Miniaturized Planar Transformer Embedded in Ferrite Substrate for Isolated High-Frequency Low-Power Applications

This paper presents the development, fabrication, and characterization of planar transformers for isolated low-power applications operating at switching frequencies between 1 MHz and 5 MHz. The planar transformer is embedded in a ferrite substrate, allowing for a high integration level, using thick-film fabrication technology. The device features a footprint of 5 mm x 5 mm and has a profile height of only 2 mm. The planar transformer is designed with turns ratio of 1:3, has a primary inductance of 1.3 mu H, a secondary side inductance of 11.8 mu H, a coupling factor of 0.8, and an isolation voltage of 3.5 kVDC for 1 Sec. Initial functionality is tested using a 5 V-to-5 V, 0.5 W, and 2.25 MHz isolated buck dc-dc converter. Also, the functionality is tested with a 5 V-to-1.2 V, 0.5 A and 2.25 MHz buck converter. The experimental results collected with both converters prove the suitability of using the embedded planar transformer in the targeted applications.