DC Transformer? Forward converter?
Isolation amp? You decide!
Michael Dunn - March 10, 2017
Sometimes, a Design Ideas submission doesn’t quite fit with the theme of that EDN section, yet still
promises interest for our readers (you). Such is the case with a DC Transformer design that reader
Newton Ball recently sent in. The title certainly is catchy.
Newton and I engaged in a bit of friendly debate over the circuit. It looked to me like the core of a
forward converter (for you power engineers), or of an isolation amp (for you analog engineers), with
explicitly driven synchronous rectification – nothing really new – just a totally generalized
implementation or exposition of the circuit.
Newton countered that any capacitance on the primary would be greatly multiplied by the turns
ratio (he envisions use in, say, a single-stage 48V-to-1V telecoms converter), reflecting and
effectively improving the capacitor characteristics viewed from the secondary, and taking advantage
of CV vs. CV2 (volume vs. energy) proportionalities.
Figure 1 Once you get past all the lines, it’s an H-bridge on the primary, and another on the
secondary (which are reversible like a “real” transformer), switched by eight floating gate drives.
He’s done some basic testing: The circuit above was constructed, using toroidal cores for the drive
and main transformers. In operation, it successfully passed substantial power in both directions.
I of course browsed the interwebs in research mode, looking at, among other things, synchronous
rectifiers.
Newton continues:
Transformers, AC or “DC”, provide a continuous, nearly instantaneous, link between ports, and are
completely reversible. This continuous linkage, or referral, or reflection, works for voltage, or for
current, in proportion to turns ratio. Near instantaneous reflection of impedance is by the square of
turns ratio.
This quality is of importance in the original intended use, 48V to <1V conversion in data centers,
inspired by an EDN article. It is difficult to get enough capacitance at the sub-1V level to support
current surge demand. The DC transformer reflects every microfarad at the 48V supply input as
≥482 microfarads at the ≤1V level. This reflection is very quick, and independent of reversal
(switching) rate.
So, do you think this generalization ad absurdum of a forward converter pulls back the curtain to
reveal a hitherto unconsidered attribute of the architecture?
Or is it what I like to call a “Partly-baked Idea”? Interesting to ponder, but not of practical value. Not
yet anyway.
Also see:
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Utilizing GaN transistors in 48V communications DC-DC converter design
Gate-drive transformer eases multi-output, isolated dc/dc-converter designs
High-performance GaN-based 48-V to 1-V conversion for PoL applications
Power inverter is bidirectional
Bi-directional DC/DC power supplies: Which way do we go?
48V direct-conversion dramatically improves data-center energy efficiency
Method provides self-timing for synchronous rectifiers
Isolated Full Bridge Converters
Secondary-side synchronous rectification boosts resonant converter efficiency