EMI Suppression Of MRI Gradient Amplifier

In the MRI system, In order to spatially locate the imaging object in the main magnetic field,A gradient magnetic field application needs to change linearly in the x, y,z three directions, to obtain magnetic resonance signals which contain position information and having to reconstruct the three-dimensional MRI images. The function of Gradient amplifier is to provide gradient coils a linear change of the intensity, low ripple, quickly turn on and off current to generate a magnetic field gradient.In the NMR gradient amplifier developed process, Due to the high frequency pulse signal generated by IGBT power switches in the switching process with high voltage and current changes( dv/ dt and d i/ d t), It will form strong electromagnetic interference(EMI), The frequency from thousands of Hz to tens of MHz, May far exceed the requirements of the Electromagnetic compatibility( EMC) limits. Based on the research and development requirements for manufacturers of MRI gradient amplifier equipment, mainly for EMI interference generated by the power amplifier circuit,focusing on the generation of theory, EMI coupling mechanism, and proposes measures for effective EMI suppression. The main contents of this design include:1. Analysis of the circuit elements gradient amplifier system at high frequency interference features, Based on the three elements of electromagnetic interference,studied the gradient amplifier electromagnetic interference source, coupling path,conduction and radiation coupling mechanism.2. According to the switching characteristics of IGBT and the project mission requirements, design a dual full-bridge parallel topology structures. Discussed the role of the gate resistor, simulation analysis and experimental testing of its resistance selection, a detailed analysis of the parasitic inductance of the bus which generate voltage spikes effects for the IGBT switching process, And electromagnetic radiation generated by the bus loop through a large current rapidly changing interference to the adjacent axial, Proposed a improvement program of power board. Taking into account the reduced nominal power requirements when used in parallel IGBT, passive average current circuit topology of the parallel design for average current.3. Discussion of the buffer circuit inhibition for IGBT switching impulse voltage,given the model and the common buffer circuit topology and analyze their work process.Designed for the needs of the subject absorption circuit, and give the theory analysis of its parameter selection. Comparative analysis the High-frequency characteristics of ordinary winding inductance and magnetic inductance, final adoption magnetic inductance and capacitance circuits constitute gradient filter. For magnetic selection,selected capacitance value and Cut-off frequency of the filter are all give the theory analysis and experimental testing, meanwhile described the install considerations of gradient filter.4. According to the EMI coupling mechanism and the propagation characteristics of electromagnetic fields. Design three-dimensional structure of the machine and build a gradient amplifier test prototype which using in permanent magnet MRI system.According to the test results, analysis of electromagnetic compatibility and chassis structure, chassis shielding, grounding and other measures for the role and significance of EMI suppression.