The core challenge facing planar transformers in high-power density applications is heat accumulation. This problem stems from copper and core losses caused by high-frequency switching, as well as the constraints imposed by compact structures on heat dissipation paths. To address this issue, a heat dissipation system must be constructed from multiple dimensions, including material selection, structural optimization, thermal conductivity design, and active cooling technologies.
The low-loss characteristics of the core material are fundamental to suppressing heat sources. In high-power density scenarios, traditional ferrite cores suffer from poor temperature stability of magnetic permeability, leading to a surge in losses at high temperatures. Using nanocrystalline alloys or cobalt-based amorphous cores can significantly reduce hysteresis losses, with their permeability variation with temperature being over 60% lower than that of ferrite. Furthermore, the core shape must be tailored to the heat dissipation requirements. For example, the cylindrical design of the EC core reduces flux concentration at the edges, improving surface temperature uniformity by 40%.
Optimizing the winding structure can reduce copper losses and improve heat conduction. Traditional spiral windings reduce their effective current-carrying area at high frequencies due to the skin effect. Using multi-strand parallel windings or Litz wire can reduce AC resistance by 30%. More importantly, alternating the primary and secondary windings through multi-layer PCB lamination shortens the heat conduction path. For example, in a six-layer PCB design, placing the primary winding on layers 1, 3, and 5, and the secondary winding on layers 2, 4, and 6, allows for rapid lateral heat dissipation through the inner copper foil of the PCB, reducing thermal resistance by 55% compared to a single-layer winding.
Optimizing the heat conduction path requires a combination of material and structural innovations. Applying thermal grease or phase change material to the contact surface between the core and the PCB can eliminate the 0.1-0.2mm air gap and triple the heat transfer efficiency. For applications exceeding 200W, a copper- or aluminum-based heat conducting plate can be embedded beneath the PCB. By matching the thermal expansion and contraction coefficients of the metals, the increase in contact thermal resistance caused by temperature cycling can be reduced. A server power supply case study demonstrated that the use of a copper-based thermal pad reduced the hotspot temperature of a planar transformer from 125°C to 95°C, while increasing power density to 28W/in³.
The integration of active cooling technology is key to breaking through the heat density barrier. A microheat pipe array uses capillary forces to circulate a liquid working fluid, achieving a heat flux density of 100W/cm² without external power. Embedding the evaporating end of the heat pipe in the base of the magnetic core and connecting the condensing end to heat sink fins reduces thermal resistance from 1.5°C/W for passive cooling to 0.3°C/W. For extreme heat density scenarios, a combination of a thermally conductive condenser (TEC) and a liquid cold plate can achieve localized hotspot temperature control, but this comes with a trade-off in energy efficiency. This approach is typically suitable for pulsed loads with large power fluctuations.
Three-dimensional packaging technology offers a new dimension in heat dissipation design. Vertical chip stacking using through-silicon vias (TSVs) increases the interconnect density between the power management IC and the planar transformer by 10 times, while also shortening the heat conduction path. For example, TI's PowerStack™ package integrates two NexFET™ MOSFETs and a controller in a 3.5mm x 3.5mm footprint, reducing on-resistance to 0.8mΩ and increasing current capability to 60A. This design reduces thermal resistance from 0.5°C/W to 0.2°C/W by embedding copper pillars in the chip stack.
Simulation-driven design optimization can proactively mitigate thermal risks. Thermal-fluid coupled simulation using ANSYS Icepak or FloTHERM accurately predicts temperature distribution under various operating conditions. Simulations for a new energy vehicle OBC (on-board charger) project revealed that the original design had localized hot spots at an ambient temperature of 40°C. Increasing the gap between the magnetic core and the PCB from 0.5mm to 1.2mm reduced the hot spot temperature by 18°C, demonstrating the value of simulation in guiding design optimization.
The essence of planar transformer thermal design is the cross-optimization of thermodynamics and electromagnetics. From low-loss core materials and flatter winding structures to shortened heat conduction paths and integrated active cooling technology, innovation at every stage must balance electrical performance and thermal reliability. With the increasing adoption of third-generation semiconductor devices, planar transformers are evolving toward "zero thermal stress," providing core support for high-power density scenarios such as data centers, 5G base stations, and new energy vehicles.
