【摘要】：在核磁共振成像(Magnetic Resonance Imaging,MRI)系統(tǒng)中,為了對主磁場中的成像物體進行空間定位,需要在x,y,z三個方向上施加線性變化的梯度磁場,獲取具有位置信息的磁共振信號,以重建出MRI三維圖像。梯度功放的功能是為梯度線圈提供強度線性變化、低紋波、可快速開啟和關(guān)閉的電流以產(chǎn)生梯度磁場。在核磁共振梯度功放研制的過程中,由于大功率開關(guān)管IGBT在開關(guān)過程中產(chǎn)生的高頻脈沖信號具有很高的電壓與電流變化率(dv/dt與di/dt),會形成很強的電磁干擾(Electromagnetic Interference,EMI),其頻率從幾千赫茲到幾十兆赫茲,可能遠(yuǎn)遠(yuǎn)超過電磁兼容(Electromagnetic Compatibility,EMC)要求的極限值。本文根據(jù)廠家核磁共振梯度功放設(shè)備的研發(fā)要求,主要針對功率放大電路工作時所產(chǎn)生的EMI干擾,重點研究了EMI的產(chǎn)生原因、耦合機理,并提出了對EMI進行有效抑制的措施。本文主要工作內(nèi)容包括:1.分析了梯度功放系統(tǒng)中電路元件在高頻干擾作用下的特性。基于電磁干擾的三要素,研究了梯度功放電磁干擾源、耦合途徑、傳導(dǎo)以及輻射耦合機理。2.根據(jù)IGBT的開關(guān)特性及項目課題任務(wù)要求,設(shè)計了全橋雙并聯(lián)結(jié)構(gòu)的梯度功放拓?fù)浣Y(jié)構(gòu)。討論了柵極電阻的作用,對其阻值的選取進行仿真分析以及實驗測試,詳細(xì)分析了母線寄生電感產(chǎn)生的尖峰電壓對于IGBT開關(guān)過程的影響、以及母線回路通過快速變化的大電流時產(chǎn)生的電磁輻射對于相鄰軸向的干擾,提出了功率電路板的改進方案�？紤]到IGBT并聯(lián)使用時的降額要求,使用無源均流電路對并聯(lián)拓?fù)浣Y(jié)構(gòu)進行均流設(shè)計。3.探討了緩沖電路對于IGBT開關(guān)脈沖電壓的抑制作用,給出了緩沖電路的模型以及常用的拓?fù)浣Y(jié)構(gòu)并分析了其工作過程。設(shè)計了適用于課題需求的吸收電路,并對其電路元件的參數(shù)選取進行了理論分析。對比分析普通繞線電感與磁環(huán)電感的高頻特性,最終采用磁環(huán)電感與濾波電容電路構(gòu)成梯度濾波器。對磁環(huán)的選型、電容容值的選取、濾波器的截止頻率進行理論分析與實驗測試,同時對梯度濾波器的安裝注意事項進行說明。4.根據(jù)EMI的耦合機理以及電磁場的傳播特性。設(shè)計了整機三維結(jié)構(gòu)圖并搭建了應(yīng)用在永磁磁共振成像系統(tǒng)的梯度功放測試樣機。根據(jù)調(diào)試結(jié)果分析了電磁兼容問題以及機箱結(jié)構(gòu),機箱屏蔽,接地等措施對于EMI抑制的作用及意義。
[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.
【學(xué)位授予單位】：電子科技大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2015
【分類號】：R445.2

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本文編號：2124840