CN115520019A - Method and system for high-voltage power down of electric automobile in limping mode - Google Patents

Abstract

This disclosure relates to the technical field of electric vehicles, and proposes a method and system for powering off electric vehicles under high voltage in limp mode, and controls the power-off function that is crucial to the safe operation of electric vehicles through a dedicated limp mode circuit, avoiding the use of expensive MCUs , which simplifies the difficulty of software design; the circuit has strong adaptability and can be used for limp mode shutdown control of other key relays or contactors; the use of analog circuits and delay control leaves enough room for users to confirm power-off operations time, avoiding the potential safety hazard of power loss caused by false triggering and power-off.

Description

Method and system for powering off high voltage of electric vehicle in limp mode

technical field

The present disclosure relates to the technical field related to electric vehicles, and in particular, relates to a method and system for powering off high voltage of an electric vehicle in a limp mode.

Background technique

The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.

In electric vehicles, the vehicle controller (VCU) generally controls the on-off of the main positive relay and the main negative relay of the high-voltage circuit to realize the power on and off of the high-voltage electrical system. When the main controller chip (MCU) of the VCU is running normally, it controls the high-voltage circuit relay according to the established logic. Big hidden danger.

The industry generally adopts the method of designing redundant circuits to solve the problem of MCU failure, that is, using two MCUs, and when one of the MCUs fails, the other MCU takes over the key control tasks. However, this method is not only costly, but also increases the difficulty of software design. Aiming at the specific problem that the failure of the MCU leads to the inability to turn off the key relay, there is a second solution, that is, to use the watchdog monitoring chip to monitor the running status of the MCU, and design a special relay shutdown circuit. The existing car limp home control circuit uses the system basis chip SBC with watchdog monitoring function to monitor the operation of the MCU. When the MCU cannot feed the dog, the SBC outputs an effective low-level "limp enable signal" to enable The limp home control circuit controls two limp output signals through two input signals.

The inventors found that the existing car limp home control method has the following disadvantages: first, the input signal of the limp home control circuit is a digital signal, which is easily disturbed under harsh working conditions and complex electromagnetic environment of the car. Second, the limp home control logic is too simple, and the limp output is triggered when the digital input signal is valid, which can easily lead to misoperation. In the case of MCU failure, if the high-voltage power-off operation is triggered by mistake, it will directly cause the electric vehicle to lose power, which will bring great safety hazards.

Contents of the invention

In order to solve the above problems, the present disclosure proposes a method and system for powering off electric vehicles under high voltage in limp mode. The power-off function that is crucial to the safe operation of electric vehicles is controlled by a dedicated limp mode circuit, which avoids the use of expensive MCUs. It simplifies the difficulty of software design; the circuit has strong adaptability and can be used for limp mode shutdown control of other key relays or contactors; it adopts the form of analog circuit and delay control, leaving enough time for the user to confirm the power-off operation Time, avoiding the potential safety hazard of power loss caused by false triggering and power off.

In order to achieve the above purpose, the present disclosure adopts the following technical solutions:

One or more embodiments provide a high-voltage power-off method for an electric vehicle in a limp mode, including the following steps:

Monitor the running status of the main control chip MCU of the vehicle controller and the status of the vehicle start-stop switch;

When the MCU is running normally, according to the state of the vehicle start-stop switch, the on-off of the controlled components is controlled through the vehicle MCU and the normal mode high-voltage relay control circuit;

When the MCU fails, the control of the high-voltage circuit relay by the MCU is disconnected through the limp mode pin of the SBC. After the time set by the shutdown signal of the stop switch, the controlled component is turned off.

One or more embodiments provide a high-voltage power-off system for electric vehicles in limp mode, including a start-stop switch tri-state detection circuit I, a high-voltage power-off confirmation circuit II in limp mode, a high-voltage relay control circuit III in normal mode, and a high-voltage power-off circuit in limp mode. Relay control circuit IV and internal power supply enabling control circuit V;

The start-stop switch tri-state detection circuit Ⅰ is used to detect the working state of the start-stop switch;

When the MCU is running normally, the vehicle internal power supply enabling control circuit Ⅴ enables the power supply of the normal mode high-voltage relay control circuit Ⅲ, disables the power supply of the high-voltage power-off confirmation circuit Ⅱ in the limp mode, and the high voltage is controlled by the MCU and the normal mode high-voltage relay control circuit Ⅲ On and off of the main positive relay;

When the MCU fails, the internal power supply enabling control circuit Ⅴ enables the power supply of the high-voltage power-off confirmation circuit II in the limp mode, and disables the power supply of the high-voltage relay control circuit III in the normal mode, and the high-voltage power-off confirmation circuit II in the limp mode and the high-voltage relay in the limp mode The control circuit IV controls the shut-off of the high-voltage main positive relay, so that the high-voltage main positive relay is turned off after the time set by the start-stop switch is pressed.

Compared with the prior art, the beneficial effects of the present disclosure are:

In the present disclosure, by adopting the limp mode high-voltage power-off confirmation circuit and the limp-mode high-voltage relay control circuit, the power-off function that is crucial to the safe operation of electric vehicles avoids the use of expensive MCUs and simplifies the difficulty of software design. The use of delay control allows enough time for the user to confirm the power-off operation, avoiding the potential safety hazard of power loss caused by false triggering of power-off.

Advantages of the present disclosure, as well as advantages of additional aspects, will be described in detail in the following specific examples.

Description of drawings

The accompanying drawings constituting a part of the present disclosure are used to provide further understanding of the present disclosure, and the exemplary embodiments and descriptions of the present disclosure are used to explain the present disclosure, but not to limit the present disclosure.

FIG. 1 is a flowchart of an overall method of an embodiment of the present disclosure;

FIG. 2 is a three-state detection circuit diagram of a start-stop switch according to an embodiment of the present disclosure;

FIG. 3 is a circuit diagram of a high-voltage power-off confirmation circuit in a limp mode according to an embodiment of the present disclosure;

FIG. 4 is a variation curve of the C2 terminal voltage _Uc in FIG. 3 under the first setting parameter of the embodiment of the present disclosure;

Fig. 5 is the unit step response curve of the resistance-capacitance network transfer function in Fig. 3 under the setting parameters of the embodiment of the present disclosure

6 is a control circuit diagram of a high-voltage loop relay according to an embodiment of the present disclosure;

FIG. 7 is an internal power supply enabling control circuit of an embodiment of the present disclosure.

detailed description

The present disclosure will be further described below in conjunction with the accompanying drawings and embodiments.

It should be noted that the following detailed description is exemplary and intended to provide further explanation of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

It should be noted that the terminology used herein is only for describing specific embodiments, and is not intended to limit the exemplary embodiments according to the present disclosure. As used herein, unless the context clearly dictates otherwise, the singular is intended to include the plural, and it should also be understood that when the terms "comprising" and/or "comprising" are used in this specification, they mean There are features, steps, operations, means, components and/or combinations thereof. It should be noted that, in the case of no conflict, various embodiments and features in the embodiments in the present disclosure can be combined with each other. The embodiments will be described in detail below in conjunction with the accompanying drawings.

Example 1

In the technical solution disclosed in one or more embodiments, as shown in FIG. 1 , a method for powering off the high voltage of an electric vehicle in a limp mode includes the following steps:

Step 1. Monitor the running state of the main control chip MCU of the vehicle controller and the state of the vehicle start-stop switch;

Step 2. When the MCU is running normally, according to the state of the start-stop switch of the vehicle, the on-off of the controlled components is controlled through the vehicle MCU and the normal mode high-voltage relay control circuit;

Step 3. When the MCU fails, disconnect the MCU from the control of the high-voltage loop relay through the limp mode pin of the SBC, and the high-voltage main positive relay is controlled by the limp-mode high-voltage power-off confirmation circuit and the limp-mode high-voltage relay control circuit to turn off the high-voltage main positive relay. After the time set by the shutdown signal of the start-stop switch is received, the controlled component is turned off.

Among them, the controlled component can be the high-voltage main positive relay of the vehicle, the key relay or contactor on the vehicle, and can realize the limp mode shutdown control of other key relays or contactors on the vehicle.

In this embodiment, by adopting the limp mode high-voltage power-off confirmation circuit and the limp-mode high-voltage relay control circuit, the power-off function that is crucial to the safe operation of electric vehicles avoids the use of expensive MCUs and simplifies the difficulty of software design. The use of delay control allows enough time for the user to confirm the power-off operation, avoiding the potential safety hazard of power loss caused by false triggering of power-off.

The above steps can be realized by designing a circuit. In step 1, the operation of the MCU is monitored by the SBC with a watchdog monitoring function, and the start-stop switch is used as the input control signal of the high-voltage power-off function in limp mode. This embodiment provides a high-voltage power-off system for electric vehicles in limp mode, as shown in Figures 2-7, the designed circuit may include: start-stop switch tri-state detection circuit I, high-voltage power-off confirmation circuit II in limp mode, Normal mode high voltage relay control circuit III, limp mode high voltage relay control circuit IV and internal power supply enabling control circuit V.

In step 2, when the MCU is running normally, the vehicle internal power supply enabling control circuit Ⅴ enables the power supply of the normal mode high-voltage relay control circuit Ⅲ, disables the power supply of the high-voltage power-off confirmation circuit Ⅱ in the limp mode, and is controlled by the MCU and the normal mode high-voltage relay Circuit III controls the on-off of the high-voltage main positive relay;

In step 3, when the MCU fails, the internal power supply enabling control circuit V enables the power supply of the high-voltage power-off confirmation circuit II in the limp mode, and disables the power supply of the high-voltage relay control circuit III in the normal mode, and the high-voltage power-off confirmation circuit II and The high-voltage relay control circuit IV in limp mode controls the shut-off of the high-voltage main positive relay, so that the high-voltage main positive relay is turned off after the time set by the start-stop switch is pressed.

Wherein, the set time may be about 4 seconds.

In some embodiments, the start-stop switch tri-state detection circuit I includes a start-stop switch circuit and a voltage dividing and current limiting circuit, the circuit diagram of which is shown in FIG. 2 .

The start-stop switch circuit and the vehicle's physical switch button are integrated in the start-stop switch module, and connected to the VCU through the SSBin interface. The signal line connecting the VCU to the start-stop switch is SSBin. The physical switch button of the vehicle is specifically the start-stop switch of the whole vehicle, which controls the start-stop switch of the whole vehicle.

Optionally, the start-stop switch circuit includes two resistors R4 and R5 connected in series. After series connection, one end is grounded, the other end is connected to the VCU, and the other end is grounded. The start-stop switch is connected in parallel to both ends of one of the resistors.

The voltage dividing and current limiting circuit can be set in the VCU. After the output signal of the SSBin interface is divided and current limited, it is connected to the ADC pin of the MCU through ADin, and the three states of the start-stop switch are determined by the MCU.

Optionally, the voltage dividing current limiting circuit includes a first resistor R1 and a second resistor R2 connected in series, and also includes a first capacitor C1 and a third resistor R3, and the connection point of the first resistor R1 and the second resistor R2 is connected through SSBin The interface is connected to the VCU, the first capacitor C1 and the third resistor R3 are connected in series and then connected to both ends of the second resistor R2, the connection point of the first capacitor C1 and the third resistor R3 is grounded, and the third resistor R3 and the second resistor R2 The connection point of is the output terminal ADin of the voltage dividing and current limiting circuit.

When the start-stop switch is in the state of input disconnection, pressing, and popping up, record the nominal voltage values of SSBin as V _open , V _press , and V _release respectively. value, so that V _open , V _press , and V _release satisfy the following relationship:

V _press ·(1±0.2)<V _release ·(1±0.2)<V _open ·(1±0.2)(1)

The R1, R2, R3, R4, and R5 adopt ordinary resistors with an accuracy of ±5%, and the resistance values are respectively recorded as R ₁ , R ₂ , R ₃ , R ₄ , and R ₅ , and the nominal voltage values are R1, R2, R3, R4, R5 take the voltage value of SSBin when the nominal resistance value is taken.

Record the voltage value of ADin as V _{ssb_mcu} and the voltage value of SSBin as V _ssb , and the two satisfy the following relationship:

The MCU judges the three states of the start-stop switch based on the analog-to-digital conversion results of ADin, and according to the size relationship of the start-stop switch input open circuit, press, and pop-up states. The switch signal is transmitted to the VCU through the wire harness, and the wire may be disconnected, which means the input is in an open circuit state.

In some embodiments, the high-voltage power-off confirmation circuit II in the limp mode includes a start-stop switch press confirmation circuit and a high-voltage power-off countdown circuit connected in sequence, and the circuit form is shown in FIG. 3 .

Optionally, the confirmation circuit for pressing the start-stop switch includes a first series resistor divider circuit, a comparator U1 and a transistor Q1, and the output terminals of the first series resistor R6 and R7 divider circuit are connected to the negative input terminal of the comparator U1 , the positive input terminal of the comparator U1 is connected to the SSBin interface of the VCU; the output terminal of the comparator U1 is connected to the base of the transistor Q1, and the state of the start-stop switch is judged by the conduction or cut-off of the transistor Q1.

Press the start-stop switch to confirm that the input of the circuit is SSBin, connect SSBin to the positive input terminal of analog comparator U1, and connect the voltage obtained by dividing Vcc through resistors R6 and R7 to the negative input terminal of U1. By selecting the resistance values of R6 and R7 reasonably, after the start-stop switch is pressed, the positive input terminal voltage is lower than the negative input terminal voltage, U1 outputs low level, transistor Q1 is turned on, and the collector is high level. When the start-stop switch is in the state of input disconnection or pop-up, the voltage at the positive input terminal of U1 is higher than the voltage at the negative input terminal, U1 outputs a high level, and Q1 is cut off.

In this embodiment, a start-stop switch circuit and a start-stop switch press confirmation circuit are provided, which can improve the accuracy of the state detection of the start-stop switch, improve the accuracy of vehicle limp power-off control, reduce system misoperation, and improve vehicle driving. security.

Optionally, the high-voltage power-off countdown circuit can use a timer circuit, or a circuit structure as shown in Figure 3, including a resistance-capacitance network, a comparator U2, a second series resistor divider circuit, and an input terminal Connect the start-stop switch and press the output terminal of the confirmation circuit, which is the emitter of the triode Q1, the second series resistor voltage divider circuit and the resistance-capacitance network are respectively connected to the input terminal of the comparator U2, and the output terminal of the comparator U2 is connected to the limp mode High voltage relay control circuit Ⅳ.

Wherein, the second series resistor voltage divider circuit includes two resistors R12 and R13 connected in series.

The resistance-capacitance network includes resistor R9, resistor R10, resistor R11 and capacitor C2. One end of resistor R9 is connected to resistor R10 and resistor R11 respectively, and the other end of R10 is grounded. Resistor R11 is connected to capacitor C2. The other end of capacitor C2 is grounded. The voltage at the terminal is the output terminal voltage.

The input of the high-voltage power-down countdown circuit is the collector of the transistor Q1, which is connected to the positive input terminal of the comparator U2 through the resistance-capacitance network, and the voltage obtained by dividing Vlimp through the resistors R12 and R13 is connected to the negative input terminal of U2. By reasonably selecting the resistance values of R9, R10, R11, R12, R13 and the capacitance value of capacitor C2, the voltage at the positive input terminal of U2 is higher than the voltage at the negative input terminal after the time interval set by the start-stop switch is pressed, for example, At about 4 seconds, U2 outputs a high level, which is connected to the high-voltage relay control circuit in limp mode through LimpCtrl. As shown in Figure 4, the change curve of the C2 terminal voltage U _c when R ₉ =47kΩ, R ₁₀ =56kΩ, R ₁₁ =130kΩ, R ₁₂ =47kΩ, R ₁₃ =47kΩ, C ₂ =10uF, V _limp =12v.

The input of the resistance-capacitance network is the collector voltage of Q1, and the output is the voltage U _c of the capacitor C2 terminal, and its transfer function is:

Wherein, R ₉ , R ₁₀ , and R ₁₁ are the resistance values of the resistors R9, R10, and R11 respectively, and C ₂ is the capacitance of C2. Fig. 5 is the unit step response curve of the RC network transfer function when R ₉ =47kΩ, R ₁₀ =56kΩ, R ₁₁ =130kΩ, and C ₂ =10uF.

In this embodiment, Vcc is the power supply of the low-voltage system of the electric vehicle, and Vlimp is the power supply of the high-voltage power-off confirmation circuit in limp mode, and the enabling and disabling of Vlimp are controlled by the internal power supply enabling control circuit V.

In some embodiments, the high voltage loop relay control circuit includes two parts, the normal mode high voltage relay control circuit III and the limp mode high voltage relay control circuit IV, as shown in FIG. 6 .

Optionally, the normal mode high voltage relay control circuit III includes a normal mode on control circuit and a normal mode off control circuit.

Normal mode conduction control circuit, including triode Q2, triode Q3, diode D4 and triode Q4, the base of triode Q2 is connected to the Mcu_on of the MCU, which is the conduction signal output terminal, and the collector of triode Q2 is connected to The base of the triode Q3 and the collector of the triode Q3 are connected to the Vnor power supply. In the limp mode, the Vnor power supply is disabled. The emitter of the triode Q3 is connected to the base of the triode Q4 through the resistor R16. The triode Q4 The collector of the transistor Q3 is connected to the power supply Vcc; the emitter of the triode Q3 is connected to the diode D4 through the resistor R15, and the cathode of the diode D4 is the control output terminal Relay_ctrl.

The input signal of the conduction control circuit in normal mode is Mcu_on from the MCU, and the output signal is Relay_ctrl, which controls the conduction of the high-voltage main positive relay.

When the MCU is running normally, Vnor supplies power normally, and Mcu_on is effective for the conduction control of Relay_ctrl. When Mcu_on is at a high level, the transistors Q2 and Q3 are turned on, the collector of Q3 outputs a high level, the one-way diode D4 is turned on, the Relay_ctrl signal outputs a high level, and the high voltage main positive relay is turned on. When entering the limp mode, Vnor power supply is disabled, Q3 is cut off, D4 is cut off, and the conduction control of Mcu_on to Relay_ctrl is invalid.

Optionally, as shown in FIG. 6 , the shutdown control circuit in normal mode includes transistors Q5 and Q6 connected in sequence, and a unidirectional diode D3. The collector of transistor Q6 is connected to Vnor for power supply. In limp mode, Vnor power supply is disabled. Yes, the cathode of the one-way diode D3 is connected to the turn-off interlock circuit.

Specifically, the turn-off interlock circuit includes a triode Q7, the collector of the triode Q7 is connected to the control output terminal Relay_ctrl through a resistor R6, the collector of the triode Q7 is connected to the power supply Vcc through a resistor R19, and the collector of the triode Q7 is connected in series The connected diode D1, resistor R18, resistor R20, and resistor R21 are connected to the base of the transistor Q7, the base and emitter of the transistor Q7 are directly connected through the capacitor C4, and the connection point of the resistor R20 and the resistor R21 is connected to the normal mode shutdown control circuit. output.

The input signal of the normal mode shutdown control circuit is Mcu_off from the MCU, and the output signal is Relay_ctrl.

Controls the shutdown of the high voltage main positive relay in normal mode. When the MCU is running normally, Vnor supplies power normally. When Mcu_off is at a high level, the transistors Q5 and Q6 are turned on, the collector of Q6 outputs a high level, the unidirectional diode D3 is turned on, the transistor Q7 is turned on, and the collector outputs a low level. The Relay_ctrl signal outputs a low level, turning off the high-voltage main positive relay.

When entering the limp mode, the Vnor power supply is disabled, the transistor Q6 is cut off, the one-way diode D3 is cut off, and the turn-off control of the Relay_ctrl by Mcu_off is invalid.

The limp mode high voltage relay control circuit IV is the limp mode shutdown control circuit, as shown in Figure 6, the input signal is LimpCtrl from the limp mode high voltage power-off confirmation circuit, and the output signal is Relay_ctrl. The high-voltage relay control circuit IV in limp mode includes a diode D2 connected to the off-on interlock circuit of the high-voltage relay control circuit III in normal mode.

In limp mode, when LimpCtrl is at high level, unidirectional diode D2 is turned on, transistor Q7 is turned on, collector Q7 outputs low level, Relay_ctrl signal outputs low level, and the high-voltage circuit relay is turned off.

In the limp mode in which Q7 is turned on, when LimpCtrl is at low level, the unidirectional diodes D1 and D2 are cut off, the base of Q7 is kept at the conduction voltage through capacitor C4, Q7 continues to be turned on, and the Relay_ctrl signal outputs low level , the high-voltage loop relay continues to remain off.

Optionally, when Relay_ctrl outputs high level, the relay is turned on, and when Relay_ctrl outputs low level, the relay is turned off.

Wherein, Vnor is the power supply of the normal mode on control circuit and the normal mode off control circuit, enabling and disabling of Vnor is controlled by the internal power supply enabling control circuit V.

In some embodiments, the internal power supply enabling control circuit V controls enabling and disabling of Vnor and Vlimp, as shown in FIG. 7 .

The internal power supply enabling control circuit Ⅴ is realized by the on-off control circuit of the triode, including the transistor Q8, the transistor Q9, the transistor Q10 and the transistor Q11 connected in sequence, the base of the transistor Q8 is connected to the limp mode pin Limp of the SBC, and the collector of the transistor Q8 Connect to the power supply Vcc, the collector of the transistor Q8 is connected to the power supply output Vnor end through a resistor, the emitter of the transistor Q8 is connected to the base of the transistor Q9 through a resistor R22, and the collector of the transistor Q9 is connected to the power supply output Vnor end; the transistor Q8 The emitter is connected to the base of the transistor Q10, the collector of the transistor Q10 is connected to the base of the transistor Q11, the collector of the transistor Q11 is connected to the power supply Vcc, the emitter of the transistor Q10 is connected to the emitter of the transistor Q11 through a capacitor C2, and the transistor The emitter of Q11 is the power supply output Vlimp terminal

When the MCU is in normal operation, the limp mode pin Limp of the SBC outputs a high level, Q8, Q9, Q10, and Q11 are cut off, enabling Vnor power supply, and disabling Vlimp power supply; when in limp mode, Limp outputs a low level , Q8, Q9, Q10, and Q11 are turned on, the Vlimp power supply is enabled, and the Vnor power supply is disabled.

The above descriptions are only preferred embodiments of the present disclosure, and are not intended to limit the present disclosure. For those skilled in the art, the present disclosure may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present disclosure shall be included within the protection scope of the present disclosure.

Although the specific implementation of the present disclosure has been described above in conjunction with the accompanying drawings, it does not limit the protection scope of the present disclosure. Those skilled in the art should understand that on the basis of the technical solutions of the present disclosure, those skilled in the art do not need to pay creative work Various modifications or variations that can be made are still within the protection scope of the present disclosure.

Claims (10)

1. A method for powering off electric vehicle under high voltage under limp mode, is characterized in that, comprises the steps: Monitor the running status of the main control chip MCU of the vehicle controller and the status of the vehicle start-stop switch; When the MCU is running normally, according to the state of the vehicle start-stop switch, the on-off of the controlled components is controlled through the vehicle MCU and the normal mode high-voltage relay control circuit; When the MCU fails, the control of the high-voltage circuit relay by the MCU is disconnected through the limp mode pin of the SBC. After the time set by the shutdown signal of the stop switch, the controlled component is turned off. 2. A method for powering off electric vehicles under high voltage in limp mode according to claim 1, characterized in that: the controlled component is a high voltage main positive relay of the vehicle, a key relay or a contactor on the vehicle. 3. A high-voltage power-off system for electric vehicles in limp mode, characterized in that it includes a start-stop switch tri-state detection circuit I, a high-voltage power-off confirmation circuit II in limp mode, a high-voltage relay control circuit III in normal mode, and a high-voltage relay control circuit in limp mode Circuit IV and internal power supply enabling control circuit V; The start-stop switch tri-state detection circuit Ⅰ is used to detect the working state of the start-stop switch; When the MCU is running normally, the vehicle internal power supply enabling control circuit Ⅴ enables the power supply of the normal mode high-voltage relay control circuit Ⅲ, disables the power supply of the high-voltage power-off confirmation circuit Ⅱ in the limp mode, and the high voltage is controlled by the MCU and the normal mode high-voltage relay control circuit Ⅲ On and off of the main positive relay; When the MCU fails, the internal power supply enabling control circuit Ⅴ enables the power supply of the high-voltage power-off confirmation circuit II in the limp mode, and disables the power supply of the high-voltage relay control circuit III in the normal mode, and the high-voltage power-off confirmation circuit II in the limp mode and the high-voltage relay in the limp mode The control circuit IV controls the shut-off of the high-voltage main positive relay, so that the high-voltage main positive relay is turned off after the time set by the start-stop switch is pressed. 4. A high-voltage power-off system for an electric vehicle in a limp mode as claimed in claim 3, characterized in that: the start-stop switch tri-state detection circuit I includes a start-stop switch circuit and a voltage dividing and current limiting circuit; The start-stop switch circuit includes two resistors connected in series, one end of the series-connected circuit is grounded, the other end is connected to the SSBin interface of the VCU, and the start-stop switch is connected in parallel to both ends of one of the resistors; The voltage dividing and current limiting circuit is set in the VCU, and the output signal of the SSBin interface is subjected to voltage dividing and current limiting. 5. A high-voltage power-off system for electric vehicles in limp mode as claimed in claim 4, wherein the voltage dividing and current limiting circuit includes a first resistor R1 and a second resistor R2 connected in series, and also includes a first capacitor C1 and the third resistor R3, the connection point of the first resistor R1 and the second resistor R2 is connected to the VCU through the SSBin interface, the first capacitor C1 and the third resistor R3 are connected in series and then connected to both ends of the second resistor R2, the first The connection point between the capacitor C1 and the third resistor R3 is grounded, and the connection point between the third resistor R3 and the second resistor R2 is the output terminal of the voltage dividing and current limiting circuit. 6 . The high-voltage power-off system for electric vehicles in limp mode according to claim 3 , wherein the detected states of the start-stop switch include input open circuit, pressed and bounced states. 7 . 7. A high-voltage power-off system for electric vehicles in limp mode according to claim 3, characterized in that: the high-voltage power-off confirmation circuit II in limp mode includes a start-stop switch press confirmation circuit and a high-voltage power-off countdown circuit connected in sequence ; The start-stop switch press confirmation circuit includes a first series resistance voltage divider circuit, a comparator U1 and a triode Q1, the output end of the first series resistance voltage divider circuit is connected to the negative input end of the comparator U1, and the positive input end of the comparator U1 It is connected to the SSBin interface of the VCU; the output terminal of the comparator U1 is connected to the base of the transistor Q1, and the state of the start-stop switch is judged by the conduction or cut-off of the transistor Q1. 8. A high-voltage power-off system for electric vehicles in limp mode as claimed in claim 7, wherein the countdown circuit for high-voltage power-off uses a timer circuit; Alternatively, the high-voltage power-off countdown circuit includes a resistance-capacitance network, a comparator U2, and a second series resistance voltage divider circuit. The resistance-capacitance network is respectively connected to the input terminals of the comparator U2, and the output terminal of the comparator U2 is connected to the limp mode high voltage relay control circuit IV. 9. A high-voltage power-off system for electric vehicles in limp mode according to claim 3, characterized in that: the normal mode high-voltage relay control circuit III includes a normal mode on control circuit and a normal mode off control circuit; The normal mode conduction control circuit includes a triode Q2, a triode Q3, a diode D4 and a triode Q4. The base of the triode Q2 is connected to the conduction signal output terminal Mcu_on of the MCU, and the collector of the triode Q2 is connected to the three The base of the transistor Q3 and the collector of the transistor Q3 are connected to the Vnor power supply. In the limp mode, the Vnor power supply is disabled. The emitter of the transistor Q3 is connected to the base of the transistor Q4 through the resistor R16, and the transistor Q4’s The collector is connected to the power supply Vcc; the emitter of the triode Q3 is connected to the diode D4 through the resistor R15, and the cathode of the diode D4 is the control output terminal; Alternatively, the shutdown control circuit in normal mode includes transistors Q5 and Q6 connected in sequence, and a unidirectional diode D3. The collector of the transistor Q6 is connected to Vnor for power supply. In limp mode, Vnor power supply is disabled, and the cathode of unidirectional diode D3 connected to an off-on interlock circuit; The off-on interlock circuit includes a triode Q7, the collector of the triode Q7 is connected to the control output terminal through a resistor R6, the collector of the triode Q7 is connected to the power supply Vcc through a resistor R19, and the collector of the triode Q7 is connected through a series connected diode D1 , the resistor R18, the resistor R20, and the resistor R21 are connected to the base of the transistor Q7, the base and the emitter of the transistor Q7 are directly connected through the capacitor C4, and the connection point of the resistor R20 and the resistor R21 is connected to the output end of the normal mode shutdown control circuit; Alternatively, the high-voltage relay control circuit IV in limp mode includes a diode D2 connected to the off-on interlock circuit of the high-voltage relay control circuit III in normal mode. 10. A high-voltage power-off system for electric vehicles in limp mode according to claim 3, characterized in that: the internal power supply enabling control circuit V realizes the enabling control of power supply output through the triode on-off control circuit.

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