Applications by Frequency

Radio Frequency <10MHz

Radio Frequency (RF) Iron PowdersPower Conversion (PC) Iron PowdersLine Filter (LF) Iron Powders

Distributed gap powder cores have become a preferred solution by RF engineers who are designing inductors or transformers to handle power in the 250 kHz to 5 MHz frequency range.  The distributed air-gap of powder cores contributes to their rather low permeability and very good stability. In applications involving low-level signals, the choice of core size, material and winding is normally based on required Q and/or packaging requirements.

Iron powder cores are commonly chosen to produce high Q inductors and transformers for selective circuits.

Low level broadband transformers and RF chokes are commonly built on high permeability ferrite cores. Ferrites are a metallic ceramic ferromagnetic compound with a spinel crystalline structure. Ferrite cores have higher permeability than iron or alloy powder cores but are less stable.  A common misconception is that core saturation is the primary limiting factor in selecting a core for RF power applications. While it needs to be determined how much voltage drop or current flow a given inductor or transformer can support before a limit is reached, this limit will be either magnetic saturation or excessive temperature rise resulting from both winding (copper) and core material losses. Magnetic saturation is the point at which an increase in magnetization force does not result in any further increase in flux density. This implies that there will be a loss of permeability. An inductor will show a decrease in inductance and a transformer will show both a decrease in impedance and will not transform the additional signal. Carbonyl iron powders typically reach their maximum permeability at about 3000 gauss and then begin to saturate. They reach full saturation at approximately 10,000 gauss.

With continuous sinewave signals, iron powder cores are limited by temperature rise resulting from losses rather than magnetic saturation. Temperature rise results from losses in both the winding and the core material. At high frequency the current carried by a conductor tends to be concentrated near the surface. This is known as  “skin effect”. The skin depth of the AC current in a copper conductor at room temperature is described by: Skin Depth (cm) = 6.62 / f.5 where f is frequency in hertz.  For example, at 1 MHz, a wire larger than #35 AWG will not be fully utilized and will thus show an increased AC resistance. Due to this, the use of a large number of strands of fine wire that are insulated from each other and interwoven can be useful in reducing the AC resistance of conductors at high frequency. Such a conductor is known as litz wire. Practical litz conductors are very effective at frequencies below 500 kHz, but begin to lose effect above 3 MHz.

Winding losses are an important consideration in properly designing higher power magnetic components. Further information on skin effect and the AC resistance of conductors can be found in the Radio Engineer’s Handbook as well as a number of other references. 

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Radio Frequency >10MHz

Radio Frequency (RF) Iron PowdersPower Conversion (PC) Iron PowdersLine Filter (LF) Iron Powders

While at low frequency the magnetic field in a coil is in its axial direction, at high frequency, each turn generates its own field concentric with the wire. These fields are coupled with fields from adjacent turns and are coupled to the core through axial fields rather than one central field. This requires cores that produce very low Q at RF, like those provided by iron powder distributed gap cores.

Carbonyl iron powders have a particle that is formed by the decomposition of pentacarbonyl iron vapor. This produces a spherical particle with an onion skin structure. This laminating affect of the onion skin produces resistivity of individual particles much higher than that of pure iron. This high resistance in conjunction with the very small particles (3 to 5 microns) greatly enhances the high frequency performance. Carbonyl iron powders permeability, and thus its inductance can be manufactured to very tight tolerance and remaining extremely stable with frequency, temperature, and applied signal level. All of this is important in high Q selective circuits. The distributed air-gap characteristic of the carbonyl iron powder produces a core with permeabilities ranging from 4 to 35. This feature in conjunction with the inherent high saturation point of iron makes it very difficult to saturate at high power RF. Normally, high power applications are limited by temperature rise due to core loss.

In high frequency applications, high Q tuned circuits, high permeability is not nearly as important as attainable Q and good stability with varying environmental and electrical conditions.

In a simplified view, Q = tanθ = ωL/R where θ is the phase angle, ωL is the inductive reactance and R is the effective series resistance. In the case of an ideal inductor, the phase angle is 90° and the Q is infinite. Likewise, an inductor with a Q of 1 has a phase angle of 45 degrees and thus its reactive and resistive elements are equal. A Q of 150 has a phase angle of 89.6 degrees. The factors that make up the effective resistance are quite complex. They involve both the core and winding losses.

The core losses vary with material, frequency, flux density and core size. The winding losses involve wire resistance, turn to turn, and turn to core Capacitive effects which are all frequency and size dependent. There are rigorous analysis of these interrelationships available, but in general are far too complex to be of much practical use when it comes to designing a high Q, high frequency inductor. Optimum Q will occur when the combined core loss equals the total winding loss. It has been shown by Legg that in general maximum attainable Q is directly related to a cores physical size for any given material. It has also been shown that the frequency at which this maximum attainable Q occurs is, in general, inversely proportional to permeability, core size, and the square root of core loss.

In arriving at the best winding for a given coil, there are two basic effects which reduce Q to be considered: resistive and capacitive. The resistance of copper wire at very low frequency is the same as its DC resistance. The skin depth of an AC current is inversely proportional to the square root of the operating frequency. Thus the AC resistance of a conductor is proportional to f1/2. Because of this, the increased resistance due to skin effect will begin to come into play at higher frequencies for smaller wire and at relatively low frequency for large wire.

As an example #30 wire will begin to see increased resistance as low as 300kHz and #40 wire is affected around 3MHz. This resistance is further increased in the case of wound coils due to the proximity effect of adjacent turns. In order to help the AC resistance of a conductor approach its DC resistance at moderate frequency, Litz wire can be used.

Litz wire is formed by a number of strands of small insulated wire connected in parallel at the ends and completely interwoven. The interweaving is essential in order for the various strands to equally share the current. There is a significant difference between true Litz sire and stranded wire. Practical Litz wire is very effective at frequencies up to 1MHz. As frequency increases, however the benefits begin to disappear. At very high frequency the reduced resistance due to the interwoven stranding is more than offset by the capacitive build-up between the strands. Since most of the work in RF today is at frequencies above 1MHz the use of Litz wire has become rather uncommon.

In a winding, the self-capacitance that is built up is a result of the turn-to-turn capacitance of adjacent wires as well as turn to core capacitance. The turn to turn capacitance is affected by wire size, number of turns, and the spacing and positioning of the turns. In general, capacitive effects on Q become increasingly important with frequency squared (f²). For a toroidal coil, one of the most important factors in controlling capacitive build-up is to limit the winding to a single layer since self-capacitance of a toroidal coil varies with the number of layers. It is seen that the addition of even a partial second layer dramatically increases the self-capacitance.

Iron powder is a core material well suited for high Q stable inductors to be used in the 100kHz to 200MHz frequency range.  For a given material, larger cores produce higher Q at lower frequency and Q peaks at lower frequency as turns are increased - there is a frequency and winding of Q optimization. For a given core size, the optimum value of Q is inversely proportional to permeability.

From the winding standpoint, in order to help optimize Q at low frequency (<500kHz) know that resistive losses are dominant and thus the use of Litz wire is advantageous.  At frequencies above 1MHz losses due to capacitive affects begin to dominate and that multi-layering is very detrimental to Q. It can generally be considered that a full single layer will provide the best result.

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Power Conversion <1MHz

Power Conversion (PC) Iron PowdersRadio Frequency (RF) Iron PowdersLine Filter (LF) Iron Powders

Distributed gap powder cores have become the most common core material solution for inductors in switch-mode power supplies.

Iron powder is one of the most common core materials that are used to produce magnetic components in today's switching power supplies. It is one of the least expensive core materials available, comparable in cost to Sendust.

Micrometals Iron Powder and Alloy Powder cores are specially formulated powders are produced from very fine, insulated particles that are compressed under extremely high pressures to produce a solid core. This process creates a magnetic structure with a distributed air-gap. The inherent high saturation flux density of iron combined with the distributed air-gap produces a core material with high-energy storage capabilities. The compaction process used to produce distributed gap powder cores is suitable to make a wide variety of configurations. Toroidal cores, E-cores, U-cores, Slugs, and Bus-Bar cores are all available in iron or alloy powder.

Micrometals cores can be provided in non-standard height variations of existing sizes through press adjustment without the need for separate tooling. Custom tooling can also be created to produce highly customized shapes with a typical cost of $1500 per inch for the major dimension. Distributed gap powder cores can be produced to rather tight tolerances both physically and electrically. They are quite stable with temperature and tolerate the stresses of encapsulation with very little change in properties.

The amount of energy an inductor stores (in microjoules) is calculated by multiplying one-half the inductance in microhenries times the current in amperes squared (1/2 LI^2 ). This energy is proportional to the operating flux density squared divided by the effective permeability under those conditions (B^2 /ueff). In the case of high permeability core materials, such as Ferrites, by introducing an air gap, a significantly lower permeability is realized. This increases the energy storage capabilities of the core by allowing additional energy to be stored in the air gap.

Energy storage inductor designs will be limited by either magnetic saturation or excessive temperature rise resulting from both winding and core losses. In the case of iron powder, due to the fairly low permeability, moderate core loss properties, and very gradual saturation characteristics; designs are almost always limited by temperature rise rather than magnetic saturation.

When selecting the required wire size to handle a given amount of current, "rule of thumb" formulations based on circular mills per amp are generally inadequate. The wound unit temperature rise resulting from copper losses are directly related to core size, wire size, and nature of the winding.

While there do exist many DC output choke applications which do not have enough AC content to generate any appreciable core loss, most of the higher voltage, higher frequency DC chokes and power factor correction inductors do need to take core loss into account. Additionally, designs for 60 Hz differential-mode inductors and AC resonant inductors are quite significantly affected by core loss.

Core losses are a result of an alternating magnetic field in a core material. The loss generated for a given material is a function of operating frequency and total flux swing (∆B). The core losses are due to Hysteresis, eddy current and residual losses in the core material. Iron powder has higher core loss than some other more expensive core materials, and it will sometimes become a limiting factor in applications with relatively high ripple current at very high frequency. Consequently, it is important to have a good understanding of the proper evaluation of core loss.

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Power Conversion >1MHz

Power Conversion (PC) Iron Powder CoresRadio Frequency (RF) Iron PowdersLine Filter Iron Powder Cores

Inductors in power factor correction boost topologies do not have the simple steady-state waveforms.  Rather, the high frequency signal is such that both the peak voltage across the inductor (E) and the "on" time (t) are constantly changing though out the period of the fundamental line frequency (50 or 60 Hz). The core loss in this case will be the time-averaged core loss of the individual pulses for the period of the line frequency. The flux density generated is dependent on the volt-seconds per pulse, while the core loss is dependent on approximately the square of the flux density. In order to estimate the high frequency core loss in this type of application, it is recommended that the RMS value of voltseconds during the period of the line frequency be approximated. This will provide the value of peak AC flux density to be used with the core loss curves. The frequency is the repetition rate of the high frequency signal. In addition to the high frequency core loss in a power factor correction inductor, the fundamental line frequency will also generate core loss. This loss should also be included when determining the total loss. Since a cores ability to dissipate heat (surface area) varies squared with its size, but a cores generation of heat due to its magnetic losses varies cubed (volume) with its size, physically small cores can dissipate more power per unit volume than physically large cores.

When both elevated AC and DC are present the increasing level of DC bias causes the core materials permeability to decrease, that as the level of AC increases it causes the permeability to climb. This property means that output chokes with elevated AC levels will require fewer turns than would be predicted by only considering the DC effects.

Power factor correction inductors contain both a bias current and a lower level high frequency signal. These inductors in typical boost topologies see both a continually changing bias current (50 or 60 Hz) as well as a continually changing high frequency ripple condition. The combination of these two effects makes the evaluation of these inductors more complicated than typical DC chokes. It is generally recommended that the bias current be treated as DC current. This will provide the most conservative design.

Another use for energy storage inductors is in AC resonant applications. This type of inductor is being driven by all high frequency AC current. In order to keep the core loss to an acceptable level, it is necessary to minimize the operating flux density. This is accomplished by utilizing lower permeability materials that will require more turns so that the same amount of voltage drop (same current flowing) will result in a lower operating AC flux density. One method of lowering effective permeability and thus lowering the operating flux density is to introduce a localized air gap. At frequencies above 100 kHz, the additional "gap loss" generated by the high frequency fringing can cause severe localized heating problems. In many instances, the "gap loss" alone can be greater than the calculated core loss. Iron powder has been produced for many years for use in high power communication circuits operating from 500 kHz to several MHz. One of the materials which is gaining popularity in resonant power supply applications is the -2 Material. This material has a permeability of 10 which helps keep the operating flux density low without creating localized gap-loss problems. At these high frequencies, the use on litz wire is essential in minimizing the AC winding losses.

While the -2 Material is the material of choice for resonant applications above 20 kHz, the -30 Material should be considered for lower frequency AC inductors in very high power UPS applications operating in the 1 kHz to 5 kHz frequency range. This material provides a good balance of permeability, core loss, saturation characteristics, and core cost.

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Line Filters

​Line Filter (LF) Iron PowdersRadio Frequency (RF) Iron PowdersPower Conversion (PC) Iron Powders

The addition of both U.S. and International regulations has increased the need to effectively filter the main power line. In order to accomplish this, both the common-mode and differential-mode (normal-mode) noise must be controlled.

Common-mode noise is interference that is common to both the positive and neutral lines in relation to earth ground and is usually a result of capacitive coupling. Differential-mode noise is the interference that is present between the positive and neutral lines and is typically generated by switching devices such as transistors, SCRs and triacs. This type of noise is more readily filtered when the choke is in close proximity to the noise source.

Common-mode filtering requires capacitors to earth ground. Safety regulations limit these capacitors to a relatively low value. This mandated low value of capacitance for common-mode filtering makes a high value of inductance essential for effective filtering. Common-mode inductors typically require a minimum inductance of 1000 mH and are most often wound in a balun configuration on a 5000 or higher permeability ferrite core. The balun winding allows the 60 Hz flux density generated by each line to cancel in the core, thus avoiding saturation. Lower permeability materials like iron powder are useful for common-mode applications involving significant line imbalance. Otherwise, for most common-mode applications, the increased core size necessary to accommodate the number of turns needed to achieve the required inductance makes this alternative less attractive.

Differential-mode chokes usually have a single winding, though it is possible to put more than one differentialmode choke on a core by connecting the windings in the additive configuration rather than in the balun configuration. This type of choke must be able to support significant 60 Hz flux density without saturating and at the same time respond to the high frequency noise. The distributed air-gap of iron powder in addition to its high saturation flux density of greater than 12,000 gauss (1.2 T) make it well-suited for this requirement.

Iron powder experiences magnetostriction. This means that as the material is magnetized it experiences a very slight change in dimensions. In applications above audible frequencies (>20 kHz) this is of no concern. In certain 60 Hz applications, however, core buzzing can be noticeable. This condition will be more noticeable with E Cores than with toroids. It will also be more significant with signals which have been chopped (light dimmers, motor controllers) than with normal sinewaves. It is also dependent on operating AC flux density. Energy storage inductor design is limited by temperature rise resulting from the combined copper and core loss, and core saturation. While the -8, -18 and -52 Materials have lower core losses at 60 Hz.

Further, the higher core loss characteristics of the -26 and -40 Materials at frequencies above 25 KHz will produce a coil with low Q at high frequency. This characteristic is an additional benefit in helping to suppress the unwanted signals. 

The -26 and -40 Materials maintain good permeability versus AC flux density characteristics. The significant increase in percent permeability for these materials can be a considerable advantage. It appears that this increase in permeability is experienced in applications such as light dimmers.

For applications where it is unclear if the high frequency signal will experience the same increase in permeability as the 60 Hz signal, it is recommended that the 60 Hz signal be treated as DC current. This will produce a significantly different result but will be the most conservative approach.

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