High frequency transformer design principles

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 the transient process of transmission, leakage inductance and distributed capacitance will cause surge current and peak voltage, as well as top oscillation, causing increased losses. Usually the leakage inductance of the transformer is controlled to be 1% to 3% of the primary inductance.

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. 

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 in the innermost layer, so that the length of each turn of the primary winding of the transformer can be minimized, thereby minimizing the use of wires in the entire winding, which effectively reduces the distribution of the primary winding itself. capacitance.

Secondary winding - after the primary winding is wound, (3 to 5) layers of insulating padding must be added and then the secondary winding is wound. This can reduce the capacitance of the distributed capacitance between the primary winding and the secondary winding, and also increase the insulation strength between the primary and secondary windings, meeting the requirements of insulation withstand voltage.

Bias winding - Whether the bias winding is wound between the primary and secondary, or on the outermost layer, is related to whether the switching power supply is adjusted based on the secondary voltage or the primary voltage.