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DC Transformer? Forward converter? Isolation amp? You decide!
2017-09-13 10:15:13

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.

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