DC / DC converter

Abstract

To provide a DC / DC converter that reduces switching noise and is compatible with a relatively wide voltage range.SOLUTION: A DC / DC converter 10 includes a first bus capacitor Cb, a first inductor circuit 1, a first full bridge circuit 2, a first resonant circuit 3, a transformer circuit 4, a second full bridge circuit 5, and a control unit 6. The control unit 6 performs on / off control of switching elements Q1 to Q4 of the first full bridge circuit 2, and as on / off control during forward power transmission, performs frequency modulation control to modulate the drive frequency of the switching elements Q1 to Q4, and also performs duty control to modulate the duty of the switching elements Q1 to Q4 using the drive frequency as a control unit.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a DC / DC converter.

Background Art

[0002] In recent years, in order to achieve carbon neutrality, development has been progressing on charging devices for charging electric vehicle batteries, V2H (Vehicle to Home) systems that supply the battery power of electric vehicles to household loads, distributed power systems that combine solar power generation devices and storage batteries, etc. There is a demand for miniaturization, cost reduction, and high efficiency of the power supplies included in these devices and systems. Therefore, as a DC / DC converter included in a power supply, an LLC-type DC / DC converter, which is a current resonance type DC / DC converter with a small number of components and capable of high efficiency, and a CLLC-type DC / DC converter in which its circuit topology is made symmetric and bidirectionalized are adopted.

[0003] On the other hand, in the CHAdeMO standard, which is an electric vehicle charging standard, a wide battery voltage range from 150 [V] to 450 [V] is required. For this reason, in electric vehicle charging devices and V2H systems, it is necessary not only to step down but also to step up the DC voltage output by the DC / DC converter during charging according to the battery voltage of the electric vehicle. In the case of a V2H system, it is further necessary to step up and down the battery voltage according to the DC voltage inside the power supply during regeneration. Therefore, compared with a charging device that performs unidirectional power transmission, the requirements for step-up / step-down operation are more severe.

[0004] In addition, in a LLC (or CLLC) type DC / DC converter, boost-buck operation is performed by frequency modulation control. However, in frequency modulation control, since the drive frequency is modulated over a wide frequency range according to the gain characteristic curve determined by the resonance constant of the resonance circuit, there is a drawback that the outputtable voltage range becomes narrow. For this reason, in a conventional LLC type DC / DC converter, it has been common to fix the drive frequency to the resonance frequency and operate it without control, and to control the input voltage to the LLC type DC / DC converter with a boost-buck chopper circuit provided in the previous stage. However, in this method, in addition to the problem that miniaturization, cost reduction, and high efficiency of the power supply are hindered by the boost-buck chopper circuit, there is also a problem that noise is generated due to hard switching.

[0005] To address these problems, there is a method aiming for miniaturization, cost reduction, and high efficiency of the power supply by temporarily short-circuiting the secondary-side rectifier circuit of the LLC type DC / DC converter to perform a boost (step-up) operation, thereby eliminating the need for the boost-buck chopper circuit provided in the previous stage. However, in this method, since a large resonance current is interrupted in the secondary-side rectifier circuit during the boost operation, it becomes hard switching and a large switching noise is generated. For this reason, it is difficult to put into practical use, and the primary-side drive circuit cannot be operated at high speed (cannot be made high-frequency) for noise countermeasures, resulting in a problem in miniaturization of the power supply.

[0006] Patent Document 1 describes a method of generating a boosted voltage from an input voltage with a boost inductor circuit having an interleaved configuration, and further transforming the boosted voltage with an LLC circuit to generate an output voltage. In this method, since two switching elements need to be added to the LLC circuit (a total of six switching elements are required), miniaturization, cost reduction, and high efficiency of the power supply are hindered, similar to a LLC type DC / DC converter provided with a boost-buck chopper circuit in the previous stage. Also, in this method, since the buck operation is performed by drive control of the LLC circuit, there is a problem that the output cannot be reduced to zero. In addition to the drive control of the LLC circuit, control of the boosted voltage by the boost inductor circuit is also required.

[0007] Non-Patent Document 1 describes a method in which a boost coil (inductor) with an interleaved configuration is provided in the input section, the boost coil is connected to an LLC full-bridge circuit, the drive frequency of the LLC full-bridge circuit is fixed at the resonance frequency, and the duty ratio is made smaller than 50% to boost the voltage and larger than 50% to step down the voltage. In this method, although there is an issue that a boost coil with an interleaved configuration is required, on the other hand, it has low input ripple, and since the buck-boost circuit can be used in common with the LLC full-bridge circuit, the number of switching elements can be reduced, and switching noise can also be reduced by the soft switching of LLC resonance. However, in the case of this method, there is an issue that the input ripple noise increases if the duty ratio is modulated too much from 50%. Therefore, the gain obtained is about 1.35 times at a duty ratio of 30% on the boost side when the duty ratio at 50% is set to 1. For this reason, there is an issue that it is not sufficient to handle a wide voltage range such as in the case of the battery voltage of an electric vehicle, which requires a maximum of about 2.5 times (output: bus voltage 380 [V] / input: battery minimum voltage 150 [V]). Also, regarding bucking, there is an issue that the output cannot be reduced to zero.

[0008] Patent Document 2 describes a control method for a boost-type full-bridge inverter. The control method described in Patent Document 2 applies a DC voltage to two boost inductors and performs a boost operation by PWM controlling the lower arms of each leg of the full-bridge circuit. In this control method, since the full-bridge circuit is used as a chopper circuit, it becomes hard switching, and there is a possibility of generating switching noise. Also, Patent Document 2 states that "it can also be applied to a series resonance type circuit using frequency control for output control, and in this case, it is possible to use a frequency modulation function and a PWM function in combination in the input voltage limiting circuit." However, even if control is performed using the disclosed control method, it also becomes hard switching, so it is not possible to reduce switching noise by soft switching.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Non-Patent Document

[0010]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present invention has been made in view of the above circumstances, and an object thereof is to provide a DC / DC converter capable of reducing switching noise and corresponding to a relatively wide voltage range.

Means for Solving the Problems

[0012] In order to solve the above problems, the DC / DC converter according to the present invention includes a main circuit section and a control section, wherein the main circuit section has a first terminal, a second terminal, and a third terminal, and includes a first boost inductor and a second boost inductor, and a first inductor circuit in which one end of the first boost inductor and one end of the second boost inductor are connected to the second terminal, A first full-bridge circuit including a first leg and a second leg connected in parallel, each leg including an upper arm and a lower arm including a switching element, a first connection point of the upper and lower arms of the first leg being connected to the other end of the first boost inductor, a second connection point of the upper and lower arms of the second leg being connected to the other end of the second boost inductor, a high-voltage side connection point of the first leg and the second leg being connected to the first end, and a low-voltage side connection point of the first leg and the second leg being connected to the third end; A first resonant circuit including a first resonant inductor and a first resonant capacitor, an input side being connected to the first connection point and the second connection point; An isolation transformer including a primary side winding and a secondary side winding, both ends of the primary side winding being connected to an output side of the first resonant circuit; A first bus capacitor connected between the first end and the third end; A second full-bridge circuit including a third leg and a fourth leg connected in parallel, each leg including an upper arm and a lower arm including a switching element and / or a diode, a third connection point of the upper and lower arms of the third leg being connected to one end of the secondary side winding, and a fourth connection point of the upper and lower arms of the fourth leg being connected to the other end of the secondary side winding; A fourth end connected to a high-voltage side connection point of the third leg and the fourth leg; A fifth end connected to a low-voltage side connection point of the third leg and the fourth leg; A DC / DC converter comprising: The control unit: Performs on / off control of at least the switching element of the first full-bridge circuit; When operating the first full-bridge circuit as a drive circuit and the second full-bridge circuit as a rectifier circuit during forward power transmission, As the on / off control, frequency modulation control for modulating the drive frequency of the switching element of the first full-bridge circuit is performed, and duty control for modulating the duty of the switching element of the first full-bridge circuit using the drive frequency as a control unit is performed.

[0013] In the DC / DC converter, During the on-off control, the control unit Causes the main circuit section to perform a boosting operation by making the drive frequency smaller than the 50% frequency corresponding to a duty ratio of 50%, Causes the main circuit section to perform a bucking operation by making the drive frequency larger than the 50% frequency, In the range from the first drive frequency corresponding to a first duty ratio smaller than 50% to the second drive frequency corresponding to a second duty ratio larger than 50%, the main circuit section can be configured to perform a buck-boost operation.

[0014] In the DC / DC converter, During the on-off control, the control unit When the drive frequency by the frequency modulation control is larger than the second drive frequency, the duty ratio control is performed by changing the increase rate of the duty ratio with respect to the change in the drive frequency so that the duty ratio becomes the maximum value when the drive frequency is the maximum drive frequency which is the substantial maximum value.

[0015] In the DC / DC converter, During the on-off control, the control unit When the drive frequency by the frequency modulation control becomes a value equal to or less than the resonance frequency of the first resonance circuit, the drive frequency is fixed to the resonance frequency and the duty ratio control is performed.

[0016] In the DC / DC converter, During the on-off control, the control unit When the drive frequency by the frequency modulation control becomes a value larger than the resonance frequency of the first resonance circuit, the duty ratio is fixed at 50%, When the drive frequency by the frequency modulation control becomes a value equal to or higher than the phase shift start frequency at which the drive frequency is higher than the resonance frequency, the phase shift amount between the first leg and the second leg can be configured to perform phase shift control that controls the drive frequency as a control unit.

[0017] In the DC / DC converter, During the on / off control, the control unit In accordance with the turn-on timing of the switching elements of the first leg and the second leg, the switching elements of the third leg and the fourth leg are turned on, so that the second full-bridge circuit can be configured to perform synchronous rectification operation.

[0018] In the DC / DC converter, A second inductor circuit including a third boost inductor and a fourth boost inductor, one end of the third boost inductor being connected to the third connection point of the second full-bridge circuit, and one end of the fourth boost inductor being connected to the fourth connection point of the second full-bridge circuit, A sixth terminal connected to the other ends of the third boost inductor and the fourth boost inductor, And a second bus capacitor connected between the fourth terminal and the fifth terminal, The control unit Performs on / off control of the switching elements of the second full-bridge circuit, When operating the second full-bridge circuit as the drive circuit and the first full-bridge circuit as the rectifier circuit during reverse power transmission, As the on / off control, frequency modulation control for modulating the drive frequency of the switching elements of the second full-bridge circuit and duty control for modulating the duty of the switching elements of the second full-bridge circuit with the drive frequency as a control unit can be configured.

[0019] In the DC / DC converter, It can be configured to further include a second resonance circuit including a second resonance inductor and a second resonance capacitor, wherein the input side during the reverse power transmission is connected to the third connection point and the fourth connection point, and the output side during the reverse power transmission is connected to both ends of the secondary side winding.

Advantages of the Invention

[0020] According to the present invention, it is possible to provide a DC / DC converter capable of reducing switching noise and corresponding to a relatively wide voltage range.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Embodiments for Carrying Out the Invention

[0022] Hereinafter, embodiments of a DC / DC converter according to the present invention will be described with reference to the accompanying drawings.

[0023] [First Embodiment] FIG. 1 shows a DC / DC converter 10 according to the first embodiment of the present invention. The DC / DC converter 10 includes a main circuit section including terminals T1 to T5, a bus capacitor Cb, a first inductor circuit 1, a first full-bridge circuit 2, a first resonance circuit 3, a transformer circuit 4 (a high-frequency isolation transformer Tr, hereinafter abbreviated as "transformer Tr"), and a second full-bridge circuit 5, and a control section 6. Note that the terminals T1, T2, T3, T4, and T5 correspond to the "first terminal", "second terminal", "third terminal", "fourth terminal", and "fifth terminal" of the present invention, respectively. The bus capacitor Cb corresponds to the "first bus capacitor" of the present invention.

[0024] A bus capacitor Cb is connected between terminals T1 and T3. In the DC / DC converter 10, an input voltage V1 is boosted to a DC bus voltage Vb, which is applied between terminals T1 and T3. The bus voltage Vb is simultaneously applied to the first full-bridge circuit 2, converted into an output voltage V2 by the DC / DC converter 10, and supplied to a load. The bus capacitor Cb is connected to stabilize the bus voltage Vb. An output terminal of a DC voltage source (for example, a DC output terminal of an AC / DC converter or a voltage terminal of a battery), not shown, is connected between terminals T2 and T3, and a DC input voltage V1 is input. A DC output voltage V2 is output from between terminals T4 and T5. A capacitor C11 for removing input ripple is connected between terminals T2 and T3, and a capacitor C22 for removing output ripple is connected between terminals T4 and T5.

[0025] The first inductor circuit 1 includes a first boost inductor Lb1 and a second boost inductor Lb2 in an interleaved configuration. The first boost inductor Lb1 and the second boost inductor Lb2 have the same inductance value, and one end of both is connected to the terminal T2.

[0026] The first full-bridge circuit 2 has a full-bridge circuit configuration including a first leg and a second leg connected in parallel, and each leg includes an upper arm and a lower arm connected in series. The upper arm of the first leg includes a switching element Q1, the lower arm of the first leg includes a switching element Q2, the upper arm of the second leg includes a switching element Q3, and the lower arm of the second leg includes a switching element Q4. Diodes D1 to D4 are connected in parallel in the reverse direction, and capacitors C1 to C4 are connected in parallel in the current paths of the switching elements Q1 to Q4.

[0027] As the switching elements Q1 to Q4, for example, power semiconductor switching elements capable of switching at high frequencies such as IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) can be used. The same applies to the switching elements Q5 to Q8 described later.

[0028] The collector (or drain) terminals of the switching elements Q1 and Q3 (the high-voltage side connection points of the first leg and the second leg) are connected to the terminal T1, and the emitter (or source) terminals of the switching elements Q2 and Q4 (the low-voltage side connection points of the first leg and the second leg) are connected to the terminal T3. The connection point (the first connection point a) between the emitter terminal of the switching element Q1 and the collector terminal of the switching element Q2 is connected to the terminal T2, which is the high-voltage end, via the first boost inductor Lb1, and the connection point (the second connection point b) between the emitter terminal of the switching element Q3 and the collector terminal of the switching element Q4 is connected to the terminal T2, which is the high-voltage end, via the second boost inductor Lb2.

[0029] Diodes D1 to D4 operate as reflux diodes and may be built-in diodes of switching elements Q1 to Q4, external diodes, or both. The same applies to diodes D5 to D8 described later. Further, capacitors C1 to C4 are partial resonance capacitors and may be the parasitic capacitances of switching elements Q1 to Q4, external capacitors, or both. The same applies to capacitors C5 to C8 described later.

[0030] The first resonance circuit 3 includes a first resonance inductor Lr1 and a first resonance capacitor Cr1. The input side of the first resonance circuit 3 is connected to the first connection point a and the second connection point b, and the output side is connected to both ends of the primary winding of the transformer Tr. In the present embodiment, the first resonance inductor Lr1 and the first resonance capacitor Cr1 form a series resonance circuit together with the exciting inductance (not shown) of the transformer Tr.

[0031] Note that if the first resonance inductor Lr1 and the first resonance capacitor Cr1 are connected to the primary winding of the transformer Tr to form a series resonance circuit, their arrangement is arbitrary. For example, the first resonance inductor Lr1 and the first resonance capacitor Cr1 may be arranged separately on the primary side and the secondary side of the transformer Tr. Further, the first resonance inductor Lr1 may be the leakage inductance of the transformer Tr, a coil having an individual core, or both. The first resonance capacitor Cr1 may be composed of individual capacitors, the parasitic capacitance of the switching elements Q1 to Q4, or both. Although the exciting inductance of the transformer Tr is not shown as being included in the primary winding of the transformer Tr, it may be a coil having an individual core.

[0032] The transformer circuit 4 is composed of one or more transformers Tr. The primary winding of the transformer Tr is connected to the first full-bridge circuit 2 via the first resonance circuit 3. The secondary winding of the transformer Tr is connected to the second full-bridge circuit 5.

[0033] The second full-bridge circuit 5 includes a third leg and a fourth leg connected in parallel, and each leg includes an upper arm and a lower arm connected in series, which is the circuit configuration of a full bridge. The upper arm of the third leg includes a switching element Q5, the lower arm of the third leg includes a switching element Q6, the upper arm of the fourth leg includes a switching element Q7, and the lower arm of the fourth leg includes a switching element Q8. Diodes D5 to D8 are connected in parallel in the reverse direction in the current paths of the switching elements Q5 to Q8, and capacitors C5 to C8 are connected in parallel.

[0034] The collector terminals of the switching elements Q5 and Q7 (the high-voltage side connection points of the third leg and the fourth leg) are connected to the terminal T4, and the emitter terminals of the switching elements Q6 and Q8 (the low-voltage side connection points of the third leg and the fourth leg) are connected to the terminal T5. The connection point between the emitter terminal of the switching element Q5 and the collector terminal of the switching element Q6 is connected to one end of the secondary winding of the transformer Tr, and the connection point between the emitter terminal of the switching element Q7 and the collector terminal of the switching element Q8 is connected to the other end of the secondary winding of the transformer Tr.

[0035] The control unit 6 includes a processing unit (including a storage unit) that generates control signals for performing on / off control of the switching elements Q1 to Q8, and a driving unit (not shown) that turns on and off the switching elements Q1 to Q8 based on the control signals. The control unit 6 may be composed of a digital circuit such as a microprocessor or a digital signal processor, or may be composed of an analog circuit, or may be composed of a circuit combining a digital circuit and an analog circuit. The control unit 6 may further include a detection unit (not shown). The detection unit includes, for example, a detection circuit such as a current sensor and / or a voltage sensor that detects current values and / or voltage values necessary for the control of the control unit 6, and its peripheral circuits.

[0036] The control unit 6 operates the first full-bridge circuit 2 as a drive circuit and the second full-bridge circuit 5 as a rectifier circuit to cause the main circuit unit to perform forward power transmission from the primary side to the secondary side. Although details will be described later, during forward power transmission, the control unit 6 performs frequency modulation control to modulate the drive frequencies of the switching elements Q1 to Q4 of the first full-bridge circuit 2 and duty control to modulate the duty of the switching elements Q1 to Q4 with the drive frequency as a control unit, as the on-off control. Further, for the switching elements Q5 to Q8 of the second full-bridge circuit 5, in this embodiment, the switching elements Q5 to Q8 are turned on and off to perform synchronous rectification control, but the switching elements Q5 to Q8 may be turned off to perform diode rectification control.

[0037] Fig. 2 shows an example of the control timings and voltage-current waveforms of the switching elements Q1 to Q8. In Fig. 2, the drive frequency of the switching elements Q1 to Q8 is set as f (= 1 / T), and the duty of the switching elements Q1 and Q3 is set as D [%] (where D < 50%). The control unit 6 performs PWM control (duty control) and frequency modulation control in an LLC method involving a boosting operation on the first full-bridge circuit 2, and performs synchronous rectification control on the second full-bridge circuit 5.

[0038] Here, the duty D is the ratio of the on-time to the period T. The voltage Vt is the voltage between the first connection point a and the second connection point b (tank voltage). The current Ib1 is the current flowing through the first boosting inductor Lb1, and the current Ib2 is the current flowing through the second boosting inductor Lb2. The current Ib is the total current of the current Ib1 and the current Ib2, and its average value is equal to the input current Iin flowing from the terminal T2 to the first inductor circuit 1. The current IL is the current flowing through the first resonance circuit 3, and is the total current obtained by adding the resonance current ILr and the exciting current ILm flowing through the exciting inductance.

[0039] The control unit 6 drives the first full-bridge circuit 2 with a duty ratio D. That is, the control unit 6 turns on the switching element Q1 with a duty ratio D, and turns on the switching element Q2 with a duty ratio (100 - D) in a reverse operation with a dead time period provided. Similarly, the control unit 6 turns on the switching element Q3 with a duty ratio D, and turns on the switching element Q4 with a duty ratio (100 - D) in a reverse operation with a dead time period provided. Also, the control unit 6 shifts the on / off operation of the second leg with respect to the first leg by a phase of 1 / 2 cycle (T / 2). In other words, centering on the time t5 of 1 / 2 cycle, the first leg and the second leg are alternately turned on and off.

[0040] The current Ib1 starts to decrease at the time t0 when the switching element Q1 is turned on, and starts to increase at the time t2 when the switching element Q2 is turned on. On the other hand, the current Ib2 starts to decrease at the time t5 when the switching element Q3 is turned on, and starts to increase at the time t7 when the switching element Q4 is turned on.

[0041] Figure 2 shows the case where the duty ratio D < 50%. When the duty ratio D is 50%, the on (off) timings of the switching elements Q1 and Q4 are the same, and the on (off) timings of the switching elements Q3 and Q2 are the same. Also, the timing at which the current Ib1 starts to decrease (the on timing of the switching element Q1) and the timing at which the current Ib2 starts to increase (the on timing of the switching element Q4) are the same. Similarly, the timing at which the current Ib1 starts to increase (the on timing of the switching element Q2) and the timing at which the current Ib2 starts to decrease (the on timing of the switching element Q3) are the same. Since the inductance values of the first boost inductor Lb1 and the second boost inductor Lb2 are equal, the magnitudes of the current Ib1 and the current Ib2 are equal, and the total current Ib becomes zero. That is, when the duty ratio D is 50%, since neither addition nor subtraction is performed on the resonance current ILr, the buck-boost operation is not performed in the main circuit section.

[0042] However, in the case of FIG. 2 where the duty D is less than 50%, the on-period of the switching element Q1 becomes shorter than the on-period of the switching element Q4, and the timing at which the current Ib2 starts to rise becomes earlier than the timing at which the current Ib1 starts to fall. For this reason, when the switching element Q1 is turned on and the resonance current ILr starts to flow, a positive current flows as the current Ib. Also, the on-period of the switching element Q3 becomes shorter than the on-period of the switching element Q2, and the timing at which the current Ib1 starts to rise becomes earlier than the timing at which the current Ib2 starts to fall. For this reason, when the switching element Q3 is turned on and the resonance current ILr starts to flow, a positive current flows as the current Ib. As a result, whether the tank voltage Vt is positive or negative, the resonance current ILr increases, and a boosting operation is performed in the main circuit section.

[0043] On the other hand, when the duty D is greater than 50%, contrary to the case where the duty D is less than 50%, the on-period of the switching element Q1 becomes longer than the on-period of the switching element Q4, and the timing at which the current Ib2 starts to rise becomes later than the timing at which the current Ib1 starts to fall. For this reason, when the switching element Q1 is turned on and the resonance current ILr starts to flow, a negative current flows as the current Ib. Also, the on-period of the switching element Q3 becomes longer than the on-period of the switching element Q2, and the timing at which the current Ib1 starts to rise becomes later than the timing at which the current Ib2 starts to fall. For this reason, when the switching element Q3 is turned on and the resonance current ILr starts to flow, a negative current flows as the current Ib. As a result, whether the tank voltage Vt is positive or negative, the resonance current ILr decreases, and a bucking operation is performed in the main circuit section.

[0044] Regarding the switching elements Q5 to Q8, the control unit 6 performs on / off control (synchronous rectification control) with a duty of 50% in synchronization with the switching elements Q1 to Q4. The control unit 6 provides at least a dead time period for the switching elements Q5 and Q6 to perform an anti-phase operation, and also provides at least a dead time period for the switching elements Q7 and Q8 to perform an anti-phase operation.

[0045] The control unit 6 starts the synchronous rectification operation of the second full-bridge circuit 5 by turning on the switching elements Q5 and Q8 in accordance with the turn-on timing of the switching element Q1. Similarly, the control unit 6 starts the synchronous rectification operation of the second full-bridge circuit 5 by turning on the switching elements Q6 and Q7 in accordance with the turn-on timing of the switching element Q3.

[0046] The control unit 6 detects the timing when the synchronous current stops flowing in the third leg or the fourth leg, and turns off the switching elements Q5 and Q8 (solid line in FIG. 2), or turns off the switching elements Q5 and Q8 at the timing when the switching element Q4 turns off before the start of the next cycle at the latest (broken line in FIG. 2), thereby ending the synchronous rectification operation. Similarly, the control unit 6 detects the timing when the synchronous current stops flowing in the third leg or the fourth leg, and turns off the switching elements Q6 and Q7 (solid line in FIG. 2), or turns off the switching elements Q6 and Q7 at the timing when the switching element Q2 turns off before the start of the next cycle at the latest (broken line in FIG. 2), thereby ending the synchronous rectification operation.

[0047] FIG. 3(A) shows the current path diagram during the time period t0 - t1 in FIG. 2. At time t0, since the switching element Q4 is on and the switching element Q1 turns on, a positive resonance current ILr flows through the first resonance circuit 3 during the time period t0 - t1. The current Ib1 decreases, and the tank voltage Vt becomes the positive bus voltage Vb (the voltage of the capacitor Cb for the bus), and the positive current Ib is added to the resonance current ILr, and a step-up operation is performed in the main circuit section. In the second full-bridge circuit 5, since the switching elements Q5 and Q8 are on, synchronous rectification is performed, and a load current flows through the second full-bridge circuit 5 due to the voltage induced on the secondary side.

[0048] Fig. 3(B) shows the current path diagram during the time period t1 - t2 in Fig. 2. When the switching element Q1 turns off at time t1 and the dead time period starts, the charge stored in the capacitor C2 discharges and partial resonance occurs. The tank voltage Vt decreases because the switching element Q1 is off and becomes zero at time t2. During this period, due to the energy stored in the first resonance inductor Lr1, a positive resonance current ILr flows through the first resonance circuit 3, and the load current continues to flow through the second full - bridge circuit 5.

[0049] Fig. 3(C) shows the current path diagram during the time period t2 - t3 in Fig. 2. When the switching element Q2 turns on at time t2 and the dead time period ends, the current Ib1 flowing through the first boost inductor Lb1 starts to increase. Since the switching elements Q2 and Q4 are in the on state, the tank voltage Vt becomes zero and the resonance current ILr decreases.

[0050] Fig. 4(A) shows the current path diagram during the time period t3 - t4 in Fig. 2. When the resonance current ILr becomes zero at time t3, only the exciting current ILm flows through the first resonance circuit 3 (the current waveform of IL during the time period t3 - t5 shown in Fig. 2 indicates that only the exciting current ILm(>0) continues to flow and IL = ILm). Since the load current becomes zero in the second full - bridge circuit 5, the control unit 6 turns off the switching elements Q5 and Q8.

[0051] Fig. 4(B) shows the current path diagram during the time period t4 - t5 in Fig. 2. When the switching element Q2 is in the on state and the switching element Q4 turns off at time t4, the charge stored in the capacitor C3 discharges while the capacitor C4 is charged. Since a reverse bus voltage Vb is applied between the terminals T1 and T3, the tank voltage Vt changes to a negative voltage.

[0052] The current path after time t5 is the same as the current path after time t0. For example, at time t5, instead of the switching element Q1, the switching element Q3 turns on, and a reverse (negative) resonance current ILr flows through the first resonance circuit 3.

[0053] By the way, in the case of a LLC-type DC / DC converter, generally, frequency modulation control for modulating the drive frequency is performed, and the gain (step-up / step-down ratio) of the frequency modulation control is adjusted according to the characteristic curve of the gain determined by the resonance constant of the resonance circuit. That is, when the turn ratio of the transformer is 1:1, when the drive frequency is smaller than the resonance frequency, the gain becomes larger than 1, when the drive frequency is the resonance frequency, the gain becomes 1, and when the drive frequency is larger than the resonance frequency, the gain becomes smaller than 1.

[0054] Also, in the method described in Non-Patent Document 1, frequency modulation control is not performed, the drive frequency is fixed to the resonance frequency, and the gain is adjusted only by duty control. However, since there is a problem that the input ripple noise increases when the duty is changed significantly from 50%, the gain adjustment range may not be sufficient only by duty control.

[0055] Therefore, a method can be considered in which frequency modulation control is performed in a frequency range where heat generation of the transformer and resonance coil does not become a problem, duty control is performed below the frequency of the lower limit of the above frequency range, and phase shift control is performed above the frequency of the upper limit of the above frequency range. However, in that method, since it is necessary to switch the control unit from the frequency to the duty unit or from the frequency to the phase shift amount along with the switching of the control mode, the control becomes complicated.

[0056] Therefore, in the DC / DC converter 10 according to the present embodiment, the complication of control is avoided by making the control unit only the drive frequency f [kHz]. Specifically, the control unit 6 performs gain adjustment by duty control in which the duty D is modulated as a function of the drive frequency f while performing gain adjustment by frequency modulation control.

[0057] Fig. 5 shows the relationship between the driving frequency f and the duty D in the DC / DC converter 10. In Fig. 5, the horizontal axis represents the driving frequency f [kHz], and the vertical axis represents the duty D [%]. Also, let the driving frequency f when the duty D is 50% be f50 [kHz]. The driving frequency f50 corresponds to the "50% frequency" of the present invention.

[0058] In the DC / DC converter 10, from the turns ratio of the transformer Tr and the resonance constant of the first resonance circuit 3 (LLC resonance characteristics), within the range including the driving frequency f50 and where the input ripple is within a practical range (a range where input ripple noise can be tolerated), the first driving frequency fL (< f50) when the first duty DL (< 50%) and the second driving frequency fH (> f50) when the second duty DH (> 50%) are preset in advance. The first driving frequency fL is a frequency that satisfies the desired maximum gain within the above range. The control unit 6 performs frequency modulation control for modulating the driving frequency f within the range from the first driving frequency fL to the second driving frequency fH, and duty control for modulating the duty D as a function of the driving frequency f.

[0059] To perform a boosting operation with the above duty control, the first duty DL should be set to a value smaller than 50%, and to perform a bucking operation with the duty control, the second duty DH should be set to a value larger than 50%. Therefore, it is desirable to make the driving frequency f50 when the duty is 50% coincide with the resonance frequency fr at which the gain by LLC becomes 1.

[0060] The second duty DH is set to a value larger than 50%, with (100 - DL)% as a guideline. For example, when the first duty DL is set to 30% considering the influence of the input ripple, the second duty DH is set to 70% (= 100% - 30%). Also, a practical second driving frequency fH is desirably determined in consideration of the losses and heat generation of passive components such as the transformer Tr, the first resonance inductor Lr1, and the first resonance capacitor Cr1, and the switching elements Q1 to Q4 mainly due to the increase in the driving frequency f.

[0061] When fL ≤ f ≤ fH, the control unit 6 calculates the duty D based on the following formula (1) and performs duty control.

Equation

[0062] Note that even if the driving frequency f is increased to the second driving frequency fH by frequency modulation control and duty control, the gain cannot be reduced to zero (the load current cannot be made zero). Therefore, when f > fH, the control unit 6 changes the increase rate of the duty D with respect to the change in the driving frequency f so that the duty D becomes the maximum duty Dmax when the driving frequency f is the maximum driving frequency fmax which is the substantial (practical) maximum value. The maximum duty Dmax is 100% duty, and strictly speaking, it is a value considering the dead time period from 100%.

[0063] As the duty D increases, the period during which the resonance current ILr flows becomes shorter, so the load current flowing through the second full-bridge circuit 5 decreases, and the load current becomes zero when the duty D is the maximum duty Dmax. That is, when fH < f ≤ fmax, the control unit 6 calculates the duty D based on the following formula (2) and performs duty control.

Equation

[0064] In the DC / DC converter 10, the driving frequency f is increased to a second driving frequency fH that is greater than the resonance frequency fr within a practical range, and the duty D is also increased to a second duty DH that is greater than 50%. Therefore, the gain by this control becomes smaller than the gain of only frequency modulation control or the gain of only conventional duty control. Further, by increasing the driving frequency f to a maximum driving frequency fmax, which is a substantial (practical) maximum value greater than the second driving frequency fH, and increasing the duty D to the maximum duty Dmax, it is expected that the output can be reduced to zero and the generated input ripple can be suppressed low.

[0065] Therefore, according to the DC / DC converter 10 according to the present embodiment, there is no need to switch the control unit from the driving frequency f to the duty D, and since there is no need to change the control mode, the control can be simplified. Also, compared to the case of only frequency modulation control or only conventional duty control, a large gain change can be obtained in a narrower frequency range, so that a wider voltage range can be supported. In a region where the driving frequency f is high, by performing duty control in which the increase rate of the duty D with respect to the change in the driving frequency f is changed and the duty D is modulated as a function of the driving frequency f, the gain can be effectively reduced to zero. Also, according to the DC / DC converter 10 according to the present embodiment, switching noise can be reduced by the soft switching of LLC resonance.

[0066] FIG. 6 shows the control flow of the on / off control performed by the control unit 6. As described above, the control unit 6 performs frequency modulation control and duty control (duty control in which the duty D is modulated as a function of the driving frequency f) as on / off control.

[0067] The control unit 6 that started the on / off control process in step S1 reads input / output information such as current values and / or voltage values necessary for control from the detection circuit through the A / D conversion circuit etc. into the processing unit within the control unit 6 (S2). After step S2, or in parallel with step S2, the control unit 6 reads control parameters (fL, DL, KD1, fH, DH, KD2, fmax, etc.) necessary for operations such as those of equations (1) and (2) from the storage unit within the control unit 6 into the processing unit (S3).

[0068] Next, the control unit 6 determines the driving frequency f of the first full-bridge circuit 2 by frequency modulation control (S4). Specifically, during frequency modulation control, the control unit 6 compares the output value (output current value, output voltage value, or output power value) included in the input / output information with the target value (target output current value, target output voltage value, or target output power value), and determines the driving frequency f of the switching elements Q1 to Q4 so that the output value approaches the target value. Generally, feedback control is performed. Note that the driving frequency f determined in step S4 is the driving frequency for processing within the control unit 6.

[0069] In frequency modulation control, when the output value is smaller than the target value, the control unit 6 decreases the driving frequency f to increase the output. On the other hand, when the output value is larger than the target value, the control unit 6 increases the driving frequency f to decrease the output. Note that the target value is either instructed separately from a higher-level device or a pre-specified value is read in step S3.

[0070] The control unit 6 compares the driving frequency f determined in step S4 with the second driving frequency fH read in step S3 (S5). When the driving frequency f ≤ the second driving frequency fH (YES in S5), the control unit 6 compares the driving frequency f determined in step S4 with the first driving frequency fL read in step S3 (S6). When the first driving frequency fL ≤ the driving frequency f (≤ the second driving frequency fH) (YES in S6), the control unit 6 starts duty control, sets the driving frequency f to the driving frequency f determined in step S4, calculates the duty D based on the above equation (1) (S7), and proceeds to step S12.

[0071] If the drive frequency f is less than the first drive frequency fL in step S6 (NO in S6), the control unit 6 starts duty control, sets the drive frequency f to the first drive frequency fL, and sets the duty D to the first duty DL (S8), and proceeds to step S12.

[0072] If the drive frequency f is greater than the second drive frequency fH in step S5 (NO in S5), the control unit 6 compares the drive frequency f determined in step S4 with the maximum drive frequency fmax read in step S3 (S9). If the drive frequency f is less than or equal to the maximum drive frequency fmax (YES in S9), the control unit 6 starts duty control, sets the drive frequency f to the drive frequency f determined in step S4, calculates the duty D based on the above formula (2) (S10), and proceeds to step S12.

[0073] If the drive frequency f is greater than the maximum drive frequency fmax in step S9 (No in S9), the control unit 6 starts duty control, sets the drive frequency f to the maximum drive frequency fmax, and sets the duty D to the maximum duty Dmax (S11), and then proceeds to step S12.

[0074] In step S12, the control unit 6 drives the first full-bridge circuit 2 with the drive frequency f and duty D set (or calculated) in the immediately previous step. In addition, the control unit 6 performs synchronous rectification control on the switching elements Q5 to Q8 of the second full-bridge circuit 5.

[0075] The control unit 6 judges whether to continue the on-off control (S13), and judges not to continue the control if there is an end command from a higher-level device (NO in S13), and ends the on-off control (S14). On the other hand, the control unit 6 judges to continue the control if there is no end command from a higher-level device (YES in S13), and repeats the processing from step S2 onwards.

[0076] According to the DC / DC converter 10 according to this embodiment, as described above, by performing frequency modulation control and duty control using the drive frequency f determined by the frequency modulation control as a control unit, in addition to the change in gain due to the duty control, the change in gain due to LLC (the gain due to frequency modulation control) is added. Therefore, a wider range of gain changes can be obtained compared to the case of only conventional duty control. In a frequency region higher than a certain drive frequency (the second drive frequency fH above), by performing duty control while changing the increase rate of the duty D with respect to the change in the drive frequency f, the output can be significantly reduced. Further, according to the DC / DC converter 10 according to this embodiment, switching noise can also be reduced by the soft switching of LLC resonance.

[0077] [Second Embodiment] FIG. 7 shows a DC / DC converter 10' according to the second embodiment of the present invention. The DC / DC converter 10' has the same configuration as that of the first embodiment except for the control unit 6'. The control unit 6' performs duty control using the drive frequency f as a control unit, frequency modulation control with a fixed duty D, and phase shift control using the drive frequency f as a control unit.

[0078] FIG. 8 shows the relationship among the drive frequency f, the duty D, and the phase shift amount θ in the DC / DC converter 10'. The phase shift amount θ [deg] represents the phase difference between the first leg and the second leg of the drive circuit (the first full-bridge circuit 2) driven with a duty D = 50%. When the phase shift amount θ = 0°, the phase difference is zero and the current flows through the drive circuit for the maximum period (half cycle). When the phase shift amount θ = the maximum value θmax, the phase difference is maximum and the period during which the current flows through the drive circuit becomes zero, and the load current does not flow. The maximum value θmax is the period obtained by excluding the dead time period from the maximum phase difference of 180°.

[0079] When the driving frequency f determined by the frequency modulation control (the driving frequency for processing within the control unit 6') is less than or equal to the resonance frequency fr and greater than or equal to the first driving frequency fL (fL ≤ f ≤ fr), the control unit 6' sets the driving frequency f to the resonance frequency fr and calculates the duty D of the first full-bridge circuit 2 based on the above formula (1) to perform duty control.

[0080] When the driving frequency f determined by the frequency modulation control is greater than the resonance frequency fr and less than the predetermined phase shift start frequency fs (fr < f < fs), the control unit 6' sets the duty D to 50% and drives the switching elements Q1 to Q4 at the driving frequency f. That is, when fr < f < fs, the control unit 6' performs only frequency modulation control.

[0081] When the driving frequency f determined by the frequency modulation control is greater than or equal to the phase shift start frequency fs and less than or equal to the maximum driving frequency fmax (fs ≤ f ≤ fmax), the control unit 6' sets the duty D to 50% and performs phase shift control with the driving frequency f as the control unit.

[0082] Here, the resonance frequency fr is the frequency fr = 1 / (2π√(Lr1·Cr1)) determined by the first resonance inductor Lr1 and the first resonance capacitor Cr1 of the first resonance circuit 3. It is desirable to match the driving frequency f50 at the time of duty D = 50% with the resonance frequency fr, which is the same as in the first embodiment.

[0083] When the driving frequency f (the driving frequency for processing) obtained by the frequency modulation control in the LLC method is smaller than the resonance frequency fr, instead of using the driving frequency f for processing as the actual driving frequency for driving the first full-bridge circuit 2, by fixing it to the resonance frequency fr, in the DC / DC converter 10' according to the second embodiment, the actual driving frequency f can always be made equal to or higher than the resonance frequency fr. As a result, the period during which the resonance current ILr flows does not become shorter than 1 / 2 cycle (T / 2), and the synchronous rectification current (load current) continues to flow within the period when the duty of the second full-bridge circuit 5 is 50%. Therefore, it is not necessary to detect the synchronous rectification current and detect the timing when the synchronous rectification current becomes zero. As a result, a detection circuit for detecting the synchronous rectification current becomes unnecessary, and miniaturization and cost reduction of the DC / DC converter 10' can be expected.

[0084] Furthermore, in the DC / DC converter 10', when the gain obtained when the driving frequency f is less than the phase shift start frequency fs is smaller than the gain obtained by the phase shift control with the driving frequency f as the control unit. In the phase shift control, the phase shift amount θ becomes 0° when the driving frequency f is the phase shift start frequency fs, and the phase shift amount θ becomes the maximum value θmax when the driving frequency f is the maximum driving frequency fmax.

[0085] When the driving frequency f satisfies fs ≦ f ≦ fmax, the control unit 6' calculates the phase shift amount θ [deg] based on the following formula (3) and performs phase shift control.

Equation

[0086] In the DC / DC converter 10', when the driving frequency f satisfies fs ≦ f ≦ fmax, the control unit 6' sets the duty D to 50% and performs phase shift control with the driving frequency f as the control unit. As a result, the output can be reduced to zero in the low output region.

[0087] In the above description, with fr < fs, the phase shift start frequency fs is provided separately from the resonance frequency fr, and only frequency modulation control is performed in the range fr < f < fs. However, with fs = fr, the region where only frequency modulation control is performed is eliminated, and when fr = fs < f ≤ fmax, phase shift control using the drive frequency f according to the above formula (3) as a control unit may be performed.

[0088] Fig. 9 shows the control flow of the on-off control performed by the control unit 6' in the DC / DC converter 10'. As described above, the control unit 6' performs duty control, frequency modulation control, and phase shift control as on-off control.

[0089] Steps S21 to S24 in Fig. 9 are the same as steps S1 to S4 in Fig. 6, so the description is omitted. Note that the control parameters read in step S23 also include the control parameters necessary for the calculation of formula (3) and the like.

[0090] The control unit 6' compares the drive frequency f determined in step S24 with the resonance frequency fr read in step S23 (S25). When the drive frequency f ≤ the resonance frequency fr (YES in S25), the control unit 6' compares the drive frequency f determined in step S24 with the first drive frequency fL read in step S23 (S26). When the first drive frequency fL ≤ the drive frequency f (≤ the resonance frequency fr) (YES in S26), the control unit 6' starts duty control, calculates the duty D of the first full-bridge circuit 2 based on the above formula (1) (S27), and proceeds to step S29.

[0091] When the drive frequency f < the first drive frequency fL in step S26 (NO in S26), the control unit 6' starts duty control, sets the duty D to the first duty DL (S28), and proceeds to step S29.

[0092] In step S29, the control unit 6' sets the drive frequency f to the resonance frequency fr and sets the phase shift amount θ to 0°, and then proceeds to step S36. That is, when the drive frequency f determined in step S24 satisfies fL ≤ f ≤ fr, the duty D of the duty control is determined based on the drive frequency f determined in step S24, and the drive frequency f of the switching elements Q1 to Q4 during actual duty control is fixed at the resonance frequency fr.

[0093] If the drive frequency f > the resonance frequency fr in step S25 (NO in S25), the control unit 6' sets the duty D to 50% (S30), and compares the drive frequency f determined in step S24 with the phase shift start frequency fs read in step S23 (S31).

[0094] If (the resonance frequency fr <) the drive frequency f < the phase shift start frequency fs in step S31 (NO in S31), the control unit 6' sets the phase shift amount θ to 0° (S32) and proceeds to step S36. That is, when the drive frequency f determined in step S24 satisfies fr < f < fs, the control unit 6' performs only frequency modulation control with the duty D = 50% and the phase shift amount θ = 0°.

[0095] If the phase shift start frequency fs ≤ the drive frequency f in step 31 (YES in S31), the control unit 6' starts phase shift control and compares the drive frequency f determined in step S24 with the maximum drive frequency fmax read in step S23 (S33).

[0096] In step 33, when the driving frequency f satisfies (resonant frequency fr < phase shift start frequency fs ≤) driving frequency f ≤ maximum driving frequency fmax (YES in step S33), the control unit 6' calculates the phase shift amount θ based on the above formula (3) (S34), and proceeds to step S36. On the other hand, when the driving frequency f > maximum driving frequency fmax in step 33 (NO in step S33), the control unit 6' sets the driving frequency f to the maximum driving frequency fmax and sets the phase shift amount θ to the maximum value θmax (S35), and then proceeds to step S36. That is, when the driving frequency f determined in step S24 satisfies (fr <) fs ≤ f ≤ fmax, the control unit 6' determines the phase shift amount θ of the phase shift control based on the driving frequency f determined in step S24, and performs the phase shift control.

[0097] In step 36, the control unit 6' drives the first full-bridge circuit 2 with the driving frequency f, duty D, and phase shift amount θ set (or calculated) in the previous step. Also, the control unit 6' performs synchronous rectification control on the switching elements Q5 to Q8 of the second full-bridge circuit 5.

[0098] The control unit 6' determines whether to continue the on-off control (S37). For example, if there is an end command from the upper-level device, it determines not to continue the control (NO in S37), and ends the on-off control (S38). On the other hand, for example, if there is no end command from the upper-level device, the control unit 6' determines to continue the control (YES in S37), and repeats the processing after step S22.

[0099] In the DC / DC converter 10' according to this embodiment, as described above, when the driving frequency f is less than or equal to the resonant frequency fr, the driving frequency f is fixed to the resonant frequency fr, and only the duty D (<50%) is modulated to cause the main circuit section to perform a boosting operation. For this reason, the driving frequency f is always greater than or equal to the resonant frequency fr, and the resonant current ILr flows during the synchronous rectification period of the second full-bridge circuit 5. Therefore, a detection circuit for detecting the synchronous rectification current of the second full-bridge circuit 5 becomes unnecessary, and miniaturization, cost reduction of the DC / DC converter 10', and simplification of the processing of the control unit 6' can be expected.

[0100] Also, in the DC / DC converter 10' according to the present embodiment, when the driving frequency f is greater than the resonance frequency fr, the duty D is fixed at 50%. When fr < f < fs, only frequency modulation control is performed. When the output decreases to a certain extent and (fr <) fs ≤ f ≤ fmax, phase shift control with the driving frequency f as the control unit is performed. Therefore, the gain can be reduced below the gain by duty control and the gain by LLC (the gain by frequency modulation control), and the output can be reduced to zero. Also, during this period, since it is phase shift control in a state where the resonance current ILr has decreased, an effect of reducing switching noise can also be expected.

[0101] As described above, the embodiment of the DC / DC converter according to the present invention has been described, but the present invention is not limited to the above embodiment.

[0102] [First Modified Example] FIG. 10 shows a DC / DC converter 10A according to the first modified example of the present invention. The DC / DC converter 10A has the same configuration as that of the first embodiment (or the second embodiment) except for the second full-bridge circuit 5A and the control unit 6A.

[0103] The second full-bridge circuit 5A is a diode bridge rectifier circuit composed only of a diode D5 for the upper arm of the third leg, a diode D6 for the lower arm of the third leg, a diode D7 for the upper arm of the fourth leg, and a diode D8 for the lower arm of the fourth leg. Also, the control unit 6A has the same configuration as that of the first embodiment (or the second embodiment) except that it does not perform synchronous rectification control on the second full-bridge circuit 5A.

[0104] The DC / DC converter 10A according to the first modified example has the same effects as those of the first embodiment (or the second embodiment) except for the effects by synchronous rectification control.

[0105] [Second Modified Example] Fig. 11 shows a DC / DC converter 10B according to a second modification of the present invention. The DC / DC converter 10B is obtained by adding a second inductor circuit 7, a terminal T6, and a bus capacitor Cb' to the configuration of the first embodiment (or the second embodiment), and changing the control unit 6 (control unit 6') to a control unit 6B. Note that the bus capacitor Cb' corresponds to the "second bus capacitor" of the present invention.

[0106] The second inductor circuit 7 includes a third boost inductor Lb3 and a fourth boost inductor Lb4 in an interleaved configuration. The third boost inductor Lb3 and the fourth boost inductor Lb4 have the same inductance value, and one end of both is connected to the terminal T6 (corresponding to the "sixth terminal" of the present invention). The other end of the third boost inductor Lb3 is connected to the connection point (third connection point c) between the emitter terminal of the switching element Q5 and the collector terminal of the switching element Q6 of the third leg. The other end of the fourth boost inductor Lb4 is connected to the connection point (fourth connection point d) between the emitter terminal of the switching element Q7 and the collector terminal of the switching element Q8 of the fourth leg.

[0107] The bus capacitor Cb' is connected between the terminals T4 and T5. Also, a ripple removing capacitor C21 is connected between the terminals T6 and T5. The output terminal of a DC voltage source (not shown) is connected between the terminals T6 and T5, and a DC voltage V2 is applied. During reverse power transmission, the input voltage V2 is boosted and applied to the bus capacitor Cb', and the applied voltage (bus voltage) of the bus capacitor Cb' becomes the applied voltage of the second full bridge circuit 5.

[0108] The control unit 6B has the same configuration as the control unit 6 (or the control unit 6') except that it corresponds to bidirectional control. That is, the main circuit part of the DC / DC converter 10B can perform forward power transmission from the first full bridge circuit 2 to the second full bridge circuit 5 and reverse power transmission from the second full bridge circuit 5 to the first full bridge circuit 2, and can perform bidirectional step-up and step-down operations.

[0109] During forward power transmission, the control unit 6B performs frequency modulation control, duty control with the driving frequency f as the control unit (and phase shift control with the driving frequency f as the control unit) on the first full-bridge circuit 2, similar to the first embodiment (or the second embodiment). During forward power transmission, the control unit 6B performs synchronous rectification control with a duty of 50% on the second full-bridge circuit 5.

[0110] Similarly, during reverse power transmission, the control unit 6B performs frequency modulation control, duty control with the driving frequency f as the control unit (and phase shift control with the driving frequency f as the control unit) on the second full-bridge circuit 5. During reverse power transmission, the control unit 6B performs synchronous rectification control with a duty of 50% on the first full-bridge circuit 2.

[0111] Note that since there is no resonance circuit like the first resonance circuit 3 on the primary side on the secondary side, there is a limit to the gain obtained during reverse power transmission. However, since the first resonance circuit 3 on the primary side can be equivalently converted to the secondary side via the transformer Tr, frequency modulation control of the LLC method can also be performed during reverse power transmission.

[0112] [Third Modification Example] FIG. 12 shows a DC / DC converter 10C according to the third modification example of the present invention. The DC / DC converter 10C is obtained by adding a second resonance circuit 8 to the configuration of the second modification example and changing the control unit 6B to a control unit 6C.

[0113] The second resonance circuit 8 includes a second resonance inductor Lr2 and a second resonance capacitor Cr2. The second resonance inductor Lr2 and the second resonance capacitor Cr2 are connected to the third connection point c and the fourth connection point d on the input side during reverse power transmission, and the output side during reverse power transmission is connected to both ends of the secondary winding of the transformer Tr. In the present embodiment, the second resonance inductor Lr2 and the second resonance capacitor Cr2 form a series resonance circuit together with the excitation inductance of the transformer Tr during reverse power transmission.

[0114] The control unit 6C performs the same control as the control unit 6B of the second modification example. Further, since the second resonance circuit 8 is provided, the control unit 6C can arbitrarily set the gain required during reverse power transmission.

[0115] [Other Modification Examples] The main circuit section of the DC / DC converter of the present invention includes a first terminal, a second terminal, and a third terminal, a first boost inductor and a second boost inductor, and a first inductor circuit in which one end of the first boost inductor and one end of the second boost inductor are connected to the second terminal, and a first leg and a second leg connected in parallel, each leg including an upper arm and a lower arm including a switching element, a first connection point of the upper and lower arms of the first leg being connected to the other end of the first boost inductor, a second connection point of the upper and lower arms of the second leg being connected to the other end of the second boost inductor, a high-voltage side connection point of the first leg and the second leg being connected to the first terminal, a low-voltage side connection point of the first leg and the second leg being connected to the third terminal, a first full-bridge circuit, a first resonance inductor and a first resonance capacitor, a first resonance circuit having an input side connected to the first connection point and the second connection point, an isolation transformer having a primary side winding and a secondary side winding, both ends of the primary side winding being connected to the output side of the first resonance circuit, a first bus capacitor connected between the first terminal and the third terminal, a third leg and a fourth leg connected in parallel, each leg including an upper arm and a lower arm including a switching element and / or a diode, a third connection point of the upper and lower arms of the third leg being connected to one end of the secondary side winding, a fourth connection point of the upper and lower arms of the fourth leg being connected to the other end of the secondary side winding, a second full-bridge circuit, a fourth terminal connected to a high-voltage side connection point of the third leg and the fourth leg, and a fifth terminal connected to a low-voltage side connection point of the third leg and the fourth leg, and if so, the configuration can be changed as appropriate.

[0116] The control unit of the DC / DC converter of the present invention performs at least on / off control of the switching elements of the first full-bridge circuit, operates the first full-bridge circuit as a drive circuit, and operates the second full-bridge circuit as a rectifier circuit during forward power transmission. During the on / off control, frequency modulation control for modulating the drive frequency of the switching elements of the first full-bridge circuit is performed, and if duty control for modulating the duty of the switching elements of the first full-bridge circuit using the drive frequency as a control unit is performed, the configuration can be changed as appropriate.

Explanation of Signs

[0117] 1 First inductor circuit 2 First full-bridge circuit 3 First resonance circuit 4 Transformer circuit 5, 5A Second full-bridge circuit 6, 6’, 6A~6C Control unit 7 Second inductor circuit 8 Second resonance circuit 10, 10’, 10A~10C DC / DC converter

Claims

1. A DC / DC converter comprising a main circuit section and a control section, wherein the main circuit section has a first terminal, a second terminal, and a third terminal, and includes a first boosting inductor and a second boosting inductor. A first inductor circuit in which one end of the first boosting inductor and one end of the second boosting inductor are connected to the second terminal, a first full-bridge circuit including a first leg and a second leg connected in parallel, each leg including an upper arm and a lower arm each including a switching element, a first connection point of the upper and lower arms of the first leg being connected to the other end of the first boosting inductor, a second connection point of the upper and lower arms of the second leg being connected to the other end of the second boosting inductor, a high-voltage side connection point of the first leg and the second leg being connected to the first terminal, and a low-voltage side connection point of the first leg and the second leg being connected to the third terminal, a first resonance circuit including a first resonance inductor and a first resonance capacitor, the input side being connected to the first connection point and the second connection point, an isolation transformer including a primary side winding and a secondary side winding, both ends of the primary side winding being connected to the output side of the first resonance circuit, a first bus capacitor connected between the first terminal and the third terminal, a second full-bridge circuit including a third leg and a fourth leg connected in parallel, each leg including an upper arm and a lower arm each including a switching element and / or a diode, a third connection point of the upper and lower arms of the third leg being connected to one end of the secondary side winding, and a fourth connection point of the upper and lower arms of the fourth leg being connected to the other end of the secondary side winding, a fourth terminal connected to a high-voltage side connection point of the third leg and the fourth leg, and a fifth terminal connected to a low-voltage side connection point of the third leg and the fourth leg, wherein, during forward power transmission in which the control section performs on / off control of at least the switching element of the first full-bridge circuit, operates the first full-bridge circuit as a drive circuit, and operates the second full-bridge circuit as a rectifier circuit, the control section performs frequency modulation control for modulating the drive frequency of the switching element of the first full-bridge circuit and duty control for modulating the duty of the switching element of the first full-bridge circuit using the drive frequency as a control unit as the on / off control. A DC / DC converter characterized by the above.

2. During the on / off control, the control section ​ ​ By making the drive frequency smaller than the 50% frequency corresponding to a duty of 50%, the main circuit section is caused to perform a boosting operation. By making the drive frequency larger than the 50% frequency, the main circuit section is caused to perform a bucking operation. In a range from a first drive frequency corresponding to a first duty smaller than 50% to a second drive frequency corresponding to a second duty larger than 50%, the main circuit section is caused to perform a buck-boost operation. The DC / DC converter according to claim 1, characterized in that.

3. During the on-off control, the control unit When the drive frequency by the frequency modulation control is larger than the second drive frequency, the duty control is performed by changing the increase rate of the duty with respect to the change in the drive frequency so that the duty becomes the maximum value when the drive frequency is the maximum drive frequency which is a substantial maximum value. The DC / DC converter according to claim 2, characterized in that.

4. During the on-off control, the control unit When the drive frequency by the frequency modulation control becomes a value less than or equal to the resonance frequency of the first resonance circuit, the drive frequency is fixed to the resonance frequency and the duty control is performed. The DC / DC converter according to claim 1, characterized in that.

5. During the on-off control, the control unit When the drive frequency by the frequency modulation control becomes a value larger than the resonance frequency of the first resonance circuit, the duty is fixed at 50%. When the drive frequency by the frequency modulation control becomes a value greater than or equal to a phase shift start frequency larger than the resonance frequency, phase shift control is performed to control the phase shift amount between the first leg and the second leg using the drive frequency as a control unit. The DC / DC converter according to claim 1, characterized in that.

6. During the on-off control, the control unit In accordance with the turn-on timing of the switching elements of the first leg and the second leg, the switching elements of the third leg and the fourth leg are turned on to cause the second full-bridge circuit to perform a synchronous rectification operation. The DC / DC converter according to claim 1, characterized in that.

7. A second inductor circuit including a third boost inductor and a fourth boost inductor, one end of the third boost inductor being connected to the third connection point of the second full-bridge circuit, and one end of the fourth boost inductor being connected to the fourth connection point of the second full-bridge circuit, a sixth terminal connected to the other end of the third boost inductor and the other end of the fourth boost inductor, and further including a second bus capacitor connected between the fourth terminal and the fifth terminal, The control unit, performs on / off control of the switching elements of the second full-bridge circuit, operates the second full-bridge circuit as the drive circuit, and when performing reverse power transmission in which the first full-bridge circuit operates as the rectifier circuit, as the on / off control, performs frequency modulation control for modulating the drive frequency of the switching elements of the second full-bridge circuit and performs duty control for modulating the duty of the switching elements of the second full-bridge circuit using the drive frequency as a control unit The DC / DC converter according to claim 1, characterized in that.

8. A second resonance circuit including a second resonance inductor and a second resonance capacitor, the input side during reverse power transmission being connected to the third connection point and the fourth connection point, and the output side during reverse power transmission being connected to both ends of the secondary winding, is further provided. The DC / DC converter according to claim 7, characterized in that.

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