Design of electron irradiation high voltage power supply based on soft switching

Introduction

The use of electron irradiation technology in food processing can improve the quality of food and enhance food safety. As an ion beam acceleration device, the accelerator tube is a key device in the electron irradiation engineering device. In order to meet the irradiation measurement requirements of different irradiated objects, the electron gun high voltage power supply needs to be continuously adjustable. At the same time, since the accelerator tube is in a pulsed working state, and the entire biological irradiation engineering device needs to be in a long-term standby state according to the transmission needs of the irradiated object, the high voltage power supply is required to have good no-load characteristics. This article introduces a practical design of an electron gun DC high voltage power supply with good characteristics.

1 Main technical parameters

The electron gun DC high voltage power supply adopts 380V/50Hz three-phase four-wire input, the maximum output DC voltage is negative polarity 70kV, and the power supply operates for a long time at about -60kV. Under the condition of meeting the stability requirements, the output DC high voltage voltage is continuously adjustable in the range of not less than -40~-70kV.

The beam load pulse current of the accelerating tube electron gun is about 1A, and the pulse width is no more than 5μs. Under the condition of the accelerating tube pulse beam load, the output voltage stability is better than 0.3%. In addition, the power supply must meet two working conditions, namely the normal use working mode of the accelerating tube and the vacuum aging working mode of the accelerating tube electron gun.

2 Design scheme and working principle

Since the output of the 70kV DC high-voltage power supply requires high stability and dynamic response, and the insulation equipment of the high-voltage part of the power supply output is strict, in order to reduce the volume of the power supply, reduce weight, reduce losses, and improve efficiency, a high-frequency switching power supply design scheme is selected. Considering that the load of this high-voltage power supply is the electron beam of the electron gun with impedance varying between MΩ and kΩ, when the beam is extracted, under the condition of a certain internal resistance of the power supply, in order to ensure that the output voltage of the power supply meets the load's empty and full load changes and ensure the high requirements of output voltage stability, a two-stage power processing circuit is used in the design of the power supply.

The input front stage of the entire power supply adopts a switch-type pre-stabilization measure, and the back stage adopts a bridge conversion circuit with soft switching characteristics. The circuit block diagram of the entire power supply is shown in Figure 1.




长尾关键字链接

The electron gun high voltage DC power supply is required to have a wide range of voltage output, which is difficult to achieve by using a general single-stage converter to adjust the duty cycle to change the output. Therefore, adding a BUCK pre-stabilization circuit can effectively adjust the output from zero voltage to the rated output. The specific circuit is shown in Figure 2.

After the three-phase AC power distribution passes through the rectifier and filter circuit, a DC voltage of about 500V is formed and applied to the input end of the BUCK pre-stabilization circuit. Among them, K1 is set to suppress the closing surge voltage, and its second-speed pull-in signal is given by the power supply control and display circuit. S2 is the BUCK power switch tube, and its working pulse is given by the PWM control circuit. In order to ensure that the power supply has sufficient dynamic characteristics, the working frequency of the power switch tube is about 100kHz. The PWM controller outputs a pulse width, which is given by the voltage setting signal. When the voltage setting signal changes from 0 to 10V, the pulse duty cycle output by the PWM controller varies in the range of 0 to 0.85. L2 is an energy storage inductor. Since the duty cycle of the BUCK circuit varies in a wide range of 0 to 0.85, in order to ensure that the circuit plays a pre-stabilization role, it is necessary to avoid discontinuous output current, and at the same time, the design margin is not too large. The calculated value under rated load conditions is used, and the actual working condition is good.

2.2 Series resonant converter

The series resonant circuit has good anti-short circuit characteristics and is more adaptable to "sparking" faults caused by a sharp drop in vacuum, such as vacuum tubes; its rectifier output does not require a filter inductor, which can reduce the withstand voltage requirements of the high-voltage rectifier silicon stack; the biggest advantage of the resonant power converter is high efficiency and low noise. However, in the electron gun high-voltage power supply, since a high voltage of 70kV is required, the transformation ratio of the high-frequency switching transformer must be very large, and the equivalent capacitance of the secondary winding of the transformer is also significantly increased. Distributed parameters such as the equivalent capacitance of the transformer will significantly affect the operation of the converter. If the traditional series resonant converter topology is adopted, the reactive power of the converter will be significantly increased. Large reactive power will bring large current stress to the power switch tube and the high-frequency switching transformer, affecting the reliable operation of the converter. To this end, considering the load characteristics and working state of the 70kV electron gun DC high-voltage power supply, an improved series resonant converter topology circuit is adopted to reduce the influence of distributed parameters on the converter. Figure 3 shows the circuit diagram of the improved series resonant converter.

The improved series resonant converter circuit has a dual slope output characteristic with current and voltage limits. Inductors L2 and L3 are resonant inductors. C1 is a resonant capacitor.

The current waveform and drive signal waveform of the resonant tank circuit of the circuit are shown in Figure 4. The output power of the converter is controlled by adjusting the α angle.

The maximum power curve (α=1) in Figure 4 corresponds to the open-loop operation of the converter, and the curve at the bottom of Figure 4 corresponds to the light load condition. When working at the normalized output current I*o=0.15% and ψ=1, the result is an overvoltage of about 30%. The output characteristic of the improved converter is a dual-slope output characteristic, which improves the inherent ability of the circuit to limit voltage and current, improves the adaptability of the high-voltage power supply at no load and full load, and is conducive to the wide range of output regulation of the high-voltage power supply.

L1 and L4 are tightly coupled inductors (L3=L4), and clamping diodes D5 to D8. Different from the inductor absorption circuit, this circuit focuses on solving the peak conduction of the circuit, and can limit the peak current to about Ist=Vindc/2(L3+M). Because a tightly coupled inductor (coupling factor k=0.95) is used, Ist is approximately equal to Vindc/4*L3. Although the introduction of inductance increases the number of components and cost of the circuit, it can significantly change the power factor of the traditional circuit under light load and limit the disturbance peak current inherent in the traditional circuit.

The entire converter circuit still adopts the PFM working mode. When the front-stage pre-stabilized power supply outputs a DC voltage of 0 to 500V, the converter drives the power switch tube of the bridge arm with a fixed pulse width. In order to ensure that the power supply can work stably when the electron gun beam is generated, a sampling closed-loop control function is designed. When the output deviates from the voltage setting value, the converter obtains a stable output through pulse frequency adjustment. The actual operating frequency of the electron gun high-voltage power supply is 3 to 25kHz. The improved series resonant converter is shown in Figure 5, and the actual operating current waveform of the converter is shown in Figure 6.

2.3 Design of high-voltage transformer

The output voltage of the power supply is relatively high, and the secondary of the high-frequency high-voltage transformer adopts a multi-stage voltage doubling circuit to reduce the transformation ratio of the high-frequency switching transformer. However, the high-voltage transformer is still a key component in the high-voltage power supply. The design focus of the high-voltage transformer is how to reduce the distributed capacitance and ensure high-voltage insulation. Based on the above considerations, the primary winding is arranged and distributed as follows.

In order to reduce the distributed capacitance between the high-voltage winding and the low-voltage winding and ensure the insulation between the high-voltage winding and the low-voltage winding, it is necessary to wind the primary winding on one side of the rectangular core and the secondary winding on the other side of the rectangular core, and leave enough distance between the primary and secondary sides, and finally use epoxy resin injection as the main insulation.

In order to reduce the distributed capacitance of the high-voltage winding, it is necessary to adopt the secondary winding segment winding method, which is composed of multiple windings connected in series.

In order to ensure the high-voltage insulation between the secondary winding and the core and the interlayer insulation of the secondary winding, the composite dielectric insulation of polyaromatic fiber paper and epoxy resin injection produced by DuPont is used.

The secondary winding starts at a low potential and is close to the core end, while the secondary winding ends at a high potential, so sufficient insulation distance from the core end is required. Finally, epoxy resin is poured for insulation.

The winding arrangement distribution diagram is shown in Figure 7.

2.4 Converter control circuit and power supply control and display circuit

The converter control circuit mainly provides control signals and protection signals for the two-way switching power converter of the power supply. Its main feature is that the PWM pulse width required by the front-stage BUCK pre-stabilization conversion circuit is related to the frequency of the PFM of the rear-stage series resonant conversion, and the respective required signals are generated based on the same voltage setting signal. In this way, the requirement of continuous adjustable output voltage is guaranteed, and at the same time, under the rated load working conditions, the two-stage converter is at the best engineering design value, reducing the unnecessary voltage and current stress of the converter, which is conducive to the stable operation of the power supply.

In terms of the protection function of the converter, fault detection such as primary overvoltage, secondary overvoltage, and converter overcurrent is added. When the primary overvoltage occurs, the power supply is in an intermittent working state. After the overvoltage fault disappears, the power supply automatically resumes working. When the secondary overvoltage occurs, the power supply stops working directly, locks the fault state, and gives a fault indication until the power supply is reset and restarted to work normally. The

power supply control and display circuit mainly completes the control of the working sequence of the entire power supply, the output voltage detection indication, and the necessary personal safety protection measures for the high-voltage power supply. The circuit uses a single-chip microcomputer as the core control device. The output power sampling signal is converted into digital quantity through a high-precision A/D converter, and used as a local indication through a four-digit digital display. The panel button signal realizes the local power on/off timing control and the local power output voltage setting through the program controlled by the single-chip microcomputer.

3 Conclusion

The entire power supply has been delivered after debugging and is in operation with the entire biological irradiation engineering device. The output voltage characteristics meet the working requirements of the accelerator tube.