EMI filter is an economical and effective device which could significantly suppress the electromagnetic interference and improve the electromagnetic environment. But its efficiency is closed related with the right model selection, here we are to share some information.

1. Electromagnetic interference(EMI) spreading route
As the large scale integrated circuits and the internet grow fast and are widely used, our electromagnetic environment are getting worse and worse. Electromagnetic pollution becomes the fourth pollution source after air, water and noise pollution. The efficient way to solve this problem is to cut off the spreading route of EMI.

Generally speaking, EMI spreads through transmission, radiation or induction. EMI transmitted by metallic conductor is called conducted interference. There must be some kind of metallic conductors while an equipment is functioning, like power line, signal line and control line. All these metallic lines would become EMI spreading route. If we install an EMI filter at the input point of these lines connecting with the equipment, we could cut off the EMI spreading route.

2. Characteristics of EMI filter

1) Two-way low-pass filter

The power line is to provide DC or 50Hz, 400Hz power for the equipment. So the EMI filters could not cut off the DC or power supply, but only the EMI. Because the EMI frequency domain is well above the DC or AC frequency domain, so an EMI filter which allows DC and working frequency to pass but attenuates the electromagnetic interference must be a low pass filter. Ideally, the attenuation curve of  low pass filter is rectangle, also because the circuit is partly symmetry, so it could suppress EMI filters bidirectionally.

2) Mismatch circuit

Filter circuit could contain a single inductor L or a single capacitor C, or single LC unit circuit, or several LC unit circuits. In short, the complexity of the circuit depends on the requirement of the insertion loss (attenuation).
Electromagnetic interference in the spreading process is often decomposed into common mode interference between line to ground and differential mode interference between line to line. So EMI filters have correspondingly, common mode circuit and differential mode circuit.

Take Figure1 which is the most widely used general-purpose circuit as an example, and it is divided into common mode circuit

(a) and differential mode circuit(b) in Figure2

Fig. 1 Circuit for FT110 Series Filter

Fig. 2  (a) Common Mode Circuit
Fig. 2  (b) Differential Mode Circuit

Because the inductive impedance of inductor L is high resistance for EMI (The higher the frequency, the higher the impedance)
The capacitor resistance of capacitor C is low resistance for EMI (The higher the frequency, the lower the impedance)So for the common mode circuit composed of one single LC unit as Fig. 2 (a), input point LG and NG are high resistance, output point L’ N’ and N’ G’ are low resistance.

For differential mode circuit composed of single C as Fig. 2 (b), both input and output points like L N and L’ N’ are low resistance.

So how to choose the circuit of EMI filters?

In order to obtain the best filtering effect, it requires mismatch between filter input end and the impedance connected with it. The same as the output end. The reason is as the following:

Mismatch is as oppose to match. For example: Spectrum analyze, used to measure the  insertion loss of EMI filter, requires the measuring object connected to the input of the spectrum analyzer and cable used for connection are all 50Ω. The display connected to the output of the spectrum analyzer and cable used for connection are 50Ω too. That is because the spectrum analyzer works under matched conditions.

Another example: the power line of a device, of which the front end connected to the grid, and the back end connected to the inverter. In order to suppress the conducted interference, we often add one EMI filter between the power line and inverter. Then the input end of EMI filter connected with power grid, the output end connected with inverter. Generally speaking, power grid is mainly inductive low resistance and inverter power is capacitive low resistance. Whether the impedance of power grid or inverter, they all change as the load and frequency change. So it’s really difficult to get the EMI filter and inverter power grid matched. In other words, EMI filters are in mismatch state while working. In this sense, the mismatch concept is generalized. However, in order to achieve the best suppressing effect, we could expand the mismatch when design a filter’s circuit so as to improve the filtering effect.

For example: the power grid which is mainly low-impedance  , if the input of EMI filter uses high impedance common mode series inductance, when the common mode EMI passing through the filter, most part of the voltage drop would be on the high impedance input side. In other words, most of the EMI loss on the filter. Shown as the left upper part of Table 1.

For power inverter which is mainly low-impedance, the concept is the same. Only difference is the voltage drop would be on the output side which is high impedance common mode series inductance when the common mode part of EMI transmitted from the input end of power inverter to the filter output end. Shown as the right upper part of Table1. All these examples suggest that EMI filter could suppress EMI bidirectionally.

For differential mode interference, the only way to improve the attenuation effect of filter is to make its capacitive reactance much lower than both the  power grid impedance at input side and the inverter power impedance at output side, so as to make the interference loaded on the power grid and the inverter output offset bywhile travel through the filters. Shown as Fig 2 (b) or the left lower part of Table1. The conditions above tend to be more in line with power grid, power inverters often difficult to comply with. So for power inverters, it often needs additional differential mode inductance Lx , as shown in Figure3.

Fig. 3  Differential mode circuit

Finally, according to the principle of mismatch, we can summarize a guidance which is in line with the general rule, see Table1.

Table1  Rules for EMI Filter Connecting

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