EMC power knowledge

Electromagnetic standard of switching power supply and its interference suppression I. Introduction Electromagnetic compatibility (EMC) means that the performance index of electronic equipment or system will not be reduced due to electromagnetic interference under the specified electromagnetic environment level, and the electromagnetic radiation generated by itself will not be greater than the specified limit level, which will not affect the normal work of other electronic equipment or systems, so as to achieve the purpose of non-interference and reliable work between equipment and systems. All countries in the world have formulated their own electromagnetic compatibility standards accordingly. For example, IEC's 1EC6 1000 and (C 1SPR series standards, European * * * equivalent FN series standards, American Federal Communications Commission's FCC series standards and China's current GT3/T 13926 series EMC standards, etc. With the increasingly strict international electromagnetic compatibility regulations, the electromagnetic compatibility of products becomes more and more strict. As a kind of power supply 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 are constantly improved, but the rapid on-off of switches causes rapid changes in voltage and current. These transient voltages and currents, together with power lines, parasitic parameters and stray electromagnetic fields, will produce a lot of electromagnetic interference. Second, the interference source analysis of switching power supply The electromagnetic interference (EMI) produced by switching power supply can be divided into conduction interference and radiation interference according to the 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 by switching power supply in the working process form interference sources, such as charging and discharging of large capacitors for power frequency rectification and filtering, voltage switching of switching tubes at high frequency, reverse recovery current of output rectifier diodes, etc. Third, the suppression measures of electromagnetic interference electromagnetic interference is produced by the combination of three basic elements: the source of electromagnetic interference; Equipment sensitive to interference energy; The medium for transmitting electromagnetic interference sources to sensitive equipment is transmission channel or coupling mode. The measures to suppress the electromagnetic interference generated by switching power supply are mainly considered from two aspects: one is to reduce the interference intensity of interference sources; The first is to cut off the transmission route of interference. Common anti-interference measures include circuit isolation, shielding, grounding, installation of 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, and isolation between digital circuit and analog circuit. The main purpose is to cut off the path of noise interference through isolation elements, so as to achieve the effect of suppressing noise interference. For the isolation of analog signal control system of switching power supply, AC signal is generally isolated by transformer, and DC signal is generally isolated by linear isolator (such as linear optocoupler). The isolation of digital circuits mainly includes: pulse transformer isolation, photoelectric coupler isolation and so on. Among them, digital input isolation mainly adopts pulse transformer isolation and photoelectric coupler isolation; The isolation mode of digital output mainly adopts photoelectric coupler isolation and high frequency transformer isolation. 2. Shielding is generally divided into two categories, 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 conductivity, and 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 compatible, working grounding is divided into different types according to the nature of the circuits. Such as DC ground, AC ground, digital ground, analog ground, signal ground, power ground, power ground, etc. In circuit design, AC power supply should be separated from DC power supply, analog circuit should be separated from digital circuit, and power supply ground should be separated from weak current ground. 4. Add EMI filter. The power filter is installed between the power line and the electronic equipment, which suppresses the conducted interference from the power line and reduces the conducted interference from the power grid, and plays an important role in improving the reliability of the equipment. The electromagnetic interference produced by switching power supply is mainly conducted interference, which is divided into differential mode interference and * * * mode interference. The basic network of switching power supply EMI filter is shown in figure 1. The filter consists of * * * mode choke L, differential mode capacitor Cx and * * * mode capacitor Cy. * * * mode choke L consists of two windings wound on the same high permeability magnetic core, and its structure makes the magnetic flux generated by differential mode current cancel each other out. This structure can obtain a larger inductance value with a smaller volume, without worrying about working current saturation. Each winding and capacitor Cy form two pairs of low-pass filters with independent ports, L-E and N-E, respectively, to form a * * * mode filter network, which is used to suppress the * * * mode interference existing on the power line. As for the values of * * * mode choke L, differential mode capacitor Cx and * * * mode capacitor Cy, the resonant frequency of the filter should be lower than the working frequency of the switching power supply as much as possible, so as to filter out the whole frequency band. Figure 1 EMI filtering of switching power supply 5. The rational layout of 5. The radiation interference between PCB board and wiring switching power supply is directly proportional to the product of the current in the current path, the loop area of the path and the square of the current frequency, that is, the radiation interference E ∝ I A F 2. The premise of applying this relationship is that the channel size is much smaller than the wavelength of frequency. The above relationship shows that reducing the channel area is the key to reduce radiation disturbance, that is to say, the elements of switching power supply should be closely arranged with each other. In the primary circuit, the input capacitor, transistor and transformer are required to be close to each other and the wiring is compact; In the secondary circuit, the diode, transformer and output capacitor are required to be close to each other. On the printed circuit board, positive load current conductors are distributed on both sides of the printed circuit board, and the two current-carrying conductors remain parallel to each other, because the external magnetic fields generated by parallel and close positive load current conductors tend to cancel each other out. Four. Conclusion In order to improve the switching frequency and the quality of switching power supply products, electromagnetic compatibility is an issue that cannot be ignored. There are still many factors that cause electromagnetic interference of switching power supply, and there is still a lot of work to be done to suppress electromagnetic interference. Only when electromagnetic compatibility is fully considered in the design can switching power supply be more widely used.