High-frequency transformers play a crucial role in power electronic systems, performing voltage conversion and energy transfer. Their performance is directly influenced by the winding process. Distributed capacitance and leakage inductance are key parameters; the former increases energy loss and exacerbates electromagnetic interference, while the latter causes voltage spikes and reduces conversion efficiency. Optimizing the winding process can effectively balance these two contradictions, improving the overall transformer performance.
Distributed capacitance originates from electric field coupling between windings. When a voltage difference exists between adjacent windings, the plate effect forms parasitic capacitance, especially during high-frequency switching, where repeated charging and discharging exacerbates energy loss. For example, during primary winding switching, the inter-turn voltage difference charges and discharges the distributed capacitance, an effect more pronounced in high-power scenarios. Leakage inductance arises from incomplete magnetic flux coupling to the secondary side, generating back electromotive force at the moment the switch is turned off, potentially damaging the device; it also forms an oscillating circuit with the distributed capacitance, radiating electromagnetic interference.
Optimizing the winding structure is the core method for reducing distributed capacitance. Layered winding, by distributing the windings across different layers and increasing the inter-layer distance, can significantly reduce electric field coupling. For example, sandwiching the secondary winding between two primary windings to form a "P-S-P" structure reduces the capacitance between the primary and secondary windings and suppresses noise through shielding. The sandwich winding method, by alternating the primary and secondary windings, increases the coupling area, reducing leakage inductance to less than one-third of the traditional structure. However, it's important to note that the capacitance between the primary and secondary windings may increase, requiring the addition of a copper foil shielding layer and grounding to suppress interference.
The winding direction and wiring method significantly affect parameters. All windings must be wound in the same direction to avoid phase cancellation leading to efficiency loss. When winding by hand, a winding machine should be used to tighten the conductor and eliminate gaps to reduce vibration noise; simultaneously, avoid leaving gaps between turns to prevent magnetic flux leakage. For high-current applications, using Litz wire or multi-strand stranded wire can counteract the skin effect and reduce AC impedance; flat wire or flat-band conductors can reduce winding length and further optimize parameters.
Insulation is crucial for balancing performance and safety. Interlayer bonding requires 0.05mm to 0.1mm polyimide tape, which is both high-temperature resistant and puncture-resistant. Special insulating tape is needed between the magnetic core and windings to prevent high-voltage breakdown. The primary-secondary spacing must meet safety regulations, typically ≥6mm, which can be achieved using barrier tape or triple-insulated wire. For high-voltage scenarios, the secondary winding can be segmented, distributing the total number of turns evenly in series to reduce interlayer voltage difference and thus reduce distributed capacitance.
Shielding design effectively suppresses electromagnetic interference. Winding a 0.9mm wide copper foil between the primary and secondary windings, overlapping and welding the ends, and grounding at a single point forms a Faraday shield, blocking common-mode interference paths. Note that the copper foil width should avoid multiples of 1mm to prevent short circuits in the magnetic lines of force that could cause a sudden drop in inductance. For multi-output scenarios, a shielding layer can be added between the auxiliary and main windings to further isolate noise.
Process details determine the final performance. When grinding the air gap in the magnetic core, it must be kept flat, and insulating sheets should be inserted to prevent short circuits caused by debris. After winding, it needs to be vacuum-impregnated with epoxy resin to enhance heat dissipation and mechanical strength. The testing process must verify the inductance and leakage inductance, ensuring that the leakage inductance is less than 5% of the primary inductance. The withstand voltage test must meet the AC 3kV/60s or DC 4.2kV standard, with a leakage current <5mA. During full-load aging tests, the coil temperature rise must be <95℃, and the magnetic core temperature rise must be <110℃ to ensure long-term reliability.
Through structural optimization, directional control, insulation reinforcement, shielding design, and process refinement, high-frequency transformers can improve efficiency and anti-interference capabilities while reducing distributed capacitance and leakage inductance. In practical designs, depending on the power level, frequency range, and cost constraints, a flexible selection of layered winding, sandwich structure, or segmented winding should be made, along with shielding and insulation treatment, to achieve a balance between performance and reliability.
