Mains filter connected wrongly

[cupoftea]

Hi,
Why on page 5 do they show the diff mode inductor upstream of the common mode choke?...this is obviously wrong......at least from an EMC point of view, some kind of transient protection feature?

LD AC module

LD30-23B05R2_Industrial Switching Power Supply Manufacturer - MORNSUN

And why on page 4 do they show the Y caps connected to the "less-optimal" side of the common mode choke?
...those y caps on page 4 will fail to offer the emissions the low impedance path to get back into the mains line and neutral...... from the earth surrounding....any emissions going through those y caps would not have gone through both branches of the common mode choke, and so would not be likely to take that particular path anyway....whats the point in a y cap if emissions do not traverse it?

The whole point of a common mode choke is that you give escaping emissions a high impedance path if they dont go through both branches of the common mode choke......thats why the y caps are needed downstream of the common mode choke....because then you are offering the emissions the low impedance path....back through the common mode choke......if the y caps are upstream of the CM Choke.....then you are "shutting the stable door after the horse has bolted."

 

[crutschow]

Common-mode choke filters suppress common-mode noise, and differential-mode filters suppress differential-mode noise.
I see no significant difference in the order they are connected, since they are suppressing different types of noise.

 

[cupoftea]

Thanks, common mode always nearer mains connector as otherwise emissions may bypass the common mode choke.....diff mode emisisons are too low frequency to bypass in this way...they dont couple out or radiate out so easily.

 

[cupoftea]

...And when the common mode choke is split into a small high frequency one, and a bigger lower frequency one.. (ie 2 common mode chokes) ....then the higher frequency one always goes nearer the mains input connector...for the same reasons as above.

 

[mtwieg]

Hi,
Why on page 5 do they show the diff mode inductor upstream of the common mode choke?...this is obviously wrong......at least from an EMC point of view, some kind of transient protection feature?

From the standpoint of just filtering, the order of the CM/DM filters don't matter, like crutschow said above.

When also considering other aspects such as safety, surge immunity, etc, there can be practical benefits to specific arrangements for the filter components.

And why on page 4 do they show the Y caps connected to the "less-optimal" side of the common mode choke?

Indeed figure 2 is strange. My best guess that it's some sort of compromise between fig 1 (which has no EMC mitigation at all, but is still titled "Typical application") and fig 3 (titled "Recommended circuit for class Ⅰ equipment"). Fig 2 would definitely provide inferior EMC mitigation than fig 3, yet fig 2 is titled "EMC application circuit with higher requirements".

 

[cupoftea]

From the standpoint of just filtering, the order of the CM/DM filters don't matter,

Thanks, well yes, consideration of the "ideal world" is always useful, but ayk, in any real offline SMPS mains input filter...the CM choke goes upstream of the DM choke...for the reasons given above......having reverse engineered a ton of offline SMPS, where the CM choke is upstream of the DM choke.....this seems to be the accepted method everywhere.

Not only that, but if you trace the path of an emission (disturbance) noise) which would otherwise become a pesky common mdoe emission, you can see that you have more chance of "inviting" it to go back into the Line/Neutral if you dont put a DM inductor upstream of the CM choke.

Ayk, the entire purpose e of a CM choke is that you are giving the emissions a lower impedance path if they return into the Line/Neutral...simply because they "see" less impedance if they go through both branches of the CM choke.
This is one of the reasons that CM chokes are even still useful if there are no Y caps.....though to be honest, there are always those stray Y caps.