (State of Charge)) of the storage battery to which charging power is supplied. That is, as the storage battery approaches full charge and the SOC approaches 100%, the charging power decreases. Charging device 220 controls DC / DC converter 222 as shown in FIG. 13 . This allows soft switching of each switching element constituting DC / DC converter 222 even when the charging power is low, reducing losses due to hard switching and preventing overheating and breakdowns.

[0073] Although the above description has been given of a case where a storage battery included in a vehicle is charged, the present invention is not limited to this. Charging device 220 may also be suitable for charging a storage battery included in an external device other than a vehicle.

[0074] As described above, charging device 220 includes AC / DC converter 224, and the DC voltage output from AC / DC converter 224 is input to DC / DC converter 222. This makes it possible to realize a charging device that operates using AC power supplied from an external source (for example, commercial AC power).

[0075] As described above, the charging device 220 is fixedly installed and supplies power to a storage battery mounted on an external device, thereby realizing a charging device that can charge a storage battery mounted on an external device (e.g., a vehicle) using commercial AC power.

[0076] The charging power supplied from the charging device 220 is changed according to the capacity of the storage battery to which the charging power is supplied, thereby enabling the storage battery to be appropriately charged.

[0077] In the above description, the duty of the control signals for switching elements Q1 to Q8 is 50%, but this is not limiting. As long as the series-connected switching elements (e.g., switching elements Q1 and Q2) are not turned on simultaneously but are turned on alternately, the duty may be a value other than 50% (e.g., 48%).

[0078] In the above description, the switching elements constituting the power conversion device 100 are N-type FETs (see FIG. 2 ), but this is not limiting. A full bridge circuit constituting the power conversion device may be configured using P-type FETs.

[0079] Although the present disclosure has been described above by explaining the embodiments, the above-described embodiments are merely examples, and the present disclosure is not limited to only the above-described embodiments. The scope of the present disclosure is defined by the claims in the scope of the claims, taking into consideration the description of the detailed description of the invention, and includes all modifications within the meaning and scope equivalent to the wordings described therein.

[0080] REFERENCE SIGNS LIST 100 Power conversion device 102 First bridge circuit 104 Transformer 106 Second bridge circuit 110, 226 Control unit 112 CPU 114 Memory 116 I / F unit 120 Core 122 First coil 124 Second coil 130 Input unit 132 Output unit 200, 202 Area 220 Charging device 222 DC / DC converter 224 AC / DC converter 228, C1, C2, C3 Capacitor 230 Automobile 300, 302, 304, 306, 308, 310, 312, 314, 316 Step A, A', B, B', C, C', D, D' Points E1, E2 DC voltage Emax, Pmax Maximum value Emin, Pmin Minimum value I0, I1 Current L1 First inductor L2 Second inductor L3 Choke coil m1, m2, m3, m4 Mode N1, N2, N3, N4, N5, N6, N7, N8 Node N Reference burst count Nb Burst count Pth, Tpth Threshold Q1, Q2, Q3, Q4, Q5, Q6, Q7, Q8 Switching element T, Ta Cycle Tp Phase difference Ts, Tb Period t Time V0 Voltage V1, V2, V3 AC voltage

Claims

1. a transformer including a first coil and a second coil; a first bridge circuit including a plurality of first switching elements and connected to the first coil; a second bridge circuit including a plurality of second switching elements and connected to the second coil; the first bridge circuit generates a first AC voltage and supplies it to the first coil; The second bridge circuit converts and outputs a second AC voltage generated in the second coil, A power conversion device that switches between phase difference control that adjusts a phase difference between the first AC voltage and the second AC voltage, and burst control that keeps the phase difference constant, fixes one control cycle, and adjusts a period during which the first AC voltage is supplied, depending on a transmission power.

2. If the transmission power is greater than a predetermined threshold, the phase difference control is performed; The power conversion device according to claim 1 , wherein the burst control is performed when the transmission power is equal to or less than the threshold value.

3. the threshold value is greater than a value corresponding to a lower limit of the phase difference, The power conversion device according to claim 2 , wherein the lower limit value is a value of the phase difference when an operation of at least a portion of the plurality of first switching elements and the plurality of second switching elements changes from soft switching to hard switching.

4. The power conversion device according to claim 1 , wherein in the burst control, a burst period, which is a period during which the first AC voltage is supplied, is changed with reference to a predetermined period.

5. Suppose N and Nb are natural numbers, and N is greater than Nb, the predetermined period is equal to N times the period of the first AC voltage, During the burst period, the first AC voltage is continuously input to the first bridge circuit, The power converter of claim 4 , wherein the burst period is equal to the Nb times the period.

6. The power conversion device according to claim 1 , wherein in the burst control, a frequency of the first AC voltage is constant.

7. The power conversion device according to any one of claims 1 to 3, The charging device outputs the DC voltage converted by the second bridge circuit as charging power.

8. further comprising an AC / DC converter; The charging device according to claim 7 , wherein the DC voltage output from the AC / DC converter is input to the first bridge circuit.

9. The charging device is Fixed installation, The charging device according to claim 7, which supplies power to a storage battery mounted in an external device.

10. The charging device according to claim 7 , wherein the charging power is changed depending on a capacity of a storage battery to which the charging power is supplied.

11. A control method for a power conversion device including a transformer including a first coil and a second coil, a first bridge circuit including a plurality of first switching elements and connected to the first coil, and a second bridge circuit including a plurality of second switching elements and connected to the second coil, causing the first bridge circuit to generate a first AC voltage and supply it to the first coil; causing the second bridge circuit to convert the second AC voltage generated in the second coil and output the converted voltage; a phase difference control step of adjusting a phase difference between the first AC voltage and the second AC voltage; a burst control step of adjusting a period during which the first AC voltage is supplied by fixing one control cycle while keeping the phase difference constant; a step of switching between the phase difference control step and the burst control step in response to a transmission power.