We'll learn how to choose the right ferrite core material for a given SMPS circuit design in this article.
Ferrite Core: What's the Point?
Ferrite is an excellent core material for transformers, inverters, and inductors operating in the 20 kHz to 3 MHz frequency range, thanks to its low core cost and low core losses.
Ferrite is a good material for high-frequency inverter power supplies (20 kHz to 3 MHz).
For low power, low-frequency processing (50 watts and 10 kHz), ferrites should be used in a saturating approach. A two-transformer layout with a tape-wrapped core as the saturating core and a ferrite core as the output transformer offers the best performance for high power applications.
The two-transformer model offers high performance, long frequency stability, and low switching drawdowns.
Ferrite cores are widely used in fly-back transformers because they have a low core cost, a low circuit cost, and a high voltage quality. Powder cores (MPP, High Flux, Kool M®) produce softer saturation, higher Bmax, and better temperature consistency, making them a popular choice for a variety of flyback applications and inductors.
In contrast to conventional 60 hertz and 400-hertz power options, high-frequency power supplies, such as inverters and converters, provide lower costs, as well as reduced weight and structure.
Several cores in this section are standard designs that are widely used in the industry.
CORE MATERIALS
For high power/high-temperature functionality, F, P, and R materials are recommended because they allow for minimal core drawbacks and maximum saturation flux density. P material core deficits decrease with increasing temperature up to 70°C, while R material losses decrease up to 100°C.
J and W materials have a higher impedance for wide transformers, making them suitable for low-level power transformers as well.
CORE GEOMETRIES
1) POT CORES
Pot Cores are made to almost completely encircle the wound bobbin. This makes it easier to protect the coil from EMI picked up from outside sources.
To ensure that pot core proportions are compatible between firms, they almost all adhere to IEC requirements. Plain and printed circuit bobbins, and also mounting and assembly hardware, are available.
The pot core is typically a more costly core than other formats of comparable size due to its shape. Pot cores for large-scale power generation are tough to obtain.
2) DOUBLE SLAB AND RM CORES
Slab-sided solid center post cores have a section minimize off on either side of the skirt, close to pot cores. Larger wires may be lodged thanks to the large exits, which also helps to reduce heat in the setup.
RM cores are similar to pot cores, but they are designed to reduce PCB area by at least 40%, resulting in a 40% reduction in installation space.
Bobbins of printed circuits or plain bobbins are both accessible. Easy-to-use 1 unit clamps make building a breeze. It is possible to achieve a narrower outline.
3) EP CORES
Except for the printed circuit board terminals, EP Cores are circular center-post cubical designs that fully enclose the coil. The unique appearance reduces the impact of airflow crevices formed at the magnetic track's mating walls, resulting in a higher volume-to-absolute-area ratio. The ability to protect against RFs is excellent.
4) PQ CORES
PQ cores are specifically designed for switched-mode power supplies. The design allows for a high bulk-to-winding-region-to-surface-area ratio.
As a result, even with the smallest core dimension, optimum inductance and winding surface can be achieved.
As a result, the cores have maximum power output with the smallest assembled transformer mass and dimension, while also taking up the smallest amount of space on the printed circuit board.
It's easy to set up with printed circuit bobbins and one-bit clamps. This cost-effective model ensures a much more homogeneous cross-sectional section; as a result, cores also operate with less hot positions than other layouts.
5) E CORES
E cores are less expensive than pot cores, and they have the advantages of easy bobbin winding and assembly. The bobbins that are wound with these cores may be gang wounds.
Regardless, self-shielding is never present in E cores. Layouts for lamination size E were meant to fit commercially available bobbins in the past, which were used to confirm the strip stampings of standard lamination measurements.
There are also metric and DIN sizes available. E cores are usually embedded to varying degrees of consistency, resulting in a wide range of cross-sectional areas. Commercially available bobbins for these various cross-sectional areas are common.
E cores are usually mounted in specific orientations to provide a low profile if desired.
Low-profile fixing can be done with printed circuit bobbins.
E cores are well-known designs because of their lower cost, ease of production and winding, and organization capable of a large range of hardware.
Planar E cores can be used in almost all IEC standard dimensions, as well as several supplementary capacities.
Due to its decreased AC core losses and limited losses at 100°C, Magnetics R material is perfectly suited to planar shapes.
In most cases, planar layouts have lower turn numbers and more agreeable thermal dissipation than regular ferrite transformers, and as a result, the best designs for space and effectiveness result in higher flux densities. The overall performance benefit of R material is mainly evident in those variations.
7) EC, ETD, EER, AND ER CORES
These patterns are a mixture of E cores and pot cores. They, like E cores, have a massive gap on both sides. This provides a place for the larger wires needed for reduced output voltage switched-mode power supplies.
Aside from that, it ensures air circulation, which makes the structure hotter.
The middle piece is round, and it resembles the pot core in shape. As compared to the wire around a square-style central pillar with the same cross-sectional area, one of the advantages of the circular central pillar is that the winding has a shorter course time (11% faster).
This reduces winding losses by 11% while also allowing the core to cope with increased output capability. Additionally, the circular central pillar reduces the spiked fold in the copper that arises when winding on a square-style central pillar.
8) TOROIDS
Toroids are the least costly of the most important core designs because they are cost-effective to manufacture. Since there is no need for a bobbin, accessory and setup costs are minimal.
Toroidal winding equipment is used to complete the winding process. Shielding is a good trait to have.
Overview:
Ferrite geometries come in a wide range of sizes and styles. The requirements mentioned in Table 1 should be considered when selecting a core for power supply applications.
TRANSFORMER CORE SIZE SELECTION
The WaAc result of a transformer core determines its power processing capacity, where Wa is the provided core window space and Ac is the useful core cross-sectional space.
GENERAL INFORMATION
A perfect transformer guarantees minimal core decay while requiring the minimum amount of space.
The core loss in a specific core is affected by the flow velocity as well as the frequency. In the case of a transformer, frequency is the most important factor. According to Faraday's Law, the flux density decreases as the frequency is increased.
When the flux density falls, the number of core losing trades drops rapidly more than when the frequency rises. For example, if a transformer is operated at 250 kHz and 2 kG on R material at 100°C, core failures are likely to be around 400 mW/cm3.
According to Faraday's law, the flux density will likely be 1kG, with core drawdowns of about 300mW/cm3.
Core loss is restricted in standard ferrite power transformers, which range from 50 to 200mW/cm3. Due to better power dissipation and significantly less copper in the windings, planar models could be run even more aggressively, up to 600 mW/cm3.
CIRCUIT Categories
The following are some examples of simple feedback on various circuits: The push-pull circuit is efficient because it uses a transformer core in both directions, resulting in a lower-rising output. Despite this, circuitry is extremely sophisticated, and when power transistors have unequal switching properties, transformer core saturation may cause transistor breakdown.
Feedforward circuits are less expensive since they only use one transistor. Since obvious stable state current flows in the transformer irrespective of whether the transistor is ON or OFF, ripple is minimal. The flyback circuit is simple and inexpensive.
PUSH-PULL CIRCUIT
Figure 2A depicts a traditional push-pull circuit. The feed voltage is the result of an IC network, or clock, that alternately turns on and off the transistors. The high-frequency square waves on the transistor output are gradually refined, resulting in dc generation.
CORE IN PUSH-PULL CIRCUIT
Equation (4) with a flux density (B) level of 2 kG max is normally a well-known method for ferrite transformers at 20 kHz.
The colored segment of the Hysteresis Loop in Figure 2B illustrates this. The B degree was chosen because core loss is a limiting factor when choosing a core with this frequency.
If the transformer is designed for a flux density of about saturation at 20 kHz (as it is for lower frequency layouts), the core will experience an uncontrolled temperature rise.
As a result, the lower operating flux density of 2 kG would in most cases reduce core losses, allowing for a more manageable core temperature rise.
Core losses increase as the frequency rises above 20 kHz. It is important to conduct the SPS at higher frequencies with core flux rates less than 2 kg. Figure 3 shows the decline in flux levels for the MAGNETICS "P" ferrite material, which is critical for contributing constant 100mW/cm3 core losses at a variety of frequencies and with a temperature surge of 25°C.
The transformer operates in the first quadrant of the Hysteresis Loop in the feed-forward circuit shown in Figure 4A.
The transformer core is operated from its BR value near saturation thanks to unipolar pulses applied to the semiconductor chip. The core returns to its BR rate as the pulses are reduced to zero.
To retain a high level of operation, the primary inductance is kept high to reduce magnetizing current and wire drawdowns. This means that the heart must have no or very little airflow opening.
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