Abstract: This paper introduces the technical requirements of a high-voltage DC power supply for an electron beam welding machine used in bimetallic saw bands. Based on the characteristics of high-voltage power supplies for electron beam welding machines, a vacuum tube direct-adjustable high-voltage DC power supply is proposed. The main circuit structure and working principle of the control circuit are described in detail. Tests and field operation show that all technical indicators of the power supply have met the expected design goals, exhibiting high stability and low ripple voltage. Keywords: Electron beam welding machine, bimetallic saw band, high-voltage DC power supply, stability, ripple 1 Introduction Electron beam welding is widely used in aerospace, nuclear energy, defense and military, automotive, and electrical instrumentation industries due to its advantages of not requiring welding rods, being less prone to oxidation, having good process repeatability, and low thermal deformation. The basic principle of electron beam welding is that the cathode in the electron gun emits electrons due to direct or indirect heating. These electrons are accelerated by a high-voltage electrostatic field and then focused by an electromagnetic field to form an electron beam with extremely high energy density. This electron beam bombards the workpiece, converting the enormous kinetic energy into heat energy, melting the workpiece at the welding point to form a molten pool, thus achieving welding. Based on the basic principle of electron beam welding, Western countries developed a new process production line for electron beam welding of bimetal saw bands in the late 1970s, replacing the traditional ordinary high-speed steel saw band production process, thus saving a lot of high-speed steel and improving the service life of the saw band. Bimetal saw band is a new type of saw band obtained by electron beam welding of spring steel with good elasticity and high-speed steel with strong cutting ability. In the late 1980s, my country successively imported several production lines from Germany to meet the needs of the rapidly developing domestic market, but it still cannot fully meet its market requirements [1]. Since electron beam welding involves multiple disciplines such as mechanics, vacuum, high voltage and electromagnetic field theory, electron optics, automatic control and computer, it is technically difficult for general domestic manufacturers, and the import cost is expensive. Therefore, Guilin Electrical Science Research Institute combined foreign technology and many years of experience in electron beam technology research and development to successfully develop my country's first domestic bimetal saw band production line equipment. Among them, high voltage power supply is one of the key technologies of bimetal saw band welding equipment. It mainly provides acceleration voltage for electron gun, and its performance directly determines the electron beam welding process and welding quality. Therefore, many electron beam welding machine manufacturers and research institutions have studied the reliability of high-voltage power supplies, high-voltage protection, and the impact of high-voltage arcing on the weldment, and have accordingly manufactured high-voltage power supplies with higher performance to meet the needs of different electron beam welding machines [2-3]. Since bimetallic welding requires parallel weld seams, high-voltage electron beam welding machines (above 100kV) are used to weld bimetallic saw blades. Currently, China cannot produce high-voltage electron beam welding machines, making the development and research of high-voltage power supplies essential. This article introduces the high-voltage power supply used in high-voltage welding machines on bimetallic saw blade production lines. Its control method and high-voltage protection technology differ from other types of high-voltage power supplies, providing valuable insights for research on high-voltage power supply regulation technology and high-voltage arcing protection. 2 Technical Requirements for High-Voltage Power Supplies for Electron Beam Welding Machines High-voltage power supplies for electron beam welding machines have different technical characteristics compared to other types of high-voltage power supplies. Based on the factory standards of foreign electron beam welding machine manufacturers, the German DIN standard, and the technical requirements of electron beam welding machines in my country, the specific requirements for high-voltage power supplies used in electron beam welding machines are as follows: 2.1 Technical Requirements Since there is no unified standard for high-voltage power supplies used in electron beam welding machines both domestically and internationally, the main technical requirements proposed by some manufacturers are ripple coefficient and stability. The ripple coefficient is required to be less than 1%, and the stability is ±1%. Almost all electron beam welding machine manufacturers have proposed these requirements. PTR of Germany has also proposed technical requirements for medium-voltage types, requiring a relative ripple coefficient of less than 0.5%, a stability of ±0.5%, and a repeatability requirement of less than 0.5%. All of these requirements are determined based on the electron beam spot and welding process. 2.2 Performance Requirements High-voltage power supplies used in electron beam welding machines must be interlocked with relevant systems during operation, mainly including vacuum interlock, cathode interlock, gate valve interlock, and focusing interlock, to ensure equipment and personal safety. The high-voltage power supply must comply with EMC standards and have a soft-start function to prevent the power supply from being impacted by sudden switching. 2.3 Other requirements include high reliability, being an indoor device requiring continuous operation, meeting industrial equipment appearance requirements, and ease of maintenance. 3. Working Principle and Main Circuit of the High-Voltage Power Supply The system block diagram of the high-voltage power supply is shown in Figure 1. The grid voltage passes through the current suppression circuit and enters the primary and secondary windings of the high-voltage step-up rectifier transformer, where it is stepped up to approximately 100kV. This AC high voltage is then rectified and filtered by a 12-phase rectifier to obtain a DC high voltage of approximately 160kV, which is applied to the high-voltage vacuum tube and electron gun. The high-voltage vacuum tube withstands a voltage of 40kV during operation, resulting in a high voltage of 120kV applied to the electron gun. The high-voltage vacuum tube is used to regulate and stabilize the high-voltage output. The main circuit diagram of the high-voltage power supply designed according to Figure 1 is shown in Figure 2. As shown in Figure 2, this power supply is a typical series-type high-voltage DC regulated power supply with direct regulation on the high-voltage side. Its main circuit consists of an overcurrent suppression circuit, a high-voltage step-up rectifier transformer, a high-voltage rectifier circuit, a high-voltage filter, and resistor-capacitor and overvoltage/overcurrent protection circuits, as well as a high-voltage vacuum tube regulation circuit. The high-voltage step-up rectifier transformer, high-voltage rectifier circuit, high-voltage filter, and RC and overvoltage/overcurrent protection circuits are all housed in an oil tank filled with transformer oil to ensure the power supply unit's insulation and heat dissipation during operation. Because the high-voltage power supply needs to operate continuously, a water cooling system is also designed inside the oil tank to ensure timely heat dissipation. The composition and function of each circuit in the power supply are as follows: The overcurrent suppression circuit consists of a three-phase bridge rectifier circuit and a choke inductor. If an overcurrent occurs in the load or a large current appears due to electromagnetic transients caused by a sudden switch-on within the transformer, the overcurrent suppression circuit effectively limits the overcurrent within the power supply to protect it from damage. Its principle mainly utilizes the characteristic that inductor current cannot change abruptly to limit overcurrent and ensure the high-voltage transformer is not damaged. Under normal conditions, the three-phase currents are balanced, and the current flowing into the overcurrent suppression circuit is very small. The secondary winding of the rectifier transformer has four coils connected in two star and two delta configurations. These rectified coils are connected in series to obtain a 12-phase DC pulsating voltage. This helps reduce harmonic current pollution to the power grid, decreases filter capacitance, reduces the power supply ripple coefficient, and improves the power supply's performance. The rectifier circuit consists of a high-voltage silicon stack and resistive-capacitive (RC) components. The RC circuit primarily prevents overvoltage generated by the high-voltage silicon stack, ensuring its safety. Current-limiting resistors and protective resistors limit internal overcurrent and overvoltage, ensuring normal operation. These resistors require high voltage withstand capabilities and can handle significant heat generation. In the event of an external short circuit, the protection circuit should operate within the shortest possible time to prevent damage to relevant components. High-voltage filter capacitors remove AC pulsating components from the DC output, ensuring a flat voltage across the electron gun and regulating tube. The capacitor cores are placed directly inside the high-voltage tank, reducing the overall size of the power supply. The regulating tube is a multi-electrode high-voltage vacuum tube with a working withstand voltage of up to 160kV. It mainly consists of an anode, a control electrode, and first, second, and third anodes. The anode of the regulating tube is connected to the positive terminal of the high-voltage rectifier, and the cathode is connected to ground through a beam sampling resistor. The regulating tube can automatically adjust and stabilize the high-voltage output under the control of the control circuit. Due to its high operating voltage, the high-voltage regulating tube generates a large amount of heat during operation. Therefore, a special heat dissipation device is designed, placing the entire regulating tube in an oil tank filled with insulating oil to ensure cooling and insulation. The oil tank also contains a dedicated water-cooling system to ensure long-term reliable operation of the regulating tube. The regulating tube has multiple auxiliary power supplies, which are also placed inside the oil tank for heat dissipation and layout considerations. The regulating tube's adjustment principle is that its cathode emits electrons due to heating. Accelerated by the high voltage at the anode, these electrons reach the second anode and the third anode. If the voltage at the second anode is very high, all the accelerated electrons will reach the second anode. At this time, the vacuum tube is in a high-resistance state, and the entire voltage from the power supply is applied to the regulating tube. By adjusting the voltage of the second anode, the voltage on the regulating tube can also be adjusted, thus regulating the high voltage applied to the electron gun, ultimately achieving stable regulation of the high voltage output. 4. Power Supply Control Circuit The power supply control circuit is shown in Figure 3. As shown in Figure 3, the control circuit consists of a feedback signal isolation circuit, a PI setpoint regulation circuit, an automatic high voltage reapplied circuit, a power amplifier circuit, and its auxiliary circuits. The composition and working principle of each circuit are as follows: 4.1 Feedback Circuit The feedback circuit consists of a high-voltage resistor divider, a signal isolation circuit, and overvoltage and overcurrent suppression components. The high-voltage resistor divider is made of two independent precision metal film resistors. One is used to measure the high voltage, and the other provides a feedback signal to the control circuit for controlling and regulating the high voltage. The high-voltage resistor divider is placed on a bracket made of plexiglass. Considering the need for insulation and heat dissipation, it is placed in a high-voltage oil tank to ensure the stability of the resistance value during power supply operation, ultimately ensuring the stability of the sampling signal. When the high voltage power supply is working, due to the discharge of the electron gun or other external reasons, the power supply will generate overvoltage. In order to prevent the overvoltage from entering the control circuit and damaging the low voltage electronic components, a high voltage discharge tube, a varistor and a capacitor absorption circuit are connected in parallel across the sampling resistor. At the same time, the sampling resistor is placed in an electromagnetic shielding box, which can effectively prevent various electromagnetic interference signals from entering the control circuit. Before the sampling signal of the high voltage divider enters the control circuit, a signal isolation circuit is also set to isolate the feedback signal from the control circuit and convert it into a standard level signal for the control circuit. The above measures ensure the accuracy of the feedback signal in the control circuit. 4.2 PI regulation circuit The PI regulation circuit [4] consists of an operational amplifier and external resistors and capacitors. It compares the given signal with the feedback signal. The difference is amplified and given to the pre-amplifier tube to control the output of the amplifier tube. A debugging given circuit is also set in the PI regulation circuit. Its purpose is for the debugging of the high voltage power supply. During the test, the given signal is supplied by Rtest. By adjusting the value of Rp, the high voltage output can be adjusted from zero to the rated value, which is beneficial for welding process test and high voltage power supply parameter adjustment. 4.3 Automatic High Voltage Reapplying Circuit The principle of the automatic high voltage reapplying circuit is to utilize the control principle of transistors to achieve rapid cutoff and on/off of high voltage. It consists of operational amplifiers and transistors, among other circuits. Its working principle is as follows: when the feedback signal exceeds the given signal, the output of proportional amplifier IC2 is high, V4 is on, IC3 outputs low, V3 is on, and V2 is off, blocking the output of the PI regulator, thereby turning off the high voltage regulating transistor to cut off the high voltage. Conversely, when the feedback signal is less than the given signal, IC2 outputs low, V4 is off, IC3 outputs high, V3 is off, the PI regulator works normally. Because the transistor's recovery time from on to off is very fast, the high voltage applied to the electron gun quickly returns to normal operation under the control circuit without stopping the machine, ensuring normal production of the electron beam welding machine. 4.4 Power Amplifier Circuit The power amplifier circuit consists of a preamplifier tube VL33 and a power amplifier tube VL32. The operation is as follows: under the influence of a negative power supply, the adjustment input from the PI regulator is amplified by V2 and sent to the control electrode of the vacuum tube VL33. The anode is connected to the positive terminal of the auxiliary power supply, and the cathode is grounded. The higher the voltage at the control electrode (negative), the higher the anode voltage of VL33 to ground, the higher the anode voltage of the high-voltage regulating tube VL32, and the lower the voltage on the electron gun. Conversely, the control circuit adjusts the high voltage on the electron gun according to the reverse process, ultimately achieving stability of the high voltage on the electron gun. 5. Technical Specifications of the High-Voltage Power Supply When applied to the bimetallic saw band welding production line, the high-voltage power supply operates stably. The specific parameters measured by the power supply are as follows: Rated accelerating voltage: 120kV, ripple coefficient <1%, stability <1%; Rated electron beam current: 50mA, ripple coefficient <1%, stability <1%. When the power supply sparks inside the electron gun, the high-voltage power supply can quickly recover without stopping the machine. 6 Conclusion The output of the high-voltage DC power supply is regulated by the high-voltage vacuum tube on the high-voltage side. It has good output characteristics, low ripple coefficient and high stability. Due to the isolation filter capacitor of the regulating tube, the energy on the capacitor will not be discharged to the workpiece and cause damage to the workpiece when the power supply is shut down by overvoltage protection. After actual operation on the bimetallic saw production line, all technical indicators of the power supply meet the process requirements of the production line. References 1 Ye Hanmin. Market development prospects of hand bimetallic saw belt. Applicable Technology Market, 1996 (6): 40-43 2 Ye Hanmin. A high-performance high-voltage power supply for electron beam welding machine. Electrical Engineering Technology Magazine, 2000 (1): 48-50 3 Ye Hanmin. High voltage technology problems in electron beam welding machine. High Voltage Technology, 1999 (2): 50-53 4 Kang Huaguang. Basic Electronic Technology. Beijing: Higher Education Press, 1987 (end)