Active DC-Bias Mitigation Method for a Single-Phase Transformer-Connected Converter through DC-Link Measurement

DC-bias in the magnetic flux is a common challenge for transformer-connected converters. Typically, in isolated switched-mode converters, it is induced by a DC current flowing through one or more windings of the transformer. This bias can increase the core losses of the transformer and drive the transformer core into saturation, which deteriorates the performance of the transformer and can even result in the malfunctioning of the converter. Conventional methods for DC-bias mitigation involve expensive sensing circuitry and/or impair the performance of the converter. The proposed method utilizes the fact the DC-bias current through the transformer will result in a switching-frequency current component in the DC-link current of the converter. A band-pass filter can for example used to indirectly measure the switching-frequency current component and then mitigate that in a closed-loop manner. Both simulation results and experimental measurements are provided to verify the theoretical analysis and effectiveness of the proposed method.