Epoxy-Coated Nanocrystalline Gapped Toroidal Core: Parameters and Applications
Sep 22, 2025
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What Is a Nanocrystalline Gapped Toroidal Core?
A nanocrystalline gapped toroidal core is a wound toroid of nanocrystalline alloy ribbon, cut to introduce a controlled air gap and coated with epoxy. The gap stabilizes permeability under DC bias, making the core suitable for large-current output inductors, PFC chokes, differential-mode chokes and hall-effect sensor concentrators. Nanocrystalline material offers high saturation flux density, low core loss and a low-cost alternative to high-flux powder cores in many applications.
Epoxy Coating and Core Cutting
After winding, the toroid is annealed, impregnated or coated with epoxy for mechanical stability and insulation, then cut to create the required gap. The cut faces are ground and re-coated so that the finished core keeps its dimensions and electrical parameters. The gap length controls the effective permeability and the energy storage capability of the inductor.
Typical Parameters
| Parameter | Value |
|---|---|
| Density | 7.3 g/cm3 |
| Curie temperature | 410 C |
| Crystallization temperature | 530 C |
| Saturation flux density | 1.56 T |
| Electrical resistivity | about 130 micro-ohm-cm |
| Saturation magnetostriction | about 2.7x10-6 |
| Effective permeability | 180-200 |
Core Dimensions
| Core ID (mm) | Core OD (mm) | Core H (mm) | Finished ID (mm) | Finished OD (mm) | Finished H (mm) | AL (uH) | AT |
|---|---|---|---|---|---|---|---|
| 12 | 20 | 5.0 | 9.7 | 22.5 | 8.0 | 0.085 | 180 |
| 20 | 32 | 10.0 | 17.5 | 34.5 | 13.5 | 0.120 | 350 |
| 32 | 50 | 15.0 | 29.0 | 55.0 | 18.0 | 0.126 | 470 |
| 40 | 64 | 15.0 | 37.8 | 67.5 | 19.0 | 0.160 | 600 |
| 56 | 92 | 25.0 | 53.0 | 97.0 | 29.0 | 0.250 | 800 |
The table lists representative standard sizes; intermediate and custom dimensions can be produced to the application requirement.
Applications
PFC choke coils for power supplies.
Output choke coils for switched-mode power supplies.
DC/DC converter chokes.
Normal-mode and differential-mode chokes.
Chokes for adapters and battery chargers.
FAQ
Why is a gap needed in the core?
The air gap increases the reluctance of the magnetic path, preventing saturation when a DC current flows and stabilizing the inductance over the operating range.
What does AL mean?
AL is the inductance factor in microhenries per turn squared; inductance is calculated as AL times N squared, where N is the number of turns.
Can the core be re-cut or adjusted?
Adjusting the gap changes the effective permeability; cores are normally supplied cut to the specified gap, and re-grinding changes parameters and should be done by the manufacturer.
What is the temperature behaviour?
Nanocrystalline material has a high Curie temperature of about 410 C and stable permeability over a wide temperature range, which is important for power inductors.
How does it compare with high-flux powder cores?
Nanocrystalline gapped cores offer higher saturation flux density and lower core loss at a lower material cost, with the trade-off that the gap is discrete rather than distributed.