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

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-dc, 122 V-ac, 750 W prototype.