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I am trying to select a flyback transformer for a prototype I'm designing. Almost all flyback transformer data-sheets I've seen provide only the saturation current for the primary winding. The saturation value for the secondary is never given.

How can I ensure that my transformer's secondary is not saturating?

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Saturation is a function of the core (there is only one core, and it links primary to secondary winding(s)). In the case of a flyback converter, energy is stored in the core and then released to the secondary. Saturation is thus only a function of the primary current, since there is no secondary current (the rectifier is reverse biased) when the magnetic field reaches the peak value.

In the general case, saturation would be related to the signed sum of the ampere-turns of each winding (each affecting the magnetic flux in the core), but that's not a factor in a flyback.

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  • \$\begingroup\$ The signed sum of ampere turns for each winding is usually zero in the general case (a conventional transformer). It's the primary inductance that makes flux and causes saturation and not load currents. I'm just mentioning this in case you meant it but didn't spell it out fully. \$\endgroup\$ – Andy aka Apr 6 '18 at 11:41
  • \$\begingroup\$ @Andyaka The primary current is the vector sum of magnetization current (reactive) and reflected secondary current (x turns ratio), plus the core loss component (real), so the total can't be zero (that would mean zero flux in the core). \$\endgroup\$ – Spehro Pefhany Apr 6 '18 at 12:45
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    \$\begingroup\$ Yeah I misread your answer into thinking you were saying it was load related! Do'h. However, core saturation is only related to mag current. \$\endgroup\$ – Andy aka Apr 6 '18 at 12:59

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