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.
Toroids have the best possible shape from the magnetic point of view. The flux path is completely closed so the capabilities of the ferrite are fully exploited. Especially for high permeability ferrites the effect of even a minor airgap in the magnetic circuit can spoil up to 50% of the effective permeability. A further advantage is the very low leakage fi eld which makes it a suitable shape for power and pulse transformers. Ring cores are mainly used for pulse- and wide band transformers and interference suppression coils but also in special power supplies.
Due to the high saturation fl ux density of iron powder (950...1600 mT) these ring cores are very suitable for output chokes carrying high DC currents. Another application is found in lamp dimmers as ballast choke. The cores are made of electrolytic iron powder, mixed with a small amount of resin for insulation. They are coated with polyamide 11 (thickness 0.1 - 0.3 mm). The isolation voltage between core and winding is up to 1500 V.
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.
EPX cores were derived from EP cores specially for pulse transformers in ISDN and ADSL applications. In comparison to EP cores they feature an increased centre pole area and achieve the same AL and THD performance in a smaller core volume. The new EPX designs, complete with SMD bobbin and clip, satisfy the need for slimmer pulse transformers. They are available in the high permeability material 3E6 for ISDN pulse transformers and in the low harmonic distortion material 3E55 for ADSL wideband applications. Power materials are introduced along with these.
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 PT design this problem is solved by cutting away the sides of one core half. This creates ample room for wires and coil former fl anges. A range of special PT coil formers is available but also most standard P core accessories can be used.
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. Planar ER cores are very suitable to build small SMD or planar power and signal tranformers. For the 3 smallest sizes matching SMD coil formers and clips are available.
The EQ core design is derived from the ER and PQ. The range is optimized for use in compact AC/DC notebook adapters and DC/DC converters. For instance, the EQ30 has the capability to handle a power range of 50 to 70 W (fl yback topology) in an enclosed casing of a notebook adapter or 100 to 150 W in low profi le DC/DC converter . The advantages of EQ cores are a simple core shape, round centre pole, high Ae value , a large winding window, low profi le and a large surface area for heat dissipation.
The ETD core design is a further development of E cores. They are optimized for use in SMPS transformers with switching frequencies between 50 and 200 kHz. The designation ETD (Economic Transformer Design) implies that this design achieves maximum throughput power related to volume and weight of the total transformer. Shielding is somewhat improved compared with E cores. The matching coil formers are suitable for many winding types and can be handled on automatic equipment.
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 PH core range consists of potcore halves specially designed for use in proximity switches. Their shape is derived from the IEC standard P-core range. Outside diameters are adapted to fi t standardized sizes of proximity switch housings. Since the cores are used as halves, their height is increased to accommodate the winding. A complete range of coil formers is available.
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.
Transformer inductor choke coil filter, smd power inductor.
Transformer.
