in the frequency range 20
kHz to 3 MHz, due to the combination of low core cost and low core losses.
Ferrite is an excellent material for high frequency (20 kHz to 3 MHz) inverter power supplies.
Ferrites may be used in the saturating mode for low power, low frequency operation (<50 watts
and 10 kHz). For high power operation a two transformer design, using a tape wound core as the
saturating core and a ferrite core as the output transformer, offers maximum performance. The two
transformer design offers high efficiency excellent frequency stability, and low switching losses.
Ferrite cores may also be used in fly-back transformer designs, which offer low core cost, low circuit cost
and high voltage capability. Powder cores (MPP, High Flux, Kool Mµ
®
) offer soft saturation, higher Bmax
and better temperature stability and may be the best choice in some flyback applications or inductors.
High frequency power supplies, both inverters and converters, offer lower cost, and lower weight
and volume than conventional 60 hertz and 400 hertz power sources.
Many cores in this section are standard types commonly used in the industry. If a suitable size for
your application is not listed, Magnetics will be happy to review your needs, and, if necessary,
quote tooling where quantities warrant.
Cores are available gapped to avoid saturation under dc bias conditions. J and W materials are
available with lapped surfaces.
Bobbins for many cores are available from Magnetics. VDE requirements have been taken into account in
bobbin designs for EC, PQ and metric E Cores. Many bobbins are also available commercially.
4.1
Section 4
Power
Design 4.2
MAGNETICS
General Cor
e Selection
CORE MATERIALS
F, P, and R materials, offering the lowest core losses and highest saturation flux density,
are most suitable for high power/high temperature operation. P material core losses
decrease with temperature up to 70C; R material losses decrease up to 100C.
J and W materials offer high impedance for broad transformers, and are
also suitable for low-level power transformers.
FERRITE
POWER MATERIALS SUMMARY
F
P
R
J
W+
µi (20 gauss)
25C
3,000
2,500
2,300
5,000
10,000
µp (2000 gauss)
100C
4,600
6,500
6,500
5,500
12,000
Saturation
25C
4,900
5,000
5,000
4,300
4,300
Flux Density
(Bm Gauss)
100C 3,700
3,900
3,700
2,500
2,500
Core Loss (mw/cm3) 25C
100
125
140
(Typical) 60C
180
80*
100
@100 kHz, 1000 Gauss 100C
225
125
70
*@80C
+@10kHz
CORE GEOMETRIES
POT CORES
Pot Cores, when assembled, nearly surround the wound bobbin. This aids in
shielding the coil from pickup of EMI from outside sources. The pot core
dimensions all follow IEC standards so that there is interchangeability
between manufacturers. Both plain and printed circuit bobbins are
available, as are mounting and assembly hardware. Because of its design,
the pot core is a more expensive core than other shapes of a comparable
size. Pot cores for high power applications are not readily available.
DOUBLE SLAB AND RM CORES
Slab-sided solid center post cores resemble pot cores, but have a section cut
off on either side of the skirt. Large openings allow large size wires to be
accommodated and assist in removing heat from the assembly. RM cores
are also similar to pot cores, but are designed to minimize board space,
providing at least a 40% savings in mounting area. Printed circuit or plain
bobbins are available. Simple one piece clamps allow simple assembly. Low
profile is possible. The solid center post generates less core loss and this
minimizes heat buildup.
EP CORES
EP Cores are round center-post cubical shapes which enclose the coil completely
except for the printed circuit board terminals. The particular shape
minimizes the effect of air gaps formed at mating surfaces in the magnetic
path and provides a larger volume ratio to total space used. Shielding
is excellent.
PQ CORES
PQ cores are designed especially for switched mode power supplies. The
design provides an optimized ratio of volume to winding area and surface
area. As a result, both maximum inductance and winding area are possible
with a minimum core size. The cores thus provide maximum power output
with a minimum assembled transformer weight and volume, in addition to
taking up a minimum amount of area on the printed circuit board. Assembly
with printed circuit bobbins and one piece clamps is simplified. This efficient
design provides a more uniform cross-sectional area; thus cores tend to operate
with fewer hot spots than with other designs.
E CORES
E cores are less expensive than pot cores, and have the advantages of simple bobbin
winding plus easy assembly. Gang winding is possible for the bobbins used
with these cores. E cores do not, however, offer self-shielding. Lamination
size E shapes are available to fit commercially available bobbins previously
designed to fit the strip stampings of standard lamination sizes. Metric and
DIN sizes are also available. E cores can be pressed to different thickness,
providing a selection of cross-sectional areas. Bobbins for these different
cross sectional areas are often available commercially.
E cores can be mounted in different directions, and if desired, provide a low-
profile. Printed circuit bobbins are available for low-profile mounting. E
cores are popular shapes due to their lower cost, ease of assembly and
winding, and the ready availability of a variety of hardware.
PLANAR E CORES
Planar E cores are offered in all of the IEC standard sizes, as well as a num-
ber of other sizes. Magnetics R material is perfectly suited to planar
designs due to its low AC core losses and minimum losses at 100°C. Planar
designs typically have low turns counts and favorable thermal dissipation
compared with conventional ferrite transformers, and as a consequence the
optimum designs for space and efficiency result in higher flux densities. In
those designs, the performance advantage of R material is especially sig-
nificant.
The leg length and window height (B and D dimensions) are adjustable for
specific applications without new tooling. This permits the designer to
adjust the final core specification to exactly accommodate the planar con-
ductor stack height, with no wasted space. Clips and clip slots are avail-
Materials and Geometries POT
DOUBLE SLAB,
EP
PQ
E
EC, ETD,
TOROIDS
CORES
RM CORES
CORES
CORES
CORES
EER, ER CORES
See Catalog Section
6
7-8
9
10
11
12
13
Core Cost
High
High
Medium
High
Low
Medium
Very Low
Bobbin Cost
Low
Low
High
High
Low
Medium
None
Winding Cost
Low
Low
Low
Low
Low
Low
High
Winding Flexibility
Good
Good
Good
Good
Excellent
Excellent
Fair
Assembly
Simple
Simple
Simple
Simple
Simple
Medium
None
Mounting Flexibility**
Good
Good
Good
Fair
Good
Fair
Poor
Heat Dissipation
Poor
Good
Poor
Good
Excellent
Good
Good
Shielding
Excellent
Good
Excellent
Fair
Poor
Poor
Good
** Hardware is required for clamping core halves together and mounting assembled core on a circuit board or chassis.
4.3
General Cor
e Selection
able in many cases, which is especially useful for prototyping. I-cores are
also offered standard, permitting further flexibility in design. E-I planar
combinations are useful to allow practical face bonding in high volume
assembly, and for making gapped inductor cores where fringing losses must
be carefully considered due to the planar construction.
EC, ETD, EER AND ER CORES
These shapes are a cross between E cores and pot cores. Like E cores, they
provide a wide opening on each side. This gives adequate space for the
large size wires required for low output voltage switched mode power
supplies. It also allows for a flow of air which keeps the assembly cooler.
The center post is round, like that of the pot core. One of the advantages
of the round center post is that the winding has a shorter path length
around it (11% shorter) than the wire around a square center post with an
equal area. This reduces the losses of the windings by 11% and enables the
core to handle a higher output power. The round center post also eliminates
the sharp bend in the wire that occurs with winding on a square center post.
TOROIDS
Toroids are economical to manufacture; hence, they are least costly of all comparable
core shapes. Since no bobbin is required, accessory and assembly costs are nil.
Winding is done on toroidal winding machines. Shielding is relatively good.
SUMMARY
Ferrite geometries offer a wide selection in shapes and sizes. When choosing a core
for power applications, parameters shown in Table 1 should be evaluated.
TABLE 1: FERRITE CORE COMPARATIVE GEOMETRY CONSIDERATIONS
m a g - i n c . c o m
Materials and Geometries 4.4
MAGNETICS
TRANSFORMER CORE SIZE SELECTION
The power handling capacity on a transformer core can be determined by its
WaAc product, where Wa is the available core window area, and Ac is the
effective core cross-sectional area.
The WaAc/power-output relationship is obtained by starting with Faradays Law:
E=4B Ac Nf x 10-8 (square wave)
(1)
E=4.44 BAc Nf x 10-8 (sine wave)
(1a)
Where:
E=applied voltage (rms)
K=winding factor
B=flux density in gauss
I=current (rms)
Ac=core area in cm2
P
i
=input power
N=number of turns
P
o
=output power
f=frequency in Hz
e=transformer efficiency
Aw=wire area in cm2
Wa=window area in cm2:
Core window for toroids
Bobbin window for other cores
C=current capacity in cm2/amp
Solving (1) for NAc
NAc= E x 10
8
(2)
4Bf
The winding factor
K= NAw thus N= KWa and NAc= KWaAc
(3)
Wa Aw Aw
Combining (2) and (3) and solving for WaAc:
WaAc= E Aw x 10
8
, where WaAc=cm
4
(4)
4B fK
In addition:
C=Aw/l or Aw=IC e= P
o
/ P
i
P
i
=El
Thus:
E Aw=