There seems to be a known phenomenon about high turns ratio transformers drawing no-load current in higher voltages with higher frequencies. enter image description here

The intrawinding capacitance in the secondary (Cds in the figure above) can get high in small size high turns ratio multilayer transformers (3000 ~ 4000 of AWG 37 in this case), but how high can it get? The secondary is gonna operate up to 10 kV and I'm trying to up the frequency (in the 10s of kHz range up to 200 kHz if possible). But all of the factors here (turns, frequency and voltage) are increasing the charge and thus the no-load current draw of the converter.

What are the limits in this? Does it even get high enough to matter? Does segmenting the secondary into 10 or so segments of a couple of hundred turns each, separated by 1 or 2 millimeters of plastic help to reduce the capacitance?

  • \$\begingroup\$ With that much capacitance, perhaps one could, using \$L_S\$ and \$C_{DS}\$, operate the secondary at its resonant frequency. Might have less loss and considerably higher voltage, if the other parasitics don't swamp it out. \$\endgroup\$
    – rdtsc
    Commented Oct 15, 2021 at 18:20
  • \$\begingroup\$ secondary having high capacitance combined with the high inductance due to the number of turns, leads to natural frequency being lower that desired if the capacitance is really that large \$\endgroup\$ Commented Oct 15, 2021 at 20:42
  • \$\begingroup\$ @SolidState I used to use (when I was doing RF as a kid) inductors that glen_geek shows you. Exactly those kinds, in fact. But have you looked at your peak dv/dt at your 200 kHz and secondary voltage hopes? Egads! Looks like 10 mA into 1 pF. May want to back off the frequency goals a bit! \$\endgroup\$
    – jonk
    Commented Oct 16, 2021 at 0:19

2 Answers 2


does segmenting the secondary into 10 or so segments of a couple of hundred turns each, separated by 1 or 2 millimeters of plastic help to reduce the capacitance?


In a transformer winding, inter-turn capacitance is generally negligible compared to inter-layer capacitance.

We can approximate the winding self-capacitance within a small factor by considering only the capacitance between winding layers, approximating the layer to a conductor sheet.

The highest capacitance arrangement is when each layer is wound in opposite directions, so left to right on the first, right to left on the next. That's because the first turn of the first layer is opposite the last turn of the next layer, so there's twice the voltage per layer between those turns. As the energy stored in the self-capacitance goes as the square of the voltage, this more than offsets the lower voltage difference at the other side of the windings.

A lower capacitance can be obtained by winding each layer in the same direction, returning between layers, so left to right, then left to right again. The voltage between layers is now uniform at the per layer voltage.

To drive the capacitance still lower, the secondary should be segmented, with an airspace between segments. Disc windings can also be used, basically very many narrow layers.

  • \$\begingroup\$ in what range the capacitance would be typically? and if airspace wasn't possible, is it ok to space the segments with plastic? or the dielectric would make it worse? \$\endgroup\$ Commented Oct 15, 2021 at 20:49
  • \$\begingroup\$ Plastic is not as good as air, but better than having no spacing at all. \$\endgroup\$
    – Neil_UK
    Commented Oct 15, 2021 at 21:38

Here are a couple of inductors used often as radio-frequency chokes. They are wound so that capacitance is minimized:

low-C inductors

Both inductors have no ferrite cores, but this kind of winding is appropriate on ferrite. These windings are divided into segments (4-segments for the smaller one, 5-segments for the larger one) - the space between segments reduces capacitance.>br?

Each segment layer has few turns so that inter-layer capacitance is minimized. Both these use Litz wire, which attempts to distribute current flow throughout the wire ; otherwise, current tends to flow only as skin current - this can also help minimize inter-wire capacitance, at least for lower frequencies.
These wires are cotton-covered, making winding bulkier than magnet wire would be, but the thicker insulation also reduces inter-wire capacitance.

Not the easiest things to wind and keep everything in place! Perhaps you can see that windings are fixed with some kind of lacquer - which detracts from desired low-capacitance.

  • \$\begingroup\$ can i space the segments with plastic from the bobbin? airospacing seems hard to get done right \$\endgroup\$ Commented Oct 15, 2021 at 20:55
  • \$\begingroup\$ Plastics having low dielectric constant would be preferred: polystyrene, polyethylene, PTFE. Styrofoam is likely better: it is full of air, consequently mechanically weak. Air is best for low capacitance. \$\endgroup\$
    – glen_geek
    Commented Oct 15, 2021 at 23:29

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