A Passive Negative Magnetic Reluctance Structure-Based kHz Transformer for Improved DC Magnetic Bias Withstanding

How to effectively withstand the dc magnetic bias is one of the key aspects of transformer design. General approaches either require additional active power electronics devices or apply complicated control strategies, causing the increased cost or reduced reliability. In this article, a passive negative magnetic reluctance structure (NMRS) is proposed and installed in the air gap of the open-core transformer, which could achieve improved dc magnetic bias withstanding capability without extra active devices. The key criterion of the proposed design is to employ the NMRS with a frequency-variation magnetic reluctance property as a magnetic bandpass filter, which shows a negative equivalent magnetic reluctance for the fundamental component of flux. Nevertheless, it presents a high magnetic reluctance for the dc component. With the proposed design, the transformer provides a strong withstanding dc bias capability and a low magnetizing current as well as high efficiency. Accordingly, both the advantages of open-core and closed-core transformers could be obtained with the proposed design. For verification, a 500-W EE55-based core prototype is constructed and verified with sinusoidal and square wave excitation. Electromagnetic analysis and experimental results are provided to demonstrate the effectiveness of the proposed design.