Switching power supply and charging system

Abstract

To turn on / off a secondary-side switching element of a transformer without detecting the time when a current flowing to a resonance element has a peak value etc.SOLUTION: A switching power supply device comprises first to fourth switching elements (Q10-Q13) on a primary side of a transformer (TR) and also comprises synchronous rectification circuits (Q20-Q23) on a secondary side of the transformer (TR). A control part (12A) controls the synchronous rectification circuits so that the synchronous rectification circuits are turned on in a period in which a current in a first direction is equal to or larger than a predetermined threshold.SELECTED DRAWING: Figure 1

Description

Technical Field

[0004] ,

[0005] , , , , ,

[0001] The present disclosure relates to a switching power supply device and a charging system.

Background Art

[0002] Patent Document 1 discloses a resonant power supply device that detects the time when the current flowing through the primary side becomes zero and the time when it becomes the peak value, and performs on / off control of the synchronous rectifier element on the secondary side based on those times.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the power supply device described in Patent Document 1, it is necessary to detect the time when the current flowing through the resonant element becomes the peak value or the like, but the control of the switching power supply device becomes complicated. One aspect of the present disclosure aims to realize the on / off operation of the synchronous rectifier circuit on the secondary side of the transformer without detecting the time when the current flowing through the resonant element becomes the peak value or the like.

Means for Solving the Problems

[0005] To solve the above problems, a switching power supply device according to one aspect of the present disclosure is a switching power supply device to which a secondary battery is connected on the load side and which can be used to charge the secondary battery, comprising: a transformer; a resonant circuit connected to the primary side of the transformer; first and second switching elements that perform on / off operations on a first current flowing in a first direction to the primary side of the transformer; third and fourth switching elements that perform on / off operations on a second current flowing in a second direction opposite to the first direction to the primary side of the transformer; and a synchronous rectifier that rectifies the current flowing to the secondary side of the transformer during the period when the first current or the second current is flowing to the primary side of the transformer. The system comprises a rectifier circuit and a control unit that controls the on / off operation of the first to fourth switching elements and the synchronous rectifier circuit, wherein the time average value of the on / off operation frequencies of the first to fourth switching elements is fixed, there is a predetermined phase difference between the on / off operation of the first switching element and the on / off operation of the second switching element, and there is the predetermined phase difference between the on / off operation of the third switching element and the on / off operation of the fourth switching element, and the control unit controls the synchronous rectifier circuit so that it is turned on during the period when the first current or the second current is above a predetermined threshold.

[0006] With the above configuration, it is possible to enable on / off operation of the synchronous rectifier circuit on the secondary side of the transformer without detecting the time when the current flowing through the resonant circuit is at its peak value.

[0007] Furthermore, in a switching power supply according to one aspect of the present disclosure, the synchronous rectifier circuit includes fifth and sixth switching elements that rectify the current flowing to the secondary side of the transformer during the period when the first current is flowing to the primary side of the transformer, and seventh and eighth switching elements that rectify the current flowing to the secondary side of the transformer during the period when the second current is flowing to the primary side of the transformer, and the control unit controls the fifth and sixth switching elements so that both the fifth and sixth switching elements are turned on during the period when the first current is greater than or equal to the predetermined threshold, and controls the seventh and eighth switching elements so that both the seventh and eighth switching elements are turned on during the period when the second current is greater than or equal to the predetermined threshold.

[0008] With the above configuration, even if four switching elements are provided on the secondary side of the transformer as a synchronous rectifier circuit, the on / off operation of the synchronous rectifier circuit can be achieved without detecting the time when the current flowing through the resonant circuit is at its peak value.

[0009] Furthermore, a switching power supply device according to one aspect of the present disclosure further includes a current detection unit that detects the first current and the second current and outputs the current values ​​of the first current and the second current to the control unit, the control unit controls the fifth and sixth switching elements so that both the fifth and sixth switching elements are turned on during the period when the current value of the first current in the first direction detected by the current detection unit is equal to or greater than the predetermined threshold, and controls the seventh and eighth switching elements so that both the seventh and eighth switching elements are turned on during the period when the current value of the second current in the second direction detected by the current detection unit is equal to or greater than the predetermined threshold.

[0010] With the above configuration, the switching power supply unit further includes a current detection unit, so the control unit can properly determine the period during which the first current is above a predetermined threshold and the period during which the second current is above a predetermined threshold.

[0011] Furthermore, a switching power supply device according to one aspect of the present disclosure further includes a current detection circuit that outputs a first signal to the control unit during a period when the first current flowing in the first direction or the second current flowing in the second direction is greater than or equal to a predetermined threshold, and outputs a second signal to the control unit during a period when the first current flowing in the first direction is less than the predetermined threshold and the second current flowing in the second direction is less than the predetermined threshold, and the control unit performs on / off operations of the fifth to eighth switching elements based on the first signal and the second signal output from the current detection circuit.

[0012] With the above configuration, the switching power supply further includes a current detection circuit, so the control unit can properly determine the period during which the first current is above a predetermined threshold and the period during which the second current is above a predetermined threshold.

[0013] Furthermore, in a switching power supply device according to one aspect of the present disclosure, the transformer has a center tap terminal on its secondary side, the synchronous rectifier circuit includes a fifth switching element that rectifies the current flowing to the secondary side of the transformer during the period when the first current is flowing to the primary side of the transformer, and a sixth switching element that rectifies the current flowing to the secondary side of the transformer during the period when the second current is flowing to the primary side of the transformer, and the control unit controls the fifth switching element so that it is turned on during the period when the first current is greater than or equal to the predetermined threshold, and controls the sixth switching element so that it is turned on during the period when the second current is greater than or equal to the predetermined threshold.

[0014] According to the above configuration, even if the transformer has a center tap terminal on its secondary side and two switching elements are provided as a synchronous rectifier circuit on the secondary side of the transformer, the on / off operation of the synchronous rectifier circuit can be achieved without detecting the time when the current flowing through the resonant circuit is at its peak value.

[0015] Furthermore, a switching power supply device according to one aspect of the present disclosure further includes a current detection unit that detects the first current and the second current and outputs the current values ​​of the first current and the second current to the control unit, the control unit controls the fifth switching element so that the fifth switching element is turned on during the period when the current value of the first current in the first direction detected by the current detection unit is equal to or greater than the predetermined threshold, and controls the sixth switching element so that the sixth switching element is turned on during the period when the current value of the second current in the second direction detected by the current detection unit is equal to or greater than the predetermined threshold.

[0016] With the above configuration, the switching power supply unit further includes a current detection unit, so the control unit can properly determine the period during which the first current is above a predetermined threshold and the period during which the second current is above a predetermined threshold.

[0017] Furthermore, a switching power supply device according to one aspect of the present disclosure further includes a current detection circuit that outputs a first signal to the control unit during a period when the first current flowing in the first direction or the second current flowing in the second direction is greater than or equal to a predetermined threshold, and outputs a second signal to the control unit during a period when the first current flowing in the first direction is less than the predetermined threshold and the second current flowing in the second direction is less than the predetermined threshold, and the control unit performs on / off operations of the fifth to sixth switching elements based on the first signal and the second signal output from the current detection circuit.

[0018] With the above configuration, the switching power supply further includes a current detection circuit, so the control unit can properly determine the period during which the first current is above a predetermined threshold and the period during which the second current is above a predetermined threshold.

[0019] A charging system according to one aspect of the present disclosure is a charging system including the switching power supply device according to any one of the above aspects and the secondary battery, and is capable of charging the secondary battery by constant current charging using the output current of the switching power supply device and constant voltage charging using the output voltage of the switching power supply device. When the battery voltage of the secondary battery is between a first voltage and a second voltage, the secondary battery is charged by the constant current charging, and after the battery voltage of the secondary battery reaches a third voltage that is equal to or higher than the second voltage, the secondary battery is charged by the constant voltage charging.

[0020] According to the above configuration, overcurrent charging can be prevented by performing constant current charging. Further, after the charging voltage of the secondary battery reaches the third voltage, overcharging can be prevented by performing constant voltage charging.

Advantages of the Invention

[0021] According to one aspect of the present disclosure, on / off control of the synchronous rectifier circuit on the secondary side can be realized without detecting the time when the current flowing through the resonant element reaches a peak value or the like.

Brief Description of the Drawings

[0022] [Figure 1] It is a diagram showing a configuration example of a charging system according to Embodiment 1 of the present disclosure. [Figure 2] [[ID=2​​​​​​​​​​​​​​​​​This is an example of a timing chart showing the operation of each part of a DC-DC converter. [Modes for carrying out the invention]

[0023] [Embodiment 1] Hereinafter, one embodiment of the present disclosure will be described in detail. Figure 1 is a diagram showing an example configuration of a charging system 1 according to Embodiment 1 of the present disclosure. The charging system 1 illustrated in Figure 1 comprises a switching power supply 10A and a secondary battery 20. The secondary battery 20 is connected to the load side of the switching power supply 10A.

[0024] The switching power supply unit 10A includes a DC-DC converter 11A and a control unit 12A. The DC-DC converter 11A is a full-bridge circuit and has a reverse-wound transformer TR. A current detection unit 100, four switching elements Q10-Q13, and a capacitor C1, a coil L1, and an excitation inductor built into the transformer TR (not shown) are connected to the primary side of the transformer TR. The current detection unit 100 detects the current ILmp flowing on the primary side of the transformer TR.

[0025] On the secondary side of the transformer TR, a smoothing capacitor C2 and four switching elements Q20-Q23, which form a synchronous rectifier circuit and are used to rectify the output current, are connected.

[0026] Switching element Q10 is an example of a first switching element and consists of a FET. Switching element Q13 is an example of a second switching element and consists of a FET. Switching element Q12 is an example of a third switching element and consists of a FET. Switching element Q11 is an example of a fourth switching element and consists of a FET. Switching element Q20 is an example of a fifth switching element and consists of a FET. Switching element Q23 is an example of a sixth switching element and consists of a FET. Switching element Q22 is an example of a seventh switching element and consists of a FET. Switching element Q21 is an example of an eighth switching element and consists of a FET. Switching elements Q10-Q13 and Q20-Q23 may be composed of IGBTs or bipolar transistors.

[0027] The on / off operation frequency of the switching elements Q10-Q13 is fixed at its time-averaged value, for example, to the resonant frequency of capacitor C1 and coil L1.

[0028] The control unit 12A consists of, for example, a microcomputer. The control unit 12A receives the current value of the current ILmp flowing on the primary side of the transformer TR, which is detected by the current detection unit 100. The control unit 12A controls the on / off state of the eight switching elements Q10-Q13 and Q20-Q23.

[0029] Figure 2 shows the charging profile of the charging system 1. The charging system 1 can charge the secondary battery 20 using three charging methods: constant current charging CC, constant power charging CP, and constant voltage charging CV.

[0030] The charging system 1 only needs to be able to charge the secondary battery 20 by at least constant current charging CC and constant voltage charging CV from among the charging methods illustrated in Figure 2. Furthermore, the charging system 1 may also charge the secondary battery 20 by charging methods not shown in Figure 2, such as pulse charging and trickle charging.

[0031] The charging system 1 charges the secondary battery 20 by constant current charging CC when the battery voltage of the secondary battery 20 is equal to or greater than the first voltage V1 and less than or equal to the second voltage V2. In constant current charging CC, the charging system 1 uses the output current of the switching power supply 10A to output a charging current with a constant current value I1 to the secondary battery 20.

[0032] The charging system 1 charges the secondary battery 20 by constant power charging CP when the battery voltage of the secondary battery 20 is above the second voltage V2 and below the third voltage V3. In constant power charging CP, the charging current decreases as the battery voltage of the secondary battery 20 increases. The third voltage V3 is the target charging value for the battery voltage of the secondary battery 20.

[0033] The charging system 1 charges the secondary battery 20 by constant voltage charging CV when the battery voltage of the secondary battery 20 reaches the third voltage V3. In constant voltage charging CV, charging is performed while maintaining a constant charging voltage. In constant voltage charging CV, the internal voltage of the secondary battery 20 increases and the charging current decreases.

[0034] Using Figure 3, the relationship between the on / off operation of the switching elements Q10-Q13 on the primary side of the transformer TR and the current ILmp flowing through the primary side of the transformer TR will be explained. Figure 3 is a diagram showing the direction of the current flowing through the DC-DC converter 11A. In Figure 1300 shown at the top of Figure 3, switching elements Q10 and Q13 are turned on, and switching elements Q11 and Q12 are turned off. At this time, a current ILmp flows through the primary side of the transformer TR in the first direction 101. At this time, a current ILms flows through the secondary side of the transformer TR in the third direction 201, and current flows through switching elements Q20 and Q23.

[0035] In Figure 301, shown at the bottom of Figure 3, switching elements Q11 and Q12 are turned on, and switching elements Q10 and Q13 are turned off. At this time, a current ILmp flows in the second direction 102 on the primary side of the transformer TR. At this time, a current ILms flows in the fourth direction 202 on the secondary side of the transformer TR, and current flows through switching elements Q21 and Q22.

[0036] As shown in Figure 1300, switching elements Q20 and Q23 may be damaged if they are not turned on when current ILmp is flowing in the first direction 101. On the other hand, as shown in Figure 2301, switching elements Q21 and Q22 may be damaged if they are not turned on when current ILmp is flowing in the second direction 102. The proper on / off operation of switching elements Q20-Q23 by the control unit 12A of the switching power supply 10A when constant current charging CC is performed in the charging system 1 will be described below.

[0037] The capacitor C1, coil L1, and excitation inductance of the built-in transformer TR (not shown) that constitute the resonant circuit shown in Figure 1 are set such that, when the secondary battery 20 is charged by constant current charging CC, the ratio of the secondary voltage Vout to the primary voltage Vin of the transformer TR is less than 1, assuming the turns ratio of the transformer TR is 1.

[0038] The on / off operation of switching elements Q20-Q23 will be explained using Figure 4. Figure 4 is a timing chart showing the operation of each part of the DC-DC converter 11A. From the top, Figure 4 shows the gate-source voltage of switching elements Q10-Q13, the current ILmp flowing on the primary side of the transformer TR detected by the current detection unit 100, the current ILms flowing on the secondary side of the transformer TR, and the drain-source voltage of switching elements Q20-Q23.

[0039] In Figure 4, the current ILmp is represented as a positive value when flowing in the first direction 101 in Figure 3, and as a negative value when flowing in the second direction 102. Similarly, the current ILms is represented as a positive value when flowing in the fourth direction 202 in Figure 3, and as a negative value when flowing in the third direction 201.

[0040] As shown in Figure 4, the control unit 12A performs phase shift control to provide a predetermined phase difference X1 between the on / off operation of switching element Q10 and the on / off operation of switching element Q13. When the current ILmp detected by the current detection unit 100 is on during the period when switching elements Q10 and Q13 are on, the control unit 12A controls the secondary switching elements Q20 and Q23 to turn on and rectify the current ILms flowing in the third direction 201. Subsequently, when the current ILmp falls below the positive threshold ITH, the control unit 12A controls the secondary switching elements Q20 and Q23 to turn off.

[0041] Furthermore, as shown in Figure 4, the control unit 12A performs phase shift control to provide a predetermined phase difference X2 between the on / off operation of switching element Q11 and the on / off operation of switching element Q12. When the current ILmp detected by the current detection unit 100 falls below a negative threshold -ITH during the period when switching elements Q11 and Q12 are ON, the control unit 12A controls the secondary switching elements Q21 and Q22 to turn ON and rectify the current ILms flowing in the fourth direction. Subsequently, when the current ILmp becomes greater than the negative threshold -ITH, the control unit 12A controls the secondary switching elements Q21 and Q22 to turn OFF.

[0042] In other words, the control unit 12A controls the switching elements of the secondary synchronous rectifier circuit through which current is flowing to turn on during periods when the absolute value of the current ILmp is greater than or equal to a predetermined threshold ITH. Conversely, the control unit 12A controls the switching elements of the secondary synchronous rectifier circuit through which current is flowing to turn off during periods when the absolute value of the current ILmp is less than a predetermined threshold ITH.

[0043] Furthermore, in Figure 4, period T1 is longer than the period DUTY during which both phase-shift controlled primary-side switching elements Q10 and Q13 are turned on. Period T2 is longer than the period DUTY during which both phase-shift controlled primary-side switching elements Q11 and Q12 are turned on.

[0044] The current ILms is smoothed by the smoothing capacitor C2 in Figure 1 and used to charge the secondary battery 20 by constant current charging CC as shown in Figure 2. Additionally, the output voltage of the switching power supply 10A is used to charge the secondary battery 20 by constant voltage charging CV.

[0045] [Effects of Embodiment 1] The switching power supply device 10A according to Embodiment 1 of this disclosure has a secondary battery 20 connected to the load side and can be used to charge the secondary battery 20. The switching power supply device 10A includes a transformer TR, a resonant circuit (capacitor C1, coil L1, and an excitation inductance built into the transformer TR, not shown) connected to the primary side of the transformer TR, first and second switching elements Q10 and Q13 that perform on / off operations of a first current (current ILmp shown in Figure 300) flowing in a first direction 101 to the primary side of the transformer TR, and a second current (shown in Figure 301) flowing in a second direction 102 opposite to the first direction 101 to the primary side of the transformer TR. The system comprises third and fourth switching elements Q12 and Q11 that perform on / off operations of the indicated current ILmp, a synchronous rectifier circuit (switching elements Q20-Q23) that rectifies the current flowing to the secondary side of the transformer TR during the period when a current ILmp flows to the primary side of the transformer TR in the first direction 101 or the second direction 102, and a control unit 12A that controls the on / off operations of the first to fourth switching elements Q10-Q13 and the synchronous rectifier circuit (switching elements Q20-Q23). The time average value of the on / off operation frequency of the first to fourth switching elements Q10-Q13 is fixed. For example, the on / off operation frequency of the first to fourth switching elements Q10-Q13 is fixed to the resonant frequency of the resonant circuit. There is a predetermined phase difference X1 between the on / off operation of the first switching element Q10 and the on / off operation of the second switching element Q13, and there is a predetermined phase difference X2 between the on / off operation of the third switching element Q12 and the on / off operation of the fourth switching element Q11. The control unit 12A controls the synchronous rectifier circuit (switching elements Q20-Q23) so that it is turned on during the period when the first current or the second current (ILmp) is greater than or equal to a predetermined threshold ITH.

[0046] With the above configuration, it is possible to perform on / off operations of the synchronous rectifier circuit (switching elements Q20-Q23) on the secondary side of the transformer TR without detecting the time when the current flowing through the resonant circuit is at its peak value.

[0047] Furthermore, in the switching power supply device 10A according to Embodiment 1 of this disclosure, the synchronous rectifier circuit (switching elements Q20-Q23) includes fifth and sixth switching elements Q20 and Q23 that rectify the current flowing to the secondary side of the transformer TR during the period when a current ILmp in a first direction 101 flows to the primary side of the transformer TR, and seventh and eighth switching elements Q21 and Q22 that rectify the current flowing to the secondary side of the transformer TR during the period when a current ILmp in a second direction 102 flows to the primary side of the transformer TR. The control unit 12A controls the fifth and sixth switching elements Q20 and Q23 so that both the fifth switching element Q20 and the sixth switching element Q23 are turned on during the period when the current ILmp in the first direction 101 is greater than or equal to a predetermined threshold ITH, and controls the seventh and eighth switching elements Q21 and Q22 so that both the seventh switching element Q21 and the eighth switching element Q22 are turned on during the period when the current ILmp in the second direction 102 is greater than or equal to a predetermined threshold ITH.

[0048] With the above configuration, even when four switching elements Q20-Q23 are provided on the secondary side of the transformer as a synchronous rectifier circuit, the on / off operation of the synchronous rectifier circuit can be achieved without detecting the time when the current flowing through the resonant element is at its peak value.

[0049] Furthermore, the switching power supply device 10A according to Embodiment 1 of this disclosure further includes a current detection unit 100 that detects the current ILmp in the first direction 101 and the second direction 102 and outputs the current value to the control unit 12A. The control unit 12A controls the switching elements such that both the fifth switching element Q20 and the sixth switching element Q23 are turned on during the period when the current value of the current ILmp in the first direction 101 detected by the current detection unit 100 is equal to or greater than a predetermined threshold ITH, and controls the switching elements such that both the seventh switching element Q21 and the eighth switching element Q22 are turned on during the period when the current value of the current ILmp in the second direction 102 detected by the current detection unit 100 is equal to or greater than a predetermined threshold ITH.

[0050] With the above configuration, the switching power supply 10A further includes a current detection unit 100, so the control unit 12A can properly determine the period during which the current ILmp in the first direction 101 is greater than or equal to a predetermined threshold ITH and the period during which the current ILmp in the second direction 102 is greater than or equal to a predetermined threshold ITH.

[0051] The charging system 1 according to Embodiment 1 of the present disclosure comprises a switching power supply 10A and a secondary battery 20. As shown in Figure 2, the charging system 1 can charge the secondary battery 20 by constant current charging CC using the output current of the switching power supply 10A and constant voltage charging CV using the output voltage of the switching power supply 10A. The secondary battery 20 is charged by constant current charging CC while the battery voltage of the secondary battery 20 is between a first voltage V1 and a second voltage V2, and after the battery voltage of the secondary battery 20 reaches a third voltage V3 that is above the second voltage V2, the secondary battery 20 is charged by constant voltage charging CV.

[0052] With the above configuration, overcurrent charging can be prevented by performing constant current charging CC. Furthermore, after the charging voltage of the secondary battery 20 reaches the third voltage V3, overcharging can be prevented by performing constant voltage charging CV.

[0053] [Embodiment 2] Other embodiments of this disclosure are described below. For the sake of clarity, components having the same function as those described in the above embodiments are denoted by the same reference numerals, and their descriptions are not repeated.

[0054] Figure 5 shows an example configuration of a charging system 2 according to Embodiment 2 of the present disclosure. The charging system 2 shown in Figure 5 comprises a switching power supply 10B and a secondary battery 20. The switching power supply 10B includes a DC-DC converter 11B, a current detection circuit 110, and a control unit 12B.

[0055] The current detection circuit 110 has a current transformer positioned at the measurement point CT1 shown in Figure 5. The current transformer detects the current ILmp.

[0056] The current detection circuit 110 outputs a first signal when the current ILmp flowing in the first direction 101 or the second direction 102 is greater than or equal to the threshold ITH. On the other hand, it outputs a second signal when the current ILmp flowing in the first direction 101 is less than the threshold ITH and the current ILmp flowing in the second direction 102 is less than the threshold ITH.

[0057] The control unit 12B controls the switching elements Q20-Q23 on the secondary side of the transformer TR based on the on / off states of the primary side switching elements Q10-Q13 and the first and second signals output from the current detection circuit 110. For example, when the primary side switching elements Q10 and Q13 of the transformer TR are ON, the control unit 12B turns on the switching elements Q20 and Q23 at the timing when the signal output from the current detection circuit 110 switches from the second signal to the first signal.

[0058] Figure 6 shows an example configuration of the current detection circuit 110. The current detection circuit 110 shown in Figure 6 has a comparator 111. The comparator 111 is designed so that the voltage at the non-inverting input terminal of the operational amplifier 605 becomes a reference voltage corresponding to the threshold ITH of the current ILmp. The voltage at the inverting input terminal of the operational amplifier 605 is input to a voltage corresponding to the current ILmp detected by the current transformer. When the current ILmp becomes greater than the threshold ITH, the operational amplifier 605 of the comparator 111 outputs a first signal.

[0059] [Effects of Embodiment 2] Furthermore, the switching power supply device 10B according to Embodiment 2 of this disclosure further includes a current detection circuit 110 that outputs a first signal to the control unit 12B during the period when the current ILmp flowing in the first direction 101 or the second direction 102 is greater than or equal to a predetermined threshold ITH, and outputs a second signal to the control unit 12B during the period when the current ILmp flowing in the first direction 101 is less than a predetermined threshold ITH and the current ILmp flowing in the second direction 102 is less than a predetermined threshold ITH. The control unit 12B performs on / off operations of the fifth to eighth switching elements Q20-Q23 based on the on / off states of the primary side switching elements Q10-Q13 of the transformer TR and the first and second signals output from the current detection circuit 110.

[0060] With the above configuration, the switching power supply 10B further includes a current detection circuit 110, so the control unit 12B can properly determine the period during which the current ILmp in the first direction 101 is greater than or equal to a predetermined threshold ITH and the period during which the current ILmp in the second direction 102 is greater than or equal to a predetermined threshold ITH.

[0061] [Embodiment 3] Other embodiments of this disclosure are described below. For the sake of clarity, components having the same function as those described in the above embodiments are denoted by the same reference numerals, and their descriptions are not repeated.

[0062] Figure 7 shows an example configuration of a charging system 3 according to Embodiment 3 of the present disclosure. The charging system 3 shown in Figure 7 comprises a switching power supply 10C and a secondary battery 20. The switching power supply 10C has a DC-DC converter 11C and a control unit 12C.

[0063] The DC-DC converter 11C has a center-tapped transformer TR2 with a center-tapped terminal 70 on its secondary side. A current detection unit 100, four switching elements Q10-Q13, and a capacitor C1, a coil L1, and an excitation inductance built into the transformer TR2 (not shown) that constitute a resonant circuit are connected to the primary side of the transformer TR2.

[0064] On the secondary side of transformer TR2, a smoothing capacitor C3 and two switching elements Q30 and Q31, which form a synchronous rectifier circuit and are used to rectify the output current, are connected.

[0065] Figure 8 is a timing chart showing the operation of each part of the DC-DC converter 11C. As shown in Figure 8, the control unit 12C performs phase shift control to provide a predetermined phase difference X1 between the on / off operation of switching element Q10 and the on / off operation of switching element Q13. During the period when switching elements Q10 and Q13 are ON, the control unit 12A controls the secondary switching element Q30 to turn ON when the current ILmp in the first direction 101 shown in Figure 3 exceeds the threshold ITH. Subsequently, the control unit 12C controls the secondary switching element Q30 to turn OFF when the current ILmp in the first direction 101 falls below the threshold ITH.

[0066] As shown in Figure 8, the control unit 12C performs phase shift control to provide a predetermined phase difference X2 between the on / off operation of switching element Q11 and the on / off operation of switching element Q12. During the period when switching elements Q11 and Q12 are ON, if the current ILmp in the second direction 102 shown in Figure 3 exceeds the threshold ITH, the control unit 12C controls the secondary switching element Q31 to turn ON. Subsequently, when the current ILmp in the second direction falls below the threshold ITH, the control unit 12C controls the secondary switching element Q31 to turn OFF. [Effects of Embodiment 3] Furthermore, in the switching power supply device 10C according to Embodiment 3 of this disclosure, the transformer TR2 has a center tap terminal 70 on its secondary side. The synchronous rectifier circuit (switching elements Q30 and Q31) includes a switching element Q30 that rectifies the current flowing to the secondary side of the transformer TR2 during the period when a current ILmp flows to the primary side of the transformer TR2 in a first direction 101, and a switching element Q31 that rectifies the current flowing to the secondary side of the transformer TR2 during the period when a current ILmp flows to the primary side of the transformer TR2 in a second direction 102. The control unit 12C controls the switching element Q30 so that it is turned on during the period when the current ILmp in the first direction 101 is greater than or equal to a predetermined threshold ITH, and controls the switching element Q31 so that it is turned on during the period when the current ILmp in the second direction 102 is greater than or equal to a predetermined threshold ITH.

[0067] According to the above configuration, even if the transformer TR2 has a center tap terminal 70 on its secondary side, and two switching elements Q30 and Q31 are provided as a synchronous rectifier circuit on the secondary side of the transformer TR2, the on / off operation of the synchronous rectifier circuit can be achieved without detecting the time when the current flowing through the resonant element is at its peak value.

[0068] Furthermore, the switching power supply device 10C according to Embodiment 3 of this disclosure further includes a current detection unit 100 that detects the current ILmp in the first direction 101 and the second direction 102 and outputs the current value to the control unit 12C. The control unit 12C controls the switching element Q30 so that the fifth switching element Q30 is turned on during the period when the current value of the current ILmp in the first direction 101 detected by the current detection unit 100 is equal to or greater than a predetermined threshold ITH, and controls the switching element Q31 so that the sixth switching element Q31 is turned on during the period when the current value of the current ILmp in the second direction 102 detected by the current detection unit 100 is equal to or greater than a predetermined threshold ITH.

[0069] With the above configuration, the switching power supply unit 10C further includes a current detection unit 100, so the control unit 12C can properly determine the period during which the current ILmp in the first direction 101 is greater than or equal to a predetermined threshold ITH and the period during which the current ILmp in the second direction 102 is greater than or equal to a predetermined threshold ITH.

[0070] [Variation] In the charging system 1 according to Embodiment 1 and the charging system 3 according to Embodiment 3 described above, the control unit 12A is configured to control the system so that the period T1 in the first direction 101 of Figure 3, during which the current ILmp detected by the current detection unit 100 is greater than or equal to the threshold ITH, includes a period in which both secondary switching elements Q20 and Q23 are turned on. Furthermore, the control unit 12A is configured to control the system so that the period T2 in the second direction 102 of Figure 3, during which the current ILmp detected by the current detection unit 100 is greater than or equal to the threshold ITH, includes a period in which both secondary switching elements Q21 and Q22 are turned on. In other words, the control unit 12A is configured to control the system so that the period T2, shown in Figure 4 as a period during which the current ILmp is less than or equal to the negative threshold - ITH, includes a period in which both secondary switching elements Q21 and Q22 are turned on. However, the periods T1 and T2 may be measured during the design and development of the charging system 1, and the control unit 12A may perform switching control of the secondary switching elements Q20-Q23 based on the periods T1 and T2 measured during the design and development.

[0071] The charging system 1 according to Embodiment 1, the charging system 2 according to Embodiment 2, and the charging system 3 according to Embodiment 3 described above are not limited to the above configuration and may further include other components besides the switching power supply units 10A and 10B and the secondary battery 20. For example, they may further include a power factor correction circuit.

[0072] In the charging system 1 according to Embodiment 1, the charging system 2 according to Embodiment 2, and the charging system 3 according to Embodiment 3 described above, the resonant circuit is composed of a coil L1, a capacitor C1, and an excitation inductance built into a transformer TR (not shown). However, the resonant circuit is not limited to this configuration and may include other elements or parasitic elements (not shown). For example, the resonant circuit may be configured using a leakage inductance built into a transformer TR (not shown) instead of the coil L1. The winding method of the transformer TR is not limited. Furthermore, it may be composed of multiple transformers.

[0073] In the charging system 1 according to Embodiment 1, the charging system 2 according to Embodiment 2, and the charging system 3 according to Embodiment 3 described above, the capacitor C1, the coil L1, and the excitation inductance built into a transformer TR (not shown) that constitute the resonant circuit are set such that, when charging the secondary battery 20 by constant current charging CC, the ratio of the secondary voltage Vout to the primary voltage Vin of the transformer TR, assuming the turns ratio of the transformer TR is 1, is less than 1. However, the capacitor C1, the coil L1, and the excitation inductance built into a transformer TR (not shown) that constitute the resonant circuit are not limited to this configuration. For example, when charging the secondary battery 20 by constant current charging CC, the inductance and capacitance may be set such that, assuming the turns ratio of the transformer TR is 1, the ratio of the secondary voltage Vout to the primary voltage Vin of the transformer TR, is 1 or more.

[0074] In the charging system 3 according to Embodiment 3 described above, a current detection unit 100 is provided on the primary side of the transformer TR2 to detect the current ILmp flowing on the primary side of the transformer TR. However, in the charging system 3 according to Embodiment 3, the method for detecting the current ILmp is not limited to the current detection unit 100. For example, the current ILmp may be detected using a current detection circuit 110, similar to the charging system 2 according to Embodiment 2. By providing the current detection circuit 110 in the switching power supply 10C, the control unit 12C can properly determine the period during which the current ILmp in the first direction 101 is greater than or equal to a predetermined threshold ITH and the period during which the current ILmp in the second direction 102 is greater than or equal to a predetermined threshold ITH.

[0075] This disclosure is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of this disclosure. [Explanation of Symbols]

[0076] 1, 2 Charging systems 10A, 10B switching power supply 11A, 11B DC-DC converters 12A, 12B Control Unit 20 Secondary battery 100 Current detection unit 110 Current detection circuit 111 Comparator C1 Capacitor C2 Smoothing Capacitor ILmp, ILms current L1 Coil Q10, Q11, Q12, Q13, Q20, Q21, Q22, Q23, Q30, Q31 Switching elements TR, TR2 transformer T1, T2 period X1, X2 phase difference

Claims

1. A switching power supply device to which a secondary battery is connected on the load side and which can be used to charge the secondary battery, Transformer and A resonant circuit connected to the primary side of the transformer, First and second switching elements that perform on / off operations on a first current flowing in a first direction to the primary side of the transformer, Third and fourth switching elements that perform on / off operations for a second current flowing in a second direction opposite to the first direction to the primary side of the transformer, A synchronous rectifier circuit that rectifies the current flowing to the secondary side of the transformer during the period when the first current or the second current is flowing to the primary side of the transformer, The system comprises a control unit that controls the on / off operation of the first to fourth switching elements and the synchronous rectifier circuit, The time average value of the on / off operation frequencies of the first to fourth switching elements is fixed. There is a predetermined phase difference between the on / off operation of the first switching element and the on / off operation of the second switching element, and there is the predetermined phase difference between the on / off operation of the third switching element and the on / off operation of the fourth switching element. The control unit, The synchronous rectifier circuit is controlled to remain ON during the period when the first current or the second current is above a predetermined threshold. A switching power supply device characterized by the following features.

2. The synchronous rectifier circuit described above is Fifth and sixth switching elements that rectify the current flowing to the secondary side of the transformer during the period when the first current is flowing to the primary side of the transformer, A seventh and an eighth switching element that rectifies the current flowing to the secondary side of the transformer during the period when the second current is flowing to the primary side of the transformer, It has, The control unit, The fifth and sixth switching elements are controlled so that both the fifth and sixth switching elements are turned on during the period when the first current is equal to or greater than the predetermined threshold. The seventh and eighth switching elements are controlled so that both the seventh and eighth switching elements are turned on during the period when the second current is equal to or greater than the predetermined threshold. The switching power supply device according to feature 1.

3. The system further includes a current detection unit that detects the first current and the second current and outputs the current values ​​of the first current and the second current to the control unit, The control unit, During the period in which the current value of the first current in the first direction detected by the current detection unit is equal to or greater than the predetermined threshold, the fifth and sixth switching elements are controlled so that both the fifth and sixth switching elements are turned on. The switching power supply device according to claim 2, characterized in that the seventh and eighth switching elements are controlled such that both the seventh and eighth switching elements are turned on during the period in which the current value of the second current in the second direction detected by the current detection unit is equal to or greater than the predetermined threshold.

4. The system further includes a current detection circuit that outputs a first signal to the control unit during a period when the first current flowing in the first direction or the second current flowing in the second direction is greater than or equal to a predetermined threshold, and outputs a second signal to the control unit during a period when the first current flowing in the first direction is less than the predetermined threshold and the second current flowing in the second direction is less than the predetermined threshold. The switching power supply device according to claim 2, characterized in that the control unit performs on / off operations of the fifth to eighth switching elements based on the first signal and the second signal output from the current detection circuit.

5. The transformer has a center tap terminal on the secondary side. The synchronous rectifier circuit described above is A fifth switching element that rectifies the current flowing to the secondary side of the transformer during the period when the first current is flowing to the primary side of the transformer, A sixth switching element that rectifies the current flowing to the secondary side of the transformer during the period when the second current is flowing to the primary side of the transformer, It has, The control unit, The fifth switching element is controlled so that it is turned on during the period when the first current is equal to or greater than the predetermined threshold. The sixth switching element is controlled so that it is turned on during the period when the second current is equal to or greater than the predetermined threshold. The switching power supply device according to feature 1.

6. The system further includes a current detection unit that detects the first current and the second current and outputs the current values ​​of the first current and the second current to the control unit, The control unit, The fifth switching element is controlled so that it is turned on during the period when the current value of the first current in the first direction detected by the current detection unit is equal to or greater than the predetermined threshold. The switching power supply device according to claim 5, characterized in that the sixth switching element is controlled so that it is turned on during the period when the current value of the second current in the second direction detected by the current detection unit is equal to or greater than the predetermined threshold.

7. The system further includes a current detection circuit that outputs a first signal to the control unit during a period when the first current flowing in the first direction or the second current flowing in the second direction is greater than or equal to a predetermined threshold, and outputs a second signal to the control unit during a period when the first current flowing in the first direction is less than the predetermined threshold and the second current flowing in the second direction is less than the predetermined threshold. The switching power supply device according to claim 5, characterized in that the control unit performs on / off operations of the fifth and sixth switching elements based on the first signal and the second signal output from the current detection circuit.

8. A switching power supply device according to any one of claims 1 to 6, A charging system comprising the aforementioned secondary battery, Constant current charging using the output current of the aforementioned switching power supply, Constant voltage charging using the output voltage of the aforementioned switching power supply, The secondary battery can be charged by this, While the battery voltage of the secondary battery is between the first voltage and the second voltage, the secondary battery is charged by constant current charging. After the battery voltage of the secondary battery reaches a third voltage equal to or greater than the second voltage, the secondary battery is charged using constant voltage charging. A charging system characterized by the following features.

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