# How to select the equivalent coupled inductor to a flyback transformer?

Given a recommended transformer with certain parameters (inductance, saturation current, rms current, dc resistance), how would you go about choosing the equivalent coupled inductor? All the coupled inductors I've seen state their values with the coupled inductors wired either in parallel or in series. However, the flyback transformers aren't wired in either configuration, so you don't seem to be able to directly compare the values.

For example, given the SRF1280-101M inductor, if I were to use it as a 1:1 transformer in a flyback regulator, what would be the equivalent inductance, sat current, rms current and dc resistance?

• is it correct to say a flyback transformer is coupled inductor with more than two windings? Commented Oct 7, 2020 at 21:21

Looking at the table: -

I note that the series column of the saturation current is exactly half of the saturation current for the parallel configuration. This gives me a good feeling about the data sheet.

In a fly-back transformer only one winding is activated at once so, given that saturation is governed by ampere-turns and only half the turns are involved in a fly-back device you can infer saturation levels from the parallel rating.

Inductance in parallel is normally one-quarter that of series wound coils on the same core and it looks like this is true of these inductors so, if the series inductance states 8.8 uH, one winding will be 2.2 uH - remember that two parallel windings (closely coupled) have exactly the same inductance as a single winding with the same turns.

RMS current for a winding has to be limited by the series column so if it says 6.23 amps then it's going to be that value for one winding too BUT some investigation into how much the core warms might allow you to increase this by (say) 1.5 times.

DC resistance should be obvious now.

• Thanks! Let me make sure I understood. Inductance: since you're only using 1 coil, it's equivalent to having half the turns as the series configuration, which would reduce the inductance by 4. But since two closely coupled parallel windings have the same inductance as a single winding with the same number of turns, then you can just take the parallel value. This also applies to the saturation current. The rms current is basically a property of the wires, so it's going to be the same value as the series rms current. Similarly, the dc resistance will be half the series DC resistance. Commented Nov 4, 2015 at 9:50
• So for the SRF1280-101M, if I were to use it in a flyback application, the equivalent values would be as follows: Inductance: 100uH, Isat: 3.65A, Irms: 0.96A, DCR: 0.35Ohms. Commented Nov 4, 2015 at 9:51
• Yeah spot on!!! Commented Nov 4, 2015 at 10:01
• is it correct to say a flyback transformer is coupled inductor with more than two windings, where the extra windings are used for voltage-sensing? Commented Oct 7, 2020 at 21:21
• @johnywhy It's not what it is it's how you drive the transformer. It's still a transformer and a transformer relies on coupling between inductors but, how you drive the transformer and take energy from it determines if it's used "in a flyback application". To make things a tad more complex, the tertiary winding (the third winding you refer to) might actually be used in a non-flyback mode. So it depends on what you do with the windings and not what the windings are. Commented Oct 7, 2020 at 21:47