Winding Flyback SMPS transformer for lowest common mode noise. - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 13, 2024, 10:02:34 am
content: Hi,The most important thing when winding a flyback SMPS transformer is to ensurethat all the "dotted" ends of the coils are at the same side of the bobbin.(ie, the coils lie "dot to dot")This reduces the dv/dt between the coils, and thus massively reduces common mode noise.But the AN17 document (below) doesn't even give mention.Why not?AN17 Flyback transformershttps://www.power.com/sites/default/files/forum/files/AN-17.pdf

author: spostma
author_rank: Regular Contributor
timestamp: July 13, 2024, 10:13:46 am
content: good suggestion.I might add, make sure that the primary switching 'hot end' is on the inside of the primary coil winding,so the primary DC bus voltage terminal is on the outside of the coil.The secunday side that is closest to the primary should be the GND connection for a sinlge phase recification.

author: uer166
author_rank: Super Contributor
timestamp: July 13, 2024, 07:16:32 pm
content: Quote from: Faringdon on July 13, 2024, 10:02:34 amThat's only true if the secondary coil voltages are ~similar to primary. That's definitely not the case for a mains 230->5V out flyback, the delta is minimal. You can definitely add a dedicated shield winding though, but it'll take up as much space as a foil screen..

author: temperance
author_rank: Super Contributor
timestamp: July 14, 2024, 01:26:33 am
content: True because the primary winding is wider in comparison the the secondary winding in a step down converter.But at the same time magnetic coupling must be maximized by spreading the winding over the full width of the bobbin because field lines escape trough the sides of the center leg and to some extend trough the sides of core. (forget about those old drawings showing field lines only traversing across the air gap with some fringing)As with everything a trade off must be made and improving coupling (by multi stranded windings for example) increases CM noise and shielding might be required. But shielding, CM inductors and other methods to reduce CM noise are all rather expensive.If you search for papers on the subject you will discover that the last word on this subject has not been written yet while fly-back supplies aren't anything new at all. As such the ideal fly-back "transformer" doesn't exist yet and that's where knowledge can turned onto cash if you can manage to save 1 cent. (the trick is not anymore on how the windings are being arranged. The "new" tricks is canceling CM noise with an extra winding which is then used to inject some current into a strategic place. But prepare yourself for a patent battle if you go that route.)Anyhow, back to drawing board t seems.

author: Faringdon
author_rank: Super Contributor
timestamp: July 14, 2024, 09:26:35 am
content: Quote..thanks, i agree its less of an issue in the case which you kindly highlight, but the reverse voltage ayk, will be more than 5v, and so it still applies......we still should do dotted ends at same end of bobbin....to coin a phrase...just because we cant have a full Big mac with fries, doesnt mean we should turn down a portion of "small fries"....its still food.