Thermal characterisation of planar transformers

Planar inductors and transformers are widely used today in various electronic applications either as stand alone devices or integrated in the printed circuit board (PCB) of the electronic assembly. In the frame of co-operation the Slovenian partner in a bilateral Romanian-Slovenian co-operation has presented an idea regarding the development of a DC/DC converter as a hybrid electronic product. The key part of the converter is a planar transformer (PT), this design being necessary due to the request for miniaturisation of the final module. The PT shall be finally integrated into the substrate of the converter but firstly shall be realized as a stand alone component. For the substrate a multilayer structure, based on FR4 substrate, shall be used. The authors have considered that the planar technology is the only one which produces high performance results, that is suitable for a modern hybrid power converter. Furthermore, planar technology provides enhanced performance and valuable manufacturing benefits over conventional technology. The planar technology shall lead to magnetic and electrical benefits in both a low leakage inductance and low windings AC resistance. On the other hand, the cooling of such transformers can be of some concern. The paper presents the modelling and simulation of the thermal regime of the transformer. From the estimated work frequencies, at about 300 kHz, the power losses are calculated and used as loads in a subsequent thermal analysis. The results will be very useful in order to estimate the thermal stresses of the magnetic core and of the PCB.