Technical Reference Notes
ALD12/18/20/25 Series
(Single Output 16
th
Brick)
MODEL: ALD12/ 18/20/ 25 SERIES SHEET 17 OF 20
AUGUST 17, 2005 – REVISION C
Input Filter for FCC Class B Conducted Noise
A reference design for an input filter that can provide FC C Class B conducted noise levels is sho wn belo w (See F igure 42). Two
common mode connected inductors are used in the circuit along with balanced bypass capacitors to shunt common mode currents
into the ground plane. Shunting noise current back to the converter reduces the amount of energy reaching the input LISN for
measurement.
The application circuit shown has an earth ground (frame ground) connected to the converter output (-) terminal. Such a
configuration is common practice to accommodate safety agency requirements. Grounding an output terminal results in much
higher conducted emissions as measured at the input LISN because a hard path for common mode current back to the LISN is
created by the frame ground. “Floating” loads generally result in much lower measured emissions. The electrical equivalent of a
floating load, for EMI measurement purposes, can be created by grounding the converter output (load) through a suitably sized
inductor(s) while maintaining the necessary safety bonding.
Figure 42: Class B Filter Circuit
PARTS LIST
CKT CODE DESCRIPTION
Common
Mode Choke
CTX01-15091
Cooper Electronic
Technologies
X-Cap 0.47 µF X 4pcs
Y-Cap 22 nF X 4 pcs
Cin 33 uF
FILTER
