Low parasitics planar transformer for LLC resonant battery chargers

Recently, high efficiency LLC resonant converters have gained popularity to implement battery chargers and employ traditional wire-wound transformers. Due to the height of traditional cores, the form factor of LLC chargers is often plump and bulky. In order to implement slim profile chargers for EVs and portable electronic applications, planar transformers can be used featuring low height, reproducibility, and low thermal resistance. Despite those advantages, planar magnetics suffer from very high parasitic capacitance resulting in severe regulation problems for LLC chargers that require wide voltage regulation. This paper analyzes LLC regulation issues with planar magnetics and proposes a novel strategy to provide wide regulation by reducing the effects of intra-winding capacitance. The proposed strategy employs a simplified capacitance network and achieves low parasitics to successfully design LLC chargers with wide regulation. Experimental results of a 1.2kW LLC converter confirms that the proposed winding layout not only provides better output voltage regulation in light load conditions, but also increases the overall efficiency of the power converter.