A Zero Net Flux Modulation Scheme for 3-Leg Core Transformer Based 3-Phase AC–DC Topologies

Abstract

Existing single-stage 3-phase ac–dc isolated topologies typically use three separate C-core high-frequency transformers (HFTs), which require significant space and reduce power density. A key challenge preventing the use of a more compact 3-leg core HFT is the lack of suitable modulation schemes, as current methods do not ensure a zero net flux condition considering all 3 legs. Violating this condition can lead to stray fluxes that induce common mode (CM) currents and high peak magnetizing currents, resulting in increased electromagnetic interferences and losses. This article proposes a new modulation scheme that, by strategic alignment of switching pulses for a dual inverter setup, both avoids CM winding voltage generation and preserves the shape of transformer secondary-side voltage waveforms for power transfer at twice the carrier frequency. These features allow the use of a single compact 3-leg core HFT. The scheme also offers: 1) high-quality 3/5-level ac pulse-width modulated grid voltages, 2) a 3-level high-frequency voltage for the power transmitted via transformer action, 3) higher overall average winding volt–seconds compared to dual active bridge converters at low modulation indices, and 4) reduced switching losses due to the discontinuous nature of the scheme. Adopting a 3-leg core can cut the size of 3 C-core designs by 35%–40% while achieving 4% total current harmonic distortion at full load. The presented modulation scheme is verified using both simulation and experimental results for a 250 V$_{{\text{dc}}}$, 122 V$_{\text{ac}}$, 750 W prototype.