1. Suppress various electromagnetic interference sources in switching power supplies
Soft switching technology is an important means to reduce the loss of switching devices and improve the electromagnetic compatibility of switching devices. Switching devices will generate surge currents and peak voltages during the switching process, which are the main causes of electromagnetic interference and switching losses. The use of soft switching technology to enable switching tubes to perform switching transitions at zero voltage and zero current can effectively suppress electromagnetic interference. Using a snubber circuit to absorb the peak voltage across the switching tube or primary coil of a high-frequency transformer can also effectively improve the electromagnetic compatibility characteristics.
2. Cut off the transmission path of electromagnetic interference - the design of common mode and differential mode power line filters
A common mode inductor consists of two windings on the same magnetic ring with opposite winding directions and the same number of turns. Generally, annular cores are used, which have small magnetic leakage and high efficiency, but are difficult to wind. When the power frequency current of the city network flows through the two windings, it enters and exits, and the magnetic field generated exactly cancels out. In this way, the common mode inductor will not hinder the power frequency current of the city network and can be transmitted without loss. If there is a common mode noise current passing through the common mode inductor in the city network, the direction of the common mode noise current will be the same. When it flows through the two windings, the generated magnetic field is superimposed on the same phase, causing the common-mode inductor to show a larger inductive reactance to the interference current, which plays a role in suppressing common-mode interference.
3. Use shielding to reduce the sensitivity of electromagnetic sensitive equipment
Shielding is an effective way to suppress radiated noise. Materials with good conductivity can be used to shield electric fields, and materials with high magnetic permeability can be used to shield magnetic fields. In order to prevent the leakage magnetic field of the transformer and ensure good primary coupling, a closed magnetic ring can be used to form a magnetic shield. For example, the leakage flux of pot cores is much smaller than that of e-cores. The connecting lines and power lines of the switching power supply should use shielded conductors to prevent external interference from coupling into the circuit. Or use EMC components such as magnetic beads and magnetic rings to filter out high-frequency interference from power supply and signal lines. However, it should be noted that the signal frequency should not be interfered by electromagnetic compatibility components, that is, the signal frequency should be within the range of the filter. The shell of the entire switching power supply also needs to have good shielding characteristics, and the connectors should meet the shielding requirements specified by EMC. The above measures ensure that the switching power supply will not be interfered by the external electromagnetic environment and will not interfere with external electronic equipment.
