I want to design a low pass filter with common-mode choke that has a cutoff frequency of 100 Hz. This is going to be used in a bus 24V DC power system that will deliver 10A to the load. I want to use it to clear EMI, RF interference and noises. Since there is no AC line in a bus, 50 or 60 Hz noise will be no problem.
- Will Butterworth LC pi filter be a good choice?
- If so, I was given 470uH and 1.8mH common-mode chokes. How to calculate capacitor values?
- Will ouput impedance of the filter matter?
Best Answer
Common-mode (CM) chokes on AC primary inputs serve in both directions to:
prevent conducted noise from device or circuit pulse currents from coupling back into the AC source and in turn causing radiated noise via the AC power cable. (CE/FCC conducted emissions spec.)
reduce switch-mode noise from other devices causing ingress noise interference to this device. In general, the range of frequencies of concern are 10KHz to 1MHz, NOT 100 Hz, as this would require huge passive values in the filter (read cost over-kill).
Generally this is handled by using high impedance, high permeability CM ferrite wire wound in a compact part with small high voltage caps that if connected to ground must not exceed leakage currents for Common mode AC test for ground fault leakage safety tests (CE/UL test usually 0.5mA max; leakage typically much less).