Filtering, shielding and grounding are the three general methods for EMC design. The filtering and grounding mainly for the conduction disturbances in the circuit, and shielding is mainly for the radiation interference in the circuit. And filtering is the most effective and economical method to suppress the conduction interference in the circuit. Due to various disturbances is the most serious at the entrance of the system, EMI filters are usually inserted into the interface of the system or the supply side.
The types of electronic ballast used EMI filter are C-type (a capacitor), L-type (an inductor and a capacitor), T-type (two inductors and one electrical), Π-type (an inductor and two capacitors), dual Π-type (a common mode inductor and two capacitors), mixed type and so on.
In general, for 9k ~ 150kHz low frequency stage, it would be more effective by using differential mode filter, while 150kHz ~ 30MHz high frequency stage using common mode filter, and sometimes is not enough to use the level common mode filter but using two common-mode filters.
When selecting the EMI filter element, we should ensure that the resonant frequency of the filter is much lower than the operating frequency, in case of magnetic saturation and cann’t work.