Switching power supply electromagnetic standards and interference suppression

　　1. Introduction

　　Electromagnetic compatibility (EMC) means that electronic equipment or systems do not degrade their performance indicators due to electromagnetic interference under specified electromagnetic environment levels. At the same time, the electromagnetic radiation they generate themselves does not exceed the specified limit level and does not affect the normal operation of other electronic equipment or systems. It also achieves the goal of non-interference between devices and systems and reliable operation.

　　Countries around the world have formulated their own EMC standards, such as the IEC 61000 and (C1SPR series standards of the International Electrotechnical Commission, the FN series standards of the European Community, the FCC series standards of the Federal Communications Commission of the United States, and the current GT3/T13926 series EMC standards in my country. With the increasingly stringent international electromagnetic compatibility regulations, the electromagnetic compatibility performance of products has received more and more attention.

　　As a power supply device, the switching power supply is increasingly widely used. With the continuous upgrading of power electronic devices, the switching frequency and switching speed of the switching power supply are constantly increasing, but the rapid on and off of the switch causes rapid changes in voltage and current. These transient voltages and currents, coupled through power lines, parasitic parameters and stray electromagnetic fields, will generate a large amount of electromagnetic interference.

　　2. Analysis of interference sources of switching power supply

　　The electromagnetic interference (EMI) generated by the switching power supply can be divided into conducted interference and radiated interference according to the coupling channel; it can be divided into peak interference and harmonic interference according to the type of noise interference source. The surge current and peak voltage generated by the switching power supply during operation form

The charging and discharging of large capacitors used in power frequency rectification and filtering, the voltage switching of the switching tube during high-frequency operation, and the reverse recovery current of the output rectifier diode are all such interference sources.

　　3. Measures to suppress electromagnetic interference

　　Electromagnetic interference is generated by the combination of three basic elements: the source of electromagnetic interference; the equipment that is sensitive to the interference energy; the medium that transmits the electromagnetic interference source to the sensitive equipment, that is, the transmission channel or coupling path. The suppression measures taken for the electromagnetic interference generated by the switching power supply are mainly considered from two aspects: one is to reduce the interference intensity of the interference source; the other is to cut off the interference propagation path.

　　Common anti-interference measures include circuit isolation, shielding, grounding, installation of EMI filters, and reasonable layout and wiring of PCB boards.

　　1. Circuit isolation

　　In the switching power supply, the isolation of the circuit mainly includes: isolation of analog circuits, isolation of digital circuits, and isolation between digital circuits and analog circuits. The main purpose is to cut off the path of noise interference through isolation components, so as to achieve the effect of suppressing noise interference. For the isolation of the analog signal control system of the switching power supply, AC signals are generally isolated by transformers, and DC signals are generally isolated by linear isolators (such as linear photocouplers). The isolation of digital circuits mainly includes: pulse transformer isolation, photocoupler isolation, etc. Among them, the digital input isolation method mainly adopts pulse transformer isolation and photocoupler isolation; while the digital output isolation method mainly adopts photocoupler isolation and high-frequency transformer isolation.

　　2. Shielding

　　Shielding is generally divided into two categories: one is electrostatic shielding, which is mainly used to prevent the influence of electrostatic fields and constant magnetic fields; the other is electromagnetic shielding, which is mainly used to prevent the influence of alternating electric fields, alternating magnetic fields and alternating electromagnetic fields. Shielding is an effective way to suppress the radiation interference of switching power supplies. Electric fields can be shielded with materials with good conductivity, while magnetic fields can be shielded with materials with high magnetic permeability.

　　3. Grounding

　　In order to prevent various circuits from interfering with each other during operation and make them work compatibly 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 supply ground, etc. In the design of the circuit, the AC power supply ground should be separated from the DC power supply ground, the power supply ground of the analog circuit and the digital circuit should be separated, and the power ground should be separated from the weak current ground.

　　4. Install EMI filter

　　The power filter is installed between the power line and the electronic equipment to suppress the conducted interference from the power line and reduce the conducted interference introduced from the power grid, which plays an important role in improving the reliability of the equipment. The electromagnetic interference generated by the switching power supply is mainly conducted interference, which is divided into differential mode interference and common mode interference. The basic network of the 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 magnetic core, and its structure makes the magnetic flux generated by the differential mode current cancel each other. This structure obtains a larger inductance value with a smaller volume, and there is no need to worry about saturation due to the working current. Each winding and the capacitor Cy respectively form a low-pass filter with two pairs of independent ports LE and NE, forming a common mode filter network to suppress the common mode interference on the power line. As for the value of the common mode choke L, the differential mode capacitor Cx and the common mode capacitor Cy, the resonant frequency of the filter should be lower than the operating frequency of the switching power supply as much as possible, so that filtering of the entire frequency band can be achieved.

　　
Figure 1 Switching power supply EMI filter

　　5. Reasonable layout and wiring of PCB board

　　The radiation interference of the switching power supply is 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·f2. The premise of using this relationship is that the path size is much smaller than the wavelength of the frequency. The above relationship shows that reducing the path area is the key to reducing radiation interference, which means that the components of the switching power supply must be arranged closely to 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, the positive load current-carrying conductors are arranged on both sides of the printed circuit board, and try to keep the two current-carrying conductors parallel to each other, because the external magnetic fields generated by the parallel and close positive load current-carrying conductors tend to cancel each other.       4. Conclusion

　　To increase the switching frequency and improve the quality of switching power supply products, electromagnetic compatibility is an issue that cannot be ignored. There are many factors that cause electromagnetic interference in switching power supplies, and there is still a lot of work to be done to suppress electromagnetic interference. Only by fully considering electromagnetic compatibility issues during design can switching power supplies be more widely used.