Protection circuit for direct-current high-voltage backflow low-voltage communication port

Abstract

The invention discloses a protection circuit for a direct-current high-voltage backflow low-voltage communication port, which is arranged between the low-voltage communication port and an MCU (Microprogrammed Control Unit), and comprises a current limiting circuit connected to the low-voltage communication port and used for limiting current when the low-voltage communication port is subjected to high-voltage backflow; the overvoltage cut-off circuit is provided with a first end, a second end and a third end, the first end is connected to the low-voltage communication port, and the second end is connected to the current limiting circuit; the overvoltage cut-off circuit is used for cutting off an external communication line when the low-voltage communication port is subjected to high-voltage backflow; the voltage limiting circuit is connected to the third end of the overvoltage cut-off circuit and is used for limiting the voltage when the low-voltage communication port is backward filled by the high voltage; and the I / O port voltage-withstanding / current-withstanding expansion circuit is connected to the voltage limiting circuit and the overvoltage cut-off circuit, is connected with the I / O port of the MCU, and is used for carrying out level isolation on the I / O port so as to improve the voltage-withstanding value and the current-withstanding value of the I / O port.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic circuits, and particularly relates to a protection circuit for a DC high-voltage backflow to a low-voltage communication port. Background Art

[0002] The traditional protection circuit uses a PTC resettable fuse in series with the COM (serial communication interface), then a resistor in series, close to the internal low-voltage communication port, and a TVS (transient voltage suppression diode) in parallel.

[0003] However, the traditional use of a PTC thermal fuse has a long response time and slow speed (about 1.5 s). Since in order to maintain communication quality, the resistance value of the resistor in series on the communication line is relatively small, it means that before the PTC is disconnected during high-voltage backflow, the power borne by the resistor will be relatively high, the reliability of the circuit is low, and the PTC will close again due to temperature reduction after disconnection. If the fault has not disappeared, the protection will start again, forming an oscillation on the communication port. Summary of the Invention

[0004] In view of this, the present invention proposes a protection circuit for a DC high-voltage backflow to a low-voltage communication port to solve the problems of long response time, low reliability of the PTC protection scheme, and periodic conduction and disconnection of the communication line when the fault persists for a long time.

[0005] To solve the above problems, the present invention proposes the following technical solutions:

[0006] A protection circuit for a DC high-voltage backflow to a low-voltage communication port, disposed between the low-voltage communication port and the MCU, includes: a current-limiting circuit connected to the low-voltage communication port for current-limiting when the low-voltage communication port is backflowed by high voltage; an overvoltage cut-off circuit having a first end, a second end, and a third end, wherein the first end is connected to the low-voltage communication port and the second end is connected to the current-limiting circuit; the overvoltage cut-off circuit is used to cut off the external communication line when the low-voltage communication port is backflowed by high voltage; a voltage-limiting circuit connected to the third end of the overvoltage cut-off circuit for voltage-limiting when the low-voltage communication port is backflowed by high voltage; and an I / O port voltage / current withstand expansion circuit connected to the voltage-limiting circuit and the overvoltage cut-off circuit and connected to the I / O port of the MCU for level isolation of the I / O port of the MCU to increase the voltage withstand value and current withstand value of the I / O port.

[0007] Further, the overvoltage cut-off circuit includes: a first switch module and a second switch module. The first switch module is connected to the low-voltage communication port through the first end. The second switch module is connected to the current-limiting circuit through the second end. The second switch module is connected to the voltage-limiting circuit and the I / O port voltage / current withstand expansion circuit through the third end. When the voltage of the low-voltage communication port exceeds a preset value and high-voltage backflow occurs, the first switch module conducts, and the second switch module turns off.

[0008] Further, the first switch module includes a first switch tube and a first voltage-dividing resistor, and the second switch module includes a second switch tube and a second voltage-dividing resistor. When the low-voltage communication port is subject to high-voltage backflow, the first switch tube is turned on by the voltage division of the first voltage-dividing resistor, and the second switch tube is turned off under the combined action of the turn-on of the first switch tube and the second voltage-dividing resistor.

[0009] Further, the first switch module includes the first switch tube, a second resistor, a sixth resistor, and a second diode, where the second resistor and the sixth resistor form the first voltage-dividing resistor. The positive electrode of the second diode is grounded, and the negative electrode is connected to the gate of the first switch tube. One end of the second resistor is connected to the low-voltage communication port, and the other end is connected to the gate of the first switch tube. One end of the sixth resistor is grounded, and the other end is connected to the gate of the first switch tube. The source electrode of the first switch tube is grounded, and the drain electrode is connected to the gate of the second switch tube.

[0010] Further, the second switch module includes the second switch tube, a first resistor, and a fourth resistor, where the first resistor and the fourth resistor form the second voltage-dividing resistor. One end of the first resistor is connected to the power supply VCC, and the other end is connected to the gate of the second switch tube. One end of the fourth resistor is grounded, and the other end is connected to the gate of the second switch tube. The drain electrode of the second switch tube is connected to the current-limiting circuit, and the source electrode is connected to the voltage-limiting circuit and the I / O port voltage / current withstand expansion circuit.

[0011] Further, the I / O port voltage / current withstand expansion circuit includes a third switch module and a fourth switch module, where the third switch module is connected to the MCU_TX port in the I / O ports of the MCU, and the fourth switch module is connected to the MCU_RX port in the I / O ports of the MCU.

[0012] Further, the third switch module includes a third switching tube and a seventh resistor, and the fourth switch module includes a fourth switching tube and an eighth resistor; one end of the seventh resistor is connected to the VCC pin of the MCU, and the other end is connected to the drain of the third switching tube and the gate of the fourth switching tube. One end of the eighth resistor is connected to the VCC pin of the MCU, and the other end is connected to the drain of the fourth switching tube; the gate of the third switching tube is connected to the MCU_TX port, the source is grounded, and the drain is connected to the voltage limiting circuit; the gate of the fourth switching tube is connected to the voltage limiting circuit, the source is grounded, and the drain is connected to the MCU_RX port.

[0013] Further, both the first diode and the second diode are TVS diodes.

[0014] Further, the current limiting circuit includes a current limiting resistor. One end of the current limiting resistor is connected to the low-voltage communication port, and the other end is connected to the second end of the overvoltage cut-off circuit; when the third switching tube is not turned on, the current limiting resistor is used to limit the current flowing from the low-voltage communication port through the current limiting resistor, the second switching tube to the first diode in sequence; when the third switching tube is turned on, the current limiting resistor is used to limit the current flowing from the low-voltage communication port through the current limiting resistor, the second switching tube to the third switching tube in sequence.

[0015] Further, the voltage limiting circuit includes a first diode. The positive electrode of the first diode is grounded, and the negative electrode is connected to the third end of the overvoltage cut-off circuit and the I / O port voltage withstand / current withstand expansion circuit.

[0016] The beneficial effects of the present invention are as follows: The protection circuit of the present invention, on the one hand, connects a current limiting circuit in series on the communication line for current limiting and a voltage limiting circuit in parallel for voltage limiting. On the other hand, an overvoltage cut-off circuit based on a switching tube is added to the communication line, which can quickly respond when it is detected that the low-voltage communication port is backflowed by high voltage, and cut off the path between the external communication circuit and the internal circuit to protect the internal circuit and components; while it is opened during normal operation and does not affect normal communication. The protection circuit of the present invention does not require the use of PTC, uses semiconductors such as MOSFET as switches, has a fast response time, high reliability when the low-voltage communication port is backflowed by high voltage, and if the fault does not disappear, the protection action can continue, and there will be no problem of periodic opening. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic block diagram of the protection circuit for the DC high-voltage backflow to the low-voltage communication port in the embodiment of the present invention.

[0018] Figure 2 is a circuit diagram of the protection circuit for the DC high-voltage backflow to the low-voltage communication port in the embodiment of the present invention.

[0019] Figure 3a And Figure 3b are the simulation schematic diagram and simulation results of the protection circuit for the DC high-voltage backflow to the low-voltage communication port in the embodiments of the present invention.

[0020] Figure 4 are the measured results of the protection circuit for the DC high-voltage backflow to the low-voltage communication port in the embodiments of the present invention. Specific Embodiments

[0021] The present invention will be further described below in conjunction with the accompanying drawings, specific embodiments, and examples. The purpose of providing the examples is only for illustration and not for any limitation. In addition, terms such as "first" and "second" are only used to distinguish elements. It should be understood that these components should not be limited by such terms, and they do not inherently mean that these elements have the aforementioned ordinal numbers, nor do they represent the arrangement order of one element and another element or the order in the manufacturing method.

[0022] Currently, the protection circuit for the conventional high-voltage DC backflow to the low-voltage communication circuit mainly relies on the PTC thermal fuse to protect against overcurrent. Before protection, it relies on the resistor in series with the communication line and the TVS in parallel with the communication line for protection. However, the PTC protection has the disadvantage of a long action response time (about 1.5 s). During this period, relying on the resistor in the communication line and the TVS in parallel with the communication line for protection, the selection is difficult and the reliability is also difficult to guarantee. Even if the PTC acts, after the temperature drops, the impedance of the PTC will decrease again. If the fault has not disappeared, the PTC needs to wait for a period of time before it will respond again. The PTC will switch periodically and form oscillations at the communication port.

[0023] In view of this, the embodiments of the present invention use semiconductor switches in cooperation with detection circuits and expand the voltage / current withstand capabilities of the I / O ports to provide reliability, while improving the response speed and the persistence of the protection action. Please refer to Figure 1 and Figure 2, an embodiment of the present invention provides a protection circuit for a DC high-voltage backflow to a low-voltage communication port, which is disposed between the low-voltage communication port COM of a communication device and a microcontroller unit MCU (not shown in the figure) to prevent the internal low-voltage components of the communication device from being damaged when the COM port is backflowed by high voltage. The protection circuit includes a current-limiting circuit 10, an overvoltage cut-off circuit 20, an I / O port withstand voltage / current withstand expansion circuit 30, and a voltage-limiting circuit 40; the current-limiting circuit 10 is connected in series between the COM port and the overvoltage cut-off circuit 20; the overvoltage cut-off circuit 20 has a first end, a second end, and a third end, wherein the first end is connected to the COM port together with one end of the current-limiting circuit 10, the second end is connected to the other end of the current-limiting circuit 10, and the third end is connected to the I / O port withstand voltage / current withstand expansion circuit 30 and the voltage-limiting circuit 40; the I / O port withstand voltage / current withstand expansion circuit 30 is connected to the I / O port MCU_TX port (transmission port) and MCU_RX (reception port) of the MCU through two switch modules respectively, and electrically isolates the I / O ports MCU_RX and MCU_TX with relatively low withstand voltage and current withstand, so that the withstand voltage and current withstand capabilities of the MCU_RX port and the MCU_TX port are improved.

[0024] In some specific embodiments, the current-limiting circuit 10 is implemented by a resistor, the voltage-limiting circuit 40 is implemented by a TVS diode (Transient Voltage Suppressor), and the overvoltage cut-off circuit 20 and the I / O port withstand voltage / current withstand expansion circuit 30 are both implemented by a switching transistor such as a MOSFET. For example Figure 2In the illustrated embodiment, the current limiting circuit 10 is a current limiting resistor R3; the voltage limiting circuit 40 is a TVS diode D1; the overvoltage cut-off circuit 20 includes a switching transistor Q1, a TVS diode D2, voltage dividing resistors R2 and R6, a switching transistor Q2, a resistor R1, and a resistor R4. The positive electrode of the diode D2 is grounded, and the negative electrode is connected to the gate of the switching transistor Q1. One end of the resistor R2 is connected to the COM port, and the other end is connected to the gate of the switching transistor Q2. One end of the resistor R6 is grounded, and the other end is connected to the gate of the switching transistor Q1. The source electrode of the switching transistor Q1 is grounded, and the drain electrode is connected to the gate of the switching transistor Q2. One end of the resistor R1 is connected to the power supply VCC, and the other end is connected to the gate of the switching transistor Q2. One end of the resistor R4 is grounded, and the other end is connected to the gate of the switching transistor Q2. The drain electrode of the switching transistor Q2 is connected to the current limiting resistor R3, and the source electrode is connected to the negative electrode of the TVS diode D1 and the I / O port voltage / current expansion circuit 30. The I / O port voltage / current expansion circuit 30 includes a switching transistor Q3, a switching transistor Q4, a resistor R7, and a resistor R8. The gate of the switching transistor Q3 is connected to the MCU_TX port of the MCU (when working normally, the signal sent from the MCU_TX port of the MCU passes through the switching transistor Q3, the switching transistor Q2 of the overvoltage cut-off circuit 20, the current limiting circuit 10, and the COM port in sequence and is sent to the external communication circuit). The drain electrode of the switching transistor Q4 is connected to the MCU_RX port of the MCU (when working normally, the COM port receives signals from the external communication circuit and passes through the current limiting circuit 10, the switching transistor Q2 of the overvoltage cut-off circuit 20, and the switching transistor Q4 in sequence and is finally received through the MCU_RX port). One end of the resistor R7 is connected to the VCC pin of the MCU, and the other end is connected to the drain electrode of the switching transistor Q3 and the gate of the switching transistor Q4. One end of the resistor R8 is connected to the VCC pin of the MCU, and the other end is connected to the drain electrode of the switching transistor Q4. The source electrode of the switching transistor Q3 is grounded, and the drain electrode is connected to the negative electrode of the diode D1. The gate of the switching transistor Q4 is connected to the negative electrode of the diode D1, and the source electrode is grounded.

[0025] In the embodiments of the present invention, the switching transistors Q1, Q2, Q3, and Q4 can be MOSFETs, such as NMOS transistors or PMOS transistors.

[0026] When the COM port is reverse-injected with a DC high voltage, the current limiting resistor R3 limits the current, the TVS D1 limits the voltage. At the same time, the voltage division values of the voltage dividing resistors R2 and R6 of the overvoltage cut-off circuit 20 cause the switching transistor Q1 to turn on (equivalent to detecting a high voltage reverse injection), and the turning on of Q1 causes Q2 to turn off, thereby cutting off the communication line. Before Q2 turns off, although the voltage on D1 will briefly increase, due to the current limiting effect of the current limiting resistor R3, it can prevent D1 from being damaged during the short time before Q2 turns off. After Q2 turns off, the communication line is cut off. On the one hand, this prevents the current limiting resistor R3 and the voltage limiting diode D1 from being damaged due to continuous response, and at the same time prevents the reverse-injected DC high voltage from damaging the switching transistors Q3 and Q4.

[0027] In an exemplary embodiment, the external high-voltage DC is 58V and the normal communication bus voltage is 5V; the communication line impedance is related to the specific frequency, and it is sufficient to ensure that communication is not affected during normal operation (the larger the better). In this example, the resistance R3 is taken as 200Ω; to ensure normal communication, the TVS cannot affect the communication bus voltage of 5V, and TVSD1 must be higher than 5V. Here, the conventional value of 6V is taken (due to the existence of the I / O port withstand voltage / current withstand extension circuit 30, the voltage can be between higher than the bus voltage and the MOSFET GS withstand voltage of ±20V. The intermediate value can be taken. If the clamping voltage of D1 is taken at the lower limit, a larger withstand voltage margin can be provided. Before the overvoltage cut-off circuit 20 and the voltage limiting circuit 40 respond, the current limiting circuit 10 and the I / O port withstand voltage / current withstand extension circuit 30 will ensure that the MCU_RX port and the MUC_TX port are not damaged).

[0028] The working principle of the protection circuit of the present invention will be described below based on specific embodiments.

[0029] Case 1:

[0030] When the output of the MCU_TX port is at a low level and the switching transistor Q3 is not turned on (cut off), when a high-voltage backflow occurs at the COM port, it can be calculated here that the external DC high voltage of 58V backflows to the COM port, and the currents flowing through the current limiting resistor R3 and TVSD1 are both

[0031]

[0032] The power loss P of TVSD1 D1 =V D1 ×I COM =1.65W. This power determines the time that D1 can withstand when a high-voltage backflow occurs. Too high a power can only be withstood for a short time. Therefore, when selecting the TVS D1, check the peak pulse power rating curve in the TVS specification book to meet this requirement within 2us.

[0033] When the output of the MCU_TX port is low and high-voltage backflow occurs, the purpose of the I / O port voltage / current withstand extension circuit 30 is that the overvoltage cut-off circuit 20 and the voltage-limiting circuit 40 have a response delay. Before the overvoltage cut-off circuit 20 and the voltage-limiting circuit 40 respond, the current-limiting circuit 10 and the I / O port voltage / current withstand extension circuit 30 provide protection; before the voltage-limiting circuit 40 responds, the voltage on TVSD1 will rise briefly. Therefore, the selection of the I / O port voltage / current withstand extension circuit 30 needs to meet the withstand voltage of the short-term voltage rise before the voltage-limiting circuit 40 responds. The voltage-limiting circuit 40 will respond earlier than the overvoltage cut-off circuit 20. Before the overvoltage cut-off circuit 20 responds, since the current-limiting circuit 10 and the voltage-limiting circuit 40 will bear a large amount of power, it is necessary to check the device specification sheet to ensure that when the overvoltage cut-off circuit 20 responds, the current-limiting circuit 10 and the voltage-limiting circuit 40 will not be damaged due to excessive power and long time. The purpose of the overvoltage cut-off circuit 20 is to ensure protection before the current-limiting circuit 10 and the voltage-limiting circuit 40 are damaged.

[0034] In this embodiment, the value range of the voltage-dividing resistors R2 and R6 of the overvoltage cut-off circuit 20 is:

[0035] And

[0036] This ensures that when the circuit is working normally, no protection malfunction will occur. The selected resistance values should meet the requirement that the power generated on resistors R6 and R2 during high-voltage backflow meets the rated power requirement of the resistor. Generally, it is easy to meet the requirement by selecting resistors of 10k level and above. Since the voltage of the COM port is much higher than MCU_VCC during high-voltage backflow, the voltage division of resistors R6 and R2 is much higher than 2.5V. Therefore, TVSD2 is required for voltage clamping. The value range of TVSD2 should not affect the normal operating point of MOSFET Q1, that is, the clamping voltage needs to be higher than 2.5V but lower than the limit of the GS withstand voltage of MOSFET Q1 (generally ±20V). It is recommended to select a TVS tube with a clamping voltage between 5V and 15V for D2.

[0037] Case 2:

[0038] When the MCU_TX port outputs a high level: MOSFET Q3 conducts, and the voltage clamping of TVSD1 fails. The current will flow through MOSFET Q3. When the external DC high voltage of 58V backflows, Therefore, in the worst case of resistor R3 and MOSFET Q3, the loss P on resistor R3 at this time R3 =I COM_TX_ON 2 ·16.82W. When selecting R3, according to the resistor specification sheet, there is a curve of pulse impact power and time. It is necessary to ensure that R3 is not damaged within 2us at this power (the theoretical response time of this protection circuit is several hundred nanoseconds).

[0039] When the output of the MCU_TX port is at a high level, the MOSFET Q3 in the I / O port withstand voltage / current expansion circuit 30 conducts. At this time, DC high-voltage backflow occurs. Then, the current-limiting circuit 10 and the I / O port withstand voltage / current expansion circuit 30 form a loop, and the current will not flow through the overvoltage-limiting circuit 40 but will flow through the MOSFET Q3 in the I / O port withstand voltage / current expansion circuit 30. At this time, the current-expansion ability of the I / O port withstand voltage / current expansion circuit 30 is demonstrated - the current-withstanding ability of the MCU_TX port is relatively poor. The current selection of the MOSFET Q3 needs to take into account the current magnitude after the backflow voltage is limited by the current-limiting circuit 10, and a margin about 1.2 times larger than the calculated value is sufficient. However, the current-limiting circuit 10 still cannot respond for a long time, and excessive power response for a long time will cause damage. Therefore, the overvoltage cut-off circuit 20 needs to respond before the current-limiting circuit 10 is damaged. The selection of the overvoltage cut-off circuit 20 is the same as that in "Case 1" and will not be elaborated here.

[0040] Taking the ordinary MOSFET 2N7002 as an example, the maximum value of the turn-off delay time index is about 40 ns, and the maximum value of the turn-on delay time index is about 20 ns. During protection, the MOSFET Q1 is in the open state, and the MOSFET Q2 is in the off state. The total delay here is 60 ns. Adding the time for the MOSFET GS to reach the on-off threshold value, this time is jointly determined by the external impedance and the input capacitance of the MOSFET. The input capacitance of the MOSFET is about 50 pF. When the backflow is 58 V, the time for the GS (gate-source voltage) of the MOFET Q1 to rise to the maximum on-state voltage threshold value of 2.5 V can be calculated. In the case of resistors R25 6 kΩ and R6 10 kΩ, the calculated value is about 150 ns. The MOSFET Q2 has a similar calculation principle, and the total time is about several hundred ns. Figure 3a and Figure 3b From the simulation example, it can be seen that it is about 300 ns theoretically. Figure 4 This is the measured graph, and the result is 1.67 μs, which includes the delay caused by parasitic effects that are not easily considered in theoretical calculations. It is relatively easy to achieve less than 2 μs.

[0041] The resistors R2 and R6 are used to set the overvoltage preset value of the COM port. It is recommended that the overvoltage preset value of the COM port be higher than 1.2 times the normal operating voltage (the normal operating voltage of the communication port is generally 5V or 3.3V. To avoid affecting normal operation, the overvoltage preset value must be higher than this. Being higher than 1.2 times is mainly for the convenience of implementation in engineering. 1.2 times of 5V is only 6V, and there is no risk of damage to the resistors R3 and TVSD1 for the internal circuit). The voltage value after the overvoltage preset value is divided by the resistors R2 and R6 is lower than the lower limit of the gate-source conduction threshold of the MOSFET Q1, so that there will be no malfunction during normal operation. For example, if the MOSFET Q1 is selected as 2n7002GS with a minimum conduction voltage of 1V, and the resistor R6 is selected as 10kΩ, the resistor Here, 56kΩ is selected, then the COM port starts the protection response when it exceeds 6.6V at least. The maximum conduction voltage of GS is 2.5V, which is easily calculated to be 2.5 times the minimum conduction voltage. Then the maximum response protection voltage of the COM port is 16.5V. At this time, the power losses of the resistors R3 and TVS D1 are easily satisfied. In theory, the protection will respond within a few hundred nanoseconds after the high-voltage backflow. The measured value is less than 2us.

[0042] It can be seen that the protection circuit of the embodiment of the present invention can quickly respond to the high-voltage backflow and protect the low-voltage components inside the COM port and the MCU I / O port of the communication internal port from being damaged.

[0043] The above content is a further detailed description of the present invention in combination with specific preferred embodiments. It cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those skilled in the technical field to which the present invention belongs, without departing from the concept of the present invention, several equivalent substitutions or obvious variations can be made, and as long as the performance or use is the same, they should all be regarded as belonging to the protection scope of the present invention.

Claims

1. A protection circuit for a DC high-voltage backflow into a low-voltage communication port, which is arranged between the low-voltage communication port (COM) and the MCU, and is characterized in that, Comprising: A current limiting circuit (10) connected to the low-voltage communication port (COM) for limiting current when high voltage is back-fed to the low-voltage communication port; An overvoltage cut-off circuit (20) having a first terminal, a second terminal and a third terminal, wherein the first terminal is connected to the low-voltage communication port (COM), and the second terminal is connected to the current limiting circuit (10); the overvoltage cut-off circuit is used to cut off the external communication line when high voltage is back-fed to the low-voltage communication port; A voltage limiting circuit (40) connected to the third terminal of the overvoltage cut-off circuit (20) for limiting voltage when high voltage is back-fed to the low-voltage communication port; and An I / O port voltage / current withstand expansion circuit (30) connected to the voltage limiting circuit (40) and the overvoltage cut-off circuit (20) and connected to the I / O port of the MCU, for performing level isolation on the I / O port of the MCU to increase the voltage withstand value and current withstand value of the I / O port.

2. The protection circuit according to claim 1, wherein The overvoltage cut-off circuit includes: a first switch module and a second switch module. The first switch module is connected to the low-voltage communication port through the first terminal, the second switch module is connected to the current limiting circuit through the second terminal, and the second switch module is connected to the voltage limiting circuit and the I / O port voltage / current withstand expansion circuit through the third terminal; when the voltage of the low-voltage communication port exceeds a preset value and high voltage is back-fed, the first switch module is turned on and the second switch module is turned off.

3. The protection circuit according to claim 2, characterized in that: The first switch module includes a first switch tube (Q1) and a first voltage dividing resistor, and the second switch module includes a second switch tube (Q2) and a second voltage dividing resistor; when high voltage is back-fed to the low-voltage communication port (COM), the first switch tube is turned on by the voltage division of the first voltage dividing resistor, and the second switch tube is turned off under the combined action of the turn-on of the first switch tube and the second voltage dividing resistor.

4. The protection circuit according to claim 3, wherein: The first switch module includes the first switch tube (Q1), a second resistor (R2), a sixth resistor (R6) and a second diode (D2), wherein the second resistor and the sixth resistor form the first voltage dividing resistor; the positive electrode of the second diode is grounded and the negative electrode is connected to the gate of the first switch tube, one end of the second resistor is connected to the low-voltage communication port and the other end is connected to the gate of the first switch tube, and one end of the sixth resistor is grounded and the other end is connected to the gate of the first switch tube; The source electrode of the first switch tube is grounded and the drain electrode is connected to the gate of the second switch tube.

5. The protection circuit according to claim 3 or 4, characterized in that: The second switch module includes the second switch tube (Q2), a first resistor (R1) and a fourth resistor (R4), wherein the first resistor and the fourth resistor form the second voltage dividing resistor; one end of the first resistor is connected to the power supply VCC and the other end is connected to the gate of the second switch tube, one end of the fourth resistor is grounded and the other end is connected to the gate of the second switch tube; the drain electrode of the second switch tube is connected to the current limiting circuit (10), and the source electrode is connected to the voltage limiting circuit (40) and the I / O port voltage / current withstand expansion circuit (30).

6. The protection circuit according to claim 5, wherein: The I / O port withstand voltage / current expansion circuit (30) includes a third switch module and a fourth switch module, wherein the third switch module is connected to the MCU_TX port of the I / O ports of the MCU, and the fourth switch module is connected to the MCU_RX port of the I / O ports of the MCU.

7. The protection circuit according to claim 6, characterized in that: The third switch module includes a third switch transistor (Q3) and a seventh resistor (R7), and the fourth switch module includes a fourth switch transistor (Q4) and an eighth resistor (R8); one end of the seventh resistor is connected to the VCC pin of the MCU, and the other end is connected to the drain of the third switch transistor and the gate of the fourth switch transistor. One end of the eighth resistor is connected to the VCC pin of the MCU, and the other end is connected to the drain of the fourth switch transistor; the gate of the third switch transistor is connected to the MCU_TX port, the source is grounded, and the drain is connected to the voltage limiting circuit; the gate of the fourth switch transistor is connected to the voltage limiting circuit, the source is grounded, and the drain is connected to the MCU_RX port.

8. The protection circuit according to claim 7, wherein: Both the first diode and the second diode are TVS diodes.

9. The protection circuit according to claim 8, wherein: The current limiting circuit (10) includes a current limiting resistor (R3), and one end of the current limiting resistor (R3) is connected to the low-voltage communication port, and the other end is connected to the second end of the overvoltage cut-off circuit. When the third switch transistor is not turned on, the current limiting resistor is used to limit the current flowing from the low-voltage communication port through the current limiting resistor, the second switch transistor to the first diode in sequence. When the third switch transistor is turned on, the current limiting resistor is used to limit the current flowing from the low-voltage communication port through the current limiting resistor, the second switch transistor to the third switch transistor in sequence.

10. The protection circuit according to any one of claims 1 to 8, characterized in that: The voltage limiting circuit (40) includes a first diode (D1), and the positive electrode of the first diode (D1) is grounded, and the negative electrode is connected to the third end of the overvoltage cut-off circuit (20) and the I / O port withstand voltage / current expansion circuit (30).

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