Economic Flat Design (EFD) power
transformer cores offer a signifi cant
advance in circuit miniaturization. Their
low build height and high throughput
power-density make them ideally suited
to applications where space is at a
premium.
Throughput power of a ferrite core
transformer is essentially proportional
to its volume. So the transformer
is one of the main limitations in a
DC-DC converter's size. Now, with
the introduction of the EFD system, a
signifi cant reduction in transformer core
height has been achieved.
EFD transformer cores combine
both extreme fl atness with a very
high throughput power-density for
frequencies up to 1 MHz and higher.
Every transformer, based on the EFD
range, has a lower building height than
any other existing low-profi le design
with the same magnetic volume. This
is achieved by placing the centre pole
of the core always in the centre of
the fi nished transformer, thus making
maximum use of the winding area.
The EP/LP core range was specially designed for wideband transformer applications where low build height is a must. The board area occupied by the assembly is almost a square, allowing high packing densities on the PCB. The bobbins have two rows of pins allowing easy design of multiple output transformers. Cores are available in high permeability materials, including the new low THD material 3E55, for wide band transformers and in power materials for small power transformers.
PM cores are a variation on classic P cores, suitable for large high power transformers and energy storage chokes. They have larger wire slots facilitating easy assembly, but still the good shielding of a closed core shape. PM cores can be found in transmission and radar equipment and in various high power industrial installations.
RM cores were designed for use in high Q, high stability fi lter inductors. Their shape allows economic utilization of surface area on the PCB. The range is standardized in IEC 431 and is available worldwide from many suppliers. The sizes are based on the standard PCB grid distance. RM 5, for instance, fi ts on a board space of 5 x 5 modules of 2.5 mm grid. Coil formers and clips were optimized for automated winding and mounting. The slots provide suffi cient space for leads of windings. Magnetic shielding is not as good as with P-cores, but still effective.
The shape of EI cores, more precisely a core set consisting of an E core and an I core, is magnetically equivalent to an E core set with shorter legs. For typical characteristics, see therefore the E core section.
A disadvantage of the classical P core design has always been the narrow wire slots, making it diffi cult to make strong coil formers with integrated solder pins. In the PTS design this problem is solved by cutting away the sides of both core halves. This creates ample room for wires and coil former fl anges. A range of special PTS coil formers is available but also most standard P core accessories can be used.
U cores, with rectangular crosssections, are easy to produce and are relatively inexpensive. For this reason they are very popular in low cost applications such as interference fi lters and output chokes in radio and TV equipment. There is no real optimization for transformer winding designs and the core is rather bulky. Large U cores like U93 and U100 are suitable for very high throughput powers. They can be stacked to form transformers, capable of handling several kW's in applications such as industrial HF welding.
The ER core design is derived from the original E core and, like ETD and EC cores, has a round centre pole and outer legs with a radius to accomodate round coil formers. These cores are mainly used for power transformers. The round centre pole allows the use of thicker wires while the shorter turn length keeps the copper losses low.
PQ cores, like RM/I cores, have round solid centre poles and round winding areas. On the outside the design is rectangular. Top and bottom of a core set are completely fl at, allowing good thermal contact with heat sinks. PQ cores are mainly used in power conversion. Therefore they are only offered in power materials. For most core sizes matching coil formers are available.
P cores with solid centre poles have approximately a 15% higher effective area than the corresponding P cores with central hole. This makes them more suitable for applications where high fl ux densities are used. This will be the case in power conversion where the P core is still popular mainly because of its excellent magnetic shielding. This helps to avoid EMI problems, especially at higher switching frequencies.
