Amorphous C Shape Magnetic Core for High Frequency Electronics

Why Amorphous Alloy Suits High-Frequency Electronics

An amorphous alloy is cast directly from the melt into a ribbon a few tens of micrometres thick, so its atoms never arrange themselves into a crystal lattice. With no grains, there are no grain boundaries to pin the domain walls and no crystallographic anisotropy to force the magnetisation into a hard direction. The practical consequences for an electronics design are a very low coercivity, a low core loss at medium frequency and a saturation flux density near 1.56 T, which is far higher than the 0.4 T to 0.5 T available from power ferrite. The material is also brittle after annealing, so it is always handled as a wound, impregnated and clamped core rather than as a loose stack.

C Shape Geometry in Converter Hardware

The C shape is produced by winding a rectangular core and cutting it into two halves, which are then re-ground and re-clamped. This gives two properties that matter in switch-mode hardware: the coil can be wound and tested on a bobbin before the core is closed, and an air gap can be introduced in a controlled way to store energy for a filter inductor or a power factor correction choke. Because the gap is placed in a defined location rather than being distributed through the magnetic material, the gapping calculation is straightforward, and the effective permeability is set by the summed gap length plus the effective path length of the core.

Loss Behaviour from 20 kHz to 100 kHz

Over the 20 kHz to 100 kHz band the loss of a thin amorphous ribbon stays well below that of grain-oriented steel of the same mass and flux density, because the eddy current loss, which rises with the square of frequency and with the square of lamination thickness, is suppressed by the very thin ribbon. At the high end of the band the limiting factors become the winding loss and the interlayer insulation rather than the magnetic material. Above roughly 100 kHz, ferrite or powder cores usually take over, because the ferrite resistivity is many orders of magnitude higher and eddy currents are suppressed further still; the correct choice is therefore a system-level comparison of total loss, occupied volume and thermal headroom, not a comparison of core loss alone.

The magnetic properties of soft magnetic metallic materials, including measurement conditions for loss and excitation, are specified in IEC 60404-8-6 and, for semi-processed amorphous core alloys, in ASTM A900/A900M. Specific loss data should always be compared at the same flux density, frequency and waveform; a figure quoted at a sinusoidal 50 Hz waveform is not transferable to a pulse-width-modulated excitation without a correction.

Applications in Electronics

Three duties dominate. Filter inductors in solar inverter output stages need high saturation flux density and low loss under a large dc bias component. Power factor correction chokes need a gapped core with a stable inductance up to the peak current and a low loss at the switching frequency. Medium-frequency transformers in charging equipment and in isolated converters need a core that carries flux at tens of kilohertz without excessive temperature rise. In all three, the C core allows the winding to be optimised separately, which matters when the conductor is foil or litz wire and when creepage and clearance dimensions are fixed by safety standards.

Assembly, Clamping and Thermal Practice

An amorphous core is supplied annealed, and the anneal is what develops the low coercivity; bending the core after annealing raises the loss permanently in the affected region, so handling rules should be written into the work instruction. The mating faces stay clean and unbent, clamping pressure is applied through a controlled torque, and the core is impregnated or varnished to prevent interlayer movement and abrasion noise. Because the ribbon is thin, the fill factor of the wound core is lower than for stacked steel, and the window area must be sized for that. Thermal design should verify the hot spot of the winding against the insulation class defined in IEC 60085, and the core temperature against the point at which the annealed properties begin to degrade.

Frequently Asked Questions

Q: Can an amorphous C core replace a ferrite core in the same footprint?
A: Rarely on a one-to-one basis, because the saturation flux density is higher but the stacking factor and the geometry differ; the comparison should be made on total loss, temperature rise and window area for the same inductance.

Q: Why do amorphous cores need impregnation or varnish?
A: To lock the ribbon layers together, to prevent mechanical movement that produces audible noise and fretting, and to protect the cut faces from corrosion during service.

Q: What gap should be used in an amorphous C core?
A: The gap is set by the required inductance factor, and the resulting effective permeability follows from the summed gap length and the effective path length; it must be measured at the specified clamping condition.

Q: How does core loss of amorphous ribbon compare with grain-oriented silicon steel?
A: At 50 Hz and 1.3 T, thin amorphous ribbon is typically below 0.3 W/kg, against about 0.85 W/kg to 1.15 W/kg for common grain-oriented grades at 1.7 T, but the two figures must be compared at the same flux density to be meaningful.

Q: Up to what frequency is an amorphous C core practical?
A: Thin ribbon cores are used from line frequency to about 100 kHz; above that range ferrite or powder cores normally give lower total loss.

Q: Does cutting the core into two halves damage the magnetic properties?
A: It introduces two mating interfaces that add reluctance and a small local loss, which is why the faces are ground flat and clamped under a controlled force and why the AL value is verified after assembly.