Filter technology is an effective measure to suppress interference, especially in dealing with switching power supply EMI signal transmission interference and some radiation interference, has obvious effect.
Any power line conduction interference signal, can be used differential mode and common mode interference signal to represent.
Differential mode interference is transmitted between two conductors, belonging to symmetry interference; common mode interference between the wire and the ground (chassis) transmission, are asymmetric interference. Under normal circumstances, the differential mode interference is small, the frequency is low, the interference caused by smaller, common mode interference amplitude, high frequency, but also through the wire to produce radiation, caused by greater interference. Therefore, to weaken the conduction interference, the EMI signal control in the relevant EMC standards below the limit level. In addition to suppressing interference sources, the most effective way is to install the EMI filter in the switching source input and output circuits. The operating frequency of the general equipment is about 10 ~ 50 kHz. EMC limits the limits of the conducted interference levels specified by many standards from 10 kHz. On the switching power supply generated high-frequency EMI signal, as long as the choice of the corresponding decoupling circuit or network structure is relatively simple EMI filter, it is not difficult to meet the EMC standard filtering effect.
Is the surge grid appears on the surge voltage, ringing voltage, spark discharge and other instantaneous interference signal, which is characterized by very short time, but the voltage amplitude is high, the transient energy. Transient interference will cause the fluctuation of the output voltage of the monolithic switching power supply. When the transient voltage is superimposed on the rectified filtered DC input voltage VI, the VI exceeds the drain-source breakdown voltage V (BR) DS of the internal power switch , But also damage the TOPSWICTH chip, it must be used to suppress measures. In general, electrostatic discharge (ESD) and electrical fast transient burst (EFT) damage to digital circuits is more than its impact on analog circuits. Electrostatic discharge produces a strong RF radiation in the frequency range of 5 - 200MHz. The peak of this radiant energy often occurs between 35MHz and 45MHz. The resonant frequencies of many I / O cables are also usually within this frequency range, and as a result, a large amount of electrostatic discharge energy is stringed in the cable. When the cable is exposed to a 4 - 8 kV electrostatic discharge environment, the I / O cable terminal load can measure the induced voltage up to 600V. This voltage is far beyond the typical digital threshold voltage of 0.4V. A typical inductive pulse duration is approximately 400 nanoseconds. The I / O cable is shielded and grounded at both ends so that the internal signal leads are all shielded. The interference can be reduced by 60 - 70dB and the induced voltage on the load is only 0.3V or less. Electrical fast transient bursts also produce relatively strong radiation emissions, which are coupled to the cable and chassis lines. The power line filter protects the power supply. The line-to-ground common-mode capacitor is an effective device that suppresses this transient interference, which bypasses the jam to the chassis while away from the internal circuitry. When the capacitance of the capacitor is limited by the leakage current and can not be too large, the common mode choke must provide greater protection. This usually requires the use of a special center-tapped common-mode choke coil, which is connected to the housing through a capacitor (the capacity is determined by the leakage current). The common mode choke is usually wound around a high permeability ferrite core with a typical inductance of 15 to 20 mH.