EMI input switching

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Hi community! I have a doubt with a common mode choke (EMI) for my switching source. I use ST1S14 chip, It work with 850 kHz and 3A output(MAX), but I´m working with 500mA.. my question is.. for select the EMI,  I use a part with high impedance in this frecuency (850kHz)? what is the parameter to evaluate for the selection?