How to Select an EMI Power Line Filter?
Power line filter is electrical equipment which effectively filters particular frequency point in the power line or the frequency out of the particular frequency point. It is a passive bi-directional network, that is, one end is power supply, and the other end is load. When selecting an EMI power line filter, what should be taken into considerations?
1. Rated voltage
Rated voltage is the working voltage of EMI power supply at specified power frequency, which is the maximum allowable voltage value of filter. If the input voltage of filter is too high, it may damage the capacitor inside.
2. Rated current
Rated current (Ir) is the maximum allowable continuous operating current at the rated voltage and specified ambient temperature.
Along with the ambient temperature rise, or operating temperature higher than indoor temperature due to inductive wire’s copper loss, magnetic core loss and ambient temperature, it is difficult to guarantee insertion loss performance. Therefore, the rated current of EMI power line filter should be selected according to the maximum operation current and ambient temperature possibly in the actual work.
3. Insertion loss
Insertion loss is one of the most important technical parameters of EMI power filter. In the premise of ensuring the safety, environment, machinery and reliability of the filter to meet the requirements of the relevant standards, the insertion loss can be achieved as high as possible.
The insertion loss of EMI Power Filter is a function of frequency, expressed in db. Insertion loss of EMI power line filter includes common mode (CM) insertion loss and differential mode (DM) insertion loss.
The factors affect insertion loss of EMI power filter including impedance matching and installation. In practical application, the impedance of EMI power filter input and output terminal is not 50Ωas tested, therefore, its attenuation to interference signal is not equal to the one as product standard or the manual mentioned. If selecting EMI power filter with reasonable network structure and parameters, and installing properly, it may achieve better insertion loss than specified in the standard. If not, the effect will not be good.
The other factor is operation temperature and rated current of filter. Since the inductive L of the line filter is made from ferrite or other magnetic materials, under large current operation, magnetic saturation state causes performance deterioration, which is much different from one at rated current in the test.
4. Operating environment
One of the main reasons affecting the operating current and environment relation is soft magnetism material in EMI line filter. EMI power line filter normally uses high permeability soft magnetism material, Mn-Zn ferrite, the initial permeability μi=7000～10000，but its Curie point temperature is not high, and high quality is around 130℃. The higher the permeability is, the lower the Curie point temperature is. The permeability drops down rapidly after the Curie point, then leading to inductive value drop down of EMI power filter, which seriously affects filtering effect.
Therefore, rated current of power filter should be properly selected according to operating temperature, or improve filter heat radiation terms (working environment) to guarantee installation and usage of filter.
The suppression capability of EMI power filter to EMI signal is not only dependent on filter insertion loss tested within 50Ω system, but also dependent on the proper terminal connect of filter network, EMI signal source and load. Therefore, when selecting a filter, the content on the label of EMI filter should be paid attention to, to check whether it shows filter’s network parameter and structure accurately.
Besides, select a filter according to allowable value of maximum leakage current, especially for some medical care equipment.