CMC for PM8805. Do I need it?

[SIlia.1]

At the datasheet at figure 7 you use CMC befor connecting it to DC2DC.

I connect the PM8805 to DC2DC LTM8027.

If I don't put the CMC will it be worng?

BR

Shmuel

[Peter BENSCH]

Welcome, @SIlia.1​, to the community!

the data sheet of the PM8805 writes about the necessity of a common mode filter in section 4.9:

In most of the PoE applications a common mode filter is required to allow the PoE converter to be compliant with the applicable EMI standards. The right position where such a filter is introduced is between PM8805 and the PoE converter, as shown in Figure 7.

Not using a common mode filter cannot therefore be said to be "wrong", but you should ensure that you comply with EMI regulations.

Does it answer your question?

Regards

/Peter

[SIlia.1]

Hi Peter,

Thanks for the fast respond.

The answer is very clear.

At the same figure there are 2 CMC.

Which one is prefered? the t? or the L? Do you have a prefered part number?

BR

Shmuel

[Peter BENSCH · Accepted solution]

Figure 7 of the data sheet is only a simplified circuit to illustrate the principle. Further information can be found in the design tip DT0149.

There is no generally applicable recommendation for the common mode filter because its dimensioning depends on factors such as the type of SMPS or PoE class.

Regards

/Peter

[SIlia.1]

Thanks a lot

[Peter BENSCH]

You're welcome!

If the problem is resolved, please mark this topic as answered by selecting Select as best under your preferred answer. This will help other users find that answer faster.

/Peter