Planar transformers, with their low height and high power density, are widely used in high-frequency power electronics. However, improper winding arrangement can easily lead to increased AC resistance and eddy current losses, thereby reducing efficiency and causing localized overheating. Optimizing the winding arrangement structure can effectively reduce the negative impacts of high-frequency effects and improve the overall performance of the planar transformer.
Under high-frequency operating conditions, the skin effect concentrates current on the conductor surface, resulting in a reduced effective conductive cross-sectional area and a significant increase in AC resistance. The proximity effect, due to the interaction of magnetic fields between adjacent windings, generates eddy currents inside the conductor, further exacerbating losses. These two effects are closely related to the winding arrangement. A reasonable structural design can weaken the uneven distribution of high-frequency current, thereby reducing AC resistance and eddy current losses. For example, using an interleaved winding layout, with primary and secondary windings alternating, can reduce the area of magnetic field superposition and suppress eddy currents caused by the proximity effect. Simultaneously, by controlling the magnetic field cancellation between winding layers, the difference in current density on the conductor surface can be reduced, mitigating the impact of the skin effect.
Interleaved winding arrangement is one of the core strategies for reducing losses. Traditional parallel windings with a non-interlaced structure, where the magnetic field direction is consistent across all layers, easily form strong eddy current loops in the conductor, leading to a surge in losses. A cross-winding arrangement, by alternating layers of primary and secondary windings, ensures that the magnetic field directions of adjacent windings are opposite, partially canceling out the magnetic field strength and thus reducing eddy current generation. For example, in a double-layer winding structure, if the primary and secondary windings are located in different layers and are cross-arranged, the magnetic field strength in the middle layer can approach zero, significantly reducing adjacent losses. Furthermore, the cross-winding arrangement optimizes the current distribution path, allowing high-frequency current to flow more evenly across the conductor cross-section, further reducing AC resistance.
Symmetrical cross-winding layouts further reduce losses by optimizing the magnetic field distribution. This structure symmetrically distributes the secondary windings on both sides of the primary winding, forming a "sandwich" arrangement, causing the induced eddy currents generated by the primary magnetic field in the secondary windings to cancel each other out. For example, in a center-tapped transformer, the secondary winding is divided into two parallel structures, arranged on the upper and lower sides of the primary winding, respectively. Through magnetic field symmetry design, induced eddy currents in the non-operating winding can be eliminated, while simultaneously balancing the current distribution in the operating winding. This layout not only reduces proximity losses but also lowers leakage inductance by increasing winding coupling, thereby suppressing voltage spikes during switching and improving system reliability.
Optimizing the number and thickness of winding layers requires balancing the skin effect with manufacturing processes. At high frequencies, when the conductor thickness exceeds twice the skin depth, the internal current density decreases significantly, leading to reduced material utilization. Therefore, using multiple layers of thin copper foil in parallel can effectively expand the conductive cross-sectional area while controlling the thickness of each layer to suppress the skin effect. For example, using three layers of 0.2 mm copper foil in parallel results in lower AC resistance than a single layer of 0.6 mm copper foil, and the total thickness more easily meets the miniaturization requirements of planar transformers. Furthermore, the multi-layer parallel structure can reduce heat concentration in a single conductor by dispersing the current path, improving heat dissipation efficiency.
Winding gaps and arrangement density are crucial for loss control. Appropriate winding gaps can reduce interlayer capacitance and capacitive losses at high frequencies, but excessive gaps can increase leakage inductance, affecting energy transfer efficiency. Therefore, a balance must be struck between leakage inductance and capacitance. Typically, a minimum safe gap is used in conjunction with high-dielectric-constant insulating materials to achieve low-loss design within a limited space. Simultaneously, increasing winding density can enhance magnetic coupling and reduce magnetic field diffusion, thereby reducing eddy current losses. For example, by using precise PCB manufacturing processes to control winding gaps at the micrometer level, window utilization can be maximized while ensuring insulation performance, thus increasing power density.
Comprehensive simulation and experimental verification are crucial for optimizing winding arrangement. Finite element analysis software can simulate the magnetic field distribution, current density, and loss characteristics under different winding structures, providing a theoretical basis for design. For example, Maxwell 3D simulation can accurately calculate the AC/DC impedance ratio of windings and evaluate the effect of cross-layout on suppressing adjacent losses; Ansys thermal simulation can analyze temperature distribution and verify the rationality of heat dissipation design. For experimental testing, building comparative prototypes and measuring the efficiency and temperature rise of different winding structures under the same operating conditions can visually verify the optimization effect. For example, in full-load testing, planar transformers employing a cross-winding layout exhibited improved efficiency and reduced temperature rise compared to traditional structures, validating the design's effectiveness.
Optimizing the winding arrangement of a planar transformer requires a comprehensive design approach encompassing multiple dimensions, including high-frequency effect suppression, magnetic field distribution control, structural parameter matching, and simulation verification. By employing strategies such as cross-winding layout, symmetrical magnetic field cancellation, multi-layer thin copper foil parallel connection, and precise gap control, AC resistance and eddy current losses can be significantly reduced, improving the efficiency and reliability of planar transformers in high-frequency applications.
