Undervoltage protection circuit and undervoltage protection method for DC / DC converters used in electric vehicles

Abstract

This application discloses an undervoltage protection circuit for a DC / DC converter used in electric vehicles. The DC / DC converter operates in three modes: a first pre-charge stage, a second pre-charge stage, and a buck mode. The undervoltage protection circuit includes: a threshold switching unit connected to a controller to receive control signals and switch between at least two preset undervoltage protection thresholds based on the control signals; and an undervoltage protection unit that outputs a signal indicating whether to activate undervoltage protection based on a comparison between the undervoltage protection threshold and a sampled voltage value on the high-voltage side of the DC / DC converter. Specifically, when operating in buck mode, if the sampled voltage value is less than or equal to the undervoltage protection threshold, the undervoltage protection unit activates undervoltage protection to suppress high current; if the sampled voltage value is greater than the undervoltage protection threshold, the DC / DC converter operates normally in buck mode. This application also discloses an undervoltage protection method for a DC / DC converter used in electric vehicles.

Description

Technical Field

[0001] The embodiments of this application generally relate to an undervoltage protection circuit and undervoltage protection method for a DC / DC converter for an electric vehicle. Background Technology

[0002] In the electric vehicle field, existing technologies apply 48V to 12V DC / DC converters to dual-power network vehicle architectures to control power exchange between a traditional 12V network (i.e., the low-voltage side) and a 48V network (i.e., the high-voltage side). Specifically, the high-voltage side of the DC / DC converter is connected to the iBSG (integrated belt starter generator) in the vehicle and via a relay to the 48V battery. The capacitors on the high-voltage side need to be charged to the 48V battery voltage using the DC / DC converter's pre-charge mechanism. Then, the electric vehicle closes the relay to connect the 48V battery to the high-voltage side. The 48V battery then powers the iBSG, which operates in starter mode to ignite the engine. The iBSG then operates in generator mode, driven by the internal combustion engine. Finally, the electric vehicle commands the DC / DC converter to operate in buck mode to achieve power exchange. The DC / DC converter's pre-charge mechanism can be divided into two stages, such as... Figure 1 As shown, the high-voltage side capacitor is charged to the low-voltage side voltage value in the first pre-charge phase, and then charged to the target voltage value, i.e., the high-voltage side voltage value, in the second pre-charge phase. Due to factors such as high-voltage side ground short circuits and engine ignition, a voltage difference (or voltage drop) may occur on the high-voltage side. Therefore, it is necessary to implement a high-voltage side undervoltage protection mechanism in the DC / DC converter. Typically, a threshold is set in the high-voltage side undervoltage protection mechanism, which is lower than the initial voltage value of the second pre-charge phase; for example, this threshold is set to 5.5 volts. Please refer to [reference needed]. Figure 1During the first pre-charge phase, the main power supply circuit is not operational. The high-voltage side capacitor is charged by the constant current source circuit, and the high-voltage side voltage increases from 0 volts to the 12-volt battery voltage. The undervoltage protection mechanism in the first pre-charge phase manifests as timeout protection; that is, if the undervoltage state lasts significantly longer than the normal time required for the first pre-charge phase, a timeout fault will be reported, and the first pre-charge phase will be stopped. Upon entering the second pre-charge phase, the main power supply circuit operates in low-power boost mode, and the high-voltage side capacitor is charged from the 12-volt battery voltage to the target voltage. If the high-voltage side voltage drops below the aforementioned threshold, the undervoltage protection will be triggered immediately, and the DC / DC converter will stop the second pre-charge phase and turn off the safety metal-oxide-semiconductor field-effect transistors (MOSFETs). Upon entering the high-power buck mode, if the high-voltage side voltage drops below the aforementioned threshold, the undervoltage protection will be triggered immediately, and the DC / DC converter will stop the buck mode and turn off the safety MOSFETs. However, when the DC / DC converter operates in high-power buck mode, if the high-voltage side voltage drops below the 12-volt battery voltage but exceeds the threshold set by the undervoltage protection mechanism, the undervoltage protection will not be triggered, and the safety MOSFET will remain on. In this case, an uncontrolled large current will flow from the low-voltage side to the high-voltage side and through the safety MOSFET, such as... Figure 2 As shown, this current could damage the main power supply circuit and the safety MOSFET. However, if the threshold for this undervoltage protection mechanism is raised to a value greater than the 12-volt battery voltage, the undervoltage protection will be triggered in the early stages of the second precharge phase, preventing the second precharge phase from being completed.

[0003] Therefore, there is an urgent need to improve the undervoltage protection mechanism for 48-volt to 12-volt DC / DC converters used in electric vehicles, at least with high efficiency, high safety and simple structure. Summary of the Invention

[0004] According to one or more specific embodiments disclosed in this invention, in one exemplary aspect, an undervoltage protection circuit for a DC / DC converter in an electric vehicle is provided. The DC / DC converter operates in a first pre-charge phase, a second pre-charge phase, and a buck mode. The undervoltage protection circuit includes: a threshold switching unit connected to a controller for receiving control signals from the controller and switching at least two preset undervoltage protection thresholds according to the received control signals; and an undervoltage protection unit for outputting a signal indicating whether to activate undervoltage protection based on a comparison between the switched undervoltage protection threshold and a sampled voltage value on the high-voltage side of the DC / DC converter. When the DC / DC converter operates in buck mode, if the sampled voltage value is less than or equal to the switched undervoltage protection threshold, the undervoltage protection unit activates undervoltage protection to suppress large current flow from the low-voltage side to the high-voltage side of the DC / DC converter; if the sampled voltage value is greater than the switched undervoltage protection threshold, the DC / DC converter operates normally in buck mode.

[0005] In some embodiments, when the DC / DC converter starts operating in the first pre-charge phase, the undervoltage status signal output by the undervoltage protection circuit will serve as the trigger signal for the timeout fault timer to start counting.

[0006] In some embodiments, when the DC / DC converter is operating in the second pre-charge phase, if the sampled voltage value is less than or equal to the switched undervoltage protection threshold, the undervoltage protection unit will activate undervoltage protection; if the sampled voltage value is greater than the switched undervoltage protection threshold, the DC / DC converter will operate normally in the second pre-charge phase.

[0007] In some embodiments, the at least two preset undervoltage protection thresholds are respectively set in the first pre-charge stage, the second pre-charge stage, and / or the buck mode. When the undervoltage protection is activated, the second pre-charge stage will be terminated, or the buck mode will be terminated.

[0008] In some embodiments, the at least two preset undervoltage protection thresholds include: a first undervoltage protection threshold for the first pre-charge phase and the second pre-charge phase; and a second undervoltage protection threshold for the buck mode; when the DC / DC converter operates in the first pre-charge phase and the second pre-charge phase, the threshold switching unit switches to the first undervoltage protection threshold; when the DC / DC converter operates in the buck mode, the threshold switching unit switches to the second undervoltage protection threshold.

[0009] In some embodiments, the threshold switching unit includes an electrical switch that generates a first undervoltage protection threshold for the first pre-charge phase and the second pre-charge phase when the electrical switch is turned on, and generates a second undervoltage protection threshold for the buck mode when the electrical switch is turned off.

[0010] In some embodiments, the control signal received by the threshold switching unit from the controller includes a high level and a low level. When the control signal is low, the electrical switch will be open; when the control signal is high, the electrical switch will be open.

[0011] In some embodiments, the undervoltage protection unit includes a comparator for receiving and comparing the sampled voltage value of the high-voltage side with the switched undervoltage protection threshold, and outputting a high level or a low level according to the comparison result to indicate whether to activate undervoltage protection. When the output is low, it indicates that undervoltage protection is activated.

[0012] In another exemplary aspect, the present invention provides an undervoltage protection method for a DC / DC converter for an electric vehicle, the DC / DC converter operating in a mode including a first pre-charge stage, a second pre-charge stage, and a buck mode. The undervoltage protection method includes: providing a threshold switching unit for receiving a control signal from a controller and switching at least two preset undervoltage protection thresholds according to the received control signal; and providing an undervoltage protection unit for outputting a signal indicating whether to activate undervoltage protection based on a comparison between the switched undervoltage protection threshold and a sampled voltage value on the high-voltage side of the DC / DC converter; wherein, when the DC / DC converter operates in buck mode, if the sampled voltage value is less than or equal to the switched undervoltage protection threshold, undervoltage protection is activated to suppress large current flow from the low-voltage side to the high-voltage side of the DC / DC converter; if the sampled voltage value is greater than the switched undervoltage protection threshold, the high-voltage side of the DC / DC converter operates normally in buck mode.

[0013] In some embodiments, the undervoltage protection method further includes: when the DC / DC converter starts operating in the first pre-charge phase, the undervoltage state signal output by the undervoltage protection circuit will serve as a trigger signal for starting a timeout fault timer; if the undervoltage state duration is significantly longer than the normal required time of the first pre-charge phase, a timeout fault will be reported and the first pre-charge phase will be terminated; when the DC / DC converter operates in the second pre-charge phase, when the sampled voltage value is less than or equal to the switched undervoltage protection threshold, the undervoltage protection unit will output a signal to indicate the activation of undervoltage protection; when the sampled voltage value is greater than the switched undervoltage protection threshold, the DC / DC converter will operate normally in the second pre-charge phase.

[0014] In some embodiments, the undervoltage protection method further includes: setting the at least two preset undervoltage protection thresholds respectively in the first pre-charge stage, the second pre-charge stage, and / or the buck mode; when the undervoltage protection is activated, the second pre-charge stage will be terminated, or the buck mode will be terminated.

[0015] In some embodiments, when the DC / DC converter operates in the first pre-charge phase and the second pre-charge phase, the threshold switching unit will switch to the first undervoltage protection threshold; when the DC / DC converter operates in the buck mode, the threshold switching unit will switch to the second undervoltage protection threshold.

[0016] In some embodiments, an electrical switch is provided in the threshold switching unit, wherein when the electrical switch is turned on, a first undervoltage protection threshold is generated for the first precharge phase and the second precharge phase; and when the electrical switch is turned off, a second undervoltage protection threshold is generated for the buck mode.

[0017] In some embodiments, when the control signal received by the threshold switching unit from the controller is low, the electrical switch will be turned off; when the control signal is high, the electrical switch will be turned on.

[0018] In some embodiments, a comparator is provided in the undervoltage protection unit for receiving and comparing the sampled voltage value of the high-voltage side with the switched undervoltage protection threshold, and outputting a high level or a low level according to the comparison result to indicate whether to activate undervoltage protection. When the output is low, it indicates that undervoltage protection is activated.

[0019] These and other features, aspects, and advantages of this application will become more readily understood with reference to the following description. The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the application and, together with the description, serve to explain the principles of the application. Attached Figure Description

[0020] This specification sets forth the complete and illustrative disclosure of this application, including its best practices, to those skilled in the art. Reference is made to the accompanying drawings, in which:

[0021] Figure 1 This is a schematic diagram of the operating status of a 48V-12V DC / DC converter with undervoltage protection for electric vehicles in the prior art.

[0022] Figure 2This is a schematic diagram of a prior art DC / DC converter with undervoltage protection generating a large current flowing from the low-voltage side of the DC / DC converter to the high-voltage side of the DC / DC converter when the high-voltage side voltage is undervoltage in buck mode.

[0023] Figure 3 This is a schematic diagram of the working state according to an exemplary embodiment of this application;

[0024] Figure 4 This is a schematic diagram of an undervoltage protection circuit according to an exemplary embodiment of this application;

[0025] Figure 5 This is a schematic diagram illustrating how a DC / DC converter, according to an exemplary embodiment of this application, does not generate a large current flowing from the low-voltage side to the high-voltage side of the DC / DC converter under undervoltage protection conditions; and

[0026] Figure 6 This is a schematic flowchart of an undervoltage protection method according to an exemplary embodiment of this application. Detailed Implementation

[0027] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the following detailed description of these specific embodiments, some well-known functions or structures are not described in detail to avoid unnecessary detail that would affect the disclosure of the invention. Each embodiment is provided to explain the present application and not to limit it. In fact, those skilled in the art will understand that various modifications and variations can be made to this application without departing from the scope or spirit of the present application. For example, a feature described or illustrated as part of one embodiment may be used with another embodiment to produce yet another embodiment. Therefore, it is intended that this application cover such modifications and variations, which are within the scope of the appended claims and their equivalents.

[0028] As used in this specification, the terms "first," "second," and similar words are used interchangeably to distinguish one element from another and are not intended to indicate the location or importance of the elements. As used in this specification, unless the context clearly indicates otherwise, the terms "a," "an," "the," and "the" are intended to indicate the presence of one or more elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be other elements besides those listed. The terms "connected" or "linked," and similar words, are not limited to physical or mechanical connections but can include electrical connections, whether direct or indirect.

[0029] Furthermore, as used herein, the term "real-time" refers to at least one of the following: the time when the associated event occurs, the time when predetermined data is measured and collected, the time when data is processed, and the time when the system responds to the event and the environment. In the embodiments described herein, these times occur substantially instantaneously.

[0030] Referring now to the accompanying drawings, in which the same numbers throughout the drawings denote the same elements. Figures 3 to 6 The following illustrations depict the undervoltage protection types, circuitry, and methods of a DC / DC converter in various operating phases according to exemplary embodiments of this application. Specifically, the DC / DC converter operates in a first pre-charge phase, a second pre-charge phase, and a buck mode. Figure 3 In the illustrated embodiment, a first undervoltage protection threshold is provided in both the first and second pre-charge phases. This threshold voltage is set below 6.0 volts and as close to 6.0 volts as possible, for example, 5.5 volts. In buck mode, a second undervoltage protection threshold is provided, set below 24.0 volts and as close to 24.0 volts as possible, for example, 23.0 volts. In this illustrated embodiment, when the DC / DC converter operates in the first pre-charge phase (i.e., the high-voltage side voltage is charged from 0 volts to 12 volts), the undervoltage protection mechanism manifests as a timeout protection. That is, if the undervoltage state lasts significantly longer than the normal time required for the first pre-charge phase, a timeout fault is reported and the first pre-charge phase is stopped. After entering the second pre-charge phase, the high-voltage side will be charged from the 12V battery voltage to the target voltage (e.g., 48V). At this time, the undervoltage protection mechanism will operate in real-time at the preset first undervoltage protection threshold. If the high-voltage side voltage drops below the first undervoltage protection threshold, i.e., below 5.5V, the undervoltage protection will be triggered immediately, and the DC / DC converter will stop the second pre-charge phase and turn off the safety MOSFET. After entering the high-power buck mode, the high-voltage side voltage will be maintained at the target voltage (e.g., 48V). At this time, the undervoltage protection mechanism will operate in real-time at the preset second undervoltage protection threshold. If the high-voltage side voltage drops below the second undervoltage protection threshold, i.e., below 23.0V, the undervoltage protection will be triggered immediately, and the DC / DC converter will stop the buck mode and turn off the safety MOSFET. Please refer to the reference... Figure 5When the DC / DC converter circuit 300 is under undervoltage protection, it will switch to idle mode, so that no current will flow from the low-voltage side to the high-voltage side. This suppresses the possible large current flowing from the low-voltage side of the DC / DC converter to the high-voltage side of the DC / DC converter, which could damage the main power supply circuit and the safety MOSFET. At the same time, it can also ensure that the second pre-charge stage is not excessively interfered with by the undervoltage protection mechanism. Furthermore, the setting of a higher undervoltage protection threshold in buck mode is conducive to more sensitive and faster execution of undervoltage protection, thereby more effectively protecting the DC / DC converter.

[0031] It is understandable that during the pre-charge phase, when the target voltage value is reached, the DC / DC converter may not immediately switch from the second pre-charge phase to buck mode. That is, there is a possibility that the target voltage value may be maintained for a period of time before switching to buck mode. In this possible case, the undervoltage protection for the second pre-charge phase will always remain within the range of the first undervoltage protection threshold.

[0032] It is understood that at least two preset undervoltage protection thresholds can be set, and these at least two undervoltage protection thresholds can be distributed in the first pre-charge stage, the second pre-charge stage, and the buck mode of the DC / DC converter.

[0033] Please refer to Figure 4 An exemplary embodiment of the undervoltage protection circuit includes a threshold switching unit 100 and an undervoltage protection unit 200. The threshold switching unit 100 includes an input terminal 110, which receives a control signal to switch between at least two preset undervoltage protection thresholds. In some embodiments, the control signal may come from a controller. The controller may be any type of programmable device, such as a controller, controller unit (MCU), digital signal processor (DSP), etc. In an exemplary embodiment, the input terminal 110 may be connected to a controller (not shown) to receive a control signal to switch between a first undervoltage protection threshold and a second undervoltage protection threshold. When the DC / DC converter operates in a first pre-charge phase, the input terminal 110 receives the control signal to cause the threshold switching unit 100 to switch to the first undervoltage protection threshold; when the DC / DC converter operates in buck mode, the input terminal 100 receives the control signal to cause the threshold switching unit 100 to switch to the second undervoltage protection threshold.

[0034] In some embodiments, the threshold switching unit 100 includes an electrical switch Q1. The electrical switch Q1 includes, but is not limited to, a MOSFET, an insulated gate bipolar transistor (IGBT), an integrated gate commutated thyristor (IGCT), an injection enhanced gate transistor (IEGT), a silicon carbide MOSFET, or other controllable electrical switches that can switch between on and off states. In the illustrated embodiment, the electrical switch Q1 is a MOSFET connected to input terminal 110 to receive control signals from the controller transmitted through input terminal 110. The output terminals 120 and 130 of the electrical switch Q1 are grounded and connected to the second voltage divider resistor R2, respectively. In the illustrated embodiment, the threshold switching unit 100 also includes a reference voltage terminal Uref, whose voltage value is set to the voltage value of a first undervoltage protection threshold, for example, 5.5 volts; and a ground terminal whose voltage value is set to the voltage value of a second undervoltage protection threshold, for example, 23.0 volts. Please refer to [reference needed]. Figure 4 The threshold switching unit 100 also includes a third voltage divider resistor R3 and a fourth voltage divider resistor R4. The third voltage divider resistor R3 is connected to the reference voltage terminal Uref, and the fourth voltage divider resistor R4 is grounded. The third voltage divider resistor R3, the fourth voltage divider resistor R4 and the second voltage divider resistor R2 together form a voltage divider circuit to determine the undervoltage protection threshold.

[0035] In some embodiments, the control signal received by the input terminal 110 of the threshold switching unit 100 from the controller includes a high level and a low level. When the control signal is high, the electrical switch Q1 will be turned on, and the second voltage divider resistor R2 and the fourth voltage divider resistor R4 will be connected in parallel. The voltage value output by the threshold switching unit 100 is the voltage value of the first undervoltage protection threshold, for example, 5.5 volts. When the control signal is low, the electrical switch Q1 will be turned off; the voltage value output by the threshold switching unit 100 is the voltage value of the second undervoltage protection threshold, for example, 23.0 volts.

[0036] Please continue to refer to this. Figure 4 The undervoltage protection circuit also includes an undervoltage protection unit 200. In the illustrated embodiment, the undervoltage protection unit 200 includes a comparator U1. One input terminal 210 of the comparator U1 is connected to the sampled voltage value UHV on the high-voltage side, and the other input terminal 220 of the comparator U1 is connected to the voltage value output by the threshold switching unit 100, that is, the switched undervoltage protection threshold, either the first undervoltage protection threshold or the second undervoltage protection threshold.

[0037] Comparator U1 receives and compares the sampled voltage value UHV from the high-voltage side with the switched undervoltage protection threshold, and outputs a high or low level at its output terminal 230 to indicate whether undervoltage protection should be activated based on the comparison result. When the sampled voltage value UHV is less than or equal to the switched undervoltage protection threshold, the undervoltage protection unit 200 will output a signal, such as a low level, to indicate that undervoltage protection should be activated; when the sampled voltage value UHV is greater than the switched undervoltage protection threshold, the undervoltage protection unit 200 will also output a signal, such as a high level, and the DC / DC converter will operate normally in the second pre-charge stage or buck mode.

[0038] Specifically, when the DC / DC converter starts operating in the first pre-charge phase, the undervoltage state signal output by the undervoltage protection circuit will serve as the trigger signal for the timeout fault timer to start counting. That is, if the undervoltage state time is significantly longer than the normal required time for the first pre-charge phase, a timeout fault will be reported and the first pre-charge phase will be stopped.

[0039] When the DC / DC converter operates in the second pre-charge phase, the threshold switching unit 100 will still output the first undervoltage protection threshold, for example, 5.5 volts, to one input terminal 210 of the comparator U1 of the undervoltage protection unit 200. At this time, the other input terminal 220 of the comparator U1 receives the sampled voltage value UHV from the high-voltage side in real time. When the sampled voltage value UHV is less than or equal to the first undervoltage protection threshold, the DC / DC converter will stop operating in the second pre-charge phase. When the sampled voltage value UHV is greater than the first undervoltage protection threshold, the DC / DC converter will operate normally in the second pre-charge phase.

[0040] When the DC / DC converter operates in buck mode, the threshold switching unit 100 outputs a second undervoltage protection threshold, such as 23.0 volts, to one input terminal 210 of the comparator U1 in the undervoltage protection unit 200. Meanwhile, the other input terminal 220 of the comparator U1 receives the sampled voltage value UHV from the high-voltage side in real time. When the sampled voltage value UHV is less than or equal to the second undervoltage protection threshold, the DC / DC converter will stop operating in buck mode. When the sampled voltage value UHV is greater than the second undervoltage protection threshold, the DC / DC converter will operate normally in buck mode. The fifth and sixth voltage divider resistors R5 and R6 divide the sampled voltage value UHV into a relatively low voltage according to a certain ratio for comparison with the voltage output from the threshold switching unit 100, i.e., the voltage at one input terminal 210 of the comparator U1.

[0041] Please continue to refer to this. Figure 4The undervoltage protection unit 200 also includes a resistor R7 to provide hysteresis and enable the protection circuit to have Schmitt characteristics. The resistor R7 is connected to another input terminal 220 and output terminal 230 of the comparator U1 to prevent the undervoltage protection unit 200 circuit from oscillating when the sampled voltage value UHV is near the undervoltage protection threshold, thereby improving the stability of the undervoltage protection unit 200 circuit.

[0042] like Figure 6 As shown, the exemplary undervoltage protection method 400 is applied to Figures 3 to 5 The undervoltage protection mechanism is shown. An exemplary undervoltage protection method 400 includes the following steps:

[0043] Steps 41 and 42: Provide a controller to output corresponding control signals according to different operating modes of the DC / DC converter; and provide a threshold switching unit to receive the control signals, switching to at least one first undervoltage protection threshold when the DC / DC converter is operating in the first pre-charge stage and the second pre-charge stage, and switching to at least one second undervoltage protection threshold when the DC / DC converter is operating in buck mode.

[0044] Steps 431 and 432: Provide an undervoltage protection unit. When the DC / DC converter starts working in the first pre-charge phase, the undervoltage status signal output by the undervoltage protection unit will serve as the trigger signal for the timeout fault timer to start counting. When the DC / DC converter is working in the second pre-charge phase and buck mode, the undervoltage protection unit will compare the magnitude of the received high-voltage side voltage value and the switched undervoltage protection threshold.

[0045] Step 44: When the sampled voltage value is less than or equal to the switched undervoltage protection threshold, the undervoltage protection will be activated to suppress a possible large current from the low-voltage side of the DC / DC converter to the high-voltage side of the DC / DC converter.

[0046] Step 45: When the sampled voltage value is greater than the switched undervoltage protection threshold, the high-voltage side of the DC / DC converter will operate normally in the second pre-charge stage or buck mode.

[0047] This specification uses examples to disclose this application, including preferred embodiments, and also enables those skilled in the art to practice this application, including making and using any apparatus or system and performing any incorporated methods. The patentable scope of this application is defined by the claims and may include other embodiments conceived by those skilled in the art. Such other embodiments are deemed to be within the scope of the claims if they include structural elements that are not different from the literal language of the claims, or if they include equivalent structural elements that are not substantially different from the literal language of the claims.

Claims

1. An undervoltage protection circuit for a DC / DC converter used in an electric vehicle, the DC / DC converter operating modes including a first pre-charge stage, a second pre-charge stage, and a buck mode, the undervoltage protection circuit comprising: Threshold switching unit, which is connected to the controller to receive control signals from the controller and switch at least two preset undervoltage protection thresholds according to the received control signals to generate a switched undervoltage protection threshold; and An undervoltage protection unit is used to output a signal to indicate whether undervoltage protection is activated based on a comparison between the switched undervoltage protection threshold and the sampled voltage value on the high-voltage side of the DC / DC converter. in, The at least two preset undervoltage protection thresholds include: a first undervoltage protection threshold for the first precharge phase and the second precharge phase; and a second undervoltage protection threshold for the buck mode, wherein the second undervoltage protection threshold is greater than the first undervoltage protection threshold and less than the target voltage value of the DC / DC converter; When the DC / DC converter operates in buck mode, if the sampled voltage value is less than or equal to the switched undervoltage protection threshold, the undervoltage protection unit will activate undervoltage protection to suppress large current flow from the low-voltage side to the high-voltage side of the DC / DC converter; if the sampled voltage value is greater than the switched undervoltage protection threshold, the DC / DC converter will operate normally in buck mode, and When the DC / DC converter operates in the first pre-charge stage and the second pre-charge stage, the threshold switching unit will switch to the first undervoltage protection threshold; when the DC / DC converter operates in the buck mode, the threshold switching unit will switch to the second undervoltage protection threshold.

2. The undervoltage protection circuit according to claim 1, characterized in that: When the DC / DC converter starts working in the first pre-charge phase, the undervoltage status signal output by the undervoltage protection circuit will serve as the trigger signal for the timeout fault timer to start counting.

3. The undervoltage protection circuit according to claim 1, characterized in that: When the DC / DC converter is operating in the second pre-charge phase, if the sampled voltage value is less than or equal to the switched undervoltage protection threshold, the undervoltage protection unit will activate undervoltage protection; if the sampled voltage value is greater than the switched undervoltage protection threshold, the DC / DC converter will operate normally in the second pre-charge phase.

4. The undervoltage protection circuit according to claim 1, characterized in that: When the undervoltage protection is activated, the second pre-charge phase will be terminated, or the buck mode will be terminated.

5. The undervoltage protection circuit according to claim 1, characterized in that: The threshold switching unit includes an electrical switch. When the electrical switch is turned on, a first undervoltage protection threshold is generated; when the electrical switch is turned off, a second undervoltage protection threshold is generated.

6. The undervoltage protection circuit according to claim 5, characterized in that: The control signals received by the threshold switching unit from the controller include high level and low level. When the control signal is low level, the electrical switch will be open; when the control signal is high level, the electrical switch will be open.

7. The undervoltage protection circuit according to claim 1, characterized in that: The undervoltage protection unit includes a comparator for receiving and comparing the sampled voltage value of the high-voltage side with the switched undervoltage protection threshold, and outputting a high level or a low level according to the comparison result to indicate whether to activate undervoltage protection. When the output is low, it indicates that undervoltage protection is activated.

8. An undervoltage protection method for a DC / DC converter used in an electric vehicle, wherein the DC / DC converter operates in a first pre-charge phase, a second pre-charge phase, and a buck mode, and the undervoltage protection method includes: A threshold switching unit is provided to receive control signals from the controller and switch at least two preset undervoltage protection thresholds according to the received control signals to generate a switched undervoltage protection threshold. and An undervoltage protection unit is provided to output a signal to indicate whether undervoltage protection is activated based on a comparison between the switched undervoltage protection threshold and the sampled voltage value on the high-voltage side of the DC / DC converter. in, The at least two preset undervoltage protection thresholds include: a first undervoltage protection threshold for the first precharge phase and the second precharge phase; and a second undervoltage protection threshold for the buck mode, wherein the second undervoltage protection threshold is greater than the first undervoltage protection threshold and less than the target voltage value of the DC / DC converter; When the DC / DC converter operates in buck mode, if the sampled voltage value is less than or equal to the switched undervoltage protection threshold, undervoltage protection will be activated to suppress large current flow from the low-voltage side to the high-voltage side of the DC / DC converter. If the sampled voltage value is greater than the switched undervoltage protection threshold, the high-voltage side of the DC / DC converter will operate normally in buck mode. When the DC / DC converter operates in the first pre-charge stage and the second pre-charge stage, the threshold switching unit will switch to the first undervoltage protection threshold; when the DC / DC converter operates in the buck mode, the threshold switching unit will switch to the second undervoltage protection threshold.

9. The method according to claim 8, characterized in that, Also includes: When the DC / DC converter starts working in the first pre-charge phase, the undervoltage state signal output by the undervoltage protection circuit will serve as the trigger signal for the start of the timeout fault timer. If the undervoltage state time is significantly longer than the normal time required for the first pre-charge phase, a timeout fault will be reported and the first pre-charge phase will be terminated. When the DC / DC converter is operating in the second pre-charge phase, if the sampled voltage value is less than or equal to the switched undervoltage protection threshold, the undervoltage protection unit will output a signal to indicate the activation of undervoltage protection; if the sampled voltage value is greater than the switched undervoltage protection threshold, the DC / DC converter will operate normally in the second pre-charge phase.

10. The method according to claim 8, characterized in that, Also includes: When the undervoltage protection is activated, the second pre-charge phase or buck mode will be terminated.

11. The method according to claim 8, characterized in that: An electrical switch is provided in the threshold switching unit. When the electrical switch is turned on, a first undervoltage protection threshold is generated; when the electrical switch is turned off, a second undervoltage protection threshold is generated.

12. The method according to claim 11, characterized in that: When the control signal received by the threshold switching unit from the controller is low, the electrical switch will be open; when the control signal is high, the electrical switch will be open.

13. The method according to claim 8, characterized in that: The undervoltage protection unit provides a comparator for receiving and comparing the sampled voltage value of the high-voltage side with the switched undervoltage protection threshold, and outputting a high level or a low level according to the comparison result to indicate whether to activate undervoltage protection. When the output is low, it indicates that undervoltage protection is activated.

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