DC-DC converter and switching power supply device

Abstract

A DC-DC converter (21) comprises a first switching element (S1), a second switching element (S2), an LC series resonant circuit (LC), a conductor, a first winding (LT21) and a second winding (LT22) that are magnetically coupled to each other in a positive manner, a first rectifying element (D1), and a second rectifying element (D2). The LC series resonant circuit is connected to the conductor. The conductor is connected to the first winding and the first rectifying element. The first winding is connected to the second winding. The second winding is connected to the second switching element and the second rectifying element.

Description

DC-DC converter and switching power supply device

[0001] The present invention relates to a DC-DC converter and a switching power supply device.

[0002] A known example of a conventional DC-DC converter is the DC-DC converter described in Patent Document 1. The DC-DC converter described in Patent Document 1 receives a voltage Vin from a DC power supply 10 and outputs a voltage Vout. The DC-DC converter described in Patent Document 1 includes switching elements 11 and 12, a capacitor 13, inductance elements 14 and 15, diodes 16 and 17, and a smoothing capacitor 18. The inductance element 14 includes a primary winding 141, a secondary winding 142, and output voltage terminals 201 and 202. The turns ratio between the primary winding 141 and the secondary winding 142 is n:1. The inductance element 15 includes a primary winding 151 and a secondary winding 152. The turns ratio between the primary winding 151 and the secondary winding 152 is n:1.

[0003] One end of a series circuit of the switching element 11 and the switching element 12 is connected to the positive electrode of the DC power supply 10. The other end of the series circuit of the switching element 11 and the switching element 12 is connected to the negative electrode of the DC power supply 10.

[0004] The switching elements 11 and 12 are controlled so that when one is on, the other is off. The connection point of the switching elements 11 and 12 is connected to the negative electrode of a DC power supply via a series circuit of a capacitor 13, a primary winding 141 of an inductance element 14, and a primary winding 151 of an inductance element 15.

[0005] One end of the secondary winding 142 of the inductance element 14 is connected to the cathode of the diode 16. The other end of the secondary winding 142 of the inductance element 14 is connected to one end of the secondary winding 152 of the inductance element 15. The other end of the secondary winding 152 of the inductance element 15 is connected to the cathode of the diode 17. The connection point between the other end of the secondary winding 142 of the inductance element 14 and one end of the secondary winding 152 of the inductance element 15 is connected to an output voltage terminal 201. The anodes of the diodes 16 and 17 are both connected to an output voltage terminal 202. A smoothing capacitor 18 is connected between the output voltage terminals 201 and 202. The voltage Vout output from the DC-DC converter is expressed as D×(1−D)×Vin / n, where D is the ratio of the on period of the switching element 11 to the switching period (the sum of the on period and off period of the switching element 11).

[0006] Patent No. 4649299

[0007] In the above configuration, if the step-down ratio of the DC-DC converter is to be increased, the number of turns of the primary winding must be increased. For example, when D = 0.5, if the step-down ratio is to be 1 / 8 (Vout = 1 / 8 Vin), n = 2. Since the turn ratio between the primary winding 141 and the secondary winding 142 is 2:1 and the turn ratio between the primary winding 151 and the secondary winding 152 is 2:1, the turn ratio between the total of the primary windings 141 and 151 and the secondary winding 142 and the secondary winding 152 is 4:1:1. When the number of turns of the primary winding is increased, the resistance value of the primary winding increases, and therefore copper loss due to the primary winding increases.

[0008] Furthermore, when switching element 11 is on and switching element 12 is off, no current flows through secondary winding 142 of inductance element 14 and diode 16, but current flows through secondary winding 152 of inductance element 15 and diode 17. When switching element 11 is off and switching element 12 is on, current flows through secondary winding 142 of inductance element 14, but no current flows through secondary winding 152 of inductance element 15. In this manner, the period (first period) during which current flows through secondary winding 142 of inductance element 14 and the period (second period) during which current flows through secondary winding 152 of inductance element 15 are biased toward a predetermined period. Therefore, for a predetermined output current, the effective value of current Irec2 flowing through secondary winding 142 of inductance element 14 becomes large during the first period. Similarly, in the second period, the effective value of the current Irec1 flowing through the secondary winding 152 of the inductance element 15 increases. Therefore, the copper loss due to the secondary winding 142 of the inductance element 14 and the copper loss due to the secondary winding 152 of the inductance element 15 increase.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a DC-DC converter and a switching power supply device that can reduce copper loss.

[0010] a first winding having a ninth end and a tenth end; a second winding having an eleventh end and a twelfth end; a first rectifying element having a thirteenth end and a fourteenth end; and a second rectifying element having a fifteenth end and a sixteenth end, the fifteenth end being connected to the thirteenth end; The tenth terminal is connected to the eleventh terminal, and the twelfth terminal is connected to the fourth terminal and the sixteenth terminal.

[0011] Because the eighth end of the conductor is connected to the ninth end of the first winding and the fourteenth end of the first rectifying element, the first winding also corresponds to a part of the primary winding. This allows the DC-DC converter according to one aspect of the present invention to increase the step-down ratio of the DC-DC converter without increasing the number of turns of the primary winding. As a result, the DC-DC converter according to one aspect of the present invention can reduce copper loss due to the primary winding.

[0012] The DC-DC converter and switching power supply device according to the present invention can reduce copper loss.

[0013] FIG. 1 is a circuit diagram showing a switching power supply device 20 including a DC-DC converter 21 and a load resistor RL1. FIG. 2 is a diagram showing an example of the drain D-source S voltage v1 of the first switching element S1, the drain D-source S voltage v2 of the second switching element S2, the first control signal CS1, the second control signal CS2, the current i1 flowing through the capacitor C, the excitation current im flowing through the first excitation inductance Lm1, the current iD1 flowing through the first diode D1, and the current iD2 flowing through the second diode D2. FIG. 3 is a cross-sectional view schematically showing the structure of the transformer TR1 and a diagram showing the wiring of the transformer TR1. FIG. 4 is a plan view schematically showing the structure of the first core CO1. FIG. 5 is a diagram showing the current flowing through the switching power supply device 20 and the load resistor RL1 during a first period P1. FIG. 6 is a diagram showing the current flowing through the switching power supply device 20 and the load resistor RL1 during a second period P2. FIG. 7 is a cross-sectional view schematically showing the structure of transformer TR2 and a diagram showing the wiring of transformer TR2. FIG. 8 is a plan view schematically showing the structure of first core CO1. FIG. 9 is a circuit diagram showing switching power supply device 20b including DC-DC converter 21b and load resistor RL1. FIG. 10 is a circuit diagram showing switching power supply device 20c including DC-DC converter 21c and load resistors RL1 and RL2. FIG. 11 is a circuit diagram showing switching power supply device 20d including DC-DC converter 21d and load resistors RL1 and RL2. FIG. 12 is a circuit diagram showing switching power supply device 20e including DC-DC converter 21e and load resistor RL1. FIG. 13 is a cross-sectional view schematically showing the structure of transformer TR4 and a diagram showing the wiring of transformer TR4. FIG. 14 is a plan view schematically showing the structure of first core CO1. FIG. 15 is a cross-sectional view schematically showing the structure of transformer TR6 and a diagram showing the wiring of transformer TR6. Fig. 16 is a plan view showing the structure of the first core CO1. Fig. 17 is a circuit diagram showing a switching power supply device 20g equipped with a DC-DC converter 21g and a load resistor RL1. Fig. 18 is a cross-sectional view showing the structure of a transformer TR7 and a diagram showing the wiring of the transformer TR7. Fig. 19 is a plan view showing the structure of a second core CO2.Fig. 20 is a cross-sectional view showing a schematic structure of the transformer TR8 and a diagram showing the wiring of the transformer TR8. Fig. 21 is a plan view showing a schematic structure of the second core CO2. Fig. 22 is a circuit diagram showing a switching circuit SW in a ninth modified example. Fig. 23 is a circuit diagram showing a switching circuit SW in a tenth modified example. Fig. 24 is a circuit diagram showing a modified example of the first rectifying element.

[0014] First Embodiment A switching power supply 20 including a DC-DC converter 21 according to a first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing the switching power supply 20 including the DC-DC converter 21 and a load resistor RL1. Note that the leakage inductance of the transformer TR1 is omitted from FIG. 1 . FIG. 2 is a diagram illustrating an example of the drain D-source S voltage v1 of the first switching element S1, the drain D-source S voltage v2 of the second switching element S2, the first control signal CS1, the second control signal CS2, the current i1 flowing through the capacitor C, the excitation current im flowing through the first excitation inductance Lm1, the current iD1 flowing through the first diode D1, and the current iD2 flowing through the second diode D2. Note that the horizontal axis in FIG. 2 represents time t. The vertical axis in FIG. 2 represents the drain D-source S voltage v1 of the first switching element S1, the drain D-source S voltage v2 of the second switching element S2, the first control signal CS1 of the first switching element S1, the second control signal CS2 of the second switching element S2, the current i1 flowing through the capacitor C, the excitation current im flowing through the first excitation inductance Lm1, the current iD1 flowing through the first diode D1, and the current iD2 flowing through the second diode D2. In FIG. 2, the positive direction of the current iD1 is from the first anode A1 to the first cathode K1. In FIG. 2, the positive direction of the current iD2 is from the second anode A2 to the second cathode K2. FIG. 3 is a cross-sectional view schematically illustrating the structure of the transformer TR1 and a diagram showing the wiring of the transformer TR1. FIG. 4 is a plan view schematically illustrating the structure of the first core CO1.

[0015] The switching power supply device 20 is used to supply a DC voltage to a load. As shown in Fig. 1, the switching power supply device 20 includes a DC power supply DCPS, a DC-DC converter 21, and first output terminals O1 and O2.

[0016] The first output terminals O1 and O2 are connected to both ends of a load resistor RL1. The first output terminal O2 is connected to ground potential. In this embodiment, the first output terminal O2 is connected to earth. Note that the first output terminal O2 does not have to be connected to earth. The load resistor RL1 is a specific example of a load. The load is not limited to a resistance component and may include a reactance component.

[0017] The DC power supply DCPS outputs a first DC voltage Vin. The negative electrode of the DC power supply DCPS is connected to the first output terminal O2. Therefore, the negative electrode of the DC power supply DCPS is connected to the ground potential. The positive electrode of the DC power supply DCPS is connected to the DC-DC converter 21. The DC power supply DCPS is, for example, a battery or an electric double layer capacitor. It is noted that the DC power supply DCPS only needs to output a DC voltage. Therefore, the DC power supply DCPS may be an AC-DC converter, a DC-DC converter, or the like.

[0018] A first DC voltage Vin is input to the DC-DC converter 21 from a DC power supply DCPS. The DC-DC converter 21 is used to supply a second DC voltage Vout, which is different from the first DC voltage Vin, to a load. The DC-DC converter 21 includes a switching circuit SW, an LC series resonant circuit LC, a smoothing capacitor SC1, a first diode D1, a second diode D2, a transformer TR1, a gate drive circuit GD, an input terminal IT, and second output terminals O3 and O4. Note that the smoothing capacitor SC1 is not essential.

[0019] The input terminal IT is connected to the positive electrode of the DC power supply DCPS. The second output terminals O3 and O4 output a second DC voltage Vout. The second output terminals O3 and O4 are connected to the first output terminals O1 and O2, respectively.

[0020] The switching circuit SW includes a first switching element S1 and a second switching element S2. The first switching element S1 is connected to an input terminal IT, the second switching element S2, and a reactor L. In this embodiment, the first switching element S1 is a metal-oxide-semiconductor field effect transistor (MOSFET). The first switching element S1 has a parasitic capacitor C1 and a parasitic diode FD1. A drain D of the first switching element S1 is connected to the positive electrode of the DC power supply DCPS via the input terminal IT. A source S of the first switching element S1 is connected to the reactor L and the second switching element S2. The drain D of the first switching element S1 corresponds to a "first terminal" in the present invention. The source S of the first switching element S1 corresponds to a "second terminal" in the present invention. The "first switching element" of the present invention is not limited to a MOSFET, but may be an element having a switching function, such as a bipolar transistor or an IGBT (Insulated Gate Bipolar Transistor).

[0021] The second switching element S2 is connected to the first switching element S1, the reactor L, the transformer TR1, and the second diode D2. In this embodiment, the second switching element S2 is a MOSFET. The second switching element S2 has a parasitic capacitor C2 and a parasitic diode FD2. The drain D of the second switching element S2 is connected to the first switching element S1 and the reactor L. The source S of the second switching element S2 is connected to the transformer TR1 and the second diode D2. The drain D of the second switching element S2 corresponds to the "third terminal" of the present invention. The source S of the second switching element S2 corresponds to the "fourth terminal" of the present invention. Note that the "second switching element" of the present invention is not limited to a MOSFET, and may be an element having a switching function, such as a bipolar transistor or an IGBT.

[0022] A first control signal CS1 is applied to the gate G1 of the first switching element S1 from the gate drive circuit GD. The first switching element S1 is controlled to be on / off by the first control signal CS1. As shown in FIG. 2 , the first switching element S1 alternates between an on state and an off state. In the on state, the first switching element S1 is conductive. In the off state, the first switching element S1 is non-conductive. In this embodiment, the ratio of the on period to the off period of the first switching element S1 is 1:1. Therefore, the ratio of the on period of the first switching element S1 to the switching period (the sum of the on period and off period of the first switching element S1) is 0.5. Note that the ratio of the on period to the off period of the first switching element S1 is not limited to 1:1.

[0023] A second control signal CS2 is applied from the gate drive circuit GD to the gate G2 of the second switching element S2. The second switching element S2 is controlled to be on / off by the second control signal CS2. The second switching element S2 alternates between on and off states. In the on state, the second switching element S2 is conductive. In the off state, the second switching element S2 is non-conductive. However, the gate drive circuit GD generates the second control signal CS2 so that the second switching element S2 is on when the first switching element S1 is off. This periodically repeats a first period P1 during which the first switching element S1 is conductive and the second switching element S2 is non-conductive, and a second period P2 during which the second switching element S2 is conductive and the first switching element S1 is conductive. In this embodiment, the ratio of the on period to the off period of the second switching element S2 is 1:1. As a result, the ratio between the first period P1 and the second period P2 is 1:1. A dead time period may be provided between the first period P1 and the second period P2, not limited to the first period P1 and the second period P2. That is, during the dead time period, the gate drive circuit GD may generate the first control signal CS1 and the second control signal CS2 so that the first switching element S1 is in the off state and the second switching element S2 is in the off state. By providing a dead time period, it is possible to more reliably prevent both the first switching element S1 and the second switching element S2 from being in the on state at the same time. Furthermore, the ratio between the on period and the off period of the second switching element S2 is not limited to 1:1.

[0024] As shown in Fig. 1, a parasitic capacitor C1 is connected between the drain D and source S of the first switching element S1. The parasitic capacitor C1 suppresses ringing when the first switching element S1 is turned on and off. Furthermore, the parasitic capacitor C1 enables the first switching element S1 to perform ZVS (Zero Voltage Switching) during dead time.

[0025] The parasitic diode FD1 is connected between the drain D and source S of the first switching element S1. The cathode of the parasitic diode FD1 is connected to the drain D of the first switching element S1. The anode of the parasitic diode FD1 is connected to the source S of the first switching element S1. When the first switching element S1 is turned off, the parasitic diode FD1 causes a current to flow back from the anode to the cathode of the parasitic diode FD1. This prevents a current from flowing from the source S to the drain D of the first switching element S1, preventing breakdown of the first switching element S1. Note that a freewheeling diode may be provided separately from the first switching element S1.

[0026] The parasitic capacitor C2 is connected between the drain D and source S of the second switching element S2. The parasitic capacitor C2 suppresses ringing when the second switching element S2 is turned on and off. Furthermore, the parasitic capacitor C2 enables the second switching element S2 to perform ZVS during the dead time period.

[0027] The parasitic diode FD2 is connected between the drain D and source S of the second switching element S2. The cathode of the parasitic diode FD2 is connected to the drain D of the second switching element S2. The anode of the parasitic diode FD2 is connected to the source S of the second switching element S2. When the second switching element S2 is turned off, the parasitic diode FD2 causes a current to flow back from the anode to the cathode of the parasitic diode FD2. This prevents a current from flowing from the source S to the drain D of the second switching element S2, preventing breakdown of the second switching element S2. Note that a freewheeling diode may be provided separately from the second switching element S2.

[0028] The LC series resonant circuit LC includes a reactor L and a capacitor C. The reactor L is connected to the connection point between the source S of the first switching element S1 and the drain D of the second switching element S2, and to the capacitor C. The capacitor C is connected to the reactor L and a transformer TR1. The capacitor C is connected in series with the reactor L. The first switching element S1 and the second switching element S2 can perform soft switching by current resonance between the reactor L and the capacitor C. The terminal TL of the reactor L, which is connected to the connection point between the source S of the first switching element S1 and the drain D of the second switching element S2, corresponds to the "fifth terminal" of the present invention. The terminal TC of the capacitor C, which is connected to the transformer TR1, corresponds to the "sixth terminal" of the present invention.

[0029] In the circuit diagram shown in FIG. 1 , the transformer TR1 includes a first excitation inductance Lm1 and windings LT11, LT21, and LT22. The windings LT11, LT21, and LT22 are magnetically coupled. The first excitation inductance Lm1 is an inductance that generates a magnetic flux φ that interlinks with all of the windings LT11, LT21, and LT22. In this embodiment, the turns ratio between the windings LT11, LT21, and LT22 is n:1:1. Also, n=2. Note that n=2 is not limited to this. The turns ratio between the windings LT11, LT21, and LT22 does not have to be n:1:1.

[0030] In the cross-sectional view shown in FIG. 3 , the transformer TR1 includes a first core CO1 and windings LT11, LT21, and LT22. The first core CO1 is made of a magnetic material. The first core CO1 has a first core portion CR1 and a second core portion CR2. The winding LT11 is wound around the second core portion CR2. The windings LT21 and LT22 are each wound around the first core portion CR1. As shown in FIGS. 3 and 4 , the first core CO1 forms a magnetic path. In this embodiment, the cross-sectional area of ​​the second core portion CR2 perpendicular to the magnetic path is equal to the cross-sectional area of ​​the first core portion CR1 perpendicular to the magnetic path. Note that an air gap may be provided between the first core portion CR1 and the second core portion CR2.

[0031] As shown in FIG. 1 , one end T1 of the winding LT11 is connected to the end TC of the capacitor C. The other end T2 of the winding LT11 is connected to the winding LT21 and the first diode D1. Therefore, the windings LT11 and LT21 are not insulated from each other. The winding LT11 corresponds to the "conductor" and the "third winding" of the present invention. The one end T1 of the winding LT11 corresponds to the "seventh end" of the present invention. The other end T2 of the winding LT11 corresponds to the "eighth end" of the present invention.

[0032] One end T3 of the winding LT21 is connected to the other end T2 of the winding LT11 and the first diode D1. The other end T4 of the winding LT21 is connected to the winding LT22 and the second output terminal O3. The winding LT21 corresponds to the "first winding" of the present invention. The one end T3 of the winding LT21 corresponds to the "ninth terminal" of the present invention. The other end T4 of the winding LT21 corresponds to the "tenth terminal" of the present invention.

[0033] One end T5 of the winding LT22 is connected to the other end T4 of the winding LT21 and the second output terminal O3. The other end T6 of the winding LT22 is connected to the source S of the second switching element S2 and the second diode D2. The winding LT22 corresponds to the "second winding" of the present invention. The one end T5 of the winding LT22 corresponds to the "eleventh terminal" of the present invention. The other end T6 of the winding LT22 corresponds to the "twelfth terminal" of the present invention.

[0034] As shown in FIG. 3 , in the transformer TR1, the direction of the magnetic flux φ generated when a current flows from one end T1 of the winding LT11 to the other end T2 of the winding LT11 coincides with the direction of the magnetic flux φ generated when a current flows from one end T3 of the winding LT21 to the other end T4 of the winding LT21, and the direction of the magnetic flux φ generated when a current flows from one end T5 of the winding LT22 to the other end T6 of the winding LT22. Therefore, the windings LT11, LT21, and LT22 are magnetically positively coupled to one another. In other words, the coupling coefficient between the windings LT11 and LT21 is positive. The coupling coefficient between the windings LT11 and LT22 is also positive. The coupling coefficient between the windings LT21 and LT22 is also positive.

[0035] As shown in FIG. 1 , the first diode D1 has a first anode A1 and a first cathode K1. The first cathode K1 is connected to the other end T2 of the winding LT11 and one end T3 of the winding LT21. The first anode A1 is connected to the second diode D2 and the second output terminal O4. The first diode D1 corresponds to the "first rectifying element" of the present invention. The first anode A1 corresponds to the "thirteenth terminal" of the present invention. The first cathode K1 corresponds to the "fourteenth terminal" of the present invention.

[0036] The second diode D2 has a second anode A2 and a second cathode K2. The second cathode K2 is connected to the source S of the second switching element S2 and the other end T6 of the winding LT22. The second anode A2 is connected to the first anode A1 and the second output terminal O4. The second diode D2 corresponds to the "second rectifier element" of the present invention. The second anode A2 corresponds to the "fifteenth terminal" of the present invention. The second cathode K2 corresponds to the "sixteenth terminal" of the present invention.

[0037] The smoothing capacitor SC1 is connected to the second output terminals O3 and O4, i.e., to the connection point between the other end T4 of the winding LT21 and the winding LT22, and to the connection point between the first anode A1 and the second anode A2.

[0038] Next, the operation of the DC-DC converter 21 will be described. Fig. 5 is an operational diagram showing the current flowing through the switching power supply device 20 and the load resistance RL1 during the first period P1. Fig. 6 is an operational diagram showing the current flowing through the switching power supply device 20 and the load resistance RL1 during the second period P2.

[0039] As shown in Fig. 5, during the first period P1, the current i1 flows in the following order: ground, the DC power supply DCPS, the input terminal IT, the first switching element S1, the reactor L, the capacitor C, the windings LT11 and LT21, the second output terminal O3, the first output terminal O1, the load resistor RL1, and ground. During the first period P1, the capacitor C is charged by the current i1. That is, the current i1 is the charging current for the capacitor C during the first period P1. When the current i1 flows through the winding LT11, the excitation current im flows through the first excitation inductance Lm1. At this time, the windings LT11 and LT22 are magnetically coupled, and therefore the current i2 flows through the winding LT22. The current i2 flows through the winding LT22, the second output terminal O3, the first output terminal O1, the load resistor RL1, the first output terminal O2, the second output terminal O4, the second diode D2, and the winding LT22 in that order. During the first period P1, the currents i1 and i2 flow, causing the potential of one end T3 of the winding LT21 to differ from the potential of the source S of the second switching element S2. More specifically, during the first period P1, the potential of one end T3 of the winding LT21 is higher than the potential of the source S of the second switching element S2.

[0040] 2, in the first period P1, no current flows through the first diode D1, whereas in the first period P1, a current flows through the second diode D2.

[0041] As shown in FIG. 6 , during the second period P2, the capacitor C discharges. The current i1 is the discharge current of the capacitor C during the second period P2. During the second period P2, the current i1 flows in the following order: capacitor C, second switching element S2, winding LT22, second output terminal O3, first output terminal O1, load resistor RL1, first output terminal O2, second output terminal O4, first diode D1, winding LT11, and capacitor C. When the current i1 flows through the winding LT11, the excitation current im flows through the first excitation inductance Lm1. At this time, the windings LT11 and LT21 are magnetically coupled, and therefore the current i2 flows through the winding LT21. The current i2 flows in the following order: winding LT21, second output terminal O3, first output terminal O1, load resistor RL1, first output terminal O2, first diode D1, and winding LT21. In the second period P2, the currents i1 and i2 flow, so that the potential of one end T3 of the winding LT21 differs from the potential of the source S of the second switching element S2. More specifically, in the second period P2, the potential of one end T3 of the winding LT21 is lower than the potential of the drain D of the first switching element S1.

[0042] 2, in the second period P2, a current flows through the first diode D1, whereas in the second period P2, no current flows through the second diode D2.

[0043] In this embodiment, the second DC voltage Vout is expressed by the following Equation 1 using the first DC voltage Vin and n.

[0044]

[0045] [Effect] The DC-DC converter 21 can reduce copper loss. More specifically, the second DC voltage Vout is expressed by the above equation 1 using the first DC voltage Vin and n. Therefore, when the ratio of the on-period of the first switching element S1 to the switching period is 0.5, if the step-down ratio of the DC-DC converter 21 is to be 1 / 8 (Vout = 1 / 8 Vin), n = 2. That is, the turns ratio between the winding LT11, the winding LT21, and the winding LT22 is 2:1:1. In the DC-DC converter disclosed in Patent Document 1 (Japanese Patent No. 4649299), the turns ratio between the total of the primary windings 141 and 151 and the secondary windings 142 and 152 was 4:1:1. Therefore, the number of turns of the winding LT11 is reduced compared to the primary windings 141 and 151 in the DC-DC converter disclosed in Patent Document 1. That is, according to the DC-DC converter 21 of this embodiment, the resistance value of the winding LT11 can be reduced by reducing the number of turns of the winding LT11. As a result, according to the DC-DC converter 21 of this embodiment, copper loss due to the winding LT11 can be reduced.

[0046] Furthermore, the DC-DC converter 21 can further reduce copper loss. More specifically, in the first period P1, a current flows through both the windings LT21 and LT22. At this time, the current flowing through the winding LT21 flows in the same direction as the current flowing through the winding LT22 with respect to the load resistance RL1. In other words, in the first period P1, the current flowing through the load resistance RL1 is shared between the current flowing through the winding LT21 and the current flowing through the winding LT22. Furthermore, in the second period P2, a current flows through both the windings LT21 and LT22. At this time, the current flowing through the winding LT21 flows in the same direction as the current flowing through the winding LT22 with respect to the load resistance RL1. In other words, in the second period P2 as well, the current flowing through the load resistance RL1 is shared between the current flowing through the winding LT21 and the current flowing through the winding LT22. Therefore, the period during which current flows through the winding LT21 and the period during which current flows through the winding LT22 are not biased toward a predetermined period. As a result, the effective values ​​of the currents flowing through the windings LT21 and LT22 do not need to be unnecessarily large for a predetermined output current. Therefore, with DC-DC converter 21, copper loss due to the windings LT21 and LT22 can be reduced.

[0047] Furthermore, the DC-DC converter 21 can extend the life of the second diode D2. More specifically, during the first period P1, the current i1 flowing through the capacitor C does not flow through the second diode D2. Furthermore, during the second period P2, no current flows through the second diode D2. Therefore, the effective value of the current flowing through the second diode D2 is relatively small. This reduces the loss generated by the second diode D2 and also reduces the amount of heat generated by the second diode D2. Therefore, the temperature of the second diode D2 can be kept low, and the life of the second diode D2 can be extended.

[0048] [First Modification] A DC-DC converter 21a and a switching power supply 20a according to a first modification of the present invention will be described below with reference to the drawings. FIG. 7 is a cross-sectional view showing a schematic structure of the transformer TR2 and a diagram showing the wiring of the transformer TR2. FIG. 8 is a plan view showing a schematic structure of the first core CO1. Note that, regarding the DC-DC converter 21a and the switching power supply 20a according to the first modification, only the parts that are different from the DC-DC converter 21 and the switching power supply 20 according to the first embodiment will be described, and the rest will be omitted.

[0049] The DC-DC converter 21a and switching power supply device 20a according to the first modified example differ from the DC-DC converter 21 and switching power supply device 20 according to the first embodiment in that they each include a transformer TR2 instead of the transformer TR1.

[0050] As shown in FIG. 7 , the first core CO1 further includes a third core portion CR3. That is, the first core CO1 has three core portions. The winding LT22 is wound around the third core portion CR3. That is, the windings LT11, LT21, and LT22 are each wound around one of the three core portions of a different first core CO1. As shown in FIG. 8 , a gap V is provided between the first core portion CR1 and the third core portion CR3. As shown in FIGS. 7 and 8 , the first core CO1 forms a magnetic path. More specifically, the first core portion CR1 and the second core portion CR2 form a magnetic path through which magnetic flux φ1 flows. Furthermore, the second core portion CR2 and the third core portion CR3 form a magnetic path through which magnetic flux φ2 flows. In this case, the relationship φ = φ1 + φ2 holds. In this modification, the cross-sectional area of ​​the first core portion CR1 perpendicular to the magnetic path and the cross-sectional area of ​​the third core portion CR3 perpendicular to the magnetic path are each half the cross-sectional area of ​​the second core portion CR2 perpendicular to the magnetic path. The sum of the cross-sectional areas of the first core portion CR1 perpendicular to the magnetic path and the third core portion CR3 perpendicular to the magnetic path is equal to the cross-sectional area of ​​the second core portion CR2 perpendicular to the magnetic path. Therefore, the magnitude of the magnetic flux φ1 and the magnitude of the magnetic flux φ2 are each half the magnetic flux φ. Note that the first core CO1 may have four or more core portions.

[0051] In this modification, the second DC voltage Vout is expressed by the following Equation 2 using the first DC voltage Vin and n.

[0052]

[0053] The DC-DC converter 21a and switching power supply 20a described above also achieve the same effects as the DC-DC converter 21 and switching power supply 20. Furthermore, the DC-DC converter 21a can reduce copper loss when the step-down ratio is the same. More specifically, the magnitude of the magnetic flux φ1 and the magnitude of the magnetic flux φ2 are each half of the magnetic flux φ. Therefore, the induced electromotive forces generated in the windings LT21 and LT22 of the DC-DC converter 21a are half of the induced electromotive forces generated in the windings LT21 and LT22 of the DC-DC converter 21. In the case of the DC-DC converter 21, in order to achieve n=2, it was necessary to set the number of turns of the winding LT11 to 2, the number of turns of the winding LT21 to 1, and the number of turns of the winding LT22 to 1, for example. On the other hand, in the case of the DC-DC converter 21a, to achieve n = 2, the number of turns of the winding LT11 should be 1, the number of turns of the winding LT21 should be 1, and the number of turns of the winding LT22 should be 1. In other words, the number of turns of the winding LT11 can be half the number of turns of the winding LT11 of the DC-DC converter 21. Therefore, according to the DC-DC converter 21a, the resistance value of the winding LT11 can be made smaller by further reducing the number of turns of the winding LT11. As a result, according to the DC-DC converter 21a, the copper loss due to the winding LT11 can be further reduced.

[0054] In other words, when the number of turns of the winding LT11, the number of turns of the winding LT21, and the number of turns of the winding LT22 are equal in DC-DC converter 21 and DC-DC converter 21a, the step-down ratio of DC-DC converter 21a is higher than that of DC-DC converter 21. DC-DC converter 21a can achieve a high step-down ratio without increasing the number of turns of the winding LT11 and without increasing the cross-sectional area of ​​the core portion perpendicular to the magnetic path.

[0055] [Second Modification] A DC-DC converter 21b and a switching power supply 20b according to a second modification of the present invention will be described below with reference to the drawings. Fig. 9 is a circuit diagram showing a switching power supply 20b including a DC-DC converter 21b and a load resistor RL1. Note that with regard to the DC-DC converter 21b and switching power supply 20b according to the second modification, only the differences from the DC-DC converter 21 and switching power supply 20 according to the first embodiment will be described, and the rest will be omitted.

[0056] A DC-DC converter 21b and a switching power supply 20b according to the second modification differ from the DC-DC converter 21 and the switching power supply 20 according to the first embodiment in that they each include a transformer TR3 instead of the transformer TR1. As shown in Fig. 9, the transformer TR3 does not include a winding LT11.

[0057] In this modified example, the DC-DC converter 21b includes a conductor CON. One end CON1 of the conductor CON is connected to the end TC of the capacitor C. The other end CON2 of the conductor CON is connected to the winding LT21 and the first diode D1. The conductor CON in this modified example corresponds to the "conductor" of the present invention. The one end CON1 of the conductor CON corresponds to the "seventh end" of the present invention. The other end CON2 of the conductor CON corresponds to the "eighth end" of the present invention.

[0058] The DC-DC converter 21b and switching power supply 20b described above also achieve the same effects as the DC-DC converter 21 and switching power supply 20. Furthermore, with the DC-DC converter 21b, the step-down ratio of the DC-DC converter 21b can be set to 1 / 4 (Vout = 1 / 4Vin) even without the winding LT11. More specifically, by substituting n = 0 into Equation 1, Vout = 1 / 4Vin. Furthermore, by eliminating the winding LT11, the DC-DC converter can be made smaller and less expensive.

[0059] [Third Modification] A DC-DC converter 21c and a switching power supply 20c according to a third modification of the present invention will be described below with reference to the drawings. Fig. 10 is a circuit diagram showing a switching power supply 20c including a DC-DC converter 21c and load resistors RL1 and RL2. Note that with regard to the DC-DC converter 21c and switching power supply 20c according to the third modification, only the differences from the DC-DC converter 21 and switching power supply 20 according to the first embodiment will be described, and the rest will be omitted.

[0060] The switching power supply device 20c according to the third modification differs from the switching power supply device 20 according to the first embodiment in that it further includes third output terminals O5 and O6, that the DC-DC converter 21c includes a transformer TR4 instead of the transformer TR1, and that the DC-DC converter 21c further includes a smoothing capacitor SC2, a third diode D3, a fourth diode D4, and fourth output terminals O7 and O8.

[0061] As shown in Fig. 10, the transformer TR4 further includes windings LT23 and LT24. The windings LT11, LT21, LT22, LT23, and LT24 are magnetically coupled. The first excitation inductance Lm1 is an inductance that generates a magnetic flux φ that interlinks with all of the windings LT11, LT21, LT22, LT23, and LT24. In the transformer TR4, the direction of the magnetic flux φ generated when a current flows from one end T1 of the winding LT11 to the other end T2 of the winding LT11 coincides with the direction of the magnetic flux φ generated when a current flows from one end T7 of the winding LT23 to the other end T8 of the winding LT23, and the direction of the magnetic flux φ generated when a current flows from one end T9 of the winding LT24 to the other end T10 of the winding LT24. Therefore, the windings LT11, LT21, LT22, LT23, and LT24 are magnetically positively coupled to one another. In this embodiment, the turns ratio between the windings LT11, LT21, LT22, LT23, and LT24 is n:1:1:1:1. However, the turns ratio between the windings LT11, LT21, LT22, LT23, and LT24 does not have to be n:1:1:1:1.

[0062] The third output terminals O5 and O6 are connected to both ends of a load resistor RL2. The third output terminal O6 is connected to ground potential. The third output terminals O5 and O6 output a third DC voltage different from the first DC voltage Vin. The load resistor RL2 is a specific example of a load. The load is not limited to a resistance component and may include a reactance component. The third DC voltage may be equal to the second DC voltage Vout or may be different from the second DC voltage Vout.

[0063] The third diode D3 has a third anode A3 and a third cathode K3. The fourth diode D4 has a fourth anode A4 and a fourth cathode K4. The connections of the smoothing capacitor SC2, the third diode D3, the fourth diode D4, the windings LT23 and LT24, the third output terminals O5 and O6, and the fourth output terminals O7 and O8 are the same as those of the smoothing capacitor SC1, the first diode D1, the second diode D2, the windings LT21 and LT22, the first output terminals O1 and O2, and the second output terminals O3 and O4, except that the third diode D3 and the winding LT23 are not connected to the other end T2 of the winding LT11, and the fourth diode D4 and the winding LT24 are not connected to the source S of the second switching element S2. Therefore, a detailed description of the connections is omitted. The third diode D3 corresponds to the "third rectifier element" in the present invention. The third anode A3 corresponds to the "21st terminal" in the present invention. The third cathode K3 corresponds to the "22nd terminal" of the present invention. The fourth diode D4 corresponds to the "4th rectifying element" of the present invention. The fourth anode A4 corresponds to the "23rd terminal" of the present invention. The fourth cathode K4 corresponds to the "24th terminal" of the present invention.

[0064] The winding LT23 corresponds to the "fourth winding" of the present invention. One end T7 of the winding LT23 corresponds to the "seventeenth terminal" of the present invention. The other end T8 of the winding LT23 corresponds to the "eighteenth terminal" of the present invention. The winding LT24 corresponds to the "fifth winding" of the present invention. One end T9 of the winding LT24 corresponds to the "nineteenth terminal" of the present invention. The other end T10 of the winding LT24 corresponds to the "twentieth terminal" of the present invention.

[0065] The DC-DC converter 21c and the switching power supply device 20c described above also have the same effects as the DC-DC converter 21 and the switching power supply device 20. Furthermore, the DC-DC converter 21c can output DC voltages in parallel.

[0066] [Fourth Modification] A DC-DC converter 21d and a switching power supply 20d according to a fourth modification of the present invention will be described below with reference to the drawings. Fig. 11 is a circuit diagram showing a switching power supply 20d including a DC-DC converter 21d and load resistors RL1 and RL2. Note that with regard to the DC-DC converter 21d and the switching power supply 20d according to the fourth modification, only the differences from the DC-DC converter 21c and the switching power supply 20c according to the third modification will be described, and the rest will be omitted.

[0067] The DC-DC converter 21d and the switching power supply device 20d according to the fourth modification differ from the DC-DC converter 21c and the switching power supply device 20c according to the third modification in that they each include a transformer TR5 instead of the transformer TR4. As shown in Fig. 11 , the transformer TR5 does not include a winding LT11.

[0068] In this modified example, the DC-DC converter 21d includes a conductor CON. One end CON1 of the conductor CON is connected to the end TC of the capacitor C. The other end CON2 of the conductor CON is connected to the winding LT21 and the first diode D1. The conductor CON in this modified example corresponds to the "conductor" of the present invention. The one end CON1 of the conductor CON corresponds to the "seventh end" of the present invention. The other end CON2 of the conductor CON corresponds to the "eighth end" of the present invention.

[0069] The DC-DC converter 21d and the switching power supply device 20d described above also have the same effects as the DC-DC converters 21b and 21c and the switching power supply devices 20b and 20c.

[0070] [Fifth Modification] A DC-DC converter 21e and a switching power supply 20e according to a fifth modification of the present invention will be described below with reference to the drawings. FIG. 12 is a circuit diagram showing a switching power supply 20e equipped with a DC-DC converter 21e and a load resistor RL1. FIG. 13 is a cross-sectional view showing the structure of a transformer TR4 and a diagram showing the wiring of the transformer TR4. FIG. 14 is a plan view showing the structure of a first core CO1. Note that, regarding the DC-DC converter 21e and the switching power supply 20e according to the fifth modification, only the differences from the DC-DC converter 21c and the switching power supply 20c according to the third modification will be described, and the rest will be omitted.

[0071] The switching power supply device 20e according to the fifth modification differs from the switching power supply device 20c according to the third modification in that it does not include the third output terminals O5 and O6 and that the DC-DC converter 21e does not include the smoothing capacitor SC2 and the fourth output terminals O7 and O8. In this modification, the third DC voltage output from the third output terminals O5 and O6 is equal to the second DC voltage Vout.

[0072] 12, the connection point between the other end T8 of the winding LT23 and one end T9 of the winding LT24 is connected to the second output terminal O3. That is, the connection point between the other end T8 of the winding LT23 and one end T9 of the winding LT24 is connected to the connection point between the other end T4 of the winding LT21 and one end T5 of the winding LT22. The connection point between the third anode A3 and the fourth anode A4 is connected to the second output terminal O4. That is, the connection point between the third anode A3 and the fourth anode A4 is connected to the connection point between the first anode A1 and the second anode A2.

[0073] As shown in FIG. 13 , the first core CO1 further includes a third core portion CR3. The windings LT23 and LT24 are wound around the third core portion CR3. As shown in FIG. 14 , a gap V is provided between the first core portion CR1 and the third core portion CR3. As shown in FIGS. 13 and 14 , the first core CO1 forms a magnetic path. More specifically, the first core portion CR1 and the second core portion CR2 form a magnetic path through which a magnetic flux φ1 flows. Furthermore, the second core portion CR2 and the third core portion CR3 form a magnetic path through which a magnetic flux φ2 flows. In this case, the relationship φ = φ1 + φ2 holds. In this modification, the cross-sectional area of ​​the first core portion CR1 perpendicular to the magnetic path and the cross-sectional area of ​​the third core portion CR3 perpendicular to the magnetic path are each half the cross-sectional area of ​​the second core portion CR2 perpendicular to the magnetic path. The sum of the cross-sectional area of ​​the first core part CR1 perpendicular to the magnetic path and the cross-sectional area of ​​the third core part CR3 perpendicular to the magnetic path is equal to the cross-sectional area of ​​the second core part CR2 perpendicular to the magnetic path. Therefore, the magnitude of the magnetic flux φ1 and the magnitude of the magnetic flux φ2 are each half of the magnetic flux φ. The third core part CR3 corresponds to the "second core part" of the present invention.

[0074] The DC-DC converter 21e and switching power supply 20e described above also achieve the same effects as the DC-DC converters 21, 21a and switching power supplies 20, 20a. That is, the DC-DC converter 21e can further reduce copper loss due to the winding LT11. In other words, the DC-DC converter 21e can achieve a high step-down ratio without increasing the number of turns of the winding LT11 and without increasing the cross-sectional area of ​​the core portion perpendicular to the magnetic path.

[0075] [Sixth Modification] A DC-DC converter 21f and a switching power supply device 20f according to a sixth modification of the present invention will be described below with reference to the drawings. FIG. 15 is a cross-sectional view showing the structure of a transformer TR6 and a diagram showing the wiring of the transformer TR6. FIG. 16 is a plan view showing the structure of a first core CO1. Note that for the DC-DC converter 21f according to the sixth modification, only those parts that differ from the DC-DC converter 21e and the switching power supply device 20e according to the fifth modification will be described, and the rest will be omitted.

[0076] The DC-DC converter 21f and switching power supply device 20f according to the sixth modified example differ from the DC-DC converter 21e and switching power supply device 20e according to the fifth modified example in that they each include a transformer TR6 instead of the transformer TR4.

[0077] As shown in FIG. 15 , the first core CO1 further includes a fourth core portion CR4 and a fifth core portion CR5. That is, the first core CO1 includes five core portions. The winding LT22 is wound around the fourth core portion CR4. The winding LT23 is wound around the fifth core portion CR5. The winding LT24 is wound around the third core portion CR3. That is, the windings LT11, LT21, LT22, LT23, and LT24 are each wound around one of the five different core portions. As shown in FIG. 16 , a gap V is provided between the first core portion CR1 and the fourth core portion CR4. A gap V is provided between the fourth core portion CR4 and the fifth core portion CR5. A gap V is provided between the fifth core portion CR5 and the third core portion CR3. As shown in Figures 15 and 16, the first core CO1 forms a magnetic path. More specifically, the first core part CR1 and the second core part CR2 form a magnetic path through which magnetic flux φ1 flows. The second core part CR2 and the third core part CR3 form a magnetic path through which magnetic flux φ2 flows. The second core part CR2 and the fourth core part CR4 form a magnetic path through which magnetic flux φ3 flows. The second core part CR2 and the fifth core part CR5 form a magnetic path through which magnetic flux φ4 flows. In this case, the relationship φ = φ1 + φ2 + φ3 + φ4 holds. In this modification, the cross-sectional areas perpendicular to the magnetic path of the first core part CR1, the third core part CR3, the fourth core part CR4, and the fifth core part CR5 are each ¼ of the cross-sectional area perpendicular to the magnetic path of the second core part CR2. The sum of the cross-sectional areas perpendicular to the magnetic path of the first core part CR1, the third core part CR3, the fourth core part CR4, and the fifth core part CR5 is equal to the cross-sectional area perpendicular to the magnetic path of the second core part CR2. Therefore, the magnitudes of the magnetic flux φ1, φ2, φ3, and φ4 are each ¼ of the magnetic flux φ. The number of core parts may be six or more.

[0078] The above-described DC-DC converter 21f and switching power supply 20f also achieve the same effects as the DC-DC converters 21a, 21e and switching power supply devices 20a, 20e. Furthermore, the DC-DC converter 21f can reduce copper loss in the winding LT11 more than the DC-DC converter 21e. In other words, the DC-DC converter 21f can achieve a higher step-down ratio than the DC-DC converter 21e without increasing the number of turns of the winding LT11 and without increasing the cross-sectional area of ​​the core perpendicular to the magnetic path.

[0079] [Seventh Modification] A DC-DC converter 21g and a switching power supply 20g according to a seventh modification of the present invention will be described below with reference to the drawings. FIG. 17 is a circuit diagram showing a switching power supply 20g including a DC-DC converter 21g and a load resistor RL1. FIG. 18 is a cross-sectional view showing the structure of a transformer TR7 and a diagram showing the wiring of the transformer TR7. FIG. 19 is a plan view showing the structure of a second core CO2. Note that, regarding the DC-DC converter 21g and the switching power supply 20g according to the seventh modification, only the differences from the DC-DC converter 21e and the switching power supply 20e according to the fifth modification will be described, and the rest will be omitted.

[0080] A DC-DC converter 21g and a switching power supply 20g according to the seventh modification differ from the DC-DC converter 21e and the switching power supply 20e according to the fifth modification in that they respectively include transformers TR1 and TR7 instead of the transformer TR4. Note that the structure of the transformer TR1 is the same as that of the transformer TR1 according to the first embodiment, except that the cross-sectional area perpendicular to the magnetic path of the first core CO1 is half the cross-sectional area perpendicular to the magnetic path of the first core CO1 according to the first embodiment, and therefore a description thereof will be omitted.

[0081] In the circuit diagram shown in FIG. 17 , the transformer TR7 includes a second excitation inductance Lm2 and windings LT12, LT23, and LT24. The windings LT12, LT23, and LT24 are magnetically coupled. The second excitation inductance Lm2 is an inductance that generates a magnetic flux φ12 that interlinks with all of the windings LT12, LT23, and LT24. In the transformer TR7, the direction of the magnetic flux φ12 generated when a current flows from one end T11 of the winding LT12 to the other end T12 of the winding LT12 coincides with the direction of the magnetic flux φ12 generated when a current flows from one end T7 of the winding LT23 to the other end T8 of the winding LT23, and the direction of the magnetic flux φ12 generated when a current flows from one end T9 of the winding LT24 to the other end T10 of the winding LT24. Therefore, the windings LT12, LT23, and LT24 are magnetically positively coupled to one another. In this modification, the turns ratio between the windings LT11, LT12, LT21, LT22, LT23, and LT24 is n / 2:n / 2:1:1:1:1. Also, n=2. Note that n is not limited to 2. Also, the turns ratio between the windings LT11, LT12, LT21, LT22, LT23, and LT24 does not have to be n:n:1:1:1:1:1. The winding LT12 corresponds to the "sixth winding" of the present invention. The winding LT23 corresponds to the "fourth winding" of the present invention. The winding LT24 corresponds to the "fifth winding" of the present invention.

[0082] The other end T2 of the winding LT11 is connected to one end T11 of the winding LT12. The other end T12 of the winding LT12 is connected to one end T3 of the winding LT21 and the first cathode K1. The one end T11 of the winding LT12 corresponds to the "25th terminal" of the present invention. The other end T12 of the winding LT12 corresponds to the "26th terminal" of the present invention.

[0083] In the cross-sectional view shown in FIG. 18 , the transformer TR7 includes a second core CO2 and windings LT12, LT23, and LT24. The second core CO2 is made of a magnetic material. The second core CO2 has a fourth core portion CR4 and a sixth core portion CR6. The windings LT22 and LT23 are wound around the fourth core portion CR4. The winding LT12 is wound around the sixth core portion CR6. As shown in FIGS. 18 and 19 , the second core CO2 forms a magnetic path. More specifically, the fourth core portion CR4 and the sixth core portion CR6 form a magnetic path through which magnetic flux φ3 flows. In this modification, the cross-sectional area of ​​the second core CO2 perpendicular to the magnetic path is half the cross-sectional area of ​​the first core CO1 perpendicular to the magnetic path according to the first embodiment. In this modification, the cross-sectional area of ​​the fourth core part CR4 perpendicular to the magnetic path is equal to the cross-sectional area of ​​the sixth core part CR6 perpendicular to the magnetic path. An air gap may be provided between the fourth core part CR4 and the sixth core part CR6. The first core part CR1 in this modification corresponds to the "first core part" of the present invention. The second core part CR2 in this modification corresponds to the "second core part" of the present invention. The fourth core part CR4 in this modification corresponds to the "third core part" of the present invention. The sixth core part CR6 in this modification corresponds to the "fourth core part" of the present invention.

[0084] The DC-DC converter 21g and switching power supply device 20g described above also have the same effects as the DC-DC converter 21e and switching power supply device 20e.

[0085] [Eighth Modification] A DC-DC converter 21h and a switching power supply device 20h according to an eighth modification of the present invention will be described below with reference to the drawings. FIG. 20 is a cross-sectional view showing a schematic structure of the transformer TR8 and a diagram showing the wiring of the transformer TR8. FIG. 21 is a plan view showing a schematic structure of the second core CO2. Note that, regarding the DC-DC converter 21h and the switching power supply device 20h according to the eighth modification, only the differences from the DC-DC converter 21f and the switching power supply device 20f according to the seventh modification will be described, and the rest will be omitted.

[0086] A DC-DC converter 21h and a switching power supply device 20h according to the eighth modification differ from the DC-DC converter 21g and the switching power supply device 20g according to the seventh modification in that they respectively include a transformer TR2 instead of the transformer TR1, and a transformer TR8 instead of the transformer TR7. Note that the structure of the transformer TR2 is the same as that of the transformer TR2 according to the first modification, except that the cross-sectional area perpendicular to the magnetic path of the first core CO1 is half the cross-sectional area perpendicular to the magnetic path of the first core CO1 according to the first modification, and therefore a description thereof will be omitted.

[0087] As shown in FIG. 20 , the second core CO2 further includes a fifth core portion CR5. That is, the second core CO2 has three core portions. The winding LT23 is wound around the fourth core portion CR4. The winding LT24 is wound around the fifth core portion CR5. That is, the windings LT12, LT23, and LT24 are each wound around one of the three core portions of a different second core CO2. As shown in FIG. 21 , a gap V is provided between the fourth core portion CR4 and the fifth core portion CR5. As shown in FIGS. 20 and 21 , the second core CO2 forms a magnetic path. More specifically, the fourth core portion CR4 and the sixth core portion CR6 form a magnetic path through which magnetic flux φ31 flows. Furthermore, the fifth core portion CR5 and the sixth core portion CR6 form a magnetic path through which magnetic flux φ32 flows. In this case, the relationship φ3 = φ31 + φ32 holds. In this modification, the cross-sectional area perpendicular to the magnetic path of the fourth core part CR4 and the cross-sectional area perpendicular to the magnetic path of the fifth core part CR5 are each half the cross-sectional area perpendicular to the magnetic path of the sixth core part CR6. The sum of the cross-sectional area perpendicular to the magnetic path of the fourth core part CR4 and the cross-sectional area perpendicular to the magnetic path of the fifth core part CR5 is equal to the cross-sectional area perpendicular to the magnetic path of the sixth core part CR6. Therefore, the magnitude of the magnetic flux φ31 and the magnitude of the magnetic flux φ32 are each half the magnetic flux φ3. Note that the second core CO2 may have four or more core parts.

[0088] The DC-DC converter 21h and switching power supply 20h described above also achieve the same effects as the DC-DC converters 21, 21a and switching power supplies 20, 20a. That is, the DC-DC converter 21h can further reduce copper loss due to the winding LT11. In other words, the DC-DC converter 21h can achieve a high step-down ratio without increasing the number of turns of the winding LT11 and without increasing the cross-sectional area of ​​the core portion perpendicular to the magnetic path.

[0089] [Ninth Modification] A DC-DC converter 21i and a switching power supply device 20i according to a ninth modification of the present invention will be described below with reference to the drawings. Fig. 22 is a circuit diagram showing a switching circuit SW in the ninth modification. Note that with regard to the DC-DC converter 21i and the switching power supply device 20i according to the ninth modification, only the parts that are different from the DC-DC converter 21 and the switching power supply device 20 according to the first embodiment will be described, and the rest will be omitted.

[0090] As shown in FIG. 22 , the switching circuit SW further includes a third switching element S3 connected in series with the first switching element S1 and a fourth switching element S4 connected in series with the second switching element S2. The on-period of the first switching element S1 and the on-period of the third switching element S3 are the same. The on-period of the second switching element S2 and the on-period of the fourth switching element S4 are the same. This configuration increases the upper limit of the first DC voltage Vin that can be input to the DC-DC converters 21, 21a-21h and the switching power supply devices 20, 20a-20h. The third switching element S3 may include a parasitic capacitor C3 and a parasitic diode FD3. The fourth switching element S4 may include a parasitic capacitor C4 and a parasitic diode FD4. In this modification, the first switching element S1 and the third switching element S3 correspond to the “first switching element” of the present invention. The source S of the third switching element S3 corresponds to the “second terminal” of the present invention. The second switching element S2 and the fourth switching element S4 correspond to the "second switching element" of the present invention. The source S of the fourth switching element S4 corresponds to the "fourth terminal" of the present invention.

[0091] [Tenth Modification] A DC-DC converter 21j and a switching power supply device 20j according to a tenth modification of the present invention will be described below with reference to the drawings. Fig. 23 is a circuit diagram showing a switching circuit SW in the tenth modification. Note that with regard to the DC-DC converter 21j and the switching power supply device 20j according to the tenth modification, only the parts that are different from the DC-DC converter 21 and the switching power supply device 20 according to the first embodiment will be described, and the rest will be omitted.

[0092] As shown in FIG. 23 , the switching circuit SW further includes a third switching element S3 connected in parallel with the first switching element S1 and a fourth switching element S4 connected in parallel with the second switching element S2. The on-period of the first switching element S1 and the on-period of the third switching element S3 are the same. The on-period of the second switching element S2 and the on-period of the fourth switching element S4 are the same. This increases the upper limit of the current that can be input to the DC-DC converter 21, 21a-21h and the switching power supply device 20, 20a-20h. The third switching element S3 may include a parasitic capacitor C3 and a parasitic diode FD3. The fourth switching element S4 may include a parasitic capacitor C4 and a parasitic diode FD4. In this modification, the first switching element S1 and the third switching element S3 correspond to the “first switching element” of the present invention. The second switching element S2 and the fourth switching element S4 correspond to the “second switching element” of the present invention.

[0093] [Eleventh Modification] A DC-DC converter 21k and a switching power supply device 20k according to an eleventh modification of the present invention will be described below with reference to the drawings. Fig. 24 is a circuit diagram showing a modification of the first rectifier element. Note that with regard to the DC-DC converter 21k and the switching power supply device 20k according to the eleventh modification, only the parts that are different from the DC-DC converter 21 and the switching power supply device 20 according to the first embodiment will be described, and the rest will be omitted.

[0094] The DC-DC converter 21k and switching power supply device 20k according to the eleventh modification differ from the DC-DC converter 21 and switching power supply device 20 according to the first embodiment in that they each include a MOSFET rectifier MOSC instead of the first diode D1.

[0095] The MOSFET rectifier MOSC includes a MOSFET 1, an operational amplifier OP, and a DC power supply Vcc. When the potential of the drain D of MOSFET 1 is equal to or lower than the potential of the source S of MOSFET 1, the operational amplifier OP applies a positive voltage between the gate G and source S of MOSFET 1, turning MOSFET 1 on. When the potential of the drain D of MOSFET 1 is higher than the potential of the source S of MOSFET 1, the operational amplifier OP does not apply a positive voltage between the gate G and source S of MOSFET 1, turning MOSFET 1 off. In other words, the MOSFET rectifier MOSC has a function similar to that of the first diode D1. In this modification, the MOSFET rectifier MOSC corresponds to the "first rectifying element" of the present invention. One terminal A11 of the MOSFET rectifier MOSC corresponds to the "thirteenth terminal" of the present invention. The other terminal K11 of the MOSFET rectifier MOSC corresponds to the "fourteenth terminal" of the present invention. By using a MOSFET with a low on-resistance as the MOSFET 1, the conduction loss generated by the "first rectifying element" can be reduced.

[0096] As shown in this modification, the "first rectifying element" of the present invention is not limited to a diode. Similarly, the "second rectifying element," "third rectifying element," and "fourth rectifying element" of the present invention are not limited to diodes.

[0097] Other Embodiments The DC-DC converter according to the present invention is not limited to DC-DC converters 21, 21a to 21k, and can be modified within the scope of the present invention. In addition, the structures of DC-DC converters 21, 21a to 21k may be combined in any manner.

[0098] The switching power supply device according to the present invention is not limited to the switching power supply devices 20, 20a to 20k, and can be modified within the scope of the present invention. Furthermore, the structures of the switching power supply devices 20, 20a to 20k may be combined in any manner.

[0099] It should be noted that the switching circuits SW in the DC-DC converters 21, 21a to 21k and the switching power supplies 20, 20a to 20jk should not be of a full-bridge type, since if the switching circuits SW were of a full-bridge type, the number of switching elements would increase, making the circuit configuration and control more complex, increasing losses, and making the DC-DC converters and switching power supplies larger and more difficult to dissipate heat.

[0100] The present invention has the following configuration.

[0101] (1) A power supply comprising: a first switching element having a first end and a second end, the first end being connected to a DC power source; a second switching element having a third end and a fourth end, the third end being connected to the second end; an LC series resonant circuit having a fifth end and a sixth end, the fifth end being connected to a connection point between the second end and the third end; a conductor having a seventh end and an eighth end; a first winding having a ninth end and a tenth end; a second winding having an eleventh end and a twelfth end; a first rectifying element having a thirteenth end and a fourteenth end; and a second rectifying element having a fifteenth end and a sixteenth end, the fifteenth end being connected to the thirteenth end; The twelfth terminal is connected to the fourth terminal and the sixteenth terminal.

[0102] (2) The DC-DC converter according to (1), wherein the conductor is a third winding, and the first winding, the second winding, and the third winding are magnetically positively coupled to each other.

[0103] (3) The DC-DC converter according to (1) or (2), further comprising: a fourth winding having a 17th terminal and an 18th terminal; a fifth winding having a 19th terminal and a 20th terminal; a third rectifier element having a 21st terminal and a 22nd terminal; and a fourth rectifier element having a 23rd terminal and a 24th terminal, the 23rd terminal being connected to the 21st terminal; wherein the first winding, the second winding, the fourth winding, and the fifth winding are magnetically positively coupled to each other; the 17th terminal is connected to the 22nd terminal; the 18th terminal is connected to the 19th terminal; and the 20th terminal is connected to the 24th terminal.

[0104] (4) The DC-DC converter according to (1) or (2), further comprising: a fourth winding having a 17th terminal and an 18th terminal; a fifth winding having a 19th terminal and a 20th terminal; a third rectifier element having a 21st terminal and a 22nd terminal; and a fourth rectifier element having a 23rd terminal and a 24th terminal, the 23rd terminal being connected to the 21st terminal; wherein the first winding, the second winding, the fourth winding, and the fifth winding are magnetically positively coupled to each other; the 17th terminal is connected to the 22nd terminal; the 18th terminal is connected to the 19th terminal; the 20th terminal is connected to the 24th terminal; a connection point between the 18th terminal and the 19th terminal is connected to a connection point between the 10th terminal and the 11th terminal; and a connection point between the 21st terminal and the 23rd terminal is connected to a connection point between the 13th terminal and the 15th terminal.

[0105] (5) The DC-DC converter according to (4), further comprising a core that forms a magnetic path, the core having a first core portion and a second core portion, the first winding and the second winding being wound around the first core portion, and the fourth winding and the fifth winding being wound around the second core portion.

[0106] (6) The DC-DC converter according to (4), further comprising a core that forms a magnetic path, the core having five or more core portions, and the first winding, the second winding, the fourth winding, and the fifth winding are each wound around one of the five or more different core portions.

[0107] (7) The DC-DC converter according to any one of (1) to (6), wherein the first rectifying element and the second rectifying element are each a diode; the thirteenth terminal and the fifteenth terminal are each an anode; and the fourteenth terminal and the sixteenth terminal are each a cathode.

[0108] (8) The DC-DC converter according to (3) or (4), wherein the third rectifying element and the fourth rectifying element are each a diode, the 21st terminal and the 23rd terminal are each an anode, and the 22nd terminal and the 24th terminal are each a cathode.

[0109] (9) The DC-DC converter according to any one of (1) to (8), wherein a first period during which the first switching element is conductive and the second switching element is non-conductive, and a second period during which the second switching element is conductive and the first switching element is non-conductive, are repeated periodically, and during the first period, the potential of the ninth terminal is different from the potential of the fourth terminal, and DC voltages are output from a connection point between the tenth terminal and the eleventh terminal and a connection point between the thirteenth terminal and the fifteenth terminal.

[0110] (10) The DC-DC converter according to (9), wherein the ratio of the first period to the second period is 1:1.

[0111] (11) A first switching element having a first end and a second end, the first end being connected to a DC power supply; a second switching element having a third end and a fourth end, the third end being connected to the second end; an LC series resonant circuit having a fifth end and a sixth end, the fifth end being connected to a connection point between the second end and the third end; a third winding having a seventh end and an eighth end; a first winding having a ninth end and a tenth end; a second winding having an eleventh end and a twelfth end; a first rectifying element having a thirteenth end and a fourteenth end; a second rectifying element having a fifteenth end and a sixteenth end, the fifteenth end being connected to the thirteenth end; a fourth winding having a seventeenth end and an eighteenth end; a fifth winding having a nineteenth end and a twentieth end; a sixth winding having a twenty-fifth end and a twenty-sixth end; and a third rectifying element having a twenty-first end and a twenty-second end. a fourth rectifier element having a 23rd terminal and a 24th terminal, the 23rd terminal being connected to the 21st terminal; wherein the 6th terminal is connected to the 7th terminal, the 8th terminal is connected to the 25th terminal, the 26th terminal is connected to the 9th terminal and the 14th terminal, the 10th terminal is connected to the 11th terminal, the 12th terminal is connected to the 4th terminal and the 16th terminal, the 17th terminal is connected to the 22nd terminal, the 18th terminal is connected to the 19th terminal, the 20th terminal is connected to the 24th terminal, a connection point between the 18th terminal and the 19th terminal is connected to a connection point between the 10th terminal and the 11th terminal, and a connection point between the 21st terminal and the 23rd terminal is connected to a connection point between the 13th terminal and the 15th terminal.

[0112] (12) The DC-DC converter according to (11), further comprising: a first core that forms a magnetic path; and a second core that forms a magnetic path, wherein the first core has a first core portion and a second core portion, wherein the first winding and the second winding are each wound around the first core portion, the third winding is wound around the second core portion, and the second core has a third core portion and a fourth core portion, wherein the fourth winding and the fifth winding are each wound around the third core portion, and the sixth winding is wound around the fourth core portion.

[0113] (13) The DC-DC converter according to (11), further comprising: a first core that forms a magnetic path; and a second core that forms a magnetic path, wherein the first core has three or more core portions, and the first winding, the second winding, and the third winding are each wound around one of the three or more different core portions of the first core, and the second core has three or more core portions, and the fourth winding, the fifth winding, and the sixth winding are each wound around one of the three or more different core portions of the second core.

[0114] (14) The DC-DC converter according to any one of (11) to (13), wherein the first rectifier element, the second rectifier element, the third rectifier element, and the fourth rectifier element are each diodes; the thirteenth terminal, the fifteenth terminal, the twenty-first terminal, and the twenty-third terminal are each anodes; and the fourteenth terminal, the sixteenth terminal, the twenty-second terminal, and the twenty-fourth terminal are each cathodes.

[0115] (15) The DC-DC converter according to any one of (11) to (14), wherein a first period during which the first switching element is conductive and the second switching element is non-conductive, and a second period during which the second switching element is conductive and the first switching element is non-conductive, are repeated cyclically, and during the first period, the potential of the 26th terminal is different from the potential of the fourth terminal, and DC voltages are output from a connection point between the 10th terminal and the 11th terminal and a connection point between the 13th terminal and the 15th terminal.

[0116] (16) The DC-DC converter according to (15), wherein the ratio of the first period to the second period is 1:1.

[0117] (17) A switching power supply device comprising: the DC-DC converter according to any one of (1) to (16); and the DC power supply.

[0118] 20, 20a to 20k: Switching power supply device 21, 21a to 21k: DC-DC converter A1: First anode A2: Second anode A3: Third anode A4: Fourth anode C1, C2, C3, C4: Parasitic capacitor C: Capacitor CO1: First core CO2: Second core CR1: First core section CR2: Second core section CR3: Third core section CR4: Fourth core section CR5: Fifth core section CR6: Sixth core section CS1: First control signal CS2: Second control signal FD1, FD2, FD3, FD4: Parasitic diodes D1: First diode D2: Second diode D3: Third diode D4: Fourth diode DCPS, Vcc: DC power supply GD: Gate drive circuit IT: Input terminal K1: First cathode K2: Second cathode K3: Third cathode K4: Fourth cathode L: Reactor LC: LC series resonant circuit LT11, LT12, LT21, LT22, LT23, LT24: Windings Lm1: First exciting inductance Lm2: Second exciting inductance O1, O2: First output terminal O3, O4: Second output terminal O5, O6: Third output terminal O7, O8: Fourth output terminal OP: Operational amplifier P1: First period P2: Second period RL1, RL2: Load resistor S1: First switching element S2: Second switching element S3: Third switching element S4: Fourth switching element SC1, SC2: Smoothing capacitor SW: Switching circuit TR1 to TR8: Transformer V: Air gap Vin: First DC voltage Vout: Second DC voltage i1, i2, iD1, iD2: Current im: Excitation current φ: Magnetic flux

Claims

1. A switching element having a first end and a second end, the first end being connected to a DC power source; a second switching element having a third end and a fourth end, the third end being connected to the second end; an LC series resonant circuit having a fifth end and a sixth end, the fifth end being connected to a connection point between the second end and the third end; a conductor having a seventh end and an eighth end; a first winding having a ninth end and a tenth end; a second winding having an eleventh end and a twelfth end; a first rectifying element having a thirteenth end and a fourteenth end; and a second rectifying element having a fifteenth end and a sixteenth end, the fifteenth end being connected to the thirteenth end; the twelfth terminal is connected to the fourth terminal and the sixteenth terminal.

2. The DC-DC converter according to claim 1, wherein the conductor is a third winding, and the first winding, the second winding and the third winding are magnetically positively coupled to each other.

3. The DC-DC converter according to claim 1 or 2, further comprising: a fourth winding having a 17th terminal and an 18th terminal; a fifth winding having a 19th terminal and a 20th terminal; a third rectifier element having a 21st terminal and a 22nd terminal; and a fourth rectifier element having a 23rd terminal and a 24th terminal, the 23rd terminal being connected to the 21st terminal; wherein the first winding, the second winding, the fourth winding and the fifth winding are magnetically positively coupled to each other; the 17th terminal is connected to the 22nd terminal; the 18th terminal is connected to the 19th terminal; and the 20th terminal is connected to the 24th terminal.

4. The DC-DC converter according to claim 1 or 2, further comprising: a fourth winding having a 17th terminal and an 18th terminal; a fifth winding having a 19th terminal and a 20th terminal; a third rectifier element having a 21st terminal and a 22nd terminal; and a fourth rectifier element having a 23rd terminal and a 24th terminal, the 23rd terminal being connected to the 21st terminal; wherein the first winding, the second winding, the fourth winding, and the fifth winding are magnetically positively coupled to each other; the 17th terminal is connected to the 22nd terminal; the 18th terminal is connected to the 19th terminal; the 20th terminal is connected to the 24th terminal; a connection point between the 18th terminal and the 19th terminal is connected to a connection point between the 10th terminal and the 11th terminal, and a connection point between the 21st terminal and the 23rd terminal is connected to a connection point between the 13th terminal and the 15th terminal.

5. A DC-DC converter as set forth in claim 4, further comprising a core forming a magnetic path, said core having a first core portion and a second core portion, said first winding and said second winding being each wound around said first core portion, and said fourth winding and said fifth winding being each wound around said second core portion.

6. The DC-DC converter as set forth in claim 4, further comprising a core forming a magnetic path, said core having five or more core portions, and said first winding, said second winding, said fourth winding and said fifth winding being each wound around one of different ones of said five or more core portions.

7. A DC-DC converter as claimed in any one of claims 1 to 6, wherein the first rectifying element and the second rectifying element are each diodes, the thirteenth terminal and the fifteenth terminal are each anodes, and the fourteenth terminal and the sixteenth terminal are each cathodes.

8. A DC-DC converter as claimed in claim 3 or 4, wherein the third rectifying element and the fourth rectifying element are each a diode, the 21st terminal and the 23rd terminal are each an anode, and the 22nd terminal and the 24th terminal are each a cathode.

9. A DC-DC converter as claimed in any one of claims 1 to 8, wherein a first period during which the first switching element is conductive and the second switching element is non-conductive, and a second period during which the second switching element is conductive and the first switching element is non-conductive are repeated cyclically, and during the first period, a potential of the ninth terminal is different from a potential of the fourth terminal, and a DC voltage is output from a connection point between the tenth terminal and the eleventh terminal, and a connection point between the thirteenth terminal and the fifteenth terminal.

10. The DC-DC converter according to claim 9, wherein the ratio of said first period to said second period is 1:

1.

11. A first switching element having a first end and a second end, the first end being connected to a DC power source; a second switching element having a third end and a fourth end, the third end being connected to the second end; an LC series resonant circuit having a fifth end and a sixth end, the fifth end being connected to a connection point between the second end and the third end; a third winding having a seventh end and an eighth end; a first winding having a ninth end and a tenth end; a second winding having an eleventh end and a twelfth end; a first rectifying element having a thirteenth end and a fourteenth end; a second rectifying element having a fifteenth end and a sixteenth end, the fifteenth end being connected to the thirteenth end; a fourth winding having a seventeenth end and an eighteenth end; a fifth winding having a nineteenth end and a twentieth end; a sixth winding having a twenty-fifth end and a twenty-sixth end; and a third rectifying element having a twenty-first end and a twenty-second end. a fourth rectifier element having a 23rd terminal and a 24th terminal, the 23rd terminal being connected to the 21st terminal; wherein the 6th terminal is connected to the 7th terminal, the 8th terminal is connected to the 25th terminal, the 26th terminal is connected to the 9th terminal and the 14th terminal, the 10th terminal is connected to the 11th terminal, the 12th terminal is connected to the 4th terminal and the 16th terminal, the 17th terminal is connected to the 22nd terminal, the 18th terminal is connected to the 19th terminal, the 20th terminal is connected to the 24th terminal, a connection point between the 18th terminal and the 19th terminal is connected to a connection point between the 10th terminal and the 11th terminal, and a connection point between the 21st terminal and the 23rd terminal is connected to a connection point between the 13th terminal and the 15th terminal.

12. A DC-DC converter as described in claim 11, further comprising: a first core forming a magnetic path; and a second core forming a magnetic path, wherein the first core has a first core portion and a second core portion, wherein the first winding and the second winding are each wound around the first core portion, wherein the third winding is wound around the second core portion, wherein the second core has a third core portion and a fourth core portion, wherein the fourth winding and the fifth winding are each wound around the third core portion, and wherein the sixth winding is wound around the fourth core portion.

13. A DC-DC converter as described in claim 11, further comprising: a first core forming a magnetic path; and a second core forming a magnetic path, wherein the first core has three or more core portions, the first winding, the second winding and the third winding are each wound around one of the three or more different core portions of the first core, the second core has three or more core portions, and the fourth winding, the fifth winding and the sixth winding are each wound around one of the three or more different core portions of the second core.

14. A DC-DC converter as described in any one of claims 11 to 13, wherein the first rectifier element, the second rectifier element, the third rectifier element and the fourth rectifier element are each diodes; the 13th terminal, the 15th terminal, the 21st terminal and the 23rd terminal are each anodes; and the 14th terminal, the 16th terminal, the 22nd terminal and the 24th terminal are each cathodes.

15. A DC-DC converter as claimed in any one of claims 11 to 14, wherein a first period during which the first switching element is conductive and the second switching element is non-conductive, and a second period during which the second switching element is conductive and the first switching element is non-conductive are repeated cyclically, and during the first period, a potential at the 26th terminal is different from a potential at the fourth terminal, and a DC voltage is output from a connection point between the 10th terminal and the 11th terminal and a connection point between the 13th terminal and the 15th terminal.

16. The DC-DC converter of claim 15, wherein the ratio of said first period to said second period is 1:

1.

17. A switching power supply device comprising: a DC-DC converter according to any one of claims 1 to 16; and the DC power supply.

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