Optical Transformer Design with Additional Common-Mode Noise Reduction Winding for Flyback DC-DC Converters

This paper proposes a method and theory for reducing conducted noise in the frequency range of 150 kHz to 30 MHz in accordance with CISPR16 for flyback converter circuits, which are the most used topologies in isolated DC/DC converters. Among conducted noise, we focus on common-mode noise, which is the most important to counteract. Choke coils and Y-capacitors are commonly used as countermeasures, but they have the disadvantage of increasing the circuit volume. Therefore, we focused on transformers used in switching power supplies and confirmed that by adding shielded windings between the primary and secondary windings and designing the optimal number of turns, the choke coil commonly used as a countermeasure for common mode noise is no longer necessary and the volume can be reduced by 6%. While other studies have theorized a simple transformer structure in their analysis, this study presents a design that considers transformer variations due to transformer design errors by theorizing so that the analysis can be performed even with complex structures, based on practical applications. A 98.3% reduction of common-mode noise was confirmed by simulation using the proposed common-mode noise reduction method. The number of shield windings optimal for common-mode noise suppression is also demonstrated through measurements on actual equipment in accordance with CISPR standards.