iron core, ferrite core, power core

Mar 06,2023

Types and comparison of iron powder cores

The inductors which application in switching power supply are made by high-frequency magnetic materials. The material of core will affect the characteristics of the inductor, ex: relationship of impedance and frequency, corresponding relationship between inductance and frequency, and the requirement of iron core saturation characteristics…etc.

Here are the comparisons for core material and core saturation characteristics as a reference for selecting power inductors:

Ferrite core: Toroid type and 3 in 1 type
Iron powder: Toroid type and EE type
MPP: Molypermalloy powder
Sendust core
High flux core

Ferrite core
General high-frequency inductors commonly used the ferrite containing Ni-Zn or Mn-Zn which have coercivity (H_C) that means the ability of magnetic materials to resist demagnetization.

Low magnetic permeability materials have good coercivity (H_C), also known as coercive force; that refers to the magnetic field strength required to reduce the magnetization to zero after the magnetic material has been magnetized to magnetic saturation.

Coercive force represents the ability of a magnetic material to resist demagnetization; low coercive force means the resist demagnetization is low, which the hysteresis loss also is low.

The ferrite materials of Mn-Zn and Ni-Zn have high relative permeability (μr), the material with high relative permeability is able to obtain energy with high inductance; but the disadvantage is unable to withstand the large current and causes magnetic saturation easily. That impacts the coil and iron core to be overheating. Magnetic saturated will impact the magnetic permeability drop dramatically and the inductance will also drop down.

If the inductor is used in high power, we will grind an air gap on the iron core which the coil can store more energy, reduce the magnetic permeability and avoid saturation caused by high current passing through.

During the manufacturing process of iron core, it can add the non-magnetic materials, like quartz powder or kaolinite, they are able to increase the internal air gap to reduce the magnetic permeability.

Generally, the magnetic permeability can be controlled at around 20-300, it can reduce the power loss and rise temperature which caused by saturation; so, it is suitable for high frequency transformers, EMI filter inductors, and energy storage inductors of power converters.

In terms of operating frequency, Ni-Zn ferrite is suitable for operating frequency which greater than 1MHz; and the Mn-Zn ferrite is suitable for low frequency which less than 2MHz; comparison of decreasing trend of magnetic permeability of ferrite and powder cores at core saturation as shown in the reference figure. The hysteresis curve of the core (B-H loop).
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B_R: Remanence
B_SAT: Saturation flux density

Iron powder
The power core is also a kind of soft magnetic ferromagnetic material. It is made by different materials of iron oxide powder and other nickel metal alloys or only irons powder; non-magnetic materials are sometimes added to the formula to make the core less saturated. The powder iron core is mainly in toroidal shape, the large current can also be replaced by large EE core to facilitate winding construction.

The common powder cores which are added with expensive and rare metal as: MPP core, Sendust core, High-flux core; and cheap Iron powder core., the ferrite of saturated magnetic flux density is around 4000Gs~5000Gs…etc, these will affect the cost of inductor.

MPP Core
A molypermalloy powder (MPP) core is a toroidal magnetic core comprised from the powder of multiple alloys. It is distributed with air gaps to help condense its magnetic field to minimize core losses. Its composition is made from approximately 79% nickel, 17% iron, and 4% molybdenum; the magnetic permeability is around 14μ~500μ, the saturated magnetic flux density is around 7500Gs

It maintains the lowest core losses and the best of temperature stabilization out of all the magnetic powdered cores used. When the applied DC current reaches the saturation current (I_SAT); when the applied DC current reaches saturation current, the inductance value will be slowly magnetically saturated according the magnitude of current, not drop sharply. MPP core has better performance, it can be used on power inductor and EMI filter of power converter.

Sendust Core
Sendust core composition is typically 85% iron, 9% silicon and 6% aluminium, the magnetic permeability is around 26μ~125μ, the saturated magnetic flux density is around 10500Gs. The iron loss is between the iron powder core, MPP core and high flux core; the saturation flux density is around 10500Gs, which is higher than MPP core.
Temperature stability and saturation current characteristics are slightly worse than MPP and high flux, but better than iron powder cores and ferrite cores.
The cost of Sendust is cheaper than MPP and high flux; it used in EMI filtering, PFC circuits and power inductors in switching power converters widely.

High flux Core
High flux core composition is typically 50–50% of nickel and iron, the iron loss and temperature stability is between MPP core and Sendust core; the magnetic permeability is around 14μ~200μ, the saturated magnetic flux density is the highest, it is around 15000Gs and it can withstand higher of DC bias current, and its DC bias characteristics are also better others.
The application as: power factor choke (PFC), energy storage inductors, filter inductors, flyback converters, and high frequency transformers…etc.

Iron powder Core

Iron powder is formed as a whole from several other iron particles, during the process, there will have non-magnetic materials are added to increase the air gap. Iron powder cores are most commonly available as toroids. Sometimes as E, EI, and rods or blocks, which used primarily in high-power and high-current parts. The magnetic permeability is around 10μ~75μ, the saturated magnetic flux density is around 1500Gs.
Powdered iron is the cheapest material. It has higher core loss than the more advanced alloys.

Comparison of Powder Core Characteristics

	MPP	High flux	Sendust	Iron powder
Core loss	Lowest	Middle	Low	High
DC bias	Better	Best	Good	Normal
Saturation magnetic flux intensity (Gs)	7,500	15,000	10,500	15,000
Relative permeability(μ)	14-550	14-200	26-125	10-75
Cost	High-tier	Mid-tier	Lower-tier	Lowest-tier
Temperature stability	Best	Better	Good	Poor

Below figure shows B-H curve that TDK Mn-Zn ferrite core that material PC47 and Micrometals iron core that material -52 and -2; the relative permeability of Mn-Zn ferrite core is higher than iron core, the saturation magnetic flux intensity is also huge difference, the ferrite core is about 5000Gs, and the iron power core is more than 10000Gs.

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Fig. B-H curves for Mn-Zn ferrite core and different materials of iron powder core

To sum up the above explanation, the saturation characteristics of each core are the same; once the saturation current is exceeded, the magnetic permeability of the ferrite core will drop sharply; iron powder core will decrease slowly. According below figure, the permeability drops characteristics of powder cores with same magnetic permeability and the ferrite core with air gap under different magnetic field intensity.

That means the inductors which assembly with ferrite core, due to the core didn’t have gap, when the core is saturated, the magnetic permeability drops sharply, that will affect the inductance to drop sharply; while the powder iron core with distributed air gap, the magnetic permeability drops when the core is saturated, it will decrease slowly, that the inductance also decreases gently, and has better characteristic of DC bias.

During design of power converters, engineers have to refer this feature; if the saturation characteristics of the inductor are poor, when the inductor current rises to magnetic saturation, the inductance will drop sharply, which will cause the reverse current of the switching crystal to rise suddenly, that will be easy to damage some parts of power supply.

The following curves are different types of iron cores with air gaps, during the magnetic is saturated, the situation of decrease slowly or drop sharply.

Reference:

TDK web site: https://www,product.tdk.com/
Micrometals web site: https://www.micrometals.com
Motocraft's Engineer Education and Training Materials
web site: https://www.motocraft.com

If you want to know more about the design application, please feel free to contact us.

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Types and comparison of iron powder cores

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Types and comparison of iron powder cores