DC-DC converter and vehicle

Abstract

To efficiently convert electric power.SOLUTION: A disclosed DC-DC converter includes a DC-AC conversion circuit, a transformer, a rectifier circuit, a smoothing circuit, and a control circuit. The DC-AC conversion circuit converts a DC input voltage into a primary AC voltage by switching the positive and the negative. The transformer generates a secondary AC voltage using the primary AC voltage applied. The rectifier circuit outputs a rectified voltage obtained by full-wave rectification of secondary AC voltage. The control circuit is configured to control the rectifier circuit to rectify with a diode so that, in the recycling period when the electric power of the DC input voltage is not transferred from the DC-AC conversion circuit to the transformer, the current does not flow through the rectifier circuit to a second intermediate terminal opposite to a first intermediate terminal connected to the smoothing circuit from the first intermediate terminal, which is one of the intermediate terminals connected to the smoothing circuit and the current flows from the second intermediate terminal to the first intermediate terminal via the rectifier circuit.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a DC-DC converter and a vehicle. [Background technology]

[0002] An insulating DC-DC converter using a transformer is known. Such a DC-DC converter is used, for example, in an on-board charger for charging the power of an on-board lithium-ion battery to a lead-acid battery. The DC-DC converter used in the on-board charger converts, for example, a DC voltage of about 360 V of the lithium-ion battery to a DC voltage of about 14 V. Patent Document 1 describes a DC-DC converter using a transformer.

[0003] The DC-DC converter adjusts the ratio between a transfer period during which power from the input side is transferred to the output side and a reflux period during which power from the input side is not transferred to the output side, depending on the deviation of the output voltage or output current from the target voltage or target current. That is, the DC-DC converter increases the ratio of the transfer period when the deviation is negative, and increases the ratio of the reflux period when the deviation is positive.

[0004] For example, DC-DC converters used in on-board chargers often use a switch element on the secondary side of the transformer to perform synchronous rectification. A synchronous rectification DC-DC converter turns on the switch element during the period when current flows, so it can reduce conduction loss compared to a diode rectification method that uses only a diode on the secondary side. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2019-161992 Summary of the Invention [Problem to be solved by the invention]

[0006] In addition, in a full-bridge synchronous rectification DC-DC converter, during the return period, all of the MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) constituting the secondary-side full-bridge switch elements are turned on to form an internal closed circuit between the smoothing circuit in the output stage and the secondary-side full-bridge switch elements. As a result, during the return period, the full-bridge synchronous rectification DC-DC converter passes a return current through the internal closed circuit, thereby keeping the output current supplied from the smoothing circuit to the load constant.

[0007] In a full-bridge synchronous rectification DC-DC converter, when the load is light, that is, when the output current output from the smoothing circuit to the outside is small, a return current flows in the reverse direction from the smoothing circuit to the secondary side full-bridge switch element during the return period. In a full-bridge DC-DC converter, the drain of each MOSFET constituting the full bridge is connected to the high potential side of the smoothing circuit, and the source is connected to the low potential side of the smoothing circuit. Therefore, when the load is light, a current flows in the direction from the drain to the source in each MOSFET constituting the full bridge.

[0008] However, when each MOSFET that configures the full bridge is switched while a current is flowing from the drain to the source, hard switching occurs, which generates large switching losses. Furthermore, when such switching occurs, the MOSFET is severely damaged.

[0009] To eliminate switching losses due to such hard switching, conventional full-bridge DC-DC converters turn off all MOSFETs in advance and perform diode rectification using body diodes instead of synchronous rectification when the load is light. This prevents reverse reflux current through the body diodes, eliminating hard switching of the MOSFETs.

[0010] However, in a conventional full-bridge DC-DC converter, when all MOSFETs are turned off and diode rectification is performed, the current flows through two body diodes connected in series, resulting in a large loss. Therefore, conventional DC-DC converters cannot efficiently convert current under light load conditions.

[0011] An object of the present disclosure is to provide a DC-DC converter and a vehicle capable of efficiently converting power. [Means for solving the problem]

[0012] The DC-DC converter according to the present disclosure includes a DC-AC conversion circuit, a transformer, a rectifier circuit, a smoothing circuit, and a control circuit. The DC-AC conversion circuit converts a DC input voltage into a primary AC voltage by switching to invert the DC input voltage between positive and negative. The transformer includes a primary coil to which the primary AC voltage is applied and a secondary coil magnetically coupled to the primary coil, and generates a secondary AC voltage in the secondary coil. The rectifier circuit outputs a rectified voltage obtained by full-wave rectifying the secondary AC voltage. The smoothing circuit outputs a DC output voltage obtained by smoothing the rectified voltage. The control circuit controls the operation of the DC-AC conversion circuit and the rectifier circuit. During a reflux period when power of the input voltage is not being transmitted from the DC-AC conversion circuit to the transformer, the control circuit rectifies the rectifier circuit using a diode so that current does not flow from a first intermediate terminal, which is one of the intermediate terminals connected to the smoothing circuit, to a second intermediate terminal opposite to the first intermediate terminal connected to the smoothing circuit, via the rectifier circuit, and current flows from the second intermediate terminal to the first intermediate terminal via the rectifier circuit. The control circuit switches the connections between the secondary coil and the first intermediate terminal and between the secondary coil and the second intermediate terminal in the rectifier circuit in synchronization with switching of the DC-AC conversion circuit so that the positive side of the secondary AC voltage is applied to the first intermediate terminal and the negative side of the secondary AC voltage is applied to the second intermediate terminal during a transfer period in which power of the input voltage is transferred from the DC-AC conversion circuit to the transformer. The rectifier circuit has a first transformer output terminal connected to one terminal of the secondary coil and a first switch element connected between the first intermediate terminal, a second switch element connected between the second intermediate terminal and the first transformer output terminal, a third switch element connected between a second transformer output terminal connected to the side of the secondary coil opposite to the terminal to which the first transformer output terminal is connected and the first intermediate terminal, and a fourth switch element connected between the second intermediate terminal and the second transformer output terminal. Each of the first switch element, the second switch element, the third switch element, and the fourth switch element is conductive when turned on, and functions as a diode when turned off. The first switch element has an anode of the diode connected to the first transformer output terminal and a cathode of the diode connected to the first intermediate terminal. The second switch element has an anode of the diode connected to the second intermediate terminal and a cathode of the diode connected to the first transformer output terminal. The third switch element has an anode of the diode connected to the second transformer output terminal and a cathode of the diode connected to the first intermediate terminal. The fourth switch element has an anode of the diode connected to the second intermediate terminal and a cathode of the diode connected to the second transformer output terminal. When set to the first mode, the control circuit turns on the first switch element, the second switch element, the third switch element, and the fourth switch element during the freewheeling period.When the second mode is set, during the freewheeling period, the control circuit turns off the first switch element and the third switch element and turns on at least one of the second switch element and the fourth switch element, or turns off the second switch element and the fourth switch element and turns on at least one of the first switch element and the third switch element. The control circuit sets the first mode when an output current output from the smoothing circuit is larger than a preset value, and sets the second mode when the output current is not larger than the set value.

[0013] A vehicle according to the present disclosure includes the DC-DC converter. Effect of the Invention

[0014] The DC-DC converter and vehicle according to the present disclosure can perform power conversion efficiently. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram showing a configuration of a DC-DC converter according to an embodiment. [Diagram 2] FIG. 2 is a diagram showing a configuration of a DC-AC conversion circuit and a control signal supplied from a control circuit to the DC-AC conversion circuit. [Diagram 3] FIG. 3 is a timing chart of the control signals supplied to the DC-AC conversion circuit. [Figure 4] FIG. 4 is a diagram showing the configuration of a rectifier circuit and a control signal supplied from a control circuit to a DC-AC conversion circuit. [Diagram 5] FIG. 5 is a diagram illustrating the switching state of the rectifier circuit in the first transfer period. [Figure 6] FIG. 6 is a diagram illustrating the switching state of the rectifier circuit in the second transfer period. [Figure 7] FIG. 7 is a diagram showing a first example of the switching state of the rectifier circuit during the freewheeling period. [Figure 8] FIG. 8 is a diagram showing a second example of the switching state of the rectifier circuit during the freewheeling period. [Figure 9] FIG. 9 is a diagram showing a third example of the switching state of the rectifier circuit during the freewheeling period. [Figure 10] FIG. 10 is a diagram showing a fourth example of the switching state of the rectifier circuit during the freewheeling period. [Figure 11] FIG. 11 is a diagram showing a fifth example of the switching state of the rectifier circuit during the freewheeling period. [Figure 12] FIG. 12 is a diagram showing a sixth example of the switching state of the rectifier circuit during the freewheeling period. [Figure 13] FIG. 13 is a schematic waveform diagram of voltage and current when synchronous switching is performed during the transfer period and all switch elements are turned on during the freewheel period in a state in which a heavy load is connected. [Figure 14] FIG. 14 is a schematic waveform diagram of voltage and current when synchronous switching is performed in the transfer period and all switch elements are turned on in the freewheel period with a light load connected. [Figure 15] FIG. 15 is a schematic waveform diagram of the voltage and current of the DC-DC converter according to this embodiment when a light load is connected. [Figure 16] FIG. 16 is a flowchart showing the mode setting process. [Figure 17] FIG. 17 is a timing chart of the control signals in the first mode. [Figure 18] FIG. 18 is a timing chart of the control signals in the second mode. [Figure 19] FIG. 19 is a diagram showing the waveform of a reference signal for generating a control signal on the secondary side. [Figure 20] FIG. 20 is a diagram showing the configuration of a logic circuit for generating a control signal on the secondary side. [Figure 21] FIG. 21 is a diagram showing the configuration of a vehicle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of a DC-DC converter 10 according to the present disclosure will be described with reference to the drawings.

[0017] 1 is a diagram showing the configuration of a DC-DC converter 10 according to a first embodiment. The DC-DC converter 10 receives a DC input voltage V I and receives the DC input voltage V I The DC output voltage V O and output voltage V O In this embodiment, the DC-DC converter 10 is a power conversion device that supplies a DC input voltage V I The DC output voltage V O The DC-DC converter 10 outputs a DC input voltage V I The boosted DC output voltage VO may be output.

[0018] The DC-DC converter 10 includes a first input terminal 22, a second input terminal 24, a first output terminal 26, a second output terminal 28, a DC-AC conversion circuit 32, a transformer 34, a rectifier circuit 36, a smoothing circuit 38, and a control circuit 40.

[0019] The DC-DC converter 10 receives a DC input voltage V between a first input terminal 22 and a second input terminal 24 from a preceding device. I A higher voltage is applied to the first input terminal 22 than to the second input terminal 24. Each of the first input terminal 22 and the second input terminal 24 may be a cable or wiring that connects to a device in a previous stage.

[0020] The DC-DC converter 10 outputs a DC output voltage V O A higher voltage is generated at the first output terminal 26 than at the second output terminal 28. Each of the first output terminal 26 and the second output terminal 28 may be a cable or wiring that connects to a downstream device.

[0021] The DC-AC conversion circuit 32 converts a DC input voltage V applied between the first input terminal 22 and the second input terminal 24 into I The input voltage is converted into a primary side AC voltage by switching to invert the positive and negative of the input voltage. Then, the DC-AC conversion circuit 32 outputs the primary side AC voltage between the first AC terminal 42 and the second AC terminal 44. The waveform of the primary side AC voltage may be a pulse waveform.

[0022] The DC-AC conversion circuit 32 is a full-bridge converter. When the DC-AC conversion circuit 32 is a full-bridge converter, the DC-AC conversion circuit 32 includes a conversion capacitor 46, a first conversion switch 52, a second conversion switch 54, a third conversion switch 56, and a fourth conversion switch 58. The first conversion switch 52, the second conversion switch 54, the third conversion switch 56, and the fourth conversion switch 58 are semiconductor elements such as MOSFETs that switch the power line between conductive and non-conductive in response to control from the control circuit 40.

[0023] The conversion capacitor 46 is connected between the first input terminal 22 and the second input terminal 24. The conversion capacitor 46 may be provided outside the input side of the DC-AC conversion circuit 32.

[0024] The first conversion switch 52 is connected between the first input terminal 22 and the first AC terminal 42. The second conversion switch 54 is connected between the second input terminal 24 and the first AC terminal 42. The third conversion switch 56 is connected between the first input terminal 22 and the second AC terminal 44. The fourth conversion switch 58 is connected between the second input terminal 24 and the second AC terminal 44. The first conversion switch 52, the second conversion switch 54, the third conversion switch 56 and the fourth conversion switch 58 are switched and controlled by the control circuit 40 so that a primary side AC voltage is generated between the first AC terminal 42 and the second AC terminal 44. As a result, the DC-AC conversion circuit 32 converts the DC input voltage V I can be converted into a primary side AC voltage.

[0025] The transformer 34 includes a primary coil 60 and a secondary coil 62 .

[0026] The primary coil 60 is connected between the first AC terminal 42 and the second AC terminal 44. The secondary coil 62 is magnetically coupled to the primary coil 60. The secondary coil 62 outputs a secondary AC voltage based on the primary AC voltage. Therefore, when the primary AC voltage is applied to the primary coil 60, the transformer 34 generates a secondary AC voltage in the secondary coil 62.

[0027] The transformer 34 also includes a first transformer output terminal 64 and a second transformer output terminal 66. The first transformer output terminal 64 is connected to one terminal of the secondary coil 62. The second transformer output terminal 66 is connected to a terminal of the secondary coil 62 opposite to the terminal to which the first transformer output terminal 64 is connected. Thus, the transformer 34 generates a secondary AC voltage between the first transformer output terminal 64 and the second transformer output terminal 66.

[0028] The rectifier circuit 36 ​​full-wave rectifies the secondary AC voltage output from the secondary coil 62. Then, the rectifier circuit 36 ​​outputs a rectified voltage obtained by full-wave rectifying the secondary AC voltage output from the secondary coil 62. The rectifier circuit 36 ​​outputs the rectified voltage between the first intermediate terminal 68 and the second intermediate terminal 70.

[0029] The rectifier circuit 36 ​​performs synchronous full-wave rectification of a full-bridge system. More specifically, the rectifier circuit 36 ​​includes a first switch element 72, a second switch element 74, a third switch element 76, and a fourth switch element 78. The first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 are, for example, semiconductor elements that are switched on or off in response to control from the control circuit 40.

[0030] Each of the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 conducts the power line when turned on. Also, each of the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 functions as a diode when turned off.

[0031] In this embodiment, the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 are MOSFETs. The MOSFETs include a body diode whose forward direction is from the source to the drain. That is, the MOSFET functions as a switch that makes the source and drain conductive or non-conductive, and when turned off, functions as a diode with the anode connected to the source and the cathode connected to the drain. Therefore, when the MOSFET is turned off, if the source potential is higher than the drain potential, the MOSFET is conductive and allows current to flow, and when the source potential is lower than the drain potential, the MOSFET is non-conductive and does not allow current to flow.

[0032] The first switch element 72 is connected between the first transformer output terminal 64 and the first intermediate terminal 68. If the first switch element 72 is a MOSFET, the source of the first switch element 72 is connected to the first transformer output terminal 64, and the drain of the first switch element 72 is connected to the first intermediate terminal 68. Therefore, the first switch element 72, which is a MOSFET, has an anode of the body diode connected to the first transformer output terminal 64, and a cathode of the body diode connected to the first intermediate terminal 68.

[0033] The second switch element 74 is connected between the second intermediate terminal 70 and the first transformer output terminal 64. If the second switch element 74 is a MOSFET, the source of the second switch element 74 is connected to the second intermediate terminal 70 and the drain of the second switch element 74 is connected to the first transformer output terminal 64. Therefore, the second switch element 74, which is a MOSFET, has an anode of the body diode connected to the second intermediate terminal 70 and a cathode of the body diode connected to the first transformer output terminal 64.

[0034] The third switch element 76 is connected between the second transformer output terminal 66 and the first intermediate terminal 68. If the third switch element 76 is a MOSFET, the source of the third switch element 76 is connected to the second transformer output terminal 66, and the drain of the third switch element 76 is connected to the first intermediate terminal 68. Therefore, the third switch element 76, which is a MOSFET, has an anode of the body diode connected to the second transformer output terminal 66, and a cathode of the body diode connected to the first intermediate terminal 68.

[0035] The fourth switch element 78 is connected between the second intermediate terminal 70 and the second transformer output terminal 66. If the fourth switch element 78 is a MOSFET, the source of the fourth switch element 78 is connected to the second intermediate terminal 70 and the drain of the fourth switch element 78 is connected to the second transformer output terminal 66. Therefore, the fourth switch element 78, which is a MOSFET, has an anode of the body diode connected to the second intermediate terminal 70 and a cathode of the body diode connected to the second transformer output terminal 66.

[0036] Furthermore, when each of the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 is a MOSFET, a control signal from the control circuit 40 is applied to the gate. Therefore, the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 are switched on or off by the control signal from the control circuit 40.

[0037] The smoothing circuit 38 smoothes the rectified voltage output from the rectifier circuit 36. For example, the smoothing circuit 38 may be an LC type low-pass filter including a smoothing inductor 80 and a smoothing capacitor 82. In this case, the smoothing inductor 80 and the smoothing capacitor 82 are connected in series between the first intermediate terminal 68 and the second intermediate terminal 70. In this case, the smoothing circuit 38 outputs the voltage across the smoothing capacitor 82 as a voltage obtained by smoothing the rectified voltage.

[0038] The first output terminal 26 is connected to a terminal of the smoothing capacitor 82 included in the smoothing circuit 38 that is connected to the smoothing inductor 80. Also, for example, the second output terminal 28 is connected to a terminal of the smoothing capacitor 82 included in the smoothing circuit 38 that is not connected to the smoothing inductor 80. The first output terminal 26 and the second output terminal 28 output a DC output voltage V O Output.

[0039] The control circuit 40 includes, for example, a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM). The control circuit 40 executes processing based on a program set in advance. For example, the control circuit 40 generates a control signal by cooperation between a processor such as a CPU and a program (software) stored in a ROM or the like. Note that the function of the control circuit 40 is not limited to being realized by software, and may be a circuit including a logic circuit or the like, or may be realized by a hardware configuration such as a dedicated circuit.

[0040] The control circuit 40 controls the output voltage V O or the measured output voltage V O The output current I supplied to the load to which O The control circuit 40 obtains a measurement value of the output voltage V O The deviation between the measured value and the target voltage or output current I O The control circuit 40 then calculates the deviation between the measured current and the target current. The control circuit 40 then controls the ratio between the transmission period and the return period so as to reduce the calculated deviation.

[0041] For example, the control circuit 40 controls the switching of the DC-AC conversion circuit 32 so as to change the ratio between the transmission period and the reflux period in accordance with the calculated deviation. For example, the control circuit 40 controls the on and off of the first conversion switch 52, the second conversion switch 54, the third conversion switch 56, and the fourth conversion switch 58 in accordance with the output voltage V OThe deviation between the measured value and the target voltage or output current I O The control is performed according to the deviation between the measured value and the target current.

[0042] 2 is a diagram showing a configuration of the DC-AC conversion circuit 32 and control signals supplied from the control circuit 40 to the DC-AC conversion circuit 32. In this embodiment, the control circuit 40 supplies to the DC-AC conversion circuit 32 a control signal (A) for turning on / off the first conversion switch 52, a control signal (B) for turning on / off the second conversion switch 54, a control signal (C) for turning on / off the third conversion switch 56, and a control signal (D) for turning on / off the fourth conversion switch 58.

[0043] FIG. 3 is a timing chart of a control signal supplied to the DC-AC conversion circuit 32. More specifically, FIG. 3 is a timing chart of a control method of a phase-shift full-bridge converter in which the first conversion switch 52 and the second conversion switch 54 are a lag leg and the third conversion switch 56 and the fourth conversion switch 58 are a lead leg. In FIG. 3, the on-duty of the first conversion switch 52, the second conversion switch 54, the third conversion switch 56, and the fourth conversion switch 58 is 0.5, but a short period (dead time) in which both the first conversion switch 52 and the second conversion switch 54 are off may be provided so that the first conversion switch 52 and the second conversion switch 54 are not simultaneously turned on during switching between the transmission period and the reflux period, causing a power supply short circuit. In that case, the on-duty is set to 0.47, for example. A dead time may also be provided for the third conversion switch 56 and the fourth conversion switch 58.

[0044] The control circuit 40 controls the first conversion switch 52, the second conversion switch 54, the third conversion switch 56 and the fourth conversion switch 58 to sequentially switch between the first transmission period, the first freewheeling period, the second transmission period and the second freewheeling period.

[0045] Each of the first and second transmission periods is a DC input voltage V I This is a transfer period during which power is transferred from the DC-AC conversion circuit 32 to the transformer 34.

[0046] The first transmission period is a period during which a voltage of a first polarity, either positive or negative, is applied to the primary coil 60 of the transformer 34 to generate a positive secondary AC voltage from the secondary coil 62 of the transformer 34. In other words, the first transmission period is a period during which a voltage higher than that of the second transformer output terminal 66 is generated from the first transformer output terminal 64.

[0047] The second transmission period is a period during which a voltage of a second polarity, either positive or negative, opposite to the first polarity is applied to the primary coil 60 of the transformer 34, and a negative secondary AC voltage is generated from the secondary coil 62 of the transformer 34. In other words, the second transmission period is a period during which a voltage higher than that of the first transformer output terminal 64 is generated from the second transformer output terminal 66.

[0048] In the first transmission period, the control circuit 40 turns on the first conversion switch 52 and the fourth conversion switch 58 by setting the control signal (A) and the control signal (D) to H logic, and turns off the second conversion switch 54 and the third conversion switch 56 by setting the control signal (B) and the control signal (C) to L logic. This allows the control circuit 40 to connect the first input terminal 22 to the first AC terminal 42 of the transformer 34 and connect the second input terminal 24 to the second AC terminal 44 of the transformer 34 in the first transmission period.

[0049] In the second transmission period, the control circuit 40 turns on the second conversion switch 54 and the third conversion switch 56 by setting the control signal (B) and the control signal (C) to H logic, and turns off the first conversion switch 52 and the fourth conversion switch 58 by setting the control signal (A) and the control signal (D) to L logic. This allows the control circuit 40 to connect the first input terminal 22 to the second AC terminal 44 of the transformer 34 and connect the second input terminal 24 to the first AC terminal 42 of the transformer 34 in the second transmission period.

[0050] The first and second freewheeling periods are each controlled by a DC input voltage V IThis is a reflux period during which no power is transferred from the DC-AC conversion circuit 32 to the transformer 34.

[0051] During the return period, the control circuit 40 switches the DC-AC conversion circuit 32 so that power is not supplied from the DC-AC conversion circuit 32 to the transformer 34 and a closed circuit is formed including the primary coil 60. This allows the control circuit 40 to hold the energy stored in the primary coil 60 in the closed circuit during the return period.

[0052] In the first return period, the control circuit 40, for example, sets the control signal (A) and the control signal (C) to logic H to turn on the first conversion switch 52 and the third conversion switch 56, and sets the control signal (B) and the control signal (D) to logic L to turn off the second conversion switch 54 and the fourth conversion switch 58. In this way, the control circuit 40 can form a closed circuit including the first conversion switch 52, the third conversion switch 56, and the primary coil 60.

[0053] In the second return period, the control circuit 40, for example, sets the control signal (B) and the control signal (D) to logic H to turn on the second conversion switch 54 and the fourth conversion switch 58, and sets the control signal (A) and the control signal (C) to logic L to turn off the first conversion switch 52 and the third conversion switch 56. In this way, the control circuit 40 can form a closed circuit including the second conversion switch 54, the fourth conversion switch 58, and the primary coil 60.

[0054] The control circuit 40 controls the output voltage V O The deviation between the measured value and the target voltage or output current I OThe control circuit 40 controls the ratio of the transmission period to the reflux period according to the deviation between the measured value of the output voltage V and the target current. For example, the control circuit 40 controls the ratio of the transmission period to be larger when the deviation is negative, and to be smaller when the deviation is positive. For example, the control circuit 40 controls the ratio of the transmission period to the reflux period by changing the phase relationship between the A signal (and the B signal) and the C signal (and the D signal). Alternatively, the control circuit 40 may fix the phase relationship and control the ratio of the transmission period to the reflux period by changing the duty ratio of the A signal (and the B signal) or the duty ratio of the C signal (and the D signal). In this way, the control circuit 40 controls the output voltage V O or the output current I O can be set as a target value.

[0055] FIG. 4 is a diagram showing the configuration of the rectifier circuit 36 ​​and a control signal supplied from the control circuit 40 to the rectifier circuit 36. As shown in FIG.

[0056] The control circuit 40 switches the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 included in the rectifier circuit 36 ​​in synchronization with the switching of the switches in the DC-AC conversion circuit 32. That is, the control circuit 40 switches the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 in synchronization with the switching of the first transmission period, the first freewheeling period, the second transmission period, and the second freewheeling period, according to an on / off pattern predetermined for each period.

[0057] In the present embodiment, the control circuit 40 supplies the rectifier circuit 36 ​​with a control signal (E) for turning on / off the first switch element 72, a control signal (F) for turning on / off the second switch element 74, a control signal (G) for turning on / off the third switch element 76, and a control signal (H) for turning on / off the fourth switch element 78. Note that, with respect to each of the control signals (E to F), the voltage that turns on the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 is H logic, and the voltage that turns them off is L logic.

[0058] Here, during the transmission period, the control circuit 40 switches the connections between the secondary coil 62 and the first intermediate terminal 68 and between the secondary coil 62 and the second intermediate terminal 70 in the rectifier circuit 36 ​​in synchronization with the switching of the DC-AC conversion circuit 32, so that the positive side of the secondary AC voltage is applied to the first intermediate terminal 68 and the negative side of the secondary AC voltage is applied to the second intermediate terminal 70. Note that a specific switching pattern of the rectifier circuit 36 ​​during the transmission period will be described later with reference to Figs. 5 and 6.

[0059] In addition, during the freewheeling period, the control circuit 40 causes the rectifier circuit 36 ​​to perform rectification using a diode so that no current flows from the first intermediate terminal 68 to the second intermediate terminal 70 via the rectifier circuit 36, and so that current flows from the second intermediate terminal 70 to the first intermediate terminal 68 via the rectifier circuit 36.

[0060] In this embodiment, the diodes are body diodes included in each of the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78. In this embodiment, the control circuit 40 switches the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 so that, during the freewheeling period, a current flows through only one body diode in the rectifier circuit 36 ​​in the direction from the second intermediate terminal 70 to the first intermediate terminal 68, and a current is blocked in the direction from the first intermediate terminal 68 to the second intermediate terminal 70 by the rectification function of the body diode in the rectifier circuit 36.

[0061] For example, during the freewheeling period, the control circuit 40 turns off the first switch element 72 and the third switch element 76, and turns on at least one of the second switch element 74 and the fourth switch element 78. Alternatively, during the freewheeling period, the control circuit 40 turns off the second switch element 74 and the fourth switch element 78, and turns on at least one of the first switch element 72 and the third switch element 76. Note that specific switching patterns of the rectifier circuit 36 ​​during the freewheeling period will be described later with reference to FIGS. 7 to 12.

[0062] By controlling the rectifier circuit 36 ​​in this manner, the control circuit 40 can eliminate power loss caused by current passing through the diode in the forward direction during the transmission period. Furthermore, the control circuit 40 can reduce power loss due to hard switching by setting the reverse return current during the return period, i.e., the current flowing through the rectifier circuit 36 ​​in the direction from the first intermediate terminal 68 to the second intermediate terminal 70, to zero. Furthermore, the control circuit 40 can reduce the number of body diodes through which the forward return current during the return period, i.e., the current flowing through the rectifier circuit 36 ​​in the direction from the second intermediate terminal 70 to the first intermediate terminal 68, passes, thereby reducing power loss during the return period to one.

[0063] 5 is a diagram showing the switching state of the rectifier circuit 36 ​​in the first transmission period. In the first transmission period, the control circuit 40 sets the control signal (E) and the control signal (H) to H logic to turn on the first switch element 72 and the fourth switch element 78. In addition, in the first transmission period, the control circuit 40 sets the control signal (F) and the control signal (G) to L logic to turn off the second switch element 74 and the third switch element 76.

[0064] During the first transmission period, the transformer 34 generates a voltage higher than that of the second transformer output terminal 66 from the first transformer output terminal 64. Therefore, by performing such switching during the first transmission period, the control circuit 40 can connect the first transformer output terminal 64 to the first intermediate terminal 68 and connect the second transformer output terminal 66 to the second intermediate terminal 70, and apply a rectified voltage obtained by rectifying the secondary side AC voltage between the first intermediate terminal 68 and the second intermediate terminal 70. As a result, the control circuit 40 can pass a current from the second intermediate terminal 70 to the first intermediate terminal 68 via the secondary side coil 62 without passing through a diode, thereby eliminating power loss.

[0065] 6 is a diagram showing the switching state of the rectifier circuit 36 ​​in the second transmission period. In the second transmission period, the control circuit 40 sets the control signal (F) and the control signal (G) to H logic to turn on the second switch element 74 and the third switch element 76. In addition, in the second transmission period, the control circuit 40 sets the control signal (E) and the control signal (H) to L logic to turn off the first switch element 72 and the fourth switch element 78.

[0066] In the second transmission period, the transformer 34 generates a voltage higher than that from the second transformer output terminal 66 to the first intermediate terminal 64. Therefore, by performing such switching in the second transmission period, the control circuit 40 can connect the second transformer output terminal 66 to the first intermediate terminal 68 and connect the first transformer output terminal 64 to the second intermediate terminal 70, and apply a rectified voltage obtained by rectifying the secondary side AC voltage between the first intermediate terminal 68 and the second intermediate terminal 70. This allows the control circuit 40 to pass a current from the first intermediate terminal 68 to the second intermediate terminal 70 via the secondary side coil 62 without passing through a diode, thereby eliminating power loss.

[0067] Fig. 7 is a diagram showing a first example of the switching state of the rectifier circuit 36 ​​during the freewheeling period. During the freewheeling period (for example, the first freewheeling period or the second freewheeling period), as an example, as shown in Fig. 7, the control circuit 40 turns off the first switch element 72, the third switch element 76, and the fourth switch element 78 by setting the control signal (E), the control signal (G), and the control signal (H) to L logic. Also, during the freewheeling period, the control circuit 40 turns on the second switch element 74 by setting the control signal (F) to H logic.

[0068] As a result, during the freewheeling period, the control circuit 40 can cause a forward freewheeling current from the second intermediate terminal 70 to the first intermediate terminal 68 to flow via the body diode of the first switch element 72 and the second switch element 74 in the on state. The control circuit 40 can block a reverse freewheeling current from the first intermediate terminal 68 to the second intermediate terminal 70 by the rectification function of the body diodes of the first switch element 72 and the third switch element 76. By switching in this manner, the control circuit 40 can set the reverse freewheeling current during the freewheeling period to zero, thereby eliminating hard switching. Furthermore, the control circuit 40 can reduce power loss by limiting the number of body diodes through which the forward freewheeling current passes during the freewheeling period to one.

[0069] Fig. 8 is a diagram showing a second example of the switching state of the rectifier circuit 36 ​​during the freewheeling period. During the freewheeling period, as an example, the control circuit 40 turns off the first switch element 72, the second switch element 74, and the third switch element 76 by setting the control signal (E), the control signal (F), and the control signal (G) to L logic as shown in Fig. 8. Also, during the freewheeling period, the control circuit 40 turns on the fourth switch element 78 by setting the control signal (H) to H logic.

[0070] As a result, during the freewheeling period, the control circuit 40 can cause a forward freewheeling current from the second intermediate terminal 70 to the first intermediate terminal 68 to flow via the body diode of the third switch element 76 and the fourth switch element 78 in the on state. The control circuit 40 can block a reverse freewheeling current from the first intermediate terminal 68 to the second intermediate terminal 70 by the rectification function of the body diodes of the first switch element 72 and the third switch element 76. By switching in this manner, the control circuit 40 can set the reverse freewheeling current during the freewheeling period to zero, thereby eliminating hard switching. Furthermore, the control circuit 40 can reduce power loss by limiting the number of body diodes through which the forward freewheeling current passes during the freewheeling period to one.

[0071] Fig. 9 is a diagram showing a third example of the switching state of the rectifier circuit 36 ​​during the freewheeling period. During the freewheeling period, as an example, the control circuit 40 turns off the second switch element 74, the third switch element 76, and the fourth switch element 78 by setting the control signal (F), the control signal (G), and the control signal (H) to L logic as shown in Fig. 9. Also, during the freewheeling period, the control circuit 40 turns on the first switch element 72 by setting the control signal (E) to H logic.

[0072] As a result, during the freewheeling period, the control circuit 40 can cause a forward freewheeling current from the second intermediate terminal 70 to the first intermediate terminal 68 to flow via the body diode of the second switch element 74 and the first switch element 72 in the on state. The control circuit 40 can block a reverse freewheeling current from the first intermediate terminal 68 to the second intermediate terminal 70 by the rectification function of the body diodes of the second switch element 74 and the fourth switch element 78. By switching in this manner, the control circuit 40 can set the reverse freewheeling current during the freewheeling period to zero, thereby eliminating hard switching. Furthermore, the control circuit 40 can reduce power loss by limiting the number of body diodes through which the forward freewheeling current passes during the freewheeling period to one.

[0073] Fig. 10 is a diagram showing a fourth example of the switching state of the rectifier circuit 36 ​​during the freewheeling period. During the freewheeling period, as an example, the control circuit 40 turns off the first switch element 72, the second switch element 74, and the fourth switch element 78 by setting the control signal (E), the control signal (F), and the control signal (H) to L logic as shown in Fig. 10. Also, during the freewheeling period, the control circuit 40 turns on the third switch element 76 by setting the control signal (G) to H logic.

[0074] As a result, during the freewheeling period, the control circuit 40 can cause a forward freewheeling current from the second intermediate terminal 70 to the first intermediate terminal 68 to flow via the body diode of the fourth switch element 78 and the third switch element 76 in the on state. The control circuit 40 can block a reverse freewheeling current from the first intermediate terminal 68 to the second intermediate terminal 70 by the rectification function of the body diodes of the second switch element 74 and the fourth switch element 78. By switching in this manner, the control circuit 40 can set the reverse freewheeling current during the freewheeling period to zero, thereby eliminating hard switching. Furthermore, the control circuit 40 can reduce power loss by limiting the number of body diodes through which the forward freewheeling current passes during the freewheeling period to one.

[0075] Fig. 11 is a diagram showing a fifth example of the switching state of the rectifier circuit 36 ​​during the freewheeling period. During the freewheeling period, as an example, the control circuit 40 turns off the first switch element 72 and the third switch element 76 by setting the control signal (E) and the control signal (G) to L logic as shown in Fig. 11. Also, during the freewheeling period, the control circuit 40 turns on the second switch element 74 and the fourth switch element 78 by setting the control signal (F) and the control signal (H) to H logic.

[0076] As a result, during the freewheeling period, the control circuit 40 can cause a forward freewheeling current from the second intermediate terminal 70 to the first intermediate terminal 68 to flow via the body diode of the first switch element 72 and the second switch element 74 in the on state, or via the body diode of the third switch element 76 and the fourth switch element 78 in the on state. Note that, in theory, the forward freewheeling current is distributed and flows through two paths, but in reality, it flows through one of the paths due to a difference in the balance of resistance values.

[0077] The control circuit 40 can block the reverse freewheel current flowing from the first intermediate terminal 68 to the second intermediate terminal 70 by the rectification function of the body diodes of the first switch element 72 and the third switch element 76. By switching in this manner, the control circuit 40 can set the reverse freewheel current during the freewheel period to zero, eliminating hard switching. Furthermore, the control circuit 40 can reduce power loss by limiting the number of body diodes through which the forward freewheel current passes during the freewheel period to one.

[0078] Fig. 12 is a diagram showing a sixth example of the switching state of the rectifier circuit 36 ​​during the freewheeling period. During the freewheeling period, as an example, the control circuit 40 turns off the second switch element 74 and the fourth switch element 78 by setting the control signal (F) and the control signal (H) to L logic as shown in Fig. 12. Also, during the freewheeling period, the control circuit 40 turns on the first switch element 72 and the third switch element 76 by setting the control signal (E) and the control signal (G) to H logic.

[0079] As a result, during the freewheeling period, the control circuit 40 can cause a forward freewheeling current from the second intermediate terminal 70 to the first intermediate terminal 68 to flow via the body diode of the second switch element 74 and the first switch element 72 in the on state, or via the body diode of the fourth switch element 78 and the third switch element 76 in the on state. Note that, in theory, the forward freewheeling current is distributed and flows through two paths, but in reality, it flows through one of the paths due to a difference in the balance of resistance values.

[0080] The control circuit 40 can block the reverse freewheel current flowing from the first intermediate terminal 68 to the second intermediate terminal 70 by the rectification function of the body diodes of the second switch element 74 and the fourth switch element 78. By switching in this manner, the control circuit 40 can set the reverse freewheel current during the freewheel period to zero, eliminating hard switching. Furthermore, the control circuit 40 can reduce power loss by limiting the number of body diodes through which the forward freewheel current passes during the freewheel period to one.

[0081] FIG. 13 is a schematic waveform diagram of voltage and current when synchronous switching is performed during the transfer period and all switch elements are turned on during the freewheel period in a state in which a heavy load is connected.

[0082] Io is the output current. Vo is the output voltage. I L is the current flowing through the smoothing inductor 80. V M is the rectified voltage applied between the first intermediate terminal 68 and the second intermediate terminal 70.

[0083] Rectified voltage (V M ) is a positive voltage during the conduction period and is zero during the freewheeling period. L ) increases during the transfer period and decreases during the reflux period.

[0084] The output voltage (Vo) is the rectified voltage (V M The output current (Io) is the average of the smoothing inductor current (IL ) is the average value.

[0085] In the example of FIG. 13, a heavy load is connected, the output current (Io) is about 30 A, and the smoothing inductor current (I L ) fluctuates between about 20A and about 40A. Therefore, when a heavy load is connected, the smoothing inductor current (I L ) does not become a negative value during the freewheeling period. That is, when a heavy load is connected, the DC-DC converter 10 does not pass a freewheeling current in the reverse direction.

[0086] FIG. 14 is a schematic waveform diagram of voltage and current when synchronous switching is performed in the transfer period and all switch elements are turned on in the freewheel period with a light load connected.

[0087] In the example of FIG. 14, a light load is connected and the output current (Io) is about 5 A. In this case, the smoothing inductor current (I L ) increases or decreases between approximately -5A and approximately 15A.

[0088] In the example of Figure 14, the smoothing inductor current (I L ) includes a period in which it is a negative value. During this period, a return current flows in the reverse direction in the rectifier circuit 36. Therefore, when switching is performed in the rectifier circuit 36 ​​while such a return current flows in the reverse direction, hard switching occurs, resulting in large switching loss.

[0089] FIG. 15 is a schematic waveform diagram of the voltage and current of the DC-DC converter 10 according to the present embodiment when a light load is connected.

[0090] The control circuit 40 according to the present embodiment controls the smoothing inductor current (I LIn addition, the control circuit 40 according to the present embodiment switches the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 so that a forward return current flows through the rectifier circuit 36 ​​during the return period.

[0091] Therefore, as shown in Figure 15, the smoothing inductor current (I L ) does not become a negative value even when a light load is connected. Therefore, the control circuit 40 can perform power conversion efficiently without causing hard switching.

[0092] 16 is a flowchart showing a mode setting process. The control circuit 40 may execute the mode setting process shown in FIG.

[0093] First, in S11, the control circuit 40 determines whether the output current (Io) output from the smoothing circuit 38 is greater than a preset value. If the output current (Io) is greater than the set value (Yes in S11), the process proceeds to S12. Then, in S12, the control circuit 40 sets the first mode. If the output current (Io) is not greater than the set value (No in S11), the process proceeds to S13. Then, in S13, the control circuit 40 sets the second mode.

[0094] FIG. 17 is a timing chart of the control signals in the first mode.

[0095] When the first mode is set, in the transmission period, the control circuit 40 switches the rectifier circuit 36 ​​in synchronization with the DC-AC conversion circuit 32 so that the positive side of the secondary AC voltage is applied to the first intermediate terminal 68 and the negative side of the secondary AC voltage is applied to the second intermediate terminal 70. Specifically, in the first transmission period, the control circuit 40 sets the control signal (E) and the control signal (H) to H logic and the control signal (F) and the control signal (G) to L logic. In addition, in the second transmission period, the control circuit 40 sets the control signal (F) and the control signal (G) to H logic and the control signal (E) and the control signal (H) to L logic.

[0096] When the first mode is set, during the reflux period, the control circuit 40 turns on the first switch element 72, the second switch element 74, the third switch element 76 and the fourth switch element 78 by setting all the control signals (E), (F), (G) and (H) to H logic.

[0097] When a heavy load is connected, the return current does not become a negative value. In other words, when the output current (Io) is greater than the set value, no return current flows in the reverse direction through the rectifier circuit 36. Therefore, when a heavy load is connected, the DC-DC converter 10 does not generate switching loss due to hard switching. Therefore, when the output current (Io) is greater than the set value, the DC-DC converter 10 turns on all of the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78, thereby eliminating power loss due to the body diode and enabling efficient power conversion.

[0098] 18 is a timing chart of the control signals in the second mode. When the second mode is set, the control circuit 40 executes the same control as in the first mode on the rectifier circuit 36 ​​during the transmission period.

[0099] When set to the second mode, during the freewheeling period, the control circuit 40 switches the rectifier circuit 36 ​​so that current flows through only one body diode in the rectifier circuit 36 ​​in the direction from the second intermediate terminal 70 to the first intermediate terminal 68, and current is blocked in the direction from the first intermediate terminal 68 to the second intermediate terminal 70 by the rectification function of the body diode in the rectifier circuit 36.

[0100] 18, in the first freewheeling period, the control circuit 40 sets the control signal (H) to H logic and the control signal (E), control signal (F), and control signal (G) to L logic, turns on the fourth switch element 78, and turns off the first switch element 72, the second switch element 74, and the third switch element 76. Also, in the example of Fig. 18, in the second freewheeling period, the control circuit 40 sets the control signal (F) to H logic and the control signal (E), control signal (G), and control signal (H) to L logic, turns on the second switch element 74, and turns off the first switch element 72, the third switch element 76, and the fourth switch element 78.

[0101] When the control circuit 40 is set to the second mode, during the reflux period, the first switch element 72, the second switch element 74, the third switch element 76, and the fourth switch element 78 may be switched using a switching pattern other than the switching pattern in FIG. 18.

[0102] When the second mode is set, the control circuit 40 controls the rectifier circuit 36 ​​in this manner, so that the diode can block reverse reflux current only when there is a possibility of reverse reflux current flowing during the reflux period.

[0103] 19 is a diagram showing the waveform of a reference signal for generating a secondary side control signal. The control circuit 40 may generate secondary side control signals (E to H) for controlling the rectifier circuit 36 ​​in synchronization with primary side control signals (A to D) for controlling the DC-AC conversion circuit 32. In this case, the control circuit 40 switches the rectifier circuit 36 ​​so as not to connect the secondary side coil 62 to the smoothing circuit 38 during a period in which no voltage is generated from the secondary side coil 62 of the transformer 34.

[0104] For example, the control circuit 40 switches the rectifier circuit 36 ​​from the reflux period to the transmission period at a timing that is later than the timing at which the DC-AC conversion circuit 32 switches from the reflux period to the transmission period by a predetermined margin time. Also, the control circuit 40 switches the rectifier circuit 36 ​​from the transmission period to the reflux period at a timing that is earlier than the timing at which the DC-AC conversion circuit 32 switches from the transmission period to the reflux period by a predetermined margin time.

[0105] For example, when the secondary side control signals (E to H) are generated by a logic circuit based on the primary side control signals (A to D), the control circuit 40 generates a corrected control signal (A') by correcting the primary side control signal (A) as shown in FIG. 19. Specifically, the control circuit 40 generates a corrected control signal (A') by delaying the timing of switching from L logic to H logic by a margin time and by advancing the timing of switching from H logic to L logic by the margin time. Similarly, the control circuit 40 generates corrected control signals (B', C', D') for the other primary side control signals (B, C, D). Then, the control circuit 40 generates the secondary side control signals (E to H) by a logic circuit based on these corrected control signals (A' to D').

[0106] 20 is a diagram showing a configuration of a signal generating circuit 110 which is a logic circuit for generating the secondary side control signals (E to H). The control circuit 40 may have a signal generating circuit 110 which is a logic circuit for generating the secondary side control signals (E to H).

[0107] The signal generating circuit 110 receives a mode signal (S) and four correction control signals (A' to D'). When the mode signal (S) is at logic H, it indicates that the circuit operates in the first mode. When the mode signal (S) is at logic L, it indicates that the circuit operates in the second mode.

[0108] The signal generating circuit 110 includes an E generating circuit 122 , an F generating circuit 124 , a G generating circuit 126 , and an H generating circuit 128 .

[0109] The E generation circuit 122 includes a first OR circuit 132, a first AND circuit 134, an inversion circuit 136, a second AND circuit 138, a third AND circuit 140, and a second OR circuit 142.

[0110] The first OR circuit 132 outputs the logical sum of A' and D'. The first AND circuit 134 outputs the logical product of A' and D'. The inversion circuit 136 inverts S. The second AND circuit 138 outputs the logical product of the output signal of the first OR circuit 132 and S. The third AND circuit 140 outputs the logical product of the output signal of the first AND circuit 134 and the output signal of the inversion circuit 136. The second OR circuit 142 outputs the logical sum of the output signal of the second AND circuit 138 and the output signal of the third AND circuit 140.

[0111] When S is logic H, i.e., in the first mode, the E generating circuit 122 outputs the logical sum of A' and D' as the secondary side control signal (E). When S is logic L, i.e., in the second mode, the E generating circuit 122 outputs the logical product of A' and D' as the secondary side control signal (E).

[0112] The F generation circuit 124 has the same circuit configuration as the E generation circuit 122. However, a first OR circuit 132 included in the F generation circuit 124 outputs the logical sum of B' and C'. A first AND circuit 134 included in the F generation circuit 124 outputs the logical product of B' and B'.

[0113] When S is logic H, that is, in the first mode, the F generation circuit 124 outputs the logical sum of B' and C' as the secondary side control signal (F). When S is logic L, that is, in the second mode, the F generation circuit 124 outputs B' as the secondary side control signal (F).

[0114] The G generation circuit 126 has the same circuit configuration as the E generation circuit 122. However, a first OR circuit 132 included in the G generation circuit 126 outputs the logical sum of B' and C'. A first AND circuit 134 included in the G generation circuit 126 outputs the logical product of B' and C'.

[0115] In the first mode, the G generating circuit 126 outputs the logical sum of B' and C' as a secondary side control signal (E). In the second mode, the G generating circuit 126 outputs the logical product of B' and C' as a secondary side control signal (G).

[0116] The H generation circuit 128 has the same circuit configuration as the E generation circuit 122. However, a first OR circuit 132 included in the H generation circuit 128 outputs the logical sum of A' and D'. A first AND circuit 134 included in the H generation circuit 128 outputs the logical product of A' and A'.

[0117] In the first mode, the H generation circuit 128 outputs the logical sum of A' and D' as a control signal (H) on the secondary side. In the second mode, the H generation circuit 128 outputs A' as a control signal (H) on the secondary side.

[0118] In the first mode, the signal generating circuit 110 configured as above can generate the secondary side control signals (E to H) as shown in Fig. 17. In the second mode, the signal generating circuit 110 can generate the secondary side control signals (E to H) as shown in Fig. 18.

[0119] As described above, the DC-DC converter 10 according to this embodiment switches the connections between the secondary coil 62 and the first intermediate terminal 68 and between the secondary coil 62 and the second intermediate terminal 70 in the rectifier circuit 36 ​​in synchronization with the switching of the DC-AC conversion circuit 32 so that the positive side of the secondary AC voltage is applied to the first intermediate terminal 68 and the negative side of the secondary AC voltage is applied to the second intermediate terminal 70 during the transmission period. This enables the DC-DC converter 10 to eliminate power loss during the transmission period.

[0120] Furthermore, in the DC-DC converter 10 according to this embodiment, during the freewheeling period, the rectifier circuit 36 ​​is rectified by a diode so that a current flows from the first intermediate terminal 68 to the second intermediate terminal 70 via the rectifier circuit 36, and a current flows from the second intermediate terminal 70 to the first intermediate terminal 68 via the rectifier circuit 36. This allows the DC-DC converter 10 to reduce power loss due to hard switching by setting the freewheeling current in the reverse direction during the freewheeling period, i.e., the current flowing in the direction from the first intermediate terminal 68 to the second intermediate terminal 70 via the rectifier circuit 36, to zero.

[0121] Furthermore, the DC-DC converter 10 of this embodiment has only one diode through which the forward return current, i.e., the current flowing in the direction from the second intermediate terminal 70 to the first intermediate terminal 68 via the rectifier circuit 36, passes during the return period, thereby making it possible to reduce power loss during the return period.

[0122] 21 is a diagram showing the configuration of a vehicle 200 to which the DC-DC converter 10 is applied. The vehicle 200 includes a first battery 212, a second battery 214, an electrical device 216, and a charging device 218.

[0123] The first battery 212 is, for example, a lithium ion battery, and is supplied with power from a charging station for electric vehicles, etc. The first battery 212 generates a DC voltage of, for example, about 360V.

[0124] The second battery 214 is, for example, a lead storage battery, and is charged by the power transferred from the first battery 212. The second battery 214 generates, for example, a DC voltage of about 12V.

[0125] The electrical equipment 216 is a device mounted on the vehicle 200. The electrical equipment 216 operates on power output from a charging device 218 or power stored in the second battery 214. For example, the electrical equipment 216 is an on-board computer, a power steering system, a headlight, an air conditioner, etc.

[0126] The charging device 218 includes the DC-DC converter 10 of either the first or second embodiment. The charging device 218 takes out power from the first battery 212 and charges the second battery 214. For example, the DC-DC converter 10 included in the charging device 218 steps down the DC voltage generated by the first battery 212 and converts it into a DC voltage that can be charged into the second battery 214.

[0127] The vehicle 200 can run by driving a motor with electric power from the first battery 212. In addition, the vehicle 200 can drive an electrical device 216 with electric power from the second battery 214, and perform various control and auxiliary operations of the vehicle 200.

[0128] Then, the vehicle 200 can perform efficient power conversion with little power loss in the DC-DC converter 10 in the charging device 218. This allows the vehicle 200 to efficiently charge the second battery 214 with the power of the first battery 212.

[0129] In addition, the above-mentioned embodiments are merely examples of the embodiment of the present disclosure, and the technical scope of the present disclosure should not be interpreted as being limited by them. In other words, the present disclosure can be embodied in various forms without departing from the gist or main features thereof. [Explanation of symbols]

[0130] 10 DC-DC converter, 22 first input terminal, 24 second input terminal, 26 first output terminal, 28 second output terminal, 32 DC-AC conversion circuit, 34 transformer, 36 rectifier circuit, 38 smoothing circuit, 40 control circuit, 42 first AC terminal, 44 second AC terminal, 46 conversion capacitor, 52 first conversion switch, 54 second conversion switch, 56 third conversion switch, 58 fourth conversion switch, 60 primary coil, 62 secondary coil, 64 first transformer output terminal, 66 second transformer output terminal, 68 first intermediate terminal, 70 second intermediate terminal, 72 first switch element, 74 second switch element, 76 third switch element, 78 fourth switch element, 80 smoothing inductor, 82 smoothing capacitor, 200 vehicle, 212 first battery, 214 second battery, 216 electrical equipment, 218 charging device

Claims

1. a DC-AC conversion circuit that converts a DC input voltage into a primary side AC voltage by switching the input voltage to invert the positive and negative of the DC input voltage; a transformer including a primary coil to which the primary AC voltage is applied and a secondary coil magnetically coupled to the primary coil, the transformer generating a secondary AC voltage in the secondary coil; a rectifier circuit that outputs a rectified voltage obtained by full-wave rectifying the secondary side AC voltage; a smoothing circuit that smoothes the rectified voltage and outputs a DC output voltage; A control circuit for controlling the operation of the DC-AC conversion circuit and the rectifier circuit; Equipped with The control circuit includes: during a reflux period in which power of the input voltage is not being transmitted from the DC-AC conversion circuit to the transformer, rectifying the rectifier circuit by a diode so that a current does not flow from a first intermediate terminal, which is one of the intermediate terminals connected to the smoothing circuit, to a second intermediate terminal opposite to the first intermediate terminal connected to the smoothing circuit, via the rectifier circuit, and a current flows from the second intermediate terminal to the first intermediate terminal via the rectifier circuit; during a transfer period in which power of the input voltage is transferred from the DC-AC conversion circuit to the transformer, a connection between the secondary coil and the first intermediate terminal and a connection between the secondary coil and the second intermediate terminal in the rectifier circuit are switched in synchronization with switching of the DC-AC conversion circuit so that a positive side of the secondary AC voltage is applied to the first intermediate terminal and a negative side of the secondary AC voltage is applied to the second intermediate terminal; The rectifier circuit includes: a first switch element connected between a first transformer output terminal connected to one terminal of the secondary coil and the first intermediate terminal; a second switch element connected between the second intermediate terminal and the first transformer output terminal; a third switch element connected between a second transformer output terminal connected to a side of the secondary coil opposite to a terminal to which the first transformer output terminal is connected, and the first intermediate terminal; a fourth switch element connected between the second intermediate terminal and the second transformer output terminal; having each of the first switch element, the second switch element, the third switch element, and the fourth switch element is conductive when turned on and functions as a diode when turned off; The first switch element has an anode of the diode connected to the first transformer output terminal and a cathode of the diode connected to the first intermediate terminal, The second switch element has an anode of the diode connected to the second intermediate terminal and a cathode of the diode connected to the first transformer output terminal, The third switch element has an anode of the diode connected to the second transformer output terminal and a cathode of the diode connected to the first intermediate terminal, The fourth switch element has an anode of the diode connected to the second intermediate terminal and a cathode of the diode connected to the second transformer output terminal, When the first mode is set, the control circuit turns on the first switch element, the second switch element, the third switch element, and the fourth switch element during the reflux period, When the second mode is set, during the reflux period, the control circuit: Turning off the first switch element and the third switch element and turning on at least one of the second switch element and the fourth switch element, or turning off the second switch element and the fourth switch element, and turning on at least one of the first switch element and the third switch element; The control circuit sets the first mode when the output current output from the smoothing circuit is greater than a preset value, and sets the second mode when the output current is not greater than the preset value. DC-DC converter.

2. The control circuit includes: during a first transmission period during which a voltage higher than that of the second transformer output terminal is generated from the first transformer output terminal, the first switch element and the fourth switch element are turned on and the second switch element and the third switch element are turned off; In a second transmission period during which a voltage higher than that of the first transformer output terminal is generated from the second transformer output terminal, the second switch element and the third switch element are turned on and the first switch element and the fourth switch element are turned off.

2. The DC-DC converter according to claim 1.

3. During the reflux period, Turning off the first switch element and the third switch element and turning on at least one of the second switch element and the fourth switch element, or The second switch element and the fourth switch element are turned off, and at least one of the first switch element and the third switch element is turned on.

3. The DC-DC converter according to claim 1 or 2.

4. Each of the first switch element, the second switch element, the third switch element, and the fourth switch element is a metal-oxide-semiconductor field-effect field effect transistor (MOSFET).

4. The DC-DC converter according to claim 1.

5. A DC-DC converter comprising the DC-DC converter according to any one of claims 1 to 4. vehicle.

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