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Within the context of isolated converters, the flyback topology is nice in its simplicity as can be the forward topology, in spite of needing a second primary reset winding. However, both make poor use of the transformer core in as much power is not continuously passed through the transformer. Why can't both topologies be combined by taking the reset windings to the secondary side as a flyback output windings?

Consider the following schematic: Proposed topology schematic

The upper secondary windings are as in a typical forward topology while the lower secondary windings are as in a flyback topology and act as reset windings. Ideally, one would remove L1 altogether by designing T1 with a large magnetizing inductance.

My questions are specifically:

1) Wouldn't this topology offer advantages such as lowered value for C1, no output inductors and better use of core materials? If yes, why isn't it more common?

2) Are there major drawbacks I haven't considered? The ones I see concerns stressed Q1 in no-load conditions and the need for a lot of primary windings so that T1 can have a high enough inductance to remove L1 altogether.

3) I'm considering such a design for a constant power PSU for use with loads that exhibit negative resistance. The idea is to have the ON-state duty cycle more proportional to total output power as the energy not used in the forward part of the cycle will be dumped in the load in the flyback part of the cycle. Would that be an acceptable use case?

I have seen similar topologies in some patents and a few papers targeting its use in solar power systems but it does not seem to be common at all and I can't see why.

P.S.: I'm a physicist by training and I suspect the reason might be first and foremost economical.

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  • \$\begingroup\$ Here's an app note on that.. onsemi.com/pub/Collateral/AND8039-D.PDF \$\endgroup\$ – AlmostDone Apr 12 '18 at 0:36
  • \$\begingroup\$ @AlmostDone Thank you for the reference, but there seems to be explicitly a reset winding on this application note. \$\endgroup\$ – G. Bergeron Apr 12 '18 at 0:48
  • \$\begingroup\$ @G.Bergeron. Flybacks are not as efficient as buck/boost topologies because they are isolated from directly using the source current. A 1MHZ buck/boost can approach 97% efficiency. No magic here-just physics. A PFC helps flyback efficiency somewhat. \$\endgroup\$ – Sparky256 Apr 12 '18 at 0:52
  • \$\begingroup\$ @Sparky256 Of course, but as this is to be operated off-line, I'm focusing on isolated topologies. \$\endgroup\$ – G. Bergeron Apr 12 '18 at 1:02
  • \$\begingroup\$ @Sparky256 How on earth would a PFC improve the efficiency? The PFC will have losses of its own. \$\endgroup\$ – winny Apr 12 '18 at 7:34

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