Low EMI Planar Transformer for an Isolated, Cascaded Buck-LLC Converter

Abstract

A key consideration in spacecraft applications is the conducted and radiated electromagnetic interference (EMI) generated by electronic components. One of the biggest sources of EMI is from the power system, caused by converters rapidly switching large amounts of current and voltage that generates unwanted noise. Without significant engineering considerations of mechanical and electrical layout, EMI can have serious impacts on other spacecraft systems such as communication equipment. Isolated supplies typically use a forward or flyback converter topology with a coil wound toroidal transformer. The transformer can experience a high voltage impulse called an inductive kickback during switching due to its leakage inductance, which contributes to overall power system radiated and conducted emissions. The transformer is a major source of EMI in currently available switching forward topologies, requiring a significant amount of input filtering, snubbing, and shielding. A high efficiency, isolated, cascaded prototype utilizing resonant switching techniques was developed in the form of a Buck-LLC utilizing a traditional wire wound transformer. With the successful testing of a 200W, 1MHz, Buck-LLC converter utilizing gallium nitride (GaN) devices, a planar transformer design was desired for an improvement in overall converter efficiency and EMI performance. In this work, a new transformer winding design is presented for a planar transformer using paired Litz winding interleaving. Ansys finite element analysis (FEA) software is used to verify design parameters. The winding configuration is designed to be compatible with a standard PCB stack-up so that the transformer windings can be directly integrated into the converter PCB even further reducing leakage inductance and increasing power density.