EMI power filter foundation selection analysis and installation precautions

EMI power filter acts as two low-pass filters: one is to attenuate common-mode interference and the other is to attenuate differential-mode interference. EMI power filter can attenuate RF energy within the stopband range, and let the power frequency pass through the EMI power filter without attenuation or with little attenuation. So how can we maximize the role of the filter? What should we pay attention to in the layout and installation of the filter? The following is a brief analysis of EMI power filter for your reference.

1、 Key points of EMI power filter selection

1. Consider rated voltage

The rated voltage of the power filter refers to the highest voltage when the filter is working. The rated voltage of the filter must be higher than the input voltage during the selection, generally higher than 30% - 50%. Take the single-phase 220V voltage of 50/60Hz as an example, the rated voltage of the filter is generally 250V or even larger.

2. Consider rated current

The rated current of the filter refers to the continuous maximum current that can pass under the rated voltage. The current inside the filter will flow through the inductor. If the current exceeds the rated current, the filter will not be damaged immediately, but the insulation layer will be damaged due to the heating of the inductor coil for a long time.

3. Consider insertion loss

For filters, insertion loss is a very important parameter. On the same load, the ratio of the power received by the load before the filter is connected to the power received by the load after the filter is the insertion loss, whose unit is dB. When selecting the type, the larger the dB, the stronger the anti-interference ability.

4. Consider certification

Nowadays, electronic products require a lot of certification, and different countries have different requirements for certification, such as UL certification in the United States and CE certification in the European Union. Products sold to these regions need to comply with their certification, so it is also very important to select filters according to the certification situation.

2、 EMI power filter installation precautions

1. EMI filter and housing are well lapped

In case of poor connection, the shell of EMI filter does not fully contact the shell of the equipment under test, resulting in the ground terminal of the common-mode filter capacitor in the filter being suspended and becoming floating ground, so the common-mode filter capacitor cannot filter the common-mode disturbance, which will generate a higher common-mode disturbance voltage on LISN. After the bonding is good, that is, after the metal shell of the filter is directly connected to the shell of the equipment under test, the common-mode filter capacitor in the filter returns to normal operation, and the common-mode disturbance voltage on the power line can be filtered, so the measured disturbance voltage can be reduced.

2. Pull the EMI filter input and output cable far away

When the filter input cable is close to the output cable, parasitic capacitance will be generated between the cables, so the disturbance with higher frequency generated on the main board in the chassis is more easily coupled to the input cable of the filter through the parasitic capacitance, which worsens the filtering performance of the power filter and cannot achieve the normal filtering function of the disturbance voltage. Therefore, it is necessary to ensure that the EMI filter input and output cables maintain a certain distance.

3. EMI filter is close to the power inlet

The input cable of the filter is too long. All kinds of high-frequency disturbances in the enclosure are directly coupled to the input cable through space. At the same time, after the electromagnetic radiation from the outside is transmitted into the cabinet from the input cable, it is coupled to the main board of the equipment in the form of space coupling without passing through the filter. After the superposition of the two factors, the disturbance voltage on the power line increases.

When the filter is installed at the enclosure outlet of the power line, all disturbance signals entering the enclosure through the power line must pass through the filter first, and all disturbance signals coming out of the enclosure will also pass through the filter. At this time, the filter can play the maximum filtering role.