Technical Reference Notes

ALD12/18/20/25 Series

(Single Output 16

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 FCC Class B conducted noise levels is shown below (See Figure 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