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IEC 62358: Standard Inductance Factor (AL) for Gapped Ferrite Cores
Every power electronics engineer knows that the turns count of a transformer or inductor is determined by the core’s ability to store magnetic energy. But how do you guarantee that a core from Manufacturer A will produce the same inductance as an ostensibly identical core from Manufacturer B? The answer lies in the inductance factor, denoted as AL—a single parameter that encapsulates the core geometry and material permeability into a simple figure of merit.
IEC 62358, formally titled “Ferrite cores – Standard inductance factor (AL) for gapped ferrite cores,” defines the standard AL values and their tolerances for the most common gapped ferrite core shapes. By standardizing AL, this IEC standard enables true interchangeability of magnetic components across the global supply chain.
📋 Understanding the Inductance Factor AL
The inductance factor AL relates the inductance L of a wound core to the number of turns N by the simple formula:
L = AL × N² × 10⁻⁹ (where L is in henries, AL in nH/N²)
This relationship allows a designer to quickly determine the number of turns required for a target inductance: N = √(L / AL × 10⁹). The AL value depends on two factors: the effective permeability of the core (influenced by the air gap) and the effective cross-sectional area and magnetic path length.
| Core Shape | Common Sizes | Typical AL Range (nH/N²) | Standard Gaps (mm) |
|---|---|---|---|
| E Cores | E20, E25, E32, E42, E55 | 250 – 4000 | 0.1, 0.2, 0.5, 1.0 |
| ETD Cores | ETD29, ETD34, ETD39, ETD49, ETD59 | 200 – 3200 | 0.1, 0.2, 0.5, 1.0, 2.0 |
| RM Cores | RM5, RM6, RM8, RM10, RM12, RM14 | 100 – 2500 | 0.05, 0.1, 0.2, 0.5 |
| PQ Cores | PQ20, PQ26, PQ32, PQ35, PQ40, PQ50 | 150 – 3000 | 0.1, 0.2, 0.5, 1.0 |
| Pot Cores (P) | P9×5, P11×7, P14×8, P18×11, P22×13, P26×16 | 63 – 1600 | 0.05, 0.1, 0.2, 0.5 |
| U Cores | U10, U20, U30, U60, U93, U100 | 500 – 10000 | 0.1, 0.2, 0.5, 1.0, 2.0 |
💡 Engineering Shortcut: To estimate the gap length needed for a desired AL, use the approximation g ≈ (μ₀ × Ae) / (AL) — where g is the gap length in meters, μ₀ = 4π × 10⁻⁷, and Ae is the effective cross-sectional area from the core datasheet. This approximation holds when the gap reluctance dominates the core reluctance, which is true for all but very small gaps (g < 0.05 mm).
🔧 Standard AL Values and Tolerance Grades
IEC 62358 defines standard AL values for each core shape and size, along with the corresponding gap length that produces that value. More importantly, the standard specifies tolerance grades that allow the designer to balance cost against precision:
| Tolerance Grade | AL Tolerance | Application | Relative Cost |
|---|---|---|---|
| Grade 1 | ±3% | Precision filters, resonant converters, tuned circuits | High |
| Grade 2 | ±5% | General-purpose SMPS transformers, coupled inductors | Medium |
| Grade 3 | ±10% | EMC filters, common-mode chokes, non-critical applications | Low |
| Grade 4 | ±15% | Power factor correction, saturable reactors | Lowest |
⚠️ Design Margin Warning: When designing a flyback transformer, always account for the AL tolerance in your worst-case air gap calculation. If your design targets AL = 2000 nH/N² with ±10% tolerance, the worst-case AL is 1800 nH/N². At the minimum AL, the primary inductance will be 10% lower, which directly affects the peak current and stored energy in a flyback topology. This could push the core into saturation at high line or full load. Always design to the minimum AL for energy storage calculations and the maximum AL for saturation current limits.
🔄 Relationship with Core Geometry Standards
IEC 62358 does not work in isolation. It is intimately connected with the dimensional standards for ferrite cores, specifically IEC 62317 (Ferrite cores – Dimensions) which defines the mechanical outlines of all standard core shapes. The effective parameters (Ae, le, Ve) defined in IEC 62317 are the inputs from which the standard AL values are calculated.
For a gapped core, the AL is calculated using the effective permeability μe of the gapped assembly:
AL = (μ₀ × μe × Ae) / le
where: 1/μe = 1/μi + g / le
where: 1/μe = 1/μi + g / le
Here μi is the initial permeability of the ferrite material (typically 2000 for power ferrites), g is the air gap length, Ae is the effective cross-sectional area, and le is the effective magnetic path length. The measurement methods for verifying AL compliance are specified in IEC 62044.
✅ Practical Application: Suppose you are designing an LLC resonant converter using an E55 core. From the datasheet, Ae = 353 mm² and le = 125 mm. You need an AL of 800 nH/N² for the desired transformer magnetizing inductance. Using the formula, the required gap is approximately g ≈ μ₀ × Ae / AL = 4π × 10⁻⁷ × 353 × 10⁻⁶ / (800 × 10⁻⁹) = 0.55 mm. IEC 62358 specifies the nearest standard gap of 0.5 mm for this core, yielding an AL of approximately 880 nH/N², which is within ±10% of your target.
📊 Measurement and Verification
IEC 62358 also provides guidance on measuring AL to verify compliance. The standard test method involves winding a specified number of turns (typically 10 or 25 turns, depending on the core size) of a specified wire gauge around the core and measuring the inductance at 10 kHz or 100 kHz at a low flux density (typically < 1 mT).
Key measurement conditions:
- Test frequency: 10 kHz for high-permeability materials (μi > 1000); 100 kHz for low-permeability materials
- Test flux density: < 1 mT to ensure operation in the Rayleigh region where permeability is essentially constant
- Winding: Single-layer, evenly distributed, minimum 25 turns or as specified in the relevant detail specification
- Clamping torque: Specified in the detail specification for each core shape (e.g., 1.0 ± 0.1 N·m for RM cores)
🚨 Common Measurement Mistake: Using too few turns for the measurement winding can produce inaccurate results because the parasitic series resistance and self-resonance of the winding dominate the impedance reading. Always use enough turns so that the inductive reactance at the test frequency is at least 10 times the DC resistance of the winding. For small cores (RM5, P9×5), 25 turns may be necessary; for large cores (E55, U100), 10 turns is usually sufficient.
❓ Frequently Asked Questions
Q1: Can I use IEC 62358 for ungapped (closed) ferrite cores?
No. IEC 62358 specifically covers gapped ferrite cores where the air gap dominates the magnetic circuit. For ungapped cores, the AL is determined by the material permeability and manufacturing tolerances of the effective geometry, not by a precision gap. Ungapped cores typically have AL tolerances of ±25% or wider, which is why precision inductor designs almost always use gapped cores from this standard.
Q3: How does temperature affect the AL value?
The AL value changes with temperature because the initial permeability μi of ferrite materials varies with temperature. For MnZn power ferrites, μi typically increases by about 15–25% from 25°C to 100°C (at the Curie point, it drops sharply). However, in a gapped core where the gap reluctance dominates (which is the intended application of this standard), the temperature dependence is significantly reduced—typically less than 5% variation over the operating temperature range.
Q3: What core shapes are NOT covered by IEC 62358?
The standard covers E, ETD, EFD, EP, ER, EQ, RM, PQ, PM, and pot (P) cores, as well as U and I cores. Toroidal cores are generally not covered because their AL is determined by material properties and distributed gaps (if any) rather than a discrete center-leg gap. For custom core shapes, manufacturers typically provide AL values on the datasheet, but these are not standardized under IEC 62358.
Q4: Can I combine cores from different manufacturers using IEC 62358?
Yes—this is precisely the purpose of the standard. If two manufacturers produce cores to the same IEC 62317 dimensional standard and specify the same IEC 62358 AL value and tolerance grade, the cores are functionally interchangeable for inductance-based applications. However, core loss (Pv) and saturation flux density (Bsat) may differ between materials, so always verify thermal performance if the application is loss-limited rather than inductance-limited.
