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We have seen many flyback SMPS circuits in the past over here in EC, some of which were fully discrete. For such models, a transformer with one primary, one secondary and one auxiliary is needed. Due to the lack of such a transformer on EC, users have used 2 transforms with their primaries in parallel, which results in an incorrect model as shown:
http://everycircuit.com/circuit/5697835620892672
The reason for that is that 2 seperate transformers with their primaries in parallel will behave exactly like 2 seperate transformers drawing different currents from the same power source, hence failing to emulate a flyback switching transformer. As you can see from the simulation, they draw completely different currents.
Obviously, that's not what we want. We want the two transformers to behave like one. Putting them in parallel isn't the solution. So I thought of putting them in series. It has been done before, but incorrectly. Users have put 2 IDENTICAL transformers in series, thus leaving each primary with only half the supply voltage.
But that is actually a step in the right direction. Putting two transformers in series results in the exact same current flowing through both primaries, which is exactly what we want, but the primaries equally share the input voltage if they are identical, which isn't what we want! So what is the correct combination, series or parallel!?
The solution: put two transformers' primaries in series. Not two identical step-downs, but one main step-down and a smaller one-to-one for the auxiliary.
The trick is simple. Let's say that we want to model a flyback transformer that has 150 primary, 30 secondary and 10 auxiliary turns. We now use one main EC transformer with 150 primary and 30 auxiliary turns and another transforms with 10 primary and secondary turns, putting the primaries in series. That is step one. Now let's say that the transformer that we want to model has a primary inductance of 1mH. We need to split that value between the 2 EC transforms in the ratio of their respective number of primary turns. So 150x + 10x = 1mH, which implies that the main transformer's primary inductance is 937.5uH (I had to set it to 920 to keep the sim from crashing. So annoying!) and that of the one-to-one is 62.5uH. And that's it. The flyback SMPS transformer is modelled:
http://everycircuit.com/circuit/4657889787248640
Now, the primaries share the same current without sharing the same voltage. The one-to-one only gets a fraction of the power supply voltage, allowing the main primary to hog on most of the input voltage. This is a more accurate model of a flyback SMPS transformer. You can see that in the following simulation:
http://everycircuit.com/circuit/5695517110304768
Thus, we have created a more accurate model.
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