This paper analyzes the main types of electromagnetic interference and its hazards in medical electrical equipment, focusing on the application of power filters in medical electrical equipment to solve the electromagnetic interference phenomenon of medical electrical equipment. In hospital outpatient rooms, operating rooms and wards, electromagnetic interference generated by a medical electrical device with strong electromagnetic radiation can destroy or reduce the performance of other medical electrical equipment; and it is harmful through the physical and chemical effects on human tissues and organs. Physiological effects, damage to human tissues and organs, endangering human health. Therefore, many countries in the world have begun to pay attention to the electromagnetic radiation problem of medical electrical equipment, and have developed corresponding standards and documents, and proposed mandatory requirements for radiation and anti-interference of medical electrical equipment. Therefore, the attention of various medical electrical equipment manufacturers to electromagnetic interference is increasing.
1 Type of electromagnetic interference
Electromagnetic compatibility refers to the ability of a product and other products to coexist in a specific electromagnetic environment without causing degradation or damage to other products or their own performance. It mainly includes radiation and immunity. There are two main types of electromagnetic interference:
1.1 Radiation interference
Radiated interference refers to an interference mode in which electromagnetic waves generated by equipment propagate through space and generate unwanted voltage/current to other equipment circuits. When the distance between the two devices is longer than the wavelength, the interference propagates in the form of electromagnetic waves. For example, when a doctor uses a mobile phone, the image of the medical device next to the medical device may appear to be shaken, because the signal emitted by the mobile phone is transmitted to the inside of the display through the space in the form of an electromagnetic field.
1.2 Conducted interference
Conducted interference is a form of interference in which voltage/current changes generated by one device are transmitted through power lines and signal lines and affect other devices. When the length of the device and the wire is shorter than the wavelength, the conducted interference is mainly the common mode interference and differential mode interference transmitted through the power line, the control line, and the signal line, and the common impedance generated by using the common power source or the common ground line by different devices. interference. In modern hospitals, due to the diagnosis of the disease and the need for surgery, the clinic and the operating room are often no longer a single medical device, but multiple devices are placed in a centralized manner, which inevitably allows multiple medical electrical devices to share a single power source. . The power cord is usually only delivering 50 Hz AC, but the current on the power line is very impure due to electromagnetic interference from the connected equipment. Conducted interference can easily enter the medical device through the power cord, making the device faulty or even damaged, thus affecting the correctness of the diagnosis. If such conduction interference occurs during the operation, it will endanger the patient's life safety in severe cases. Therefore, it is very important to suppress the conducted interference of the power line in medical electrical equipment. In general, the main and effective method we use is to install a power filter on the power line of the victim device.
2 specific application of power filter in medical equipment
Medical surgical microscopes need to be exported to the European Union for electromagnetic compatibility testing in accordance with IEC 60601-1-2. The results of the conducted interference test of the AC power port of the product are shown in Figure 1. As can be seen from the figure, the conducted interference of the product's power port does not pass the Class B limit. After analysis and discussion, it was decided to install a power filter at the power input end of the product.
2. 1 selection of power filter
A filter is a two-port network with a frequency-selective feature that allows certain frequencies to pass smoothly while blocking other frequencies. The power filter is an interference suppression filter. It is also a low-pass filter from the perspective of frequency selection. It can transmit the power of the DC power supply and the AC power supply to the device without attenuation, and at the same time enable high-frequency interference. The signal is greatly attenuated to protect the device from damage. When selecting a specific model, in order to achieve the maximum possible mismatch in the frequency range of the suppressed electromagnetic interference signal, and to maximize the possible attenuation of the electromagnetic interference signal to be suppressed, it is necessary to according to the impedance of the power supply to be connected at both ends of the filter. The load impedance is used to select the network structure and parameters of the power filter to achieve satisfactory interference suppression. After selection, we chose the internal power supply filter of the passive low-pass network (6EHG1-2 model produced by TYCO). One end of the filter is connected to the interference source, and the load is connected to the interfered device. The common mode EMI signal existing on the power line can be suppressed, attenuated, and controlled to a very low level.
2.2 Power Filter Installation
After the filter is properly selected, the power filter is installed at the input end of the surgical microscope power supply, but the product has not passed the conducted interference test. Analyze the reason: First, it is suspected that the filter grounding is poor and the filter effect is deteriorated. So the chassis is placed on the ground plane, and the filter power supply is directly grounded with a short line. The result is still not improved. After observation and testing, it was found that the power filter installation position in the product was different, and the conducted interference test results were different. When the power supply filter is installed as shown in Figure 2(a), the conducted interference test results are not significantly improved; when the power supply filter is installed as shown in Figure 2(b) and filtered with metal screws and star spring washers When the shield of the device is firmly fixed to the metal casing of the machine at the power inlet of the device, the conducted interference test data is significantly reduced. After analysis, we found that the installation of Figure 2(a) results in an electromagnetic coupling path between the input and output conductors of the filter, which creates crosstalk in the conductor, if at this point the cable at the input and output of the filter Bundling together will further aggravate the electromagnetic coupling between the input and output of the filter, seriously damaging the suppression of the interference signal by the filter and the device shielding, so that the EMI signal at one end of the filter will escape the limitation of the filter, without going through The attenuation of the filter is directly coupled to the other end of the filter. The installation of Figure 2(b) is to minimize the electromagnetic coupling (ie, crosstalk) that may exist between the two ends by effectively isolating the input and output of the power supply filter by means of shielding of the device. And the filter is installed on the device casing on the inlet side of the power supply line, and the shield casing of the filter is in good contact with the equipment casing (grounding of the equipment casing), so that the filter is well grounded.
As a result, the correct installation of the filter is also an important part of preventing electromagnetic interference. The purpose of the power supply filter is to prevent conducted interference from the power supply from directly entering the medical electrical equipment. If the filter is not properly installed, even in the shielded cabinet, the interference from the power cord can affect the medical electrical equipment by radiation or induction, so the correct installation method is very important.
2.3 Improvement measures
According to the above theoretical analysis, a power supply filter is installed at the input end of the surgical microscope power supply, and good isolation between the input and output of the filter is ensured, and some internal circuit changes are assisted, and the test is performed again. The test result is shown in FIG. . The conducted interference of the product's power port is specified by the ClassB limit. The surgical microscope was mass-produced according to the modified scheme, and the electromagnetic compatibility test results of each product all met the standard requirements.
3 Conclusion
The power filter is a commonly used device to meet the electromagnetic compatibility requirements of the product. After it is connected to the inlet of the power line of the device, it can effectively suppress the interference signal from the power grid from entering the device and suppress the interference signal generated by the internal circuit of the device. It is transmitted to the grid through the power line. In the production process of medical electrical equipment, in order to make the electromagnetic compatibility design meet the requirements, it is not only necessary to select a power supply filter with good filtering effect, but also to correctly install the filter for electromagnetic compatibility requirements.
Suizhou simi intelligent technology development co., LTD , https://www.msmvape.com