EMC filter all on DC side is best?
Hello,
In an offline SMPS , would it not be better to put ALL of the EMC filtration “after” the Mains rectifier (DC side) instead of putting some on the AC side?
I mean, the common mode choke and the X caps should best go on the DC side of the filter?
This SMPS schematic has a common mode choke and a 220nF EMC capacitor on the AC side, which would appear to be a complete waste of time. ?
Offline SMPS schematic:-
eg page 11 fig 13 of http://www.powerint.com/sites/default/files/product-docs/an43.pdf
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I can see that a small amount of very high frequency filtration (such as ferrite cylinders or beads) around the AC input cable might be needed,
-but certainly not the ~100nF X capacitors and ~10mH common mode chokes which are commonly seen on the AC side of most offline SMPS’s.
After all, we want the high frequency switching harmonics to be traversing as small a loop as possible, so what’s the point in putting the mains rectifier in that loop and making it bigger than it needs to be?
The ONLY reason that I can think of for putting a filter on the AC side of the rectifier is to dampen down the peak of any overvoltage transient such that the rectifier and other components are slightly protected from it.
However, unless an AC-side filter was unreasonably large, it would not really give enough attenuation of an overvoltage peak anyway. ?
So, taking that into account, the ONLY reason that I can now think of to have an AC side filter is to slow up an overvoltage transient’s rising edge prior to clamping it with a MOV or transorb (the MOV or transorb also being placed on the AC side).
-The AC-side filter would be useful in this case because MOV’s and transorbs are relatively slow to turn ON, and so would benefit from having some filtration before them.
Otherwise, I can see utterly NO reason to have an AC side filter in an SMPS.
Both X capacitors AND common-mode-choke would surely be better situated on the DC-side of the mains rectifier ?
Comments
Thankyou Spock,
Since the X cap is purely for filtering mains diode bridge noise, would this mean that the X cap should really only be small since diode bridge noise is just a small blip occurring every 10ms?
Yes that is correct. If infact you have 2 X caps in your power supply its better to have the larger one on the DC side to provide you better differential filtering along with the CM choke and bulk capacitor.
However since most sub 100W supplies use only one X-cap it needs to be on the AC side.
Not entirely true. You need at least 1 X capacitor to the left of the bridge (AC side) This is because bridge diodes can be very noisy and during their commutation, their turn off (snap off) event creates differential currents which are filtered by the X cap. In absence of the AC side capacitor you will see that your EMI increases significantly.
Regarding the common mode choke - You are correct you can place it on the DC side. The reason we show it on AC side for high power designs is to make sure that the physical location of the filter is far away from the switching transformer. Otherwise the capacitive coupling between the two will lead to higher common mode noise. In principle though, yes the CMC can be on the DC side.