At present time, the appearance of magnetic materials with improved characteristics leads to a creation of new devices and an upgrade of earlier developed electronics operating in different conditions when being exposed by strong magnetic fields and mechanical loads.
[1]. Among famous and well-studied soft magnetic alloys used in cores of transformers and chokes along with Ni Zn and MnZn ferrites it is powder materials (Powdered Iron , alloys based on Fe and Ni, permalloys) that have required characteristics.
Electromagnetic devices based on powder materials are applied in switched-mode power supplies, network filters and also in devices where a high saturation induction is required. For example, when designing cores for components used in power equipment and PFC modules , in smoothing or limiting chokes, permalloys are preferred. These materials are characterized by high permeability and resistivity. Permalloys magnetic saturation is determined by a percentage content of Ni in the alloy and also by a additives quantity (Cr, Mo, Si, Cu and others). The doping allows to obtain alloys with increased magnetic permeability and electrical resistivity. Moreover, a high saturation induction of permalloys promotes the passing through the cross-sectional area of the largest magnetic flux, which allows to reduce the size of the magnetic circuit.
A wide range of powder cores of both domestic and foreign production is represented on the Russian market. Among the possible configurations the rings are the most common. Such a constructional design of the magnetic core allows to achieve the best application of the material due to the provision of high magnetic permeability, the greatest resistance to external influences of electromagnetic fields and minimal external field dispersion. Among ring cores made of permalloy, products of Magnetics are of particular interest. The company developed the series with improved properties. The company has developed a series of powder materials with improved characteristics, among which Kool Mμ®, HighFlux, MPP, XFlux® are widespread.
According to the data[2] Kool Mμ® toroids are produced on the base of Fe (85%) with Al (6%) and Si (9%) alloy, which is characterized by low loss at high operating frequencies. Thanks to low cost Kool Mμ® toroidsare applied in power supplies circuits, PFC systems, invertors and others. Magnetostriction in cores based on Kool Mμ® is practically not fixed, which allowsto use this material in sound noise filters.
![Figure 1 – Thermal stability of powder materials [2]:
Powdered Iron, МРР](uploads/images/products/magnetics/powderediron_en.png)
Another material produced by Magnetics, - MPP. This alloy is based on Fe (17%), Ni (79%) and Mo (4%) and is characterized by improved thermal stability and small variations of permeability (figure 1). In cores made of MPP the aging effect
does not exhibit itself even with prolonged exposure to high temperatures. Unlike a core made of Powdered Iron, these cores has no organic binders. The air gap presence, in turn, makes possible to achieve extremely low loss and high saturation induction. Molybdenum-doped permalloys are often used in resonant circuits, high-power chokes, audio frequency filters, power.
A more economical material in comparison with MPP is HighFlux alloy consisting of iron and nickel in the ratio 50:50. Despite the high core loss, HighFlux has a sufficiently high saturation induction and temperature stability . Cores based on such alloy, as a rule, are made in the form of rings and are often used in flyback and pulse transformers. [3].
Along with Kool Mμ®, HighFlux and MPP the material XFlux® with distributed gap is of interest. As well as other Magnetics powder materials, this alloy (Fe Si B) is characterized by rather low core loss and is a general purpose type material for using in industrial power sources, computer equipment and other equipment where power factor correction is required and output DC (DC) chokes. Table 1 shows the characteristics of some powder materials.[2]. According to characteristics represented in the table 1 one can see , that maximum of saturation induction (1,6 Т) corresponds XFlux®.
| Table 1 - Characteristics of Magnetics permalloys | ||||||
| Material | Kool Мμ® | Amoflux® | MPP | XFlux® | HighFlux | Powdered Iron |
| Alloy's composition | Fe Si Al | Fe SI B | Fe Ni Mo | FeSi | FeNi | Fe |
| Core loss | Low | Low | Very low | Low | High | The highest |
| Initial permeability | 26…125 | 60 | 14…550 | 26…60 | 14...160 | 10…100 |
| Temperature Curie, °С | 500 | 400 | 460 | 700 | 500 | 770 |
| Operating temperatures, °С | -55...200 | -55...155 | -55...200 | -55...200 | -55...200 | -30...75 |
| Saturation induction, Т | 1 | 1.5 | 0.75 | 1.6 | 1.5 | 1.2-1.5 |
| Permeability vs DC Bias | Medium | Good | Good | The best | The best | Good |
| Relative cost | Low | Middle | High | Low | Low | The lowest |
It should be noted that considered Magnetics powder materials are characterized by rather low core loss, which at a frequency of 100 kHz and a flux density of 100 mT for XFlux® alloy is about 1100 mW /cm³, for Kool Мμ® - about 800 mW/cm³ , and in case of MPP- about 590 mW /cm³. According to the curves (Figure 2) the Powdered Iron loss (6000 mW /cm³) are much higher than those of Kool Мμ® (850 mW /cm³) alloy. Since permalloy-based ring cores are often used in coils of switch-mode power supplies and power factor correction circuits, it is necessary to take into account the operating parameters of the circuit (output constant voltage, ripple currents, etc. .) and characteristics of chokes and transformers (inductance, rated current, number of turns, magnetic induction, etc.) in order to achieve high technical and economic indicators and reduce the risk of equipment breakdown.
The choice of a required material produced by Magnetics is simplified in general due to reference data and method for coils calculating represented in [4]. in a point of fact, diagrams (Figure 3) for almost all powder materials are represented at Magnetics web site. These data allow to allow to determine an ordering code, core dimensions using
L∙I² parameter, where L – choke's induction (mH), and I - current (А) and permeability value (μ).
) The inductance factor AL for the core is obtained from the core data sheet. The minimum AL is determined by using the worst case negative tolerance (generally -8%). With this information one can calculate the number of turns:
,the number of turns is being calculated [4]:
где L – minimal inductance, μH;
ALmin – minimal value of AL, nH/T².
The calculation of the bias is carried out [4].
:
where N – number of turns;
I – current, А;
Le – magnetic Path length, sm.
In the table 2 calculation results for a choke applied in filters (EMI) are represented. The initial data: L=700μH, I=6 A, U(AC)=25 V, f=50/60 kHz, U(DC)=350 V. Permalloys of different composition as choke's core were used.
| Table 2 - Comparison of the characteristics of permalloy Magnetics for a selection of the power choke's core material (L=700 μH, I=6A) | ||||
| Materials characteristics | Kool Мμ® | MPP | XFlux® | HighFlux |
| AL nH/ |
81±8% | 56±8% | 81±8% | 168±8%i |
| Permeability | 60 | 60 | 60 | 125 |
| Core loss at 100kHz и 100mT | 1000 | 950 | 950 | 1275 |
| Core dimensions А (Outside diameter), mm х B(Inside diameter), mm х С (Height), mm | 40,7х23,3х14,5 | 36,7х21,5x11.4 | 40,7х23,3х15,4 | 40,7х23,3х14,5 |
| Number of turns | 167 | 148 | 128 | 88 |
| Magnetic field strength , А∙Т/sm | 102 | 99 | 78 | 54 |
| Weight, g | 61 | 43 | 87 | 87 |
| Ordering code | 0077083A7 | 0078076A7 | C055083A2 | C058254A2 |
Analyzing the data in the table, it becomes obvious that, using XFlux® alloy, it is possible to achieve a significant reduction in core loss in comparison with other materials, as well as to reduce the size and weight of the final product (from 87 grams to 43).
The reduction of cost and coils geometrical dimensions is also an important task when power factor correction modules (PFC) developing. The increased interest to such devices is primarily caused by the fact,that practically all electronic devices create high frequency electromagnetic interference and other dynamic changes in the network when consuming energy. The application of chokes in the PFC circuits allows to reduce the reactive component of the current flowing through the power supply networks, as well as inductive components of the load. The cores of such coils are mainly made of powder ring-shaped cores and are characterized by a rather low magnetic permeability.
Today, it is actual to use new material Amoflux® (Fe Si B)manufactured by Magnetics (table 1) along with permalloys and Powdered Iron in industrial power sources, computer equipment and other equipment where power factor correction and DC output chokes installation are required. Due to the presence of a distributed gap, it is possible to reduce significantly core loss and achieve high values of saturation induction (1.5 T). In addition, this alloy boasts a stability of such parameters as saturation induction and core loss at temperatures up to 155 °C. Cores made of Amoflux® material are produced in the form of rings with a magnetic permeability 60. The dimensions and characteristics of products made of Amoflux alloy are shown in Table 3.
| Table 3 - Dimesnions and magnetic characteristics of cores, made of Amoflux® (Magnetics) [2]. | |||||||||
| Dimensions after coating | Ordering code | Magnetic permeability | AL±8% (nH/T² | Weight, g | Effective parameters | ||||
| Outside diameter, mm | Inside diameter, mm | Height,mm | Ve (Effective volume), mm³ | Ae (Effective cross-section), mm² |
Le (Magnetic path length), mm |
||||
| 24,4 | 13,7 | 9,66 | 0088351A7 | 60 | 51 | 14 | 2,280 | 38,8 | 58,8 |
| 27,69 | 14,1 | 12,0 | 0088894A7 | 60 | 75 | 26 | 4,150 | 65,4 | 63,5 |
| 33,66 | 19,4 | 11,5 | 0088071A7 | 60 | 61 | 33 | 5,340 | 65,6 | 81,4 |
| 40,77 | 23,3 | 15,4 | 0088083A7 | 60 | 81 | 65 | 10,600 | 107 | 98,4 |
| 47,63 | 23,3 | 19,0 | 0088439A7 | 60 | 135 | 131 | 21,300 | 199 | 107 |
| 58,04 | 25,57 | 16,2 | 0088192A7 | 60 | 138 | 173 | 28,600 | 229 | 125 |
In such a case, main parameters of cores made of Kool Mµ®, XFlux®, MPP, HighFlux и Amoflux® were determined using the calculation method and formulas 1 and 2 mentioned above. The typical choke used in PFC circuits with initial data L=20 000 µH, I=1 A, U(AC)=250 V, f=50/60 kHz, U(DC)=350 V (Table 4) served as an example.
| Table 4 – The comparison of the Magnetics permalloys characteristics for choosing of the choke's material (L=20 000 µH, I=1 A) | ||||||
| Material | Kool Мμ® | XFlux® | MPP | HighFlux | Amoflux® | |
| AL µH/T² | 81±8% | 56±8% | 81±8% | 117±8%i | 61±8%i | |
| Permeability | 60 | 60 | 60 | 125 | 60 | |
| Core loss at 100 kHz и 100 mT | 1000 | 950 | 950 | 1275 | 1000 | |
| Core dimensions А (Outside diameter), mm х B(inside diameter), mm х С (Height), mm | 40,7х23,3х14,5 | 36,7х21,5x11.4 | 40,7х23,3х15,4 | 36,7х21,5х11,4 | 33,7х19,4х11,5 | |
| Number of turns | 797 | 751 | 656 | 634 | 807 | |
| Magnetic field strength , А·Т/sm | 81 | 84 | 67 | 71 | 99 | |
| Weight, g | 61 | 43 | 87 | 49 | 33 | |
| Ordering code | 0077083A7 | 0078076A7 | C055083A2 | C058324A2 | 0088071A7 | |
When comparing data represented in the table 3 one can note, that when choosing the core's material for the choke with L=20 µH and I=1 A the largest practical interest is in Amoflux® alloy. The application of a new alloy allows to achieve low loss when reducing the inner and outer diameters of the core from 41 to 34 mm and from 23 to 19 mm, respectively, and reducing its weight from 87 to 33 grams. Along with Amoflux®, more economical material XFlux® can be used in coils of PFC circuits. XFlux® is also characterized by low core loss, rather high saturation induction, but has a slightly higher magnetic core weight (49 g).
Thus, this article hows the magnetic properties of permalloys produced by Magnetics, their field of application. A significant influence of the distributed gap presence in Magnetics alloys on increasing of thermal stability and decreasing of the aging effect was noted. Using the example of calculations carried out for cores made of different alloys and used in chokes of EMI filters and PFC modules , it was shown that the application of Amoflux® and XFlux® materials with magnetic permeability ( µ = 60) can significantly reduce the size, weight and cost of a ring-shaped core while maintaining high values of saturation induction and low core loss.
Literature
1. Compact magnetic cores/ I. N. Sidorov, А. А. Khristinin, S. V. Skornyakov . – М.: Radio and signal communication, 1989. – P. 384.
2. DataSheets from http://www.mag-inc.com/
3. A.V. Kunevich, А.А. Maximov. Modern soft magnetic materials for power electronics /"Power electronics". 2006 - №5. – P. 34-35
4. Inductor Design with Magnetics Powder Cores. Design guides. http://www.mag-inc.com/
