Learn More about Ferrite Cores

1. Learn More about Ferrite Cores

2. Overview Ferrite cores are dense, homogeneous ceramic structures composed of iron oxide (Fe2O3) mixed with oxides or carbonates of one or more metals such as manganese, zinc, nickel, or magnesium. They are pressed, then fired in a kiln to 1300o C before being machined to meet various operational requirements. Because of their high electrical resistivity and low eddy current losses over a wide frequency range, ferrites have an advantage over other types of magnetic materials. These properties, combined with high permeability, make ferrite ideal for use in high frequency transformers, wide band transformers, adjustable inductors, and other high frequency circuitry starting at 10 kHz to 50kHz.

3. Importance of permeability in power materials? Permeability is calculated as flux density (B) divided by drive level (H). Power materials are typically used in high frequency transformer applications; thus, high flux density and/or low core losses are important characteristics. Permeability is less important due to its variability across an operating flux range.

4. Disaccommodation Disaccommodation occurs in ferrites and is defined as a decrease in permeability over time after a core has been demagnetized. This demagnetization can be caused by heating above the Curie point with a decreasing amplitude alternating current or by mechanically shocking the core. The permeability increases towards its original value in this phenomenon, then begins to decrease exponentially. If no extreme conditions are expected in the application, permeability changes will be minor because the majority of the change will have occurred within the first few months of the core's manufacture. The decrease in permeability is accelerated by high temperatures.

5. Difference between nickel-zinc and manganese-zinc ferrites The permeability of MnZn materials is high, whereas that of NiZn ferrites is low. Manganese-zinc ferrites are used in applications with a frequency of operation less than 5 MHz. Nickel-zinc ferrites have a higher resistivity and are used at frequencies ranging from 2 MHz to hundreds of megahertz. The exception is common mode inductors, where the impedance of MnZn material makes it the best choice up to 70 MHz and NiZn from 70 MHz to several hundred GHz.

6. Ferrite Applications Ferrite cores have two broad applications that differ in size and frequency of operation: signal transformers (small size and higher frequencies) and power transformers (large size and lower frequencies). Cores can also be classified based on their shape, such as toroidal, shell, or cylindrical cores. Power transformer ferrite cores operate in the low frequency range (1 to 200 kHz[2]) and are fairly large in size. They can be toroidal, shell, or shaped like the letters ‘C', ‘D', or ‘E'. They are useful in all types of electronic switching devices, particularly power supplies ranging from 1 Watt to 1000 Watts. because more powerful applications are usually beyond the capabilities of ferritic single cores and necessitate grain oriented laminated cores The ferrite cores used for signals have applications ranging from 1 kHz to many MHz, possibly as much as 300 MHz, and have found their primary application in electronics, such as AM radios and RFID tags.

7. Ferrite Applications(Properties, materials and shapes) Applications Preferred Materials Available Shapes Desired Properties Pot cores, Toroids, E, U & I cores, RM, EP cores Low loss, High µ (permeability), Good frequency response Broadband Transformers J, W, M* Common Mode Chokes Very high µ J, W, M* Toroids, E cores Converter and Inverter Transformers Toroids, E, U & I cores, Pot cores, RS cores, Planar cores Low losses, High saturation F, L, P, R, T Low losses, High temperature stability, Good stability across load conditions Gapped pot cores, EP cores, E cores, RM cores, Planar cores, PQ cores Differential Mode Inductors F, P, R, T Pot cores, Toroids, RM cores, EP cores Narrow Band Transformers Moderate Q, High µ, High stability F, J

8. Continued…. Applications Preferred Materials Available Shapes Desired Properties Noise Filters High µ, Good frequency response J, W, M Toroids Low losses at high flux densities and temperatures, High saturation, Good stability across load conditions Pot cores, E cores, PQ cores, RM cores, Planar cores Power Inductors F, L, P, R High µ and low losses at high flux densities and temperatures, High saturation, Low exciting currents Ungapped pot cores, E, U & I cores, Toroids, EP cores, RS cores, DS cores, PQ cores, Planar cores Power Transformers F, L, P, R, T Pulse Transformers High µ, Low loss, High B saturation J, W, M Toroids Low losses, High temperature stability, Good stability across load conditions Pot cores, EP cores, E cores, RM cores, Planar cores Telecom Inductors F, P, R, T

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