What is the difference between a coupled inductor and an actual transformer? The LT3757 datasheet mentions that I need a coupled inductor for the SEPIC to work
Electronic – Coupled inductor vs an actual transformer
inductorsepic
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The two are basically the same class of device, although each will have parameters optimized differently. The two names are to explain the different intended usage, which also gives you a quick guess of how some of the parameters may differ. Of course only the datasheets would tell you what the parameters are for sure.
A transformer is specifically intended for transferring power from one winding to another. You want the coupling between windings to be as good as possible, the leakage inductance zero, and the absolute inductance of each winding with the other open is often not a large concern.
With coupled inductors, each winding is still used for its inductance alone, although of course some coupling is being utilized else there would be two separate inductors. Generally leakage inductance is less of a issue. In fact, it can be useful to have some minimum guaranteed individual (non-coupled, or leakage) inductance for each winding. The absolute inductance of each winding with the other open is also a important parameter that will be well specified.
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.
Best Answer
You don't have to use coupled inductors in a SEPIC configuration. The main advantage is that you can get by with lower inductance windings. If the two inductors are closely coupled, the ripple current is divided between them and the required inductance is halved. So if you want to replace a coupled inductor which has two 3.3 uH windings, you would need two individual inductors of 6.6 uH each at the same current rating.