AN296 Silicon_Laboratories, AN296 Datasheet - Page 20

• Manufacturer Part Number: AN296
• Description: Using THE Si3400 AND Si3401 POE PD Controllers IN Isolated AND Non-isolated Designs
• Manufacturer: Silicon_Laboratories

AN296
The loops carrying high ac current are as follows:
These loops should be kept small in diameter to avoid EMI. An effective way of doing this is to route the return for
the loop underneath the source for the loop as much as possible, effectively shrinking the loop diameter to almost
zero.
Ferrite beads L2–L5 are primarily for conducted emissions reduction, although they have been found to improve
radiated emissions as well.
Using these methods, the test result shown in Figure 16 has been obtained for EMI.
Lab testing of conducted emissions has shown that if the power output is not referenced to earth ground (as is the
case in many applications), conducted emissions are not a problem because there are no path-induced common-
mode disturbances on the loop. However, it has been found that if the output has an earth ground reference,
common-mode current can be quite high at lower frequencies.
20
The loop from the input filter, C1–C4, to the transformer retuning from the transformer to SWO, then to V
back to the input filter capacitors.
The loop from the transformer secondary to D3, the filter capacitor C6, and returning to the transformer
secondary.
The loop associated with the primary side snubber and transformer.
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0
10.0M
Figure 16. Radiated Emissions Test Result
Frequency (Hz)
Rev. 0.8
100.0M
SS
1.0G
and