Research On EFT Coupling Characteristics And Immunity Of Focused Ultrasound Treatment Equipment

BackgroundThe electromagnetic compatibility(EMC)performance of medical electrical equipment directly affects the safety of patients and operators.YY0505-2012 ‘Medical electrical equipment-Part 1-2: General requirements for Safety-parallel standard electromagnetic compatibility requirements and tests’ was promulgated by the State Food and Drug Administration(CFDA)in 2012,requiring that all medical electrical equipment must be tested by EMC standard,which has been enforced since January 2014.Electrical fast transient(EFT)test is an important test item in conducted disturbance test of electromagnetic compatibility(EMC),Electrical fast pulse group(EFT)test is an important test item in the EMC conducted immunity test,the purpose of which is to test the performance of medical electrical equipment under the influence of transient disturbance.As the small space and structure,the system layout and wiring of the small HIFU treatment equipment are relatively compact,and the transient interference coupling between cables is serious.Therefore,the EFTimmunity problems are often more prominentPurpose This paper aims to analyze the internal EFT coupling characteristics and evaluate the EFT immunity of small HIFU treatment equipment,then,improve the EFT immunity of small HIFU treatment equipment or other medical electrical equipment,to make the equipment meet EMC standard test and YY0505-2012.Methods1.The characteristic analysis of EFT coupling between cablesIn order to analyze the characteristics of cable coupling of EFT,a calculation model about double parallel cables was established based on the transmission line theory,and numerical calculations about EFT coupling characteristic between cables were done by using CST software.The characteristics of cable coupling of EFT under different exciting voltage,cable spacing,cable height and load impedance were researched systematically.2.Research on EFT immunity of MCU data communication system with multi-protocol based on statistical analysisFor an end that simulating the EFT coupling between power line and data communication line and the affect to data communication on system or equipment,a MCU data communication system which contains four kinds of data communication protocol including I2 C,USART,SPI and RS232 was designed for EFT immunity test by means of cable coupling,and an EFT immunity test platform was established.A lot of EFT interference threshold voltage of MCU data communication system under different communication protocols were obtained by many times EFT immunity test based on step-by-step method.As the randomicity was discovered in the EFT immunity test frequently.In order to evaluate the immunity of the system more reasonably,the mathematical statistics method is used to study the threshold distribution of EFT interference of MCU data communication system with multi-protocol,and the threshold voltage of EFT interference of system is determined.A lot of EFT interference threshold voltage obtained by many times EFT immunity test were analyzed by mathematical statistics method.The optimal Gamma model is used for researching distribution law of EFT interference threshold voltage and EFT immunity of MCU data communication system under different communication protocols,which was selected by chi-square goodness of fit test and Hannan Quinn information criterion through hypothetical models.The model parameters were estimated by maximum likelihood method based on test data.Results1.The peak of coupled voltages at two ends of the tested cable gradually increases with the exciting voltage increasing.The waveform shapes of coupled voltages under different exciting voltages are basically identical. The coupled voltage equals 12.04 V when exciting voltage is 250 V,but coupled voltage rises to 47.63 V as exciting voltage equals 1000 V.And the coupled voltage tends to decrease gradually with the cable spacing d increasing.The coupled voltage reach up to 22.03 V which means that the radiated influence of EFT and coupling between cables are still very strong as cable spacing equals 130 mm.2.The coupled voltage is weak When cable height is small.Subsequently,The coupled voltage rises gradually with the addition of cable height.But,when cable height rises to a certain height,the peak tends to be stable.The peak of coupled voltage is up to 158 V as cable height equals 2.5m.The coupled current is proportional to the exciting voltage when the load on the end of tested cable is changed.The coupled voltage is proportional to the load resistance as the load on the two terminals of tested cable.3.The EFT interference voltage of MCU data communication system under I2C、SPI、USART、RS232 respectively equal 920 V,1259V,995 V,1686V when baud rate is 9600 bps.The probability of system under I2 C,SPI,USART,RS232 interfered by 1000 V EFT voltage respectively equal62.7%,3.21%,46.2%,0.0000796%Conclusion1.The coupling characteristics of EFT cables are greatly affected by the factors such as the excitation voltage,the cable spacing,cable height and cable load.The cable coupling caused by EFT radiation is strong even cable spacing is wide.The cable height has great influence on EFT coupling.Thus,whether there is a grounded metal frame or metal case has a greatly influence on EFT coupling.The peak value of the coupled voltage can be appropriately reduced by reducing grounded resistance of the cables or the pull-up resistor between power and cables because the load impedance can affect the coupled voltage.2.Using mathematical statistics method to acquire The EFT interference threshold of MCU data communication system with multi-protocol,the randomicity in the EFT immunity test was avoided perfectly,and the EFT immunity of system would be evaluated very reasonably.Different data communication protocols have different electrical characteristics and different data transmission modes,and their EFT immunity is also different.For MCU communication protocols,EFT immunity from low to high is in the order of I2 C,USART,SPI,RS232 under the same baud rate.Therefore,in order to improve the EFT immunity of data communication then enhance the EFT immunity of the whole system,SPI communication should be the first choice for data communication within the system or in the circuit board and RS232 data communication should be the first choice for data communication between systems or circuit boards.In addition,the EFT coupling energy between cables is directly related to cable length and cable spacing.Hence,reasonable wiring design is very important for improving the EFT immunity of electronic system or equipment.