TECHNICAL FIELD

[0001] The present invention relates to the field of electronic circuits, and in particular, to a DC-DC converter, a bi-directional DC-DC converter, and an uninterrupted power supply comprising the same.

BACKGROUND

[0002] DC-DC converters are electrical devices widely used in an uninterrupted power supplies. The operating principle of a DC-DC converter is as follows: an input terminal of the DC-DC converter is connected to a rechargeable battery, and an output terminal of the DC-DC converter is connected to positive and negative direct-current buses in the uninterrupted power supply. When a mains supply fails, the DC-DC converter boosts the direct current in the rechargeable battery and then outputs the direct current to the positive and negative direct-current buses.

[0003] FIG. 1 is a circuit diagram of a first DC-DC converter in the prior art connected between direct-current buses of an uninterrupted power supply and a rechargeable battery and in a charging mode. As shown in FIG. 1 , the DC-DC converter 1 includes an inductor L11 , a capacitor C11 , and an inductor L13 connected in sequence between a positive direct-current bus 11 and a negative electrode of a rechargeable battery B; and an insulated gate bipolar transistor T 11 and a diode D12. A node formed by connecting the inductor L11 and the capacitor C11 is connected to a collector of the insulated gate bipolar transistor T11 ; and a node formed by connecting the capacitor C11 and the inductor L13 is connected to an anode of the diode D12. Both an emitter of the insulated gate bipolar transistor T11 and a cathode of the diode D12 are connected to a negative direct-current bus 12 and a positive electrode of the rechargeable battery B. [0004] The DC-DC converter 1 shown in FIG. 1 can only be controlled to transmit electric energy in the capacitor between the positive and negative direct-current buses to the rechargeable battery B, and cannot transmit electric energy in the rechargeable battery B to the capacitor between the positive and negative direct-current buses, and thus cannot achieve bi-directional energy transmission. In order to achieve bi-directional energy transmission, costs of circuit modules of the uninterrupted power supply would be increased. [0005] In practical application of the uninterrupted power supply, in order to increase power density, a plurality of uninterrupted power supply power modules often need to be connected in parallel.

[0006] FIG. 2 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 1 that are connected in parallel. As shown in FIG. 2, a positive electrode of a rechargeable battery B is simultaneously connected to negative direct-current buses 121 , 122 of the two uninterrupted power supplies. As a result, each control device (not shown in FIG. 2) cannot independently control the voltage on a negative direct-current bus of a corresponding uninterrupted power supply. Thus, the DC-DC converter 1 shown in FIG. 1 cannot be used in a plurality of uninterrupted power supplies connected in parallel.

[0007] FIG. 3 is a circuit diagram of a second DC-DC converter in the prior art connected between direct-current buses of an uninterrupted power supply and a rechargeable battery. As shown in FIG. 3, the DC-DC converter 2 includes an inductor L21 , a capacitor C21 , and an inductor L23 connected in sequence between a positive direct-current bus 21 and a negative electrode of a rechargeable battery B; and an insulated gate bipolar transistor T22 and a diode D21. A node formed by connecting the inductor L21 and the capacitor C21 is connected to a cathode of the diode D21 ; a node formed by connecting the capacitor C21 and the inductor L23 is connected to an emitter of the insulated gate bipolar transistor T22; and both an anode of the diode D21 and a collector of the insulated gate bipolar transistor T22 are connected to a negative direct-current bus 22 and a positive electrode of the rechargeable battery B.

[0008] The DC-DC converter 2 shown in FIG. 3 can only be controlled to transmit electric energy in the rechargeable battery B to the capacitor between the positive and negative direct-current buses, and cannot transmit electric energy in the capacitor between the positive and negative direct-current buses to the rechargeable battery B, and thus cannot achieve bi-directional energy transmission. In order to achieve bi-directional energy transmission, costs of circuit modules of the uninterrupted power supply would be increased. [0009] FIG. 4 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 3 that are connected in parallel. As shown in FIG. 4, a positive electrode of a rechargeable battery B is simultaneously connected to negative direct-current buses 221 , 222 of the two uninterrupted power supplies. Each control device (not shown in FIG. 4) also cannot independently control the voltage on a negative direct-current bus of a corresponding uninterrupted power supply. Thus, the DC-DC converter 2 shown in FIG. 3 cannot be used in a plurality of uninterrupted power supplies connected in parallel. SUMMARY

[0010] In view of the aforementioned technical problems existing in the prior art, the present invention provides a DC-DC converter, comprising:

a first inductor, a first switching tube and a second inductor connected in sequence;

a third inductor, a first diode and a fourth inductor connected in sequence; a first capacitor, wherein one terminal of the first capacitor is connected to a node formed by connecting the first inductor and the first switching tube, and the other terminal of the first capacitor is connected to a node formed by connecting the first diode and the third inductor; and

a second capacitor, wherein one terminal of the second capacitor is connected to a node formed by connecting the first switching tube and the second inductor, and the other terminal of the second capacitor is connected to a node formed by connecting the first diode and the fourth inductor. [0011] Preferably, when the first switching tube is turned ON, the first inductor, the first switching tube and the second inductor form a first current path, and the third inductor, the first capacitor, the first switching tube, the second capacitor and the fourth inductor form a second current path; and when the first switching tube is turned OFF, the first inductor, the first capacitor, the first diode, the second capacitor and the second inductor form a third current path, and the third inductor, the first diode and the fourth inductor form a fourth current path.

[0012] Preferably, the first switching tube is a first insulated gate bipolar transistor, a collector of the first insulated gate bipolar transistor is connected to a node formed by connecting one terminal of the first inductor and the first capacitor, and an emitter of the first insulated gate bipolar transistor is connected to a node formed by connecting one terminal of the second inductor and the second capacitor, wherein the other terminal of the first inductor and the other terminal of the second inductor are respectively used for connecting to a positive electrode and a negative electrode of a first direct-current power supply device, and the third inductor and the fourth inductor are respectively used for connecting to a negative electrode and a positive electrode of a second direct-current power supply device. [0013] Preferably, the DC-DC converter further comprises a second diode connected in anti-parallel to the first switching tube and a second switching tube connected in anti-parallel to the first diode.

[0014] Preferably, the DC-DC converter further comprises a control device for providing a pulse-width modulation signal to the first switching tube so that the first switching tube is alternately turned ON and OFF.

[0015] Preferably, the DC-DC converter further comprises a control device for the following:

controlling the second switching tube to turn OFF, and providing a pulse-width modulation signal to the first switching tube so that the first switching tube is alternately turned ON and OFF; or

controlling the first switching tube to turn OFF, and providing a pulse-width modulation signal to the second switching tube so that the second switching tube is alternately turned ON and OFF.

[0016] The present invention further provides a bi-directional DC-DC converter, comprising:

a first inductor, a first capacitor and a second inductor connected in sequence;

a first switching tube having a first anti-parallel diode, wherein a cathode of the first diode is connected to a node formed by connecting the first inductor and the first capacitor; and

a second switching tube having a second anti-parallel diode, wherein an anode of the second diode is connected to a node formed by connecting the first capacitor and the second inductor, wherein

an anode of the first diode is connected to a cathode of the second diode.

[0017] Preferably, the first switching tube is a first insulated gate bipolar transistor, a collector of the first insulated gate bipolar transistor is connected to a node formed by connecting one terminal of the first inductor and the first capacitor, and an emitter of the first insulated gate bipolar transistor and the other terminal of the first inductor are respectively used for connecting to a negative electrode and a positive electrode of a first direct-current power supply device; and the second switching tube is a second insulated gate bipolar transistor, an emitter of the second insulated gate bipolar transistor is connected to a node formed by connecting the first capacitor and one terminal of the second inductor, and a collector of the second insulated gate bipolar transistor and the other terminal of the second inductor are respectively used for connecting to a positive electrode and a negative electrode of a second direct-current power supply device.

[0018] Preferably, the bi-directional DC-DC converter further comprises: a second capacitor connected between the anode of the first diode and the cathode of the second diode;

a third inductor connected to the anode of the first diode; and a fourth inductor connected to the cathode of the second diode.

[0019] Preferably, the bi-directional DC-DC converter further comprises a control device for the following:

controlling the second switching tube to turn OFF, and providing a pulse-width modulation signal to the first switching tube so that the first switching tube is alternately turned ON and OFF; or

controlling the first switching tube to turn OFF, and providing a pulse-width modulation signal to the second switching tube so that the second switching tube is alternately turned ON and OFF.

[0020] The present invention further provides an uninterrupted power supply, comprising:

the DC-DC converter described above or the bi-directional DC-DC converter described above, wherein the DC-DC converter or the bi-directional DC-DC converter is connected between positive and negative direct-current buses and a rechargeable battery;

a power factor correction circuit, wherein an input terminal of the power factor correction circuit is used for connecting to an alternating-current power supply, and an output terminal of the power factor correction circuit is connected to the positive and negative direct-current buses; and

an inverter, wherein an input terminal of the inverter is connected to the positive and negative direct-current buses, and an output terminal of the inverter is used for providing an alternating current.

[0021] The DC-DC converter of the present invention can buck or boost charge a rechargeable battery, or make the rechargeable battery buck or boost discharge, and can be used in uninterrupted power supplies connected in parallel. The bi-directional DC-DC converter of the present invention can also achieve bi-directional energy transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Embodiments of the present invention are further described below with reference to the accompanying drawings:

FIG. 1 is a circuit diagram of a first DC-DC converter in the prior art connected between direct-current buses of an uninterrupted power supply and a rechargeable battery and in a charging mode;

FIG. 2 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 1 that are connected in parallel;

FIG. 3 is a circuit diagram of a second DC-DC converter in the prior art connected between direct-current buses of an uninterrupted power supply and a rechargeable battery and in a discharging mode;

FIG. 4 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 3 that are connected in parallel;

FIG. 5 is a circuit diagram of a DC-DC converter according to a first embodiment of the present invention; FIG. 6 and FIG. 7 are circuit diagrams of the DC-DC converter shown in FIG. 5 connected between direct-current buses of an uninterrupted power supply and a rechargeable battery and in a charging mode;

FIG. 8 and FIG. 9 are circuit diagrams of the DC-DC converter shown in FIG.

5 connected between direct-current buses of an uninterrupted power supply and a rechargeable battery and in a discharging mode;

FIG. 10 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 6 that are connected in parallel;

FIG. 11 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 8 that are connected in parallel;

FIG. 12 is a circuit diagram of a DC-DC converter according to a second embodiment of the present invention;

FIG. 13 is a circuit diagram of the DC-DC converter shown in FIG. 12 connected between direct-current buses of an uninterrupted power supply and a rechargeable battery;

FIG. 14 is an equivalent circuit diagram of the DC-DC converter shown in FIG.

13 in a charging mode;

FIG. 15 is an equivalent circuit diagram of the DC-DC converter shown in FIG.

13 in a discharging mode;

FIG. 16 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 12 that are connected in parallel;

FIG. 17 is a circuit diagram of a bi-directional DC-DC converter according to a third embodiment of the present invention;

FIG. 18 is a circuit diagram of the bi-directional DC-DC converter shown in FIG. 17 connected between direct-current buses of an uninterrupted power supply and a rechargeable battery;

FIG. 19 and FIG. 20 are equivalent circuit diagrams of the DC-DC converter shown in FIG. 18 in a charging mode;

FIG. 21 and FIG. 22 are equivalent circuit diagrams of the DC-DC converter shown in FIG. 18 in a discharging mode; and

FIG. 23 is a circuit diagram of a bi-directional DC-DC converter according to a fourth embodiment of the present invention. DETAILED DESCRIPTION

[0023] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention is further described in detail below through specific embodiments with reference to the accompanying drawings.

[0024] FIG. 5 is a circuit diagram of a DC-DC converter according to a first embodiment of the present invention. The DC-DC converter 3 includes an inductor L31 , an insulated gate bipolar transistor T31 and an inductor L32 connected in sequence; an inductor L33, a diode D32 and an inductor L34 connected in sequence; and a capacitor C31 and a capacitor C32. One terminal of the capacitor C31 is connected to a node formed by connecting the inductor L31 and a collector of the insulated gate bipolar transistor T31 , and the other terminal of the capacitor C31 is connected to an anode of the diode D32; and one terminal of the capacitor C32 is connected to a node formed by connecting the inductor L32 and an emitter of the insulated gate bipolar transistor T31 , and the other terminal of the capacitor C32 is connected to a cathode of the diode D32. [0025] One terminal of the inductor L31 and one terminal of the inductor

L32 are respectively used for connecting to a positive electrode and a negative electrode of a direct-current power supply device (for example, a capacitor or a rechargeable battery), and one terminal of the inductor L34 and one terminal of the inductor L33 respectively serve as a positive output terminal and a negative output terminal of the DC-DC converter 3 and are used for connecting to a positive electrode and a negative electrode of another direct-current power supply device (for example, a capacitor or a rechargeable battery). [0026] With reference to FIG. 1 and FIG. 5, the DC-DC converter 3 differs from the DC-DC converter 1 shown in FIG. 1 in that the DC-DC converter 3 further includes the inductor L32 connected to the emitter of the insulated gate bipolar transistor T31 ; the capacitor C32 connected between the emitter of the insulated gate bipolar transistor T31 and the cathode of the diode D32; and the inductor L34 connected to the cathode of the diode D32. [0027] In addition, with reference to FIG. 3 and FIG. 5, the DC-DC converter 3 differs from the DC-DC converter 2 shown in FIG. 3 in that the DC-DC converter 3 further includes the inductor L31 connected to the collector of the insulated gate bipolar transistor T31 ; the capacitor C31 connected between the collector of the insulated gate bipolar transistor T31 and the anode of the diode D32; and the inductor L33 connected to the anode of the diode D32.

[0028] The working principle of the DC-DC converter 3 will be described below with reference to circuit diagrams of the DC-DC converter 3 in a charging mode and a discharging mode.

[0029] FIG. 6 and FIG. 7 are circuit diagrams of the DC-DC converter shown in FIG. 5 connected between direct-current buses of an uninterrupted power supply and a rechargeable battery and in a charging mode. As shown in FIG. 6 and FIG. 7, the other terminal of the inductor L31 and the other terminal of the inductor L32 are respectively connected to a positive direct-current bus 31 and a negative direct-current bus 32, and the other terminal of the inductor L33 and the other terminal of the inductor L34 are respectively connected to a negative electrode and a positive electrode of a rechargeable battery B.

[0030] In the charging mode, a pulse-width modulation signal is provided to a gate of the insulated gate bipolar transistor T31 (namely, a control terminal thereof) so that the insulated gate bipolar transistor T31 is alternately turned ON and OFF. [0031] When the insulated gate bipolar transistor T31 is turned ON, as shown in FIG. 6, the positive direct-current bus 31 , the inductor L31 , the insulated gate bipolar transistor T31 , the inductor L32, and the negative direct-current bus 32 form a current path, a current direction of which is indicated by the dashed single-headed arrow in FIG. 6. At this time, the inductor L31 and the inductor L32 store energy. At the same time, the inductor L33, the capacitor C31 , the insulated gate bipolar transistor T31 , the capacitor C32, the inductor L34, and the rechargeable battery B form another current path, a current direction of which is indicated by the dashed double-headed arrows in FIG. 6. At this time, the capacitor C31 and the capacitor C32 release and store energy in the inductor L34, the rechargeable battery B and the inductor L33.

[0032] When the insulated gate bipolar transistor T31 is turned OFF, the positive direct-current bus 31 , the inductor L31 , the capacitor C31 , the diode D32, the capacitor C32, the inductor L32, and the negative direct-current bus 32 form a current path, a current direction of which is indicated by the dashed single-headed arrow in FIG. 7. The inductor L31 and the inductor L32 release and store energy in the capacitor C31 and the capacitor C32. At the same time, the inductor L33, the diode D32, the inductor L34, and the rechargeable battery B form another current path, a current direction of which is indicated by the dashed double-headed arrows in FIG. 7. At this time, the inductor L33 and the inductor L34 release and store energy in the rechargeable battery B. [0033] With reference to FIG. 6 and FIG. 7, electric energy in the capacitor between the positive direct-current bus 31 and the negative direct-current bus 32 is finally stored in the rechargeable battery B, thereby achieving charging of the rechargeable battery B. [0034] Assume that a voltage value between the positive and negative direct-current buses is Udc; a voltage value of the rechargeable battery is Uo; the voltage at a node formed by connecting the inductor L31 and the capacitor C31 is UBI ; the voltage at a node formed by connecting the inductor L32 and the capacitor C32 is U B2; the voltage at a node formed by connecting the inductor L33 and the capacitor C31 is UAI ; and the voltage at a node formed by connecting the inductor L34 and the capacitor C32 is UA2. The voltage at two terminals of the capacitor C31 is Uci , and the voltage at two terminals of the capacitor C32 is Uc2. A period of the pulse-width modulation signal is T, a duty cycle of the pulse-width modulation signal is d, and an ON time and an OFF time of the insulated gate bipolar transistor T31 in one pulse-width modulation signal period are respectively Ton and Toff. One period of the pulse-width modulation signal is used as an example for description.

[0035] When the insulated gate bipolar transistor T31 is turned ON, the following equations are satisfied:

When the insulated gate bipolar transistor T31 is turned OFF, the following equations are satisfied:

Within one switching period T, the following equations are satisfied:

The average voltage of the inductors L31 , L32, L33, L34 within one switching period is 0, and thus:

[0036] Thus, Uo/Udc=d/(1 -d). When the duty cycle d is less than 0.5, buck charging of the rechargeable battery B is achieved. When the duty cycle d is greater than 0.5, boost charging of the rechargeable battery B is achieved.

[0037] FIG. 8 and FIG. 9 are circuit diagrams of the DC-DC converter shown in FIG. 5 connected between direct-current buses of an uninterrupted power supply and a rechargeable battery and in a discharging mode. The inductor L31 and the inductor L32 shown in FIG. 6 and FIG. 7 are respectively connected to a positive electrode and a negative electrode of a rechargeable battery B, and the inductor L33 and the inductor L34 shown in FIG. 6 and FIG. 7 are respectively connected to a negative direct-current bus 32 and a positive direct-current bus 31 so as to obtain the circuit shown in FIG. 8 and FIG. 9.

[0038] In the discharging mode, a pulse-width modulation signal is provided to a gate of an insulated gate bipolar transistor T32' so that the insulated gate bipolar transistor T32' is alternately turned ON and OFF. [0039] When the insulated gate bipolar transistor T32' is turned ON, as shown in FIG. 8, the rechargeable battery B, an inductor L34', the insulated gate bipolar transistor T32', and an inductor L33' form a current path, a current direction of which is indicated by the dashed single-headed arrow in FIG. 8. At this time, electric energy in the rechargeable battery B is stored in the inductor L33' and the inductor L34'. At the same time, the negative direct-current bus 32, an inductor L32', a capacitor C32', the insulated gate bipolar transistor T32', a capacitor C31 ', an inductor L31 ', and the positive direct-current bus 31 form a current path, a current direction of which is indicated by the dashed double-headed arrows in FIG. 8. At this time, the capacitor C31 ' and the capacitor C32' release and store energy in the inductor L31 ', the inductor L32' and a capacitor between the positive direct-current bus 31 and the negative direct-current bus 32.

[0040] When the insulated gate bipolar transistor T32' is turned OFF, as shown in FIG. 9, the rechargeable battery B, the inductor L34', the capacitor C32', a diode D32', the capacitor C31 ', and the inductor L33' form a current path, a current direction of which is indicated by the dashed single-headed arrow in FIG. 9. At this time, the inductor L34' and the inductor L33' release and store energy in the capacitor C32' and the capacitor C31 '. At the same time, the negative direct-current bus 32, the inductor L32', the diode D32', the inductor L31 ', and the positive direct-current bus 31 form another current path, a current direction of which is indicated by the dashed double-headed arrows in FIG. 9. The inductor L32' and the inductor L31 ' release and store energy in a capacitor between the positive direct-current bus 31 and the negative direct-current bus 32.

[0041] With reference to FIG. 8 and FIG. 9, electric energy in the rechargeable battery B is finally stored in the capacitor between the positive and negative direct-current buses. [0042] Assume that a voltage value between the positive and negative direct-current buses is Udc; a voltage value of the rechargeable battery is Uo; the voltage at a node formed by connecting the inductor L31 ' and the capacitor C31 ' is U BI ; the voltage at a node formed by connecting the inductor L32' and the capacitor C32' is UB2; the voltage at a node formed by connecting the inductor L33' and the capacitor C31 ' is UAI ; and the voltage at a node formed by connecting the inductor L34' and the capacitor C32' is UA2. The voltage at two terminals of the capacitor C3T is Uci, and the voltage at two terminals of the capacitor C32' is Uc2. A period of the pulse-width modulation signal is T, a duty cycle of the pulse-width modulation signal is d, and an ON time and an OFF time of the insulated gate bipolar transistor T32' in one pulse-width modulation signal period are respectively Ton and Toff. One period of the pulse-width modulation signal is used as an example for description.

[0043] When the insulated gate bipolar transistor T32' is turned ON, the following equations are satisfied:

When the insulated gate bipolar transistor T32' is turned OFF, the following equations are satisfied:

Within one switching period T, the following equations are satisfied:

The average voltage of the inductors L3T, L32', L33', L34' within one switching period is 0, and thus:

[0044] Thus, Udc/Uo=d/(1 -d). When the duty cycle d is less than 0.5, buck discharging of the rechargeable battery B is achieved. When the duty cycle d is greater than 0.5, boost discharging of the rechargeable battery B is achieved.

[0045] FIG. 10 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 6 that are connected in parallel. As shown in FIG. 10, a negative direct-current bus of each uninterrupted power supply is connected to a positive electrode of a rechargeable battery B through an inductor, a capacitor, and an inductor in sequence; negative direct-current buses of the plurality of uninterrupted power supplies are isolated from each other; and each control device (not shown in FIG. 10) can independently control the voltage on a negative direct-current bus of a corresponding uninterrupted power supply, so as to charge the rechargeable battery B using electric energy in a capacitor between the positive and negative direct-current buses of the uninterrupted power supply. [0046] FIG. 11 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 8 that are connected in parallel. As shown in FIG. 11 , a negative direct-current bus of each uninterrupted power supply is connected to a positive electrode of a rechargeable battery B through an inductor, a capacitor, and an inductor in sequence; negative direct-current buses of the plurality of uninterrupted power supplies are isolated from each other; and each control device (not shown in FIG. 11 ) can independently control the voltage on a negative direct-current bus of a corresponding uninterrupted power supply, so that the rechargeable battery B discharges and stores electricity in a capacitor between the positive and negative direct-current buses of the corresponding uninterrupted power supply.

[0047] FIG. 12 is a circuit diagram of a DC-DC converter according to a second embodiment of the present invention. As shown in FIG. 12, the DC-DC converter 4 differs from the DC-DC converter 3 shown in FIG. 5 in that the DC-DC converter 4 further includes a diode D41 connected in anti-parallel to an insulated gate bipolar transistor T41 and an insulated gate bipolar transistor T42 connected in anti-parallel to a diode D42.

[0048] One terminal of the inductor L41 and one terminal of the inductor L42 are respectively used for connecting to a positive electrode and a negative electrode of a direct-current power supply device (for example, a capacitor or a rechargeable battery), and one terminal of the inductor L44 and one terminal of the inductor L43 respectively serve as a positive output terminal and a negative output terminal of the DC-DC converter 4 and are used for connecting to a positive electrode and a negative electrode of another direct-current power supply device (for example, a capacitor or a rechargeable battery). Or, one terminal of the inductor L44 and one terminal of the inductor L43 are respectively used for connecting to a positive electrode and a negative electrode of a direct-current power supply device (for example, a capacitor or a rechargeable battery), and one terminal of the inductor L41 and one terminal of the inductor L42 respectively serve as a positive output terminal and a negative output terminal of the DC-DC converter 4 and are used for connecting to a positive electrode and a negative electrode of another direct-current power supply device (for example, a capacitor or a rechargeable battery).

[0049] FIG. 13 is a circuit diagram of the DC-DC converter shown in FIG. 12 connected between direct-current buses of an uninterrupted power supply and a rechargeable battery. As shown in FIG. 13, the inductors L41 , L42 are respectively connected to a positive direct-current bus 41 and a negative direct-current bus 42, and the inductors L43, L44 are respectively connected to a negative electrode and a positive electrode of a rechargeable battery B.

[0050] The working principle of the DC-DC converter 4 is described below with reference to equivalent circuit diagrams of the DC-DC converter 4 in a charging mode and a discharging mode.

[0051] In the charging mode, a control device (not shown in FIG. 13) controls the insulated gate bipolar transistor T42 to turn OFF, and provides a pulse-width modulation signal to a gate of the insulated gate bipolar transistor T41 so that the insulated gate bipolar transistor T41 is alternately turned ON and OFF. FIG. 14 is an equivalent circuit diagram of the DC-DC converter shown in FIG. 13 in a charging mode, which is the same as the circuit shown in FIG. 6 and FIG. 7. The specific control process is not described herein again, and buck charging or boost charging of the rechargeable battery B can also be achieved.

[0052] In the discharging mode, a control device (not shown in FIG. 13) controls the insulated gate bipolar transistor T41 to turn OFF, and provides a pulse-width modulation signal to a gate of the insulated gate bipolar transistor T42 so that the insulated gate bipolar transistor T42 is alternately turned ON and OFF. FIG. 15 is an equivalent circuit diagram of the DC-DC converter shown in FIG. 13 in a discharging mode, which is the same as the circuit shown in FIG. 8 and FIG. 9. The specific control process is not described herein again, and boost discharging or buck discharging of the rechargeable battery B can also be achieved.

[0053] With reference to FIG. 14 and FIG. 15, the DC-DC converter 4 is a bi-directional DC-DC converter and does not need to be additionally connected to a charger or a direct-current converter, thereby saving costs. Boost discharging or buck discharging of the rechargeable battery B can be achieved, and buck charging or boost charging of the rechargeable battery B can also be achieved. During charging of the rechargeable battery, by changing a duty cycle d of a pulse-width modulation signal, a capacitor between positive and negative direct-current buses can be deeply discharged and charge the rechargeable battery B at a voltage of approximately 0 volts without producing any impact current.

[0054] FIG. 16 is a circuit diagram of two uninterrupted power supplies including the DC-DC converter shown in FIG. 12 that are connected in parallel. Each control device (not shown in FIG. 16) can separately control a DC-DC converter in a corresponding uninterrupted power supply so as to independently control the voltage on a negative direct-current bus of each uninterrupted power supply.

[0055] FIG. 17 is a circuit diagram of a bi-directional DC-DC converter according to a third embodiment of the present invention. As shown in FIG. 17, the bi-directional DC-DC converter s includes an inductor L51 , a capacitor C51 and an inductor L53 connected in sequence; an insulated gate bipolar transistor T51 having an anti-parallel diode D51 ; and an insulated gate bipolar transistor T52 having an anti-parallel diode D52. A cathode of the diode D51 is connected to a node formed by connecting the inductor L51 and the capacitor C51 ; an anode of the diode D52 is connected to a node formed by connecting the capacitor C51 and the inductor L53; and an anode of the diode D51 is connected to a cathode of the diode D52.

[0056] One terminal of the inductor L51 and an emitter of the insulated gate bipolar transistor T51 (namely, the anode of the diode D51 ) are respectively used for connecting to a positive electrode and a negative electrode of a direct-current power supply device (for example, a capacitor or a rechargeable battery), and a collector of the insulated gate bipolar transistor T51 (namely, the cathode of the diode D52) and one terminal of the inductor L53 respectively serve as a positive output terminal and a negative output terminal of the bi-directional DC-DC converter 5 and are used for connecting to a positive electrode and a negative electrode of another direct-current power supply device (for example, a capacitor or a rechargeable battery). Or, the cathode of the diode D52 and one terminal of the inductor L53 are respectively used for connecting to a positive electrode and a negative electrode of a direct-current power supply device (for example, a capacitor or a rechargeable battery), and one terminal of the inductor L51 and the anode of the diode D51 respectively serve as a positive output terminal and a negative output terminal of the bi-directional DC-DC converter 5 and are used for connecting to a positive electrode and a negative electrode of another direct-current power supply device (for example, a capacitor or a rechargeable battery).

[0057] With reference to FIG. 1 and FIG. 17, the bi-directional DC-DC converter 5 differs from the DC-DC converter 1 shown in FIG. 1 in that the bi-directional DC-DC converter 5 further includes a diode connected in anti-parallel to the insulated gate bipolar transistor T11 and an insulated gate bipolar transistor connected in anti-parallel to the diode D12. [0058] In addition, with reference to FIG. 3 and FIG. 17, the bi-directional

DC-DC converter 5 differs from the DC-DC converter 2 shown in FIG. 3 in that the bi-directional DC-DC converter 5 further includes an insulated gate bipolar transistor connected in anti-parallel to the diode D21 and a diode connected in anti-parallel to the insulated gate bipolar transistor T22.

[0059] FIG. 18 is a circuit diagram of the bi-directional DC-DC converter shown in FIG. 17 connected between direct-current buses of an uninterrupted power supply and a rechargeable battery. The inductor L51 is connected to a positive direct-current bus 51 ; the inductor L53 is connected to a negative electrode of a rechargeable battery B; and a positive electrode of the rechargeable battery B, the emitter of the insulated gate bipolar transistor T51 , the anode of the diode D51 , the cathode of the diode D52, and a collector of the insulated gate bipolar transistor T52 are all connected to a negative direct-current bus 52.

[0060] The working principle of the bi-directional DC-DC converter 5 will be described below with reference to equivalent circuit diagrams of the bi-directional DC-DC converter 5 in a charging mode and a discharging mode.

[0061] In the charging mode, a control device (not shown in FIG. 18) controls the insulated gate bipolar transistor T52 to turn OFF, and provides a pulse-width modulation signal to a gate of the insulated gate bipolar transistor T51 so that the insulated gate bipolar transistor T51 is alternately turned ON and OFF. FIG. 19 and FIG. 20 are equivalent circuit diagrams of the DC-DC converter shown in FIG. 18 in a charging mode. [0062] As shown in FIG. 19, when the insulated gate bipolar transistor T51 is turned ON, the positive direct-current bus 51 , the inductor L51 , the insulated gate bipolar transistor T51 , and the negative direct-current bus 52 form a current path, a current direction of which is indicated by the dashed single-headed arrow in FIG. 19. At this time, the inductor L51 stores energy. At the same time, the inductor L53, the capacitor C51 , the insulated gate bipolar transistor T51 , and the rechargeable battery B form another current path, a current direction of which is indicated by the dashed double-headed arrows in FIG. 19. At this time, the capacitor C51 releases and stores energy in the rechargeable battery B and the inductor L53.

[0063] As shown in FIG. 20, when the insulated gate bipolar transistor T51 is turned OFF, the positive direct-current bus 51 , the inductor L51 , the capacitor C51 , the diode D52, and the negative direct-current bus 52 form a current path, a current direction of which is indicated by the dashed single-headed arrow in FIG. 20. At this time, the inductor L51 releases and stores energy in the capacitor C51. At the same time, the inductor L53, the diode D52, and the rechargeable battery B form another current path, a current direction of which is indicated by the dashed double-headed arrows in FIG. 20. At this time, the inductor L53 releases and stores energy in the rechargeable battery B. [0064] With reference to FIG. 19 and FIG. 20, electric energy in the capacitor between the positive direct-current bus 51 and the negative direct-current bus 52 is finally stored in the rechargeable battery B, thereby achieving charging of the rechargeable battery B. [0065] Assume that a voltage value between the positive direct-current bus

51 and the negative direct-current bus 52 is Udc; a voltage value of the rechargeable battery B is Uo; the voltage at a node formed by connecting the inductor L51 and the capacitor C51 is U BI ; the voltage at a node formed by connecting the inductor L53 and the capacitor C51 is UAI ; and the voltage at the emitter of the insulated gate bipolar transistor T51 and the cathode of the diode D52 is U2. The voltage at two terminals of the capacitor C51 is Uci, a period of the pulse-width modulation signal is T, a duty cycle of the pulse-width modulation signal is d, and an ON time and an OFF time of the insulated gate bipolar transistor T51 in one pulse-width modulation signal period are respectively Ton and Toff. One period of the pulse-width modulation signal is used as an example for description. [0066] When the insulated gate bipolar transistor T51 is turned ON, the following equations are satisfied:

When the insulated gate bipolar transistor T51 is turned OFF, the following equations are satisfied:

Within one switching period T, the following equations are satisfied:

The average voltage of the inductor L51 and the inductor L53 within one switching period is 0, and thus:

[0067] Thus, Uo/Udc=d/(1 -d). When the duty cycle d is less than 0.5, buck charging of the rechargeable battery B is achieved. When the duty cycle d is greater than 0.5, boost charging of the rechargeable battery B is achieved.

[0068] In the discharging mode, a control device (not shown in FIG. 18) controls the insulated gate bipolar transistor T51 to turn OFF, and provides a pulse-width modulation signal to a gate of the insulated gate bipolar transistor T52 so that the insulated gate bipolar transistor T52 is alternately turned ON and OFF. FIG. 21 and FIG. 22 are equivalent circuit diagrams of the DC-DC converter shown in FIG. 18 in a discharging mode.

[0069] As shown in FIG. 21 , when the insulated gate bipolar transistor T52 is turned ON, the rechargeable battery B, the insulated gate bipolar transistor T52, and the inductor L53 form a current path, a current direction of which is indicated by the dashed single-headed arrow in FIG. 21. At this time, the inductor L53 stores energy. At the same time, the negative direct-current bus 52, the insulated gate bipolar transistor T52, the capacitor C51 , the inductor L51 , and the positive direct-current bus 51 form another current path, a current direction of which is indicated by the dashed double-headed arrows in FIG. 21. At this time, the capacitor C51 releases and stores energy in the inductor L51 and a capacitor between the positive direct-current bus 51 and the negative direct-current bus 52.

[0070] As shown in FIG. 22, when the insulated gate bipolar transistor T52 is turned OFF, the rechargeable battery B, the diode D51 , the capacitor C51 , and the inductor L53 form a current path, a current direction of which is indicated by the dashed single-headed arrow in FIG. 22. The rechargeable battery B and the inductor L53 release and store energy in the capacitor C51. At the same time, the negative direct-current bus 52, the diode D51 , the inductor L51 , and the positive direct-current bus 51 form another current path, a current direction of which is indicated by the dashed double-headed arrows in FIG. 22. At this time, the inductor L51 releases and stores energy in a capacitor between the positive direct-current bus 51 and the negative direct-current bus 52.

[0071] With reference to FIG. 21 and FIG. 22, discharging of the rechargeable battery B is achieved, and electric energy of the rechargeable battery B is finally stored in the capacitor between the positive direct-current bus 51 and the negative direct-current bus 52.

[0072] Assume that a voltage value between the positive direct-current bus 51 and the negative direct-current bus 52 is Udc; a voltage value of the rechargeable battery is Uo; the voltage at a node formed by connecting the inductor L51 and the capacitor C51 is U BI ; the voltage at a node formed by connecting the inductor L53 and the capacitor C51 is UAI ; and the voltage at the diode D51 and the collector of the insulated gate bipolar transistor T52 is II2. The voltage at two terminals of the capacitor C51 is Uci , a period of the pulse-width modulation signal is T, a duty cycle of the pulse-width modulation signal is d, and an ON time and an OFF time of the insulated gate bipolar transistor T52 in one pulse-width modulation signal period are respectively Ton and Toff. One period of the pulse-width modulation signal is used as an example for description. [0073] When the insulated gate bipolar transistor T52 is turned ON, the following equations are satisfied:

When the insulated gate bipolar transistor T52 is turned OFF, the following equations are satisfied:

Within one switching period, the following equations are satisfied:

The average voltage of the inductor L51 and the inductor L53 within one switching period is 0, and thus:

[0074] Thus, Udc/Uo=d/(1 -d). When the duty cycle d is less than 0.5, buck discharging of the rechargeable battery B is achieved. When the duty cycle d is greater than 0.5, boost discharging of the rechargeable battery B is achieved. [0075] The bi-directional DC-DC converter 5 of the present invention can controllably transmit electric energy in the capacitor between the direct-current buses to the rechargeable battery B, and transmit electric energy in the rechargeable battery B to the capacitor between the direct-current buses, thereby achieving bi-directional energy transmission.

[0076] FIG. 23 is a circuit diagram of a bi-directional DC-DC converter according to a fourth embodiment of the present invention. As shown in FIG. 17, the bi-directional DC-DC converter 6 differs from the bi-directional DC-DC converter 5 in FIG. 17 in that the bi-directional DC-DC converter 6 further includes an inductor L62 connected to an anode of a diode D61 ; a capacitor C62 connected between the anode of the diode D61 and a cathode of a diode D62; and an inductor L64 connected to the cathode of the diode D62. [0077] With reference to the DC-DC converter 4 shown in FIG. 12, the bi-directional DC-DC converter 6 in this embodiment has the same topology structure as that of the DC-DC converter 4 shown in FIG. 12, and the working principle thereof will not be described herein again. Thus, the bi-directional DC-DC converter 6 can also be used in a plurality of uninterrupted power supplies connected in parallel.

[0078] In other embodiments of the present invention, a switching tube such as a metal-oxide-semiconductor field effect transistor (MOSFET) is adopted in place of the insulated gate bipolar transistor in the aforementioned embodiment.

[0079] The present invention further provides an uninterrupted power supply, which includes the DC-DC converter or bi-directional DC-DC converter in the aforementioned embodiment of the present invention, a power factor correction circuit (PFC) and an inverter, wherein the DC-DC converter or the bi-directional DC-DC converter is connected between positive and negative direct-current buses and a rechargeable battery; an input terminal of the PFC is connected to an alternating-current power supply (for example, a mains supply); an output terminal of the PFC is connected to the positive and negative direct-current buses; an input terminal of the inverter is connected to the positive and negative direct-current buses; and an output terminal of the inverter is used for providing an alternating current to a load.

[0080] Although the present invention has been described through preferred embodiments, the present invention is not limited to the embodiments described herein, but includes various changes and variations made without departing from the scope of the present invention.