1. Working Principle

High frequency transformer is a component that converts alternating voltage, current, and impedance. When an alternating current flows through the primary coil, an alternating magnetic flux is generated in the iron core (or magnetic core). At the same time, the voltage (or current) is induced in the secondary coil.

The transformer consists of a transformer core (or core) and a coil. The coil has two or more windings. Where the winding connects to the power supply is called the primary coil. The other windings are called the secondary coils.

2. Design

Let's talk about designing high frequency transformer. The transformer in the switching power supply transmits a high-frequency pulse square wave signal. The leakage inductance and distributed capacitance of the transformer must be minimized.

During the transient process of transmission, leakage inductance and distributed capacitance cause surge current and peak voltage, as well as gyro oscillation, resulting in increased loss. Usually the leakage inductance of the transformer is controlled at 1% ~ 3% of the primary inductance.

· Inductance leakage of primary coil

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

· Distributed capacitance

What is it? It is the capacitance between the turns of the transformer winding, between the upper and lower layers of the same winding, between different windings, between the winding and the shielding layer. You can find it in many places.

· Primary winding

The primary winding places in the innermost layer, so that the length of each turn is the shortest. Thereby it minimizes the wires in the entire winding and effectively reduces the distributed capacitance of the primary winding.

· Secondary winding

After the primary winding is wound, add 3-5 layers of insulating lining. Then wind the secondary winding. In this way, the capacitance of the distributed capacitance between the primary winding and the secondary winding reduces. The insulation strength between the primary winding and the secondary winding increases. The requirement of insulation withstand voltage can be met.

· Bias winding

The bias winding may be wound between the primary and secondary or the outermost layer. It determines whether the switching power supply is adjusted according to the secondary voltage or the primary voltage.

3. Application

The high frequency transformer is a power transformer whose operating frequency exceeds the intermediate frequency (10 kHz). It is mainly used as a high-frequency switching power supply transformer in a high-frequency switching power supply. It also works as an inverter power transformer in a high-frequency inverter power supply and a high-frequency inverter welding machine.

The component divides into several grades according to the working frequency: 10k Hz-50k Hz, 50k Hz-100k Hz, 100k Hz-500k Hz, 500k Hz-1M Hz, above 1M Hz. When the transmission power is large, the IGBT is generally used as the power device. The operating frequency will be low due to the tail phenomenon of the shutdown current. If the transmission power is small, MOSFET is available. The operating frequency is high.

4. Case

The conversion efficiency of conventional transformer is low. The size is large. If it is made into a mobile phone charger, the volume is very large and inconvenient to carry. In order to solve this situation, high frequency transformers appeared.

As the name implies, the input end of the high-frequency transformer is high-frequency alternating current. The alternating current is rectified into direct current after passing through the rectifier bridge. The direct current is turned into a very high-frequency alternating current under the action of the PWM controller.

The alternating current is stepped down by a high-frequency transformer. Under the condition of the same power, the iron core and number of turns of the high-frequency transformer are less than those of the power frequency transformer, which reduces the size of the transformer. It also reduces the size of the charger.