Design method for electromagnetic compatibility of power adapter

　　Electromagnetic compatibility has become an important part of modern electronic product design. The testing and certification of electromagnetic compatibility has been developed from a country to a global area, making electromagnetic compatibility and safety and environmental adaptability equally important. For example, the European Community incorporates the electromagnetic compatibility requirements of products into technical regulations, enforces Directive 89/336/EEC, stipulates that electrical and electronic products must comply with electromagnetic compatibility requirements from January 1, 1996, and affix the CE mark Only then can it be sold in the market. In order to be in line with international standards, the Ministry of Foreign Affairs of my country and the State Entry-Exit Inspection Bureau implemented mandatory electromagnetic compatibility testing on personal computers, monitors, printers, switching power adapters, televisions and audio equipment in January 1999. The State Bureau of Technical Supervision has stipulated that since October 2002, sound and television broadcasting equipment, information technology equipment, household appliances, power tools, power adapters, lighting appliances, electric ignition drives, financial settlement electronic equipment, security electronic products and low-voltage Electrical appliances implement compulsory certification of electromagnetic compatibility. The power supply adapter in the system often becomes a source of electromagnetic interference due to factors such as the on-off transition process of the load, the nonlinearity of semiconductor components, the coupling of pulse equipment and lightning. The electromagnetic compatibility design method of the power supply adapter system is:

　　1. Shield the power adapter transformer

　　Due to the distributed capacitance between the primary and secondary of the power adapter transformer, high-frequency interference entering the primary of the power adapter transformer can be coupled to the secondary of the power adapter transformer through the distributed capacitance. After the shield is added between the primary and secondary of the power adapter transformer, a new distributed capacitance is formed between the shield and the winding. Grounding the shielding can introduce high-frequency interference to the ground through this new distributed capacitance, thereby playing a role in anti-electromagnetic interference. The shield should be made of a material with good conductivity, and the end and the end cannot be closed to avoid short circuit.

　　2. Use a low-pass filter

　　The low-pass filter does not hinder the passage of power frequency electric energy, but is in a high-impedance state to high-frequency electromagnetic interference, and has a strong suppression ability. When using an AC low-pass filter, the type of filter should be selected according to its impedance at both ends and the required insertion attenuation coefficient. Pay attention to its ability to withstand voltage and conduct current. The shield and the case should be in good electrical contact, the ground wire should be as short as possible, the cross section should be large enough, the incoming and outgoing wires should be far away, and the filter should be as close as possible to the power supply adapter.

　　3. Absorption of pulse voltage

　　The electromagnetic interference to the pulse voltage can be absorbed by a varistor, a solid discharge tube or a transient voltage suppression diode. When the pulse voltage absorption device is subjected to a high-energy transient overvoltage pulse, its operating impedance can be immediately reduced to a very low level, allowing a large current to pass through, absorbing a large amount of power, and thus clamping the voltage within an allowable level.

　　Varistors or solid discharge tubes can be used in DC or AC circuits. Unidirectional transient voltage suppression diodes are used in DC circuits, while bidirectional transient voltage suppression diodes are used in AC circuits. When using a pulse voltage absorption device, the rated voltage should be slightly higher than the maximum working voltage of the equipment to ensure that the power consumption of the absorption device is the least when there is no pulse voltage; when there is a pulse voltage, its clamping voltage should be lower than The highest insulation voltage of the equipment to ensure the safety of the equipment; its current capacity should be greater than the current generated by the pulse voltage.

　　4. Other electromagnetic compatibility measures for power adapters

　　The control circuit and power circuit adopt split-phase power supply or adopt different power adapters; adopt UPS (uninterruptible power adapter) for power supply; adopt power adapter voltage monitoring integrated circuits.

　　5. EMC measures for DC power adapters

　　The high-frequency filtering of the rectifier circuit is to connect a small capacitor (0.01μF) in parallel with the rectifier tube to further filter the high-frequency interference from the transformer. DC decoupling means connecting two capacitors in parallel between the DC power adapter and the ground. The large capacitor (10～100μF) filters out low frequency interference, and the small capacitor (0.01～0.22μF) filters out high frequency interference.

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