How Do You Choose the Windings and Conductors for an SMPS Transformer?

Published by West Coast Magnetics, June 2026, based on our 2010 switch-mode transformer application note (available for download below); content reviewed and confirmed current as of publication.

Once the core, bobbin, and turns count are set, the winding is what decides how much copper loss an SMPS transformer actually carries. The choice comes down to four conductor types (solid wire, litz wire, high-voltage isolation wire, and copper foil), and the right one for each winding depends on that winding’s current, operating frequency, voltage, and isolation requirement. This is the winding half of the design; for the core, geometry, material, and thermal budget that come first, see the SMPS transformer design guide.

Winding-conductor selection is the step in switch-mode transformer design where each winding is assigned a conductor type (solid wire, litz wire, HV isolation wire, or copper foil) to meet its current, frequency, and isolation needs at the lowest practical loss. AC copper loss is the frequency-dependent part of winding loss, driven by skin effect and proximity effect, that at switch-mode frequencies frequently exceeds the DC loss and can overwhelm a design if it is not managed.

How Are the Secondary Turns Determined?

Unlike a line-frequency transformer, an SMPS transformer is duty-cycle regulated, so the turns ratio is not simply the voltage ratio. It is set by the input-to-output transfer function for the chosen topology, which relates input voltage, output voltage, turns ratio, and the primary on-time together.

Many designers approximate the ratio by dividing the output voltage by the input voltage and adding about 10%, and that works reasonably well. Using the actual transfer function for the topology produces a better-informed and superior choice. Most topologies are limited to a 50% maximum duty cycle, though the forward converter and the flyback can run duty cycles above 50%. For a worked example of setting primary turns from the volt-microsecond product, and of the flyback converter transformer specifically, see those guides.

Regulation applies to only one secondary output. When a design has multiple outputs, two things matter: choose a topology that gives good cross-regulation between windings, and pick the turns ratio carefully so every output lands as close as possible to its target.

What Are the Four Winding-Conductor Types?

There are four basic classes of winding conductor, and the best choice depends on the specific requirements of each winding.

Solid wire is typically used for transformers operating at 100 kHz and below and at RMS currents under 20 A. Its advantages are low material cost and low winding time. Its disadvantage is loss: solid wire can develop excessive AC winding resistance at SMPS frequencies, so a solid-wire winding should always be kept to a single layer where possible.

Litz wire is many fine, individually insulated strands twisted into one bundle. Strands are available from 24 AWG down to 50 AWG; at SMPS frequencies most designers use 36 to 44 AWG, with 36 to 40 AWG below 100 kHz and 42 to 44 AWG at higher frequencies. Dividing the conductor this way suppresses the eddy-current losses that drive AC resistance.

HV wire is a safety-isolation conductor with two or three thin layers of Teflon or a similar polymer (Tex-E and Rubadue are common). Its advantage is that it can reach a 4000 Vac hypot and a reinforced-insulation rating with only a modest increase in overall diameter, which vastly simplifies the build because no additional insulation, creepage, or clearance has to be engineered into the winding. Its disadvantages are the same as litz: higher cost and lower winding-window utilization.

Copper foil is an excellent choice for low-voltage windings. It typically gives far greater window utilization and the lowest DCR of any option, and it can be the lowest-loss choice. Its low-profile cross-section helps minimize copper loss, because the magnetic field in a transformer winding tends to draw AC current to the top and bottom of the winding. The disadvantage is that each turn is a single layer, so with many turns winding capacitance and AC copper loss add up quickly. For how WCM reshapes foil to cut that AC resistance in gapped inductors, see reducing winding losses in high-frequency inductors.

How Do You Estimate Winding Resistance and Loss?

Winding loss in an SMPS transformer has both an AC and a DC component. A useful first-order approximation of copper loss is to compute each winding’s DC resistance, multiply by that winding’s RMS current squared, and sum across windings. The DC resistance of a winding is its number of turns times the mean length per turn times the conductor’s resistance per unit length:

DCR = N x (mean length per turn) x (conductor resistance per unit length)

That captures the DC term. The AC term is harder. At switch-mode frequencies, AC losses frequently exceed DC copper losses and can overwhelm a design if they are not managed. Predicting AC copper loss precisely is complex, but the driver is skin depth. At 250 kHz, for example, the skin depth in copper is about 0.0148 cm, so a designer would need 35 AWG wire to keep the conductor within one skin depth. A single strand of 35 AWG carries very little current, which is exactly why litz wire is required in most higher-frequency designs.

A few rules of thumb keep AC copper loss under control:

• Minimize the number of layers in each winding. One layer is ideal, though not always possible.
• Interleave primaries and secondaries where the topology allows it.
• Keep a single-layer conductor’s diameter to one skin depth or less. Windings with more than one layer need even smaller conductors.
• Use litz or copper foil for any winding of two or more layers, or at 100 kHz and above.

For predicting litz copper loss specifically, the Thayer School of Engineering at Dartmouth maintains a free online tool, the same collaboration behind WCM’s shaped-foil work.

One more effect is easy to forget: copper resistance rises with temperature. It increases about 8% from 20 to 40 degrees C, 16% from 20 to 60 degrees C, and 24% from 20 to 80 degrees C, and that same percentage applies to both AC and DC copper loss. For temperature-rise-limited designs, calculate core and copper losses and the resulting temperature rise at the worst-case operating condition rather than at room temperature.

When Should You Let WCM Design the Winding?

Conductor selection, turns ratio, layer count, and interleaving all interact, and the AC-loss penalty for getting them wrong grows with frequency. WCM publishes data sheets that give the power-handling capability of many cores as a function of frequency, based on a 40 degrees C rise for a conventional push-pull transformer with minimal parasitic losses. If the application needs high isolation, very high efficiency, or an input or output voltage above 200 V, expect to size up from the baseline curve.

When the requirement is specific, WCM engineers these as custom switch-mode transformers, choosing the core, bobbin, turns, and winding conductor together rather than from a catalog, and delivering a specification sheet that certifies the key transformer properties.

FAQ