What Is the Best Winding Design for Lower AC Loss in High-Frequency Transformers？

When I work with engineers designing high-frequency power supplies, one challenge appears again and again: the transformer works in theory, but the efficiency drops and the temperature rises unexpectedly during operation. In many cases, the real cause is excessive AC loss in the transformer windings. As switching frequencies increase in modern converters, traditional winding designs often suffer from strong skin effect and proximity effect, which significantly increase copper losses.

The problem becomes more serious in compact power electronics where thermal margins are limited. If AC loss is not carefully controlled, the transformer may run hotter, reduce system efficiency, or even shorten the product’s lifetime.

From my experience in transformer manufacturing, the good news is that proper winding design can dramatically reduce AC loss. In this article, I will explain which winding structures work best for high-frequency transformers and how engineers can optimize their designs for better efficiency and reliability.

Why AC Loss Is a Critical Issue in High-Frequency Transformers

Discussing the best winding structures for reducing AC loss, it is important to understand why AC loss becomes a critical issue in high-frequency transformers. In many switching power supply designs, engineers initially focus on core material, turns ratio, or switching topology. However, during prototype testing, the transformer may run hotter than expected even when the DC resistance of the winding appears acceptable. In my experience working with high-frequency transformer projects, this problem is often caused by AC loss in the winding conductors. At higher frequencies, current distribution inside the conductor changes significantly due to electromagnetic effects, which increases effective resistance and heat generation. Understanding how AC loss occurs and why it increases with switching frequency is the first step toward selecting the right winding design.

1. What Is AC Loss in Transformer Windings

AC loss in transformer windings refers to the additional copper loss that occurs when alternating current flows through the conductor at high frequency. Unlike DC current, which distributes evenly across the conductor cross-section, AC current tends to concentrate in certain regions of the conductor.

Two main electromagnetic effects cause this behavior.

Skin effect occurs when alternating current flows mainly near the outer surface of the conductor. As the frequency increases, the effective depth of current penetration becomes smaller. This reduces the effective conducting area of the wire and increases the apparent resistance of the winding.

Proximity effect occurs when magnetic fields generated by nearby conductors force the current to redistribute unevenly inside the wire. In transformer windings, adjacent turns and layers produce strong alternating magnetic fields that push current toward certain regions of the conductor.

In my experience designing high-frequency transformers, these two effects can significantly increase the effective resistance of the winding compared with the DC resistance measured at low frequency.

2. Why AC Loss Increases Rapidly at High Switching Frequencies

In switching power supplies, transformers typically operate at frequencies ranging from tens of kilohertz to several hundred kilohertz. At these frequencies, the skin depth of copper becomes very small, meaning current flows only in a thin outer layer of the conductor.

For example, as frequency increases, the usable cross-section of a solid copper wire effectively decreases. This means that even if the conductor size appears large enough for the required current under DC conditions, the effective resistance under AC conditions becomes much higher.

In addition, the compact winding structures used in high-frequency transformers place conductors very close together. This increases the proximity effect, which further distorts the current distribution and increases AC resistance.

In our factory, we often see cases where engineers initially select solid wire based on DC current calculations. During testing, however, the transformer experiences unexpected heating because the AC copper loss is much higher than anticipated.

3. Impact of AC Loss on Efficiency, Temperature Rise, and Reliability

Excessive AC loss has several direct consequences for transformer performance.

First, it reduces overall power conversion efficiency. Higher winding resistance means more electrical energy is dissipated as heat instead of being transferred to the load.

Second, increased copper loss leads to higher winding temperature rise. In high-frequency transformers, the winding is often the main heat-generating component. If the temperature exceeds the insulation rating, the lifetime of the transformer may be significantly shortened.

Third, excessive heating can affect long-term reliability and stability. Insulation materials degrade faster at higher temperatures, and thermal stress may cause mechanical damage to the winding structure over time.

In my experience working with production transformers, many overheating issues can be traced back to inadequate consideration of AC loss during the design stage. This is why selecting the right winding design, conductor type, and layout becomes essential when designing high-frequency transformers.

What Are Common Winding Designs Used in High-Frequency Transformers?

Selecting the best winding structure to reduce AC loss, it is helpful to understand the common winding designs used in high-frequency transformers. In my experience working with SMPS transformer production, different winding structures offer different trade-offs between AC loss, power density, manufacturability, and thermal performance. The choice of winding design often depends on the switching frequency, current level, and space constraints of the power supply. Some winding methods are simpler and easier to manufacture, while others are designed specifically to reduce proximity effects and improve magnetic coupling.

1. Single-Layer Winding

Single-layer winding is one of the simplest winding structures used in high-frequency transformers. In this design, the conductor is wound in a single layer across the bobbin, so each turn lies next to the previous one without stacking multiple layers.

One major advantage of this structure is that it reduces the proximity effect significantly. Because the conductors are not stacked vertically, the magnetic interaction between adjacent layers is minimized. As a result, the current distribution inside each conductor remains more uniform, which helps keep the AC resistance relatively low.

In my experience, single-layer winding is often used in low-power or very high-frequency applications, where minimizing AC loss is a priority. However, it also has some limitations. Since only one layer is used, the number of turns that can fit on the bobbin is limited, which restricts the achievable power level and voltage ratio. For higher-power transformers, this design may not provide enough winding space.

2. Multi-Layer Winding

Multi-layer winding is widely used when the transformer needs higher power density or more turns than a single layer can provide. In this configuration, conductors are wound in multiple stacked layers, allowing a much larger number of turns to be placed within the available window area.

From a manufacturing perspective, this structure is practical and efficient because it maximizes the utilization of the core window space. In many real production projects, especially in medium-power SMPS transformers, multi-layer winding becomes a common solution.

However, this design also introduces a significant challenge: increased AC loss due to proximity effect. When several winding layers are stacked together, the alternating magnetic field from neighboring layers forces current to concentrate in certain regions of the conductor. This effect increases the effective AC resistance and leads to additional copper losses.

In our factory testing experience, transformers with poorly optimized multi-layer windings often show unexpected temperature rise, even when the calculated DC copper loss appears acceptable. This is why additional design measures such as using Litz wire or optimized layer arrangement are often necessary to control AC loss.

3. Sectional or Sandwich Winding

Sectional winding, often called sandwich winding, is a more advanced structure frequently used in high-performance high-frequency transformers. In this design, the primary and secondary windings are divided into sections and interleaved with each other.

For example, the winding structure might look like:

Primary – Secondary – Primary

This arrangement provides several advantages.

First, it improves magnetic coupling between the windings, which helps reduce leakage inductance. Lower leakage inductance is particularly important in switching power supplies because it reduces voltage spikes and improves energy transfer efficiency.

Second, sectional winding can also help control AC loss. By distributing the current more evenly across multiple winding sections and reducing the magnetic field concentration between layers, the proximity effect can be reduced compared to a traditional multi-layer winding.

In my experience, sandwich winding is commonly used in high-efficiency SMPS transformers, especially in applications such as flyback or forward converters where tight coupling between primary and secondary is critical.

However, this design is more complex to manufacture, and careful attention must be paid to insulation spacing, creepage distance, and winding consistency during production.

unicreed High-Frequency Transformers wininding

What Are Main Causes of AC Loss in High-Frequency Windings?

Optimal winding design to reduce AC loss, it is important to understand what actually causes AC loss in high-frequency transformer windings. In my experience working with high-frequency magnetics for switching power supplies, AC loss rarely comes from a single factor. Instead, it usually results from a combination of electromagnetic effects inside the conductors and magnetic interactions between adjacent windings. When switching frequencies increase, these effects become much more pronounced, significantly increasing the effective resistance of the winding. Understanding these mechanisms helps transformer designers choose the right conductor type and winding layout to minimize losses.

1. Skin Effect in High-Frequency Conductors

One of the primary causes of AC loss in high-frequency windings is the skin effect.

When alternating current flows through a conductor at high frequency, the current no longer distributes evenly across the entire cross-section of the wire. Instead, the current tends to concentrate near the outer surface of the conductor.

This phenomenon occurs because the changing magnetic field generated by the alternating current induces internal eddy currents within the conductor. These eddy currents oppose the flow of current in the center of the conductor, forcing most of the current toward the surface.

As the switching frequency increases, the skin depth, which represents the depth at which most of the current flows, becomes smaller. This effectively reduces the usable cross-sectional area of the conductor.

In practical transformer design, this means the effective AC resistance of the conductor becomes much higher than its DC resistance, which directly increases copper loss.

In my experience, this is why solid round wires that work well at low frequencies often become inefficient in high-frequency transformers.

2. Proximity Effect Between Adjacent Windings

Another major contributor to AC loss is the proximity effect.

In a transformer, multiple turns of wire are wound closely together. Each conductor generates its own magnetic field when current flows through it. These magnetic fields interact with neighboring conductors and influence how current distributes inside them.

Because of this interaction, current inside a conductor tends to concentrate in regions that minimize magnetic field opposition. This leads to uneven current distribution within the conductor, even if the conductor itself is relatively thin.

The proximity effect becomes particularly severe in multi-layer windings, where conductors are stacked close together. In these cases, the magnetic fields from adjacent turns and layers force current into narrow regions of the conductor, further increasing AC resistance.

From my experience in transformer design, poorly optimized multilayer windings can produce significant additional copper loss, even when the DC resistance of the winding appears low.

3. Leakage Flux and Its Influence on AC Loss

Leakage flux also plays an important role in AC loss within transformer windings.

In an ideal transformer, all magnetic flux generated by the primary winding would link perfectly with the secondary winding through the core. In reality, some portion of the magnetic flux does not follow the core path and instead spreads through the surrounding space. This is known as leakage flux.

Leakage flux creates additional magnetic fields around the winding conductors. These fields can induce circulating currents inside the conductors, which further contribute to AC loss.

The amount of leakage flux is strongly influenced by the transformer geometry and winding layout. For example, when the primary and secondary windings are placed far apart, the leakage flux tends to increase. Similarly, certain winding arrangements may expose conductors to stronger leakage fields.

In my experience, careful winding design can significantly reduce the influence of leakage flux. By improving magnetic coupling and optimizing the physical arrangement of windings, designers can reduce both leakage inductance and the associated AC losses.

Understanding these three main factors, skin effect, proximity effect, and leakage flux, is essential when designing high-frequency transformers. By addressing these mechanisms through appropriate winding structures and conductor selection, engineers can achieve lower AC loss, higher efficiency, and better thermal performance in high-frequency power converters.

Best Winding Design for High-Frequency Transformers

What Are The Key Winding Design Strategies To Reduce AC Loss?

In our factory, when we optimize a transformer for high-frequency operation, we usually evaluate several design factors together, especially wire type, layer count, winding arrangement, and conductor dimensions. By carefully balancing these elements, it is possible to significantly reduce both skin effect loss and proximity effect loss, which improves efficiency and reduces temperature rise in real operating conditions.

1. Using Litz Wire to Mitigate Skin Effect

One of the most effective methods to reduce AC loss at high frequencies is the use of Litz wire. Litz wire consists of many thin insulated strands woven together in a specific pattern so that each strand periodically moves between different positions in the conductor bundle.

This structure allows current to distribute more evenly across the strands, which helps reduce the impact of skin effect. Because each strand has a smaller diameter, typically smaller than the skin depth at the operating frequency, the conductor can utilize most of its cross sectional area effectively.

In my experience, Litz wire is especially beneficial in transformers operating above 100 kHz, where solid conductors would suffer significant AC resistance increase. However, designers must also consider cost, manufacturability, and winding space, because Litz wire is usually more expensive and occupies slightly more volume than solid wire.

2. Interleaving Primary and Secondary Windings

Another important design technique is interleaving the primary and secondary windings, often called sandwich winding.

Instead of placing the entire primary winding on one side and the secondary on the other, the windings are divided into sections. For example:

Primary → Secondary → Primary

This structure improves magnetic coupling between windings, which significantly reduces leakage inductance. Lower leakage inductance helps minimize stray magnetic fields inside the winding window. As a result, proximity effect losses are also reduced.

In our production experience, interleaved winding structures are commonly used in high efficiency SMPS transformers, especially in applications where both low leakage inductance and high efficiency are required.

However, interleaving must be carefully designed because it can also introduce challenges such as:

Increased insulation requirements
Higher manufacturing complexity
Creepage and clearance constraints for safety standards

3. Reducing the Number of Layers in the Winding

The number of winding layers has a strong influence on AC loss, especially due to proximity effect.

When multiple layers are stacked on top of each other, the magnetic fields generated by neighboring layers interact strongly with the conductors. This interaction causes uneven current distribution inside the wires, increasing effective AC resistance.

In my experience, transformers with many tightly packed layers often show much higher AC copper loss than expected from simple calculations. Whenever possible, reducing the number of layers helps minimize this effect.

Design approaches may include:

Using larger core sizes to increase window space
Splitting windings into parallel strands
Using foil or Litz conductors instead of thick round wire

These methods help maintain lower AC loss while still meeting current and voltage requirements.

4. Optimizing Conductor Thickness Relative to Skin Depth

Another critical design factor is choosing the appropriate conductor thickness relative to the skin depth at the operating frequency.

Skin depth decreases as frequency increases. If the conductor diameter becomes significantly larger than twice the skin depth, most of the current will flow only near the outer surface of the conductor. The inner copper will not be fully utilized.

For this reason, in high frequency transformer design we usually ensure that the effective strand diameter is comparable to or smaller than the skin depth. This principle explains why Litz wire or multi strand conductors are widely used in SMPS transformers.

In our factory design process, we often evaluate conductor dimensions during the early design stage and compare them with the expected operating frequency and RMS current. This helps ensure that the conductor is efficiently utilized while avoiding unnecessary copper loss.

How Custom Transformer Manufacturers Optimize Winding Design

In my experience working with OEM power supply projects, standard transformer designs do not always deliver the best efficiency or thermal performance, especially when switching frequencies and load profiles vary from one application to another. This is why many companies work with custom transformer manufacturers who can refine the winding structure, conductor type, and manufacturing process. With proper engineering collaboration and testing, the winding design can be optimized to minimize AC loss and ensure reliable operation.

1. Application-Specific Winding Optimization

High-frequency transformers used in switching power supplies often operate under very different electrical conditions. The optimal winding structure for a 50 kHz converter may not work well in a 200 kHz or higher frequency design.

For this reason, custom transformer manufacturers usually optimize the winding based on the specific operating frequency, current waveform, and power level.

In many projects I have worked on, this optimization includes selecting the appropriate conductor type, such as Litz wire, foil conductors, or parallel strands, depending on the current level and frequency. The winding layout may also be adjusted by using sectional winding, interleaving, or other arrangements to reduce leakage inductance and proximity effects.

By tailoring the conductor structure and winding geometry to the real application, it is often possible to significantly reduce AC loss while maintaining compact transformer size.

2. Advanced Manufacturing and Testing

Optimizing winding design is only effective if the manufacturing process can reproduce the design consistently.

In our factory, careful control of winding tension, insulation placement, and layer alignment is essential for maintaining the intended electromagnetic performance. Even small variations in winding placement can affect leakage inductance and AC resistance in high-frequency transformers.

To ensure consistent performance, transformers are usually verified through a series of electrical tests during production. These tests may include measurements of turns ratio, no-load parameters, load performance, and insulation verification.

In my experience, combining optimized winding design with reliable manufacturing processes and electrical testing is essential to ensure that transformer performance remains stable from prototype to mass production.

3. Working With Experienced Manufacturers

Working with experienced transformer manufacturers can greatly simplify the process of optimizing winding design for high-frequency applications.

An experienced manufacturer can evaluate the electrical requirements, switching frequency, and thermal constraints of the application and recommend suitable winding structures. Through prototype development and testing, the design can be refined to achieve the best balance between efficiency, size, and reliability.

At Unicreed, we have been manufacturing transformers since 2008 and regularly support OEM customers in developing customized transformer solutions for industrial equipment and power electronics. Our engineering team works closely with customers to adjust winding structures, conductor selection, and layout to improve efficiency and reduce AC loss in high-frequency transformers.

In my experience, this type of engineering collaboration often leads to better thermal performance, higher efficiency, and more reliable transformer operation in real applications.

Conclusion

When I evaluate transformer performance, I always pay close attention to the wiring layout, because the right winding design can greatly reduce AC loss and improve efficiency in high-frequency transformers. By optimizing conductor size, winding structure, and magnetic coupling, I can achieve better electrical performance and thermal stability.

In my experience, the best results come from working with experienced transformer manufacturers who can support engineering collaboration, prototype testing, and real-world validation.

If you are developing a high-frequency power supply and want to improve efficiency, I invite you to work with us at Unicreed. We specialize in custom transformer design and optimized winding solutions to help your equipment perform reliably.

Contact Unicreed today to discuss your project.