Introduction of Insertion Loss
Insertion loss is the most important indicator of the performance of EMI filter. People always wrongly think that the insertion loss is the higher the better. Actually it depends on the nature of the electromagnetic interference, the purpose is to solve the electromagnetic interference(EMI) problem or electromagnetic susceptibility(EMS) problem, and the requirement of the standards or specifications.
For example, switch power supply is a source of electromagnetic interference, so we should learn its switching frequency and its power. To learn its switching frequency is because it decides the effective starting frequency to be suppressed(Only the fundamental frequency being effectively suppressed, it is possible to suppress its higher harmonics). For military standard the starting frequency is very low, so to learn the switching frequency is specially important. To learn its power is because the greater the power, the higher the switch current and switch voltage, of course the greater the noise generated, so it is the EMI problem to be solved. Commonly, noise created by switch power is mainly differential mode interference, so for large power switch power supply, its EMI filter often adopts differential mode inductance. Crystals is also a source of electromagnetic interference, its oscillation frequency is much higher than the switching frequency of switch power supply, but its power is much lower than that of the switch power supply, so the EMI filter needed to suppress its interference may only need a capacitor.
Another example, remote automatic meter reading device is a power metering device, it is very sensitive to the pulse group interference from the surrounding area (FET). Because the spectrum of pulse group interference is extremely wide, above 300MHZ, the effect of using conventional EMI filter circuit to suppress the noise is not ideal, while absorbing circuit will be much better, so it is electromagnetic susceptibility(EMS) problem to be solved. But for switching power supply, EMI filter can solve the general EMS problem since EMI filter is two-way suppression. So the reasonable insertion loss should be fixed according to the actual situation.
From the specification we can see that common mode and differential mode insertion loss of different kind of EMI filter is different, generally speaking: performance of insertion loss of single stage LC cell circuit is worse, while that of the two-stage LC cell circuit is better; filter with lower rated working current is better then filter with higher rated current; single-phase circuit is better than three-phase circuit. When the rated current is the same, three-phase three-line circuit is better than three phase four-line circuit.
How to Test Insertion Loss
Insertion loss indicates the performance of a filter. It is the ratio of terminal voltage before filter is inserted to that when filter is inserted (measured in dB). It is tested with a system of empty load and 50Ω-50Ω according to the international standard UL1283，the formula is as the following:
Insertion loss is composed of common mode and differential mode, its testing electrical schematics is as the following:
Notes: The insertion loss of filters provided by our company is measured according to the UL1283 Standard, using Spectrum Network Analyzer under the ideal condition of empty load, 50Ω-50Ω. The actual effect please follow the field data of real application.