| The discovery of the X-Ray is extremely significant for the development of medical history, which opens up a new path for the medicine and science research. The application of X-Ray firstly got responses from clinical doctors, applied to looking inside the human body structure. The X-Ray machine has become the first medical imaging equipment in clinic, which has been100years of development history.The first X-Ray generator was discovered in a year later after the discovering of X-ray, developed by Roentgen, the discoverer of X-Ray, and their research team. Richard Seifert and Co manufactured the world’s first medical X-Ray machine. In the beginning of the20th century, the conventional X-Ray machines were first to be applied to clinic. Digital technology was applied to X-Ray equipment in the end of the1970s, and eventually developed into the dominant technology in modern imaging. In the late of1990s, high frequency high voltage generator came out, which is a huge leap in the history of X-Ray equipment’s development. Under the work environment of high frequency, the request of output filter capacitor reduced, and the ripple of the output high voltage waveform became smaller, improving the efficiency of the high voltage generator while decreasing its volume and weight.With the rapid development of science and technology, the high precision imaging equipment among the hospital equipments, such as ultrasound, MRI, X-Ray equipment (including ordinary X-Ray machine, digital X-Ray machine, and computed tomography (CT)) and so on. In these equipments, because the technology of X-Ray equipment is more mature, so it has become the major medical diagnostic equipment in each big hospital.X-Ray equipment not only is the foremost diagnostic device, but also will still play the most important part in medical diagnostic devices for quite a long period of time. In the process of accelerated urbanization in China, the hospitals are developing and the equipment is updated, which would need large amount of X-Ray equipment.Traditional X-Ray machine operating frequency is fifty hertz with the high voltage generator. With the technology progressing and new electronic components and electronic technology applied to high voltage generator, the middle frequency generator appeared, then the high frequency generator.The high frequency generator has been the mainstream technology of X-Ray equipment since its advent. It will not even change largely in the coming short time, unless the theories and methods of X-Ray imaging equipment generate revolution on theory.Medical X-Ray equipment is mainly composed of a high frequency high voltage generator, an X-Ray tube, an X-Ray detector and so on. The high-frequency and high-voltage generator, one of the core components of the X-Ray equipment, plays the role of the heart and brain in the X-Ray equipment. So the research and design of high frequency high voltage generator in this paper which is suitable for all types of X-Ray equipment has an important practical significance.In the research and design of high frequency and high voltage generator, the products abroad have advanced technology, good performance and well craft. Compared to the mainstream companies, the high frequency high voltage generator made in our country exists a large disparity, no matter in the performance or the manufacturing craft.Our country has a late start in the development of high frequency high voltage generator. Only some manufacturers began to research and develop independently in about2000. After decade or so to catch up, there is still a disparity in the products similar to foreign countries, and difficult to achieve the standard of developed countries.The high-frequency and high-voltage generator designed in this study is based on high frequency switching power supply technology and SCM control technology, which mainly uses ATmega128AVR microcontroller. There are four technical difficulties in the implementation of system solutions, which we base on to study.The first difficult is the full bridge inverter technology. We need to rectify the power frequency AC voltage to smooth DC voltage, in order to change the power frequency to high frequency. Then the inversed full bridge is composed of high-speed switches, inversing the DC voltage to high frequency AC voltage.The second difficult is the voltage increasing convert technology. We make full use of the high-frequency and high-voltage convert to increase the AC voltage obtained from the inversed full bridge.The third is doubling voltage rectifier technology. The voltage output from the voltage increasing convert is still AC voltage, while the X-Ray tube needs DC voltage. So the sub-output voltage from voltage convert should be rectified. If we directly rectify the voltage, the voltage may not achieve the standard. That’s the reason why the doubling voltage convert is needed.The fourth is FM control technology. The high output voltage which is a feedback for the control circuit should be sampled, achieving precise control and automatic adjustment of the output voltage to stabilize it at the voltage value set.In this paper, I expound the rationale of high frequency high voltage generator. I have talked about the principle of X-Ray in physics. I also explain the external condition for generating X-Ray from a macro point of view, citing a commonly used measure to evaluate X-Ray. What’s more, we elaborate the principle of adjustment to control the output voltage from a mathematical point. Finally we design a series of system plans to accomplish. I have detailed every module such as power system module, control system module, inverse circuit module, high frequency high voltage transformer, doubler rectifier filter, sampling feedback circuit modile and so on.The power system module in the design is a key switch electromechanical road and pre-charging detection circuit to control the system power supply. A key switch circuit can control system power distribution. When pressing the power on button, the generator first provides low voltage to guarantee the normal work of the control circuit, then does not provide high voltage until the system stable. When press the power off button, the equipment connected to the grid can be cut off. The high voltage power supply of the charging circuit can be first on the advance charge, and doesn’t charge until the voltage reaching a certain voltage value, in order to prevent the instant surge current when switching on. And power supply system takes multiple measures such as fuses, relays, and so on to ensure the safety of equipment.Some system modules of the high frequency high voltage generator work normally with low voltage, but they usually need many power supply. In the allocation of power supply, the power supply which has high requirement is the switch power with mature technology bought directly in the market. This is not only beneficial to improve the design efficiency, but also beneficial to reduce the cost. While the power supply with relatively low requirement will be designed and producted by myself.The biggest advantage of the pulse frequency modulation (PFM) is that the transformer at work will not generate DC magnetic component. The work loop current of the transformer is sine wave to ensure the high work efficiency. So now the mainstream of the high frequency high voltage generator largely adopts frequency modulation control technology.The pulse frequency modulation used in scenario is MC33067chip, which is an integration of PFM chip. Its different control voltage corresponds to the different output frequency. The chip is based on the summation of the reference voltage and error voltage to adjust the output waveform. The reference voltage to determine the operating frequency of the circuit; error voltage to be adjusted to the operating frequency of the circuit, under the joint action of the two, to complete the frequency control, and when the error voltage is zero, indicating that the actual output and the value of the set are equal.The full bridge inverter inverses the DC to high frequency AC. The four bridge arms of the whole bridge inverter consist of four identical bridge arm units, which is composed of high-speed switching frequency switching tube controlled by the driving sequence.Because output voltage value of X-Ray equipment is very high, the primary and secondary transformer ratio of the high frequency high voltage transformer is rather big. According to the requirement of the design, we design the primary and secondary windings of the high frequency transformer, the type of the magnetic core and the structure of the transformer.This scheme takes LC resonant circuit as the output load of the inverter bridge, takes the output high frequency alternating current (ac) of the inverter bridge as the input signal of the resonant circuit. The high frequency and voltage transformer is equivalent to leakage inductance and an ideal transformer in series. So the transformer leakage inductance is just used as the resonant inductance of resonance loop circuit, and the ideal transformer as the load, transmitting the energy to the transformer secondary.In the case of the constant output dc voltage amplitude, it takes full use of the characteristics that the capacitor can store energy to realize voltage doubling rectifying filter. The voltage doubling rectifying filter technology can reduce the output secondary voltage of the transformer. Thus it transfered part of transformer pressure requirements for voltage to doubling rectifying filter circuit, reducing the pressure requirement of transformer. Transformer is designed for two separate secondary output, further reducing the withstand voltage requirements, at the same time reducing the withstand voltage requirement of the voltage doubling rectifier diodes and filtering capacitance in the rectifier filter circuit.The research has focused on four key technology of the high frequency high voltage generator systems, and has obtained the certain effect in the full bridge inverter, frequency control and voltage doubling rectifying filter. Only the booster transformer technology is rather immature. Though it can be used to do experiments, but it still need further improvement.The high frequency high voltage generator is an equipment combined with the current all kinds of advanced technology. So it always develops synchronously with various new technologies, including semiconductor technology, computer and network technology, power technology and so on. The emergence of a new technology will soon be able to improve the high frequency high voltage generator, which reflected from power frequency machine to high frequency machine. So the high frequency high voltage generator technology will have greater breakthroughs in the future. Doctors and patients will also propose higher requirements in the process of using it, which will inevitably promote the X-Ray equipment technology to improve again. So these new requirements are also the new issues for researchers on X-Ray equipment to face. |
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