The transformer uses magnetic coupling to achieve electromagnetic induction between the coil and the coil to achieve the purpose of voltage conversion. The AC voltage can help the transformer to achieve the purpose of increasing and decreasing the voltage, so as to solve the problem of the input and output equipment due to the voltage mismatch. Use and other difficult problems, with the help of transformers, you can achieve voltage conversion at a soft cost.

Transformers mainly realize voltage conversion. In addition to being used for mains conversion, transformers have more applications. Due to different applications, the classification and functions of transformers are also very different.

Transformers are also used in electronic power supplies, electronic integrated circuits and other places. And the application of the transformer, according to its classification, the shape and structure are related. Transformers have various shapes, sizes and configurations. Transformers can be classified according to core material, geometry and structure, voltage level and purpose.

Ⅰ. According to the core structure, the classification is as follows:

1. Transformers can be divided into laminated cores, ferrite cores, iron powder cores, and air cores according to their cores.

2. Transformers can be divided into solenoid cores, toroidal cores, and pot cores according to their appearance.

3. According to the voltage level, the transformer can be divided into: step-up, step-down, and isolation.

4. Transformers can be divided into power transformers, voltage regulators, reactors, mutual inductors, and special transformers according to their uses.

When manufacturing any transformer, the manufacturer tries to achieve maximum magnetic coupling between the two inductors. By using ferromagnetic materials or iron powder as the magnetic core, the magnetic coupling can be increased many times. Compared with air-core transformers, a pair of inductors wound on a ferromagnetic core has a better coupling coefficient. However, the use of ferromagnetic cores has its own limitations. Ferromagnetic cores have some energy loss due to hysteresis and eddy currents, and are also limited by current carrying capacity. In addition to these limitations, the choice of core material also limits the frequency range of the transformer.

Ⅱ. Transformers are classified according to the type of core used:

1. Laminated iron transformers: These transformers use silicon steel as the core material. Silicon steel is also called transformer iron or iron for short. Silicon steel is laminated into layers to avoid losses due to eddy currents and hysteresis. Eddy current is a circular current that flows in a magnetic material during magnetization. Eddy currents cause the core to lose energy in the form of heat. Hysteresis is the tendency of the magnetic core to accept fluctuating magnetic flux. Due to the loss of hysteresis and eddy currents, these transformers are only suitable for 60 Hz frequencies and other low frequencies in the audio range. When the frequency increases above several kilohertz, the internal loss of the core will increase beyond the feasible limit.

2. Ferrite cores: Ferrite cores have high permeability and require fewer coil turns. However, at frequencies above a few megahertz, due to eddy currents and hysteresis, such magnetic cores begin to show significant energy loss. This is why these transformers are suitable for audio frequencies up to several megahertz.

3. Iron powder core: Compared with ferrite core, iron powder also has higher magnetic permeability and lower loss due to hysteresis and eddy current. As the frequency increases, the need for high permeability decreases. Transformers using powdered iron cores are suitable for extremely high frequencies up to 100 MHz. Since there is no need to achieve high magnetic permeability at extremely high frequencies higher than 100 MHz, air-core transformers are more suitable due to their higher energy efficiency.

4. Air-core transformer: In an air-core transformer, both the primary coil and the secondary coil are wound on diamagnetic materials. The magnetic coupling in this transformer takes place through air. In such a transformer, not only the inductance of the two coils is low, but the mutual inductance is also very low, so the magnetic coupling between the coils is very small. These transformers do not lose energy due to hysteresis or eddy currents, and can also regulate large currents. This type of transformer is suitable for high-voltage applications where energy efficiency is the primary concern, such as distribution transformers. These are also suitable for ultra-high RF applications above 100 MHz. At high radio frequencies, the required inductance value is very low, which can be easily achieved by air core inductors. 

Ⅲ. The shape and structure of the transformer: 

Transformers can also be classified by their shape and geometry. The shape of the transformer depends on the type of inductor used in its construction and the shape of its core. Any transformer is essentially a pair of inductors wound on the same magnetic core. The classification is as follows:

1. Practical transformers: utility transformers are power transformers that use laminated iron sheet as the core material. These iron core transformers have various iron core shapes, such as E, L, U, I, etc., and are bulky and heavy. The most commonly used core shape in these transformers is E core or EI core, because the shape of the laminated core is the letter "E" and a rod is placed at the open end of the "E" to complete the structure. The coil is wound on the core by the shell method or the core method. In the shell method, both coils are wound on top of each other on the middle strip of "E". This ensures maximum magnetic coupling between the coils, but at the expense of high coil-to-coil capacitance. The shell method also limits the current carrying capacity of the transformer. In the magnetic core method, one coil is wound on the top strip of "E" and the other coil is wound on the bottom. The magnetic coupling between the coils only occurs due to the magnetic flux passing through the iron core. The magnetic core method greatly reduces the coil-to-coil capacitance and makes it possible to handle high voltages. Utility transformers with EI cores and shell or core windings are most commonly used as 60 Hz transformers and other audio transformers.

2. Electromagnetic coil transformer: Electromagnetic coil transformer is usually used as the loop antenna of radio frequency circuit. These transformers have primary and secondary windings on a cylindrical core (ferrite or iron powder). The coils are wound around each other or separately wound. In such a transformer, the primary winding captures the radio signal, while the secondary winding provides impedance matching for the first amplifier stage of the radio circuit. Such transformers have become very common in portable radio communication equipment.

3. Toroidal core transformer: The primary and secondary windings of the toroidal core transformer are both wound on the toroidal core, and the coils may be wound around each other or separately. Toroidal cores are a better alternative to solenoid cores in radio frequency circuits. They contain the magnetic flux in the iron core, so as long as the coil is insulated, these transformers can be installed directly without any other shielding. In addition to the absence of electromagnetic interference, each coil of the toroidal core also provides higher inductance. Since the magnetic flux is still contained in the core, the toroidal core transformer has better magnetic coupling between the coils.

4. Tank core transformer: The main winding and secondary winding of the tank core transformer are superimposed on each other or adjacent to each other. The core can provide the highest inductance and has the obvious advantage of self-shielding. One of the main disadvantages of core transformers is the coil-to-coil capacitance. Since the capacitance between the coil and the coil and the inductance of the two coils are abnormally high, the pot-shaped iron core transformer is only suitable for low frequencies. At high frequencies, the required inductance value is very low, and the capacitive reactance must be basically reduced.

Ⅳ.transformer voltage level transformer

The most common application is to regulate AC voltage. The transformer can step up, step down or maintain a complete AC voltage level. This is the simplest but most important classification of transformers.

1. Step-up transformer: In the step-up transformer, the number of turns of the secondary coil is higher than that of the primary coil. When the primary to secondary turns ratio is less than 1, the voltage applied to the primary will rise to a higher voltage in the secondary. Therefore, this comes at the cost of a lower current level on the secondary winding. Step-up transformers are used in stabilizers and inverters, where a lower AC voltage needs to be converted to a higher voltage. They are also used in the power grid to increase the AC voltage level before distribution.

2. Step-down transformer: In a step-down transformer, the number of turns of the primary coil is higher than that of the secondary coil. When the turns ratio of the primary to secondary winding is greater than 1, the secondary voltage will be lower than the primary voltage. Step-down transformers are commonly used in electronic applications. Electronic circuits usually require 5V, 6V, 9V, 12V, 18V or 24V to operate. The step-down transformer is usually used in the power circuit, before the rectifier to step down the 120V or 240V AC power to the required low voltage level. In power distribution, a step-down transformer is used to reduce the high voltage to supply power to the two poles. This ensures the energy efficiency and cost-effectiveness of power distribution.

3. Isolation transformer: The primary and secondary turns of the isolation transformer are the same. Since the ratio of the number of turns of the primary to the secondary is exactly 1, the voltage level on the two windings remains the same. These transformers are used to provide electrical isolation between electronic circuits or to eliminate the transmission of noise from one circuit to another. The isolation transformer needs to have high inductive coupling and minimum capacitive coupling. This is why these transformers are designed to have a minimum number of turns on independent coils wound on a highly magnetic and self-shielded core.

Isolation transformers are also used to connect balanced and unbalanced circuits. A balanced circuit refers to a circuit that can be connected in any way across ports. An unbalanced circuit refers to a circuit that needs to be connected across ports in a specific way. Balanced and unbalanced loads can be connected through an isolation transformer by grounding the center tap of the balance center. If the balanced load and the unbalanced load have the same impedance, the turns ratio of the isolation transformer should be 1. If the balanced load and the unbalanced load have different impedance ratios, the turns ratio should match the square of the impedance ratio.