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1、Conduction Interference
  Conduction is one of the main interference coupling paths between interference source and sensitive equipment. Conduction interference can propagate through conductors like power line, signal line, interconnections, shield configuration, grounding conductors etc.
  The effective way of preventing conduction interference is to filter before interference conducted into sensitive circuit.
  Conduction coupling, namely interference existing in one electrical network couples into another one, belongs to relatively low frequency (low 30MHz).
2、Electromagnetic Interference Filter
  Electromagnetic interference filter (namely EMI filter) including signal line filter and power line filter, is the most effective way of suppressing conduction interference.
  Signal line filter greatly suppresses clutter interference without suppression of useful signals.
  Power line filter can also be called power network filter. DC, AC of 50Hz and 400Hz can betransmitted to equipment with very low attenuation. On the contrary, EMI conducted frompower supply is greatly attenuated in the procession. At the same time, it can prevent EMI produced by equipments polluting electromagnetic environment and doing harm to other equipments.
2.1 Characters Preface
  EMI filter is not equal to the normal low pass filter. Concerns on filter index and application environment are greatly distinct between them.
  Low pass filter pays more attention to characteristics such as amplitude frequency characteristic, phase index, group delay, waveform distribution, while EMI filter puts more emphasis on the characteristics of insertion loss, energy attenuation, cut off frequency etc.

  Compared from the application environment, low pass filter works in environment of low level, low current and single character of source and load terminal. However, EMI filter and can endure temporary impact of high current.   What¨s more, source and load terminal character variant greatly with working environment. Differences dismounted above determinate the different design technologies between them.
2.2 Function
  Cutting off interference path and building perfect protection from interference together with shield onfiguration.

2.3 Principle
  Cut-off frequency, defined as the frequency under which insertion loss is 3dB, is of high importance to choose reasonable EMI filter.
  Requirement that working frequency is below cut-off frequency must be strictly meet.
3、EMI filter analysis
3.1 Network structure of EMI filter (one)

  Common mode filter performance of EMI filter is highly restricted by the common mode capacitor (Cy), because grounding leakage current is relative significantly to Cy. For the importance of grounding leakage current to
personal safety, it is extraordinarily restricted in many countries.Given the maximum value of leakage current of equipment, the maximal value allowable of grounding capacitor (common mode capacitor) can be preliminarily calculated by the following formula:

  Differential mode capacitor (Cx) is the capacitor whose failure will not result in electril shock phenomenon and endanger personal safety.
If safety performance, namely voltage withstanding properties, is not satisfactory, filter performance of EMI filter will be greatly weakened, so withstand voltage test of 1500-1700 DC for one minute is absolutely necessary.
4、Leakage current test principle of EMI filter
Leakage current under the voltage of 250VACis the current flowing through phase line, mid line and shell of EMI filter (grounding line), which mainly determined by the value of grounding capacitor (common mode capacitor). Excellent performance of common mode capacitor in high insertion loss is normally at the cost of large leakage current. In order to provide outstanding combined properties, filter should be designed according to the working environment and give overall consideration to electrical properties and safety. Test circuit of leakage current is as follows:

5、Installation of EMI filter CThe third primary function of EMI filter
5.1 Incorrect Installation

5.2 Incorrect Installation of EMI filter

5.3 Correct Installation

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