How do you test for AL? Explain why testing at 5 gauss is important.
Inductance is measured on a bridge with variable
voltage and frequency. The flux density should be less than 10 gauss. Cores
have to be properly mounted around the measuring coil, or for toroids, wound with
the correct number of turns.
Testing at low gauss level is important. Magnetic material characteristics
change considerably at higher drive levels. Since all applications are
different, it is necessary for manufacturers to normalize characteristics at a low
level to insure a degree of consistency in magnetic properties and that "apples
are compared to apples."
Why is corner radius important on a toroid?
A radius is important on a toroid because if the core has sharp edges, the wire
insulation could be scraped during rigorous winding operations. Magnetics
takes special pains to insure that toroids have some radius. Ferrite toroid dies
are made with a built-in radius, and cores are tumbled to remove any sharp
edges. Powder cores have a radius on one side of the tool and the other side is
deburred. Additionally many cores are painted or coated to provide not only a more
blunt corner radius, but also a smoother winding surface. As with ferrites,
a coating provides additional edge coverage.
Why do you provide AL for ferrites and powder cores
and not for tape cores?
Tape wound cores
are generally used in
transformer or square loop applications in which AL
is not critical.
The desirable characteristics are high flux density, low core losses, and in some
cases high squareness in the B-H loop. In square B-H loop materials such as those
used in tape wound cores, the permeability varies widely as the loop is traversed;
a consistent and repeatable inductance measurement is not obtainable. In rounded
B-H loop materials, such as ferrites
and powder cores
, the permeability is more
constant along the loop. AL
is a measure of permeability at low drive
levels, where the permeability is relatively stable in round loop materials.
Can I get tighter dimensional tolerances on cores?
During the sintering operation, ferrite parts shrink to their final dimensions.
Different material and processing techniques result in variance in this linear shrinkage
which can range from 10 to 20% of the pressed dimensions. The resulting variation
in fired dimensions results in final tolerances in the range of 1-4%.
Some dimensions cannot be held to a tighter tolerance. But dimensions that
can be machined after firing can certainly be held to tighter tolerances. Powder
cores generally have the dimensions held to the best available tolerances, though
custom dimensions can be quoted on special specification parts.
What is the best core shape?
There is no "best shape" It depends on the application, space constraints,
temperature limitations, winding capabilities, assembly, and a number of other factors;
this means compromises must be made.
Why are AL tolerances wide for ferrites and narrow for powder cores?
Ferrites are sensitive to chemistry and kiln conditions. Fired cores have
wide inductance tolerances, but machining an air gap in the core
provides a tighter
. Process control in ferrite manufacturing results
in inductance ranges ±20% - ±30% (without 100% selection), in
the case of ungapped cores. Process control in powder core manufacturing results
in inductance ranges of ±8% to ±15%.
Can you tighten electrical tolerances on toroidal cores?
While a production batch of toroids may have a wide tolerance, the cores can be
graded into narrower inductance bands. Powder cores are all ±15% or
better. MPP and High Flux are graded into 2% bands. Due to the equipment limitations,
this is not possible on all sizes and permeabilities. Check with Magnetics for specific
information and costs.
What about availability of parylene core coatings for toroids?
is a vacuum deposited coating providing good resistance
to moisture and organic solvents. Electrical characteristics are superior to other
coatings. Because this is an expensive coating, the size range is economically limited
to an outside diameter of 14 mm or less.
Why are cores not like magnets?
Permanent magnets are considered "hard" magnetic materials because their magnetism
is permanently retained, a result that has been achieved by their manufacturing
prooess. Cores are considered "soft" magnetic materials because they
are magnetically biased only when wound with current-carrying wire. Hard magnets
are fixed at one point on the B-H (or hysteresis) curve. Soft magnetic materials
can be cyclically driven along portions of their B-H curves, making them usable
for transformers and inductors.
What are effective
Magnetic cores, particularly ferrites, oome in a variety of geometries. In
order to apply the many formulas that are used in calculations for designs, core
physical parameters are calculated to minimize geometry effects. These parameters
are the magnetic path length, effective area and effective volume.
How does Magnetics measure toroid coating insulation and make
voltage breakdown guarantees?
Core finishes on toroids are measured for voltage breakdown by inserting the core
between two weighted wire mesh pads. The force is adjusted to produce a uniform
pressure of 10 psi, simulating winding pressure. The test is conducted
using a 60 Hz r.m.s. or DC voltage. Consult the toroid product catalog for
specific finishes and their guaranteed voltage breakdowns. Users should be careful
to note that their actual windings, especially when heavy wire is used, can cause
mechanical stresses that are not present in the standard breakdown test; excessive
stresses here can result in a lower-than expected breakdown. On the other hand,
magnet wire is insulated, unlike the wire mesh test pads.
What is the best material to use?
There is no universal answer to this question, as selection depends on the application
and application frequency. Any material selected is subject to tradeoffs.
For instance, some materials may keep heat rise to a minimum and are expensive,
but if one is willing to put up with more heat, perhaps a larger component or less
costly one will do the job. The best material selection first depends on whether
you have an inductor or a transformer application. From this point, the operating
frequency and cost are important. Different materials are optimal at different
frequency ranges, operating temperatures, and
flux densities. After narrowing the core selection to particular types, it
is advisable to sample the
different ones that could fill the bill, then make a final selection. For additional
information, refer to Magnertics'
All Products Bulletin,
Magnetic Cores for Switching Power Supplies, or
Ferrite Material Selection Guide.
How do you demagnetize a core?
Drive the core under 60 HZ conditions (saturating alternately in a positive and
negative direction) then slowly reduce the drive level over several cycles until
it is reduced to zero. This action will reduce the remanence point to the
What happens to a core if you go above the Curie temperature?
Curie temperature is the temperature at which a material loses all of its magnetic
properties. Beyond the Curie temperature, the core loses all useful properties
in a circuit. Many cores have an insulated coating which would be ruined long
before the Curie temperature is reached. (Coating temperature restrictions can be
found in Magnetics' Design Manuals.)
Strip wound cores
can have their magnetic
characteristics permanently altered during exposure to the Curie temperatures.
Conventional strip wound cores and powder cores generally have such high Cuire temperatures
(>450°C) that the materials may be damaged due to oxidation well below
the Curie temperature. Manganese-zinc ferrites, on the other hand, will not
be affected, except for the insulated coating on them. This is due to the
low Curie temperatures (120°C to 300°C) of ferrites. Exceeding these
temperatures is generally not high enough to alter the ceramic material structure.
In general, the core's magnetic properties will be restored when the temperature
is reduced to below the Curie temperature, as long as the material has not been
oxidized or held at high temperature for extended period of times.
What is the maximum frequency at which you can operate a magnetic
Primarily, this depends on the type of material. Strip wound cores generally
will have a maximum usable frequency lower than ferrites, because the resistivity
is lower, resulting in high eddy currents and higher core losses. The thinner
the strip material, the higher the usable frequency. On the other hand, core
losses depend on the operational flux density of the design; thus, by reducing the
flux density, a higher operating frequency can be achieved. Often in power
magnetics, it is not the saturation flux density (Bsat) of the material that
limits the drive level, but rather the maximum tolerable losses at the specific
operating frequency. Consult the Magnetics' Design Manuals to see the relationships
among core losses, frequency, and flux densities.
What information does a B-H loop provide?
It defines the flux density of the material, coercive force, the amount of drive
level required to saturate the core, and the permeability (or the ability to change
the magnetic lines of force). The B-H loop changes with frequency and drive
level. How a material reacts to the frequency and excitation level (current
and voltage) is very important in determining its effectiveness to meet the needs
of a particular application. Consult the
Magnetics' Design Manuals
to see the relationships among core losses, frequency,
and flux densities.
What is the relative cost of different magnetic materials?
Much of the cost is related to the basic cost of raw materials. Magnetic materials
containing high percentages of nickel or cobalt have a higher cost than those containing
primarily iron. In between these two extremes are the variety of compositions
that comprise the many types of materials and geometries. Material cost impacts
large cores more significantly than small ones. Relative costs can be compared
In powder cores, iron powder ranges from x-3x
---ranges from 4x-5x • XFLUX®
6x-8x • High Flux
10x • MPP
In ferrrtes, F, L, P, R, and J materials, roughly equivalent (y)
W material -------1.25 -1.75 y
cost is a function of geometry
Toroids---------least • E Cores---------Mid • Other Shapes-----Most
Why do you put an air gap into cores?
Introducing an air gap into a core, "tilts" or "shears" the B-H loop, making it
possible to use the core at higher H levels. It is desirable, for many applications
such as inductors, to delay this saturation. Air gaps have an added advantage
in allowing for tighter control on inductance.
What is magnetostriction?
When a magnetic material
is magnetized, a small change in dimension occurs. The relative change is
in the order of several parts per million, and is called "magnetostriction".
For applications like ultrasonic generators, the mechanical motion produced by magnetic
excitation through magnetostriction is used to good advantage. In other applications,
operating in the audible frequency range, an unwanted audible hum is observed.
For this reason, low magnetostrictive materials such as Permalloy 80, Kool
Mµ and MPP powder cores may be used to limit or remove audible noise.
How do you determine proper core size?
Two elements are useful in determining core size: core window (winding) area
and core cross sectional area. The product of these two elements (area product,
) relates to the power handling capability
of a core. The larger the Wa
the higher the power able to
be handled. As operating frequency increases, the area product can be reduced,
thus reducing the core size. Magnetics publishes the area products as
a useful design tool. Consult the
Magnetics Design Manuals
to see the relationships among core losses,
frequency, and flux densities.
What are the differences between using a distributed gap (powder
core) versus a discrete gap (ferrite)?
A distributed gap material such as Kool Mµ has each alloy grain insulated
from the others. This allows for soft saturation over increasing current,
giving inherent fault protection. Discrete gap cores
hold high inductance
out to a knee in the curve resulting in sharp saturation. Distributed gap
cores hold better Bmax
and DC bias at high temperatures. Discrete gap
cores have fringing flux around the gap adding significantly to the losses.
Where can I find core standards?
The organization recognized for new and existing core standards is the International
Commission (IEC). Specifically the group TC-51 works with core specifications.
Multiply By To
Oersteds 0.79577 ampere-turns/ern
Oersteds 79.577 ampere-turns/rn
Ampere-turns/em 1 2566 oersteds
Gausses 10-4 teslas
What types of cores are used for transformers? What type for inductors?
Desirable materials for transformer cores are those that have a high flux density
and keep the temperature rise within desirable limits. High permeability materials
are desirable to limit the exciting current (air gap minimized). For lower
frequency applications (under 20kHz), strip materials have the highest flux density;
for higher frequency applications (above 20kHZ), ferrites are desirable because
the materials are designed to have low core losses (lower heat rise) at these higher
frequencies. For inductors, cores that have discrete or distributed air gaps are
desirable because they can maintain their constant permeability levels up to high
DC or AC drive levels. Ferrites and strip wound cores can be gapped
Powder cores have a built-in distributed air gap. See Magnetics Core
for selection by application.
Why does one consider single layer windings on toroid cores?
Single layer windings are less costly to wind. The distributed capacitance
is kept to a minimum. Temperature rise due to copper loss is minimized. For
common mode chokes, symmetry between the opposing windings is much easier to maintain
when only one layer is used.
What is a bifilar winding?
Two strands of wire, usually twisted together. The dual wire is then wound
on the core or bobbin to produce two equal and parallel windings which take the
plaoe of one large single strand.
Why does the inductance decrease after winding and potting?
Ferrite materials are susceptible to mechanical stress, both from winding the core
and from encapsulation. High permeability materials are particularly affected. Suggested
remedies: (1) after winding, bake, and or temperature cycle, (2) thin out epoxy
used for encapsulation or dope with an inert material such as sand or ground mica,
(3) cushion with tape.
How do I know the ferrite hardware will fit on the core?
Cores are manufactured to standards that have been agreed to in the industry
Tolerances have been assigned to the critical dimensions. Generally, hardware fit
should not be a problem. When possible hardware and cores should be purchased from
the same source.
Can you press powder cores to different heights?
Many cores can be pressed to different heights. Dies are made so that the
cavities can accommodate a range of heights. Height variation is relative
to core size. One advantage this offers is the ability to produce alternate
core sizes with out the expense of additional tooling. Consult Magnetics for
specific questions on the size of interest.
In powder cores, why is measured inductance different from calculated?
Magnetics measures Inductance in a Kelsall Permeameter Cup. Actual wound inductance
outside a Kelsall Cup is greater than the value calculated due to leakage flux and
flux developed in the winding. The difference depends on the core size, permeability,
core finish thickness, wire size, and number of turns, in addition to the way windings
are put on the core. The difference is negligible for 125µ and higher
and turns greater than 500. The following table is a guide to the differences
that one might experience:
No. of Turns Actual L No. of Turns Actual
1000 0% 100 +3.0%
+0.5% 50 +5.0%
300 +1.0% 25 +8.5%
The following formula can be used to approximate the leakage flux to add the expected
inductance. This formula was developed from historical data of cores tested at Magnetics.
Be aware that this will only give an approximation based on evenly spaced
windings. You might expect as much as ±50% deviation from this
L LK = 292N1065
where L LK = leakage inductance (nH/Turn2
N = number of turns
= core cross-section (mm2
= core magnetic path (mm)
What is the main advantage of each distributed gap material?
MPP has the lowest losses, and the best Q HF has the highest DC bias. Powdered Iron
is the least expensive. Kool M® is a mix of the advantages of the previous three;
lower losses than Iron Powder,near zero magnetostriction, and much lower cost than
What is the adhesive recommended for Powder Cores?
BondmasteESP 309 is a one part epoxy adhesive that is recommended for use
on powder core materials. This adhesive has good strength at room temperature and
retains strength at high temperatures.
How are properties affected when cores are stacked?
Stacking cores will increase the cross section (Ael by the multiple of the number
of oores in the stack. The magnetic path length (lel will remain constant. The AL
can be estimated by the same method as for single sets, where a leakage adjustment
is estimated based on the ratio of window area 0N f); tocore area (Ae•l Because
that ratio decreases as cores are stacked, the AL of n stacked sets are
slightly less than n times the AL of a single set.
What is soft saturation?
Soft saturation is a distributed gap material advantage over a ferrite. The DC bias
curve does not have the traditional saturation point that a ferrite core does, rather,
as the drive level increases the permeability slowly rolls off in a predictable
How important is permeability in power materials?
Permeability is flux density, (B), divided by drive level, (H) Power materials
are generally used for high frequency transformer applications. Hence, the important
characteristics are high flux density and/or low core losses. Permeability is of
lower importance because of its variability over an operating flux range.
What is disaccommodation?
Disaccommodation, occurring in ferrites, is the reduction of permeability with time
after a core is demagnetized. This demagnetization can be caused by heating above
the Curie point, by applying an alternating current of diminishing amplitude, or
by mechanically shocking the core. In this phenomenon, the permeability increases
towards its original value, then starts to decrease exponentially. If no extreme
conditions are expected in the application, permeability changes will be small because
most of the change has occurred during the first few months after manufacture
of thecore. High temperature accelerates the decrease in permeability. Disaccommodation
is repeatable with
each successive demagnetization; thus, it is not the same as aging.
Why are actual core losses larger than calculated?
When calculating the core losses, 1t is assumed that the structure is homogeneous.
In reality, when core halves are mated, there is leakage flux (fringing flux) at
the mating surfaces, and the gap losses contnbute to the total losses. G3p losses
are caused by flux concentration in the core and eddy currents generated in
the windings. When a core is gapped, this gap loss can drastically increase overall
losses. Additionally, because the cross-sectional area of many core geometnes is
not uniform, local "hot spots" can develop at points of minimum cross section. This
creates localized areas of increased flux density, resulting in higher losses at
What is the difference between nickel-zinc and manganese-zinc ferrites?
MnZn materials have a high permeability, while NiZn ferrites have a low permeability.
Manganese-zinc ferrites are used in applications where the operating frequency is
less than 5 MHz. Nickel-zinc ferrites have a higher resistivity and are used
at frequencies from 2 MHz to several hundred megahertz. The exception is common
mode inductors where the impedance of MnZn material makes it the best choiceup to
70 MHz and NiZn is recommended from 70 MHz to several hundred GHz.
Why, in some cases, is only the minimum AL listed in the core datasheet?
Permeability (and ALl varies with drive level. For power applications, there is
no need to place a limit on the maximum AL. A minimum AL translates into maximum
What is the proper clamping pressure for ferrites?
Generally, a recommended figure is about 700 kg/m2 (100 lbs/sq. in.) of mating surface.
For specific recommended pressures tor RM, PO, EP, and pot cores, consult the MAGNETICS
Ferrite Design Manual.
Why do you flat-grind ferrite cores?
Cores are flat-ground on the mating surface because of the uneven suriace
produced during the firing process. It is important for cores to mate with a minimum
air gap to keep the gap losses low and to achieve optimum inductance.
Why do cores get lapped? What is the surface finish?
Lapping is an additional production process used to improve the mating suriace.
It Is typicadlone on cores with material permeability of 5000 and greater
in order to achieve the maximum AL value for a given material. A mirror-like finish
is the result. The sutiace finish for a normafllat-ground sutiace is0.5 to 1.0 microns
and for a lapped core is 0.1 to 0.2 microns.
Why is the ferrite gapped tolerance not always ±3%?
Due to limitations of the machine petiorming the gapping
, as the gap dimension decreases
it is increasingly difficult to hold tight tolerances. As AL increases the gap gets
smaller and the tolerances increase. As the gap gets smaller, the mechanical tolerance
becomes proportionately larger, in addition the influence of variation in the material
permeability becomes greater A gap specified by its AL value yields a tighter
tolerance than a gap specified by its physical dimensions.
How do you glue ferrite cores?
Gluing should be done with thermosetting epoxy resin adhesives. The available range
is very large. Important factors in the choice are the required temperature and
viscosity. The economic curing temperature must not be above the maximum temperature
to which the assembly may be safely raised. High viscosity resin can be difficult
to apply Low viscosity resin may run out of a poorfitted joint or may be absorbed
by the porous ferrite material. Follow the manufacturer's instructions for a particular
resin. Take care not to thermaslhock ferrites; raising or lowering the oore temperature
toorapidly is dangerous. Ferrites will crack if changes in temperature exoeed
5-10°C/min. In addition, care
must be taken to match the adhesives' ooefficient of thermal expansion (CTE)
to that of the ferrite material. Otherwise, the resin may expand or contract more
quickly than the ferrite; the result can be cracks that will degrade core properties.
What is MAGNETICS specification for out of roundness on a ferrite toroid?
Out of roundness is controlled by mandating that cores meet overall dimensional
tolerances for OD and ID while keeping enough cross section to meet the specified
AL. Refer to the MAGNETICS Ferrite Design Manual for toroid physical dimension tolerances.
What is the difference between nylon and epoxy coatings for ferrite toroids?
They are similar. Nylon is thicker, and can stand temperatures up to 155°C.
Epoxy is rated at 200°C. Nylon finishes are generally applied to cores ranging
in 0.D. (outside diameter) from 12. 7 mm to 29 mm. Very large and very small oores
are coated with a epoxy finish. The voltage breakdown guarantee of nylon and epoxy
coating is 1000 volts wire to wire. Nylon cushions better and is more resistant
to solvents. Both finishes are held to the same electrical specifications.