The winding thickness of a planar transformer is one of the core parameters affecting its power density and temperature rise. Its design requires a balance between current carrying capacity, high-frequency losses, heat dissipation efficiency, and manufacturing cost. The winding thickness directly determines the cross-sectional area of the conductor, thus affecting the resistance and thermal effects when current flows through it. In low-voltage, high-current applications, insufficient winding thickness significantly increases conductor resistance, leading to a sharp increase in copper losses (I²R losses). This not only reduces efficiency but also causes insulation aging due to localized overheating, shortening the transformer's lifespan. Conversely, simply increasing the thickness to reduce resistance, while reducing copper losses, may increase high-frequency losses due to the skin effect and proximity effect, while also occupying more core window area, limiting the improvement of power density.
The skin effect is a key constraint in winding thickness design under high-frequency operating conditions. As the AC frequency increases, the current concentrates on the conductor surface, resulting in a decrease in the effective conductive cross-sectional area and an increase in the equivalent resistance. If the winding thickness exceeds twice the skin depth (Δ=√(2/(μωσ)), where μ is permeability, ω is angular frequency, and σ is conductivity, the inner conductors will not be able to effectively participate in conduction, resulting in material waste and increased losses. For example, in MHz-level high-frequency applications, the skin depth may only be tens of micrometers. In this case, using excessively thick windings will actually reduce efficiency due to the low current density in the inner layers. Therefore, the winding thickness should be selected according to the operating frequency during design, making it close to or slightly less than the skin depth to maximize the utilization of conductor material.
The proximity effect further complicates the relationship between winding thickness and losses. When multiple windings are closely arranged, the alternating magnetic fields of adjacent conductors will interfere with each other, leading to uneven current distribution and a significant increase in local current density. This effect is particularly pronounced in thick windings and may cause local overheating and additional losses. For example, in parallel winding structures, if the thickness of each layer is inconsistent or the spacing is too small, the proximity effect will cause current to concentrate in certain areas, forming hot spots and threatening the reliability of the transformer. Therefore, optimizing winding layout (such as using interleaved winding or sandwich structures) and controlling interlayer spacing are important means to reduce proximity effect losses.
Temperature rise is a direct feedback indicator of winding thickness design. Heat generated by winding losses needs to be conducted to the external environment through heat dissipation structures (such as PCB copper foil, core surface). If the winding is too thick, although it can reduce resistance, it may lead to excessive temperature rise due to insufficient heat dissipation area or increased thermal resistance. For example, in multilayer PCB planar transformers, the inner layer windings typically have a higher temperature rise than the outer layers due to their longer heat dissipation paths. In this case, if the winding thickness is not properly designed, the inner layer may experience accelerated aging of the insulation material due to excessive temperature, or even insulation failure. Therefore, thermal simulation and experimental verification are necessary to ensure that the winding thickness meets power density requirements while keeping the temperature rise within a safe range.
Power density is one of the core advantages of planar transformers, and winding thickness is a key factor affecting its improvement. Within a limited core window area, increasing the winding thickness can improve current carrying capacity, thereby increasing power density. However, excessively thick windings occupy more space, limit the number of turns, and may reduce performance due to core saturation or increased leakage inductance. Therefore, it is necessary to control winding thickness while maintaining power density by optimizing the winding structure (e.g., using multiple layers of thin copper foil in parallel) and the core material (e.g., high-saturation magnetic flux density ferrite).
Manufacturing cost and process feasibility are also important considerations in winding thickness design. While thicker copper foil can reduce resistance, it may increase costs due to increased processing difficulty (e.g., etching precision, interlayer alignment). Therefore, a trade-off must be struck between performance and cost, choosing standard thickness copper foil (e.g., 1 oz or 2 oz) and compensating for insufficient thickness through design optimization (e.g., increasing the number of parallel layers).
