DC / DC converter

Abstract

To provide a DC / DC converter capable of reducing switching noise and compatible with a relatively wider voltage range.SOLUTION: A bidirectional DC / DC converter 11 includes a main circuit part and a control part 10. The control part 10 performs at least one of: control on-PWM control of turning on / off a first leg of a first full-bridge circuit 3, turning on / off a second leg of the first full-bridge circuit 3, and turning on / off a first bidirectional switch 5 synchronizing with the on / off of the second leg, so as to increase resonance current during an on-duty period in which the second leg turns on; and off-PWM control of turning on / off the first leg, turning off the second leg, and turning off the first bidirectional switch 5 synchronizing with the turning on of the first leg, so as to suppress generation of the resonance current during an off-duty period in which the first bidirectional switch 5 turns off.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Generally, a power supply device used in a charger for an electric vehicle or the like includes an AC / DC converter that converts an AC input voltage input from an AC power supply into a DC input voltage, and a DC / DC converter that converts the DC input voltage into a desired DC output voltage or DC output current. Further, due to the demands for miniaturization, cost reduction, and high efficiency of the power supply device, as the DC / DC converter, 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 the circuit topology is made symmetric and bidirectional are adopted.

[0003] On the other hand, the CHAdeMO standard, which is a charging standard for electric vehicles, requires a wide battery voltage range from 150 [V] to 450 [V]. However, the current resonance type DC / DC converter of the LLC type or the CLLC type has a problem that the output voltage range is narrower than that of the PWM type DC / DC converter. For this reason, various methods for expanding the output voltage range, such as a bus voltage variable method and a full bridge / half bridge switching method, have been proposed for the LLC type or CLLC type current resonance type DC / DC converter.

[0004] In particular, in a CLLC-type DC / DC converter that performs bidirectional operation, if the turns ratio (boost ratio) of the transformer is determined under unidirectional boost conditions, the boost conditions cannot be satisfied in the reverse direction, making it difficult to satisfy the boost conditions bidirectionally. Therefore, Patent Document 1 proposes a method of short-circuiting a part of the rectifier-side circuit for a short period during the period when the resonant current flows (i.e., performing a boost operation). However, when performing the boost operation, a large resonant current flows, and the short circuit of the rectifier-side circuit is released without waiting for the end of the resonant current, resulting in hard switching and generating switching noise, which causes the problem that high-speed switching cannot be achieved. Therefore, performing the boost operation has become a factor that hinders the stable operation and miniaturization of the power supply device.

[0005] Patent Document 2 describes a resonant-type DC / DC converter in which the drive side is composed of a full-bridge type inverter, a bidirectional switch, and a resonant circuit. However, in this configuration, since the voltage applied to the resonant circuit is always the input voltage Vin, there is a problem that it cannot cope with a wide voltage range even if the conduction period of the inverter and the conduction period of the bidirectional switch are fixed at a constant frequency and shifted from each other.

[0006] Patent Document 3 describes an LLC resonant converter in which the driving side is a full-bridge circuit, a voltage-dividing capacitor and a bidirectional switch are provided between the input terminal and the full-bridge circuit, and the rectifying side is composed of a center-tapped full-wave rectifier circuit with a transformer in between. Patent Document 3 also describes a method in this LLC resonant converter where the driving frequency of the full-bridge circuit is set as the resonant frequency, and with constant-frequency PWM control, when the input voltage changes from the minimum value Vmin to the maximum value Vmax, the duty ratio is changed from 0.5 to 0, one leg on one side of the full-bridge circuit is turned on and off with a 50% duty ratio, the leg connected to the remaining bidirectional switch is turned on and off with PWM control, and one side of the bidirectional switch is turned off in accordance with the PWM control. However, this method has a practical problem that it cannot cope with the change amount of the output voltage or output current even when the input voltage is constant (it cannot obtain the desired output voltage or output current), and as a result, there is a problem that it cannot cope with a wide voltage range.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0008] In view of the above circumstances, the present invention has been made, and the problem thereof is to provide a DC / DC converter capable of reducing switching noise and coping with a relatively wide voltage range.

Means for Solving the Problems

[0009] In order to solve the above problems, the DC / DC converter according to the present invention is It includes a main circuit section and a control section, The main circuit section has a first input / output terminal and a second input / output terminal, a first voltage dividing circuit composed of two first capacitor sections that divide the voltage of the first input / output terminal, a first full-bridge circuit having a first leg and a second leg connected in parallel, each leg having an upper arm and a lower arm including a switching element, and both ends of each leg being connected to the first input / output terminal, a first resonant circuit having a first resonant inductor and a first resonant capacitor, with one end side connected to a first connection point that is a connection point of the upper and lower arms of the first leg and a second connection point that is a connection point of the upper and lower arms of the second leg, a first bidirectional switch provided between a connection point of the two first capacitor sections and the second connection point, an isolation transformer having a primary winding and a secondary winding, with both ends of the primary winding connected to the other end side of the first resonant circuit, a second full-bridge circuit having a third leg and a fourth leg connected in parallel, each leg having an upper arm and a lower arm including a switching element, and both ends of each leg being connected to the second input / output terminal, and is a DC / DC converter comprising The main circuit section Under the control of the control section, the first full-bridge circuit operates as a drive circuit, and the second full-bridge circuit operates as a rectifier circuit to perform forward power transmission from the first input / output terminal to the second input / output terminal. During the forward power transmission, the control section turns on and off the first leg with a duty ratio of 50%, turns on and off the second leg with a duty ratio less than 50% synchronously with the on and off of the first leg, turns on and off the first bidirectional switch synchronously with the on and off of the second leg to increase the resonant current flowing through the first resonant circuit during the duty period when the second leg is on, which is on-PWM control, Turn on and off the first leg with a 50% on-duty cycle, turn off the second leg, and turn off the first bidirectional switch synchronously with the turn-on of the first leg, and suppress the generation of the resonance current during the off-duty period when the first bidirectional switch is off, and perform at least one of the off-PWM control.

[0010] In the DC / DC converter, During the forward power transmission, the control unit When the input-output voltage ratio between the voltage of the first input-output terminal and the voltage of the second input-output terminal is equal to or higher than a certain voltage ratio, or when the driving frequency of the drive circuit is lower than a certain frequency, perform the on-PWM control, while When the input-output voltage ratio is lower than the certain voltage ratio, or when the driving frequency is equal to or higher than the certain frequency, it can be configured to perform the off-PWM control.

[0011] In the DC / DC converter, During the on-PWM control, the control unit When the driving frequency becomes equal to or lower than a predetermined first driving frequency, increase the on-duty period in accordance with the decrease in the driving frequency, and when the on-duty period reaches a predetermined maximum on-duty value, fix the on-duty period at the maximum on-duty value.

[0012] In the DC / DC converter, for example, The on-duty of the on-duty period is a value obtained by multiplying the difference between the first driving frequency and the driving frequency by a predetermined gain.

[0013] In the DC / DC converter, The control unit According to the input-output voltage ratio, when the input-output voltage ratio is high, increase the gain, and when the input-output voltage ratio is low, decrease the gain.

[0014] In the DC / DC converter, the control unit when the drive frequency becomes equal to or higher than a predetermined second drive frequency, increases the off-duty period during the off-PWM control in accordance with an increase in the drive frequency, and when the off-duty period reaches a predetermined maximum off-duty value, can be configured to fix the off-duty period at the maximum off-duty value.

[0015] In the DC / DC converter, for example, the off-duty of the off-duty period is a value obtained by multiplying a difference obtained by subtracting the second drive frequency from the drive frequency by a predetermined gain.

[0016] In the DC / DC converter, the main circuit unit a second voltage dividing circuit composed of two second capacitor units that divide the voltage of the second input / output terminal, a second resonance circuit including a second resonance inductor and a second resonance capacitor, with one end side connected to a third connection point that is a connection point of the upper and lower arms of the third leg and a fourth connection point that is a connection point of the upper and lower arms of the fourth leg, and the other end side connected to both ends of the secondary winding, and a second bidirectional switch provided between a connection point of the two second capacitor units and the fourth connection point, under the control of the control unit, the second full-bridge circuit operates as the drive circuit, and the first full-bridge circuit operates as the rectifier circuit to perform reverse power transmission from the second input / output terminal to the first input / output terminal, when the on-PWM control during the forward power transmission is defined as the first on-PWM control and the off-PWM control during the forward power transmission is defined as the first off-PWM control, the control unit during the reverse power transmission Turn on and off the third leg with a duty cycle of 50%, turn on and off the fourth leg with a duty cycle less than 50% synchronously with the on and off of the third leg, turn on and off the second bidirectional switch synchronously with the on and off of the fourth leg, and increase the resonance current flowing through the second resonance circuit during the duty cycle period when the fourth leg is on, which is the second on PWM control; Turn on and off the third leg with a duty cycle of 50%, turn off the fourth leg, turn off the second bidirectional switch synchronously with the on of the third leg, and suppress the generation of the resonance current during the off duty cycle period when the second bidirectional switch is off, which is the second off PWM control. It can be configured to perform at least one of the controls.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a DC / DC converter that reduces switching noise and can handle a relatively wide voltage range.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying out the Invention

[0019] Hereinafter, embodiments of the DC / DC converter according to the present invention will be described with reference to the accompanying drawings.

[0020] FIG. 1 shows a bidirectional DC / DC converter 11 according to an embodiment of the present invention. The bidirectional DC / DC converter 11 is a CLLC-type current resonance bidirectional DC / DC converter and includes a main circuit section and a control section 10.

[0021] The main circuit section includes a transformer circuit 1 (high-frequency isolation transformer Tr, hereinafter abbreviated as "transformer Tr"), a first resonance circuit 2, a first full-bridge circuit 3, a first voltage division circuit 4, a first bidirectional switch 5, a second resonance circuit 6, a second full-bridge circuit 7, a second voltage division circuit 8, a second bidirectional switch 9, a first input / output terminal T1, T2, and a second input / output terminal T3, T4. A DC voltage V1 (for example, the output voltage of an AC / DC converter) is applied to the first input / output terminal T1, T2, and a DC voltage V2 (for example, the battery voltage of an electric vehicle) is applied to the second input / output terminal T3, T4.

[0022] The main circuit section performs forward power transmission from the first input / output terminals T1 and T2 to the second input / output terminals T3 and T4 and reverse power transmission from the second input / output terminals T3 and T4 to the first input / output terminals T1 and T2 under the control of the control section 10. Also, since the main circuit section is configured to include an A-NPC (Advanced Neutral Point Clamped) circuit on the primary side and the secondary side of the transformer circuit 1, the bidirectional DC / DC converter 11 may also be referred to as an A-NPC type or T-type full-bridge CLLC method bidirectional DC / DC converter.

[0023] The transformer circuit 1 is composed of one or a plurality of transformers Tr. The primary winding of the transformer Tr is connected to the first full-bridge circuit 3 via the first resonance circuit 2. The secondary winding of the transformer Tr is connected to the second full-bridge circuit 7 via the second resonance circuit 6.

[0024] The first resonance circuit 2 includes a first resonance inductor (also simply called an induction coil or coil) Lr1 and a first resonance capacitor (also called a capacitor) Cr1. One end side of the first resonance circuit 2 is connected to the first full-bridge circuit 3, and the other end 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 constitute a series resonance circuit together with the exciting inductance of the transformer Tr during forward power transmission. The first resonance inductor Lr1, the first resonance capacitor Cr1, and the exciting inductance are called resonance constants. Although the exciting inductance of the transformer Tr is included in the primary winding of the transformer Tr and its illustration is omitted, it may also be a coil having an individual core.

[0025] If the first resonant inductor Lr1 and the first resonant capacitor Cr1 are connected to the primary winding of the transformer Tr to form a series resonant circuit, their arrangement is arbitrary. For example, the first resonant inductor Lr1 and the first resonant capacitor Cr1 may be arranged separately on the primary side and the secondary side of the transformer Tr. Also, the first resonant inductor Lr1 may be the leakage inductance of the transformer Tr, a coil with an individual core, or both. The first resonant capacitor Cr1 may be composed of individual capacitors, the parasitic capacitances of the switching elements Q1 to Q4 described later, or both. The same applies to the second resonant inductor Lr2 and the second resonant capacitor Cr2 described later.

[0026] The first full-bridge circuit 3 includes 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 the switching element Q1, the lower arm of the first leg includes the switching element Q2, the upper arm of the second leg includes the switching element Q3, and the lower arm of the second leg includes the 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] The collector 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 first input / output terminal T1, and the emitter 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 first input / output terminal T2. The connection point a between the emitter terminal of the switching element Q1 and the collector terminal of the switching element Q2 (corresponding to the "first connection point" of the present invention) and the connection point b between the emitter terminal of the switching element Q3 and the collector terminal of the switching element Q4 (corresponding to the "second connection point" of the present invention) are connected to one end side of the first resonant circuit 2.

[0028] The switching elements Q1 to Q4 can use 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). The same applies to the switching elements Q5 to Q12 described later.

[0029] The diodes D1 to D4 are freewheeling diodes, and can be built-in diodes of the switching elements Q1 to Q4, external diodes, or both. The same applies to the diodes D5 to D12 described later. Also, the capacitors C1 to C4 are partial resonance capacitors, and can be the parasitic capacitances of the switching elements Q1 to Q4, external capacitors, or both. The same applies to the capacitors C5 to C12 described later.

[0030] The first voltage dividing circuit 4 is composed of capacitors Co11 and Co12 connected in series, and is connected between the first input / output terminals T1 and T2. The capacitors Co11 and Co12 respectively correspond to the "first capacitor section" of the present invention and have the same capacitance.

[0031] The first bidirectional switch 5 is provided between the connection point c and the connection point b of the capacitors Co11 and Co12, and includes switching elements Q5 and Q6 connected in series in the reverse direction. Diodes D5 and D6 are connected in parallel in the reverse direction in the current paths of the switching elements Q5 and Q6, and capacitors C5 and C6 are connected in parallel. Note that the first bidirectional switch 5 may be composed of switching elements Q5 and Q6 connected in parallel in the reverse direction.

[0032] The second resonance circuit 6 includes a second resonance inductor Lr2 and a second resonance capacitor Cr2. One end side of the second resonance circuit 6 is connected to the second full-bridge circuit 7, and the other end side 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 constitute a series resonance circuit together with the exciting inductance of the transformer Tr during reverse power transmission.

[0033] The second full-bridge circuit 7 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. The upper arm of the third leg includes a switching element Q7, the lower arm of the third leg includes a switching element Q8, the upper arm of the fourth leg includes a switching element Q9, and the lower arm of the fourth leg includes a switching element Q10. Diodes D7 to D10 are connected in parallel in the reverse direction to the current paths of the switching elements Q7 to Q10, and capacitors C7 to C10 are connected in parallel.

[0034] The collector terminals of the switching elements Q7 and Q9 (the high-voltage side connection points of the third leg and the fourth leg) are connected to the second input / output terminal T3, and the emitter terminals of the switching elements Q8 and Q10 (the low-voltage side connection points of the third leg and the fourth leg) are connected to the second input / output terminal T4. The connection point d between the emitter terminal of the switching element Q7 and the collector terminal of the switching element Q8 (corresponding to the "third connection point" of the present invention) and the connection point e between the emitter terminal of the switching element Q9 and the collector terminal of the switching element Q10 (corresponding to the "fourth connection point" of the present invention) are connected to one end side of the second resonance circuit 6.

[0035] The second voltage dividing circuit 8 is composed of a capacitor Co21 and a capacitor Co22 connected in series, and is connected between the second input / output terminals T3 and T4. The capacitors Co21 and Co22 respectively correspond to the "second capacitor portion" of the present invention and have the same capacitance.

[0036] The second bidirectional switch 9 is provided between the connection point f of the capacitor Co21 and the capacitor Co22 and the connection point e, and includes switching elements Q11 and Q12 connected in series in the reverse direction. Diodes D11 and D12 are connected in parallel in the reverse direction to the current paths of the switching elements Q11 and Q12, and capacitors C11 and C12 are connected in parallel. Note that the second bidirectional switch 9 may be composed of switching elements Q11 and Q12 connected in parallel in the reverse direction.

[0037] The control unit 10 includes a processing unit (including a storage unit) that generates control signals for performing on / off control of the switching elements Q1 to Q12, and a driving unit (not shown) that turns on / off the switching elements Q1 to Q12 based on the control signals. The control unit 10 may be composed of a digital circuit such as a microprocessor or a digital signal processor, an analog circuit, or a circuit combining a digital circuit and an analog circuit. The control unit 10 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 a current value and / or a voltage value necessary for the control of the control unit 10, and its peripheral circuits. The control unit 10 performs on PWM control, off PWM control, frequency modulation control, and synchronous rectification control as on / off control.

[0038] FIG. 2 shows waveforms of the on / off timings of the switching elements Q1 to Q12, the voltage Vab between the connection points a and b, and the current IL flowing through the first resonance circuit 2 during forward power transmission and during on PWM control. The current IL is the total current of the resonance current ILr flowing through the first resonance circuit 2 and the exciting current ILm flowing through the exciting inductance (Lm).

[0039] The control unit 10 performs on PWM control, turns on / off the first leg (switching elements Q1, Q2) of the first full-bridge circuit 3 which is a driving circuit with an on duty of 50%, turns on the second leg (switching elements Q3, Q4) for a period of duty D (periods of modes 1 and 5) in synchronization with the on timing of the first leg, and turns off / on the first bidirectional switch 5 (switching elements Q5, Q6) in synchronization with the on / off timing of the second leg. In the first bidirectional switch 5, the switching elements Q5 and the switching element Q6 alternately turn off during the period of duty D and the dead time periods before and after it. The duty D is sometimes also referred to as the on duty D.

[0040] In addition, the control unit 10 performs synchronous rectification control, and turns on and off the third leg (switching elements Q7, Q8) and the fourth leg (switching elements Q9, Q10) of the second full-bridge circuit 7, which is a rectifier circuit, in synchronization with the first leg (switching elements Q1, Q2). Note that the third leg and the fourth leg can perform synchronous rectification by turning on in synchronization with the first leg. However, the turn-off timing of the third leg and the fourth leg may be such that when the current flowing through the rectifier circuit or the voltage of the switching elements Q7 to Q10 constituting the rectifier circuit is detected and it is determined that the current has become zero or is below a certain value, they are turned off (dashed line in FIG. 2).

[0041] The voltage Vab applied to the first resonant circuit 2 is the input voltage V1 during the period of mode 1 (the period of on-duty D) in the on-period of the switching element Q1, and is 1 / 2 of the input voltage V1 during the period of mode 3. Also, the voltage Vab is the reverse voltage of the input voltage V1 during the period of mode 5 (the period of on-duty D) in the on-period of the switching element Q2, and is the reverse voltage of 1 / 2 of the input voltage V1 during the period of mode 7.

[0042] The current IL flowing through the first resonant circuit 2 greatly increases the resonant current ILr during the period of mode 1, which is the period of on-duty D, and the increase of the resonant current ILr is suppressed during the period of mode 3. Similarly, the reverse resonant current ILr greatly increases during the period of mode 5, which is the period of on-duty D, and the increase of the reverse resonant current ILr is suppressed during the period of mode 7. That is, in the on-PWM control, the resonant current ILr increases or decreases by increasing or decreasing the on-duty D. Therefore, in the bidirectional DC / DC converter 11, in addition to the change in the gain of the LLC due to the change in the driving frequency of the driving circuit, the output can be increased or decreased by the change in the on-duty D of the on-PWM control.

[0043] FIGS. 3 to 5 show current path diagrams in each mode of FIG. 2. FIG. 3(A) shows the current path in mode 1, FIG. 3(B) shows the current path in mode 2, FIG. 3(C) shows the current path in mode 3, FIG. 4(A) shows the current path in mode 4, FIG. 4(B) shows the current path in mode 5, FIG. 4(C) shows the current path in mode 6, FIG. 5(A) shows the current path in mode 7, and FIG. 5(B) shows the current path in mode 8, respectively.

[0044] In mode 1 of FIG. 3(A), since the switching elements Q1 and Q4 of the first full-bridge circuit 3 are turned on, the input voltage V1 is applied to the first resonant circuit 2, a large resonant current ILr flows, and magnetic energy is stored in the first resonant inductor Lr1. In the first bidirectional switch 5, the switching element Q5 is off and no current flows through the first bidirectional switch 5. In the second full-bridge circuit 7, in synchronization with the first full-bridge circuit 3, since the switching elements Q7 and Q10 are turned on, a synchronous rectification current flows through the switching elements Q7 and Q10, and current flows to a load (not shown) connected to the second input / output terminals T3 and T4.

[0045] In mode 2 of FIG. 3(B), although the switching element Q4 is turned off, since the resonant current ILr continues to flow, the charge stored in the capacitor C3 is discharged, charge is stored in the capacitor C4, and partial resonance occurs. Therefore, the turn-off of the switching element Q4 becomes soft switching by ZVS (zero voltage switching).

[0046] In mode 3 of FIG. 3(C), the switching element Q5 of the first bidirectional switch 5 is turned on and the first bidirectional switch 5 conducts, so a voltage of 1 / 2 of the input voltage V1 is applied to the first resonant circuit 2 and the resonant current ILr continues to flow. In the second full-bridge circuit 7, the synchronous rectification current continues to flow through the switching elements Q7 and Q10, and current flows to the load. The resonant current ILr ends after a certain time has elapsed, and thereafter only the exciting current ILm flows. When the load current no longer flows through the switching elements Q7 and Q10, the control unit 10 turns off the switching elements Q7 and Q10.

[0047] In mode 4 of Fig. 4(A), since the switching elements Q1 and Q6 turn off, only the exciting current ILm flows on the primary side. No current flows on the secondary side. When the switching element Q1 turns off, the charge accumulated in the capacitor C2 is discharged, the charge is accumulated in the capacitor C1, and partial resonance occurs. Therefore, the turn-off of the switching element Q1 becomes soft switching by ZVS (Zero Voltage Switching).

[0048] In mode 5 of Fig. 4(B), since the switching elements Q2 and Q3 of the first full-bridge circuit 3 turn on, an input voltage V1 in the direction opposite to that in mode 1 is applied to the first resonance circuit 2, a large resonance current ILr in the reverse direction flows, and magnetic energy in the reverse direction is accumulated in the first resonance inductor Lr1. In the first bidirectional switch 5, the switching element Q6 is off and no current flows through the first bidirectional switch 5. In the second full-bridge circuit 7, in synchronization with the first full-bridge circuit 3, since the switching elements Q8 and Q9 turn on, a reverse synchronous rectification current flows through the switching elements Q8 and Q9, and current flows through the load. Since the switching elements Q2 and Q3 turn on when only the exciting current is flowing, it becomes ZCS (Zero Current Switching) and soft switching.

[0049] In mode 6 of Fig. 4(C), although the switching element Q3 turns off, since the resonance current ILr of the first resonance circuit 2 continues to flow in the reverse direction, the charge accumulated in the capacitor C4 is discharged, the charge is accumulated in the capacitor C3, and partial resonance occurs. Therefore, the turn-off of the switching element Q3 becomes soft switching by ZVS (Zero Voltage Switching).

[0050] In mode 7 of FIG. 5(A), the switching element Q6 of the first bidirectional switch 5 turns on and the first bidirectional switch 5 conducts. Therefore, a voltage of 1 / 2 of the input voltage V1 in the direction opposite to that in mode 3 is applied to the first resonance circuit 2, and a reverse resonance current ILr continues to flow. In the second full-bridge circuit 7, a reverse synchronous rectification current continues to flow through the switching elements Q8 and Q9, and a current flows through the load. The resonance current ILr ends after a certain period of time, and thereafter only the excitation current ILm flows. When the load current stops flowing through the switching elements Q8 and Q9, the control unit 10 turns off the switching elements Q8 and Q9.

[0051] In mode 8 of FIG. 5(B), since the switching elements Q2 and Q5 turn off, only the excitation current ILm flows on the primary side. No current flows on the secondary side.

[0052] FIG. 6 shows a waveform diagram of the on / off timings of the switching elements Q1 to Q12 and the voltage Vab between the connection points a and b during forward power transmission and off-PWM control. Note that the description of the current IL is omitted.

[0053] The control unit 10 performs off-PWM control, turns on and off the first leg (switching elements Q1, Q2) of the first full-bridge circuit 3, which is a drive circuit, with a 50% on-duty, turns off the second leg (switching elements Q3, Q4), and turns off the first bidirectional switch 5 (switching elements Q5, Q6) in synchronization with the on-timing of the first leg. In the first bidirectional switch 5, the switching element Q5 and the switching element Q6 alternately turn off during the period of duty*D and the dead time period before that. The duty*D is sometimes also referred to as the off-duty*D.

[0054] Further, the control unit 10 performs synchronous rectification control to turn on and off the third leg (switching elements Q7, Q8) and the fourth leg (switching elements Q9, Q10) of the second full-bridge circuit 7, which is a rectifier circuit, in synchronization with the first leg (switching elements Q1, Q2). Note that the third leg and the fourth leg can perform synchronous rectification by turning on in synchronization with the first leg. However, the off-timing of the third leg and the fourth leg may be set to turn off when it is determined that the current flowing through the rectifier circuit or the voltage of the switching elements Q7 to Q10 constituting the rectifier circuit becomes zero or equal to or less than a certain value.

[0055] The voltage Vab applied to the first resonance circuit 2 becomes zero during the period of mode 1 (the period of off-duty * D) within the on-period of the switching element Q1, and becomes 1 / 2 of the input voltage V1 during the period of mode 2. Also, the voltage Vab becomes zero during the period of mode 4 (the period of off-duty * D) within the on-period of the switching element Q2, and becomes the reverse voltage of 1 / 2 of the input voltage V1 during the period of mode 5.

[0056] Regarding the current IL flowing through the first resonance circuit 2, no resonance current ILr is generated during the period of mode 1, which is the period of off-duty * D, and similarly, no resonance current ILr is generated during the period of mode 4, which is also the period of off-duty * D. That is, by increasing or decreasing the off-duty * D of the off-PWM control, the period during which the generation of the resonance current ILr is suppressed increases or decreases (as a result, the resonance current ILr increases or decreases). For this reason, in the bidirectional DC / DC converter 11, in addition to the change in the gain of the LLC due to the change in the driving frequency of the driving circuit, the output can be increased or decreased by changing the off-duty * D of the off-PWM control.

[0057] Figs. 7 and 8 show current path diagrams in each mode of Fig. 6. Fig. 7(A) shows the current path in mode 2, Fig. 7(B) shows the current path in mode 3, Fig. 7(C) shows the current path in mode 4, Fig. 8(A) shows the current path in mode 5, Fig. 8(B) shows the current path in mode 6, and Fig. 8(C) shows the current path in mode 1.

[0058] In mode 2 of FIG. 7(A), the switching element Q1 of the first full-bridge circuit 3 is on, and the switching element Q5 of the first bidirectional switch 5 is turned on to conduct the first bidirectional switch 5. Therefore, a voltage of 1 / 2 of the input voltage V1 is applied to the first resonant circuit 2, and a resonant current ILr flows through the first resonant circuit 2. In the second full-bridge circuit 7, since the switching elements Q7 and Q10 are on in synchronization with the first full-bridge circuit 3, a synchronous rectification current flows through the switching elements Q7 and Q10, and a current flows through a load (not shown) connected to the second input / output terminals T3 and T4. Note that the turn-off timing of the switching elements Q7 and Q10 may be set to turn off when it is determined that the current flowing through the rectifier circuit or the voltage of the switching elements Q7 to Q10 constituting the rectifier circuit becomes zero or falls below a certain value by detecting the current or voltage.

[0059] In mode 3 of FIG. 7(B), the switching elements Q1 and Q6 are turned off, and the switching elements Q7 and Q10 are turned off. Therefore, only the exciting current ILm flows on the primary side, and no current flows on the secondary side. The exciting current ILm flows through the switching element Q5 of the first bidirectional switch 5 and the diode D6.

[0060] In mode 4 of FIG. 7(C), the switching element Q2 is turned on, only the exciting current ILm flows on the primary side, and no current flows on the secondary side. The exciting current ILm flows through the switching element Q5 of the first bidirectional switch 5 and the diode D6.

[0061] In mode 5 of FIG. 8(A), the switching element Q2 of the first full-bridge circuit 3 is on, and the switching element Q6 of the first bidirectional switch 5 is turned on to conduct the first bidirectional switch 5. Therefore, a voltage of 1 / 2 of the input voltage V1 in the reverse direction to that in mode 2 is applied to the first resonant circuit 2, and a reverse resonant current ILr flows through the first resonant circuit 2. In the second full-bridge circuit 7, since the switching elements Q8 and Q9 are on in synchronization with the first full-bridge circuit 3, a reverse synchronous rectification current flows through the switching elements Q8 and Q9, and a current flows through the load.

[0062] In mode 6 of FIG. 8(B), since the switching elements Q2 and Q5 turn off and the switching elements Q8 and Q9 turn off, only the exciting current ILm flows on the primary side and no current flows on the secondary side. The exciting current ILm flows through the switching element Q6 and the diode D5 of the first bidirectional switch 5.

[0063] In mode 1 of FIG. 8(C), the switching element Q1 turns on, only the exciting current ILm flows on the primary side, and no current flows on the secondary side. The exciting current ILm flows through the switching element Q6 and the diode D5 of the first bidirectional switch 5.

[0064] FIG. 9 shows the relationship between the drive frequency f of the drive circuit in the bidirectional DC / DC converter 11, the on-duty D of the on-PWM control, and the off-duty *D of the off-PWM control.

[0065] In FIG. 9, the horizontal axis is the drive frequency f [kHz], the left vertical axis is the on-duty D [%] of the on-PWM control, and the right vertical axis is the off-duty *D [%] of the off-PWM control. Also, fDs is the start drive frequency of the on-PWM control (corresponding to the "first drive frequency" of the present invention), and f*Ds is the start drive frequency of the off-PWM control (corresponding to the "second drive frequency" of the present invention). fDh is the drive frequency at which the on-duty maximum value Dh [%] of the on-PWM control is reached, and f*Dh is the drive frequency at which the off-duty maximum value *Dh [%] of the off-PWM control is reached. The on-duty maximum value Dh [%] and the off-duty maximum value *Dh [%] are values less than 50% excluding the dead time period, but may also be 50%.

[0066] The drive frequency f is controlled between the minimum drive frequency fmin that is greater than the resonance frequency determined by the inductances of the first resonance inductor Lr1 and the exciting inductor (Lm) and the capacitance of the first resonance capacitor Cr1, and the maximum drive frequency fmax determined by the characteristics of the first resonance inductor Lr1, the transformer Tr, and the switching elements Q1 to Q4 during forward power transmission.

[0067] That is, when the drive frequency f is equal to or lower than the first drive frequency fDs during on-PWM control, the control unit 10 increases the on-duty D as the drive frequency f decreases. When the on-duty D reaches the maximum on-duty value Dh, the control unit 10 fixes the on-duty D at the maximum on-duty value Dh. On the other hand, when the drive frequency f is equal to or higher than the second drive frequency f*Ds during off-PWM control, the control unit 10 increases the off-duty *D as the drive frequency f increases. When the off-duty *D reaches the maximum off-duty value *Dh, the control unit 10 fixes the off-duty *D at the maximum off-duty value *Dh.

[0068] Specifically, the control unit 10 calculates the on-duty D based on the following formulas (1) and (2) to perform on-PWM control, and calculates the off-duty *D based on the following formulas (3) and (4) to perform off-PWM control.

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[0069] When the drive frequency f is greater than the first drive frequency fDs and less than the second drive frequency f*Ds (fDs < f < f*Ds), the control unit 10 performs frequency modulation control to control the output gain of the LLC based on the drive frequency f.

[0070] Figure 10 shows the control flow of the on / off control of switching elements Q1 to Q12 performed by the control unit 10 during forward power transmission. As described above, as on / off control, the control unit 10 performs on-PWM control in which the on-duty D is modulated as a function of the driving frequency f, frequency modulation control, and off-PWM control in which the off-duty *D is modulated as a function of the driving frequency f.

[0071] The control unit 10 that starts 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 or the like into the processing unit in the control unit 10 (S2). After step S2, the control unit 10 reads control parameters necessary for operations such as those of formulas (1) to (4) from the storage unit in the control unit 10 into the processing unit (S3). Note that the process of step S3 may be performed in parallel with step S2 or may be performed before step S2.

[0072] Next, the control unit 10 determines the driving frequency f of the first full-bridge circuit 3 by frequency modulation control according to the output characteristics of LLC determined by the circuit constants of the first resonance circuit 2 and the load conditions (S4). Specifically, during frequency modulation control, the control unit 10 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 in the control unit 10.

[0073] In frequency modulation control, when the output value is smaller than the target value, the control unit 10 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 10 increases the driving frequency f to decrease the output. Note that the target value is instructed from a higher-level device separately or is read in step S3 as a previously specified value.

[0074] The control unit 10 compares the drive frequency f determined in step S4 with the second drive frequency f*Ds read in step S3 (S5). If the drive frequency f is not greater than or equal to the second drive frequency f*Ds (NO in S5), the control unit 10 compares the drive frequency f with the first drive frequency fDs read in step S3 (S6).

[0075] When the drive frequency f is less than or equal to the first drive frequency fDs (YES in S6), the control unit 10 compares the drive frequency f with the drive frequency fDh read in step S3 (S7). If the drive frequency f is not less than or equal to the drive frequency fDh (NO in S7), the control unit 10 calculates the on-duty D by on-PWM control based on Equation (1) (S8). If the drive frequency f is less than or equal to the drive frequency fDh (YES in S7), the on-duty D is fixed at the on-duty maximum value Dh (S9), and the process proceeds to step S10.

[0076] Here, KD in Equation (1) may be a single value or a plurality of values. In the latter case, KD can be a value associated with the input / output voltage ratio (V1 / V2). For example, KD can be the first KD when the input / output voltage ratio is included in a predetermined first range, and a second KD with a value different from the first KD when the input / output voltage ratio is included in a predetermined second range different from the first range. Specifically, it is preferable that the control unit 10 stores a plurality of KDs associated with the input / output voltage ratio in the storage unit in advance and determines the KD corresponding to the current input / output voltage ratio based on the input / output information read in step S2. Thereby, KD can be increased when the input / output voltage ratio is high, and KD can be decreased when the input / output voltage ratio is low. Similarly, K*D in Equation (3) can also be a plurality of values, and for example, it can be a value associated with the input / output voltage ratio.

[0077] In step S10, the control unit 10 performs on-PWM control, turns on and off the first leg (switching elements Q1, Q2) at a driving frequency f and an on-duty ratio of 50%, turns on the second leg (switching elements Q3, Q4) for a period of on-duty ratio D in synchronization with the on-timing of the first leg, and turns on and off the first bidirectional switch 5 (switching elements Q5, Q6) in synchronization with the on-off timing of the second leg. In the first bidirectional switch 5, the switching element Q5 and the switching element Q6 alternately turn off during the period of duty ratio D and the dead time periods before and after it.

[0078] Also in step S10, the control unit 10 performs synchronous rectification control, turns on and off the third leg (switching elements Q7, Q8) and the fourth leg (switching elements Q9, Q10) in synchronization with the first leg (switching elements Q1, Q2), and turns off the switching element Q11 and the switching element Q12 of the second bidirectional switch 9.

[0079] When the driving frequency f is smaller than the second driving frequency f*Ds and larger than the first driving frequency fDs (NO in S6), the control unit 10 performs frequency modulation control (S11), turns on and off the first leg (switching elements Q1, Q2) at a driving frequency f and an on-duty ratio of 50%, turns off the second leg (switching elements Q3, Q4), and turns on the switching element Q5 and the switching element Q6 of the first bidirectional switch 5. That is, it operates with an on-duty ratio D = 0 in on-PWM control, or an off-duty ratio *D = 0 in off-PWM control.

[0080] Also in step S11, the control unit 10 performs synchronous rectification control, turns on and off the third leg (switching elements Q7, Q8) and the fourth leg (switching elements Q9, Q10) in synchronization with the first leg (switching elements Q1, Q2), and turns off the switching element Q11 and the switching element Q12 of the second bidirectional switch 9.

[0081] When the driving frequency f is equal to or higher than the second driving frequency f*Ds (YES in S5), the control unit 10 compares the driving frequency f with the driving frequency f*Dh read in step S3 (S12). If the driving frequency f is not equal to or higher than the driving frequency f*Dh (NO in S12), the control unit 10 calculates the off duty *D by off PWM control based on Equation (3) (S13). If the driving frequency f is equal to or higher than the driving frequency f*Dh (YES in S12), the off duty *D is fixed at the maximum off duty *Dh (S14), and the process proceeds to step S15.

[0082] In step S15, the control unit 10 performs off PWM control, turns on and off the first leg (switching elements Q1, Q2) at the driving frequency f and an on duty of 50%, turns off the second leg (switching elements Q3, Q4), and turns off the first bidirectional switch 5 (switching elements Q5, Q6) in synchronization with the on timing of the first leg during the dead time period and the period of the off duty *D.

[0083] Also in step S15, the control unit 10 performs synchronous rectification control, turns on and off the third leg (switching elements Q7, Q8) and the fourth leg (switching elements Q9, Q10) in synchronization with the first leg (switching elements Q1, Q2), and turns off the switching elements Q11 and Q12 of the second bidirectional switch 9.

[0084] Next, the control unit 10 determines whether to continue the on / off control (S16). For example, if there is an end command from a higher-level device, it determines not to continue the control (NO in S16) and ends the on / off control (S17). On the other hand, if there is no end command from a higher-level device, for example, the control unit 10 determines to continue the control (YES in S16) and repeats the processing after step S2.

[0085] As described above, in the bidirectional DC / DC converter 11, the control unit 10 performs on-PWM control during the period when the resonance current ILr flows, so that the period during which the voltage Vab is applied to the first resonance circuit 2 can be divided into a period in which Vab = V1 and a period in which Vab = (1 / 2)·V1. During the period when Vab = V1 (the period of the on-duty D), the resonance current ILr increases significantly, and during the period when Vab = (1 / 2)·V1, the increase in the resonance current ILr is suppressed. That is, by increasing or decreasing the on-duty D of the on-PWM control, the resonance current ILr increases or decreases. Therefore, in the bidirectional DC / DC converter 11, in addition to the change in the gain of the LLC due to the change in the drive frequency f, the output can be increased or decreased by changing the on-duty D of the on-PWM control.

[0086] Also, the control unit 10 performs off-PWM control to turn off the second leg (switching elements Q3, Q4), and at the same time, turns off the first bidirectional switch 5 (switching elements Q5, Q6) for a period of off-duty *D in synchronization with the on-timing of the first leg, so that the voltage Vab can be set to zero during the period of off-duty *D. During the period when the voltage Vab is zero, the resonance current ILr does not occur. That is, by increasing or decreasing the off-duty *D of the off-PWM control, the period during which the generation of the resonance current ILr is suppressed increases or decreases (as a result, the resonance current ILr increases or decreases). Therefore, in the bidirectional DC / DC converter 11, in addition to the change in the gain of the LLC due to the change in the drive frequency f, the output can be increased or decreased by changing the off-duty *D of the off-PWM control.

[0087] From these facts, according to the bidirectional DC / DC converter 11, it is possible to correspond to a wide range of output voltages V2 within a narrow drive frequency range, and at the same time, it is possible to correspond to soft switching, so that the generation of switching noise is suppressed, high-speed switching is possible, and miniaturization and cost reduction of the power supply device are possible.

[0088] Furthermore, in the bidirectional DC / DC converter 11, since on-PWM control is performed by modulating the on-duty D as a function of the driving frequency f, the control unit becomes only the driving frequency f, simplifying the control. As a result, as the driving frequency f decreases, the gain of the LLC increases. Therefore, as described above, in addition to increasing the on-duty D of the on-PWM control, the gain of the LLC can be increased. Similarly, in the bidirectional DC / DC converter 11, since off-PWM control is performed by modulating the off-duty *D as a function of the driving frequency f, the control unit becomes only the driving frequency f, simplifying the control. As a result, as the driving frequency f increases, the gain of the LLC decreases. Therefore, in addition to increasing the off-duty *D of the off-PWM control, the gain of the LLC can be decreased.

[0089] Note that although the above description was about forward power transmission, the same applies to reverse power transmission. The control unit 10 performs the control that was performed on the first full-bridge circuit 3 during forward power transmission on the second full-bridge circuit 7 during reverse power transmission, performs the control that was performed on the first bidirectional switch 5 during forward power transmission on the second bidirectional switch 9 during reverse power transmission, performs the control that was performed on the second full-bridge circuit 7 during forward power transmission on the first full-bridge circuit 3 during reverse power transmission, and performs the control that was performed on the second bidirectional switch 9 during forward power transmission on the first bidirectional switch 5 during reverse power transmission. The on-PWM control during reverse power transmission corresponds to the "second on-PWM control" of the present invention, and the off-PWM control during reverse power transmission corresponds to the "second off-PWM control" of the present invention.

[0090] As described above, the embodiments of the DC / DC converter according to the present invention have been described, but the present invention is not limited to the above embodiments.

[0091] For example, in the above embodiment, when the drive frequency f is greater than the first drive frequency fDs and less than the second drive frequency f*Ds (fDs < f < f*Ds), the control unit 10 performs frequency modulation control to control the output gain of the LLC based on the drive frequency f. However, a configuration that does not perform frequency modulation control may also be used. For example, with the first drive frequency fDs = the second drive frequency f*Ds, when f < fDs (= f*Ds), the control unit 10 may perform the same on-PWM control as in the above embodiment, and when f ≥ fDs (= f*Ds), the control unit 10 may perform the same off-PWM control as in the above embodiment.

[0092] In the above embodiment, the control unit 10 performs synchronous rectification control on the second full-bridge circuit 7, but may perform diode rectification control instead.

[0093] In the above embodiment, the control unit 10 performs on-PWM control in which the on-duty D is modulated as a function of the drive frequency f. However, the on-PWM control of the present invention can be appropriately changed as long as, during forward power transmission, the first leg is turned on and off at an on-duty of 50%, the second leg is turned on and off at an on-duty less than 50% in synchronization with the on and off of the first leg, and the first bidirectional switch 5 is turned off and on in synchronization with the on and off of the second leg to increase the resonance current ILr during the on-duty period D when the second leg is on.

[0094] Similarly, in the above embodiment, the control unit 10 performs off-PWM control in which the off-duty *D is modulated as a function of the drive frequency f. However, the off-PWM control of the present invention can be appropriately changed as long as, during forward power transmission, the first leg is turned on and off at an on-duty of 50%, the second leg is turned off, and the first bidirectional switch 5 is turned off in synchronization with the on of the first leg to suppress the generation of the resonance current ILr during the off-duty period when the first bidirectional switch 5 is off.

[0095] In the above embodiment, the control unit 10 determines the control mode based on the driving frequency f determined by the frequency modulation control, but it is not limited thereto. For example, the input / output voltage ratio (V1 / V2) may be used instead of the driving frequency f. For example, when the input / output voltage ratio (V1 / V2) is equal to or higher than a predetermined threshold value (a predetermined constant voltage ratio), the on-PWM control of the present invention is performed, and when the input / output voltage ratio (V1 / V2) is smaller than the above threshold value, the off-PWM control of the present invention may be performed. Alternatively, when the input / output voltage ratio (V1 / V2) is equal to or lower than the first threshold value, the on-PWM control of the present invention is performed, when the input / output voltage ratio (V1 / V2) is equal to or higher than the second threshold value that is larger than the first threshold value, the off-PWM control of the present invention is performed, and when the input / output voltage ratio (V1 / V2) is larger than the first threshold value and smaller than the second threshold value, the frequency modulation control may be performed.

[0096] Also, in the above embodiment, the description is made on the premise of the bidirectional DC / DC converter 11 shown in FIG. 1. However, regarding the forward power transmission, since the second resonance circuit 6, the second bidirectional switch 9, and the second voltage dividing circuit 8 have no influence, it may be a unidirectional DC / DC converter in which the second resonance circuit 6, the second bidirectional switch 9, and the second voltage dividing circuit 8 are not provided in the main circuit section.

[0097] Also, in the above embodiment, as shown in FIG. 9, on-PWM control and off-PWM control were combined, but it is not necessarily required to be combined. Either on-PWM control or off-PWM control alone may be used, or it may be combined with another control method. For example, in addition to on-PWM control and frequency modulation control, instead of off-PWM control, phase shift control of the drive-side full-bridge circuit may be combined. In this case, for frequencies equal to or higher than the second drive frequency, phase shift control may be performed to shift the first leg and the second leg of the first full-bridge circuit 3 by a predetermined phase shift amount with a duty of 50%, turn on the first bidirectional switch 5, and control to increase the phase shift amount as the drive frequency increases. Also, during reverse power transmission, similarly, it is not necessarily required to combine the second on-PWM control and the second off-PWM control. Either the second on-PWM control or the second off-PWM control alone may be used, or it may be combined with another control method.

Explanation of Reference Numerals

[0098] 1 Transformer circuit 2 First resonance circuit 3 First full-bridge circuit 4 First voltage dividing circuit 5 First bidirectional switch 6 Second resonance circuit 7 Second full-bridge circuit 8 Second voltage dividing circuit 9 Second bidirectional switch 10 Control unit 11 Bidirectional DC / DC converter

Claims

1. Comprising a main circuit section and a control section, The main circuit section A first input / output terminal and a second input / output terminal, A first voltage dividing circuit composed of two first capacitor sections that divide the voltage of the first input / output terminal, A first full-bridge circuit comprising a first leg and a second leg connected in parallel, each leg having an upper arm and a lower arm including a switching element, and both ends of each leg being connected to the first input / output terminal, A first resonant circuit comprising a first resonant inductor and a first resonant capacitor, with one end side connected to a first connection point that is the connection point of the upper and lower arms of the first leg and a second connection point that is the connection point of the upper and lower arms of the second leg, A first bidirectional switch provided between the connection point of the two first capacitor sections and the second connection point, An isolation transformer comprising a primary winding and a secondary winding, with both ends of the primary winding connected to the other end side of the first resonant circuit, A second full-bridge circuit comprising a third leg and a fourth leg connected in parallel, each leg having an upper arm and a lower arm including a switching element, and both ends of each leg being connected to the second input / output terminal, A DC / DC converter comprising The main circuit section Under the control of the control section, the first full-bridge circuit operates as a drive circuit, and the second full-bridge circuit operates as a rectifier circuit to perform forward power transmission from the first input / output terminal to the second input / output terminal, During the forward power transmission, the control section Turns on and off the first leg with a duty ratio of 50%, turns on and off the second leg with a duty ratio less than 50% synchronously with the on and off of the first leg, and turns on and off the first bidirectional switch synchronously with the on and off of the second leg to increase the resonant current flowing through the first resonant circuit during the duty period when the second leg is on, i.e., on PWM control, Turns on and off the first leg with a duty ratio of 50%, turns off the second leg, and turns off the first bidirectional switch synchronously with the on of the first leg to suppress the generation of the resonant current during the off-duty period when the first bidirectional switch is off, i.e., off PWM control, and performs at least one of these controls A DC / DC converter characterized by this.

2. During the forward power transmission, the control section When the input / output voltage ratio between the voltage of the first input / output terminal and the voltage of the second input / output terminal is equal to or higher than a certain voltage ratio, or when the driving frequency of the driving circuit is lower than a certain frequency, the on-PWM control is performed, while when the input / output voltage ratio is lower than the certain voltage ratio, or when the driving frequency is equal to or higher than the certain frequency, the off-PWM control is performed The DC / DC converter according to claim 1, characterized in that.

3. During the on-PWM control, the control unit when the driving frequency becomes equal to or lower than a predetermined first driving frequency, increases the on-duty period in accordance with the decrease in the driving frequency, and when the on-duty period reaches a predetermined maximum on-duty value, fixes the on-duty period at the maximum on-duty value The DC / DC converter according to claim 2, characterized in that.

4. The on-duty of the on-duty period is a value obtained by multiplying a difference obtained by subtracting the driving frequency from the first driving frequency by a predetermined gain The DC / DC converter according to claim 3, characterized in that.

5. The control unit in accordance with the input / output voltage ratio, increases the gain when the input / output voltage ratio is high, and decreases the gain when the input / output voltage ratio is low The DC / DC converter according to claim 4, characterized in that.

6. The control unit when the driving frequency becomes equal to or higher than a predetermined second driving frequency, increases the off-duty period during the off-PWM control in accordance with the increase in the driving frequency, and when the off-duty period reaches a predetermined maximum off-duty value, fixes the off-duty period at the maximum off-duty value The DC / DC converter according to claim 2, characterized in that.

7. The off-duty of the off-duty period is a value obtained by multiplying a difference obtained by subtracting the second driving frequency from the driving frequency by a predetermined gain The DC / DC converter according to claim 6, characterized in that.

8. The main circuit unit a second voltage dividing circuit composed of two second capacitor units that divide the voltage of the second input / output terminal, A second resonant circuit including a second resonant inductor and a second resonant capacitor, having one end connected to a third connection point which is a connection point between the upper and lower arms of the third leg and a fourth connection point which is a connection point between the upper and lower arms of the fourth leg, and having the other end connected to both ends of the secondary winding; A second bidirectional switch provided between a connection point of the two second capacitor portions and the fourth connection point; and further comprising: Under the control of the control unit, the second full-bridge circuit operates as the drive circuit, and the first full-bridge circuit operates as the rectifier circuit to perform reverse power transmission from the second input / output terminal to the first input / output terminal; When the on PWM control during the forward power transmission is defined as the first on PWM control and the off PWM control during the forward power transmission is defined as the first off PWM control; During the reverse power transmission, the control unit: A second on PWM control for turning on and off the third leg with a duty ratio of 50%, turning on and off the fourth leg with a duty ratio less than 50% in synchronization with the on and off of the third leg, and turning on and off the second bidirectional switch in synchronization with the on and off of the fourth leg to increase the resonant current flowing through the second resonant circuit during the duty period when the fourth leg is on; A second off PWM control for turning on and off the third leg with a duty ratio of 50%, turning off the fourth leg, and turning off the second bidirectional switch in synchronization with the on of the third leg to suppress the generation of the resonant current during the off duty period when the second bidirectional switch is off; and performing at least one of the controls; The DC / DC converter according to claim 1, characterized in that.

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