Planar transformer design requires optimizing the core structure to improve power density and efficiency. The core's core strength lies in balancing the properties of the core material, its geometric parameters, and the electromagnetic-thermal coupling relationship. The choice of core material directly affects losses and efficiency. In high-frequency applications, low-loss ferrites or nanocrystalline alloys should be prioritized. These materials exhibit lower hysteresis and eddy current losses in high-frequency alternating magnetic fields, significantly reducing energy dissipation. Simultaneously, the core geometry must be well-suited to the planar structure. A flattened core design shortens the magnetic circuit length, reduces magnetic reluctance, and thus improves magnetic flux density uniformity, preventing efficiency degradation caused by local saturation. For example, using a core structure with a cylindrical center post optimizes winding layout, reduces copper losses, improves magnetic shielding, and suppresses electromagnetic interference.
The air gap design of the core is crucial for optimizing leakage inductance and losses. While a concentrated air gap can prevent magnetic saturation, it can lead to excessively high local magnetic flux density, resulting in a significant increase in core temperature and consequently exacerbating losses. By introducing a distributed air-gap structure, a single long air gap is divided into multiple short air gaps, effectively dispersing magnetic flux, reducing local hot spot temperature, and maintaining stable excitation inductance. Furthermore, the distributed air gap requires precise calculation using a magnetoresistance model to avoid uneven magnetic flux distribution caused by diffused magnetic flux. To address the edge effects caused by the air gap, the design of the core corner areas can be optimized, employing rounded transitions or core block splicing to reduce magnetic flux concentration and further reduce losses.
The utilization rate of the core window directly affects power density. Traditional planar transformers often suffer from low copper fill factor due to a mismatch between the core window area and the winding cross-sectional area. Customized core design, such as using EIR or cylindrical cores, can achieve a precise match between the window height and the PCB stack thickness, increasing the copper fill factor to over 85%. For example, the coordinated design of multi-layer PCB windings and the core window can significantly increase the conductor cross-sectional area by increasing the number and thickness of parallel copper foil layers, thereby reducing DC resistance and AC impedance. Furthermore, integrated design of the magnetic core and PCB, such as embedded core structures, can further shorten the heat dissipation path, improve heat conduction efficiency, and ensure high power density.
Controlling parasitic parameters at high frequencies is another key aspect of core structure optimization. Planar transformers, due to their small winding interlayer spacing, exhibit significant distributed capacitance and leakage inductance issues. By employing an interleaved winding layout, alternating between primary and secondary windings, the coupling coefficient can be enhanced, reducing leakage inductance energy storage. Simultaneously, optimizing the relative position of the core and PCB, such as placing the core close to the winding area, can reduce magnetic field leakage and suppress common-mode noise. For distributed capacitance, conductive materials can be coated on the core surface or a shielding layer can be added to create a Faraday cage effect, blocking high-frequency current paths and thus reducing electromagnetic interference and energy loss.
Thermal management is a crucial constraint for core structure optimization. Due to the high power density of planar transformers, temperature rise control is particularly critical. The core material must possess low thermal resistance and high thermal conductivity; for example, using a metal-based composite core or surface nickel plating can improve heat dissipation efficiency. Meanwhile, the core structure needs to be designed in conjunction with the heat dissipation system. For example, adding a thermally conductive silicone pad or embedding copper heat dissipation channels at the bottom of the core can quickly conduct heat to the casing or heat sink. For high-power scenarios, liquid cooling or forced air cooling solutions can be used. By optimizing the flow field distribution on the core surface, the convective heat transfer coefficient can be improved, ensuring that the temperature rise remains within a safe range.
Integrated core design is an innovative direction for improving power density. For example, in a three-phase CLLC resonant converter, a cylindrical planar core structure can be used to integrate six resonant inductors and three transformers, significantly reducing the size of magnetic components. This type of design, through joint optimization of the magnetic circuit model and loss model, ensures balanced load current in each phase, improving system stability and efficiency. Furthermore, co-packaging designs of the core and power devices, such as directly integrating the core into the PCB substrate, can further shorten interconnection distances, reduce parasitic parameters, and achieve a balance between high power density and high efficiency.
Optimization of the planar transformer core structure requires collaborative advancement from multiple dimensions, including material selection, geometric design, parasitic parameter control, thermal management, and integrated innovation. By employing customized core design, distributed air gap technology, staggered winding layout, and integrated packaging, power density and efficiency can be significantly improved, meeting the stringent requirements of high-frequency, high-power-density power electronic systems.
