Switching power supply and power supply system

Abstract

A switching power source device according to one embodiment of the present invention comprises: an input terminal pair; an output terminal pair; a transformer; an inverter circuit constituted by including first to fourth switching elements, a first capacitor, and a changeover switch; a rectification and smoothing circuit constituted by including a rectification circuit having a plurality of rectification elements, an inductor, and a second capacitor; and a drive unit. The first and second switching elements are connected in series to each other between a pair of connection lines, which are individually connected to the input terminal pair, and the third and fourth switching elements are connected in series to each other between a pair of connection lines. The first capacitor and a primary-side winding are connected to each other in series in no particular order between a first connection point, which is a connection point between the first and second switching elements, and a second connection point, which is a connection point between the third and fourth switching elements, and the changeover switch is connected in parallel with the first capacitor.

Description

[Technical field]

[0001] The present invention relates to a switching power supply device that performs voltage conversion using a switching element, and to a power supply system including such a switching power supply device. [Background technology]

[0002] As an example of a switching power supply device, various DC-DC converters have been proposed and put into practical use (for example, see Patent Document 1). This type of DC-DC converter generally includes an inverter circuit including a switching element, a power conversion transformer, and a rectifying and smoothing circuit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2010-148227 A Summary of the Invention

[0004] However, in general, there is a demand for miniaturization and low loss in switching power supply devices such as DC-DC converters. It is desirable to provide a switching power supply device that can achieve both miniaturization and low loss, and a power supply system including such a switching power supply device.

[0005] According to one embodiment of the present invention 1.The switching power supply device includes an input terminal pair to which an input voltage is input, an output terminal pair to which an output voltage is output, a transformer having a primary winding and a secondary winding, an inverter circuit disposed between the input terminal pair and the primary winding and including first to fourth switching elements, a first capacitor, and a changeover switch, a rectifying and smoothing circuit disposed between the output terminal pair and the secondary winding and including a rectifying circuit having a plurality of rectifying elements and a smoothing circuit having an inductor and a second capacitor, and a drive unit that performs switching drive to control the operations of the first to fourth switching elements in the inverter circuit and controls the operation of the changeover switch. The first and second switching elements are connected in series to each other between a pair of connection lines individually connected to the input terminal pair, and the third and fourth switching elements are connected in series to each other between the pair of connection lines. The first capacitor and the primary winding are connected in series with each other in no particular order between a first connection point which is a connection point between the first and second switching elements and a second connection point which is a connection point between the third and fourth switching elements, and the changeover switch is connected in parallel to the first capacitor. In addition, when the input voltage is equal to or higher than a predetermined threshold voltage, the drive unit sets the change-over switch to an off state and performs switching drive so that half-bridge operation is performed in the inverter circuit, and when the input voltage is less than the threshold voltage, the drive unit sets the change-over switch to an on state and performs switching drive so that full-bridge operation is performed in the inverter circuit. A second switching power supply according to an embodiment of the present invention includes an input terminal pair to which an input voltage is input, an output terminal pair to which an output voltage is output, a transformer having a primary winding and a secondary winding, an inverter circuit disposed between the input terminal pair and the primary winding and including first to fourth switching elements, a first capacitor, a changeover switch, a discharge resistor element, and a discharge switch, a rectifying and smoothing circuit disposed between the output terminal pair and the secondary winding and including a rectifier circuit having a plurality of rectifying elements and a smoothing circuit having an inductor and a second capacitor, and a drive unit that performs switching drive to control the operations of the first to fourth switching elements in the inverter circuit and controls the operation of the changeover switch. The first and second switching elements are connected in series to each other between a pair of connection lines individually connected to the input terminal pair, and the third and fourth switching elements are connected in series to each other between the pair of connection lines. The first capacitor and the primary winding are connected in series in any order between a first connection point which is a connection point between the first and second switching elements and a second connection point which is a connection point between the third and fourth switching elements, the changeover switch is connected in parallel to the first capacitor, and the discharge resistance element and the discharge switch which are connected in series to each other are each connected in parallel to the first capacitor. When switching the operating state of the inverter circuit from a half-bridge operation state in which the changeover switch is set to an OFF state to a full-bridge operation state in which the changeover switch is set to an ON state, the drive unit sets the discharge switch to an ON state, thereby discharging the charge stored in the first capacitor via the discharge resistance element, and then switches the changeover switch from an OFF state to an ON state.

[0006] A power supply system according to an embodiment of the present invention includes the switching power supply device according to the embodiment of the present invention described above, and a power supply that supplies the input voltage to the input terminal pair.

[0007] According to one embodiment of the present invention 1st and 2nd Switching Power Supply Unit And The present invention According to one embodiment According to the power supply system, it is possible to reduce loss while also achieving miniaturization. [Brief description of the drawings]

[0008] [Figure 1]1 is a circuit diagram illustrating a schematic configuration example of a switching power supply device according to an embodiment of the present invention. [Diagram 2] 1 is a circuit diagram illustrating a schematic configuration example of a switching power supply device according to a comparative example. [Diagram 3] 5A and 5B are diagrams illustrating examples of operation of switching power supply devices according to a comparative example and an embodiment. [Figure 4] 2 is a circuit diagram illustrating a schematic example of an operation of the switching power supply device illustrated in FIG. 1 in a half-bridge operating state. [Diagram 5] 2 is a circuit diagram illustrating a schematic example of an operation of the switching power supply device illustrated in FIG. 1 in a full-bridge operation state. [Figure 6] 1 is a circuit diagram illustrating a schematic configuration example of a switching power supply device according to a first modification. [Figure 7] 7 is a circuit diagram illustrating an example of an operation of the inverter circuit illustrated in FIG. 6 when the operating state is switched. [Figure 8] FIG. 8 is a circuit diagram illustrating a schematic operation example following FIG. 7. [Figure 9] FIG. 9 is a circuit diagram illustrating a schematic operation example following that of FIG. 8. [Figure 10] FIG. 11 is a circuit diagram illustrating a schematic configuration example of a switching power supply device according to Modification 2. [Figure 11] 11 is a circuit diagram illustrating an example of an operation of the inverter circuit illustrated in FIG. 10 when the operating state is switched. [Figure 12] FIG. 12 is a circuit diagram illustrating a schematic operation example following FIG. 11. [Figure 13] FIG. 11 is a circuit diagram illustrating a schematic configuration example of a switching power supply device according to Modification 3. [Figure 14] FIG. 11 is a circuit diagram illustrating a schematic configuration example of a switching power supply device according to Modification 4. [Figure 15] FIG. 13 is a circuit diagram illustrating a schematic configuration example of a switching power supply device according to Modification 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The description will be given in the following order. 1. Embodiment (Example of a case where a center-tap type rectifier circuit is used) 2. Variations Modifications 1 and 2 (Examples in which a discharge resistor element and a switch are further provided) Modification 3 (Example of using a bridge-type rectifier circuit) Modifications 4 and 5 (Examples of the embodiment and modification 3 in which a synchronous rectification circuit is used) 3. Other Modifications

[0010] <1. Preferred embodiment> [composition] 1 is a circuit diagram showing an example of the schematic configuration of a switching power supply device (switching power supply device 1) according to an embodiment of the present invention. This switching power supply device 1 functions as a DC-DC converter that converts a DC input voltage Vin supplied from a DC input power source 10 (e.g., a battery) into a DC output voltage Vout and supplies power to a load 9. Note that examples of this load 9 include electronic devices and batteries. Note that the mode of voltage conversion in the switching power supply device 1 may be either up-conversion (step-up) or down-conversion (step-down).

[0011] Here, the DC input voltage Vin corresponds to a specific example of the "input voltage" in the present invention, and the DC output voltage Vout corresponds to a specific example of the "output voltage" in the present invention. Moreover, the DC input power supply 10 corresponds to a specific example of the "power supply" in the present invention, and a system including this DC input power supply 10 and the switching power supply device 1 corresponds to a specific example of the "power supply system" in the present invention.

[0012] The switching power supply device 1 includes two input terminals T1, T2, two output terminals T3, T4, an inverter circuit 2, a transformer 3, a rectifying and smoothing circuit 4, and a drive circuit 5. A DC input voltage Vin is input between the input terminals T1, T2, and a DC output voltage Vout is output between the output terminals T3, T4.

[0013] Here, the input terminals T1, T2 correspond to a specific example of an "input terminal pair" in the present invention, and the output terminals T3, T4 correspond to a specific example of an "output terminal pair" in the present invention.

[0014] For example, an input capacitor may be arranged between the primary high-voltage line L1H connected to the input terminal T1 and the primary low-voltage line L1L connected to the input terminal T2. Specifically, at a position between the inverter circuit 2 described later and the input terminals T1 and T2, a first end (one end) of the input capacitor may be connected to the primary high-voltage line L1H and a second end (the other end) of the input capacitor may be connected to the primary low-voltage line L1L. Such an input capacitor is a capacitor for stabilizing the DC input voltage Vin input from the input terminals T1 and T2.

[0015] (Inverter circuit 2) The inverter circuit 2 is disposed between input terminals T1, T2 and a primary winding 31 of a transformer 3, which will be described later. The inverter circuit 2 has four switching elements S1 to S4, a capacitor C1, and switches S5 and S6, and is a so-called "bridge-type" inverter circuit. As will be described in detail later, the operating state (operating mode) of the inverter circuit 2 can be switched between an operating state in which a half-bridge operation is performed and an operating state in which a full-bridge operation is performed.

[0016] Here, the primary high-voltage line L1H and the primary low-voltage line L1L each correspond to a specific example of a "pair of connection lines" in the present invention. Moreover, the switching element S1 corresponds to a specific example of a "first switching element" in the present invention, and the switching element S2 corresponds to a specific example of a "second switching element" in the present invention. Similarly, the switching element S3 corresponds to a specific example of a "third switching element" in the present invention, and the switching element S4 corresponds to a specific example of a "fourth switching element" in the present invention. Moreover, the switches S5 and S6 correspond to specific examples of a "changeover switch" and a "bidirectional switch" in the present invention, respectively. Moreover, the capacitor C1 corresponds to a specific example of a "first capacitor" in the present invention.

[0017] As the switching elements S1 to S4, for example, switching elements such as metal oxide semiconductor-field effect transistors (MOS-FETs) and insulated gate bipolar transistors (IGBTs) are used. In the example shown in Fig. 1, the switching elements S1 to S4 are each configured with a MOS-FET. In this way, when a MOS-FET is used as the switching elements S1 to S4, the capacitors and diodes (not shown in Fig. 1) connected in parallel to each of the switching elements S1 to S4 can be configured with the parasitic capacitance or parasitic diode of the MOS-FET.

[0018] In this inverter circuit 2, two switching elements S1, S2 are connected in series to each other in this order between the input terminals T1, T2 (between the primary high-voltage line L1H and the primary low-voltage line L1L). Specifically, the switching element S1 is arranged between the primary high-voltage line L1H and the connection point P1, and the switching element S2 is arranged between the connection point P1 and the primary low-voltage line L1L. Similarly, two switching elements S3, S4 are connected in series to each other in this order between these input terminals T1, T2. Specifically, the switching element S3 is arranged between the primary high-voltage line L1H and the connection point P2, and the switching element S4 is arranged between the connection point P2 and the primary low-voltage line L1L.

[0019] In the inverter circuit 2, the capacitor C1 and a primary winding 31 of the transformer 3 described below are connected in series between a connection point (connection point P1) between the switching elements S1 and S2 and a connection point (connection point P2) between the switching elements S3 and S4. Specifically, in the example of Fig. 1, a first end of the capacitor C1 is connected to the connection point P2, a second end of the capacitor C1 is connected to the connection point P3, one end of the primary winding 31 is connected to the connection point P3, and the other end of the primary winding 31 is connected to the connection point P1.

[0020] Furthermore, in this inverter circuit 2, switches S5 and S6 connected in series to each other are connected in parallel to the capacitor C1. Specifically, as shown in FIG. 1, one end of the switch S5 is connected to a connection point P2, the other end of the switch S5 is connected to one end of the switch S6, and the other end of the switch S6 is connected to a connection point P3. In the example of FIG. 1, these switches S5 and S6 are configured as bidirectional switches made up of a pair of MOS-FETs connected in series in the opposite directions. In addition, although details will be described later, each of these switches S5 and S6 functions as a changeover switch used when switching between the operating states (half-bridge operating state and full-bridge operating state) of the inverter circuit 2 described above.

[0021] The above-mentioned connection point P1 corresponds to a specific example of a "first connection point" in the present invention, and the above-mentioned connection point P2 corresponds to a specific example of a "second connection point" in the present invention.

[0022] In the inverter circuit 2 configured as described above, the switching elements S1 to S4 perform switching operations (on / off operations) in accordance with drive signals SG1 to SG4 supplied from a drive circuit 5 (described later), as follows: That is, the DC input voltage Vin applied between the input terminals T1 and T2 is converted into an AC voltage (voltage Vp) and output to the transformer 3 (primary winding 31).

[0023] (Trans 3) The transformer 3 has one primary winding 31 and two secondary windings 321 and 322.

[0024] In the primary winding 31, a first end (one end) of the primary winding 31 is connected to the aforementioned connection point P3, and a second end (the other end) of the primary winding 31 is connected to the aforementioned connection point P1.

[0025] In the secondary winding 321, a first end of the secondary winding 321 is connected to a cathode of a rectifier diode 41 (to be described later) via a connection line L21 (to be described later), and a second end of the secondary winding 321 is connected to a center tap P6 in a rectifier smoothing circuit 4 (to be described later). In the secondary winding 322, a first end of the secondary winding 322 is connected to a cathode of a rectifier diode 42 (to be described later) via a connection line L22 (to be described later), and a second end of the secondary winding 322 is connected to the center tap P6. In other words, the second ends of the secondary windings 321 and 322 are commonly connected to the center tap P6.

[0026] The transformer 3 converts the voltage generated by the inverter circuit 2 (voltage Vp, which is input to the primary winding 31 of the transformer 3 and has been converted into a rectangular pulse wave: see FIG. 1) and outputs an AC voltage (voltage Vs) from each end of the secondary windings 321 and 322. Specifically, the secondary winding 321 outputs a voltage Vs1, and the secondary winding 322 outputs a voltage Vs2 (see FIG. 1). In this case, the degree of voltage conversion of the DC output voltage Vout relative to the DC input voltage Vin is determined as follows. That is, the degree of such voltage conversion is determined by the turn ratio between the primary winding 31 and the secondary windings 321 and 322, and the time ratio of the on-period of each switching element S1 to S4 relative to the switching cycle (=1 / switching frequency).

[0027] (Rectifier smoothing circuit 4) The rectifying smoothing circuit 4 has two rectifying diodes 41 and 42, one choke coil Lch, and one output smoothing capacitor Cout. Specifically, the rectifying smoothing circuit 4 includes a rectifying circuit having the rectifying diodes 41 and 42, and a smoothing circuit having the choke coil Lch and the output smoothing capacitor Cout.

[0028] The two rectifier diodes 41 and 42 correspond to a specific example of "plurality of rectifier elements" in the present invention. The choke coil Lch corresponds to a specific example of "inductor" in the present invention, and the output smoothing capacitor Cout corresponds to a specific example of "second capacitor" in the present invention.

[0029] The above-mentioned rectifier circuit is a so-called "center tap type" rectifier circuit. That is, the anodes of the rectifier diodes 41 and 42 are connected to the ground line LG, the cathode of the rectifier diode 41 is connected to the first end of the secondary winding 321 via the connection line L21, and the cathode of the rectifier diode 42 is connected to the first end of the secondary winding 322 via the connection line L22. As described above, the second ends of the secondary windings 321 and 322 are commonly connected to the center tap P6, and the center tap P6 is connected to the output terminal T3 via the output line LO and the choke coil Lch. The above-mentioned ground line LG is connected to the output terminal T4.

[0030] In the above-mentioned smoothing circuit, an output smoothing capacitor Cout is connected between the above-mentioned output line LO and ground line LG (between the output terminals T3 and T4). That is, a first terminal of the output smoothing capacitor Cout is connected to the output line LO, and a second terminal of the output smoothing capacitor Cout is connected to the ground line LG.

[0031] In the rectifier smoothing circuit 4 configured as above, the rectifier circuit including the rectifier diodes 41 and 42 rectifies and outputs the AC voltage (voltage Vs) output from the transformer 3. In addition, in the smoothing circuit including the choke coil Lch and the output smoothing capacitor Cout, the voltage rectified by the rectifier circuit is smoothed to generate a DC output voltage Vout. Note that the DC output voltage Vout thus generated supplies power to the load 9 from the output terminals T3 and T4.

[0032] (Drive circuit 5) The drive circuit 5 is a circuit that performs switching drive and controls the operations of the switching elements S1 to S4 in the inverter circuit 2. Specifically, the drive circuit 5 controls the switching operations (on / off operations) of the switching elements S1 to S4 by individually supplying drive signals SG1 to SG4 to the switching elements S1 to S4, respectively.

[0033] Here, the drive circuit 5 performs, for example, pulse width control when controlling the switching operation of each of the switching elements S1 to S4 (performing switching drive), i.e., performs PWM (Pulse Width Modulation) control on the drive signals SG1 to SG4.

[0034] Further, the drive circuit 5 performs the above-mentioned switching drive such that the switching frequencies of the switching elements S1 to S4 are the same (almost the same) and constant (almost constant), for example.

[0035] Furthermore, the drive circuit 5, which will be described in detail later, controls the operation of the above-mentioned switches S5 and S6 (changeover switches) (setting them to an ON state or an OFF state) according to the magnitude of the DC input voltage Vin, thereby switching the above-mentioned operation state of the inverter circuit 2. In other words, which will be described in detail later, the drive circuit 5 performs switching drive so that the above-mentioned half-bridge operation or full-bridge operation is switched in the inverter circuit 2 according to the detection value of the DC input voltage Vin (see FIG. 1). Note that such operation control of the switches S5 and S6 is performed using drive signals SG5 and SG6 supplied individually from the drive circuit 5 (see FIG. 1).

[0036] [Operation and Effects] (A.Basic movement) In this switching power supply device 1, a DC input voltage Vin supplied from a DC input power supply 10 via input terminals T1 and T2 is switched in an inverter circuit 2 to generate a rectangular pulse voltage (voltage Vp). This rectangular pulse voltage is supplied to a primary winding 31 in a transformer 3 and transformed in the transformer 3, so that a transformed AC voltage (voltage Vs) is output from secondary windings 321 and 322.

[0037] In the rectifying and smoothing circuit 4, the AC voltage output from the transformer 3 (the above-mentioned transformed AC voltage) is rectified by the rectifying diodes 41 and 42 in the rectifying circuit, and then smoothed by the choke coil Lch and the output smoothing capacitor Cout in the smoothing circuit. As a result, a DC output voltage Vout is output from the output terminals T3 and T4. Then, power is supplied to the load 9 by this DC output voltage Vout.

[0038] (B.Detailed operation) Next, with reference to FIGS. 2 to 5 in addition to FIG. 1, a detailed operation of the switching power supply device 1 (such as the switching operation of the above-mentioned operating state in the inverter circuit 2) will be described in comparison with a comparative example.

[0039] Here, FIG. 2 is a circuit diagram showing a schematic configuration example of a switching power supply device (switching power supply device 101) according to a comparative example. Also, FIG. 3(A) shows an operation example of the switching power supply device 101 of the comparative example, and FIG. 3(B) shows an operation example of the switching power supply device 1 of this embodiment. Specifically, FIG. 3(A) and FIG. 3(B) show the setting states of the switching elements S1 to S4 and the switches S5 and S6 in each of the above-mentioned operation states (half-bridge (HB) operation state and full-bridge (FB) operation state). Also, FIG. 4 is a schematic circuit diagram showing an operation example of the switching power supply device 1 of this embodiment in the half-bridge operation state, and FIG. 5 is a schematic circuit diagram showing an operation example of the switching power supply device 1 in the full-bridge operation state.

[0040] (B-1. Comparative example) The comparative switching power supply device 101 shown in FIG. 2 corresponds to the switching power supply device 1 of the present embodiment shown in FIG. 1, in which an inverter circuit 102 and a drive circuit 105 are provided instead of the inverter circuit 2 and the drive circuit 5, respectively.

[0041] This inverter circuit 102 is the inverter circuit 2 (FIG. 1) of this embodiment, which is modified as follows. That is, first, the above-mentioned capacitor C1 and switches S5 and S6 are not arranged between the above-mentioned connection points P1 and P2. Instead, in this inverter circuit 102, a capacitor C1 is arranged between the primary high-voltage line L1H and the connection point P9, and a capacitor C2 is arranged between the connection point P9 and the primary low-voltage line L1L. Then, switches S5 and S6 connected in series to each other are arranged between the connection point P9 and the connection point P2.

[0042] The drive circuit 105 is a circuit that performs switching drive to control the operations of the switching elements S1 to S4 in the inverter circuit 102, and also controls the operations of the switches S5 and S6 in the inverter circuit 102. Specifically, the drive circuit 105 controls the switching operations (on / off operations) of the switching elements S1 to S4 by individually supplying drive signals SG1 to SG4 to the switching elements S1 to S4, respectively. Similarly to the drive circuit 5 described above, the drive circuit 105 also controls the operations of the switches S5 and S6 (setting them to an on state or an off state) in accordance with the magnitude of the DC input voltage Vin, thereby switching the operating state of the inverter circuit 102.

[0043] Specifically, as shown in FIG. 3(A), the drive circuit 105 of this comparative example switches between the half-bridge (HB) operation state and the full-bridge (FB) operation state in the inverter circuit 102 as follows. That is, when the DC input voltage Vin is equal to or higher than a predetermined threshold voltage Vth (Vin≧Vth), the drive circuit 105 sets each of the switches S5 and S6 (switching switches) to the ON state and performs switching drive so that the half-bridge operation is performed in the inverter circuit 102. Further, when the DC input voltage Vin is lower than the threshold voltage Vth (Vin<Vth), the drive circuit 105 sets each of the switches S5 and S6 to the OFF state and performs switching drive so that the full-bridge operation is performed in the inverter circuit 102. Note that, as an example of such a threshold voltage Vth, 300V can be mentioned.

[0044] Specifically, in the state of the half-bridge operation, the drive circuit 105 performs switching drive so that each of the switching elements S1 and S2 performs a predetermined switching (SW) operation and each of the switching elements S3 and S4 is set to the OFF state. On the other hand, in the state of the full-bridge operation, the drive circuit 105 performs switching drive so that each of the switching elements S1 to S4 performs a predetermined switching operation.

[0045] (B-2. This Embodiment) On the other hand, as shown in FIG. 3(B), the drive circuit 5 of the present embodiment switches between the half-bridge (HB) operation state and the full-bridge (FB) operation state in the inverter circuit 2 as follows. That is, when the DC input voltage Vin is equal to or higher than a predetermined threshold voltage Vth (Vin≧Vth), the drive circuit 5 sets each of the switches S5 and S6 (switching switches) to the OFF state and performs switching drive so that the half-bridge operation is performed in the inverter circuit 2. Further, when the DC input voltage Vin is less than the threshold voltage Vth (Vin<Vth), the drive circuit 5 sets each of the switches S5 and S6 to the ON state and performs switching drive so that the full-bridge operation is performed in the inverter circuit 2. Note that, as in the case of the above comparative example, an example of such a threshold voltage Vth is 300V.

[0046] Specifically, as shown in FIGS. 3(B) and 4, in the half-bridge operation state, the drive circuit 5 performs switching drive so that, for example, each of the switching elements S3 and S4 performs a predetermined switching operation (see the dashed arrows in FIG. 4). Further, in the half-bridge operation state, the drive circuit 5 performs switching drive so that the switching element S1 is set to the OFF state and the switching element S2 is set to the ON state. As shown in FIG. 3(B), in this half-bridge operation state, conversely, the drive circuit 5 may set the switching element S1 to the ON state and the switching element S2 to the OFF state.

[0047] Also, as shown in FIGS. 3(B) and 5, in the full-bridge operation state, the drive circuit 5 performs switching drive so that, for example, each of the switching elements S1 to S4 performs a predetermined switching operation (see the dashed arrows in FIG. 5).

[0048] Note that, in the half-bridge operation state shown in FIG. 3(B), for example, the set states of the switching elements S1 and S2 and the set states of the switching elements S3 and S4 may be interchanged (set to opposite states).

[0049] (C. Actions and Effects) Next, the operation and effects of the switching power supply device 1 of the present embodiment will be described in detail while comparing it with the above-mentioned comparative example.

[0050] (C-1. About general switching power supplies) First, in a conventional general AC-DC converter (general switching power supply device), during the output holding time when the input is cut off (for example, during an emergency such as a power outage), the operation is performed as follows. That is, during such output holding time, the AC-DC converter operates by utilizing the energy (charge) stored in the input capacitor. Furthermore, the capacity required for this input capacitor is specified based on the lower limit specification value of the input voltage and the minimum operating voltage (minimum regulation) (based on the voltage difference between the lower limit specification value of the input voltage and the minimum operating voltage). Therefore, for example, when the lower limit specification value of the input voltage is low and the minimum operating voltage is high (when the voltage difference between the lower limit specification value of the input voltage and the minimum operating voltage is small), it is necessary to increase the capacity of the input capacitor. As a result, there is a risk that the AC-DC converter itself as a switching power supply device and the power supply system equipped with this AC-DC converter will become large in size.

[0051] Thus, it can be said that it is difficult to reduce the size of a typical switching power supply device (AC-DC converter).

[0052] (C-2. Comparative example) On the other hand, in the switching power supply device 101 of the above comparative example, by controlling the operations of the switches S5 and S6 according to the magnitude of the DC input voltage Vin, the operating state of the inverter circuit 102 is switched between a half-bridge operating state and a full-bridge operating state. That is, as described above, when the DC input voltage Vin is relatively high (Vin ≧ Vth), the drive circuit 105 enters the half-bridge operating state, and when the DC input voltage Vin is relatively low (Vin < Vth), the drive circuit 105 performs switching so as to enter the full-bridge operating state.

[0053] As a result, in the above comparative example, compared with the above-described general switching power supply device, the input voltage range (the range of the DC input voltage Vin) can be expanded or the minimum operating voltage can be set lower, so that the capacitances of the capacitors C1 and C2 (input capacitors) can be reduced. As a result, in this comparative example, the switching power supply device 101 itself and the power supply system including this switching power supply device 101 can be miniaturized.

[0054] However, as indicated by the broken-line circles in Fig. 3(A), in this comparative example, when the DC input voltage Vin is relatively high (Vin ≧ Vth), the switches S5 and S6 (switching switches) are set to the on state, resulting in the half-bridge operating state. That is, in the operating input voltage range of the switching power supply device 101, conduction losses occur in the switches S5 and S6, so that such conduction losses always occur during normal operation. Therefore, in the switching power supply device 101 of this comparative example, the losses increase, and it may be difficult to achieve low losses.

[0055] (C-3. This Embodiment) On the other hand, in the switching power supply device 1 of this embodiment, for example, compared with the above comparative example and the like, for example, the following operations and effects can be obtained.

[0056] That is, first, in the present embodiment, since the inverter circuit 2 has the circuit configuration described above, similar to the above-described comparative example, switching of the operating state of the inverter circuit 2 according to the magnitude of the DC input voltage Vin can be easily realized.

[0057] Specifically, in the present embodiment, when the DC input voltage Vin is equal to or higher than a predetermined threshold voltage Vth, the drive circuit 5 is set to the half-bridge operation state, and when the DC input voltage Vin is less than the threshold voltage Vth, it is set to the full-bridge operation state.

[0058] Thereby, in the present embodiment, similar to the above-described comparative example, compared with the above-described general switching power supply device, the input voltage range (the range of the DC input voltage Vin) can be expanded or the minimum operating voltage can be set lower, so that the capacitance of the capacitor C1 (input capacitor) can be reduced.

[0059] Further, in the present embodiment, in such a half-bridge operation state and full-bridge operation state, respectively, for example, by setting the switches S5 and S6 as follows, the occurrence of the above-described conduction loss can be suppressed compared with the above-described comparative example.

[0060] Specifically, in the present embodiment, when the DC input voltage Vin is relatively high (Vin ≧ Vth), contrary to the above-described comparative example, the switches S5 and S6 (switching switches) are set to the off state, resulting in the half-bridge operation state (see the dashed arrow P10 in FIG. 3(B)). Also, when the DC input voltage Vin is relatively low (Vin < Vth), contrary to the above-described comparative example, the switches S5 and S6 are set to the on state, resulting in the full-bridge operation state (see the dashed arrow P10 in FIG. 3(B)).

[0061] That is, in this embodiment, unlike the comparative example, no conduction loss occurs in the switches S5 and S6 within the operating input voltage range of the switching power supply 1, and such conduction loss is avoided during normal operation. On the other hand, in this embodiment, conduction loss occurs in the switches S5 and S6 during the output holding time when the input is cut off (for example, during an emergency such as a power outage), but such an output holding time is very short (occurs only rarely), so that the following occurs. That is, in this embodiment, the conduction loss in the switches S5 and S6 is reduced compared to the comparative example, and as a result, loss in the switching power supply 1 is suppressed.

[0062] For the above reasons, in this embodiment, it is possible to achieve miniaturization and low loss in the switching power supply device 1 itself and in a power supply system including this switching power supply device 1, compared to the above comparative examples, etc.

[0063] In addition, in this embodiment, the switches S5 and S6 serving as the changeover switches are configured as bidirectional switches made up of a pair of MOS-FETs connected in series in the opposite directions, so that the following applies: By utilizing the gate voltages (drive signals SG5 and SG6) applied to these MOS-FETs, it becomes possible to easily control the operation of such changeover switches.

[0064] Furthermore, in this embodiment, since each of the switching elements S1 to S4 in the inverter circuit 2 is configured with a MOS-FET, the switching frequency can be increased, and the components can be made smaller.

[0065] In addition, in this embodiment, the rectifier circuit in the rectifying and smoothing circuit 4 is a center-tap type rectifier circuit, so the number of rectifier elements is two (rectifier diodes 41 and 42), which is fewer than in, for example, Modification 1 described below. As a result, it is possible to reduce the size, loss, and cost of the rectifier circuit.

[0066] <2. Modifications> Next, modified examples (modifications 1 to 5) of the above embodiment will be described. In the following, the same components as those in the embodiment will be given the same reference numerals, and the description will be omitted as appropriate.

[0067] [Variation 1] (composition) FIG. 6 is a circuit diagram showing an example of the schematic configuration of a switching power supply device (switching power supply device 1A) according to the first modification.

[0068] As in the embodiment, a system including the DC input power supply 10 and this switching power supply device 1A corresponds to one specific example of the "power supply system" of the present invention.

[0069] The switching power supply device 1A of this variant 1 corresponds to the switching power supply device 1 of the embodiment in which an inverter circuit 2A and a drive circuit 5A are provided instead of the inverter circuit 2 and the drive circuit 5, respectively, and the other configurations are similar.

[0070] The inverter circuit 2A is configured by further providing each discharge element (resistance element R1 and switches S7, S8) described later in the inverter circuit 2, and the other configuration is the same. The resistance element R1 and the switches S7, S8 are connected in parallel to the capacitor C1 (and the switches S5, S6). Specifically, as shown in FIG. 6, one end of the resistance element R1 is connected to a connection point P2, the other end of the resistance element R1 is connected to one end of the switch S7, the other end of the switch S7 is connected to one end of the switch S8, and the other end of the switch S8 is connected to a connection point P3. Similarly to the switches S5, S6 described above, in the example of FIG. 6, the switches S7, S8 are configured by bidirectional switches consisting of a pair of MOS-FETs connected in series in the opposite directions.

[0071] The resistor R1 corresponds to a specific example of a "discharging resistor" in the present invention. Furthermore, the switches S7 and S8 each correspond to a specific example of a "discharging switch" in the present invention.

[0072] The drive circuit 5A is a circuit that basically performs the same operation as the drive circuit 5. However, this drive circuit 5A controls the operation of each of the switches S7 and S8 (discharge switches) (sets them to an ON or OFF state) in addition to controlling the operation of each of the switching elements S1 to S4 and each of the switches S5 and S6. The operation control of the switches S7 and S8 is performed using drive signals SG7 and SG8 that are individually supplied from the drive circuit 5A (see FIG. 6).

[0073] Specifically, when switching the operating state of the inverter circuit 2A from the half-bridge operating state (switches S5, S6: off state) to the full-bridge operating state (switches S5, S6: on state), the drive circuit 5A controls the operation of switches S7, S8, etc., for example, as follows.

[0074] Here, each of Figs. 7 to 9 is a schematic circuit diagram showing an example of operation (example of operation of each of the switches S5 to S8, etc.) when the operating state of such an inverter circuit 2A is switched.

[0075] When switching from the half-bridge operation state to the full-bridge operation state, the drive circuit 5A first controls the operation of the switches S5 to S8, for example, as shown in Fig. 7. That is, the drive circuit 5A switches the switches S7 and S8 from the off state to the on state (see the dashed arrows in Fig. 7), thereby discharging the charge stored in the capacitor C1 via the resistor element R1 and the switches S7 and S8 (see the discharge current Ic1 shown in Fig. 7). At this time, the switches S5 and S6 remain in the off state that they were in during the half-bridge operation state.

[0076] 8 and 9, after the charge stored in the capacitor C1 is discharged, the drive circuit 5A switches the switches S5 and S6 from the OFF state to the ON state (see the dashed arrows in FIG. 9). As a result, after the charge stored in the capacitor C1 is discharged, the operation state of the inverter circuit 2A switches to a full-bridge operation state.

[0077] In this example, the switches S7 and S8 are each switched from the on state back to the off state (see the dashed arrow in FIG. 8), and then the switches S5 and S6 are each switched from the off state to the on state, but this is not limited to the example. That is, for example, the switches S7 and S8 may be switched from the on state back to the off state after the switches S5 and S6 are switched from the off state to the on state, and it is preferable to switch in this order. However, if the switches S7 and S8 are each switched from the on state back to the off state, then the switches S5 and S6 may be switched from the off state to the on state as in the above example.

[0078] (Action and effect) The switching power supply device 1A of the first modification thus configured basically operates in the same manner as the switching power supply device 1 of the embodiment, and thus can obtain the same effects.

[0079] In particular, in this modified example 1, the elements for discharging (resistance element R1 and switches S7, S8) connected in series to each other in the inverter circuit 2A are connected in parallel to the capacitor C1, so that the following occurs: That is, the charge stored in the capacitor C1 can be easily discharged by utilizing these elements.

[0080] Furthermore, in this modified example 1, by setting the switches S7 and S8 to the ON state, the accumulated charge in the capacitor C1 is discharged via the resistive element R1, and then the switches S5 and S6 are switched from the OFF state to the ON state, so that it is possible to avoid the adverse effects on the switches S5, S6, etc. (such as an increase in the conduction loss in the switches S5 and S6) caused by the discharge of the accumulated charge in the capacitor C1.

[0081] [Variation 2] (composition) Fig. 10 is a circuit diagram showing a schematic configuration example of a switching power supply device (switching power supply device 1B) according to Modification 2. Note that in Fig. 10, for convenience of illustration, the above-mentioned switches S5 to S8 are shown simplified in the shape of a switch.

[0082] As in the embodiment, a system including the DC input power supply 10 and this switching power supply device 1B corresponds to one specific example of the "power supply system" of the present invention.

[0083] The switching power supply device 1B of the second variant corresponds to the switching power supply device 1A of the first variant, in which an inverter circuit 2B and a drive circuit 5B have been provided instead of the inverter circuit 2A and the drive circuit 5A, respectively, but the other configurations are similar.

[0084] The inverter circuit 2B corresponds to the inverter circuit 2A in which the positions of the switches S5 and S6 as changeover switches are changed, but the other configurations are the same. Specifically, as shown in FIG. 10, the inverter circuit 2B differs from the inverter circuits 2 and 2A as follows. That is, the switches S5 and S6 are each connected in parallel to the resistor element R1 of the resistor element R1 and the switches S7 and S8 connected in series with each other. In detail, as shown in FIG. 10, one end of the switch S5 is connected to the connection point P2, the other end of the switch S5 is connected to one end of the switch S6, and the other end of the switch S6 is connected to the connection point P4 (the connection point between the other end of the resistor element R1 and one end of the switch S7).

[0085] The drive circuit 5B is a circuit that basically operates in the same manner as the drive circuit 5A. However, when the drive circuit 5B switches the operation state of the inverter circuit 2B from the half-bridge operation state (switches S5, S6: OFF state) to the full-bridge operation state (switches S5, S6: ON state), the drive circuit 5B controls the operation of the switches S7, S8, etc., for example, as follows.

[0086] Here, FIGS. 11 and 12 each show a schematic circuit diagram of an operation example (operation example of each of the switches S5 to S8, etc.) when the operating state of the inverter circuit 2B is switched.

[0087] When switching from the half-bridge operation state to the full-bridge operation state, the drive circuit 5B first controls the operation of each of the switches S5 to S8, for example, as shown in FIG. 11. That is, the drive circuit 5B switches each of the switches S7 and S8 from the OFF state to the ON state (see the dashed arrows in FIG. 11). As a result, in this modification 2 as well as in modification 1, the charge stored in the capacitor C1 is discharged via the resistance element R1 and each of the switches S7 and S8 (see the discharge current Ic1 shown in FIG. 11). At this time, in modification 2 as well as in modification 1, the switches S5 and S6 are each set to the OFF state that was set in the half-bridge operation state.

[0088] Next, as shown in Fig. 12, after the charge stored in the capacitor C1 is discharged, the drive circuit 5B switches the switches S5 and S6 from the OFF state to the ON state (see the dashed arrows in Fig. 12). That is, in this modification 2, unlike the modification 1, the operation state of the inverter circuit 2B switches to a full-bridge operation state while the switches S7 and S8 remain set to the ON state (without being returned to the OFF state). In this way, in the modification 2, the switches S7 and S8 function as discharge switches and also function as changeover switches similar to the switches S5 and S6.

[0089] (Action and effect) The switching power supply device 1B of the second modification thus configured basically operates in the same manner as the switching power supply device 1A of the first modification, and therefore can provide the same effects as those of the first modification.

[0090] [Variation 3] (composition) FIG. 13 is a circuit diagram showing a schematic configuration example of a switching power supply device (switching power supply device 1C) according to Modification 3.

[0091] As in the embodiment, a system including the DC input power supply 10 and this switching power supply device 1C corresponds to one specific example of the "power supply system" of the present invention.

[0092] The switching power supply 1C of this variant example 3 corresponds to the switching power supply 1 of the embodiment in which a transformer 3C and a rectifying and smoothing circuit 4C are provided instead of the transformer 3 and the rectifying and smoothing circuit 4, but the other configurations are similar.

[0093] The transformer 3C has one primary winding 31 and one secondary winding 32. That is, the transformer 3 has two secondary windings 321 and 322, whereas the transformer 3C has only one secondary winding 32. A first end of the secondary winding 32 is connected to a connection point P7 in a rectifying and smoothing circuit 4C (described later), and a second end of the secondary winding 32 is connected to a connection point P8 in the rectifying and smoothing circuit 4C.

[0094] Like the transformer 3, the transformer 3C converts the voltage (voltage Vp converted into a rectangular pulse wave) generated by the inverter circuit 2, and outputs an AC voltage (voltage Vs) from the end of the secondary winding 32. In this case, the degree of voltage conversion of the output voltage with respect to the input voltage is determined by the turns ratio between the primary winding 31 and the secondary winding 32, and the time ratio of the on-period of each of the switching elements S1 to S4 with respect to the switching cycle described above.

[0095] The rectifying and smoothing circuit 4C has four rectifying diodes 41-44, one choke coil Lch, and one output smoothing capacitor Cout. Specifically, the rectifying and smoothing circuit 4C includes a rectifying circuit having the rectifying diodes 41-44, and a smoothing circuit having the choke coil Lch and the output smoothing capacitor Cout. That is, the rectifying and smoothing circuit 4C is obtained by changing the configuration of the rectifying circuit in the rectifying and smoothing circuit 4.

[0096] The four rectifier diodes 41 to 44 correspond to a specific example of the "plurality of rectifier elements" of the present invention.

[0097] The rectifier circuit of the third modification is a so-called "bridge type" rectifier circuit, unlike the rectifier circuits of the embodiment and the first and second modifications (so-called "center tap type" rectifier circuits). That is, the cathodes of the rectifier diodes 41 and 43 are connected to the output line LO via the choke coil Lch, and the anode of the rectifier diode 41 is connected to the cathode of the rectifier diode 42 and the above-mentioned first end of the secondary winding 32 at the connection point P7. The anodes of the rectifier diodes 42 and 44 are connected to the ground line LG, and the cathode of the rectifier diode 44 is connected to the anode of the rectifier diode 43 and the above-mentioned second end of the secondary winding 32 at the connection point P8.

[0098] In the rectifying / smoothing circuit 4C configured as above, similarly to the rectifying / smoothing circuit 4, the rectifying circuit including rectifying diodes 41 to 44 rectifies the AC voltage (voltage Vs) output from the transformer 3C and outputs the rectified voltage.

[0099] (Action and effect) In the switching power supply device 1C of the third modification thus configured, basically, it is possible to obtain the same effects as those of the switching power supply device 1 of the embodiment due to the same actions.

[0100] In particular, in this modification 3, the rectifier circuit in the rectifying and smoothing circuit 4C is a bridge-type rectifier circuit, so that the number of windings (number of secondary windings) in the transformer 3C is one (secondary winding 32), which is smaller than in the above-mentioned embodiment and modifications 1 and 2. As a result, it is possible to reduce the size and loss of the transformer 3C.

[0101] Also in this modified example 3, instead of the inverter circuit 2, the inverter circuit 2A described in the modified example 1 or the inverter circuit 2B described in the modified example 2 may be provided.

[0102] [Variations 4 and 5] (composition) The switching power supply devices of Modifications 4 and 5 (switching power supply devices 1D and 1E) are similar to those of the above-mentioned embodiment and Modification 3, except that the rectifier circuits in the rectifying and smoothing circuits 4 and 4C are so-called synchronous rectifier circuits, as described below.

[0103] Specifically, FIG. 14 is a circuit diagram showing an example of the schematic configuration of a switching power supply device (switching power supply device 1D) according to Modification 4.

[0104] A switching power supply device 1D of this modification 4 corresponds to one in which a rectifying and smoothing circuit 4D is provided instead of the rectifying and smoothing circuit 4 in the switching power supply device 1 of the embodiment, and other configurations are similar.

[0105] In the synchronous rectifier circuit (rectifier smoothing circuit 4D) in this modified example 4, as shown in Fig. 14, the rectifier diodes 41 and 42 described in the embodiment are each configured with a MOS-FET (MOS transistors M9 and M10) as a switching element. In this synchronous rectifier circuit, the MOS transistors M9 and M10 themselves are controlled to be turned on (perform synchronous rectification) in synchronization with the period during which the parasitic diodes of the MOS transistors M9 and M10 are conductive. Specifically, the drive circuit 5 in this modified example 4 uses drive signals SG9 and SG10 to control the on / off operation of the MOS transistors M9 and M10 (see Fig. 14).

[0106] FIG. 15 is a circuit diagram showing a schematic configuration example of a switching power supply device (switching power supply device 1E) according to Modification 5.

[0107] The switching power supply device 1E of the fifth modification corresponds to the switching power supply device 1C of the third modification in that a rectifying and smoothing circuit 4E is provided instead of the rectifying and smoothing circuit 4C, and other configurations are similar.

[0108] In the synchronous rectifier circuit (rectifier smoothing circuit 4E) in the fifth modification, as shown in FIG. 15, the rectifier diodes 41-44 described in the third modification are each configured with a MOS-FET (MOS transistors M11-M14) as a switching element. In the synchronous rectifier circuit in the fifth modification, similarly to the synchronous rectifier circuit in the fourth modification, the MOS transistors M11-M14 themselves are controlled to be turned on (to perform synchronous rectification) in synchronization with the period during which the parasitic diode of each of the MOS transistors M11-M14 is conductive. Specifically, the drive circuit 5 in the fifth modification uses drive signals SG11-SG14 to control the on / off operation of each of the MOS transistors M11-M14 (see FIG. 15).

[0109] As in the embodiment, a system including DC input power supply 10 and switching power supply device 1D or switching power supply device 1E corresponds to one specific example of the "power supply system" of the present invention.

[0110] (Action and effect) The switching power supply devices 1D and 1E of the fourth and fifth modifications thus configured basically function in the same manner as the switching power supply devices 1 and 1C of the embodiment and the third modification, and therefore can provide the same effects.

[0111] In particular, in these modified examples 4 and 5, each of the rectifier elements in the rectifier circuit is configured with a switching element, and the rectifier circuit is configured as a synchronous rectifier circuit, as described below. That is, such a synchronous rectifier circuit reduces the conduction loss during rectification, making it possible to reduce the size and loss of the rectifier circuit. Incidentally, examples of such switching elements include, in addition to the above-mentioned MOS-FET, HEMT ((High Electron Mobility Transistor) = HFET (Heterostructure Field-Effect Transistor)), IGBT or bipolar transistor with a diode added in parallel, etc.

[0112] In these fourth and fifth modified examples, too, the inverter circuit 2A described in the first modified example or the inverter circuit 2B described in the second modified example may be provided in place of the inverter circuit 2.

[0113] <3. Other Modifications> Although the present invention has been described above by way of the embodiment and modifications, the present invention is not limited to these embodiments and can be modified in various ways.

[0114] For example, in the above embodiment, the configuration of the inverter circuit is specifically described, but the present invention is not limited to the above embodiment, and for example, the inverter circuit may have another configuration. Specifically, the arrangement of the capacitor C1 and the primary winding 31, which are connected in series, is not limited to the arrangement described in the embodiment, and may be in any order. In addition, in the above embodiment, the switches S5 to S8 (changeover switches and discharge switches) are each configured by the bidirectional switches described above, but the present invention is not limited to this example. In other words, the changeover switches and discharge switches may each be configured by other switch elements, such as relays and triacs.

[0115] In addition, in the above embodiments, the configuration of the transformer (primary winding and secondary winding) has been specifically given and described, but the present invention is not limited to the examples of the above embodiments, and for example, a transformer (primary winding and secondary winding) with a different configuration may be used.

[0116] Furthermore, in the above embodiment, etc., the configuration of the rectifying smoothing circuit (rectifying circuit and smoothing circuit) has been specifically described, but the present invention is not limited to the above embodiment, etc., and for example, a rectifying smoothing circuit (rectifying circuit and smoothing circuit) with another configuration may be used. Specifically, for example, in the above embodiment, etc., the choke coil Lch is disposed on the output line LO, but the present invention is not limited to this example, and for example, the choke coil Lch may be disposed on the ground line LG.

[0117] In addition, in the above-mentioned embodiment, etc., a method of controlling the operation of each switching element by a drive circuit (switching drive) has been specifically described, but the present invention is not limited to the above-mentioned embodiment, etc., and other methods may be used as the switching drive method. Also, for example, a method of switching the operation state (half-bridge operation state and full-bridge operation state) of the inverter circuit according to the magnitude of the input voltage (switching method using a changeover switch) is not limited to the above-mentioned embodiment, etc., and other methods may be used.

[0118] In addition, in the above embodiment, a DC-DC converter has been described as an example of a switching power supply device according to the present invention, but the present invention can also be applied to other types of switching power supply devices, such as an AC-DC converter.

[0119] Furthermore, the configuration examples described so far may be applied in any combination.

Claims

1. an input terminal pair to which an input voltage is input; an output terminal pair for outputting an output voltage; a transformer having a primary winding and a secondary winding; an inverter circuit disposed between the input terminal pair and the primary winding, the inverter circuit including first to fourth switching elements, a first capacitor, and a changeover switch; a rectifying and smoothing circuit disposed between the output terminal pair and the secondary winding, the rectifying circuit having a plurality of rectifying elements, and a smoothing circuit having an inductor and a second capacitor; a drive unit that performs switching drive for controlling the operations of the first to fourth switching elements in the inverter circuit, and also controls the operation of the changeover switch; Equipped with the first and second switching elements are connected in series to each other between a pair of connection lines that are individually connected to the input terminal pair; the third and fourth switching elements are connected in series to each other between the pair of connection lines, the first capacitor and the primary winding are connected in series in no particular order between a first connection point which is a connection point between the first and second switching elements and a second connection point which is a connection point between the third and fourth switching elements, the changeover switch is connected in parallel to the first capacitor, The drive unit is When the input voltage is equal to or higher than a predetermined threshold voltage, the changeover switch is set to an OFF state, and the switching drive is performed so that a half-bridge operation is performed in the inverter circuit; When the input voltage is less than the threshold voltage, the changeover switch is set to an on state, and the switching drive is performed so that a full-bridge operation is performed in the inverter circuit. Switching power supply.

2. the inverter circuit further includes a discharge resistor element and a discharge switch, The discharge resistor element and the discharge switch, which are connected in series with each other, are each connected in parallel with the first capacitor.

2. The switching power supply device according to claim 1.

3. An input terminal pair to which an input voltage is input; an output terminal pair for outputting an output voltage; a transformer having a primary winding and a secondary winding; an inverter circuit disposed between the input terminal pair and the primary winding, the inverter circuit including first to fourth switching elements, a first capacitor, a changeover switch, a discharge resistor element, and a discharge switch; a rectifying and smoothing circuit disposed between the output terminal pair and the secondary winding, the rectifying circuit having a plurality of rectifying elements, and a smoothing circuit having an inductor and a second capacitor; a drive unit that performs switching drive for controlling the operations of the first to fourth switching elements in the inverter circuit, and also controls the operation of the changeover switch; Equipped with the first and second switching elements are connected in series to each other between a pair of connection lines that are individually connected to the input terminal pair, the third and fourth switching elements are connected in series to each other between the pair of connection lines, the first capacitor and the primary winding are connected in series in no particular order between a first connection point which is a connection point between the first and second switching elements and a second connection point which is a connection point between the third and fourth switching elements, the changeover switch is connected in parallel to the first capacitor, the discharge resistance element and the discharge switch, which are connected in series with each other, are each connected in parallel with the first capacitor, The drive unit is When switching the operation state of the inverter circuit from a half-bridge operation state in which the changeover switch is set to an off state to a full-bridge operation state in which the changeover switch is set to an on state, By setting the discharge switch to an on state, the charge stored in the first capacitor is discharged via the discharge resistor element, and then: The changeover switch is switched from an OFF state to an ON state. Switching power supply.

4. The drive unit is When the input voltage is equal to or higher than a predetermined threshold voltage, the changeover switch is set to an OFF state, and the switching drive is performed so that a half-bridge operation is performed in the inverter circuit; When the input voltage is less than the threshold voltage, the changeover switch is set to an on state, and the switching drive is performed so that a full-bridge operation is performed in the inverter circuit.

4. The switching power supply device according to claim 3.

5. The changeover switch is a bidirectional switch made up of a pair of MOS-FETs connected in series in opposite directions.

5. The switching power supply device according to claim 1.

6. The first to fourth switching elements are each composed of a MOS-FET.

6. The switching power supply device according to claim 1.

7. The rectifier circuit is a center tap type rectifier circuit.

7. The switching power supply device according to claim 1.

8. The rectifier circuit is a bridge type rectifier circuit.

7. The switching power supply device according to claim 1.

9. Each of the plurality of rectifier elements is configured by a switching element, and the rectifier circuit is a synchronous rectifier circuit.

9. The switching power supply device according to claim 1.

10. A switching power supply device according to any one of claims 1 to 9, a power supply for supplying the input voltage to the input terminal pair; A power supply system comprising:

Images