Improving Thermal Performance of High Frequency Power Transformers using Bobbinless Transformer Design

Power transformer windings in most Switch Mode Power Supplies (SMPS) are conventionally wound around a plastic bobbin. Bobbins are support structures for the windings and provide electrical insulation between the winding and core. They are generally made from plastics like Polytetrafluoroethylene (PTFE), characteristic of low thermal conductivity (k similar to 0.25 W/mK). Hence, using a bobbin creates a large thermal resistance between the windings and the core. The present paper investigates the benefit of removing the bobbin on the thermal performance of a 3.3kW power transformer fabricated on a PQ4040 core. The modelling is carried out through coupled 3D multiphysics Finite Element Analysis (FEA). First, the electromagnetic model accurately evaluates the spatial distribution of core losses for given operating conditions (core material, frequency, primary voltage and current, etc.) and the evaluated core losses are coupled to the thermal model. The winding losses are also evaluated. Next, the thermal model predicts the temperature distribution in the power transformer based on the loss inputs from the electromagnetic simulation as well as the material and geometrical properties of the transformer. The thermal model accounts for all modes of heat transfer. Further, feedback is established between the electromagnetic and thermal model to account for changes in temperature dependent electromagnetic properties. The evaluated thermal performance of the bobbin and bobbinless transformers are then compared. The FEA result shows that bobbinless transformers have lower hot spot temperatures as compared to transformers with bobbins. Further, bobbinless transformers have a smaller temperature difference between windings and core (T-winding-T-core) as compared to transformers with bobbins, indicating lower thermal resistance between winding and core. Finally, the findings are experimentally validated on a 3.3kW SMPS platform with thermocouple measurements and Infrared (IR) Thermography. Bobbinless transformers are shown to be low cost and thermally superior alternatives to their conventional counterparts.