High frequency transformer principle

1. High frequency transformer principle - Introduction
High frequency transformer is the most important component of switching power supply, and the ratio of turns of each winding coil of high frequency transformer determines the output voltage. It is a power transformer whose operating frequency exceeds the intermediate frequency, mainly used as a high-frequency switching power transformer in high-frequency switching power supplies. The transmission power is relatively large, and the operating frequency is relatively low; The transmission power is relatively small, and the operating frequency is relatively high. In this way, there are differences in both operating frequency and transmission power. The design methods of power transformers with different operating frequencies and grades are different, and it should also be self-evident.
2. High frequency transformer principle - operating principle
High frequency transformer is the most important component of switching power supply. Open connection power supplies typically use a half bridge power conversion circuit. During operation, two switching triodes are turned on in turn to generate a high-frequency pulse wave of 100 kHz. Then, a high-frequency transformer is used to reduce the voltage and output low voltage alternating current. The ratio of the turns of each winding coil of the high-frequency transformer determines the output voltage.
The main reasons for electromagnetic interference in high-frequency power transformers are also the attraction between the magnetic cores and the musical force between the winding wires. The variation frequency of these forces is consistent with the operating frequency of the high-frequency power transformer. Therefore, a high-frequency power transformer with a working frequency of about 100 khz will not generate audio noise below 20 khz for no special reason.
3. High frequency transformer principle - design principle
In the design of high-frequency transformers, the leakage inductance and distributed capacitance of the voltage transformer must be minimized, because high-frequency transformers in switching power supplies transmit high-frequency pulse square wave signals. During transmission transients, leakage inductance and distributed capacitance can bow, surge current, peak voltage, and top oscillation, resulting in increased losses. Generally, the leakage inductance of a transformer is controlled at 1% to 3% of the primary inductance.
Leakage inductance in the primary wire country - The leakage inductance of the voltage transformer is caused by incomplete coupling of magnetic flux between the primary and secondary coils, between layers, and between turns.
Distributed capacitance - The capacitance formed between the windings of a transformer, between the upper and lower layers of the same winding, between different windings, and between the windings and the shield is called distributed capacitance.
Primary winding: The primary winding should be placed at the innermost layer, which can minimize the length of wire used for each turn of the transformer primary winding, thereby minimizing the wire used for the entire winding, effectively reducing the distributed capacitance of the primary winding itself.
Secondary winding fine - After winding the primary winding, add (3-5) layers of insulating pads before winding the secondary winding. This can reduce the capacitance of the distributed capacitance between the primary and secondary windings, and also increase the insulation strength between the primary and secondary windings, meeting the requirements for insulation and withstand voltage.
Bias winding - Whether the bias winding is wound between the primary and secondary layers, or the outermost layer, depends on whether the adjustment of the switching power supply is based on the secondary voltage or the primary voltage.