Is a SEPIC converter truly isolated?

Question

I have been in two separate design reviews where the SEPIC topology was rejected for an isolated < 40W application. These meetings were at 2 different companies and with different principal engineers that both stated that the SEPIC design did not meet the isolation requirement.

In the end the size constraints, mainly height, pushed one of the designs away from a flyback and the principal engineer okayed the design so long as a common mode choke was added to the output of the SEPIC in question.

Most of the textbooks and articles that I have read categorize the SEPIC as an isolated topology, thus adding to my confusion.

What I do know:

• "You won’t find a complete analysis of the Sepic converter anywhere in printed literature. What you will find are application notes with comments like, “the Sepic is not well-understood.” Despite the lack of documentation for the converter, engineers continue to use it when applicable" Analyzing the Sepic Converter-BASSO
• All of the reference designs on the TI website list the capacitor isolated topologies as non-isolated!. (PMP10070,PMP30373,PMP30373,)
• Energy/isolation is handled by different means with a capacitor vs transformer. (High level only though, i.e. charge/discharge vs magnetic field transfer.)

My questions are as follows:

1. Is a SEPIC,CUK, or ZETA converter truly an isolated converter?
2. How does capacitive isolation compare to transformer isolation?
3. How could the common mode choke alleviate any isolation concerns?

Answer 1

Is a SEPIC,CUK, or ZETA converter truly an Isolated Converter?

All three of them are not isolated. The output shares its ground with the input.

(I think you could build a Cuk that's isolated, but never done so)

How is capacitive isolation compare to transformer isolation?

You can't call it "capacitive isolation" if there's a shared ground.

Also, I don't think I'd call a capacitor isolation; it's meant to pass current.

How could the Common Mode Choke alleviate any isolation concerns?

By looking at it and then deciding to build an isolated flyback, maybe?

If a lead engineer would be OK with a common mode choke after a non-isolated converter, then it's possible you don't need isolation, but common mode rejection; but: that's something that the requirements engineering would have told you instead of telling you to build an isolated converter, so this all remains a mystery.