Introduction to Transformer Core Classifications

1. High-Frequency Category: Ferrite Cores
Ferrite cores are used in high-frequency transformers. They are ceramic materials with a spinel crystal structure, composed of iron oxide and other divalent metal compounds (e.g., kFe₂O₄, where "k" represents another metal). Commonly used metals 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 characteristics, making them suitable for operating frequencies ranging from 1 kHz to over 200 kHz.

2. Low-Frequency Category: Silicon Steel Laminations
Silicon steel laminations are used in low-frequency transformers. Based on manufacturing processes, they are divided into two types: A: Annealed (black sheets) and N: Non-annealed (white sheets). Based on their shape, they are categorized into EI-type, UI-type, C-type, and Square (口)-type.

Square-type silicon steel is commonly used in high-power transformers. It offers excellent insulation, easy heat dissipation, and a short magnetic path. It is primarily used for transformers with power ratings greater than 500–1000W and in high-power applications. A set of silicon steel formed by combining two C-type sheets is called CD-type. For power transformers made with CD-type laminations, under identical cross-sectional area conditions, a taller window height allows for higher power capacity.

As transformer power increases, coils can be installed separately on both sides of the core. This allows the total number of turns to be distributed across two winding bobbins, thereby reducing the average turn length per bobbin and decreasing copper losses. Additionally, if two symmetrical coils are wound on separate bobbins, perfect symmetry can be achieved. A set formed by four C-type sheets is known as ED-type. Transformers made with ED-type laminations have a flat, wide profile; under the same power rating, ED-type transformers are shorter but wider than CD-type ones. Furthermore, because the coils are mounted in the center of the laminations with an external magnetic path, leakage flux is minimized, resulting in lower overall electromagnetic interference. However, since all coils are wound on a single thick bobbin, the average turn length is longer, leading to higher copper losses.

C-type cores offer superior performance, resulting in transformers that are compact, lightweight, and highly efficient. From an assembly perspective, C-type laminations require fewer parts and offer strong versatility, leading to high production efficiency. However, C-type laminations involve numerous processing steps and complex manufacturing procedures requiring specialized equipment, making their current costs relatively high.

E-type silicon steel, also known as shell-type or Japanese-standard (日型) laminations, has the primary advantage of housing both primary and secondary windings on a single common bobbin, achieving a high window space factor (Km: the ratio of the net cross-sectional area of the copper wire to the window area). The laminations form a protective shell around the windings, preventing mechanical damage. Meanwhile, the large surface area facilitates better heat dissipation, and the magnetic field divergence is minimal. However, it suffers from higher primary-to-secondary leakage inductance and greater susceptibility to external magnetic interference. Additionally, due to the longer average perimeter of the windings, EI-type core transformers require more copper wire for the same number of turns and core cross-sectional area.

Common thicknesses for silicon steel laminations are 0.35 mm and 0.5 mm.

There are two main assembly methods for silicon steel laminations: interleaving and butt-stacking. Interleaving involves alternating the open ends of the laminations on opposite sides one by one. Although this method is labor-intensive, it minimizes the air gaps between laminations and reduces magnetic reluctance, which helps increase magnetic flux; therefore, it is widely adopted in power transformers. Butt-stacking is typically used in applications carrying DC current. To prevent saturation caused by the DC current, an air gap must be maintained between the laminations. In this method, E-pieces and I-pieces are placed on opposite sides, and the gap between them can be adjusted using paper shims.

3. Coil Core Category: Three Main Types

A. Toroidal Core: Formed by stacking O-shaped laminations or by winding silicon steel strips. Winding coils onto this type of core is quite challenging.
B. Rod Core.
C. Drum Core.