Application of EMI Filter in Medical Devices

Introduction

The role of biomedical engineering in modern hospitals is becoming more and more important. The updating of medical device is changing with each passing day. High-end and sophisticated medical equipment also determines the level of medical services. The types, scale and advancedness of medical instruments and equipment have become an important symbol of modern hospitals. However, modern science and technology has quietly shrouded the shadow of electromagnetic pollution over the hospital. In hospital outpatient rooms, operating rooms, and wards, the electromagnetic interference generated by a medical electrical equipment with strong electromagnetic radiation will destroy or reduce the working performance of other medical electrical equipment; Physiological effects cause damage to human tissues and organs and endanger human health. Therefore, many countries in the world have begun to pay attention to the electromagnetic radiation of medical electrical equipment, and have formulated corresponding standards and documents, and proposed mandatory requirements for the radiation and anti-interference of medical electrical equipment. The European Union Medical Device Directive 93/42 / EEC-Medical Device Directive clearly stipulates that medical electrical equipment that needs to be CE-certified and exported to member states of the European Community must pass the electromagnetic compatibility specified in the IEC 60601-1-2 standard EMC testing. Therefore, various medical electrical equipment manufacturers are increasingly concerned about electromagnetic interference.

1.  Types of Electromagnetic Interference (EMI)

Electromagnetic compatibility (EMC) 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 its own performance. It mainly includes two areas of radiation and immunity. There are two main types of electromagnetic interference (EMI):

1.1 Radiated Interference

Radiated interference refers to a way in which electromagnetic waves generated by equipment propagate through space and produce useless 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 computer display of the medical device next to it will jitter, because the signal emitted by the mobile phone when it is working is transmitted to the interior of the display in the form of electromagnetic field through the space.

1.2 Conducted Interference

Conducted interference is a type of interference where voltage / current changes generated by one device are conducted through power lines and signal lines and affect other devices. When the length of the device and the conductor is shorter than the wavelength, the conducted interference is mainly the common mode interference and differential mode interference transmitted through the power line, control line, and signal line, as well as the common impedance interference generated by different devices using a common power supply or a common ground line. In modern hospitals, due to the need for diagnosis and surgery, outpatient rooms and operating rooms often do not place a single medical device, but multiple devices are placed centrally, which inevitably makes multiple medical electrical equipment share a common power supply. Originally, the power cord was only used to transmit 50Hz AC power, but due to the electromagnetic interference of the connected equipment, the current on the power cord was very impure. Conducted interference can easily enter the medical equipment through the power cord, making the equipment wrong or even damaged, thereby affecting the accuracy of the diagnosis results. If such conducted interference occurs during the operation, it will endanger the patient’s life safety in severe cases. Therefore, it is very important to suppress conducted interference of power cords in medical electrical equipment. Usually, the main and effective method we adopt is to install a power line filter/EMI filter on the power line of the interfered equipment.

2. The Specific Application of EMI Filter in Medical Devices

Medical operating microscopes need to be conducted an EMC test according to the IEC 60601-1-2 standard. It is found that the conducted interference test result of the AC power port of this product is shown in Figure 1. It can be seen from the figure that the conducted interference of the power port does not pass the requirements of Class B limits. After analysis and discussion, it was decided to install a power noise filter/EMI filter on the power input of the product.

Figure 1

2.1 Selection of an EMI Filter/Power Filter

A power line filter is a two-port network that has the characteristic of selecting frequencies. It allows certain frequencies to pass smoothly while blocking others. The power line filter belongs to the interference suppression filter. From the perspective of frequency selection, it is also a low-pass filter. It can deliver the power of the DC power supply and the AC power supply to the device without attenuation. The high frequency interference signal is greatly attenuated to protect the device from damage. When choosing a specific model, in order to achieve the maximum possible mismatch within the frequency range of the suppressed electromagnetic interference signal, so that the electromagnetic interference signal that needs to be suppressed is subject to the maximum possible attenuation, it must be based on the power supply impedance and Load impedance, to select the network structure and parameters of the power filter, in order to achieve a satisfactory effect of suppressing interference. After selection, we choose the power filter which is a passive low-pass network. Connect one end of the filter to the interference source and the load to the interfered equipment, which can suppress the common mode EMI signal existing on the power line, make it attenuated, and control to a very low level.

2.2 Installation of Power Line Filter/EMI Filter

After correctly selecting the filter, the power filter was installed on the power input of the surgical microscope, but the product still failed the conducted interference test. Analyze the reason: first suspect that the poor grounding of the filter causes the filter effect to deteriorate. So put the chassis on the ground plane, the power supply of the filter is directly grounded with a short wire, and the result is still not improved. After observation and experiment, it is found that the installation position of the power filter in the product is different, and the conducted interference test results are different.

When the power filter is installed as shown in Figure 2 (a), the conducted interference test results are not significantly improved; When the power filter is installed as shown in Fig. 2 (b), and the metal screw and star spring washer are used to tightly fix the filter shell on the metal enclosure of the equipment power inlet, the conducted interference test data are obviously reduced.

After analysis, we found that the installation in the mode of Figure 2 (a) will cause an electromagnetic coupling path between the input end wire of the filter and the output end wire, thereby forming crosstalk in the wire. If the cables at the input and output of the filter are bundled together at this time, the electromagnetic coupling between the input and output of the filter will be exacerbated, which will seriously damage the suppression ability of the filter and the equipment shielding to the interference signal, making The EMI signal will escape the limitation of the filter, and directly coupled to the other end of the filter without attenuation of the EMI filter.

The installation method in Figure 2 (b) is to effectively isolate the input and output of the power filter by means of the shielding of the equipment, and to minimize the electromagnetic coupling (ie crosstalk) that may exist between the two ends. And the filter is installed on the equipment shell of the power inlet line, the shielding shell of the filter is in good contact with the equipment shell (the equipment shell is grounded), so that the filter is well grounded.

Figure 2

Thus it can be seen, the correct installation of the power line filter is also a very important link to prevent electromagnetic interference (EMI). The purpose of the EMI filter is to prevent conducted interference from the power supply from directly entering the medical electrical equipment. If the power filter is improperly installed, even in the shielded cabinet, the interference of the power cord will affect medical electrical equipment through radiation or induction, so the correct installation method is very important.

2.3 Improvement Measures

According to the above theoretical analysis, install a power line filter on the power input end of the surgical microscope, and ensure good isolation between the filter input and output, and at the same time assist some internal circuit changes, and test again. The test results are shown in Figure 3. The conducted interference of the power port of the microscope passes the requirements of Class B limits. According to the modified plan, the surgical microscope was mass produced, and the electromagnetic compatibility (EMC) test results of each product all met the standard requirements.

Figure 3

3. Conclusion

The power line filter/EMI filter is a device commonly used to meet the electromagnetic compatibility (EMC) requirements of the product. When it is connected to the entrance of the power line of the device, it can effectively suppress the interference signal from the power grid from entering the device and the interference signal generated by the internal circuit of the device pass the power line to the power grid. In the production process of medical electrical equipment, in order to make the electromagnetic compatibility design meet the requirements, not only need to choose a power filter/EMI filter with good filtering effect, but also to correctly install the filter for electromagnetic compatibility (EMC)requirements.

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