A core datasheet contains six numbers that decide whether a design works: AL value, saturation flux density, permeability and its tolerance, core loss at a stated frequency and flux, Curie temperature, and the effective core parameters (Ae, le, Ve). Reading them in the wrong context is the most common source of a design that measures correctly on the bench and fails in the field.
1. AL value is not inductance
AL is the inductance per turn squared, expressed in nH/N². Inductance is L = AL × N². Two consequences that catch people out:
- AL is specified for a specific core and material combination, and for gapped cores it is set by the gap, not by the material.
- AL carries a tolerance, typically ±25% for ungapped ferrite and ±3–10% for a precisely gapped core. A ±25% core with 40 turns gives a ±25% inductance, which may exceed the converter’s ripple budget.
If the design needs tight inductance, specify the gapped AL tolerance — and expect to pay for it, because the gap has to be ground and measured per piece.
2. Saturation flux density depends on temperature
Bsat for Mn-Zn ferrite falls as temperature rises: a material rated at 510 mT at 25 °C may reach only 380–400 mT at 100 °C. Design the flux swing against the Bsat at the hot-spot temperature, not at room temperature. Powder cores saturate far more softly — their permeability rolls off gradually — which is why they tolerate overload without the abrupt current spike of a gapped ferrite.
| Material | Bsat (25 °C) | Bsat (100 °C) | Behaviour at saturation |
|---|---|---|---|
| Mn-Zn ferrite (power grade) | ~510 mT | ~380–400 mT | Sharp |
| Ni-Zn ferrite | ~400–480 mT | ~350–420 mT | Sharp |
| Iron powder | ~1.2–1.4 T | Similar | Soft |
| Sendust | ~1.0–1.2 T | Similar | Soft |
| High flux | ~1.5 T | Similar | Soft |
| Nanocrystalline | ~1.2 T | ~1.1 T | Sharp |
3. Permeability, and the difference between µi and µe
Initial permeability µi is a material property measured at low flux. Effective permeability µe is what the wound core presents, and for a gapped core it is much lower than µi because the gap dominates the magnetic path. Datasheets list both, and using µi in a gapped-core calculation underestimates the turns required.
4. Core loss needs three coordinates, not one
A core loss figure is meaningless without frequency, flux density and temperature. “100 kW/m³ at 100 kHz” tells you nothing unless you know it is at 100 mT and 100 °C. Always read the loss curve at the actual operating point, and remember that the Steinmetz parameters change with temperature — a material optimised for 100 °C can be notably worse at 25 °C.
5. Curie temperature and the derating margin
Above the Curie point the core loses its magnetic properties entirely. For ferrite the figure is typically 100–250 °C depending on grade. A design running a 120 °C hot spot on a 130 °C Curie material has no margin for a fan failure. Keep at least 30 K below the Curie point at worst-case ambient.
6. Effective parameters and the winding window
Ae and le let you convert between flux density, inductance and turns; Ve gives the loss volume. The window area Aw determines how much copper fits, and the product Ae × Aw is the area-product used to size a core from a power requirement. A datasheet that omits Aw cannot be used to check whether the winding fits — ask for the full dimensional drawing.
7. A five-minute datasheet audit
- Is AL specified with a tolerance, and at what frequency?
- Is Bsat given at the operating temperature, or only at 25 °C?
- Is the loss curve stated at your frequency, flux and temperature?
- Are Ae, le, Aw and the bobbin dimensions available?
- Is the material grade named explicitly so it can be locked in a purchase specification?
Shaanxi SiRui Technology publishes grade-specific datasheets for the Magnetic Core series — Mn-Zn and Ni-Zn ferrite, iron powder, sendust, high flux, MPP, nanocrystalline and amorphous — including bias curves and loss tables at the operating points designers actually use.
Ask for a datasheet set at sales@sirui-tech.com.