Step-down vehicle-mounted charging system

Abstract

The step-down vehicle-mounted charging system comprises an AC-DC converter, a DC-DC converter and a step-down converter which are connected in sequence. The step-down converter comprises: an input winding formed by one phase of winding in three phase windings of the motor; the output winding is composed of windings except the input winding in the three-phase windings of the motor; the voltage regulation bridge arm is composed of a traction inverter bridge arm coupled with the output winding; the traction switch is arranged in the positive bus between the traction inverter and the positive electrode of the power battery and divides the positive bus into an upstream section and a downstream section; the filter capacitor is arranged between the downstream section of the positive bus and the negative bus; and the charging switch is arranged between the positive electrode of the power battery and the input winding. The charging voltage of the charging system can be matched with the voltage of the power battery.

Description

Technical Field

[0001] This application belongs to the technical field of on-board charging. Specifically, this application relates to a step-down on-board charging system for vehicle power batteries. Background Technology

[0002] New energy vehicles, such as hybrid electric vehicles (HEVs) and electric vehicles (EVs), are equipped with on-board charging systems. These systems convert AC power from the power grid into DC power to charge the vehicle's battery (on-board battery).

[0003] In existing technologies, on-board charging systems typically consist of a two-stage topology, primarily composed of an AC-DC converter and an isolated DC-DC converter. The AC-DC converter converts AC power from the grid into DC power, while the isolated DC-DC converter regulates the DC output from the AC-DC converter before outputting it to charge the battery. During charging, the battery voltage fluctuates significantly, requiring the isolated DC-DC converter to have a correspondingly large output voltage range. Widely used isolated DC-DC converter topologies, such as LLC converters, exhibit high efficiency at their rated output voltage, but efficiency is severely affected and heat generation increases once the voltage gain exceeds a certain limit. Configuring an isolated DC-DC converter with a large output voltage range necessitates higher cooling capabilities during both low-voltage and high-voltage output phases. Furthermore, a large output voltage range requires matching a wide frequency range, which not only affects circuit efficiency but also the miniaturization of magnetic components and EMC circuitry. Utility Model Content

[0004] This application aims to provide an improved on-board charging system that can avoid or mitigate the aforementioned problems of existing two-stage on-board charging systems.

[0005] According to one embodiment of this application, a step-down on-board charging system is provided for charging a power battery. The power battery supplies power to a drive motor with three-phase windings via a traction inverter. The traction inverter includes three arms, each arm having an upper arm switch coupled to the positive bus of the traction inverter and a lower arm switch coupled to the negative bus of the traction inverter. The intermediate node of the line connecting the upper arm switch and the lower arm switch in each arm is coupled to a corresponding phase winding of the drive motor. The step-down on-board charging system includes three conversion stages connected in sequence: a first conversion stage includes an AC-DC converter, a second conversion stage includes a DC-DC converter, and a third conversion stage includes a step-down converter. The positive bus of the traction inverter... A step-down converter is configured with a positive output terminal between the DC-DC converter and the positive terminal of the power battery, and a negative bus between the negative output terminal of the DC-DC converter and the negative terminal of the power battery. The step-down converter includes: an input winding composed of one phase of the three-phase windings; an output winding composed of the three-phase windings excluding the input winding; a voltage regulating arm composed of an arm coupled to the output winding; a traction switch disposed between the traction inverter and the positive terminal of the power battery on the positive bus, dividing the positive bus into an upstream and downstream segment; a filter capacitor disposed between the downstream segment of the positive bus and the negative bus; and a charging switch disposed between the positive terminal of the power battery and the input winding.

[0006] In one embodiment, the first terminal of the charging switch is connected to the downstream section of the positive bus, or to the positive side of the filter capacitor, or to the connection line between the positive side of the filter capacitor and the downstream section of the positive bus.

[0007] In one embodiment, the second terminal of the charging switch is connected to a terminal of the input winding, or to the middle node of the bridge arm connected to the input winding, or to the connection line between the input winding and the connected bridge arm.

[0008] In one embodiment, the DC-DC converter is a fixed output voltage DC-DC converter.

[0009] In one embodiment, in the charging mode of the power battery, the charging switch is in the ON state and the traction switch is in the OFF state; in the traction mode of the power battery, the charging switch is in the OFF state and the traction switch is in the ON state.

[0010] In one embodiment, during the charging mode of the power battery, the upper bridge arm switch in the voltage regulating bridge arm is in a periodically on / off state, and the lower bridge arm switch is in a periodically on / off state with the opposite on / off state to the upper bridge arm switch.

[0011] In one embodiment, the output winding is a single-phase winding other than the input winding in the three-phase winding; the voltage regulating arm includes a single arm coupled to the single-phase winding.

[0012] In one embodiment, the output winding is two phase windings other than the input winding in the three-phase winding; the voltage regulating bridge arm includes two voltage regulating bridge arms respectively coupled to the two phase windings.

[0013] In one embodiment, in the charging mode of the power battery, the upper bridge arm switch in each voltage regulating bridge arm is in a periodic on / off state, and the lower bridge arm switch is in a periodic on / off state with the opposite on / off state to the upper bridge arm switch. The on / off operation of the upper and lower bridge arm switches of one voltage regulating bridge arm is staggered from the on / off operation of the upper and lower bridge arm switches of the other voltage regulating bridge arm by a certain deviation time.

[0014] In one embodiment, the deviation time is set such that the on-time of the upper arm switch in one voltage regulating arm has a predetermined overlap ratio with the on-time of the upper arm switch in the other voltage regulating arm.

[0015] According to the buck-type on-board charging system with a three-stage topology of this application, a buck circuit is formed by a motor inverter and windings to achieve voltage reduction during the charging process of the power battery, so that the charging voltage of the charging system matches the voltage of the power battery. Furthermore, during the charging process of the power battery, the second-stage DC-DC converter can have a substantially constant output voltage (preferably a fixed rated output voltage), thus enabling it to always operate at high efficiency, helping to reduce the heat generation of the DC-DC converter and alleviate the pressure on the cooling system. In addition, since the second-stage voltage DC-DC converter has a fixed output voltage, it can output at a fixed frequency, which helps to reduce the size of magnetic components and EMC circuits. Attached Figure Description

[0016] The technical solution of this application will become clearer from the following detailed description taken in conjunction with the accompanying drawings. It is understood that these drawings are for illustrative purposes only and are not intended to limit the scope of protection of this application.

[0017] Figure 1 An exemplary structure of a step-down on-board charging system according to this application is shown.

[0018] Figure 2 The charging modes of this step-down on-board charging system are shown.

[0019] Figure 3 , Figure 4An exemplary on / off timing sequence of the traction inverter arm switch in the third conversion stage of this buck on-board charging system is shown in two charging modes. Detailed Implementation

[0020] See Figure 1 This application provides a step-down on-board charging system for charging a power battery 1 of a new energy vehicle. The power battery 1 supplies power to the vehicle's drive motor M. As is known in the art, the drive motor M has three-phase windings and is equipped with a traction inverter circuit, which converts the DC power from the power battery 1 into AC power to supply the three-phase windings of the motor M.

[0021] The charging power source used by this step-down on-board charging system is AC power source 2, such as the power grid. Figure 1 The image illustrates an exemplary topology of this buck-type on-board charging system. As can be seen, the buck-type on-board charging system has a three-stage topology connected in sequence: a first conversion stage S1, including an AC-DC converter 3; a second conversion stage S2, including an isolated DC-DC converter 4 (preferably a fixed output voltage type); and a third conversion stage S3, including a buck converter 5 based on a traction inverter and motor windings, used to output charging voltage to both sides of the power battery 1.

[0022] The input terminal of AC-DC converter 3 is configured to be connected to AC power supply 2 for inputting AC power, and the output terminal of AC-DC converter 3 is connected to the input terminal of DC-DC converter 4 for outputting DC power with a fixed voltage to DC-DC converter 4.

[0023] The DC-DC converter 4 preferably outputs a substantially constant rated voltage at its output terminal, for example, a fixed rated voltage (i.e., the output voltage always has a rated gain). Regardless of changes in the battery voltage of the power battery 1, the output voltage of the DC-DC converter 4 remains at a substantially constant value higher than the battery voltage. Compared to isolated DC-DC converters used in the prior art, the constant output voltage (preferably fixed output voltage) isolated DC-DC converter 4 used in this application can improve power conversion efficiency and reduce the number of semiconductor devices required. Furthermore, at the rated output voltage, the circuit has lower switching losses, thus allowing for a fixed switching frequency, thereby enabling a reduction in the size of magnetic components and EMC circuitry.

[0024] The step-down converter 5 includes a charging switch Ka and a traction switch Sa (both are electronic switches, such as relays), a filter capacitor Ca, a traction inverter for powering the drive motor M, and a three-phase winding of the drive motor M.

[0025] A traction inverter is coupled between the power battery 1 and the three-phase windings of the drive motor M. The traction inverter is configured to convert DC power from the power battery 1 into AC power in traction mode and supply this AC power to the three-phase windings of the drive motor M to drive the vehicle. The traction inverter includes three arms: a first arm, a second arm, and a third arm. Each arm includes an upper arm switch and a lower arm switch connected in series, with the upper arm switch coupled to the positive bus and the lower arm switch coupled to the negative bus. Specifically, the first arm has upper arm switch Q1 and lower arm switch Q2, the second arm has upper arm switch Q3 and lower arm switch Q4, and the third arm has upper arm switch Q5 and lower arm switch Q6. The positive terminal of the power battery 1 is electrically connected to the end of the positive bus, and the negative terminal of the power battery 1 is electrically connected to the end of the negative bus.

[0026] The intermediate node of the connection between the upper and lower bridge arm switches of each bridge arm is coupled to the terminal of the corresponding phase winding of the drive motor M. Specifically, the intermediate node between the upper bridge arm switch Q1 and the lower bridge arm switch Q2 of the first bridge arm is coupled to the terminal of the first phase winding; the intermediate node between the upper bridge arm switch Q3 and the lower bridge arm switch Q4 of the second bridge arm is coupled to the terminal of the second phase winding; and the intermediate node between the upper bridge arm switch Q5 and the lower bridge arm switch Q6 of the third bridge arm is coupled to the terminal of the third phase winding.

[0027] Each of the bridge arm switches Q1 through Q6 can be implemented as a controllable power semiconductor switching device including a freewheeling diode. For example, each bridge arm switch can be implemented as a metal-oxide-semiconductor field-effect transistor (MOSFET). This transistor includes a parasitic diode with freewheeling capability. In another embodiment, each bridge arm switch can be implemented as including an insulated-gate bipolar transistor (IGBT) and a diode, wherein the cathode of the diode is connected to the collector of the transistor, and the anode is connected to the emitter of the transistor. The IGBT is a controllable switching device. The diode is an uncontrollable switching device and can provide freewheeling functionality.

[0028] When the vehicle is driven by power battery 1 supplying power to drive motor M in traction mode, the DC power output of power battery 1 can be converted into three-phase AC power and supplied to the three-phase windings of drive motor M by controlling the on / off state of each bridge arm switch Q1 to Q6.

[0029] In the buck-type on-board charging system of this application, the traction inverter, the three-phase winding of the motor, and the filter capacitor Ca are used to form the buck converter 5 in the third conversion stage S3 of the buck-type on-board charging system. In other words, the traction inverter and the three-phase winding of the motor are used not only for power supply from the power battery 1 to the drive motor M, but also for charging the power battery 1.

[0030] To enable the traction inverter to be reused for charging power battery 1, a filter capacitor Ca is connected in parallel with power battery 1 between the positive and negative busbars. A traction switch Sa is installed on the positive busbar. The traction switch Sa divides the positive busbar into an upstream segment connected to each upper arm of the traction inverter and a downstream segment connected to the positive terminal of power battery 1. A filter capacitor Ca (DC bus capacitor) is installed between the downstream segment of the positive busbar and the negative busbar. The two terminals of the filter capacitor Ca form and maintain electrical connections with the positive and negative terminals of the power battery, respectively, without any other electrical components between them. One phase winding (referred to as the input winding) of the three-phase winding of the drive motor M is connected to the downstream segment of the positive busbar. Figure 1 A charging switch Ka is installed between the terminals of the third phase winding (in the middle section) to control the connection between the positive bus and the input winding. The charging switch Ka can be connected between the positive bus and the input winding in any feasible wiring configuration. For example, the first terminal of the charging switch Ka can be directly connected to the downstream section of the positive bus, connected to the positive side of the filter capacitor Ca, connected to the line between the positive side of the filter capacitor Ca and the downstream section of the positive bus, etc. The second terminal of the charging switch Ka can be directly connected to the terminal of the input winding, connected to the bridge arm connected to the input winding (in the middle section). Figure 1 The middle node (of the third bridge arm) is connected to the line between the input winding and the connected bridge arm, etc. Figure 1 In the example shown, the first end of the charging switch Ka is connected to the line between the positive side of the filter capacitor Ca and the downstream segment of the positive bus, and the second end is connected to the middle node of the third bridge arm.

[0031] In the three-phase winding, one or both of the two phase windings other than the input winding constitute the output winding. The bridge arm connected to the output winding among the three bridge arms constitutes the voltage regulating bridge arm, used to reduce the output voltage of the DC-DC converter 4 and supply it to the two terminals of the power battery 1 during charging. Therefore, the fourth conversion stage S3 can contain only one voltage regulating bridge arm and one output winding, for example... Figure 1 The first bridge arm and the second winding, or a pair of parallel regulating bridge arms and a pair of parallel output windings, for example... Figure 1 The first bridge arm and first winding are connected to the second bridge arm and second winding. Based on different numbers of regulating bridge arms and output windings, the buck-type on-board charging system of this application can operate in different charging modes. As an example, when there is one regulating bridge arm and one output winding in the fourth conversion stage S3, the fourth conversion stage S3 can execute an exemplary first charging mode. When there is a pair of regulating bridge arms and a pair of output windings in the fourth conversion stage S3, the fourth conversion stage S3 can execute an exemplary second charging mode.

[0032] When the power battery 1 is in traction mode (i.e., supplying power to the drive motor M), the AC-DC converter 3 and DC-DC converter 4 are not working, the charging switch Ka is open, and the traction switch Sa is closed. This connects the traction inverter and the power battery 1 in the conventional manner, and disconnects the electrical connection between the traction inverter and the DC-DC converter 4. The switching of each arm switch Q1 to Q6 is controlled in the conventional manner, converting the DC output power of the power battery into AC power to supply the three-phase windings of the motor M. The filter capacitor Ca absorbs the fluctuations in the charging voltage during the charging process.

[0033] When the power battery 1 is in charging mode, AC-DC converter 3 and DC-DC converter 4 operate, and as follows: Figure 2 As shown, the charging switch Ka is turned on and the traction switch Sa is turned off, thereby establishing an electrical connection between the traction inverter and motor windings multiplexed in the third conversion stage S3 and the power battery 1. The upper and lower bridge arm switches in the bridge arm connected to the input winding remain off. In the two windings connected to the input winding, the upper and lower bridge arm switches in the bridge arm connected to a phase winding not used as an input winding (if any) (not constituting a voltage regulating bridge arm, if any) remain off, and the upper bridge arm switch in the voltage regulating bridge arm (one or two) connected to the winding used as the input winding (one or two phases) is controlled to periodically turn on and off, while the lower bridge arm switch is not controlled (i.e., off), or the lower bridge arm switch is controlled with a signal complementary to the upper bridge arm switch to periodically turn on and off in an on / off state opposite to (or complementary to) the upper bridge arm switch, thereby reducing the charging voltage output by the third conversion stage S3 to both sides of the power battery 1.

[0034] Taking the first charging mode as an example, such as Figure 2 As shown, in the third bridge arm connected to the input winding, the upper bridge arm switch Q5 and the lower bridge arm switch Q6 remain off, and in the second bridge arm connected to the second phase winding not used as the input winding, the upper bridge arm switch Q3 and the lower bridge arm switch Q4 remain off. In the second bridge arm connected to the first phase winding used as the input winding, the upper bridge arm switch Q1 is controlled to periodically turn on and off with a certain duty cycle and switching frequency, while the lower bridge arm switch is not controlled (i.e., off), or the lower bridge arm switch Q2 is controlled to periodically turn on and off in a manner synchronized with but opposite to the on / off state of the upper bridge arm switch Q1 (i.e., when the upper bridge arm switch Q1 is on, the lower bridge arm switch Q2 is off, and when the upper bridge arm switch Q1 is off, the lower bridge arm switch Q2 is on), as shown. Figure 3 As shown.

[0035] When the upper arm switch Q1 is on and the lower arm switch Q2 is off, the positive output terminal of the DC-DC converter 4 is connected to the positive side of the filter capacitor Ca and the power battery 1 via the upper arm switch Q1, the first phase winding, the third phase winding, and the on-state voltage regulating switch Ka of the first arm. The negative terminal of the power battery 1 on the positive side of the filter capacitor Ca is connected to the negative output terminal of the DC-DC converter 4 via the negative bus. When the upper arm switch Q1 is off and the lower arm switch Q2 is on, the negative output terminal of the DC-DC converter 4 is connected to the positive side of the filter capacitor Ca and the power battery 1 via the lower arm switch Q2, the first phase winding, the third phase winding, and the on-state voltage regulating switch Ka. By alternately switching the upper arm switch Q1 (and the lower arm switch Q2) on and off, the charging voltage applied to both sides of the power battery 1 by the third conversion stage S3 can be reduced.

[0036] The duty cycle of the upper arm switch Q1 is positively correlated with the charging voltage applied to both sides of the power battery 1 by the third conversion stage S3; that is, the larger the duty cycle of the upper arm switch Q1, the higher the charging voltage applied to both sides of the power battery 1 by the third conversion stage S3. Therefore, the duty cycle of the upper arm switch Q1 can be set or adjusted based on the desired charging voltage. Duty cycle adjustment can be achieved through closed-loop control.

[0037] In the second charging mode, the upper bridge arm switch Q5 and the lower bridge arm switch Q6 in the third bridge arm connected to the input winding remain off. In the first bridge arm connected to the first phase winding used as a phase input winding, the upper bridge arm switch Q1 is controlled to periodically turn on and off with a certain duty cycle and switching frequency, while the lower bridge arm switch Q2 is not controlled (off), or is controlled by a signal complementary to the upper bridge arm switch Q1 to periodically turn on and off in a manner synchronized with but opposite to the on / off state of the upper bridge arm switch Q1 (i.e., when the upper bridge arm switch Q1 is on, the lower bridge arm switch Q2 is off, and when the upper bridge arm switch Q1 is off, the lower bridge arm switch Q2 is on). In the second bridge arm connected to the second phase winding used as the input winding of another phase, the upper bridge arm switch Q3 is controlled to periodically turn on and off with a certain duty cycle and switching frequency, while the lower bridge arm switch Q4 is not controlled (off), or is controlled by a signal complementary to the upper bridge arm switch Q3 to periodically turn on and off in a manner synchronized with but opposite to the on / off state of the upper bridge arm switch Q3 (i.e., when the upper bridge arm switch Q3 is on, the lower bridge arm switch Q4 is off, and when the upper bridge arm switch Q3 is off, the lower bridge arm switch Q4 is on). Figure 4As shown. Preferably, in each switching cycle, the switching operations of the upper bridge arm switch Q3 and the lower bridge arm switch Q4 in the second bridge arm are not synchronized with the switching operations of the upper bridge arm switch Q1 and the lower bridge arm switch Q2 in the first bridge arm, but are staggered (delayed or advanced) by a deviation time Δt. The deviation time Δt can be set to a proportion (e.g., 20%, or 40%) or less of the conduction time of the upper bridge arm switches Q1 and Q3 in each switching cycle, so that the conduction time of the upper bridge arm switch Q1 coincides with the conduction time of the upper bridge arm switch Q3 to a certain extent, for example, set to mostly coincide (e.g., 60%, 80%, or more), and this deviation time Δt is adjustable.

[0038] When the upper arm switches Q1 and Q3 are on and the lower arm switches Q2 and Q4 are off, the positive output terminal of the DC-DC converter 4 is connected to the filter capacitor Ca and the positive side of the power battery 1 via the upper arm switches Q1 and Q3 of the first and second arms, the first and second phase windings, the third phase winding, and the voltage regulating switch Ka which is in the on state. When the upper arm switches Q1 and Q3 are off and the lower arm switches Q2 and Q4 are on, the negative output terminal of the DC-DC converter 4 is connected to the filter capacitor Ca and the positive side of the power battery 1 via the lower arm switches Q2 and Q4, the first and second phase windings, the third phase winding, and the voltage regulating switch Ka which is in the on state. By alternately switching the upper arm switches Q1 and Q3 (and the lower arm switches Q2 and Q4) on and off in this way, the charging voltage applied to both sides of the power battery 1 by the third conversion stage S3 can be reduced.

[0039] The duty cycles of the upper arm switches Q1 and Q3 are positively correlated with the charging voltage applied to both sides of the power battery 1 by the third conversion stage S3. That is, the higher the duty cycle of the upper arm switches Q1 and Q3, the higher the charging voltage applied to both sides of the power battery 1 by the third conversion stage S3. Therefore, the duty cycles of the upper arm switches Q1 and Q3 can be set or adjusted based on the desired charging voltage. Duty cycle adjustment can be achieved through closed-loop control.

[0040] Based on the principles of this application, those skilled in the art can make modifications to the buck on-board charging system described above. For example, in the illustrated example, the three-phase windings of the drive motor M are shown in a star connection; however, the three-phase windings of the drive motor M can also be connected in a delta connection, or the three-phase windings can be switched between star and delta connections (e.g., based on the speed switching of the drive motor).

[0041] For example, in the example described above, DC-DC converter 4 is preferably a fixed output voltage type DC-DC converter; however, the buck on-board charging system of this application may use other forms of DC-DC converters with approximately constant voltage output.

[0042] The buck-type on-board charging system according to this application has a three-stage topology: a first-stage AC-DC converter, a second-stage DC-DC converter, and a third-stage buck converter based on a traction inverter and motor windings. The third stage utilizes the motor inverter and windings to form a buck circuit, achieving voltage reduction during battery charging and matching the charging system voltage with the battery voltage. Furthermore, during battery charging, the second-stage DC-DC converter can have a substantially constant output voltage (preferably a fixed rated output voltage), enabling it to always operate at high efficiency, thus reducing heat generation and easing the pressure on the cooling system. Additionally, since the second-stage DC-DC converter has a fixed output voltage, its switching frequency can be essentially fixed, helping to reduce the size of magnetic components and EMC circuitry.

[0043] While some embodiments have been described above, these embodiments are given by way of example only and are not intended to limit the scope of this application. The appended claims and their equivalents are intended to cover all modifications, substitutions, and alterations made within the scope and spirit of this application.

Claims

1. A step-down on-board charging system for charging a power battery (1), wherein the power battery (1) supplies power to a drive motor (M) with three-phase windings via a traction inverter, the traction inverter comprising three arms, each arm having an upper arm switch coupled to the positive bus of the traction inverter and a lower arm switch coupled to the negative bus of the traction inverter, the intermediate node of the line connecting the upper arm switch and the lower arm switch in each arm being coupled to a corresponding phase winding of the drive motor (M); Its features are, The step-down on-board charging system includes three conversion stages connected in sequence. The first conversion stage includes an AC-DC converter (3), the second conversion stage includes a DC-DC converter (4), and the third conversion stage includes a step-down converter (5). The positive bus of the traction inverter is located between the positive output terminal of the DC-DC converter (4) and the positive terminal of the power battery (1), and the negative bus is located between the negative output terminal of the DC-DC converter (4) and the negative terminal of the power battery (1). The buck converter (5) includes: The input winding is composed of one phase winding of the three-phase winding; The output winding is composed of the three-phase windings excluding the input winding; A voltage regulating bridge arm, which is composed of a bridge arm coupled to the output winding; A traction switch (Sa) is disposed in the positive bus between the traction inverter and the positive terminal of the power battery (1) and divides the positive bus into an upstream segment and a downstream segment; A filter capacitor (Ca) is disposed between the downstream section of the positive busbar and the negative busbar; A charging switch (Ka) is disposed between the positive terminal of the power battery (1) and the input winding.

2. The step-down on-board charging system as described in claim 1, characterized in that, The first terminal of the charging switch (Ka) is connected to the downstream section of the positive bus, or to the positive side of the filter capacitor (Ca), or to the connection line between the positive side of the filter capacitor (Ca) and the downstream section of the positive bus.

3. The step-down on-board charging system as described in claim 1, characterized in that, The second end of the charging switch (Ka) is connected to the terminal of the input winding, or to the middle node of the bridge arm connected to the input winding, or to the line connecting the input winding and the connected bridge arm.

4. The step-down on-board charging system as described in claim 1, characterized in that, The DC-DC converter (4) is a fixed output voltage type DC-DC converter.

5. The step-down on-board charging system as described in claim 1, characterized in that, In the charging mode of the power battery (1), the charging switch (Ka) is in the ON state and the traction switch (Sa) is in the OFF state; in the traction mode of the power battery (1), the charging switch (Ka) is in the OFF state and the traction switch (Sa) is in the ON state.

6. The step-down on-board charging system as described in any one of claims 1-5, characterized in that, In the charging mode of the power battery (1), the upper bridge arm switch in the voltage regulating bridge arm is in a periodic on / off state, and the lower bridge arm switch is in a periodic on / off state with the opposite on / off state to the upper bridge arm switch.

7. The step-down on-board charging system as described in any one of claims 1-5, characterized in that, The output winding is a single-phase winding other than the input winding in the three-phase winding; the voltage regulating bridge arm includes a single bridge arm coupled to the single-phase winding.

8. The step-down on-board charging system as described in any one of claims 1-5, characterized in that, The output winding consists of two phases of the three-phase winding, excluding the input winding; the voltage regulating bridge arm includes two voltage regulating bridge arms that are respectively coupled to the two phase windings.

9. The step-down on-board charging system as described in claim 8, characterized in that, In the charging mode of the power battery (1), the upper bridge arm switch in each voltage regulating bridge arm is in a periodic on / off state, and the lower bridge arm switch is in a periodic on / off state with the opposite on / off state to the upper bridge arm switch. The on / off operation of the upper and lower bridge arm switches of one voltage regulating bridge arm is staggered by a certain deviation time relative to the on / off operation of the upper and lower bridge arm switches of the other voltage regulating bridge arm.

10. The step-down on-board charging system as described in claim 9, characterized in that, The deviation time is set such that the on-time of the upper arm switch in one voltage regulating arm has a set overlap ratio with the on-time of the upper arm switch in the other voltage regulating arm.

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