How do I Turn a Hard Switching Cuk into a Resonant Cuk

Google "resonant mode power supply"?

I think you're looking at the wrong resonant mode. In the little work that I've done with resonant mode power supplies, the resonance that was being leveraged was the resonances of the various L's and C's in the circuit. The thing that made them "resonant mode" was that instead of going at a fixed frequency, they would wait for either the zero-current point or the zero-voltage point and turn the switch on or off at that point.

I can't remember _which_ of those two points it was, but the one that I _really_ worked at had some elaborate snubbers on the primary side, so I'm pretty sure it was switching at a zero-voltage point. But, like I said, it's been ages...

The idea is that you reduce switching losses by choosing at least one switching point where there will be almost no loss.

Both the Cuk and Sepic depend on controlled coupling when isolated.

Articles looking at soft switching (which is probably the most you can expect to achieve date from the mid-90s. eg

"The Effect of the Magnetizing Inductance on the Small-Signal Dynamics of the Isolated Cuk Converter" Vorperian

"Soft-Commutated Cuk and SEPIC Converters as Power Factor Preregulators" Spiazzi

RL

Thanks. There's just something that kills me with smps development. I do wonder if it's the hardest branch in electronics.

Not sure what you mean by controlled coupling when isolated. I do know that an ideal Cuk with an isolation transformer functions correctly with leakage inductance = 0. But pop in a real world transformer and a Cuk can ring like a bell.

Thanks for pointers. I'll check'm out.