In transformer and inductor design, one of the most powerful yet subtle design parameters is the air gap.
A small non-magnetic space — sometimes less than a millimeter — can completely change how magnetic flux flows through the core, directly affecting inductance, energy storage, and saturation performance.
This article explains how the air gap works, why it’s necessary, and how to size it correctly for efficient magnetic design.
(📎 If you haven’t read our related articles, check out Why Do Transformers Use Air Gaps Instead of Low-Permeability Cores? and Why Does a Transformer Core Saturate? for deeper background on flux behavior and core saturation physics.)
1. What Is an Air Gap in a Magnetic Core?
An air gap is an intentional, non-magnetic space inserted into a magnetic core — typically between two ferrite halves — to control the overall magnetic characteristics.
Although air has a much lower permeability (μ0 ≈ 4π×10⁻⁷ H/m) compared to ferrite (μr ≈ 2000–5000), its presence dramatically affects how the magnetic circuit stores and transfers energy.
In practical terms, adding an air gap:
- Reduces effective permeability (μe)
- Increases energy storage capacity
- Improves linearity and prevents saturation
- Allows precise inductance adjustment
Thus, a tiny change in the air-gap length can make the difference between a stable, efficient converter and a transformer that overheats or saturates.
2. The Magnetic Circuit Model
The magnetic circuit of a core can be described similarly to an electrical circuit:
Φ=F/Rm=N*I/Rm
Where:
- Φ = magnetic flux
- F = magnetomotive force (MMF = N*I)
- Rm = magnetic reluctance
The total magnetic reluctance is the sum of the reluctance of the core and the air gap:
Rm=Rcore+Rgap=lc/(μ0*μr*Ac)+lg/(μ0*Ac)
Because μᵣ of ferrite is much higher than 1, even a small air gap (l_g) dominates total reluctance.
This means the air gap effectively controls the flux and inductance of the magnetic circuit.
3. How the Air Gap Affects Inductance
The inductance of a winding is inversely proportional to total reluctance:
L=N2/Rm
Thus, increasing the air gap length reduces inductance, but makes the inductor more tolerant of DC bias and saturation.
In other words:
- No air gap: High inductance, low DC tolerance, easily saturates
- Large air gap: Low inductance, high DC tolerance, stable operation
Designers choose an air gap that balances these factors based on the converter topology (flyback, forward, buck, boost, etc.) and desired current ripple.
4. Energy Storage in the Air Gap
Most engineers intuitively think energy is stored in the magnetic core — but in reality, the majority of energy is stored in the air gap.
The energy stored in a magnetic circuit is:
E=1/2*L*I2=1/2*B*H*V
When an air gap is present, the flux density (B) remains relatively constant, but the magnetizing field (H) increases dramatically within the gap due to its low permeability.
This results in much higher energy storage per unit volume in the gap region.
This property makes air-gapped ferrite cores ideal for inductors, flyback transformers, and PFC chokes, where large energy swings occur each switching cycle.
5. How the Air Gap Prevents Saturation
As discussed in our previous article Why Does a Transformer Core Saturate?, a magnetic material saturates when flux density (B) exceeds its saturation limit (Bs).
By adding an air gap:
- The total flux Φ decreases for a given magnetizing current (I).
- Effective permeability (μe) decreases.
- The slope of the B–H curve flattens.
The result is a wider linear operating range — meaning the core can handle more current before reaching saturation.
This is why air gaps are crucial for energy-storage inductors and flyback transformers, which carry large DC components in their magnetizing current.
6. Quantitative Example: Effect of Air-Gap Length
Consider a ferrite core with:
- μr = 2500
- l_c = 50 mm
- A_c = 100 mm2
- N = 20 turns
Without an air gap:
L=(N2*μ0*μr*Ac)/lc=202*4π*10-7*2500*1*10-4/0.05≈2mH
If we introduce a 0.5 mm air gap:
Rgap=Lg/(μ0*Ac)=0.5*10-3/(4π*10-7*1*10-4)=3.98*107
The new total reluctance increases by over 10×, reducing inductance to approximately 0.18 mH — but now the core can handle 10× higher current before saturating.
This illustrates how precisely controlling the air gap defines both the inductance value and the energy-handling capacity.
7. The Trade-Off: Loss vs. Control
While air gaps improve linearity and energy storage, they also introduce localized fringing flux — magnetic field lines that spread outside the core and may induce eddy currents in nearby copper windings or components.
To minimize this effect:
- Use distributed air gaps (e.g., powder cores) for lower fringing.
- Increase winding spacing or use electrostatic shields.
- Choose cores with smaller cross-section discontinuities.
Thus, air-gap design is a balance between magnetic control and EMI performance.
8. Air-Gap Design in Different Converter Topologies
| Converter Type | Air Gap Purpose | Typical Location |
|---|---|---|
| Flyback | Store magnetic energy | Main core center leg |
| Forward | Prevent DC bias saturation | Small gap at joint surface |
| Buck/Boost Inductor | Energy storage and ripple control | Core center leg |
| PFC Choke | High-current energy storage | Ferrite or powdered core center gap |
Different topologies require tailored gap sizes — flybacks often need the largest gaps to store significant energy, while forward transformers use smaller ones primarily to improve stability.
9. Key Design Guidelines
- Determine Energy Requirement (E = ½·L*I2)
- Select Core Geometry and Material suitable for frequency and power.
- Calculate Air-Gap Length (lg) using magnetic circuit equations.
- Validate Inductance and Saturation Margin via simulation and measurement.
- Check Fringing and EMI Impact to ensure system compliance.
By following these steps, designers can achieve optimal inductance, efficiency, and reliability in high-frequency power converters.
✅ Conclusion
The air gap is one of the most powerful tools in magnetic design.
By precisely controlling this tiny space, engineers can regulate inductance, prevent core saturation, and increase energy storage — all while maintaining stability across load and temperature variations.
In modern SMPS applications, where power density and reliability are paramount, air-gap optimization defines magnetic performance.
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