MAGNETIC CORES -MATERIALS AND TYPES

Core is the medium for magnetic flux in inductor, transformer or coil. It decides the inductive behaviour along with coils wound around it. Magnetic field strength, flux path and intensity are a function of its structure, permeability and nature or composition. Geometry of core and air gaps in magnetic path through core affect inductor behaviour in major way.

Most basic core material is air (or vacuum), when there is no physical material present (core is hollow), or a material like ceramic or plastic is used, having permeability very close to vacuum. These materials are nonmagnetic, with permeability close to vacuum (relative permeability nearly 1.0).

In practice, a large variety of magnetic / ferromagnetic materials are used to form the core, having permeability far above that of vacuum.  These materials enable high magnetic flux and small inductor sizes for most practical uses, each with its own advantage and limitations. One has to select these judiciously during designing. Core selection is among the most important design factor.

Materials used for Magnetic Core

• Soft Iron: Basic ferromagnetic material. Soft iron core increases the strength of magnetic field, but causes heavy eddy and hysteresis losses in AC components.
• Laminated steel: Commonly used in stacks. Lamination divides the iron mass into insulated layers. Eddy currents are restricted to individual lamination layers, and are thus greatly restricted.
• Solid iron: greatly inductance compared to air, but suffers from heavy wasteful eddy current losses (causing high heat) and easy saturation. It works only for very low frequencies. It is rarely used.
• Powder cores (or distributed-gap material): Tiny alloy cores are mixed with a binder resin to keep high-frequency losses low, while restraining saturation. These cores tolerate substantial DC bias and store more energy, but may have higher core loss.
• Ferrites: Low core loss at high frequency, but relatively low saturation flux density; common in SMPS inductors and transformers.

SOFT IRON

Soft iron is used as a magnetic core in DC applications. It has very high permeability of over 50,000 Whole volume of core acts as a conductor. Changing magnetic field induces large eddy currents circulating within core (closed loops of electric current) causing significant power losses. Solid iron cores are therefore not used in AC applications. They are replaced by laminated or powdered iron cores, or nonconductive cores like ferrite. Use of solid iron core is limited to DC applications and few low voltage AC devices.

LAMINATED STEEL CORES

Laminating the core interrupts eddy current paths through it, reducing heating and loss. These cores are made by stacking thin, electrically insulated magnetic sheets under pressure. Laminated cores are mainly used for low- to medium-frequency, high-current inductors, such as line reactors, filter chokes, smoothing inductors, and power-frequency equipment. Common geometries include E-I, E-E, U-I, C constructions.

Typical electrical-steel lamination thicknesses are 0.1–0.5 mm, depending on frequency and steel alloy composition. Thinner laminations are better as frequency increases, although they cost more. Joints between laminations offer an effective reluctance. In E-I or U-I cores, Staggering the joints can reduce effective unwanted air gap.

Laminated steel materials

Core material	Typical use	Advantages	Permeability
Silicon-steel laminations	Power transformers / line freq. inductors	High saturation flux density, low cost, robust	1000 to 40,000
Grain-oriented silicon steel	AC reactors, inverters etc.	Higher μ and lower loss along rolling direction	1800 to 6000
Non-oriented electrical steel	Rotating machines and general-purpose inductors	More uniform magnetic properties	2000 to 10,000
Permalloy (80%Ni 20% iron)	Audio transformers, current sensors, magnetic shielding	High permeability and good sensitivity	50,000 to >100,000
Amorphous or nanocrystalline strip	SMPS, Transformers, high efficiency motors, inductors	Extremely Low loss, very thin strips, High resistance	10,000 to 15,000

For silicon-steel laminated cores, practical frequency range is up to about 1 kHz. Core loss depends strongly on frequency, flux swing, DC bias, waveform, material, and temperature. Core saturation and temperature rise are usually the primary constraints in power-inductor design.

A ring shape core is used for toroid winding. The core is made by winding a very thin continuous strip of ferromagnetic steel. This gives very low losses, high energy efficiency, very low Electromagnetic Interference (EMI). The shape ensures minimum flux leakage, and inductor size and weights are much smaller than laminated steel.

Advantages of laminated steel cores

Advantages

• High saturation flux density.
• Suitability for high-current high-energy inductors.
• Sturdy construction and relatively cheap.
• Easy fabrication in large physical sizes.
• Good performance at power frequency and low switching frequency.

Limitations

• Higher eddy-current loss than ferrite at high frequency.
• Air-gap fringing can cause additional loss.
• Joints and air gap tolerances affect inductance.
• Audible hum and mechanical vibration may occur under AC excitation.
• Higher core loss with increases in frequency.

For operation at tens or hundreds of kilohertz, ferrite, powdered iron, sendust, amorphous, or nanocrystalline materials are usually more common, depending on DC bias and energy-storage requirements.

Gapped core

A deliberate air gap reduces effective permeability but improves DC-bias capability and energy storage. The gap is designed for peak current, not merely RMS current. Fringing flux near a gap can increase winding losses, especially if the winding is close to the gap.

Introducing gaps in magnetic path increases reluctance, reduces effective permeability, improves energy storage, and makes inductance less sensitive to core permeability. However, a gap creates fringing flux and increases resistive losses.

FERRITE CORES

Ferrite cores are usually classified in two ways:

1. By magnetic material: MnZn and NiZn soft ferrites.
2. By physical geometry: toroidal, E, EE, EI, ETD, PQ, RM, pot, U, rod, drum, and bead cores.

Ferrites used for inductors are generally soft ferrites, which have low coercivity and low eddy-current loss compared with laminated steel at high frequency.

Ferrite core material

Material	Main characteristics	Typical applications
MnZn ferrite	Higher permeability and generally higher saturation flux density; lower resistivity than NiZn	Power inductors, SMPS transformers, filter chokes, common-mode chokes
NiZn ferrite	Higher electrical resistivity and better high-frequency behavior; usually lower permeability and saturation flux density than MnZn	EMI suppression, RF inductors, broadband transformers, high-frequency beads

MnZn is normally preferred for power magnetics at relatively lower high frequencies, while NiZn is often selected for RF and EMI applications above approximately 1 MHz. The actual usable frequency depends strongly on the manufacturer’s material grade and loss curves.

Ferrite core shape

Core type	Salient features	Typical inductor use
Toroid	Closed magnetic path, low leakage flux, good magnetic efficiency	Power inductors, EMI chokes, current transformers
E / EE / EI	Simple, economical, easy bobbin winding, often available with a center-leg gap	SMPS inductors, output chokes, transformers
ETD	Round center leg and relatively large window	Power converters and high-current inductors
EFD	Low-profile, flat construction	Compact PCB inductors and low-profile power supplies
PQ	High magnetic-core volume relative to winding area; compact	Energy-storage inductors and SMPS magnetics
RM	Good shielding and efficient PCB mounting	Filter inductors, signal transformers, compact power magnetics
Pot	Winding is substantially enclosed by the core	Shielded inductors, tuned circuits, small transformers
U / UI	Large window and good insulation distance	High-voltage inductors, output chokes, pulse applications
Rod or bar	Simple and inexpensive, but has a substantial external magnetic field	RF coils, antenna coils, adjustable inductors
Drum	Compact and suitable for surface mounting; may be shielded or unshielded	DC-DC converter power inductors
Bead / sleeve	Suppresses high-frequency noise rather than storing significant energy	EMI suppression on wires and PCB traces

A toroid has an efficient closed magnetic path and low external leakage, while a pot core offers stronger shielding because it encloses much of the winding. E, RM, PQ, and EFD types are often selected when bobbin winding, PCB mounting, cooling, and manufacturing convenience are important.

POWDERED CORES

Powdered cores are made from compressed magnetic powder particles, usually with insulation between particles, which behave like distributed air-gap core and can work well with DC bias. They are popular in power inductors, SMPS output filters, chokes and some transformers, since they have good energy storage, low loss, and inductance stability under SC and DC conditions. These offer lower loss, stability of inductance under DC and AC currents. Their distributed air gap helps avoid sharp saturation behaviour unlike ferrite cores. These cores are available in similar sizes and shaped as ferrite cores.

Commonly used materials are as follows:            

!. Iron Powder: High flux density, strong DC bias behaviour, and have permeability beteen 15 to 150. Iron powders are inexpensive.

2. MPP (Molypermalloy powder): This is Nickel-iron-molybdenum alloy. It offers very low losses and permeability range between 14 to 550.

3. Sendust/ Kool Mu/ FeSiAl: Invented in 1936 as an alternative to permalloy, composition is typically 85% iron, 9% silicon and 6% aluminium. The material offers economy with good loss and DC bias behaviour. Their permeability is very high, going up t0 140,000, with very low losses.

4. Few other alloy powders are also available with higher saturation flux density