Introduction to Transformer Core Classification

1. High-Frequency Type: Ferrite Cores

Ferrite cores are used in high-frequency transformers. They are ceramic magnets with a spinel crystal structure, composed of iron oxide and other divalent metal compounds, such as kFe₂O₄ (where "k" represents other metals). The metals currently in common use include manganese (Mn), zinc (Zn), nickel (Ni), magnesium (Mg), and copper (Cu).

Common combinations include the manganese-zinc (MnZn) series, nickel-zinc (NiZn) series, and magnesium-zinc (MgZn) series. These materials possess high magnetic permeability and impedance properties, with an operating frequency range from 1 kHz to over 200 kHz.

2. Low-Frequency Type: Silicon Steel Laminations

Silicon steel sheets are used in low-frequency transformers. They come in various types and can be classified in two main ways:

• By manufacturing process: A: Annealed (black sheets) and N: Non-annealed (white sheets).

• By shape: EI-type, UI-type, C-type, and square (mouth-type) cores.

Square-type silicon steel sheets are commonly used in high-power transformers (typically above 500–1000 W). They offer excellent insulation, easy heat dissipation, and a short magnetic path. A set composed of two C-type silicon steel sheets is referred to as a CD-type core. For power transformers made with CD-type cores of the same cross-sectional area, a taller window allows for a greater power rating. Since coils can be installed separately on both sides of the core, the total turns can be distributed across two bobbins. This reduces the average turn length per bobbin and decreases copper losses. Additionally, winding two symmetrical coils on separate bobbins achieves perfect symmetry.

A set composed of four C-type silicon steel sheets is known as an ED-type core. Transformers made with ED-type cores have a shorter, wider profile compared to CD-type transformers of the same power rating. Because the coils are mounted in the center of the core with an external magnetic path, ED-type transformers exhibit low magnetic flux leakage and minimal interference. However, since all coils are wound on a single, thicker bobbin, the average turn length is longer, resulting in higher copper losses.

C-type cores offer excellent performance, making the resulting transformers compact, lightweight, and highly efficient. From an assembly perspective, C-type cores require very few components and offer strong versatility, leading to high production efficiency. However, the manufacturing process for C-type silicon steel sheets is complex and requires specialized equipment, making their current production costs relatively high.

E-type silicon steel sheets, also known as shell-type cores, have the primary advantage of placing both primary and secondary coils on the same bobbin, yielding a high window space factor (Space Factor Km: the ratio of the net cross-sectional area of the copper wire to the window area). The silicon steel sheets form a protective shell around the windings, preventing mechanical damage. They also provide a larger heat dissipation area and reduce magnetic field radiation. However, this design results in higher primary-to-secondary leakage inductance and greater susceptibility to external magnetic field interference. Furthermore, due to the longer average winding circumference, EI-type transformers require more copper wire for the same number of turns and core cross-sectional area.

The most common thicknesses for silicon steel sheets are 0.35 mm and 0.5 mm.

There are two assembly methods for silicon steel sheets: interleaving and butting. The interleaving method alternates the openings of the sheets on both sides. Although this method is cumbersome, it minimizes the gaps and magnetic reluctance between the sheets, which helps increase magnetic flux; therefore, it is widely used in power transformers. The butting method is commonly used in applications with direct current (DC). To prevent saturation caused by DC, a gap must be left between the sheets. In this method, the E-pieces and I-pieces are placed on opposite sides, and the gap between them can be adjusted using paper shims.

3. Coil Types

Coil-type cores are divided into three main categories:
A. Toroidal Core: Made by stacking O-shaped laminations or winding silicon steel strips. Winding these cores is quite challenging.
B. Rod Core: Rod-shaped magnetic cores.
C. Drum Core: Drum-shaped magnetic cores.