When do I need litz wire?

When the skin depth at your switching frequency is smaller than the radius of a solid conductor that would carry your current. Beyond that point the centre of the wire stops conducting and you are paying for copper that only adds weight. Litz splits the conductor into many insulated strands, each thinner than the skin depth, and transposes them so they share the current.

Is more litz strands always better?

No. More strands means more enamel and more air in the same window, so the copper fill factor falls and the DC resistance rises. There is an optimum for every frequency and every window, and past it the extra strands cost more than they save. ConverNova picks strand diameter and count from your frequency and your harmonic content.

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Winding technologies, and why frequency decides

Q: Why does interleaving reduce leakage inductance?

Leakage inductance is stored in the magnetic field between primary and secondary. Splitting the primary and putting the secondary between the halves reduces the volume that field occupies, so the leakage falls. The price is a large increase in interwinding capacitance, which is why leakage and capacitance are traded against each other rather than both minimised.

ConverNova winds custom magnetics in litz from 0.03 mm strands, planar, copper foil, edge wound flat wire, triple insulated wire and round magnet wire. Below a few tens of kilohertz the winding is a detail. Above a few hundred, it is most of the loss, and the choice of conductor stops being a cost question and becomes the design. This page is the reasoning we would walk you through.

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The two effects

Skin depth and proximity effect.

Everything on this page follows from these two, so they are worth stating plainly.

Skin effect

Alternating current pushes itself towards the surface of a conductor. The depth it usefully occupies shrinks as frequency rises, so past a point the centre of a thick wire carries almost nothing and you are paying to ship dead copper. In copper at room temperature the skin depth is roughly 0.21 mm at 100 kHz and about 0.066 mm at 1 MHz.

Proximity effect

The field from neighbouring turns and layers drives eddy currents in the conductor, crowding the current further still. In a multilayer winding this usually dominates skin effect, which is why the layer order and the interleaving matter as much as the wire.

The conductors

Six ways to carry the current.

Each one is the right answer somewhere, and we will tell you when the expensive one is not buying you anything.

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Litz: From 0.03 mm strands. Many insulated strands, each thinner than the skin depth, transposed so they share the current. Strand diameter and count chosen from your frequency and harmonic content, because more strands is not automatically better
Planar: Etched or stamped turns in a fixed geometry. Very repeatable leakage inductance and a low profile. See planar transformers
Copper foil: A full width sheet per turn. Excellent DC resistance and a direct thermal path out of the winding, and the current distributes itself across the width. Sensitive to fringing field at the ends, so gap placement matters
Edge wound flat wire: Rectangular wire wound on edge. High fill factor and tight bends, for chokes where every millimetre of window counts
Triple insulated wire: Reinforced isolation carried in the wire, so no margin tape and no barrier layer. The window goes back to copper
Round magnet wire: Still correct at line frequency and low ripple. Not everything needs litz, and the cheapest part that meets the spec is the right part

The trade

Leakage against capacitance.

You cannot minimise both. Interleaving reduces leakage inductance by shrinking the field volume between primary and secondary, and increases interwinding capacitance by putting more conductor area close together. Which one hurts you more depends on your converter.

Winding arrangements and their effect on leakage and capacitance
Arrangement	Leakage	Interwinding capacitance	Suits
Simple, primary then secondary	High	Low	Flyback, where leakage is a design parameter and common mode noise matters
Interleaved, P S P	Much lower	Higher	Forward and bridge converters that want tight coupling
Fully interleaved	Lowest	Highest	Where efficiency wins and a shield can handle the noise
Sectored, side by side	Very high	Very low	High isolation, and resonant tanks that use the leakage as the series inductor
Interleaved with an electrostatic shield	Low	Managed	The usual answer when both matter and the budget allows the extra layer

In an LLC the leakage is not a defect, it is the resonant inductor. In a phase shift bridge it is what your control loop has to live with. We ask which one you are building before we choose.

Straight answers

Windings, the usual questions.

When do I actually need litz?

When the skin depth at your switching frequency is smaller than the radius of the solid conductor that would carry your current. Past that point the middle of the wire stops conducting. Litz splits the conductor into strands thinner than the skin depth and transposes them so they share the current.

Are more strands always better?

No. Every strand brings its own enamel and its own air, so the copper fill factor falls and the DC resistance rises. There is an optimum for each frequency and each window, and past it the extra strands cost more than they save. We pick the strand diameter and count rather than reaching for the finest available.

Why does interleaving cut leakage inductance?

Leakage is energy stored in the field between primary and secondary. Splitting the primary and putting the secondary between the halves shrinks the volume that field occupies, so the leakage falls. What rises is interwinding capacitance, which is the trade.

What is triple insulated wire for?

It carries reinforced isolation in the wire itself, so the winding needs no margin tape and no barrier layer. That gives the window back to copper, which usually means a smaller part or lower loss at the same size.

What design information will I get?

The first proposal carries the estimated core and winding losses, the leakage and magnetising inductance, the dimensions and the expected hotspot temperature. The prototype then arrives with those values measured on your own waveform.

Construction

The three that come up most.

Foil for high current and few turns, litz where the frequency demands it, planar where the height does.

A foil winding on a bobbin over a gapped stacked core

A board mount transformer with litz wire brought out to flying leads

A heavy winding filling the window of a taped core

Start here

Send the waveform, not just the inductance.

Switching frequency, ripple, RMS and peak current, and the harmonic content if you have it. That is what decides the winding, and it is what we need to choose it well.

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