Power electronics teams often run into this problem. A transformer or inductor might seem efficient on paper, with low DC resistance and promising simulation results. However, when the prototype runs at switching frequency, temperatures rise quickly, and losses are higher than expected. This is usually due to the proximity effect in custom magnetics, which appears at high frequencies, increases AC copper losses, and puts greater strain on thermal limits.
As switching frequencies rise and power density requirements increase, this problem becomes more obvious. Some engineers group the proximity effect with the skin effect, but they have different causes and need different solutions. Understanding these differences helps with choosing conductors, planning winding layouts, and ensuring reliability. This article covers how the proximity effect works, where it causes issues, and how good magnetic design can reduce its impact. Addressing this early in development helps teams avoid problems during validation.
What Is Proximity Effect in Custom Magnetics?
Before looking at ways to reduce it, it’s important to define what the proximity effect means in wound components.
Definition
The proximity effect happens when alternating magnetic fields from nearby conductors change how current flows inside a conductor. Instead of spreading out evenly, the current gets crowded into certain areas. This raises the effective AC resistance much higher than the DC resistance.
In custom magnetics, windings are often packed closely, sometimes separated by just thin insulation. Each conductor’s magnetic field affects its neighbors, changing current paths with every AC cycle. This causes extra copper loss and more heat.
Why It Matters
In designs that use medium or high switching frequencies, the proximity effect can cause more loss than DC copper loss. The extra heat impacts several key performance factors:
- Electrical efficiency
- Thermal rise within windings
- Achievable power density
- Long-term reliability under continuous load
If designers ignore the proximity effect, they often must lower ratings or make unexpected changes later.
Where It Appears Most
Certain configurations experience stronger proximity effects than others:
- Transformers with multilayer windings
- High-current inductors carrying ripple-heavy waveforms
- Closely packed coils in compact assemblies
- Litz wire windings that lack proper strand sizing or transposition
For example, a well-known case involved a high-frequency transformer in a compact DC-DC converter. Not accounting for the proximity effect led to unexpected overheating and efficiency losses during prototype testing (Chae et al., 2003). Cases like this show that the proximity effect in custom magnetics needs careful analysis throughout the design process, not just a quick mention in loss calculations.
Skin Effect vs Proximity Effect: What’s the Difference?
Both effects increase AC resistance, but they have different causes and need different ways to control them. Treating them as the same can lead to incomplete designs.
Skin Effect
Skin effect refers to the tendency of alternating current to concentrate near the surface of a conductor as frequency increases.
Characteristics include:
- Dependence on frequency and conductor diameter
- Presence even in isolated conductors
- Predictable penetration depth related to material properties
You can often manage the skin effect by choosing the right conductor size or strand type.
Proximity Effect
The proximity effect comes from magnetic fields created by nearby conductors, not just from the current in the conductor itself.
Its characteristics include:
- Strong dependence on winding geometry and spacing
- Sensitivity to current direction in adjacent turns
- Significant variation across multilayer structures
This difference is why skin and proximity effects should be analyzed separately when designing magnetics.
Why the Distinction Matters
Designs that only consider skin depth can end up with higher AC losses than expected. Even if the conductor diameter matches the skin depth, nearby turns can still cause current crowding. In dense windings, the proximity effect is often the main source of copper loss.
The influence of the proximity effect varies with component type and operating conditions.
Proximity Effect in Transformers
Transformers experience interactions between primary and secondary windings, as well as among layers within the same winding. These interactions cause several observable issues:
- Increased AC copper losses at switching frequency
- Uneven current sharing in parallel windings
- Localized heating within the inner layers
- Reduced efficiency under high-frequency operation
In tightly coupled designs, magnetic fields from one winding have a strong effect on nearby conductors. This makes the proximity effect a major challenge in transformers, especially in compact power converters.
If you want a deeper understanding of transformers and the different types used in real applications, you can read our detailed guide.
Proximity Effect in Inductors
Inductors often carry large ripple currents on top of a DC bias. This ripple interacts with nearby turns and changes how current flows inside the conductor.
Typical effects include:
- Elevated winding losses under ripple conditions
- Higher temperature rise compared to DC estimates
- Reduced saturation margin due to thermal stress
The most affected applications are power-factor-correction inductors, DC-link inductors, and high-current filter inductors. In these cases, the proximity effect often sets the thermal limit before core losses matter.
If you want to learn more about inductors and how different inductor types are used in practical designs, you can read our detailed article about Inductors.
Key Factors That Influence Proximity Effect Losses
Several design factors influence how much the proximity effect adds to losses. Knowing these helps engineers make better trade-offs.
Conductor Geometry
Conductor shape plays a large role in current redistribution:
- Round conductors tend to experience uneven crowding near adjacent turns.
- Rectangular conductors concentrate current near faces exposed to stronger magnetic fields.
- Foil windings exhibit predictable field interactions but require thickness control.
- Strand diameter relative to skin depth affects current sharing inside Litz constructions.
The conductor’s shape and size directly affect AC copper losses at the operating frequency.
Winding Arrangement
How turns and layers sit relative to one another matters as much as the conductor type:
- Increased layer count intensifies field interaction.
- Tight turn-to-turn spacing increases current distortion.
- The relative direction of current between adjacent conductors changes the field strength.
Careful winding design often cuts losses more than just changing the conductor type.
Operating Frequency
As frequency increases, proximity losses rise faster than DC losses. Switching waveforms with many harmonics make this worse. In high-frequency magnetics, the proximity effect is a major source of loss.
Current Waveform
Non-sinusoidal waveforms have harmonics that make current spread unevenly inside the conductor. High ripple increases losses, even if the average current is low.
Design Strategies to Reduce Proximity Effect
Reducing the proximity effect takes changes in geometry, materials, and testing. There’s no single fix.
Optimized Winding Layout
Layout choices shape magnetic field interaction:
- Reduced layer stacking where space allows
- Physical separation between high-current conductors
- Intentional alignment of current directions to limit opposing fields
Even small layout changes can noticeably improve temperature control.
Conductor Selection
Material choices must align with operating conditions:
- Properly constructed Litz wire with appropriate strand count
- Strand diameters matched to frequency range.
- Foil windings with thickness chosen to limit internal crowding
Custom transformer design usually balances conductor cost with the need to reduce thermal stress.
Layer Interleaving
Interleaving can improve coupling and reduce leakage, but it also increases field interaction. Careful modeling helps decide when the benefits outweigh the added proximity losses.
Simulation and Modelling
Analytical formulas provide insight, but modern design relies on modeling:
- Finite element analysis for current distribution
- Frequency-domain loss calculations
- Thermal simulations to confirm temperature rise under load
Simulation helps optimize magnetic losses before building and testing hardware.
Practical Design Considerations in Custom Magnetics
Reducing losses usually means making trade-offs. Engineers have to balance performance with different constraints in each industry.
Trade-offs
Reducing the proximity effect often influences:
- Physical size of the component
- Manufacturing complexity
- Material cost
A good design meets performance goals without adding extra cost or complexity.
Industry-Specific Constraints
Different sectors impose different priorities:
- Aerospace applications emphasize weight and thermal limits.
- Defense programs focus on reliability and qualification standards.
- Industrial electronics often balance performance against cost sensitivity.
These factors determine how much effort goes into reducing proximity losses.
Why Custom Design Matters
Standard catalog parts rarely match the exact frequency, waveform, and thermal needs of a project. Custom magnetics let you tailor conductor shape and layout to real operating conditions. Teams needing high reliability can control losses better with custom designs than with off-the-shelf parts.
How to Design Magnetics with Proximity Effect in Mind
A step-by-step approach keeps the proximity effect in mind throughout the development process.
Step 1 – Define Operating Conditions
Start by defining clear electrical and thermal requirements. Set the frequency range, current waveform, duty cycle, and allowed temperature rise as your starting points.
Step 2 – Select Appropriate Conductor Technology
Choose a conductor type that fits the frequency and current density. Think about strand diameter, foil thickness, and insulation needs early in the process.
Step 3 – Engineer the Winding Geometry
Plan the spacing, layering, and orientation of windings carefully. The layout affects losses as much as the materials you choose.
Step 4 – Validate Through Testing
Test AC resistance, temperature rise, and efficiency under real operating conditions. Comparing test results with simulations builds confidence before moving to production.
Following this process keeps the proximity effect in mind at every stage, so you don’t find problems late in validation.
Conclusion
The proximity effect in custom magnetics is a major cause of AC copper losses in transformers, inductors, and wound assemblies. Its impact grows with higher frequencies, compact layouts, and waveforms with lots of ripple. If you treat it the same as the skin effect, you risk lower efficiency and tighter thermal margins.
Knowing the differences between skin and proximity effects, along with smart conductor choices, careful winding design, and simulation validation, helps you create magnetics that work well in real conditions. In demanding power electronics and high-reliability systems, handling the proximity effect is a key part of good magnetic design, not just an extra step.
Need help optimizing transformer or inductor designs for high-frequency performance?
Contact Velatron Technologies to engineer custom magnetics that minimize losses and maximize reliability.