核磁共振梯度功放EMI抑制
发布时间：2018-07-15 17:34
【摘要】：在核磁共振成像(Magnetic Resonance Imaging,MRI)系统中,为了对主磁场中的成像物体进行空间定位,需要在x,y,z三个方向上施加线性变化的梯度磁场,获取具有位置信息的磁共振信号,以重建出MRI三维图像。梯度功放的功能是为梯度线圈提供强度线性变化、低纹波、可快速开启和关闭的电流以产生梯度磁场。在核磁共振梯度功放研制的过程中,由于大功率开关管IGBT在开关过程中产生的高频脉冲信号具有很高的电压与电流变化率(dv/dt与di/dt),会形成很强的电磁干扰(Electromagnetic Interference,EMI),其频率从几千赫兹到几十兆赫兹,可能远远超过电磁兼容(Electromagnetic Compatibility,EMC)要求的极限值。本文根据厂家核磁共振梯度功放设备的研发要求,主要针对功率放大电路工作时所产生的EMI干扰,重点研究了EMI的产生原因、耦合机理,并提出了对EMI进行有效抑制的措施。本文主要工作内容包括:1.分析了梯度功放系统中电路元件在高频干扰作用下的特性。基于电磁干扰的三要素,研究了梯度功放电磁干扰源、耦合途径、传导以及辐射耦合机理。2.根据IGBT的开关特性及项目课题任务要求,设计了全桥双并联结构的梯度功放拓扑结构。讨论了栅极电阻的作用,对其阻值的选取进行仿真分析以及实验测试,详细分析了母线寄生电感产生的尖峰电压对于IGBT开关过程的影响、以及母线回路通过快速变化的大电流时产生的电磁辐射对于相邻轴向的干扰,提出了功率电路板的改进方案。考虑到IGBT并联使用时的降额要求,使用无源均流电路对并联拓扑结构进行均流设计。3.探讨了缓冲电路对于IGBT开关脉冲电压的抑制作用,给出了缓冲电路的模型以及常用的拓扑结构并分析了其工作过程。设计了适用于课题需求的吸收电路,并对其电路元件的参数选取进行了理论分析。对比分析普通绕线电感与磁环电感的高频特性,最终采用磁环电感与滤波电容电路构成梯度滤波器。对磁环的选型、电容容值的选取、滤波器的截止频率进行理论分析与实验测试,同时对梯度滤波器的安装注意事项进行说明。4.根据EMI的耦合机理以及电磁场的传播特性。设计了整机三维结构图并搭建了应用在永磁磁共振成像系统的梯度功放测试样机。根据调试结果分析了电磁兼容问题以及机箱结构,机箱屏蔽,接地等措施对于EMI抑制的作用及意义。
[Abstract]:In the magnetic resonance imaging (MRI) system, in order to locate the imaging object in the main magnetic field, it is necessary to apply a gradient magnetic field with linear variation in three directions to obtain the magnetic resonance signal with position information. To reconstruct three-dimensional MRI images. The function of gradient power amplifier is to provide gradient coil with linear variation of intensity, low ripple, and can quickly open and close the current to generate gradient magnetic field. In the process of development of nuclear magnetic resonance gradient power amplifier, Because the high frequency pulse signal produced by high power switch tube (IGBT) has high voltage and current change rate (dv/dt and di/dt), it will form strong electromagnetic interference (EMI), its frequency range from several thousand hertz to dozens of megahertz. May far exceed the limit required for electromagnetic compatibility (EMC). According to the R & D requirements of nuclear magnetic resonance gradient power amplifier equipment, this paper focuses on the cause and coupling mechanism of EMI, and puts forward some effective measures to restrain EMI. The main work of this paper includes 1: 1. The characteristics of circuit components in gradient power amplifier system under high frequency interference are analyzed. Based on the three elements of electromagnetic interference, the electromagnetic interference source, coupling path, conduction and radiation coupling mechanism of gradient power amplifier (GPA) are studied. According to the switching characteristics of IGBT and the task requirements of the project, the gradient power amplifier topology of the full-bridge double-parallel structure is designed. The function of gate resistance is discussed, the selection of resistance value is simulated and tested, and the effect of peak voltage generated by parasitic inductor on IGBT switching process is analyzed in detail. The improvement scheme of power circuit board is put forward because of the interference of electromagnetic radiation to the adjacent axis caused by the fast changing high current in the bus circuit. Considering the requirement of reducing the amount of IGBT in parallel operation, the current sharing design of parallel topology using passive current-sharing circuit is carried out. The suppression effect of buffer circuit on IGBT switching pulse voltage is discussed. The model of buffer circuit and the common topology are given and its working process is analyzed. The absorption circuit suitable for the task is designed, and the parameter selection of the circuit component is analyzed theoretically. The high frequency characteristics of common winding inductors and magnetic loop inductors are compared and analyzed. Finally, the gradient filter is composed of magnetic loop inductors and filter capacitors. The selection of magnetic ring, capacitance capacity, cut-off frequency of filter are theoretically analyzed and tested. At the same time, the attention of the installation of gradient filter is explained. According to the coupling mechanism of EMI and the propagation characteristics of electromagnetic field. The three-dimensional structure diagram of the whole machine is designed and the test prototype of gradient power amplifier used in permanent magnet magnetic resonance imaging system is built. According to the test results, the effect and significance of the electromagnetic compatibility problem, the structure of the chassis, the shielding of the chassis and the grounding on the EMI suppression are analyzed.
【学位授予单位】：电子科技大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：R445.2
本文编号：2124840
[Abstract]:In the magnetic resonance imaging (MRI) system, in order to locate the imaging object in the main magnetic field, it is necessary to apply a gradient magnetic field with linear variation in three directions to obtain the magnetic resonance signal with position information. To reconstruct three-dimensional MRI images. The function of gradient power amplifier is to provide gradient coil with linear variation of intensity, low ripple, and can quickly open and close the current to generate gradient magnetic field. In the process of development of nuclear magnetic resonance gradient power amplifier, Because the high frequency pulse signal produced by high power switch tube (IGBT) has high voltage and current change rate (dv/dt and di/dt), it will form strong electromagnetic interference (EMI), its frequency range from several thousand hertz to dozens of megahertz. May far exceed the limit required for electromagnetic compatibility (EMC). According to the R & D requirements of nuclear magnetic resonance gradient power amplifier equipment, this paper focuses on the cause and coupling mechanism of EMI, and puts forward some effective measures to restrain EMI. The main work of this paper includes 1: 1. The characteristics of circuit components in gradient power amplifier system under high frequency interference are analyzed. Based on the three elements of electromagnetic interference, the electromagnetic interference source, coupling path, conduction and radiation coupling mechanism of gradient power amplifier (GPA) are studied. According to the switching characteristics of IGBT and the task requirements of the project, the gradient power amplifier topology of the full-bridge double-parallel structure is designed. The function of gate resistance is discussed, the selection of resistance value is simulated and tested, and the effect of peak voltage generated by parasitic inductor on IGBT switching process is analyzed in detail. The improvement scheme of power circuit board is put forward because of the interference of electromagnetic radiation to the adjacent axis caused by the fast changing high current in the bus circuit. Considering the requirement of reducing the amount of IGBT in parallel operation, the current sharing design of parallel topology using passive current-sharing circuit is carried out. The suppression effect of buffer circuit on IGBT switching pulse voltage is discussed. The model of buffer circuit and the common topology are given and its working process is analyzed. The absorption circuit suitable for the task is designed, and the parameter selection of the circuit component is analyzed theoretically. The high frequency characteristics of common winding inductors and magnetic loop inductors are compared and analyzed. Finally, the gradient filter is composed of magnetic loop inductors and filter capacitors. The selection of magnetic ring, capacitance capacity, cut-off frequency of filter are theoretically analyzed and tested. At the same time, the attention of the installation of gradient filter is explained. According to the coupling mechanism of EMI and the propagation characteristics of electromagnetic field. The three-dimensional structure diagram of the whole machine is designed and the test prototype of gradient power amplifier used in permanent magnet magnetic resonance imaging system is built. According to the test results, the effect and significance of the electromagnetic compatibility problem, the structure of the chassis, the shielding of the chassis and the grounding on the EMI suppression are analyzed.
【学位授予单位】：电子科技大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：R445.2
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1 王娟;朱晓华;田松亚;孙烨;;全桥逆变电路中IGBT电压浪涌产生的机理分析[J];河海大学常州分校学报;2006年02期
相关硕士学位论文 前1条
1 陶陈彬;开关电源共模EMI抑制技术研究[D];天津大学;2008年
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