T
treez
Guest
Hi,
In an isolated offline, 30W Flyback SMPS, we all have to add a Y capacitor from secondary to primary in order to mitigate common mode emissions. This is ususally a 1nF radial film Y cap.
In a non-isolated Flyback, such a capacitor is obviously not needed.
As such, do you agree, we can say that a non-isolated offline Flyback is easier to get through EMC conducted emissions testing? (ie, easier than an isolated offline flyback)
…This is obviously because its easier (in a non-isolated flyback) for noise at the flyback secondary to return by going back across the shorted primary and secondary grounds and then back down the input mains wires, rather than coupling off externally to earth and causing common mode noise (as it would with an isolated flyback).
From this observation, a postulation obviously follows, …that is, if we also include low value ceramic SMD Y capacitors (eg 100pF) between primary and secondary of an isolated flyback, aswell as the usual 1nF radial film Y capacitor, then common mode noise will be better mitigated?
This is because the SMD Y capacitor will have less stray inductance than the radial film Y cap, and so will provide that low impedance “bridge” back across the isolation barrier, that would otherwise be absent if just a radial film Y cap were used.
Do you agree with this, and that since there is always space at the isolation barrier, this is a better idea than eg using shield windings in the flyback transformer?
In an isolated offline, 30W Flyback SMPS, we all have to add a Y capacitor from secondary to primary in order to mitigate common mode emissions. This is ususally a 1nF radial film Y cap.
In a non-isolated Flyback, such a capacitor is obviously not needed.
As such, do you agree, we can say that a non-isolated offline Flyback is easier to get through EMC conducted emissions testing? (ie, easier than an isolated offline flyback)
…This is obviously because its easier (in a non-isolated flyback) for noise at the flyback secondary to return by going back across the shorted primary and secondary grounds and then back down the input mains wires, rather than coupling off externally to earth and causing common mode noise (as it would with an isolated flyback).
From this observation, a postulation obviously follows, …that is, if we also include low value ceramic SMD Y capacitors (eg 100pF) between primary and secondary of an isolated flyback, aswell as the usual 1nF radial film Y capacitor, then common mode noise will be better mitigated?
This is because the SMD Y capacitor will have less stray inductance than the radial film Y cap, and so will provide that low impedance “bridge” back across the isolation barrier, that would otherwise be absent if just a radial film Y cap were used.
Do you agree with this, and that since there is always space at the isolation barrier, this is a better idea than eg using shield windings in the flyback transformer?