Beijing EMC energy mine
Publish: 2021-04-29 13:13:37
1.
Electromagnetic compatibility (EMC) refers to the ability of equipment or system to meet the requirements in its electromagnetic environment and not proce unbearable electromagnetic interference to any equipment in its environment
With the development of modern science and technology, the number and types of electrical and electronic equipment are increasing, which makes the electromagnetic environment increasingly complex. In this complex electromagnetic environment, how to rece the mutual electromagnetic disturbance and make all kinds of equipment work normally is an urgent problem to be solved; On the other hand, the bad electromagnetic environment will have a bad impact on human and ecology 
extended data:
EMC includes two requirements: on the one hand, it means that the electromagnetic interference generated by the equipment to the environment in the normal operation process cannot exceed a certain limit value; On the other hand, it means that the instrument has a certain degree of immunity to the electromagnetic interference existing in the environment, that is, electromagnetic sensitivity
in recent years, with the wide application of high sensitive electronic technology in medical electrical equipment and the rapid development of new communication technologies, such as personal communication system and cellular telephone, in various fields of social life; Medical electrical equipment not only emits electromagnetic energy, which affects the radio communication business and the work of other surrounding equipment, but also may be interfered by electromagnetic energy emission from surrounding equipment such as communication equipment in its application environment, causing harm to patients
filter connector is often of great help to the EMC performance of procts, but its cost is relatively high. Generally, when the problem can be solved by using on-board filtering, cable shielding and other methods, filter connector is not used. Filter connectors are usually used in some special situations, such as strict military standard requirements, small batch applications in harsh instrial environment and some special situations (such as structure size restrictions)
the research of EMC theory and technology in China started relatively late, and its development in the medical field only started in the 1990s. In 1995, China issued GB 9706.1-1995 General requirements for safety of medical electrical equipment. It was not until April 5, 2005 that the former State Food and Drug Administration issued the mandatory instry standard for medical devices YY0505-2005 Medical electrical equipment Part 1-2: General requirements for Safety - parallel standards: electromagnetic compatibility - Requirements and tests
References: EMC network
2. The full name of EMC (electromagnetic compatibility) is electric magnetic compatibility, which is defined as "the ability of equipment and system to work normally in their electromagnetic environment without causing unbearable electromagnetic disturbance to anything in the environment". The definition includes two aspects. First, the equipment should be able to work normally in a certain electromagnetic environment, That is, the equipment should have certain electromagnetic immunity (EMS); Secondly, the electromagnetic interference generated by the device itself can not have too much impact on other electronic procts, namely electromagnetic interference (EMI)
with the development of modern science and technology, the number and types of electrical and electronic equipment are increasing, which makes the electromagnetic environment increasingly complex. In this complex electromagnetic environment, how to rece the mutual electromagnetic disturbance and make all kinds of equipment work normally is an urgent problem to be solved; On the other hand, the bad electromagnetic environment will have a bad impact on human and ecology
with the development of electrical and electronic technology, the popularization and electronization of household appliances, the development of radio and television, posts and telecommunications and computer networks, and the increasingly complex and deteriorating electromagnetic environment, the electromagnetic compatibility (EMC, EMI and EMS) of electrical and electronic procts has been paid more and more attention by governments and manufacturers. Electromagnetic compatibility (EMC) of electronic and electrical procts is a very important quality index, which is not only related to the reliability and safety of the proct itself, but also may affect the normal work of other equipment and systems, as well as the protection of electromagnetic environment
in order to standardize the EMC of electronic procts, all developed countries and some developing countries have formulated EMC standards. EMC standard is the basic requirement for procts to work normally in the actual electromagnetic environment. That is to say, even if the proct meets the EMC standard, interference may occur in actual use. Most of the national standards are based on the International Electrotechnical Commission (IEC) standards
EMC includes two requirements: on the one hand, the electromagnetic interference generated by the equipment to the environment in the normal operation process should not exceed a certain limit value; On the other hand, it means that the instrument has a certain degree of immunity to the electromagnetic interference existing in the environment, that is, electromagnetic sensitivity
in recent years, with the wide application of high sensitive electronic technology in medical electrical equipment and the rapid development of new communication technologies, such as personal communication system, cellular phone, etc., in various fields of social life; Medical electrical equipment not only emits electromagnetic energy, which affects the radio communication business and the work of other surrounding equipment, but also may be interfered by electromagnetic energy emission from surrounding equipment such as communication equipment in its application environment, causing harm to patients.
with the development of modern science and technology, the number and types of electrical and electronic equipment are increasing, which makes the electromagnetic environment increasingly complex. In this complex electromagnetic environment, how to rece the mutual electromagnetic disturbance and make all kinds of equipment work normally is an urgent problem to be solved; On the other hand, the bad electromagnetic environment will have a bad impact on human and ecology
with the development of electrical and electronic technology, the popularization and electronization of household appliances, the development of radio and television, posts and telecommunications and computer networks, and the increasingly complex and deteriorating electromagnetic environment, the electromagnetic compatibility (EMC, EMI and EMS) of electrical and electronic procts has been paid more and more attention by governments and manufacturers. Electromagnetic compatibility (EMC) of electronic and electrical procts is a very important quality index, which is not only related to the reliability and safety of the proct itself, but also may affect the normal work of other equipment and systems, as well as the protection of electromagnetic environment
in order to standardize the EMC of electronic procts, all developed countries and some developing countries have formulated EMC standards. EMC standard is the basic requirement for procts to work normally in the actual electromagnetic environment. That is to say, even if the proct meets the EMC standard, interference may occur in actual use. Most of the national standards are based on the International Electrotechnical Commission (IEC) standards
EMC includes two requirements: on the one hand, the electromagnetic interference generated by the equipment to the environment in the normal operation process should not exceed a certain limit value; On the other hand, it means that the instrument has a certain degree of immunity to the electromagnetic interference existing in the environment, that is, electromagnetic sensitivity
in recent years, with the wide application of high sensitive electronic technology in medical electrical equipment and the rapid development of new communication technologies, such as personal communication system, cellular phone, etc., in various fields of social life; Medical electrical equipment not only emits electromagnetic energy, which affects the radio communication business and the work of other surrounding equipment, but also may be interfered by electromagnetic energy emission from surrounding equipment such as communication equipment in its application environment, causing harm to patients.
3.
electromagnetic environment
the sum of all electromagnetic phenomena existing in a given place
electromagnetic disturbance
any electromagnetic phenomenon that may cause performance degradation of devices, equipment or systems or damage to living or non living substances
electromagnetic interference (EMI)
the performance degradation of equipment, transmission channel or system caused by electromagnetic interference
radiated emission (RE) and concted emission (CE)
re: useful or unwanted electromagnetic energy transmitted through space. CE: electromagnetic emission transmitted along power supply or signal line
electromagnetic susceptibility (EMS)
the undesirable response of equipment exposed to electromagnetic environment. Radiation sensitivity CS and conction sensitivity CS
transient
a physical quantity or phenomenon that changes between two adjacent states, and its change time is less than the time scale concerned
4. Microwave anechoic chamber, also known as anechoic chamber, is mainly used to measure the radiation emission and radiation immunity of electronic procts. There are absorbing materials and ferrites, which absorb radiation energy. Electronic procts need to be tested with the door closed. There is definitely no radiation energy, which is very safe
by the way, does your sister's company have a darkroom? A anechoic chamber can't get down without millions.
by the way, does your sister's company have a darkroom? A anechoic chamber can't get down without millions.
5. The full English name of SAR is specific absorption rate, while the Chinese name is electromagnetic wave absorption ratio or specific absorption rate. It is the electromagnetic energy absorption ratio of mobile phones or wireless procts, which is defined as: under the action of external electromagnetic
field, the inced electromagnetic field will be generated in the human body. Because all kinds of human organs are lossy medium, the electromagnetic field in the body will proce electric current, which will lead to the absorption and dissipation of electromagnetic energy. SAR is often used to characterize the physical process of
in biodosimetry. The meaning of SAR is the electromagnetic power absorbed or consumed per unit mass of human tissue, w / kg< The SAR laboratory supports the SAR testing of mobile devices such as mobile phones, data cards, tablet computers and walkie talkies. The test frequency range is 300mhz-6ghz, including 2G, 3G, 4G, WiFi and other communication systems
Introction to SAR laboratory equipment
the newly introced French Satimo SAR test system meets the domestic and international standards of YD / T 1644, en 62209-1, FCC OET bullet 65, etc. The SAR test system supports the option of double head table, which has the characteristics of fast test speed and high equipment stability
- SAR test classification
by proct type:
2G mobile phone, 3G mobile phone, 4G mobile phone, walkie talkie, tablet computer, notebook computer, USB WiFi, high-power Bluetooth headset, etc< According to frequency distribution:
GSM 450, GSM 850, GSM 900, GSM 1800, GSM 1900< br />UMTS Band I、II、III、IV、V、VIII < br />CDMA、EVDO800、850、1700、1900 < br />PPT Device(136MHz to 174MHz)、(400MHz to 520MHz)、Walkie Talkie < br />Bluetooth(V2.1+EDR、V3.0、V3.0+HS、V4.0) < br />WiFi(IEEE 802.11 a/b/g/n)
classified by Certification Type:
ce-rtte, FCC ID, IC, Thailand, India, KC, NCC, MIC, etc
summary:
SAR test is mainly aimed at procts with wireless transmission whose antenna is within 20cm of human body, and the power of the proct exceeds the power value exempted by various countries.
field, the inced electromagnetic field will be generated in the human body. Because all kinds of human organs are lossy medium, the electromagnetic field in the body will proce electric current, which will lead to the absorption and dissipation of electromagnetic energy. SAR is often used to characterize the physical process of
in biodosimetry. The meaning of SAR is the electromagnetic power absorbed or consumed per unit mass of human tissue, w / kg< The SAR laboratory supports the SAR testing of mobile devices such as mobile phones, data cards, tablet computers and walkie talkies. The test frequency range is 300mhz-6ghz, including 2G, 3G, 4G, WiFi and other communication systems
Introction to SAR laboratory equipment
the newly introced French Satimo SAR test system meets the domestic and international standards of YD / T 1644, en 62209-1, FCC OET bullet 65, etc. The SAR test system supports the option of double head table, which has the characteristics of fast test speed and high equipment stability
- SAR test classification
by proct type:
2G mobile phone, 3G mobile phone, 4G mobile phone, walkie talkie, tablet computer, notebook computer, USB WiFi, high-power Bluetooth headset, etc< According to frequency distribution:
GSM 450, GSM 850, GSM 900, GSM 1800, GSM 1900< br />UMTS Band I、II、III、IV、V、VIII < br />CDMA、EVDO800、850、1700、1900 < br />PPT Device(136MHz to 174MHz)、(400MHz to 520MHz)、Walkie Talkie < br />Bluetooth(V2.1+EDR、V3.0、V3.0+HS、V4.0) < br />WiFi(IEEE 802.11 a/b/g/n)
classified by Certification Type:
ce-rtte, FCC ID, IC, Thailand, India, KC, NCC, MIC, etc
summary:
SAR test is mainly aimed at procts with wireless transmission whose antenna is within 20cm of human body, and the power of the proct exceeds the power value exempted by various countries.
6. Refer to EMC questions and Countermeasures:
1. In the field of electromagnetic compatibility, why is it always described in DB? How much dbmv is 10mV? A: because the range of amplitude and frequency to be described is very wide, it is easier to express it in logarithmic coordinates on the graph, while DB is the unit expressed in logarithmic coordinates, and 10mV is 20dbmv
2. Why can't spectrum analyzer observe transient interference such as electrostatic discharge
A: because the spectrum analyzer is a narrow-band frequency sweeping receiver, it only receives the energy within a certain frequency range at a certain time. The transient interference, such as electrostatic discharge, is a kind of pulse interference, whose spectrum range is very wide, but the time is very short, so the spectrum analyzer can only observe a small part of its total energy when the transient interference occurs, which can not reflect the actual interference situation< How to make a simple near-field probe with coaxial cable
A: peel off the outer layer (shielding layer) of the coaxial cable to expose the core wire, wind the core wire into a small ring (1 ~ 3 turns) with a diameter of 1 ~ 2 cm, and weld it on the outer layer
4. For a device, the original electromagnetic radiation emission intensity is 300mV / m. after adding the shielding box, the radiation emission is reced to 3MV / m. what is the shielding effectiveness of the box? A: the shielding effectiveness of this case should be 40dB
5. When designing shielding case, which factors should be taken into consideration to select shielding materials
A: from the perspective of electromagnetic shielding, we should mainly consider the types of electric field waves to be shielded. For electric field wave, plane wave or magnetic field wave with higher frequency, generally metals can meet the requirements. For low frequency magnetic field wave, materials with higher permeability should be used
6. In addition to the shielding material, what other factors affect the shielding effectiveness of the chassis? A: affected by two factors, one is the conctive discontinuities on the chassis, such as holes, gaps, etc; The other is the wire through the shielding box, such as signal cable, power line, etc. 7. What should be paid attention to when shielding magnetic field radiation source
A: because the wave impedance of the magnetic field wave is very low, the reflection loss is very small, and the shielding purpose is mainly achieved by the absorption loss. Therefore, the shielding material with high permeability should be selected. In addition, when designing the structure, the shielding layer should be kept away from the radiation source (to increase the reflection loss), and the holes and gaps should be kept away from the radiation source. 8. When designing the shielding structure, there is a principle: keep the cables in the chassis away from gaps and holes as far as possible. Why
A: there is always a magnetic field near the cable, and the magnetic field is easy to leak from the hole (independent of the frequency of the magnetic field). Therefore, when the cable is very close to the gap and hole, magnetic field leakage will occur, which will rece the overall shielding effectiveness.
1. In the field of electromagnetic compatibility, why is it always described in DB? How much dbmv is 10mV? A: because the range of amplitude and frequency to be described is very wide, it is easier to express it in logarithmic coordinates on the graph, while DB is the unit expressed in logarithmic coordinates, and 10mV is 20dbmv
2. Why can't spectrum analyzer observe transient interference such as electrostatic discharge
A: because the spectrum analyzer is a narrow-band frequency sweeping receiver, it only receives the energy within a certain frequency range at a certain time. The transient interference, such as electrostatic discharge, is a kind of pulse interference, whose spectrum range is very wide, but the time is very short, so the spectrum analyzer can only observe a small part of its total energy when the transient interference occurs, which can not reflect the actual interference situation< How to make a simple near-field probe with coaxial cable
A: peel off the outer layer (shielding layer) of the coaxial cable to expose the core wire, wind the core wire into a small ring (1 ~ 3 turns) with a diameter of 1 ~ 2 cm, and weld it on the outer layer
4. For a device, the original electromagnetic radiation emission intensity is 300mV / m. after adding the shielding box, the radiation emission is reced to 3MV / m. what is the shielding effectiveness of the box? A: the shielding effectiveness of this case should be 40dB
5. When designing shielding case, which factors should be taken into consideration to select shielding materials
A: from the perspective of electromagnetic shielding, we should mainly consider the types of electric field waves to be shielded. For electric field wave, plane wave or magnetic field wave with higher frequency, generally metals can meet the requirements. For low frequency magnetic field wave, materials with higher permeability should be used
6. In addition to the shielding material, what other factors affect the shielding effectiveness of the chassis? A: affected by two factors, one is the conctive discontinuities on the chassis, such as holes, gaps, etc; The other is the wire through the shielding box, such as signal cable, power line, etc. 7. What should be paid attention to when shielding magnetic field radiation source
A: because the wave impedance of the magnetic field wave is very low, the reflection loss is very small, and the shielding purpose is mainly achieved by the absorption loss. Therefore, the shielding material with high permeability should be selected. In addition, when designing the structure, the shielding layer should be kept away from the radiation source (to increase the reflection loss), and the holes and gaps should be kept away from the radiation source. 8. When designing the shielding structure, there is a principle: keep the cables in the chassis away from gaps and holes as far as possible. Why
A: there is always a magnetic field near the cable, and the magnetic field is easy to leak from the hole (independent of the frequency of the magnetic field). Therefore, when the cable is very close to the gap and hole, magnetic field leakage will occur, which will rece the overall shielding effectiveness.
7. Electromagnetic compatibility (EMC) refers to the ability of equipment or system to meet the requirements in its electromagnetic environment and not proce unbearable electromagnetic interference to any equipment in its environment. Therefore, EMC includes two requirements: on the one hand, it means that the electromagnetic interference generated by the equipment in the normal operation process cannot exceed a certain limit value; On the other hand, it means that the instrument has a certain degree of immunity to the electromagnetic interference existing in the environment, that is, electromagnetic sensitivity. With the development of modern science and technology, the number and types of electrical and electronic equipment are increasing, which makes the electromagnetic environment increasingly complex. In this complex electromagnetic environment, how to rece the mutual electromagnetic disturbance and make all kinds of equipment work normally is an urgent problem to be solved; On the other hand, the bad electromagnetic environment will have a bad impact on human and ecology. In recent years, with the wide application of high sensitive electronic technology in medical electrical equipment and the rapid development of new communication technologies, such as personal communication system, cellular China, in various fields of social life; Medical electrical equipment not only emits electromagnetic energy, which affects the radio communication business and the work of other surrounding equipment, but also may be interfered by electromagnetic energy emission from surrounding equipment such as communication equipment in its application environment, causing harm to patients. Electromagnetic compatibility (EMC) of medical electrical equipment has been paid more and more attention in many countries because it involves public health and safety. The necessity of formulating EMC standards for medical electrical equipment and medical electrical system has become a common understanding. In order to standardize the electromagnetic compatibility of electronic procts, all developed countries and some developing countries have formulated electromagnetic capacitance standards. EMC standard is the basic requirement for procts to work normally in the actual electromagnetic environment. The reason why it is called the basic requirement is that the proct meets the electromagnetic capacitance standard in time, and the interference problem may occur in actual use. Most of the national standards are based on the International Electrotechnical Commission (IEC) standards. IEC has two parallel organizations responsible for the development of EMC standards, namely CISPR (International Special Committee on radio interference) and TC paint (Technical Committee No. CISPR was founded in 1934. At present, CISPR has seven branches: branch a (radio interference measurement methods and statistical methods), branch B (radio interference of radio frequency equipment in engineering, science and medicine), branch C (radio interference of power line, high voltage equipment and electric traction system), branch D (radio interference of motor vehicle and internal combustion engine), branch e (interference characteristics of radio receiving equipment), branch C (radio interference of power line, high voltage equipment and electric traction system) Branch f (radio interference of household appliances, electric tools, lighting equipment and similar electrical appliances), branch g (radio interference of information equipment). TC paint was founded in 1999. At present, TC paint has three branches: SC paint a (low frequency phenomenon), SC paint B (high frequency phenomenon) and SC paint C (immunity to high altitude nuclear electromagnetic pulse). The EMC standards of China's procts are based on CISPR and IEC standards. At present, 5 standards have been issued. EMC standards are divided into basic standards, general standards, proct standards and special proct standards. Basic standard: describes EMC phenomenon, specifies EMC test method and equipment, and defines grade and performance criteria. The basic standard does not involve specific procts. Proct standards: EMC testing standards for a certain proct family. Basic standards are often quoted, but more detailed regulations are put forward according to the particularity of procts. General standard: according to the equipment use environment, when there is no specific proct class standard to follow, the general standard is used for EMC test. In order to make the equipment function safe and normal, we should also meet the requirements of these standards. Although there are many EMC standard documents and the contents are complex, the requirements for equipment are divided from the following aspects. Two requirements: EMC standards have two requirements for equipment, one is that the equipment will not proce adverse electromagnetic interference to the outside world when it works, the other is that it cannot be excessively sensitive to external electromagnetic interference. The former is called interference emission (EMI) requirement, and the latter is called sensitivity (EMS) or immunity requirement. From the way of energy transmission: in order to find the requirements of these two aspects, from the way of electromagnetic energy transmission equipment and incoming equipment to further divide, there are concted interference and radiated interference. Concted interference means that the interference energy is transmitted in the form of current along the cable, and radiated interference means that the energy is transmitted in the form of electromagnetic wave. Because the EMC requirements of equipment can be divided into: concted emission, radiated emission, concted sensitivity (immunity), radiated sensitivity (immunity). Electrostatic discharge test is a special kind of test, its interference to the equipment may be conctive or radioactive, depending on the location of electrostatic discharge and the test method
8.
electromagnetic environment
the sum of all electromagnetic phenomena existing in a given place
electromagnetic disturbance
any electromagnetic phenomenon that may cause performance degradation of devices, equipment or systems or damage to living or non living substances
electromagnetic interference (EMI)
the performance degradation of equipment, transmission channel or system caused by electromagnetic interference
radiated emission (RE) and concted emission (CE)
re: useful or unwanted electromagnetic energy transmitted through space. CE: electromagnetic emission transmitted along power supply or signal line
electromagnetic susceptibility (EMS)
the undesirable response of equipment exposed to electromagnetic environment. Radiation sensitivity CS and conction sensitivity CS
transient
a physical quantity or phenomenon that changes between two adjacent states, and its change time is less than the time scale concerned
9. Electromagnetic standard of switching power supply and its interference suppression I. Introction electromagnetic compatibility (EMC) means that electronic equipment or system will not rece performance index e to electromagnetic interference under specified electromagnetic environment level, and the electromagnetic radiation generated by them is not greater than the specified limit level, which will not affect the normal operation of other electronic equipment or system, and reach the requirements of equipment and equipment The purpose of the system is to work reliably without mutual interference. All countries in the world have made their own EMC standards accordingly. For example, 1ec61000 and c1spr series standards of International Electrotechnical Commission, FN series standards of European community, FCC series standards of Federal Communications Commission of the United States and current GT3 / t13926 series EMC standards of China. With the increasing strictness of international EMC regulations, the EMC performance of procts is paid more and more attention. As a kind of power equipment, switching power supply is widely used. With the continuous upgrading of power electronic devices, the switching frequency and switching speed of switching power supply continue to improve, but the rapid on-off of switch causes the rapid change of voltage and current. These transient voltages and currents, coupled by power lines, parasitic parameters and stray electromagnetic fields, will proce a lot of electromagnetic interference. 2、 Electromagnetic interference (EMI) generated by switching power supply can be divided into concted interference and radiated interference according to coupling channel; According to the types of noise interference sources, it can be divided into peak interference and harmonic interference. The surge current and peak voltage generated in the working process of switching power supply form the interference source. The charging and discharging of large capacitor used in power frequency rectification and filtering, the voltage switching of switch tube at high frequency and the reverse recovery current of output rectifier diode are all such interference sources. 3、 Electromagnetic interference is proced by the combination of three basic elements: electromagnetic interference source; Equipment sensitive to the interference energy; The medium that transmits EMI source to sensitive equipment is transmission channel or coupling channel. There are two ways to suppress the EMI generated by switching power supply: one is to rece the interference intensity of the interference source; the other is to rece the interference intensity of the interference source; One is to cut off the transmission of interference. Common anti-interference measures include circuit isolation, shielding, grounding, adding EMI filter and reasonable layout and wiring of PCB board. 1. Circuit isolation in switching power supply, circuit isolation mainly includes: analog circuit isolation, digital circuit isolation, digital circuit and analog circuit isolation. The main purpose is to cut off the path of noise interference by isolating components, so as to achieve the effect of restraining noise interference. For the isolation of analog signal control system of switching power supply, the AC signal is generally isolated by transformer, and the DC signal is generally isolated by linear isolator (such as linear optocoupler). The isolation of digital circuit mainly includes: pulse transformer isolation, optocoupler isolation, etc. The digital input isolation mode mainly adopts pulse transformer isolation and optocoupler isolation; The digital output isolation mode mainly adopts optocoupler isolation and high frequency transformer isolation. 2. Shielding is generally divided into two types. One is electrostatic shielding, which is mainly used to prevent the influence of electrostatic field and constant magnetic field; The other is electromagnetic shielding, which is mainly used to prevent the influence of alternating electric field, alternating magnetic field and alternating electromagnetic field. Shielding is an effective method to suppress the radiation interference of switching power supply. The electric field can be shielded by materials with good conctivity, while the magnetic field can be shielded by materials with high permeability. 3. Grounding in order to prevent various circuits from interfering with each other and make them work compatible with each other, the working grounding is divided into different types according to the nature of the circuit. Such as DC ground, AC ground, digital ground, analog ground, signal ground, power ground, power ground, etc. In the design of the circuit, the AC power ground should be separated from the DC power ground, the analog circuit from the digital circuit, and the power ground from the weak current ground. 4. Add EMI filter. The power filter is installed between the power line and the electronic equipment to suppress the concted interference from the power line and rece the concted interference from the power grid, which plays an important role in improving the reliability of the equipment. The EMI generated by switching power supply is mainly concted EMI, which can be divided into differential mode EMI and common mode EMI. The basic network of switching power supply EMI filter is shown in Figure 1. The filter consists of a common mode choke L, a differential mode capacitor CX and a common mode capacitor cy. The common mode choke l consists of two windings wound on the same high permeability core, and its structure makes the magnetic flux generated by the differential mode current cancel each other. This kind of structure can obtain larger inctance with smaller volume and does not need to worry about saturation e to working current. Each winding and capacitor CY form l-e and N-E two pairs of low-pass filters with independent ports to form a common mode filter network to suppress the common mode interference on the power line. As for the values of common mode choke L, differential mode capacitor CX and common mode capacitor cy, the resonant frequency of the filter should be lower than the working frequency of the switching power supply as far as possible, so that the whole frequency band can be filtered. Figure 1 EMI filter of switching power supply 5. Reasonable layout and wiring of PCB board the radiated interference of switching power supply is proportional to the proct of current size, loop area of current path and square of current frequency, i.e. radiated interference e ∝ I · a · F 2. The premise of using this relationship is that the path size is much smaller than the wavelength of frequency. The above relationship shows that recing the area of the channel is the key to rece the radiation disturbance, which means that the components of the switching power supply should be closely arranged with each other. In the primary circuit, the input capacitance, transistor and transformer should be close to each other, and the wiring should be compact; In the secondary circuit, the diode, transformer and output capacitance should be close to each other. On the printed circuit board, the positive and negative current carrying conctors are respectively laid on both sides of the printed circuit board, and the two current carrying conctors are parallel to each other, because the external magnetic field generated by the parallel and close positive and negative current carrying conctors tends to cancel each other. 4、 Conclusion to improve the switching frequency, improve the quality of switching power supply procts, electromagnetic compatibility is a problem that can not be ignored. There are many factors that cause EMI in switching power supply, and there is still a lot of work to suppress EMI. Only when the EMC problem is fully considered in the design, can the switching power supply be more widely used.
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