Transformers flyback

Figure 3-19 Flyback transformer secondary arrangements (a) center-tapped secondaries (b) isolated secondaries.

Figure 11-5 reveals two low-noise construction techniques, as applied to a typical flyback transformer. We should compare the right-hand electrical schematic with its equivalent winding version to the left. In the discussion below, we note that though transformers with split windings are not being explicitly discussed here, the same principles can be easily extended and applied to them, too. Here are some observations ... 

Blog Entry 3 A more simple solution to implement is to use the 3478 Low-side N-channel controller in a SEPIC configuration. This application utilizes two inductors (instead of a Flyback transformer) to attain the Buck-Boost function. The 3478 requires an external user-selectable Fet switch, so you can choose the one that suits your load current requirement. The datasheet provides an application rationale for SEPIC configuration on page 19, Figure 13. The output voltage can be set to 12V by changing the value of the feedback resistor. 

Figure 11-5 Two Alternative Low-noise Winding Arrangements for a Flyback Transformer...

Coming to the question of how currents actually reflect from one side of the transformer to the other, it must be pointed out that even though the final current scaling equations of a flyback transformer are exactly the same as in the case of an actual transformer, this is not strictly classic transformer action. The difference from a conventional transformer is, that in the flyback, the primary and secondary windings do not conduct at the same time. So in fact, it is a mystery why their currents are related to each other at all ... 

For this a good guideline/rule-of-thumb for the current density of flyback transformers... 

Question 25 Why is it commonly stated that in a flyback transformer, the air gap carries... 

The parasitic capacitance between the primary and secondary windings transfers noise across the isolation boundary. Since the secondary side ground is usually connected to the chassis, this noise returns via the earth plane, in the form of CM noise. The situation is very similar to the tradeoffs required in heatsink mounting issues. In this case, we wish to couple the primary and secondary very close to each other in order to reduce leakage inductance (especially in flyback transformers), but this also increases their mutual capacitance, and thus the CM noise. 

Flyback transformer 15.75 kHz MnZn High permeability High magnetic flux density Low loss... 

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