Hysteresis Loop
An electric current flowing through a conductive material generates a magnetic field. Magnetic fields are strongest at the conductor’s surface and weaken farther from the conductor.
When you magnetize a core material, it won’t stop having a magnetic charge when you stop magnetizing it. You must reverse the magnetization to get it back to zero magnetization. This is the hysteresis loop, when alternating magnetic fields (positive and negative) are applied to a material.
Applying a magnetic field on magnetic core materials is called magnetization force or “H.”
Toroidal Magnetic Field and Flux
Magnetic flux is the total magnetic field flowing through an area. Its intensity is referred to as flux density.
The variable for flux is “B.” The hysteresis loop is the BH curve. Analyzing the BH curve is essential when designing transformers, chokes and inductors.
Flux will decrease if you increase the area of the cores, the number of turns or the switching frequency.
Permeability
A magnetic core material’s permeability is its ability to increase the flux density if an electric current runs through it.
The greater the permeability, the greater the flux density.
Choke, Conductor and Transformer Design
When designing a choke or inductor, you don’t want to cause saturation of the toroidal core by increasing the AC or DC current. Generally, it’s the DC current that saturates the core since it’s constant and moves the cores to a certain flux level.
In a transformer design, you must ensure that maximum AC currents are well below the saturation point.
Another way to reach saturation is by increasing the flux density, normally by increasing the voltage. When permeability is high, cores saturate faster. Conversely, when permeability is low, cores become saturated at a higher flux density.
Transformers transfer power, so you want minimum losses when transferring power from the primary to secondary side. That’s why ferrite cores are common for high frequency designs. We use high-permeability, grain-oriented silicon steel for low frequency transformers. Inductors and chokes store energy, so you want high flux cores in them.
Toroidal Core Losses
There are always power losses in transformers, inductors and chokes, which generate heat and cause thermal problems. Toroidal core losses result from the following:
- Hysteresis loss comes from the moving of flux from positive to negative and the area enclosed by the loop. You can lower this loss by using more expensive materials.
- Eddy current loss comes from the difference in flux voltage in the cores causing circulating currents in the magnetic material. The higher the switching frequency, the greater this current loss.
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