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Introduction
Selecting the proper conductor is just as important as selecting the magnetic core itself.
Even the best magnetic design can fail if the winding method is not chosen correctly.
The wire affects:
- DC resistance (DCR)
- Copper losses
- Temperature rise
- Efficiency
- Manufacturability
- Cost
As switching frequencies increase, conductor selection becomes even more critical because additional AC losses begin to appear.
In this chapter, we will explore the most common conductor types used in inductors and transformers and learn how to select the best winding method for a given application.
The Purpose of the Winding
The winding creates the magnetic field that stores and transfers energy.
When current flows through the wire, a magnetic field is produced.
The magnetic field strength is proportional to:
Where:
- H = Magnetic Field Strength
- N = Number of Turns
- I = Current
- l = Magnetic Path Length
Increasing either current or turns increases the magnetic field.
Because the winding carries current continuously, conductor selection has a major impact on efficiency and thermal performance.
DC Resistance (DCR)
Every conductor has resistance.
The resistance of a wire is:
Where:
- R = Resistance
- ρ = Resistivity
- l = Length
- A = Cross-sectional Area
Resistance increases with:
- Longer wire length
- Smaller conductor area
Resistance decreases with:
- Larger wire diameter
- Parallel conductors
Lower resistance reduces power loss and improves efficiency.
Copper Losses
Current flowing through the winding produces heat.
Copper loss is:
Notice that current is squared.
Doubling current results in four times the copper loss.
This is one reason high-current inductors often use very large conductors or multiple parallel strands.
Round Magnet Wire
Round enamel-coated copper wire is the most common conductor used in magnetic components.
Advantages
- Lowest cost
- Easy to obtain
- Easy to wind
- Available in many sizes
Disadvantages
- Limited packing efficiency
- Higher AC losses at high frequency
Typical Applications
- Power inductors
- Flyback transformers
- General-purpose magnetics
For most switching power supplies below a few hundred kilohertz, round wire remains an excellent choice.
Parallel Conductors
Instead of using one large wire, multiple smaller wires can be connected in parallel.
Advantages:
- Lower resistance
- Improved winding flexibility
- Better window utilization
Example:
Three strands of AWG18 wire may fit more easily than a single conductor with equivalent copper area.
Parallel conductors are commonly used in:
- High-current inductors
- Power converters
- Automotive electronics
Litz Wire
Litz wire consists of many individually insulated strands woven together in a specific pattern.
The purpose of Litz wire is to reduce AC resistance caused by:
- Skin effect
- Proximity effect
Advantages:
- Lower high-frequency losses
- Improved efficiency
Disadvantages:
- Higher cost
- More difficult termination
- Larger overall conductor diameter
Typical Applications:
- Wireless charging
- High-frequency transformers
- RF magnetics
- Resonant converters
Understanding Skin Effect
As frequency increases, current tends to flow near the outer surface of a conductor.
This phenomenon is called skin effect.
The effective depth of current penetration is known as skin depth.
Skin depth is:
Where:
- δ = Skin Depth
- ρ = Resistivity
- ω = Angular Frequency
- μ = Permeability
As frequency increases:
- Skin depth decreases
- Effective conductor area decreases
- AC resistance increases
This is one of the primary reasons Litz wire is used in high-frequency applications.
Proximity Effect
Proximity effect occurs when magnetic fields from nearby conductors force current to crowd into specific regions of the conductor.
This can significantly increase AC resistance.
Proximity effect becomes important in:
- Multi-layer windings
- Transformers
- High-current inductors
Often proximity effect causes more loss than skin effect.
Good winding layout can significantly reduce these losses.
Foil Windings
Copper foil can be used instead of round wire.
Advantages:
- Very low resistance
- Excellent current handling
- Reduced proximity losses
Disadvantages:
- More difficult manufacturing
- Additional insulation requirements
Typical Applications:
- High-current power inductors
- High-power transformers
- Industrial converters
Fill Factor
The winding window can never be filled completely with copper.
Insulation, bobbins, and manufacturing tolerances all consume space.
Fill factor is:
Typical practical fill factors:
| Construction Type | Typical Fill Factor |
|---|---|
| Hand Wound | 25–35% |
| Machine Wound | 35–50% |
| Optimized Designs | 50–70% |
Attempting to exceed realistic fill factors usually results in manufacturing problems.
Insulation Systems
Wire insulation serves several important purposes:
- Prevents shorts between turns
- Meets safety requirements
- Improves reliability
Common insulation classes include:
| Class | Maximum Temperature |
|---|---|
| A | 105°C |
| B | 130°C |
| F | 155°C |
| H | 180°C |
Higher temperature ratings generally increase cost but improve thermal margin.
Winding Layout Considerations
Good winding layout helps improve:
- Efficiency
- EMI performance
- Thermal behavior
Designers should consider:
Layer Count
More layers increase winding resistance and AC losses.
Turn Spacing
Spacing affects insulation and EMI.
Interleaving
Transformer windings are often interleaved to reduce leakage inductance.
Lead Routing
Proper lead placement improves manufacturability and reduces unwanted coupling.
SolidMag Engineering Note
Bigger Wire Is Not Always Better
Many new designers assume that larger wire is always the best choice.
In reality, wire size must be balanced against:
- Available window area
- Number of turns required
- Fill factor limitations
- Manufacturing constraints
- High-frequency effects
An oversized conductor may actually prevent the winding from fitting inside the core window.
The best design is usually the conductor that minimizes total losses while remaining manufacturable.
What You’ve Learned
In this chapter you learned:
- How wire size affects resistance
- Why copper losses occur
- The differences between round wire, parallel conductors, Litz wire, and foil windings
- How skin effect impacts high-frequency designs
- Why proximity effect increases losses
- What fill factor means
- How insulation systems affect reliability
- How winding layout influences performance
Continue Reading
Chapter 10: Understanding Core Losses
Now that we understand how winding choices affect copper losses, the next step is learning how magnetic materials themselves dissipate energy and how core losses influence efficiency, temperature rise, and overall magnetic component performance.
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