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RE: Since parafeed is the "new kid in town"

Posted by Paul Joppa (P) on November 22, 2016 at 18:36:01

In Reply to: RE: Since parafeed is the "new kid in town" posted by Tre' on November 22, 2016 at 17:02:02:

I was afraid someone would ask that :^) It's hard to explain, which is why I didn't.

I am talking about the capacitances in the primary winding. The primary is usually in sections which are in series, interleaved with secondary windings which are basically at AC ground. Each primary section has its own self-capacitance, and a capacitance between it and each of the adjacent secondary windings and/or core. The inter-winding capacitance is adjusted by the thickness and permittivity of the dielectric material between them. A thicker insulation reduces the inter-winding capacitance, but increases the leakage inductance.

If you draw out the circuit, say for four primary winding sections (not uncommon in high-end P-P transformers), you will see that it's a bear to analyze. You do not need a thick insulator adjacent to the center tap, since it is already at AC ground, but that same insulator has serious requirements if the AC ground is moved to one end.

I once worked on a project with Magnequest Mike, in the course of which I have seen the drawings (which Mike now owns) for one of the best golden-age P-P transformers. Each insulation segment has a different material and a different thickness, specifically to manage the high frequency leakage/parasitic capacitances to avoid HF resonances. High frequency resonances in the primary winding wreak havoc with global feedback, so golden-age P-P transformers had to take these issues seriously.

For another example, there was some years ago a popular large SE transformer from another manufacturer which did not manage these capacitances well; it had a serious problem in the high frequencies and was fairly quickly redesigned.

Hope that helps! There is a useful section on transformer design in the RDH4, and Reuben Lee's "Electronic Transformers and Circuits" is a classic reference. The first edition has some information on parallel feed, which was abandoned in the second edition. FWIW.

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