In high-frequency transformer design, skin effect and proximity effect are key factors contributing to increased AC losses. Both are closely related to current frequency, conductor structure, and magnetic field distribution. Skin effect concentrates high-frequency current on the conductor surface, reducing the effective conducting area and increasing AC resistance. Proximity effect, due to magnetic field interactions between adjacent conductors, forces current to concentrate in specific areas, further exacerbating losses. When these two factors work together, copper losses in a high-frequency transformer can increase exponentially, directly impacting efficiency and temperature rise. Therefore, loss balance requires a systematic design approach.
To address skin effect, high-frequency transformer design prioritizes optimizing conductor structure. Traditional single thick conductors experience a sharp decrease in effective cross-sectional area due to skin effect at high frequencies. Litz wire, by twisting multiple thin insulated conductors together, reduces the diameter of each conductor to a much smaller diameter than the skin depth, significantly expanding the current distribution area. For example, at frequencies of hundreds of kHz, Litz wire can reduce AC resistance to less than one-third that of a single conductor. Furthermore, the use of flat copper foil can reduce the proximity effect by increasing the width-to-thickness ratio and reducing the number of layers, making it particularly suitable for high-current secondary windings.
To mitigate the proximity effect, efforts must be made to address winding layout and magnetic field coupling. Using a "sandwich winding" method (e.g., primary-secondary-primary cross-winding) allows for close coupling between the primary and secondary windings, reducing magnetic flux leakage. In this structure, the magnetic fields generated by adjacent windings are directed in opposite directions, partially offsetting the eddy current effect and reducing proximity losses. Furthermore, reducing the number of winding layers and the distance between them can further mitigate the magnetic field superposition effect. For example, a two-layer winding structure can reduce proximity losses by approximately 40% compared to a multi-layer design.
The choice of core material is crucial for balancing these two effects. Ferrite cores are the preferred choice for high-frequency transformers due to their low eddy current losses. Their high resistivity effectively suppresses eddy currents within the core, reducing additional losses due to the proximity effect. Furthermore, ferrite cores have excellent B-H curve linearity, which reduces hysteresis losses, thereby indirectly alleviating temperature rise issues caused by the skin effect. At switching frequencies exceeding 200kHz, the overall losses of ferrite cores are significantly lower than those of metal powder cores or amorphous materials.
Optimizing the operating frequency requires a balance between efficiency and losses. Both skin effect and proximity effect losses increase with increasing frequency, but lowering the frequency may affect the power supply's dynamic response. Therefore, high-frequency transformer design must select the optimal frequency range while meeting system bandwidth requirements. For example, in an LLC resonant topology, adjusting the resonant frequency can balance the transformer's skin effect and proximity effect, achieving the optimal solution for both loss and efficiency.
Heat dissipation design is crucial for balancing losses. The excess heat generated by skin effect and proximity effect in a high-frequency transformer must be dissipated promptly through thermally conductive materials or forced air cooling. Filling the gap between the core and winding with thermally conductive silicone improves heat transfer efficiency. For miniaturized designs, integrated cooling plates or fans are essential. For example, in server power supply high-frequency transformers, optimizing the heat dissipation structure can keep temperature rise within a reasonable range, preventing efficiency degradation caused by overheating. Simulation and experimental verification are the ultimate guarantee of design effectiveness. Electromagnetic field simulation tools such as Maxwell 3D can accurately predict the distribution of skin effect and proximity effect losses, guiding winding structure optimization. For example, simulations have shown that a cross-winding structure can reduce proximity losses by over 25% compared to traditional layered winding methods. Actual prototype testing requires a combination of temperature rise experiments and efficiency measurements to verify that the design parameters meet the expected targets and ensure the reliability of the high-frequency transformer under complex operating conditions.
