Q: What are the best EMC filter designs for power input of embedded systems?
Answer
Power input EMC filter design: (1) Common-mode filter: two line-to-ground Y-capacitors (2.2-22 nF Y2 rated) + common-mode choke (1-10 mH). Common-mode current is the main radiated emission source on power cables. (2) Differential-mode filter: X-capacitors (0.1-1 uF film capacitor) across line and neutral. (3) Pi filter (C-L-C): place capacitors on both sides of the inductor for best attenuation. (4) MOV (varistor): 275V AC for surge protection in parallel with line-neutral. (5) TVS diode: for fast transient protection. Filter placement: filter should be as close to the connector as possible with a solid ground plane beneath. The filter should be enclosed in a shielded compartment. Calculate filter cutoff: fc = 1/(2*pi*sqrt(L*C)) for LC filter. For CISPR 32 Class B (residential) target >40 dB attenuation from 150 kHz to 30 MHz. Use NDC series common-mode chokes (TDK DCT series) sized for the current.
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