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Old 14th Feb 2019, 11:23 am   #3
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,060
Default Re: Transformer winding article.

Interesting article!

For interstage transformers, winding self-capacitance is a significant limiting factor for high frequency response (together with leakage inductance). Even with just a 3:1 step-up ratio, 1pF extra secondary capacitance hurts as much as 9pF of primary capacitance.

I therefore - and I have only wound a couple of intervalve transformers - wind the secondary FIRST. Being nearer the core, the average turn length is less than for the outer winding. So, the wire length for the same number of turns is less, and as the capacitance between one turn and an adjacent turn is proportional to the length of each turn, the shorter turn length reduces winding self-capacitance.

The primary (wound last) of course then has more self-capacitance than if wound first, but with fewer turns, as above, this is much less significant.

From a design point of view, an intervalve coupling transformer is a really unhappy thing to design. The valve driving it is likely to have a fairly high ra, which causes a HF roll-off with shunt capacitance, and an LF roll-off with the shunt inductance. So, for good LF response you need a high inductance. But this means lots of turns hence high capacitance. Using a valve with lower ra helps (LF roll-off goes down and HF roll-off goes up) but that is rarely an option! Using an exotic core material helps get high inductance with fewer turns - but if you have DC flowing through the primary (as you would, if the transformer carries anode current) you can't really take advantage of this as you have to add a gap to prevent saturation. So you just have to be super-clever and use a low-capacitance winding layout.
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