Trailer protection method and trailer system of new energy commercial vehicle

Abstract

The embodiment of the invention provides a trailer protection method and a trailer system of a new energy commercial vehicle, an electric drive and electric control system of the vehicle is self-checked before trailer, and the trailer protection method comprises the following steps: a drive motor controller receives a trailer activation signal and judges whether to enter a trailer self-check mode, and after entering the trailer self-check mode, the drive motor controller drives the trailer activation signal to start the trailer self-check mode; whether the vehicle can be freely dragged or not is judged according to the self-inspection result of the driving motor controller, if the vehicle can be freely dragged, a trailer request instruction is sent to the transmission controller, the transmission controller controls the gear to be shifted to the N gear and waits for trailer, and if the vehicle cannot be freely dragged, the trailer request instruction is sent to the transmission controller; and if not, sending an instruction that the vehicle cannot be freely dragged to a vehicle controller and reporting the instruction to a vehicle instrument. When the new energy commercial vehicle is towed, the situation that the vehicle is towed freely in a fault state, and consequently an electric drive system is damaged can be avoided.

Description

Technical Field

[0001] The embodiments of the present invention relate to the field of electric drive technology for new energy vehicles, and in particular to a trailer protection method and trailer system for new energy commercial vehicles. Background Art

[0002] At present, most of the drive motors used in pure electric, hybrid, fuel cell and other new energy vehicles are permanent magnet synchronous drive motors. Their inherent properties determine that the drive motor will generate back electromotive force when it is dragged and rotated. The key power devices in the drive motor controller, such as IGBT, bus capacitors, etc. are all voltage-sensitive devices. Therefore, when designing the drive motor controller, it is necessary to consider the possible damage to the voltage-sensitive power devices caused by the back electromotive force generated by the drive motor when the vehicle is not driving normally and is being towed.

[0003] The main difference between the electric drive system of new energy commercial vehicles (including drive motor controller, permanent magnet synchronous drive motor, and transmission) and the electric drive system of new energy passenger vehicles is that almost all new energy commercial vehicles use two-speed or multi-speed transmissions, especially medium and heavy-duty new energy vehicles (load capacity 18 tons to 49 tons). Through the use of two-speed transmissions or multi-speed applications, the vehicle can adapt to more complex working conditions, improve climbing power, coordinate the motor to work in the high-efficiency area, and help drive the motor volume and weight.

[0004] The existing trailer protection measures are to lift the drive tires or limit the motor speed when towing. When towing, lift the drive wheel of the drive motor. The motor will not rotate during the towing process, so no back electromotive force will be generated. However, this method can only be implemented by a special trailer. Limiting the speed of the drive motor when towing will cause difficulties in the design of the whole vehicle. At the same time, limiting the speed of the drive motor will seriously affect the towing time, which is a problem that professional towing personnel cannot tolerate. The towing objects of special trailers are new energy passenger cars, which are difficult to apply to new energy commercial vehicles with relatively large load and body size, especially medium and heavy new energy vehicles (load capacity 18 tons to 49 tons). Summary of the invention

[0005] The present invention provides a trailer protection method and a trailer system for a new energy commercial vehicle, which can prevent damage to the electric drive system caused by free towing of the vehicle in a fault state when the new energy commercial vehicle is towing.

[0006] In a first aspect, the present application provides a trailer protection method for a new energy commercial vehicle, which performs a self-check on the electric drive and electric control system of the vehicle before towing. The trailer protection method includes:

[0007] The drive motor controller receives the trailer activation signal and determines whether to enter the trailer self-test mode;

[0008] After entering the trailer self-test mode, determine whether the vehicle can be towed freely based on the self-test results of the drive motor controller;

[0009] If it is determined that the vehicle can be towed freely, a towing request instruction is sent to the transmission controller, and the transmission controller controls the gear to be shifted to N gear to wait for the tow vehicle;

[0010] If it is determined that free towing is not possible, a no free towing instruction is sent to the vehicle controller and reported to the vehicle instrument.

[0011] Optionally, the drive motor controller receives a trailer activation signal and determines whether to enter a trailer self-test mode, the steps are:

[0012] Step S1, the driving motor controller starts self-test, and then executes step S2;

[0013] Step S2, the driving motor controller determines whether it receives the trailer activation signal, if yes, then executes step S3; if no, then executes step S4;

[0014] Step S3, driving the motor controller to enter trailer self-test;

[0015] Step S4: driving the motor controller to enter a driving mode.

[0016] Optionally, determining whether the vehicle can be freely towed according to the self-test result of the drive motor controller includes:

[0017] Step S5, the driving motor controller determines whether free towing is possible, if yes, then executing step S6; if no, then executing step S7;

[0018] Step S6, the driving motor controller sends a trailer request instruction to the transmission controller, and then executes step S8;

[0019] Step S7, the driving motor controller sends a command that it cannot be freely towed to the vehicle controller, and reports it to the vehicle instrument;

[0020] Step S8: The transmission controller controls the vehicle gear to be shifted to N gear, and waits for towing.

[0021] Optionally, the driving motor controller sends a command to the vehicle controller that it cannot be freely towed, further comprising:

[0022] Step S9, the drive motor controller determines whether the vehicle is mistakenly free-towing by detecting the speed signal of the permanent magnet synchronous drive motor, if yes, execute step S10; if no, execute step S11;

[0023] Step S10, the driving motor controller sends an alarm signal to the vehicle controller;

[0024] Step S11, the driving motor controller sends a signal to wait for the dedicated trailer to tow.

[0025] Optionally, the self-checking method of the drive motor controller is:

[0026] The drive motor controller sends a request for the N gear self-test result to the transmission controller;

[0027] When the transmission controller can execute the command to shift into N gear, it will feedback that the N gear self-test is successful;

[0028] When the transmission controller cannot execute the command to shift into N gear, it will feedback that the N gear self-test has failed.

[0029] Optionally, the fault in the self-test result of the drive motor controller that causes the vehicle to be unable to be towed freely includes a short circuit fault in the power switching device.

[0030] Optionally, before the driving motor controller receives the trailer activation signal and determines whether to enter the trailer self-test mode, it also includes:

[0031] The main positive relay and the main negative relay are controlled to be turned on, and the drive motor controller exchanges signals with the transmission controller and the vehicle controller through CAN communication.

[0032] Based on the same inventive concept, the embodiment of the present application further provides a trailer system for a new energy commercial vehicle, which is used to execute the trailer protection method for a new energy commercial vehicle provided in the first aspect, comprising:

[0033] High-voltage power battery, main positive relay, main negative relay, drive motor controller, permanent magnet synchronous drive motor, transmission, transmission controller and vehicle controller;

[0034] The high-voltage power battery is connected to the positive input terminal and the negative input terminal of the DC bus of the drive motor controller through the main positive relay and the main negative relay;

[0035] The AC output terminal of the drive motor controller is connected to the input terminal of the permanent magnet synchronous drive motor;

[0036] The output end of the permanent magnet synchronous drive motor is connected to the transmission;

[0037] The transmission controller controls the transmission;

[0038] The drive motor controller is connected to the transmission controller and the vehicle controller via CAN communication to perform signal interaction.

[0039] Optionally, the drive motor controller includes a three-phase bridge inverter circuit, which is composed of a bus capacitor and three bridge arms in parallel, each bridge arm is composed of two power switch tubes in series, and each power switch tube is also connected in reverse parallel with a diode.

[0040] In summary, the trailer protection method for new energy commercial vehicles provided in this application performs a self-check on the electric drive and electronic control system of the vehicle before towing. First, the drive motor controller receives the trailer activation signal and determines whether to enter the trailer self-check mode, and determines whether the vehicle can be towed freely based on the self-check result of the drive motor controller. If it is determined that it can be towed freely, a trailer request instruction is sent to the transmission controller, and the transmission controller controls the gear to be shifted to N gear and waits for the trailer. If it is determined that it cannot be towed freely, a non-free towing instruction is sent to the vehicle controller and reported to the vehicle instrument. When towing a new energy commercial vehicle, damage to the electric drive system caused by the vehicle being towed freely in a fault state can be avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of a trailer system for a new energy commercial vehicle provided by the present application;

[0042] Figure 2 This is a flow chart of a trailer protection method for a new energy commercial vehicle provided by the present application;

[0043] Figure 3 This is a logic diagram of a trailer protection method for a new energy commercial vehicle provided in this application. DETAILED DESCRIPTION

[0044] The present invention will be further described in detail below in conjunction with the accompanying drawings and examples. It will be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings. It is obvious to those skilled in the art that various modifications and changes can be made in the present invention without departing from the spirit or scope of the present invention. Therefore, the present invention is intended to cover modifications and changes of the present invention that fall within the scope of the corresponding claims (technical solutions for protection) and their equivalents. It should be noted that the implementation methods provided in the embodiments of the present invention can be combined with each other without contradiction.

[0045] In view of one or more of the above-mentioned problems existing in the prior art, the present invention proposes a trailer system and trailer protection method for a new energy commercial vehicle. Before towing, the vehicle's electric drive and electronic control system is self-checked to determine the towing method and protect the vehicle. Figure 1 This is a schematic diagram of a trailer system for a new energy commercial vehicle provided by this application, with reference to Figure 1A trailer system for a new energy commercial vehicle provided in an embodiment of the present application includes a high-voltage power battery 1, a main positive relay 2, a main negative relay 3, a drive motor controller 4, a permanent magnet synchronous drive motor 5, a transmission 6, a transmission controller 7 and a vehicle controller 8.

[0046] Among them, high-voltage power batteries refer to battery systems used in electric vehicles (EV), hybrid electric vehicles (HEV) and other devices that require high energy density and high voltage power supply. They have high energy density, high voltage and high power output capabilities, and can provide a large amount of energy in a short time.

[0047] The Main Positive Relay is commonly used in the battery management system of electric vehicles (EV) or hybrid electric vehicles (HEV) to control the connection between the battery pack and the vehicle's electrical system. The main function of the Main Positive Relay is to control the connection or disconnection of the positive terminal of the battery when the vehicle is started, thereby protecting the battery and the vehicle's electrical system.

[0048] The Primary Negative Relay is an important component in an electric vehicle (EV) or hybrid electric vehicle (HEV) that controls the connection between the negative electrode (ground) of the battery and the vehicle body or chassis. The main function of the Primary Negative Relay is to connect the negative line when the vehicle starts and disconnect the negative line when the vehicle stops, thereby improving safety and simplifying circuit design.

[0049] Permanent Magnet Synchronous Motor (PMSM) is a high-performance AC motor widely used in electric vehicles, hybrid vehicles, industrial automation, household appliances, etc. PMSM generates a magnetic field through permanent magnets, which interacts with the magnetic field generated by the stator winding to generate torque to drive the motor.

[0050] The drive motor controller is an integrated circuit that actively controls the motor to work in a set direction, speed, angle, and response time. In electric vehicles, the drive motor controller can convert the electric energy stored in the power battery into the electric energy required by the drive motor according to the gear position, throttle, brake and other instructions to control the start-up, forward and backward speed, climbing strength and other driving conditions of the electric vehicle, or help the electric vehicle brake and store part of the braking energy in the power battery.

[0051] The transmission is a key component in the vehicle's transmission system, used to change the speed and torque of the power output from the engine to the wheels. Through the different gears of the transmission, it can adapt to different driving conditions and improve the vehicle's fuel efficiency and power performance.

[0052] The Transmission Control Unit (TCU) is an electronic control unit used to control the transmission in modern cars. The TCU monitors various vehicle parameters (such as vehicle speed, throttle position, engine speed, etc.) and automatically adjusts the gear position and shift timing of the transmission according to the preset control logic to achieve the best driving performance and fuel economy.

[0053] The main functions of the vehicle control unit (VCU) are to control driving torque, optimize braking energy, manage vehicle energy, maintain CAN network, diagnose and handle faults, and monitor vehicle status, etc., which are crucial to the stable operation and safety of the vehicle. Its performance directly affects the stability and safety of the vehicle. In pure electric vehicles, the AC charging controller exchanges information and handshakes with the vehicle control unit (VCU) and the battery management system (BMS) through specific messages. In new energy vehicles, VCU stands for vehicle control unit, BMS stands for battery management system, and PEU may represent other key components. These controllers and technologies are the core of new energy vehicles and play a vital role in the vehicle's power, economy, reliability and safety.

[0054] refer to Figure 1 In the embodiment of the present application, the transmission controller high-voltage power battery 1 is connected to the positive input terminal and negative input terminal of the DC bus of the drive motor controller 4 through the main positive relay 2 and the main negative relay 3, so as to provide electric energy to the drive motor controller 4. The AC output terminal (U, V, W) of the drive motor controller 4 is connected to the input terminal of the permanent magnet synchronous drive motor 5. The output terminal of the permanent magnet synchronous drive motor 5 is connected to the transmission 6. The transmission controller 7 controls the transmission 6. The drive motor controller 4 is connected to the transmission controller 7 and the vehicle controller 8 through CAN communication for signal interaction.

[0055] Based on the above embodiment, the drive motor controller 4 includes a three-phase bridge inverter circuit (not shown in the figure), which is composed of a bus capacitor and three bridge arms in parallel, each bridge arm is composed of two power switch tubes in series, and each power switch tube is also reverse-parallel connected with a diode.

[0056] Based on the same inventive concept, an embodiment of the present invention also provides a trailer protection method for a new energy commercial vehicle, which uses the trailer system provided by the above embodiment to perform a self-inspection on the vehicle's electric drive and electronic control system before towing. Figure 2 This is a flowchart of a trailer protection method for a new energy commercial vehicle provided by this application, refer to Figure 1 and Figure 2 The trailer protection method for a new energy commercial vehicle provided in the embodiment of the present application includes:

[0057] S101 . A driving motor controller receives a trailer activation signal and determines whether to enter a trailer self-test mode.

[0058] Among them, the trailer activation signal generally refers to activating the trailer mode through vehicle settings or operations to provide better traction when towing a vehicle or heavy objects.

[0059] Specifically, when the drive motor controller receives a trailer activation signal, it enters a trailer self-check mode.

[0060] S102: Determine whether the vehicle can be towed freely according to the self-test result of the drive motor controller.

[0061] Specifically, after the drive motor controller 4 receives the trailer activation signal and enters the trailer self-test mode, the self-test method of the drive motor controller is:

[0062] The drive motor controller sends a request for the N gear self-test result to the transmission controller;

[0063] When the transmission controller can execute the command to shift into N gear, it will feedback that the N gear self-test is successful.

[0064] When the transmission controller cannot execute the command to shift into N gear, it will feedback that the N gear self-test has failed.

[0065] Among them, the main function of N gear is to cut off the power transmission between the input shaft and the output shaft inside the transmission, putting the vehicle in neutral state.

[0066] It can be understood that if the vehicle transmission can be shifted into N gear, then the feedback is that the N gear self-test is successful, and it is determined that the car can be towed freely; if the vehicle transmission cannot be shifted into N gear, then the feedback is that the N gear self-test failed, and it is determined that the car cannot be towed freely and requires a professional tow truck.

[0067] S103: If it is determined that the vehicle can be towed freely, a towing request instruction is sent to a transmission controller, and the transmission controller controls the gear to be shifted to N gear to wait for the towing vehicle.

[0068] The trailer request instruction may be in the form of voice prompt, information prompt, etc.

[0069] Specifically, after determining that the vehicle can be towed freely, the drive motor controller 4 sends a towing request instruction to the transmission controller 7, and the transmission controller 7 controls the transmission 6 to shift to the N gear and wait for the towing vehicle.

[0070] S104: If it is determined that the vehicle cannot be towed freely, a command indicating that the vehicle cannot be towed freely is sent to the vehicle controller and reported to the vehicle instrument.

[0071] Among them, the faults in the self-test results of the drive motor controller that caused the vehicle to be unable to tow freely included a short circuit failure of the power switch device, which affected the safety of the entire vehicle.

[0072] Specifically, after determining that free towing is not possible, the drive motor controller 4 sends a command that free towing is not possible to the vehicle controller 8 through CAN communication, and reports it to the vehicle instrument to issue an alarm.

[0073] The inability to freely drag command may be in the form of voice prompt, information prompt, etc.

[0074] The trailer protection method provided in the embodiment of the present application has the following positive effects on the vehicle self-check before towing:

[0075] 1) Before towing, the drive motor controller performs a self-check and determines whether the vehicle can be towed freely based on the self-check result, thus avoiding damage to the vehicle caused by free towing when the electric drive system fails.

[0076] 2) Taking advantage of the two-speed or multi-speed transmission configuration of the commercial vehicle electric drive system, the drive motor controller sends a trailer request instruction to the transmission controller to shift the gear to N gear. This can avoid the damage to voltage-sensitive power devices caused by the back electromotive force generated by the permanent magnet synchronous drive motor when it is towed and rotates.

[0077] 3) Without adding any additional controllers or equipment, the vehicle design and cost are not affected, the trailer protection function is realized, and the competitiveness of the vehicle in specific scenarios is improved.

[0078] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the description, claims and drawings.

[0079] A specific embodiment is listed below to illustrate a trailer protection method for a new energy commercial vehicle provided by an embodiment of the present application. Figure 3 This is a logic diagram of a trailer protection method for a new energy commercial vehicle provided by this application, refer to 1- Figure 3 The trailer protection method for a new energy commercial vehicle provided in the embodiment of the present application includes:

[0080] Step S1, drive the motor controller to start self-check, and execute step S2.

[0081] Step S2: The driving motor controller determines whether a trailer activation signal is received. If yes, step S3 is executed; if not, step S4 is executed.

[0082] Step S3: driving the motor controller to enter the trailer self-test mode.

[0083] Step S4: The driving motor controller enters the driving mode. The trailer protection method ends.

[0084] Step S5: After entering the trailer self-check mode, the drive motor controller determines whether free towing is possible. If yes, step S6 is executed; if not, step S7 is executed.

[0085] Step S6: The driving motor controller sends a trailer request instruction to the transmission controller, and then executes step S8.

[0086] Step S7: the driving motor controller sends a command that it cannot be freely towed to the vehicle controller, and reports it to the vehicle instrument.

[0087] Step S8: The transmission controller controls the vehicle gear to be shifted to N gear.

[0088] Step S9, the drive motor controller determines whether the vehicle is erroneously free-towing by detecting the speed signal of the permanent magnet synchronous drive motor. If yes, execute step S10; if not, execute step S11.

[0089] Among them, when the vehicle is mistakenly free-towing, the permanent magnet synchronous drive motor will generate a speed signal. The drive motor controller obtains the speed signal in real time through CAN communication, and determines that the vehicle is mistakenly free-towing based on the speed signal. This free-towing will cause irreversible damage to the vehicle.

[0090] Step S10: The driving motor controller sends an alarm signal to the vehicle controller. The trailer protection method ends.

[0091] Step S11: The driving motor controller sends a signal to wait for the special trailer to tow. The trailer protection method ends.

[0092] Based on the above embodiment, before executing step S101, when the driving motor controller receives the trailer activation signal and determines whether to enter the trailer self-test mode, it also includes:

[0093] The main positive relay and the main negative relay are controlled to be turned on, and the drive motor controller exchanges signals with the transmission controller and the vehicle controller through CAN communication.

[0094] By executing this step, the trailer system of the new energy commercial vehicle can be started to ensure the circuit connectivity and CAN communication connection of the trailer system.

[0095] In summary, the trailer protection method provided in the embodiment of the present application performs a self-check on the vehicle before towing, performs a self-check through the drive motor controller, and determines whether the vehicle can be towed freely based on the self-check result, thereby avoiding damage to the vehicle caused by being towed freely when the electric drive system fails. Taking advantage of the two-speed or multi-speed transmission configuration of the commercial vehicle electric drive system, the drive motor controller sends a trailer request instruction to the transmission controller, and the gear is shifted to N gear, which can avoid the permanent magnet synchronous drive motor from generating back electromotive force when it rotates while being towed, causing damage to voltage-sensitive power devices. Without adding additional controllers or equipment, there is no impact on the design and cost of the entire vehicle, and the trailer protection function can be realized to improve the competitiveness of the entire vehicle in specific scenarios.

[0096] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, combinations and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may include more other equivalent embodiments without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A trailer protection method for a new energy commercial vehicle, which performs a self-check on the vehicle's electric drive and electric control system before towing, characterized in that: The trailer protection method comprises: The drive motor controller receives the trailer activation signal and determines whether to enter the trailer self-test mode; After entering the trailer self-test mode, determine whether the vehicle can be towed freely based on the self-test results of the drive motor controller; If it is determined that the vehicle can be towed freely, a towing request instruction is sent to the transmission controller, and the transmission controller controls the gear to be shifted to N gear to wait for the tow vehicle; If it is determined that free towing is not possible, a no free towing instruction is sent to the vehicle controller and reported to the vehicle instrument.

2. The trailer protection method according to claim 1, characterized in that: The drive motor controller receives the trailer activation signal and determines whether to enter the trailer self-test mode, including: Step S1, the driving motor controller starts self-test, and then executes step S2; Step S2, the driving motor controller determines whether it receives the trailer activation signal, if yes, then executes step S3; if no, then executes step S4; Step S3, driving the motor controller to enter the trailer self-test mode; Step S4: driving the motor controller to enter a driving mode.

3. The trailer protection method according to claim 1, characterized in that: Determine whether the vehicle can be towed freely based on the self-test results of the drive motor controller, including: Step S5, the driving motor controller determines whether free towing is possible, if yes, then executing step S6; if no, then executing step S7; Step S6, the driving motor controller sends a trailer request instruction to the transmission controller, and then executes step S8; Step S7, the driving motor controller sends a command that it cannot be freely towed to the vehicle controller, and reports it to the vehicle instrument; Step S8: The transmission controller controls the vehicle gear to be shifted to N gear, waiting for the trailer.

4. The trailer protection method according to claim 3, characterized in that: When the driving motor controller sends a command to the vehicle controller that it cannot be freely towed, it also includes: Step S9, the drive motor controller determines whether the vehicle is mistakenly free-towing by detecting the speed signal of the permanent magnet synchronous drive motor, if yes, execute step S10; if no, execute step S11; Step S10, the driving motor controller sends an alarm signal to the vehicle controller; Step S11, the driving motor controller sends a signal to wait for the dedicated trailer to tow.

5. The trailer protection method according to claim 1, characterized in that: The self-test method of the drive motor controller is: The drive motor controller sends a request for the N gear self-test result to the transmission controller; When the transmission controller can execute the command to shift into N gear, it will feedback that the N gear self-test is successful; When the transmission controller cannot execute the command to shift into N gear, it will feedback that the N gear self-test has failed.

6. The trailer protection method according to claim 1, characterized in that: The faults in the self-test results of the drive motor controller that cause the vehicle to be unable to be towed freely include a short circuit fault of a power switch device.

7. The trailer protection method according to claim 1, characterized in that: Before the driving motor controller receives the trailer activation signal and determines whether to enter the trailer self-test mode, the method further includes: The main positive relay and the main negative relay are controlled to be turned on, and the drive motor controller exchanges signals with the transmission controller and the vehicle controller through CAN communication.

8. A trailer system for a new energy commercial vehicle, used to execute the trailer protection method for a new energy commercial vehicle according to any one of claims 1 to 7, characterized in that: include: High-voltage power battery, main positive relay, main negative relay, drive motor controller, permanent magnet synchronous drive motor, transmission, transmission controller and vehicle controller; The high-voltage power battery is connected to the positive input terminal and the negative input terminal of the DC bus of the drive motor controller through the main positive relay and the main negative relay; The AC output terminal of the drive motor controller is connected to the input terminal of the permanent magnet synchronous drive motor; The output end of the permanent magnet synchronous drive motor is connected to the transmission; The transmission controller controls the transmission; The drive motor controller is connected to the transmission controller and the vehicle controller via CAN communication to perform signal interaction.

9. The trailer system according to claim 8, characterized in that: The drive motor controller comprises a three-phase bridge inverter circuit, which is composed of a bus capacitor and three bridge arms in parallel, each bridge arm is composed of two power switch tubes in series, and each power switch tube is also connected in reverse parallel with a diode.

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