What is a High Frequency Transformer?

Features

A transformer is a device that transforms AC voltage, current and impedance. When an AC current flows through the primary coil, an AC magnetic flux is generated in the iron core (or magnetic core), which induces a voltage (or current) in the secondary coil. The transformer consists of an iron core (or magnetic core) and a coil. The coil has two or more windings. The winding connected to the power supply is called the primary coil, and the rest of the windings are called secondary coils. In the generator, whether the coil moves through the magnetic field or the magnetic field moves through the fixed coil, an electric potential can be induced in the coil. In both cases, the value of the magnetic flux is unchanged, but the amount of magnetic flux that intersects the coil is different. Change, this is the principle of mutual induction. A transformer is a device that uses the electromagnetic mutual inductance effect to transform voltage, current and impedance.

Definition of high frequency transformer

High-frequency transformer is a kind of transformer, as the most important part of switching power supply. There are many topologies in switching power supplies. For example, in a half-bridge power conversion circuit, the two switching transistors are turned on in turn to generate a high-frequency pulse wave of 100 kHz, which is then transformed by a high-frequency transformer to output alternating current. The ratio of the turns of each winding of the high-frequency transformer is determined by the output voltage. The most conspicuous in a typical half-bridge transformer circuit are three high-frequency transformers: the main transformer, the drive transformer and the auxiliary transformer (standby transformer). Each transformer has its own measurement standards in national regulations, such as the main transformer, As long as it is a power supply of more than 200W, the core diameter (height) must not be less than 35mm. As for the auxiliary transformer, when the power supply does not exceed 300W, its core diameter is 16mm.

Working principle of high frequency transformer

A high-frequency transformer is a device that converts AC voltage, current and impedance. When there is an AC current in the primary coil, an AC magnetic flux is generated in the iron core (or magnetic core), so that a voltage (or current) is induced in the secondary coil. .

A high-frequency transformer consists of an iron core (or magnetic core) and a coil. The coil has two or more windings. The winding connected to the power supply is called the primary coil, and the rest of the windings are called secondary coils.

The high-frequency transformer is the most important part of the switching power supply. The switching power supply generally adopts a half-bridge power conversion circuit. When working, the two switching transistors are turned on in turn to generate a high-frequency pulse wave of 100 kHz, and then step down through a high-frequency transformer to output low-voltage alternating current. Each winding coil of the high-frequency transformer The ratio of turns determines the output voltage.

The main reasons for the electromagnetic interference of high-frequency power transformers are the suction between the magnetic cores and the repulsion between the winding wires. The frequency of change of these forces is consistent with the operating frequency of the high frequency power transformer. Therefore, a high-frequency power transformer with an operating frequency of about 100khz will not produce audio noise below 20khz for no special reason.

High frequency transformer design principle

When designing a high-frequency transformer, the leakage inductance and distributed capacitance of the transformer must be minimized, because the high-frequency transformer in the switching power supply transmits a high-frequency pulse square wave signal. During transmission transients, leakage inductance and distributed capacitance cause inrush current and voltage spikes, as well as top ringing, resulting in increased losses. Usually the leakage inductance of the transformer is controlled to be 1% to 3% of the primary inductance.

The leakage inductance of the primary coil—the leakage inductance of the transformer is caused by the incomplete coupling of the magnetic flux between the primary coil and the secondary coil, between layers, and between turns and turns.

Distributed capacitance----The capacitance formed between the turns of the transformer winding, between the upper and lower layers of the same winding, between different windings, and between the winding and the shielding layer is called distributed capacitance.

Primary winding - The primary winding should be placed on the innermost layer, so that the length of each turn of the primary winding of the transformer can be the shortest, so that the wire used in the entire winding is the least, which effectively reduces the distribution of the primary winding itself capacitance.

Secondary winding ---- After the primary winding is wound, add (3-5) layers of insulating gasket and then wind the secondary winding. In this way, the capacitance of the distributed capacitance between the primary winding and the secondary winding can be reduced, and the dielectric strength between the primary and secondary windings can also be increased, which meets the requirements of insulation withstand voltage.

Bias winding—The bias winding is wound between the primary and the secondary, or is wound on the outermost layer, which is related to whether the adjustment of the switching power supply is performed according to the secondary voltage or the primary voltage.
Classification of high frequency transformers

High-frequency transformers are power transformers whose operating frequency exceeds the intermediate frequency (10kHz). power transformer. According to their operating frequency, we divide high-frequency transformers into the following categories:

1. According to the frequency range

(1) kHz-level high-frequency transformer, which refers to a high-frequency transformer with an operating frequency of 20 kHz to several hundred kHz;

(2) MHz-level high-frequency transformer, which refers to a high-frequency transformer whose operating frequency is above 1MHz.

2. According to the working frequency band

(1) Single-frequency or narrow-frequency high-frequency transformer, which refers to a single-frequency or a very narrow frequency band, such as converter transformers, oscillator transformers, etc.;

(2) Broadband transformer, which refers to transformers that work in a wide frequency range, such as impedance converter transformers, communication transformers, broadband power amplifier transformers, etc.

When the transmission power of the high-frequency transformer is relatively large, the power device generally adopts IGBT. Since the IGBT has the phenomenon of tailing off current, the operating frequency is relatively low; if the transmission power is relatively small, MOSFET can be used, and the operating frequency is relatively high.

The role of high frequency transformer

1. To improve the conversion efficiency, the power supply of the high-frequency transformer is used. Because the power tube works in an instant on-off state, that is, a state of continuous conversion when one is on and one off, we call it "switching power supply". The loss is about 30% lower than the traditional iron core transformer. Unfortunately, the failure rate is relatively high.

2. It can transform high-frequency pulses. The iron core of ordinary transformers does not have the high permeability of ferrite cores. can't do it

3. To improve the conversion efficiency, the power supply of the high-frequency transformer is used. Since the power tube works in an instant on-off state, that is, a state of continuous conversion when it is turned on and off, we call it "switching power supply". The loss is about 30% lower than the traditional iron core transformer. Unfortunately, the failure rate is relatively high. What can be learned from is the comparison between old-fashioned ballasts and electronic ballasts.

The use of high frequency transformers

High-frequency transformers are power transformers whose operating frequency exceeds the intermediate frequency (10kHz). power transformer. According to the working frequency, it can be divided into several grades: 10kHz-50kHz, 50kHz-100kHz, 100kHz-500kHz, 500kHz-1MHz, and above 1MHz. When the transmission power is relatively large, IGBTs are generally used for power devices. Since the IGBTs have a tailing off current phenomenon, the operating frequency is relatively low; when the transmission power is relatively small, MOSFETs can be used, and the operating frequency is relatively high.

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